Wednesday, May 31, 2006

This week in cond-mat

Two preprints that caught my eye this week:

cond-mat/0604528 - Elimination of the supersolid state through crystal annealing, Rittner et al.
This paper is the work of John Reppy's group at Cornell, and is part of a large effort going on by a number of people to verify or refute the observations of Moses Chan's group - that there's a "supersolid" state of helium (4He) under high pressure (tens of bars) and low temperatures (below 1 K). A supersolid is a solid that exhibits "nonclassical rotational inertia". Put another way, in a superfluid, the atoms in the system form a kind of condensate - a macroscopic quantum phase where all the atoms behave cooperatively. If the atoms are weakly interacting bosons, the system can be described as a Bose-Einstein condensate. In a supesolid, the vacancies in the crystal lattice are thought to undergo some kind of condensation into a single quantum phase. This new paper reproduces the results of Chan et al., and finds that the supersolid behavior in 4He crystals can be eliminated entirely by annealing the crystals near their melting point. It would appear that the disorder responsible for the supersolidity can be removed by annealing. Nice paper.

cond-mat/0605739 - Landau level spectroscopy of ultrathin graphite layers, Sadowski et al.
This paper shows some beautiful cyclotron resonance data taken on graphene sheets as a function of carrier density. As I've mentioned before, graphene is a very funky model system, in which the electrons and holes act just like (apparently) massless Dirac fermions, because of the peculiarities of the graphene band structure. This work is very pretty, and is a cool example of an experiment that, in some ways, is analogous to electron-positron pair production (!). I'm a big fan of solid state systems that are models of more general physics.

Monday, May 22, 2006

Fraud follow-up

I just received the following email from Phys Rev Letters:

Dear Dr. Natelson,

We are in the process of considering the issues you raise about the
Letter by XXXXX et al. Such consideration often takes a substantial
amount of time. Fortunately, in the present case, in which the paper
at issue was published six years ago, there does not appear to be
cause for time pressure. We will apprise you of our conclusion when
we reach it.


Reinhardt B. Schuhmann
Physical Review Letters
Well, I guess we'll see what happens. It'll be interesting to see if anything comes of this. I'm quite sure there's something fishy about the particular paper, but it may be very hard to ever prove.

Friday, May 19, 2006

Possible fraud....

In the course of serving on a committee for a graduate oral presentation, I noticed something very strange looking in a Phys Rev Letter from a few years ago. While unlikely to be seen in a casual glance at the printed version of the journal, it was very striking when the figures were blown up to 4' on a side by a digital projector. Basically, it looks like someone used what I shall delicately term the "Photoshop operator" to massage their data. This paper has been cited 65 times since its publication, and has been milked heavily by its authors.

So, what is the right course of action? I've got no actual proof of fraud, just a very suspicious figure. I've now emailed the editors at PRL twice about this, and received no response from any human being - just the form letter generated by their mail system. Since this is circumstantial, I'm certainly not going to accuse anyone publicly. Next I'm going to call PRL on the phone. Updates as events warrant.

Wednesday, May 03, 2006

This week in cond-mat

There are three papers I'd like to bring up from the past week or so that I think are pretty neat pieces of physics:

cond-mat/0605061 - Boulant et al., Bloch oscillations in a Josephson circuit.
This is the most recent paper from the Quantronics (quantum electronics) group at Saclay, a collaborative effort that routinely cranks out some of the most elegant and pretty physics experiments using nanodevices. Consider a tunnel junction with some capacitance C. To move a single electron across the junction would generically require an amount of energy (in the form of eV, where e is the electronic charge and V is the dc bias voltage across the junction) that exceeds the capacitive charging energy of the junction, ~ e^2/2C. If such a junction is hooked up to a constant current source, the voltage across the junction is expected to vary like a sawtooth pattern: rising linearly with time until it hits that threshold, and then dropping quickly as the electron tunnels. If one does this with a superconductor, the relevant particles are Cooper pairs with charge 2e, but the effect is the same: a constant current bias should lead to an ac voltage across the junction, with a dominant frequency proportional to the current. These ac voltage wiggles are called Bloch oscillations, and have not been measured directly yet. There's all kinds of reasons why doing so is hard, most related to the fact that it's hard to really make a true constant current source at the relevant frequency scale. Remember, one microamp of current would lead to THz oscillations. Anyway, these folks made a more complicated structure with two junctions, and use that structure to terminate an rf line. When they send rf power into the line and look at the reflected rf coming back, they can see sidebands in the reflected signal offset from the input frequency by the Bloch frequency. It's a very pretty experiment.

cond-mat/0604654 - van der Wolen et al., The Magneto-Coulomb effect in spin valve devices.
This paper is an interesting theory paper by the group of van Wees, who has helped to define the field of mesoscopic physics. It's an examination of the interplay of magnetic effects and Coulomb charging effects in single-electron tunneling structures incorporating ferromagnetic metals. In the absence of the charging effects, the connection between magnetization and electronic transport is responsible for many useful effects like giant magnetoresistance, the basis for the read-head in your hard drive.

cond-mat/0604608 - Onac et al., Using a quantum dot as a high frequency shot noise detector.
Another beautiful and clever experiment from the mesoscopics group at Delft. It is often very challenging to measure high frequency dynamics in nanostructures, since the relatively high impedances of the devices are typically a poor match for most commercial rf electronics and coaxial cables. Life is even more difficult at very low temperatures, where most of the interesting physics often happens, because there are painful experimental constraints that must be obeyed. One method of studying rapid charge variations in quantum dots has been to use a quantum point contact as a charge detector. A QPC is a region of 2d electron gas that has been constricted using gates down to the point where only one or a couple of channels of transmission are left. Sitting on the edge of depletion of a channel, the presence or absence of charge on a nearby quantum dot can strongly change the conductance (and therefore rf impedance) of the QPC. Using rf reflectance methods like those above, this can be monitored at high frequencies. This experiment is the complement of that - by gating and biasing the quantum dot appropriately, dc transport through the dot can be strongly modified by the high frequency fluctuations of the current (shot noise) in the nearby QPC. This is a great approach for studying back-action and measurement: is the dot the detector and the QPC the system, or vice versa?