A Material World: a Renaissance at the Atomic Scale

so hello

actually it's not even me you should be applauding i'm just giving the introduction

so i'd like to welcome you all to the slack public lectures this installment will be about the

material world it'll be given by rob moore who is

one of our staff members here rob got his undergraduate degree in mechanical engineering from tennessee

tech then he went into the navy and spent two years in a submarine

and from that experience whatever it does to you decided to go into physics

he has his phd from the university of tennessee and he came here in 2006 to

work at our x-ray laboratory now he's the assistant director of symes

which is the slack stanford laboratory for materials research and

energy research and he's going to tell you about the future of materials

so let me introduce rob and uh

all right thank you thank you oh my public i love it so um

i'd like to thank you for the introduction and you know it's a thrill to be here and it's it's it's gonna be a lot of fun talking about some of our

latest research and some of the things that we're doing here at the lab now there there is one thing though you

know i i would also like to thank sly communications for hosting the event and they also made our nice wonderful poster

here now but i i do have to you know wonder you know with the materials theme and

the material world and and and all of the fashions you know i just wanted to to make

sure you're aware that you know we're not bringing back the 80s that's not where we're going okay so you

know i find i kind of feel like we should have a disclaimer down here so just to make sure it's it's not going to be the 80s

but you know i did actually request sort of a a poster with a fashion theme because my daughter's actually here

watching me tonight and um so that means that i'm actually competing with her cell phone and the gains on us so i'm

trying to have something that sort of appeals to her fashion sets so we'll see how i pull it off we'll see

how it works so now when you think of a material world what what what images come to your

mind i mean do you think about madonna being a material girl in a material world

all of the fashions and all of the trends and latest gadgets and all of the stuff

that we fill our lives with or do you envision some sort of world to where the boundary between technology

and human existence is becoming so narrow that it's blurred as we you know go through and scour

under every other rock around the earth trying to find that last ultimate material unobtainium

so you know these are pretty extreme examples but you know but we can't think about for a second what is the boundary

between technology and our lives and so how far away are you from a

technological device so think about even when you're sleeping i mean you may have a cell phone on your

nightstand maybe your alarm clock what about a pacemaker

and so it comes down to it that technology is everywhere and everything that you see comes down to being built

out of something that was made from superconductors i mean not superconductors but semiconductors

and so you know it's so that's what we'll be talking about tonight and so for me you know i like to know how

things work and so that's the physics background but i also like to build things i like to build things that

can make an impact on the world around us and so that's the engineering background

and so you know when i go to dinner parties people often ask me well what is it i do and you know it's kind of hard to

explain you know within the the few minutes that are typically allowed for a casual conversation at a dinner party

but you know now you know i have your attention for an hour and i've got slides

so um so yeah this is what we do all right so this will be fun so

so there's basically two sides to my job one of them is making materials

and then the other one is actually looking at the electronic structure of the materials by shining light on it

now this seems rather straightforward right but but let's dig just a little bit deeper

but i want to start with just a very brief history to kind of give us sort of a perspective

of where we're going and and what materials mean to our lives

so about 3.3 million years ago are the first historical records of rudimentary

tools made out of stone now stonehenge was made around 3000 bc

and so what does this mean so it takes about 3.3 million years for us to go

from very rudimentary simple objects to being masters of stone from rudimentary

to ornate about 3.3 million years to become masters of all things stone

but then about 6000 bc we learn how to smelt copper extract

copper metal from you know oral that we find in shallow mines and such

and so then about 3000 bc we actually learn how to take copper and mix it with

tin to form bronze now bronze has an age made you know named after it simply because you know

bronze is a strong enough material to where you can make very you know useful farming implements hunting tools weapons

for defense and so you know that that's what really gives it its age

and so that was about 3000 bc and then we come up to about 1200 bc to where we can actually learn how to start

working with iron we developed the technologies to hot work iron control the carbon content

and so this is a situation to where we can get to the point to where we can make just about anything that we want

out of metals and so while this spans several different ages this takes about 5 000

years from rudimentary to becoming masters of every all things metal

five thousand years now the end of the iron age is is you know

was with the development of writing as we go from pre-history to history

but from our perspective as we actually start looking at the journey of different materials and and how they impact our lives

then we need to come to semiconductors and so semiconductors the the very first idea of a transistor

started to appear in the 1930s and semiconductors really started to have some sort of practical applications in

radar technologies in the 40s so but it wasn't until 1947 when the

very first transistor actually became a reality and so from that we actually go from 47

in the mid 70s we actually start having a lot of computer technology and then in the mid 80s we start having the internet

the propagation of all of these internet service providers and the birth of the information age

and so semiconductors from the very first devices made out of just a chunk of a semiconductor all the way

to masters of all things semiconductor about 60 years so to get an idea of today's materials

and sort of the development cycle for today's materials let's talk about something that we call giant magneto

resistance now this is a material property to where you actually can

change the resistance of a material by small changes in a magnetic field

now this is one of the key technologies that allowed us to shrink hard drives down to write smaller and smaller bits and to

read smaller and smaller bits in magnetic media and so we go for about 10 years in 1997

as the first hard drives with the gmr head and then in 2006 we actually start

having micro drives small enough that we can fit in you know pocket held devices

now i really like this picture because it's cute but i pulled it off the web several

years ago and so by now the chicken has already flown the coop and i'm not talking about the chicken

so this is a situation to where now today we actually have solid state hard drives that far exceed the capacity

of a lot of these these magnetic media but it's based on a completely different material technology

but this is sort of where we are today now to gain some perspective

does everybody remember these raise your hand if you know what this is wow yeah all right maybe this is going

to be the 80s night all right i love it so floppy disks

so these were all the rage back in the 80s and so the floppy disk if we actually think about well how much

information can we store here compared to the floppy disks and well

it's about 750 000. now if we actually stack all of these

floppy disks up here on the stage it would fill a box that's about 10 feet wide 10 feet long and 12 feet high

but what's even more interesting is the simple fact that that box of information

would weigh more than eight toyota camrys it's hard to believe that our

information weighs that much

so as we look at this brief evolution of materials and how it impacts our lives there's two things that i really want

you to take home one of them is that the time between materials

discovery and becoming masters and implement implementing that material that time is shrinking exponentially

now the other thing that i'd really like to point out is the fact that fashions have dramatically changed along with the

materials and the technologies so i mean i would really i feel sorry for the person who had to wear this hat

