Really? Seriously?

Sometimes I read a science article online or in the newspaper that I think is poor. This one I just don't know how to interpret. Lawrence Krauss is a solid guy, a very strong public advocate for science, and a very good popularizer of physics. Still, the idea that our observations of dark energy have somehow collapsed the quantum state of the entire universe is, umm, nuts on the same level as saying that the moon doesn't exist if no one is looking at it. There's no question that there are subtleties in worrying about applying quantum mechanics to the universe as a whole. Still, this carries the "spooky action at a distance" idea a bit far.

This week in cond-mat

Two papers this week. I'll write about our own at a later date. These two are both connected to on-going long-term controversies in condensed matter/mesoscopic/nanoscale physics.

arxiv:0711.1810 - Capron et al., Low temperature dephasing in irradiated metallic wires
In the orthodox picture of metals (thought to be valid for relatively weak disorder), the quantum coherence time of electrons is expected to diverge toward infinity as the temperature approaches zero. Think about a two-slit experiment for electrons. If the electrons are well isolated from their environment, they can diffract off the slits and land on the screen, producing an interference pattern. If the electrons are coupled to environmental degrees of freedom that can change their state when the electron goes by, the relative phase of the electron wavefunctions going through each of the slits gets scrambled by that interaction, washing out the interference. In the usual 2-slit experiment, the degrees of freedom are those of detectors at the slits. Within a disordered metal, those environmental degrees of freedom can be lattice vibrations, other electrons, or magnetic impurities. For a decade now there has been an ongoing controversy about whether the coherence time (as inferred from some quantum correction to the classical electronic conductance) really does diverge, or whether it saturates as T -> 0. Intrinsic saturation would be a big deal - it would imply that the quasiparticle picture of electrons (Fermi liquid theory) fails at the low T limit. In this paper, the authors perform a very careful control experiment, looking at whether structural damage to silver nanowires can, by itself, introduce extra degrees of freedom that cause decoherence. They get this damage by ion-implanting Ag ions into Ag nanowires. The results show no sign of extra decoherence due to this irradiation.

arxiv:0711.1464 - Baenninger et al., Low-temperature collapse of electron localisation (sic) in two dimensions
Another ongoing brouhaha has been about whether electrons confined to two dimensions have an insulating or metallic ground state in the presence of any disorder. Without interactions, the "Gang of Four" (Anderson, Abrahams, Ramakrishnan, and Licciardello) showed that even infinitesimal disorder leads to localization and an insulating ground state for an infinite 2d system. Of course, real electrons do interact with each other, and real systems are of finite size. One big complication in this whole discussion is in trying to tell the difference between a real, uniform, insulating state and the breakup of your system into inhomogeneous "puddles" of electrons due to the disorder potential. The Cambridge group has done some careful experiments in mesoscopic samples of rather clean 2d electron gas, and they've found that small regions with higher temperature resistances far exceeding the quantum of resistance (~ h/e^2 ~ 26 kOhms) can show a crossover at low temperatures to what looks like a metallic state. I haven't been following this controversy in detail, but these data look very interesting, and I will have to read this closely.

Potpourri

A small selection of links....

This game is very addictive, educational, and as you play, you feed the hungry (albeit extremely slowly).

Now this is a nanotube radio! Rather than having the nanotube just be the nonlinear element responsible for demodulating the AM signal on the carrier wave, this one has the nanotube acting as the antenna and amplifier as well, effectively. I heard Alex Zettl get interviewed on NPR about it.

The FSP has an interesting post about ambition. Physics as a discipline has issues with this, with an historical attitude that anything less than a tenured job at Harvard is somehow inadequate - a notion that's wrongheaded and sociologically unhealthy.

Schlupp has a post about a little frustrating science journalism. It is a shame that sometimes the media can't tell the difference between good science or engineering and crackpottery. On a plane last week I had someone (who realized I was a physicist from my reading material) ask me about the guy who can get hydrogen from seawater by hitting it with microwaves. Kind of cool, yes. Source of energy? Of course not - it takes more microwave power to break the water into hydrogen and oxygen than you can get back by burning the resulting hydrogen. It's called thermodynamics.

