Sunday, April 29, 2018

What is a quantum point contact? What is quantized conductance?

When we teach basic electrical phenomena to high school or college physics students, we usually talk about Ohm's Law, in the form \(V = I R\), where \(V\) is the voltage (how much effort it takes to push charge, in some sense), \(I\) is the current (the flow rate of the charge), and \(R\) is the resistance.  This simple linear relationship is a good first guess about how you might expect conduction to work.  Often we know the voltage and want to find the current, so we write \(I = V/R\), and the conductance is defined as \(G \equiv 1/R\), so \(I = G V\). 

In a liquid flow analogy, voltage is like the net pressure across some pipe, current is like the flow rate of liquid through the pipe, and the conductance characterizes how the pipe limits the flow of liquid.  For a given pressure difference between the ends of the pipe, there are two ways to lower the flow rate of the liquid:  make the pipe longer, and make the pipe narrower.  The same idea applies to electrical conductance of some given material - making the material longer or narrower lowers \(G\) (increases \(R\)).   

Does anything special happen when the conductance becomes small?  What does "small" mean here - small compared to what?  (Physicists love dimensionless ratios, where you compare some quantity of interest with some characteristic scale - see here and here.  I thought I'd written a long post about this before, but according to google I haven't; something to do in the future.)  It turns out that there is a combination of fundamental constants that has the same units as conductance:  \(e^2/h\), where \(e\) is the electronic charge and \(h\) is Planck's constant.  Interestingly, evaluating this numerically gives a characteristic conductance of about 1/(26 k\(\Omega\)).   The fact that \(h\) is in there tells you that this conductance scale is important if quantum effects are relevant to your system (not when you're in the classical limit of, say, a macroscopic, long spool of wire that happens to have \(R \sim 26~\mathrm{k}\Omega\).   
Example of a quantum point contact, from here.

Conductance quantization can happen when you make the conductance approach this characteristic magnitude by having the conductor be very narrow, comparable to the spatial spread of the quantum mechanical electrons.  We know electrons are really quantum objects, described by wavefunctions, and those wavefunctions can have some characteristic spatial scale depending on the electronic energy and how tightly the electron is confined.  You can then think of the connection between the two conductors like a waveguide, so that only a handful of electronic "modes" or "channels" (compatible with the confinement of the electrons and what the wavefunctions are required to do) actually link the two conductors.  (See figure.) Each spatial electronic mode that connects between the two sides has a conductance of \(G_{0} \equiv 2e^{2}/h\), where the 2 comes from the two possible spin states of the electron.  

Conductance quantization in a 2d electron system,
from here.
A junction like this in a semiconductor system is called a quantum point contact.  In semiconductor devices you can use gate electrodes to confine the electrons, and when the conductance reaches the appropriate spatial scale you can see steps in the conductance near integer multiples of \(G_{0}\), the conductance quantum.  A famous example of this is shown in the figure here.  

In metals, because the density of (mobile) electrons is very high, the effective wavelength of the electrons is much shorter, comparable to the size of an atom, a fraction of a nanometer.  This means that constrictions between pieces of metal have to reach the atomic scale to see anything like conductance quantization.  This is, indeed, observed.

For a very readable review of all of this, see this Physics Today article by two of the experimental progenitors of this.  Quantized conductance shows up in other situations when only a countable number of electronic states are actually doing the job of carrying current (like along the edges of systems in the quantum Hall regime, or along the edges of 2d topological materials, or in carbon nanotubes).   

Note 1:  It's really the "confinement so that only a few allowed waves can pass" that gives the quantization here.  That means that other confined wave systems can show the analog of this quantization.  This is explained in the PT article above, and an example is conduction of heat due to phonons.

Note 2:  What about when \(G\) becomes comparable to \(G_{0}\) in a long, but quantum mechanically coherent system?  That's a story for another time, and gets into the whole scaling theory of localization.  

4 comments:

Anonymous said...

I haven't worked in this field for quite some time now, but has there been an accepted explanation for the 0.7 anomaly since I left 15 years ago?

Douglas Natelson said...

Hi Anon - Good question. (For those not familiar with this story, the "0.7 anomaly" is the name given to a plateau in the conductance of a quantum point contact that appears at about 0.7 G0 as the QPC is pinched off.). As Anon recalls, the explanation proposed by Cronenwett et al. back in 2002 (https://arxiv.org/abs/cond-mat/0201577) was that this feature may result from Kondo physics - that a single spin can be localized in the point contact during pinch-off, and that the 0.7 feature (and how the conductance evolves as the bias voltage is cranked up) is a signature of resonant conduction via coherent tunneling processes involving that spin (a many-body correlation effect). Meir has a more recent discussion here. Another proposal (here) is that this feature comes from enhanced density of states at pinch-off effectively due to the remnants of a van Hove singularity at the 1d band edge (in principle a single-electron effect). Recently, the argument is that the latter scenario is closely connected to the former (different limits), and that spin fluctuations are the key. There is some evidence in the form of phase shifts through the QPC that many-body (Kondo) physics is at work. Still a subtle business!

Anonymous said...

Hi Prof. Natelson, thank you very much for the thorough overview on the current research of the 0.7 anomaly. Some familiar names in the author lists that you presented. Glad to see it's still active research.

Avishai Benyamini said...

Hey Prof. Douglas,

Thanks for blogging and giving us physical insights.

A relevant topic and which I think is a bit confusing is where is the resistance/voltage drop in a 1D ballistic channel. As a ballistic channel doesn't have dissipation and has no potential drop in it. As far as I understand the answer to 'where the voltage drop is?' depends if the contacts to the 1D channel are resistive (allow for inelastic scattering) or tunneling (no energy dissipation/elastic scattering). In the first case, I think there should be two voltage drops on each of the contacts, while for the tunneling case all the voltage drop will be in the drain electrode (elastic tunneling into the 1D channel, no energy loss in the channel, elastic tunneling out of the 1D channel and then inelastic processes in the drain electrode).

Maybe the confusing thing is what is the correct question to ask. Where is the resistance/voltage drop? or How much current can I have in a 1D channel for a given bias?. The 2nd question seems less confusing and will give I=e^2/h*V. The 1st is a bit confusing as it gives a finite resistance for a non-dissipative system.

If it is about the contacts, the main point may be the difference between a resistor and a tunnel junction. I think it would be nice if you could comment on this and shed light on your blog :)

Best,
Avishai Benyamini