i mean that had to be uncomfortable and the shirt you know that's a great looking shirt but i just don't see me

being able to pull that off i just don't think i could do it

so as we go through the history and we come up to today

and so as we look at today what are we in the state of the art now there are a number of different materials that are

emerging in different technological markets today you have electronic devices flexible

devices sensors batteries wearable devices smart building materials

and so a whole vast array of different materials that are coming on the market and making the dramatic impact on our

world and today i'm actually going to talk about three of these one of them is in the opto electronics

industry to where you can think about coupling light and all the information that you can carry with light think of

fiber optics and how we couple that with our electronic devices the other one is is regarding smart

windows these are windows that can change the transparency to with the flip of a switch in order to help save energy

and last is actually trying to look at superconductors and so trying to figure out how to make better

materials better wire better you know transport energy transport materials and

so if we can actually find a superconductor we could save the world trillions of dollars of wasted energy

just due to the resistance that are in the wires today but the next generation the next age

is going to be down at the level of atoms and electrons and so when you get down to this level there's actually many

more exotic properties that are out there we have superconductivity but we also have things like topological

insulators multi-feroics thermoelectrics photovoltaics

photonics spentronics catalysis there are a lot of very exotic interesting

properties that happen at the level of atoms and electrons and the key is is how can we capture those properties and

make useful devices and interesting things out of them but to start to do that we have to understand them on the

most fundamental of levels so how do we actually make these

materials and how do we design you know look at these exotic properties so here at slack we've actually

developed a program that is dedicated to the investigation of fundamental material

properties that actually generate and responsible for all of the exotic

properties that we see that are not only interesting from a fundamental perspective but could actually be useful for something we

could do something with now this is a program that is actually a collaboration between two different

divisions or a user facility in the the materials science division here at slack

and so this collaboration we have symes the material science division at slack which is actually looking at the growth

of these materials and then ssrl the user facilities to where we can actually map out the electronic structure

and so some of the people in the audience you might actually recognize this instrument here because it was one of the instruments on the recent global

physics photo walk and so this is a photo walk to where amateur and professional photographers tour the different labs and take

pictures of certain instruments and so one of our instruments was the highlight but it didn't win

i know i'm just gonna have to get over that because i mean we have by far the prettiest instrument here at slack i

mean but anyway i'm just gonna have to get over that and so slack is a department of energy

national lab and of course this program is funded also by the department of energy this is

your national lab this is what we are doing for you

so let's dig a little deeper and let's have some fun so let's see if we can understand how materials work on the level of atoms and

electrons let's start with the atom so in the atom you have a nucleus that's

formed of protons and neutrons and protons are positively charged and electrons are negatively charged that

actually orbit around the nucleus and you can see that the electrons are much much lighter than

the protons even though the charge is of the same magnitude

now these electrons take on different energy levels inside the atom and so you know the the higher the orbit

the higher the energy the you know the more energy the electron has as it whizzes around the nucleus but in

addition to that we normally think about you know an electron when it orbits you know the

nucleus we often think about you know an analogy with the the planets orbiting the sun

or the moon orbiting earth but here the electrons actually have

very strange shaped orbits and the higher the energy level you go the more different types of orbits you

can actually find the electron in

so for now let's just think about the simplest of these orbits the s orbital which is just a spherical cloud of

charge if you will now think about this as as a cloud simply because um if you ask well where

is the electron well then you can find it say it's right there but if you don't ask where's the

electron well you'll never know because it's at all places in the cloud at all times

now i know that sounds kind of strange but that's just quantum mechanics for you and so you know i i know it's weird but

you know i i'm sure there's a lot of cat lovers in the audience and so i'm not going to go into detail about what schrodinger

proposed to do to his poor cats you know in a thought experiment trying to explain the quantum phenomena

so we'll just think of this as clouds for now and so if you actually have several atoms together with these overlapping

clouds and you you know this electron right there if you found one right there which atom does it belong to

it's hard to tell what about here you just don't know

you can't tell and the electron can't tell and so if you find an electron here then

you know you can't tell what atom it is and so it's just a basically a cloud of charge that has these periodic array of

atoms inside and so in these conditions the electron is just free to move throughout the

material just wherever it wants to go but now what about here

so in this situation you know if you find an electron right there well it belongs to that atom but if you find one

in here well it's hard to tell but these are different you know these are representing the p orbitals and so the

different orbitals have different sizes different shapes and different overlaps and so in this particular condition if

you have an electron here it kind of hops one atom to the next

now you can tell that it would be easier to hop in one direction than the other

and so the orbits actually play an important role in determining how the electronic structure works how the

electrons actually move through the material so we start making materials we can start taking our atoms

and then we start taking all these different kinds of atoms and start moving them next to each other and you have all of these overlapping energy

levels and what we end up with is something that we call an energy band

now when you think of an energy band think of it in terms of like a road map

and so what this does is it's energy band in momentum and in energy so you know an electron right there it's

basically saying that it's moving in a particular direction with a particular mass and a particular speed

through the material and it takes that much energy for that to happen for that electron to

do that and so just think of the energy bands as a road map if you will

now the shape of this inner the the shape of these electron these energy bands are actually determined by the

types of atoms we have in our material and the locations of the atoms this is what kind of generates the shape of these energy bands

so now we have all of these energy bands we have all of these electronic states but each atom only has a certain number of

electrons in it and so there's always going to be some you know empty states that are of the

same energy and our little example here you know the p orbitals can hold up to six electrons but there's only one right

up here so there's a bunch of extra states in which you could put electrons

and they would be at the same energy as this and so what this means in our materials

while you can think of this as a road map it's kind of a strange road map and the fact that if you add another

electron you know exactly where it's going to go it's going to go to the lowest energy state that's not occupied

by some other electron and so it's a strange road map but it always fills up from the bottom up up to

the maximum level and the electrons that have the maximum energy at the very top we

call this the fermi level or the fermi energy

now i show here also the fact that you know the electrons have spin to it you can kind of see in you know the arrows

pointing up and arrows pointing down now spin think of it as you know the magnetic moment of the electron

and so if you have a material to where you have electrons that are on all of the atoms then they're all pointing in

the same direction you're going to generate a net magnetic moment and this is a magnet just like on your

refrigerator however if you actually have them to where they're pointed the opposite way

then this is an anti-ferromagnetic and so these spins actually determine the magnetic properties of the material

itself so

when we actually have these energy bands and we actually have the fermi level that cuts through one of these energy

