Studying Superconductors

Using an incredibly sensitive scanning tunneling microscope that resolves features smaller than an atom, Cornell University physicist J. C. Séamus Davis studies the electronic structure of exotic materials like superconductors to find out how they work and how they can be improved.

Transcript

00:00:04 the materials we use for our present computing communications and information technology are very simple materials silicon gold aluminum copper and we understand how they work very well we've understood since the 1950s and in those materials the electrons are just free to move around like a tenuous

00:00:25 gas when the electrical current passes from one region to the other all that's happening is a fluid of electrons is flowing smoothly through the material now at the beginning of the 21st century we we have started to discover many new electronic materials and most of them a big majority of them have

00:00:44 inexplicable properties completely unexpected properties for example high temperature superconductivity in which you can transmit electricity with no loss of power is discovered in this new class of materials in these materials we don't understand how the electrons operate we understand

00:01:03 that they're interacting strongly with each other so instead of being a tenuous gas of non-interacting particles they're much more like a crowd of human beings at a football game or at a dance or something and as with many complicated problems it helps tremendously to be able to

00:01:22 visualize the thing that you want to understand but actually there is no way to visualize electrons moving around in a material at least that was true until about 10 years ago at that time i got an idea for how to develop an instrument which would allow us to visualize the no matter how complicated they are

00:01:43 the arrangements and interactions of electrons in these complex new materials and that machine is called a spectroscopic imaging stm i built the first one when i was in berkeley around 1999. um it was widely expected to be an expensive failure but in fact it was a great success it

00:02:02 worked as advertised and then i had the privilege of moving to cornell and since we've been here we've built uh three different ones which have different charac different properties uh different specifications so that we can examine different properties of different classes of materials

00:02:20 so here we are we're in the underground basement laboratory space in clark hall um on the cornell campus this is where most of the experimental physics is done uh in this university and here i'm standing outside uh one of our uh very specialized scanning tunneling microscopes i'm going to refer to it as

00:02:45 an stm from now on it's got a very special environment a chamber which is somewhat like a recording studio which prevents acoustic and electromagnetic noise from getting into the experimental chamber one other thing we must do before the measurements is that this whole room is must be floated on cushions of air in order to vibe

00:03:10 to vibrationally isolated from the outside world so we lift up this 35 ton block of concrete with the experimental apparatus and its room on top by about a quarter of an inch and then it sits there in an extremely vibrationally quiet situation

00:03:30 so inside the chamber we have a second layer this large blue triangular table is a second layer of vibrational isolation it looks reasonably light and airy but it's actually filled with lead shot and it weighs about five tons and when it's in operation the motions of this large object are no more than the diameter of an atom

00:03:56 and then this vertical cylinder you see here is a very large thermos bottle and it's full of liquid helium 4 at a temperature of minus 269 degrees below the freezing temperature of water or just about four degrees above the absolute zero of temperature it's one of the coldest places in the

00:04:18 universe in fact however inside there is a refrigerator called a dilution refrigerator which cools down the experiment by almost to absolute zero and then right down here at this level inside the center of this cylinder is a little uh device called a spectroscopic imaging

00:04:38 scanning tunneling microscope it's about two inches high and two inches in diameter and it contains the technology to image the electronic wave functions of the electrons that we study in this apparatus if you think back to how a phonograph worked the old machine for playing music on on an lp

00:04:58 you had a plastic disc and there are little grooves and with wiggles in them on that disc you take a sharp needle and you put it down the disc is moving and the needle conforms to the wiggles on the disc and sends a signal to a audio amplifier and that's the music that you hear now imagine taking that device and

00:05:18 shrinking it down say by a million times so that you have little features on the surface of a material but each feature is an atom and you have a very fine needle which has one atom on the end of the needle and now you raster that needle back and forth over the surface and by measuring the the electrical

00:05:38 current from the end of the needle to the surface you can actually take an image of where the atoms were in the surface are in the surface that was invented around 1982 by binig and rohrer in ibm in zurich and they got the nobel prize for inventing that machine uh quite soon thereafter because it was

00:05:57 revolutionary you can see where the atoms are in materials the challenge that i faced was everyone could see where the atoms were but nobody could see where the electrons were so i had to find the scheme where using the same rastering of the tip over the surface but measuring a different thing

00:06:15 i i would based on a theoretical argument come up with an image a movie actually of electronic wave functions they're called because electrons are actually quantum mechanical waves and how they move through the material and how they interact with each other and that scheme involved just measuring a different probab

00:06:33 different property of the electrical current going from the end of the needle to the surface and then measuring it under conditions which were really quite different than which are used in a commercial stm namely the vibrational and the acoustic noise levels have to be about a million times better for my scheme to work

00:06:52 one thing which we would really really like to do is to cut short the so-called virtuous cycle for discovering new materials the traditional way is you have some genius chemist or physical chemist they put in a whole bunch of different element elements they cook it up they take out some new material

00:07:12 and then they measure you measure its properties my gosh it's a magnet amazing it's a superconductor astonishing it's a semiconductor okay and then you you tried to figure out well why is it that it has the properties that it had and if if we and we know the answer to that for materials like silicon and gold

00:07:32 and aluminum and platinum and things like that but we don't know the answer to that for tailor-made and exotic materials which are being developed for 21st century technology and the process is slow someone makes a new material then you have to do all kinds of measurements which may take years and

00:07:52 years and then you have to cook up a theory to explain the suite of the results of the different measurements and then you would use that theory to help you figure out how to improve the material or to get an even better one but you'd like to cut short that process so that it only takes a few days rather

00:08:09 than a few years so we imagine with our technique new material would be fabricated let's say by one of our colleagues here at cornell university daryl schlom is world famous for developing new materials which haven't existed in nature before but instead of spending years and years doing measurements you would take that

00:08:29 material and put it in a spectroscopic stm and you would just image the way the electronic structure is working what the electrons are doing how they're interacting with each other and how they're producing the properties of the material with our present generation machines you could do that in about 12 hours

00:08:45 so you could go home at night and in the next morning you could have cut short that cycle of many years to figure out the electronic structure so that you know this is why the material has the properties it has then you go to your fabrication your colleague and say we've we we understand why it has these weird properties and if you just tweak this

00:09:04 component of your fabrication of the material we would predict that you would get this important and exotic new property then if they're working hard maybe they could do that in a few days or a few weeks so the virtuous cycle for finding and making unimaginable new materials could be

00:09:21 altered by this approach