This week in cond-mat

Several entries from the arxiv this week. My descriptions here are a bit brief b/c of continued real-world constraints.

arxiv:0711.0343 - Dietl, Origin and control of ferromagnetism in dilute magnetic semiconductors and oxides
arxiv:0711.0340 - Dietl, Origin of ferromagnetic response in diluted magnetic semiconductors and oxides
These are two review articles by Tomasz Dietl, one of the big names in the dilute magnetic semiconductor (DMS) game. DMS are semiconductor materials that exhibit ferromagnetic order usually because of doping with transition metal atoms that contain unpaired d electrons, such as manganese. The idea of integrating magnetic materials directly with semiconductor devices, and ideally controlling magnetism via electrical or optical means, is quite appealing. However, it is very challenging to achieve high magnetic ordering temperatures (e.g., room temperature) and decent electronic properties at the same time. In many systems the high doping levels required for the magnetism go hand in hand with lots of disorder, in part because crystal growth must be performed under nonequilibrium conditions to force enough transition metal atoms to sit on the appropriate lattice sites. Anyway, these articles (one coming out in J. Phys.: Cond. Matt.
and the other coming out in J. Appl. Phys.) should give you plenty of reading material if you're interested in this area.

arxiv:0711.0218 - Leek et al., Observation of Berry's phase in a solid state qubit
In basic quantum mechanics we learn that particles are described by a complex wavefunction that has a phase factor. Propagation of a particle in space racks up phase at a rate proportional to the particle's momentum. As Feynman would tell us, each possible trajectory of a particle from A to B then contributes some complex amplitude (with a phase). The total probability of finding the particle at B is the squared magnitude of the sum of all of those amplitudes, rather than the classical sum of the probabilities of each path. Phase differences between paths lead to interference terms, and are the sort of thing responsible for electro diffraction, for example. Besides propagating through space, there are other ways of accumulating phase. In the case of the Aharanov-Bohm effect, the vector potential leads to an additional phase factor that depends on trajectory. In the general case of Berry's Phase, the slow variation of some external parameters (such as electric fields) can lead to a similar geometrical phase factor. The intro to this paper gives a nice discussion of the classical analog of this in terms of moving a little vector on the surface of a sphere. Anyway, this team has used a solid-state superconducting qubit to demonstrate this geometric phase explicitly. Quite nice.

arxiv:0710.5515 - Castelnovo et al., Magnetic monopoles in spin ice
One of the things that I find so interesting about condensed matter physics is the idea of emergent degrees of freedom. For example, phonons (quantized sound waves) are quantum mechanical quasiparticles in solids that can have well-defined quantum numbers, and arise because of the collective motion of large numbers of atoms. In a more exotic example, Cooper pairs in ordinary superconductors are objects with spin 0, charge -2e, yet are "built" out of electrons plus phonons. In a very exotic example, the quasiparticles in the fractional quantum Hall effect can have fractional charges and obey exotic statistics. In an even more extreme case, these authors propose that there are quasiparticle excitations in a kind of magnetically ordered insulator that act like magnetic monopoles. It seems that magnetic monopoles do not exist as elementary particles. Indeed, they would require a modification of Maxwell's equations. (In this solid state system the argument is that they exist as monopole/antimonopole pairs, so that the net divergence of the magnetic field is still zero). "Forbidden" particles emerging from the collective action of many electrons - a very neat idea, and it would appear that there may even be some experimental evidence for this already.

In honor of Halloween....

Three of my favorite science-related quotes from the movies, all from Ghostbusters:

Dean Teager: Your theories are the worst kind of popular tripe; your methods are sloppy, and your conclusions are highly questionable. You are a poor scientist, Dr. Venkman.
---
Ray Stantz: Personally, I like the University. They gave us money and facilities, we didn't have to produce anything. You've never been out of college. You don't know what it's like out there. I've worked in the private sector. They expect results.
---
Peter Venkman: Back off, man! I'm a scientist!

Any other good ones to share? (Real science post coming in a day or two....)

Jobs jobs jobs

I figure it's probably a good idea to take advantage of the staggeringly enormous readership of this blog to point out several searches going on at Rice right now.

First, three searches are going on here at Rice in the Physics and Astronomy department at the moment. These are:

• A tenure-track faculty position in atomic/molecular/optical theory, ultracold atoms in particular.
• A tenure-track faculty position in nuclear experiment, heavy ion collisions in particular.
• A prestigious named instructor position.

There is also an experimental nanophotonics search going on in Electrical and Computer Engineering.

Finally, the Chemistry department is doing a search for inorganic or physical chemists, broadly defined. The ad is on the departmental homepage.

Share and enjoy! If you want to discuss what Rice is like as a faculty member, please feel free to contact me and I'll be happy to talk.

Three papers and a video.

Three interesting papers on ASAP at Nano Letters at the moment:

http://dx.doi.org/10.1021/nl0717715 and http://dx.doi.org/10.1021/nl072090c are both papers where people have taken graphite flakes, oxidized them to make graphite oxide, and then suspended the graphene oxide sheets in solvent. They then deposit the sheets onto substrates and made electronic devices out of them after trying to reduce the graphene oxide back to just graphene. There are a couple of people here at Rice trying similar things from the chemistry side. Interesting that a number of groups are all working on this at about the same time. That's one reason why it can be dangerous to try to jump into a rapidly evolving hot topic - it's easy to get scooped.