bands so that you have unoccupied states just right above the occupied states

then you have a situation to where the electrons can actually use these little unoccupied states to kind of hop through or freely move through the materials

and so in these particular cases these are metals if you have a situation to where the

fermi level is actually between these two energy bands to where you have all of this band here which is occupied but

this band which is completely empty then you're going to have to have a certain amount of energy to kind of jump

the gap and to go from one road to another road and so we call this a band gap

and in this situation none of the electrons can move because there's no free states for them to move in

and so this is an insulator and so the filled states we call it the valence band up here to where if you

could put electrons up there they would move around we call this the conduction band now if the gap is small enough

to where if you have just a little bit of heat or just add a few extra electrons in it then it's going to start

to conduct electricity then this is what we call a semiconductor

now we've been talking about some toy models here and it's it's fairly you know um

simple to go you know with the toy models but as you start looking at real materials things get really complicated

really quick and so this is actually a theoretical calculation of the band structure of the

road maps of a particular material and showing the energies and the momentums of of the electrons as they go

in different directions in different momenta through the material so things can get pretty complicated

really quick now we talked about heat and how it can sometimes add energy to

electrons so what is this so we have what we know is is is phonons

now think of the lattice as an array of just balls connected with springs

and so what happens if you just whack that array of atoms right there what's going to happen well you're going to

have this wave that just starts propagating through this chain of atoms and springs

and so all of these different ways the atoms can move all of these different waves that propagate through these are

different phonon modes and so this is how the lattice can actually transmit energy through the

system now for more complicated materials to where you actually have a lot more complicated structures inside

the materials then the phonons are going to be even stranger and more complicated as you

actually have sort of a all these different atoms moving in concert together representing these

different phonon modes so phonons are how sound propagates through material and it's how heat

propagates through material oh there so

what types of players do we have so we have the band structure and the band structure is determined by the position

of the atoms and the types of atoms that are in our material and then we have all of these players that actually influence the electron the charge of the electron

the spin which orbital it is it is in and all of this lattice energy all of these things

can influence how the electron behaves but there's one other player and i already gave you a hint of it there's

one there's one other player which we haven't talked about yet which is critical to our story here

and that is light and so electrons can absorb light

and electrons can emit light so now that we know the players

what is the game so the idea is we talk about being able to look at materials and control

materials on the atomic level with atomic precision so how do we do that and so we do that by using a technique

called molecular beam epitaxy and so molecular beam epitaxy just think of boiling boiling water in a pot

so as you heat the water up you start to form steam and steam comes out the spout

so what we do is we have something similar to this we have little small pots where we put really pure

materials inside then we stick these pots inside of a chamber and pump out all of the air

and then we just heat up the pots of different materials and outcomes are atoms

and so we shine these atoms on some sort of crystalline substrate some of these things come on and just bounce off and

absorb and some of them hit the surface and they'll start rattling around until they find an ideal crystalline position to

where they can have a chemical reaction to form the new material that we're trying to form

and so in this way we can actually start with the substrate and start growing materials layer by layer by layer

now the neat thing about this technique is the fact that we can stack up atoms in so many different configurations

and you know so it's um and so we can you know start looking at

you know how to control all of these properties and we can actually make things that you're just never going to find in nature

and so that's the beauty with this technique but now one of the things that we need to do

is to understand you know the electronic structure of these materials now this is a very well established technique a lot

of the semiconductors and a lot of the technology that we know today you know has roots in a lot of these

thin film techniques and molecular beam epitaxy is one of them but there's a caveat as we normally can't look at a

lot of the materials created by mbe we can't directly look at the electronic structure

and it's not because of a material problem it's actually because of the electron itself and so when we're

trying to look at the behavior of the electrons the electrons of the material if they come out if you actually take the material out and take it out into

atmosphere then all of the atmospheric contaminants moisture co2 co hydrocarbons anything that's

going to sit on the surface it may not affect the material but it's going to destroy the signal that we're trying to measure

and so that's the trick is how do we get around that

so let's look at the flip side of our program how do we actually look at the electronic structure

and so with the synchrotron at ssrl so we take a long chain of

electrons let's say about a meter long and then we accelerate these two really close to the

speed of light now from our perspective sitting in the laboratory this whole chain of atoms

gets shrunk to just a fraction of its original size and this is relativity

and so as we whizz the these bunches of electrons around the ring in certain

sections of the ring we pass them through a series of magnets that are you know oriented opposite of each other

and so what this does is the magnetic field actually interacts with the electrons and they start to

oscillate as they move through the undulator now remember i said that you know

electrons can actually emit light as it turns out whenever you accelerate charge it radiates light

and so we use that simple fact by having these high energy electrons go through our undulators and by

controlling how they oscillate we can actually generate high intensity x-ray beams

with specific energies and then we just take these beams of light and then

shine it onto our sample now as we shine it onto our sample

that you know the the photon can come in which is just a quantum of light

and it can actually you know couple with the electron the electron absorbs the energy and it can jump to a higher

energy state now the caveat is is the phone is the the the quantum of

light or the photon and absorb is actually equal to the difference between those two energy levels

now the opposite can happen if you have an atom that has an electron in an excited state

as it decays back down to that ground state the state of lowest energy it can

emit photons as it actually jumps from energy level to energy level and so the game that we play is we

actually shine the x-rays onto our sample and we actually look at how much

of the the photons are absorbed and we actually can look at you know how much of the photons are actually emitted

afterwards and what this does is it tells us about the electronic structure and hello

okay all right don't go to sleep now all right

okay uh all right intermission i guess right okay so um

and so what we do is we can actually collect you know the photons coming out and we can look at how much of the x-rays are actually absorbed and it

tells us about the electronic structure the energy spacing between the different levels of the atoms inside the material

now there's a lot of different flavors of this technique and the ones we're talking about today one of them is called x-ray photoelectron spectroscopy

this is basically where we take a photon it excites an electron to an excited state but this electron has so much

energy it just leaves the material and as it leaves the material we actually collect it and resolve how much energy

it had below the fermi level and so you can see it it has very distinct fingerprints and these are

distinct spectroscopic signatures of the materials that we're looking at and then x-ray absorption spectroscopy

we actually sweep the uh the energy of the incoming light and look at how much of the energy is actually absorbed and

this is another way in which we can actually look at the electronic structure and the spacing between the actual energy states and the positions

