Saturday, June 09, 2007

This week in cond-mat

Two more papers that look interesting.

arxiv:0706.0792 - Koop et al., Persistence of the 0.7 anomaly of quantum point contacts in high magnetic fields
One of the neatest results (in my opinion) in mesoscopic physics is the appearance of conductance quantization in quantum point contacts, first shown in the late 1980s. The basic idea is simple. Start with a two-dimensional electron gas such as that formed at the interface between GaAs and modulation-doped AlGaAs. Metal gates on top of such a structure can be used to deplete the electron gas in particular places. Two closely spaced gates may be used to create a narrow constriction between two large reservoirs of 2d electron gas. As the constriction width is reduced until it is comparable to the Fermi wavelength of the confined electrons, the conductance through the constriction is quantized (at zero magnetic field) in integer multiples of G0 = 2e^2/h, the quantum of conductance (about 1/(13 kOhms)). That is, each spatial mode (each transverse subband of the constriction) can transport e^2/h worth of conductance per spin degree of freedom. Indeed, at very large magnetic fields, the conductance is quantized as integer multiples of G0/2, as one would expect if the different subbands are spin-split due to the Zeeman effect. This is all well explained by single-particle theory and the Landauer-Buttiker picture of conduction through small systems. In very clean quantum point contacts, additional structure is seen at 0.7 G0 - this is the so-called 0.7 anomaly. In the presence of a little bit of in-plane magnetic field, this approaches 0.5 G0, and therefore looks like there is some spontaneous spin-splitting, and this is a many-body effect that is the result of some kind of electron-electron correlation physics. This paper is an extensive study of 14 such point contacts, fully mapping out their magnetic field dependence and nonequilibrium (large bias voltage) properties.

arxiv:0706.0906 - Clark et al., Nonclassical rotational inertia in single crystal helium
The controversy over whether 4He has a true supersolid phase continues. This week this article appeared in Science, summarizing a number of recent experiments, and strongly suggesting that single crystals of pure 4He should not show a real supersolid phase - basically the claim is that the effects ascribed to such a phase are really due to disorder (glassy 4He at grain boundaries between crystals? 3He impurities somehow?). Now comes this paper from Moses Chan's group, arguing from new experiments that even carefully nucleated and grown single crystals of 4He show evidence of supersolid behavior (in the form of a nonclassical moment of rotational inertia). Hmmm. Neat, clever experimental design. It'll be interesting to see how this all pans out.

4 comments:

Incoherent Ponderer said...

Doug, thanks for highlights.
Maybe I am being too critical, but the supersolid paper should be titled: "NCRI in single crystal (likely, perhaps, maybe, at least we think so) helium".

Growth of single crystals is more art than science, it's a bit like cooking - and following certain recipe does not in fact guarantee that you end up with the same result.

I agree with conclusions of Phillips and Balatsky paper - structural measurements - x-ray tomography or microscopy, or neutron scattering is really crucial to resolving the current controversy. Without some sort of crystallography data arguing that this paper reports "NCRIF in (very likely) single crystal samples" - an actual quote from Clark/Chan paper - is likely to cause only more controversy on this subject.

Don't get me wrong - I think Chan and his group are very careful and professional, but in this case they could have pushed the envelope a bit further, perhaps with some help from x-ray/neutron folks, or some other structural characterization. Otherwise, I am (for one) not really convinced.

Incoherent Ponderer said...

btw - this is one example how arxiv could benefit from "comments" section

Doug Natelson said...

IP - Yeah, the 4He guys have it rough. With 3He crystals, you can do NMR with field gradients (basically MRI) and figure out if you really have a single crystal. Since 4He has no nuclear moment, that's not an option here. Crystallography is tough, too, of course, since the crystals have to be grown at high pressures inside metal containers - x-rays are basically out; neutrons are a possibility, but cross-sections for 4He are low. Hmmm. Ultrasound? I remember at some point that at least one group was looking at 4He crystals with an optical camera, but I can't find the reference anywhere. Definitely a hard experimental problem. Even if one could see large crystal boundaries, I'm not sure how one could ever rule out disordered layers at the walls.

Incoherent Ponderer said...

I agree that the experiments proving order vs. disorder effects will be challenging. But it seems to me that a whole lot of weight is currently hanging on this issue - is this simple premelting at grain boundaries/dislocations/other deffects, or is there a true supersolidity component?

I also wonder how much collaborative effort can contribute to resolving these controversies - for example, can someone from Chan group travel to Reppy's group in Cornell, or even go to France or Canada to see what they are doing and tell them first-hand how they grow their crystals? This would obviously work only if everyone truly wants to resolve this puzzle/controversy as quickly as possible. From purely selfish motives it may be more beneficial for everyone to keep exchanging high profile papers with "it's supersolid" "nope, it's not!" "it is so!". Not saying this is what's happening here, but often it seems to be the case that even a sniff of controversy provides more visibility to the issue than some boring measurement that everyone agrees on.