The Quantum Spin Hall Effect and its importance
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Gamma rayAngeregter ZustandMaterialSpaceflightPaperRRS DiscoveryYearEffects unitBandstrukturHose couplingSpin (physics)ChannelingTiefdruckgebietSuperconductivitySemiconductorMetalPattern (sewing)Power (physics)VakuumphysikPhysicistMobile phoneDirect currentGroup delay and phase delayIndustrieelektronikCosmic microwave background radiationMeasurementElementary particleLadungstrennungAudio frequencyProximity-EffektElektrische IsolierungQuantumQuantum Hall effectMolekularstrahlepitaxieSpin Hall effectHall effectTransfer functionMusical ensembleSpintronicsTypesettingBook designPower inverterPercussion malletFord MercurySuperconductivityQuantization (physics)NanotechnologyCrystal structureMachineUltraCosmic distance ladderSchalttransistorCryostatFinger protocolKickstandStandard cellSingle (music)Quality (business)ViseFood storageElectrodeMode of transportLecture/ConferenceMeeting/Interview
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Transcript: English(auto-generated)
00:04
The question really is about the quantum spin-haul effect. And then I probably should say a bit more about the spin-haul effect first, which is an effect that semiconductor transport people were after, say, in the early 2000s. An effect due to spin-orbit coupling in these materials
00:21
that could lead to new functionalities for spintronic devices, which would be then switches that dissipate very small energy. So we were working on our Wurzburg material to study this effect, when there came a prediction from two colleagues from Philadelphia, two theorists,
00:41
that there might also be a quantized version of the spin-haul effect, where actually you would get a quantum-haul effect at zero magnetic field. The question really was, how do you do such an experiment? What is the right material to use it in? The suggestion of Kane and Mallet at that time was to use graphene,
01:02
but then a graphene with an artificially high spin-orbit coupling, also of next nearest neighbor type, which was not very physical. I knew at that time that our material not only had a mercury tellerite, not only had a strong spin-orbit coupling, but it also had a weird inverted band structure. And because of this inverted band structure,
01:21
it had a surface state occurring even when the bulk was insulating, a metallic surface state. I thought this might be something related to this Kane and Mallet proposal, and I discussed this band structure with Xu Chengzheng, a theorist from Stanford, gave him a thesis, a PhD thesis describing our band structure calculations.
01:43
Very soon he came back with a theory, which is now known as the BHZ theory, showing that indeed if you make a quantum wall of mercury tellerite, you can expect to see the quantum spin-haul effect. Well before this paper was published, we were already doing the experiment, because we were talking with the guys.
02:00
Around the same time the BHZ paper was published, in our experiments around Christmas, we saw quantization of the spin-haul effect quantized at conductance in very small nanostructures made out of mercury tellerite quantum wells. The method that we use in the end is transport physics.
02:21
We measure the conductance of electronic devices. This starts with the growth of the material. Our materials are pretty special semiconductors. We have to grow them layer by layer in something we call a molecular beam epitaxy machine, which is ultra-high vacuum technology. So that's very extensive growth technology.
02:40
Then we have to pattern these samples into very small structures, just like the transistors in the chips that you use in your cell phone. We use very similar structures to do our, what we call transport experiments. To really see the effects that we want to see, because they occur at very small distances, so we have to make very small devices.
03:01
For that we have a lithography lab in Wurzburg. And once we have our devices, we do these conductance measurements, which we call transport experiments. We usually need to do them at very low temperatures, so we use very extensive cryogenic equipment, cryostats, to perform them.
03:22
The key findings of these first experiments were, well, actually pretty prosaic. What we saw is a quantized conductance of these devices in this so-called quantum spin hall regime. Quantized means quantized in the sense of a quantum hall effect. There is this thing that physicists have discovered,
03:43
which is the conductance quantum, which is the square of the electron charge divided by Planck's constant. And that conductance actually is what we also observed in our devices. Later on, if you develop topology further in other materials, we have done things like discovery of the quantum hall effect
04:04
in a three-dimensional topological insulator. Again, this was possible because of the high crystalline quality of our materials. More recently, we have been looking at topological superconductivity. There we have found a so-called four-pi dependence
04:21
of the superconducting proximity effect on the difference between the superconducting electrodes you have in these devices. And that's a pretty sure sign that you have what some people call Majorana modes in your material, which are the modes that give rise to Majorana bound states at zero energy
04:41
if you can localize them. So that is a sign that indeed you have topological superconductivity going on in these materials and that in principle you could be able to use it for topological quantum computing. The main relevance of these findings is the realization that physicists miss something
05:02
when they develop band structure theory in the 30s. It's a rather big omission. You can really look at papers from the 30s and you see that people are talking about surface states and are very close to the realization there could be surface states that are intrinsically linked to the band structure
05:21
but they never really made that step. The surface states of our material, they also were something that the community knew about for something like 20 years before this connection came with topology. And this connection with topology of course is a very, very strong thing because now you can use all kinds of topological mathematics
05:41
on the description of these band structures. And you can describe further effects. The important discovery really was that band structures can have topological properties and that they lead to totally new physics in these materials. There also may be applications but that's secondary I guess.
06:01
The applications could be that these S-channels we saw in the two-dimensional quantum spin hall effect could give you very low power possibilities for computation but you have to get the effect at room temperature and that's not so easy. The other thing is that these Majorana modes
06:20
I talked about in the topological superconductivity could be used to try topological quantum computing which some people see as a very promising road to go for quantum computing. But again these are things that still have to be demonstrated and we have to see how far all this develops.
06:40
Doing the physics of course is very exciting of all these novel aspects. So one of the things that we're working on very extensively right now is this topological superconductivity. Our group actually is kind of a newbie in this field. We have little superconducting background.
07:01
It's close enough to us of course because we are transport physicists. So we're developing this in many different directions, many different systems. One important thing is to take it to high frequencies where things are a lot more stable for topological superconductivity. Another direction that's becoming very important is looking at Dirac and Weyl systems
07:21
where we can make topological band structures that mimic the dispersion of elementary particles. This allows us to actually go after some effects that particle theorists have predicted for imaginary particles. We can actually now create a band structure in our semiconductor devices that mimics the Hamiltonians that these people study
07:42
and we can try and demonstrate the effects that they have been predicting. A final road is that we work with a different material system which is a magnetic topological insulator which shows something called the quantum anomalous hall effect which is a single spin version of the quantum spin hall effect.
08:01
This shows very good quantum hall quantization at zero magnetic fields but still at very low temperatures. This is something that the metrology people are very interested in because developing a quantum metrology standard that works at zero magnetic fields
08:20
can have big implications for their labs. So we collaborate for example with BTB Braunschweig about this.