of the atoms inside the material there's one other technique that we'll

be talking about angle resolve photoemission spectroscopy now this is a situation to where we

shine our light on the material the electrons have so much energy that they just fly out of the material and then we

pass those electrons through our detector and actually map them onto a two-dimensional sort of ccd camera if

you will and so on our ccd camera we have one axis that's energy and then the other

axis is momentum now we can stack all of these plots together to generate a multi-dimensional

representation of the electronic structure inside the material itself

now if you actually look at it here up at the the fermi level here if you actually look at the multi-dimensional

map of the fermi level these are all the states and we call this the fermi surface this is the the all of the electronic uh

states that are at the maximum energy in the multi-dimensional space it becomes a surface and so this

is a a fermi surface

now we know that the players are very limited the number of players

that actually interact with the electron are very limited but we do need to think about a bit of perspective here

and so in order to get some perspective let's think about a one centimeter cube

of stuff you know just one centimeter or just some other little shape that you want to

actually think about that's one centimeter cube and then ask yourself you know well how many atoms are in here

that's a lot of atoms now for metals you're going to have a very similar number of electrons

in there as well free electrons able to move around but

now electrons are all negatively charged and they repel each other very dramatically

and so if you have all of these electrons in there that are repelling each other and squeezing them into the space it doesn't take too much in

intuition to to realize that strange things are going to happen

and so if we want to actually think about what is going to happen we have all of these

different interacting particles and this is what we call mini body fixes because there's so many of them

but now out of this you can actually get collective modes you can actually get sort of of

you know uniform behavior for all the individuals now and since this is you know 80s night

you know in order to try to understand this concept you know i thought i'd help you know bring someone in to help explain what this concept is and what do

you mean by collective excitations and collective modes and so you know without further ado you

know i would like to introduce our guest speaker and so ladies and gentlemen please welcome madonna

i'm sorry is this on hello uh guys in the control booth did madonna

not get our invitation no no oh okay um so well i guess she's not going to be

here tonight but nonetheless you know she still helped us explain what it meant by collective behavior

because as soon as i said her name everybody's eyes went to the door and so just a very small amount of

interaction we can have a room full of individuals that all did the exact same thing

this is what we mean by collective behavior now out of this collective behavior you can actually have a lot of

emergent properties that form in the material and these emergent properties is what generates a lot of the exotic

things that we see in the material physics and we want to learn how to understand them and be able to control

them so in our materials

we actually look at the electrons coming out but are protected detectors are so sensitive enough that we not only look

at the energy and the momentum electron coming out but we can have indications fingerprints of how all of

these different parameters affect the electronic structure

how it affects the behavior of the electrons inside the material and so these are the emergent phenomena

the collective information and the collective modes that we're actually looking at in the materials and and what does this look like

so this this has very different shapes and fingerprints and such in our our data you can appear as band gaps

you can look at band kinks you can have shifts of spectral intensity you can have folded bands you can have split

bands you can have replica bands and all of these things are indications of the different players that are influencing

the behavior of the electrons to generate these exotic properties that we see

and so at ssrl we take the latest and greatest rpes synchrotron we shine the light down it and and shine it on of our

samples and this is how we actually map out the electronic structure of the materials itself

but now in order to look at a lot of the thin films that we're growing via molecular beam epitaxy

we combine these two techniques together and so this is a massive system but we actually have growth systems here to

where we can develop different films and then we move them just straight into our rpes system to where we can map out in

very precise detail the electronic structure of the materials

and so with that i'd like to take um some time to actually look at some of

our results so now that we've seen what the electronic structure is and how can we measurement what does this mean what are some things

we can do with it so the first little story here is is something that you know i'd like to talk about

smarter than you think windows and so what this is these are

electrochromic nano composite nickel oxide materials i tried to say that one 10 times in a

row really fast i'd like to see that so what these are is these are basically

these are materials that that'll change color when you apply a voltage and so the idea is you can coat them on windows

and so um and this is something to where you know if it's a hot day you can flip a switch it blocks the sun and it's nice

and cool inside so these are are you know used for smart buildings

in order to help save energy now the material the material study in this particular case was lithium nickel

tungsten oxide and this is a collaboration with the national renewable energy laboratory to where

they actually made the materials and then we investigated them at ssrl

and just to give you a scale that the actual material is right here the little color green and it's about 80 nanometers

thick so this is the x-ray spectrum that we actually got from the materials now you

can kind of see that that you know the top curves look kind of like each other the bottom curves are pretty close to

each other but comparing the two they're just completely different and the same with our xps spectrum it

just looks like different materials and so with this it this actually reminds me

of um one of my favorite science quotes and it's actually by a a a

scientific or a science fiction writer and the quote is you know the most exciting phrase to hear in science the

one that heralds new discoveries is not eureka but that's funny

now this was kind of weird we weren't expecting this but now what's even more weird is this

peak right here i mean that's just strange why is that there that just doesn't make any sense what is that

and so the what it turns out is the techniques the techniques that we were using are very surface sensitive xps and

if you look at the total electron yield of x-ray absorption spectroscopy is very surface sensitive

and so as we would start to cycle these materials start to add charge take charge out then it actually started to

form lithium peroxide on the surface and this peak here is a very unique fingerprint for lithium peroxide it

can't be anything else and so what this is telling us is we actually start looking at sort of a

charged and discharge state or a transparent and opaque state

then we're starting to form this lithium peroxide on the surface and so what happens then the lithium comes from inside the

material to form lithium peroxide on the surface and then as we start to discharge it the

the um or charge it back i guess the lithium peroxide dissolves and the lithium goes back into the material

and so what this does is it actually shifts the amount of charge inside the material which actually you know

generates some extra states inside the material that allows the absorption of

light so we learned from a very fundamental level what makes it change color and why

but also what's interesting about this particular case is the fact that the the movement of charge the movement of

lithium ions into and out of material in the formation of lithium peroxide this is exactly like a lithium-air

battery and so what we're saying is we have an auto tinting window that acts like a

battery you shine the light on it it changes how many electrons like to hang out on the

nickel changing the number of states available for the absorption of light

but also it's it's acting just like a solid-state lithium-air battery

so let's take just a second and kind of go from the world to science into science fiction and what does this mean

so let's envision we have a small building or a house that has lots of glass on it

and let's put solar cells on top of the building so as the sun comes up the solar cells

will start to generate energy and it gets the hot part of the day your solar cells will generate so much energy

that you don't know where to put it where you're going to stick it stick it in your windows

