## Wednesday, January 27, 2010

### Potpourri

The Female Science Professor is doing a great series of posts about interviewing for faculty jobs.  See here, here, and here, with two more parts to come.  On a related note, Massimo has a post about negotiating faculty job offers that is a follow-up to another post by Professor in Training.  I did not bother to re-write my post about applying for faculty jobs this year, and here's an old post I wrote about the interview process.  It also never hurts to remind people about suggestions on how to give talks.

I agree completely with Chad Orzel that Dennis Overbye at the NY Times needs to remember that "physics" extends beyond just high energy theory.

Finally, according to The Onion ("America's Finest News Source"), physics is done.  Guess it's time to re-evaluate that career choice.

## Tuesday, January 26, 2010

### Science and spending freezes

Tonight in the State of the Union address, President Obama will supposedly propose a freeze on discretionary federal spending for the next three years.  For those not familiar with the term, discretionary spending leaves out defense and debt service, as well as social security and medicare, but includes NIH, NSF, DOE, NIST, and NASA.  It will be interesting to see if, after a strong start on recovering from the funding morass (cuts in real dollars for several years in a row under the Bush administration and the budgetary mess from two wars), what will happen to federal scientific research support in such a climate.  Note that Pres. Obama is having this year's Intel Science Talent Search winner as a guest at the address.

## Thursday, January 21, 2010

### Wow. Impressive room-temperature single-electron device

Looking at the arxiv this evening, I came across this paper, in which the authors demonstrate a silicon-based single-electron transistor that operates at room temperature.  The device is fabricated from a "finFET", a transistor design put forward for ultrascaled CMOS electronics.  In a finFET, the silicon channel is surrounded on three sides by a wrap-around gate, to achieve comparatively efficient gate coupling.  A single-electron transistor is very different from an ordinary field-effect transistor.  The channel in a SET is an "island" connected via tunnel barriers to source and drain electrodes.  The island has some capacitance, C, and therefore there is an energy cost associated with putting an additional electron on the island given by e2/2C.  If that energy cost is large compared to kBT, and the tunnel barriers are sufficiently opaque (so that lifetime broadening doesn't smear out the island spectrum), then one can see single-electron charging effects in the conduction.  When the island is very small, one has to worry not just about the Coulomb charging energy, but also about the particle-in-a-box level spacing on the island.  (Note that all of this discussion is assuming that electron-electron interactions can be lumped together simply, via the capacitance.)

The impressive part of this work is just how clean the SET characteristics look at 300 K.  Getting clean SET signatures in conduction requires the thermal energy scale to be at least 20 times smaller than the charging energy scale, and at 300 K that's a tall order!  The data in Fig. 2c are spectacular for a room temperature SET device.  Very very pretty.  If they can figure out how to do this reliably, there are many exciting possibilities....

## Tuesday, January 19, 2010

### Inelastic electron tunneling spectroscopy

Motivated in part by this recent paper and ensuing conversation here, I thought it might be useful to say a few words about inelastic electron tunneling spectroscopy (IETS).  Mysterious kinks in the current as a function of voltage were first observed over 40 years ago in oxide tunnel junctions between superconductors.  As the voltage passed certain threshold values, the conductance (slope of I vs. V) increased suddenly.  A kink in I vs. V could also be plotted as a step in dI/dV vs. V, or as a peak in d2I/dV2 vs. V.  When plotted this way, and converting V into units of energy, Jaklevic and Lamb realized that what they saw looked remarkably like an infrared or Raman spectrum of some organic compound.  They were right - using inelastic electron tunneling, they had measured the vibrational spectrum of organic compounds that had been trapped in their tunnel barrier during the fabrication process.  IETS has undergone a major resurgence in the last decade, in part because of Wilson Ho's group's beautiful demonstration that one can see these effects at the single molecule level, and because it's a way of confirming that fabricated molecular junctions actually contain what they're supposed to.

In IETS, current flows via a second-order tunneling process, in which an electron tunnels on to the vibrational ground state of a molecule, and in the same coherent process tunnels of the vibrationally excited state of that molecule, leaving behind a vibrational quantum of energy.  This can only happen of the voltage applied is large enough to supply the necessary energy; hence the thresholds seen in experiment.  The voltage positions of the features correspond directly with the energies of the modes being excited.  (In the single-electron transistor world, this process would be called "inelastic cotunneling" via vibrationally excited states.)  The requirement that there be a nonzero amplitude for this process gives rise to selection rules, so that not every mode can be pumped this way.  More recently, it's been realized that IETS may not necessarily always lead to simple peaks in d2I/dV2 vs. V, because the IETS process can interfere coherently with other tunneling processes.  This is supported by data in the paper mentioned at the top of this post.

IETS is pretty amazing, when you think about it.  Even though the tunneling electrons never "really" occupy the molecule (such a state is classically forbidden due to energy conservation), nonetheless the molecule "feels" the effects of the electrons as they tunnel past.