so as the sun goes up the heat comes in you store the charge in the windows to

help minimize how much heat is getting inside the building and then as the sun goes down and your solar cells are

starting to generate less charge you can just pull the charge out of the windows and they become clear again in the

evening now this is pure science fiction this doesn't exist and a lot of it is probably because capacities there's only

going to be so much charge you can store in windows and you know buildings require a lot of energy to operate

but nonetheless this is something to wear a lot of the latest battery storage technologies these this type of scenario

is on the horizon but it's going to be much better than this

so let's take a look at our next little example but now for this one we have to take a short quiz

yeah i know you gotta love it right so what are these things should i start asking questions and point to people

so remember our energy bands so we have our energy bands you know occupied unoccupied states

now the way that we draw this is necessarily so i mean there's no reason that these things have to be lined up

with each other and so if we have a situation like this

and you know we shine a photon on this well then we can actually excite an electron into this energy

level and the reason for this is because the photon has negligible momentum

compared to the electron and so it can generate these atomic transitions these energy band

transitions but it can't change direction which means this has to be a straight line up and down

and so on this side we can actually generate a transition but you require a much more energetic photon one with much

shorter wavelength much more energy to reach from this state all the way up to there

and so is is there no way if we say we only have this wavelength of photons is

there no way we can actually get over to those states it turns out there is

and this is how we can start thinking about electron phonon coupling how the electrons and all these lattice vibrations

coupled together now one way to think about electron phonon coupling is to actually start

with the hamiltonian and then you know let's not think about it this way

let's think about it this way oh yeah let's think about the physics of the

alley-oop

and that cool i don't care who you are these guys are impressive it just it's awesome

so if you think about the the idea of an alley oop you have one player that just lobs the ball up somewhere close to the

goal and then you have another player comes over and just finishes the job so let's

think about this in perspective of electrons so let's just think of the basketball is our electron

and so you know one player it goes up you know it has the energy to make it it

has the distance but all of a sudden a phonon comes by and just makes it change direction and goes into the state that

we want right through the basket and so this is sort of what we mean by

electron phonon coupling so we have a situation a photon comes in it starts to excite an electron but then

all of a sudden a phonon comes by and just gives it a push of momentum to actually make it over into the state

and so now this is possible however it takes a really concerted effort of the phonons the

lattice the electron the light and so it's possible but it's just a very low probability of it actually

happening that way not when it is compared to something like this

now in our sort of loose basketball analogy here you know a direct band gap transition

is something to where you can envision yourself right above the basketball goal now i

cannot play basketball but i could probably make that shot

and i could probably make it most of the time and so this is a situation to where you

know you don't need any change of direction it's just a straight energy shift without changing the direction of

the electron now the neat thing about quantum mechanics is this is actually backwards

the scenario that we're actually talking about you would have to envision the basketball setting on the floor all of a sudden getting light absorbed a photon

and it just jumps up through the hoop right into your hands that's the cool thing about quantum mechanics because this is how energy is

exchanged on the quantum mechanical level and so this is a direct band gap transition and if it's over to the side

we call that an indirect band gap transition and so for these next materials

transition metal dicalcogenides and so these are materials that are very interesting because

if you look at a material that's just one layer thick then you have this very large non-linear

response to light now this is pl means photoluminescence so this is basically a technique to where you absorb you know

you shine light on a particular wavelength it the electronic sights up and then it comes right back down for

that energy it's just a straight direct transition and so you can see for one layer you have this very large response to the

light but if you grow it two layers thick or thicker nothing happens

it just doesn't you know the light just doesn't do anything to it now theory has suggested that this is

actually responsible this is actually due to a transition from an indirect to a direct band gap as you shrink down

this material to one layer thick and so if we actually so we grew some of

these materials and if we actually look at some of our data now this is eight layers of molybdenum disillinide

and so here is our rpes spectrum now one of the tricks that we do in order to understand because here you

have a situation where here's the valence band maximum but you don't see anything up here because our fermi level

is inside that gap now one of the tricks that we do is we take just a little bit of potassium and

sprinkle it on the surface now potassium is an alkali metal and so it has one electron in its outermost

shell and it doesn't like to be there so as soon as you put the the alkali metal on the surface the electron jumps

off and actually goes into the material so we're actually shifting

the position of the fermi level due to adding this potassium and so when we do this and take a look at our

spectrum then all of a sudden you can see the bottom of this conduction band right

there up at the fermi level but now you can see that yeah these are not lined up with each other this is an

indirect band gap but now as we actually shrink this thing

down to one layer and do the same trick where we add potassium on it then all of a sudden you

can see this is a straight shot from the valence band maximum to the minimum of the conduction band

and you can see what happens is just the shape of these bands actually change as you shrink down to one single layer one

single unit cell of the material but now something else i want to point

out is the fact that you see the little blue arrow and a little red arrow here that's because

this band the spins of one orientation and the spins of a different orientation have

different energies and this has to do with the symmetry inside the material but the splitting of

spin states can even be observed inside our spectra

now these and this is interesting not only from just a fundamental materials perspective

this is a situation to where you can actually think about using these types of materials and devices

because we can expand the bandwidth and the efficiency in our opto electronics the coupling of the

information that's inside light and how it interacts with our actual devices

these can actually improve greatly improve on the level of quantum mechanics greatly improve

the efficiencies of these materials and it has to do with just this simple fact you're going to a direct band gap

as you shrink down this material to one layer thick and theory suggests that this could be

an ideal and there's these little transition metal dicatrice there's a whole zoo of these materials

that have similar properties that can be explored but what's even more interesting than just the fact that you have this tunable

direct band gap is the fact that because of the symmetry inside the system

you can think of energy bands at different locations in the material and you can actually couple to different

energy bands depending on the light that you shine on them whether it's right polarized light or left polarized light

you can couple to either wait which all right the red's right all right so you can couple to the red

bands or you can couple to the blue bands now you couple that or you combine that

with the fact that you know our spins in our valence band are at different energies and now all of a sudden you

have different channels to which you can couple to the electronic states in the materials

and so this is very unique in improving not only the efficiency of the transport of the information but also information

storage and allows for things like valetronics and spintronics where you're actually

looking at the different valleys and the different spins of the material and so this is a very rich phase space

for us looking at the next generation of opto-electronic devices and storage and it's all possible with this

direct band gap resistance is futile

super conductivity so our last little topic today is based on superconductors now superconductivity is electrical

current without resistance it's the free flow of electrons and they

don't have any interactions with the lattice that would help you know destroy that energy it's free-flowing without

resistance now superconductivity was discovered back in 1911

and then in 1957 bcs theory came about that kind of

explained how these superconductors work and to think about how a superconductor

works is think about our lattice array here and as the electron is moving through

well then you have this negatively charged electron interacting with these positively charged lattice

and the lattice will start to distort this is that electron phonon coupling if you will

however you know in this scenario think about you know the atoms are moving very slowly to where the electrons are

zipping through there very fast and so as this electron leaves the material then there is going to be a net

positive charge if you will right in this area which is going to attract another electron

and so in this way these electrons actually will start to pair up and can move through

the material without resistance due to the assistance of this phonon this electron phonon coupling

now this is how conventional superconductors work but then in 1986 there was discovery of the cuprates and

what we call high tc high temperature superconductors now these materials the way that they

work yeah i don't know it is still complete mystery today we don't understand from

the most fundamental level how these materials work

although that doesn't mean we can't do useful things with them still nonetheless and so the material that i'm going to be

talking about was actually um discovered in 2012 by a Chinese group and it's basically these iron selenide films now

it says films but basically bulk iron selenite thicker materials a

chunk of iron solenoid is indeed a superconductor but the problem is is the fact that its

superconducting transition temperature is really low to where these films if you shrink it

down to a single unit cell it's an order of magnitude larger

in temperature that you actually have these superconducting properties now that's just kind of weird and we

want to understand why that is and how can we take advantage of that

so these iron-based superconductors there's a whole you know zoo of these different superconductors and they have all of

these different crystalline structures but all of the action all of the superconducting properties

lives right inside this iron selenium layer right here and so as we go through the different

materials you can kind of see the the changes in the shape of the fermi surface actually actually start putting

different amounts of electrons on the iron atoms as they move through the material

so this is how we can actually change the shape of the fermi surface and see how it interacts and then you

know changes the properties of the material now arp has on these iron solenoid thin

films angle result photomission spectroscopy so this is some of our data and so you can kind of see that if you

look at just one layer and even two layers of iron selenide i they're different the electronic

structures are just completely different and so that in itself is sort of strange enough as it is

but then you know there was actually something else that kind of appeared that kind of puzzled us a little bit

these little replica bands now these red club commands actually reminds me of one of my favorite science

quotes by a science fiction writer the most exciting phrase to hear in science

the one that heralds new discoveries is not eureka but that's funny

so this is a situation to where this is really it's not supposed to be there and it's never been seen before in

a solid-state material with such clarity and so and these are key are key to what

is going on in these materials and how they behave and so we also did our little trick to

where we can sprinkle some potassium on it to change how many electrons are in the band structure and actually

change the electronic structure and so if we take something just three layers thick to where it looks like a

bulk like electronic structure we sprinkle a little potassium on it and all of a sudden we recover this

electronic structure of that single unit cell that single layer thick film

the difference is there's no replica bands

now the reason i really point that out is because

is because the fact that you know this is itself is actually a superconductor

and it actually has an enhancement of tc but it's just not near as big as that single layer thick film is and so what

are these replica bands we can go back to our our electron phonon coupling here but this is a completely different type

of electron phonon coupling in this case the phonon doesn't come in

and just change the direction of the electron in this particular case

the phonon comes in and just gives a slight kick in energy to the electron

but doesn't change its direction and this is very unique property and

this is what's generating that replica ban that unique signature inside our material

and so what we have is we have bulk materials that have a very low transition temperature for

superconductivity and then we can kind of sprinkle a few electrons on that change the electronic

structure and we have some slight enhancement but then it's coupling

with these phonons inside the substrate that is giving this turbo boost if you

will to the superconducting properties of these materials

now this is sort of of of unique and and you know just you know for the experts in the audience

i'd like to kind of push our little analogy with the soccer ball one step further because remo before when i was

talking about superconductors remember i said that we normally think about the ions moving very slowly while the electrons move very fast

and so typically the electrons are at a much higher energy scale than these very low energy lattice vibrations

but in our particular case in our particular case

the bottom of this band is only about 60 millivolts while the phonon energy is much higher than that

and so this is a scenario to where the phonon energy scale is actually larger than the electron energy scale

and so that's strange in itself and there's concerns about our adiabatic approximation but

my theorist friends tell me not to worry so i'm not going to worry so we have a situation to where we can

actually control the substrate where we actually have coupling between the phonons and the oxides that move across

the interface and actually couple to the electrons that are moving in this two-dimensional material

and it comes down basically for the experts it comes down to screening basically if you have an oxide substrate

here it's where you have these phonons you don't have electrons floating around that can screen

the oscillation of that charge and so that means that it gets really amplified and really connects to these

electrons flowing in this two-dimensional plane now

what's really really interesting about this is there's nothing special about iron selenide

so our theories will show that it's this anisotropy and the coupling in this dielectric

constant that is actually generating this turbo boost for the superconducting properties

so in principle we can actually stack together a lot of different materials and even think about cuprates

and still be able to generate this cross-interface coupling between these different parameters these different

players in our material and so this is sort of the direction that we would like to go

with here with those materials so um

i see that i'm right now out of time and i don't want to be you know any more than fashionably late

so i'd like to go ahead and just you know wrap it up here and so i'm hoping you have a little bit under better

understanding of what this image means and so you know here we actually grow

materials via molecular beam epitaxy and then be able to look at them at the electronic structure

very precisely using the user facilities here at ssrl

now there's one thing that i've kind of you know neglected in this study here and that is theory

so with our theory today and this is one of the things that sets our age apart from

previous ages is the fact that our theory is becoming ever and ever more sophisticated and

we're combining the theory with a lot of the advances in the information age we

have giant supercomputers bigger and better that can calculate more complex systems and our theory is getting better

so that as we calculate those more complex systems the answers are even more accurate

and so what we do is we can actually think about a feedback to where we have a theory that can predict a particular

material we can grow the material and then we can benchmark the theory to see if it's right or not and use this

to do a sort of a rapid materials developments to generate the properties that we're

looking for and so what we're doing is we're actually taking the toolbox the tools

that we have that nature has given us but we're actually generating materials that just don't form in nature

so these are more akin to supernatural materials if you will

so our program offers insights that will define the next age

and with the exponentially shrinking development cycle between discovery materials and implementation

into devices and such this age will happen within our

lifetimes now in a program like this can really

only happen at a national lab to where you can couple the material science division with the actual user facilities here at

the national lab this is your national lab this is your slack and where we're going with it

so now that we're actually here at the end i thought i'd have a few final thoughts about what the future may hold

now i was going to make a joke about you know these futuristic machines that you have in science fiction films

to where you go up to the computer and you ask for a cup of coffee a cheese sandwich and a light bulb and they all

just materialize right in front of you but then i read an article where we're starting to make sports cars with 3d

printed aluminum parts and so i'm not even going to try to make any predictions anymore

but now nature has given us a tremendous toolkit for solving some very tough problems

everything in your life is touched by fundamental materials research in some way

now the rabbit hole is deep there's a lot we don't understand alice has nothing on me or my colleagues

so as this is you know a fascinating world where such few players can actually generate the complexity of

everything we see around us but as we shrink the length scales of

our technology and be able to gain efficiency minimizing cost minimizing carbon footprints

we're going to shrink down to the two dimensional limits now this is where the electronic

structure and the material surfaces and interfaces will dominate the material properties in the physics that we see

and so the fundamental understanding of how the players interact on this level is how we can win the game

and so merging the information age with the material age leading to a materials renaissance and

we are just at the dawn of it and so here is slack we're at the forefront of the wave our mbe arpas

program is now built it is completed you saw the assembly of it and we already have our first data

so let the games begin and with the rapid materials developments that we see within our lifetimes it is going to be a fun ride

but now with regard to what future societies will look like it's going to be hard we

can see the the impact we can make on materials and the positive impact we can make in

everybody's lives but predicting future societies is going to be tricky just as it is with trying to predict

google facebook and twitter would have been the results from the discovery and the development of a semiconductor a

transistor it's hard to say but now there is one thing i do know

about the future and when that next material age happens i do know one thing

i know what i'm going to be wearing because i can totally pull that off

and so with that i would like to thank all of my collaborators who worked tirelessly on all this research and of

course our funding agency the department of energy and with that i would like to thank you

and i'd be happy to take any questions

all right so i guess these are new microphones we're all figuring out how to do it so um yes so

yes we have time for some questions now right now we have these new microphones

we would like you to make sure the microphone is working before you ask your question because

this lecture is being recorded it'll appear on youtube and so we'd like to record your question

as well as the answer what you're supposed to do is to push the button in the middle at the bottom and then the

microphone a red button will appear the microphone will turn on and then you can ask your question and please don't do

that until you're recognized by the speaker so that we don't have multiple people talking at once and we'll start

with this lady right here yes

oh wow okay that works all right okay um my question

is about um photons i was wondering is there a way to measure photons because i

know that they're like uh they're a wave that like propagates through an atom and that uh there's like

the photon push in elements with like electron coupling so can photons be

measured as a as an entity now it actually turns out that photons

can be measured as an entity itself um the the techniques of getting to the

point now to where you can actually start looking at to the point of single photon spectroscopy if you will

now a lot of the detectors and things that we use don't go down to that granularity now we do with the electrons

but not with the photons so our detectors in our pes is detecting one electron at a time

and it cascades into where we can actually generate the image on the detector so there we're actually looking

at single particle physics we're measuring one electron at a time and so while it is possible to actually

measure photons and the techniques that we use we're not going to quite that granularity for looking at electronic

structures thank you sure yes in the back

um so i was wondering like what percentage of time currently do you get

sort of like a that's funny reaction to your work versus oh that did what i expected it to do

like i'm just curious like like are you how often yeah like as your theory improves are you get are you able

to better expect what's going to happen or is it still at the stage where things are completely unpredictable um

well yes or no or actually i guess yes and yes um so this is a situation to where you can kind of see that about two

out of three times something strange happens we don't understand but our theory is becoming much more

sophisticated and much better improved and so with this we can actually you

know start thinking about having theories help us develop materials which is you know sort of what gives us this

this new dawn into the renaissance because our theory is becoming so sophisticated but that being what it is

i'm not worried as an experimentalist you know i i still have job security because we're always going to find

things that just doesn't match the theory that we just don't understand and so it really depends on how deep you

go uh is to there's always going to be something that that you know the theories in the end are models if you

will that helps us understand how the materials behave but there's always going to be properties and there's always going to

be aspects that just pop up that we don't yet understand and so from the very fundamental physics

of electrons and atoms there is still so much we don't know it's a very very deep rabbit hole

but it's nonetheless fascinating to investigate but for a practical sense for the actual

use-inspired materials of actually making things to make devices and useful technologies out of

we're at the point now to where our theory is becoming sophisticated enough that we're having a much more rapid turnaround time for

investigating materials and actually the predictive capabilities of new properties of materials

and so the transition method like dicalcinites was one that was led by theory topological insulators is another

class of material that's really led by a lot of the theoretical efforts and so there's now starting to turn to have

that turning point to where theory can start leading the development of a lot of new materials and this is what's new

we're actually to that point now but that doesn't mean that theory knows at all we're always going to have some cool

surprises that they have to figure out yes

hello

after you're done with your mic and you press the button to turn it off it looks like it's off

me maybe yes thank you oh okay sorry i guess i'll

take your question how about that i'll get yours in a second yes yeah it's like gay

and my question is number one if you can put back the chart with all the

materials it shows the temperature on the side you know how you showed the years versus

the temperature for superconductivity right so most of them are at much lower temperatures than normal room

temperature obviously yes is that one of the limitations

now you know one of the the caveats was super conductivity this one yes yes this

one and so yeah i i kind of glazed over that to not go into any detail but you know these temperatures

are really cold much well below room temperature

and so the ultimate goal the ultimate thing that we would love to do is to be able to design and build a

material that will be a superconductor at room temperature but we are very far away from that

that's the main limitation i would assume that is still we are very we're still very far away from that and a lot

of that has to do with the fact that still that the fundamental mechanism generating the superconductivity

generating the glue that pairs these electrons together so that they can propagate through the material

it's still unknown that's one of the things we can help but the hope here and the new avenue that

we're able to presume pursue now is the fact that with this study is we're starting to find new ways

to actually enhance some of these properties like superconductivity

and so you know for this film you know we can actually look at you know a dramatic superconducting enhancement and

so the idea is can we apply a lot of these techniques and a lot of these ideas to some of these other materials

to get the same similar boost in transition temperature now it may not get all the way up to room temperature

but the hope is that we can eventually get something at a high enough temperature to where we can use it for practical devices

like you know energy transport and transmission lines for electricity

now just to say that even today there are some substations and such that are

have current flowing between them using superconducting wires and so this technology is there today

but because the temperatures are still so low it's not really widespread and so this is where we'd like to be and

this is hopefully a path that will help us boost up getting closer to that ultimate goal

actually i know you let's see if we get the still no work there it is all right you'll not believe it but that was my

question okay thank you all right two questions with one answer excellent

yes that's a pretty interesting machine you have there and is it what's the

throughput and the cycle time between depositing with molecular uh eptaxy

a new material characterizing it generating this arpa's spectrum and then

understanding what that means with decades of education i'm sure

to recognize that something is unusual there and is there hope to automate that process to uh in the same way that in

the huge phase space of genetics and biology nowadays we just do shotgun shotgun approaches to find interesting

things do you have hope for that here and how can i help we'll probably not go that far um now

there is been a lot of of automation in the semiconducting industry for a lot of

these these thin film techniques like molecular beam epitaxy and so even the different versions of

these machines can have robots that move samples in and out and manipulate everything

and so if you actually start trying to think about scaling up a material in order to you know generate some sort of

technological device or something like that then this is a scalable type of technique to where you can do that

because it's it's already been done once but now what we do is we actually look at just the very

fundamental material properties and so we pretty much have to do this at a material at a time

and so for each of our materials it will take a while because you know

the material electronic structure is just so sensitive to the position of the atoms that we have to get really good at

controlling the position of those atoms in a particular material before we actually look at it in our rpes you know

in station and so from that perspective you know it'll take quite a long time to look at

a particular material system and for us to investigate the electronic structure and so for us it's a bit more labor

intensity because it's a material at a time but we're really learning how to control the electronic structure and

once we do that and once we have that knowledge then that could be directly transferable out into industry in order

to generate you know new materials for new devices for example what's the cycle time for one sample

i you know there it's hard to say i mean because it's just for some of our samples you know for uh you know in this

little guy here the couch and i'm looking to be mepitaxi system um it's a looks like a small baby mbe

system so we call it bambi um so in this system to grow a film it takes 30 seconds

and so then it'll take you know maybe a few hours to map out the electronic structure

but now some of the more complicated structures like you know heterostructure is made of titanates and cuprates

you know we're spending months trying to figure out how to get these pieces of the puzzle to fit together precisely

enough before we even stick it into the beam line and take a look at it and so there but once we develop those

recipes and and we've done this with with a lot of different header structures of some tight knits and cuprates

once we develop the recipes it's a matter of growing a material in an hour and then taking a couple hours to

actually look at the electronic structure but that's only after months of developing of the material itself

let's just take two more questions so anyway yes

change of subject the pictures of people working on the gear at the end yes they all had purple

gloves on yes what happens if you don't use the gloves uh we don't get the vacuum we need

so we work under ultra high vacuum conditions and just like i was saying before to where if you actually have a

material any atmospheric contaminants you know moisture carbon dioxide carbon monoxide anything

like that can actually sit on the material it may not affect the material but it will interact with those

electrons as they come out and destroy the signal that we're looking for and so in order to actually do this

technique to have the electrons come out of the material and into the protector we have to pump out all of the air

and we actually have to get to ultra high vacuum conditions on the order of 10 to the minus 11 torr

now i think in the middle of space i think it's 10 to the minus 13 torr

and so it's it's you know incredibly low vacuums no atmosphere whatsoever inside

those chambers now in order for us to actually reach that we have to make sure that we don't have

fingerprints any of our oils as soon as you start pumping it out it just outgasses

and so we have to make sure we keep the insides of these chambers and everything absolutely pristine clean

and after we assemble it we actually wrap this whole thing up and bake it

we actually raises the temperature of this entire system all the way up to 150 degrees celsius or about 300 degrees

fahrenheit just because water is a polar molecule and so it likes to stick to everything

as we actually have to heat these things up to actually get even the water off of the wall so that our pumps can pump it

out in order for us to achieve the vacuum levels that we need and so the purple gloves are just to

make sure that we don't contaminate the inside of the chamber or any of those you know components on the chamber

that would prevent us from actually achieving the vacuum levels that we need in order to actually develop these materials

yes um back to that superconductor chart okay you didn't talk at all about

the thing way up in the upper right corner which looked like it was getting close to room temperature that green dot

in the upper right um that is yeah and the reason for that is

let's see if i can do this real quick we can just take this guy and move it out of the way and you can kind of see that

this is still ah you've got it minus 50. did i really just do that that was 200

kelvin that you saw up there and so it's still you know significantly below room temperature it's it's still

well below and so you know it's something to wear you know it's one of those things that we'd like to achieve but it's something

to where we're going to have to find the tricks in order to actually have this quantum coherence state live at such

high temperatures because a lot of the exotic properties that we see they're easy to appear at very low

temperatures these are the ground state basically the lowest energy level configuration of the materials

themselves but once you start adding heat to it then a lot of times you can scramble a

lot of these quantum mechanical states and so the trick is is how can we actually enhance these quantum

mechanical states and so that they survive at higher temperatures and that how to do that is is the 64 000

question and so by learning how to couple a lot of these different properties together

across interfaces and such like that we can help to try to find ways to enhance these quantum mechanical

properties and so these ground states these exotic properties that we see survive at much higher temperatures that

can be much you know useful for applications and superconductivity is a perfect example of that

so it's going to be a long time before we start having you know superconductors in our cell phones and in our lights and

stuff like that but this is something to where if we could boost the tc up we could start thinking about you know just electricity

transmission over great distances because i think about what is it seven

to ten percent of the total electricity that's made in the world is just wasted

it just floats off into space as heat because as you start moving current through the wires the resistance of the

wire just generates waste heat and it just gets wasted and so if we could find you know a way

to transmit large amounts of current without that resistance the world will forever be a different

place so that's the ultimate goal but we are still quite a ways away from it but

we're starting to understand the players and how they interact with each other to give us hopes of being able to develop

those materials okay let's thank rob again okay thank you

so um rob wants to clean up here but he'll be out in the lobby in just a minute if you have more questions he'll

be out there you can ask him privately the next one of these lectures will be in november

the week before thanksgiving we'll tell you how to discover dark matter with large tanks of liquid xenon

so we hope to see you there and thank you very much for coming tonight

SLAC National Accelerator Laboratory