This one is a cute paper titled "Carbon nanotube radio". The science is nicely done, though not exactly surprising. AM radio works by taking an rf carrier signal and demodulating it to get back just the envelope of that carrier signal. Back in the early 20th century (or more recently, if you bought an old kit somewhere), people used to do the demodulating using a diode made semi-reliably by jamming a metal needle (a "cat's whisker") into a lead sulfide crystal - hence the term "crystal radio". It's simple trig math to see that a nonlinear IV curve (one with a nonzero d^2I/dV^2) can rectify an ac signal of amplitude V0 to give a dc signal of (1/4)(d^2I/dV^2)V0^2. Well, in this case the nonlinear element is a nanotube device. Cute, though I have to admit that I found the media hype a bit much. Wilson Ho did the same essential thing very nicely with an STM, but didn't talk about atomic-scale radio receivers....

Lastly, via Scott Aaronson, a link to a fantastic math presentation. Watch the whole thing - this really is a model of clarity and public outreach. On a bitter-sweet note, in the credits at the end I realized that one of the people responsible for this was an acquaintance from college who has since passed away. Small world.

This week in cond-mat

Real life continues to be very busy this semester. Two interesting papers on the arxiv this week....

arxiv:0710.2845 - Fratini et al., Current saturation and Coulomb interactions in organic single-crystal transistors
The technology finally exists to do what He Who Must Not Be Named claimed to have done: use a field-effect geometry to gate significant charge densities (that is, a good fraction of a charge carrier per molecule) into the surface of a clean single crystal of an organic semiconductor. The Delft group has used Ta2O5 as a high-k gate dielectric, and are able to get 0.1 holes per rubrene atom in a single-crystal FET geometry. In typical organic FETs, increasing the charge density in the channel improves transport by filling trap states and by moving the chemical potential in the channel toward the mobility edge in the density of states. Surprisingly, Fratini et al. have found that the channel conductance actually saturates at very high charge densities instead of continuing to increase. The reason for this appears to be Coulomb interactions in the channel due to the high carrier density and the polaronic nature of the holes. The strong coupling between the carriers and the dielectric layer leads to a tendency toward self-trapping; add strong repulsion and poor screening into the mix, and you have a more insulating state induced by this combination of effects. Very interesting!

arxiv:0710.2323 - Degen et al., Controlling spin noise in nanoscale ensembles of nuclear spins
Dan Rugar at IBM has been working on magnetic resonance force microscopy for a long time, and they've got sensitivity to the point where they can detect hundreds of nuclear spins (!). (That may not seem impressive if you haven't been following this, but it's a tour de force experiment that's come very far from the initial work.) The basic idea of MRFM is to have a high-Q cantilever that is mechanically resonant at the spin resonance frequency and coupled via magnetic interactions to the sample - that way the polarized spins precess, they drive the cantilever resonance mode. When they look at such a small number of spins, the statistical fluctuations in the spin polarization are readily detected. This is a problem for imaging, actually - the timescale for the natural fluctuations is long enough that the signal bops around quite a bit during a line scan. Fortunately, Degen et al. have demonstrated in this paper that one can deliberately randomize the magnetization by bursts of rf pi/2 pulses, and thus suppress the fluctuation impact on imaging by making the effective fluctuations much more rapid. This is a nice mix of pretty physics and very clever experimental technique.

Giant magnetoresistance

I think it's great that the physics Nobel this year went for giant magnetoresistance (GMR). GMR is intrinsically a quantum mechanical effect, an example of a nanoscale technology that's made it out of the lab and into products, and one of the big reasons that you can buy a 500GB hard drive for $100. (Good job, Sujit, for the advanced pick!).

The story in brief: Back in the ancient past (that is, the 1980s), the read heads on hard drives operated based on the anisotropic magnetoresistance (AMR). For band structure reasons, the electrical resistivity of ferromagnetic metals depends a bit on the relative orientations of M, the magnetization, and J, the current density. In the common NiFe alloy permalloy, for example, the resistivity is about 2% larger when M is parallel to J than when M is perpendicular to J. To read out the bits on magnetic media, a strip of very coercible magnetic material was used, and the fringing fields from the disk media could alter the direction of that strip's M, leading to changes in the resistance that were translated into voltage changes that correspond to 1s and 0s.

In the late 1980s, Fert and Grunberg demonstrated that stacks of nanoscale layers of alternating magnetic and nonmagnetic metals had remarkable magnetoresistive properties. When the M of the FM layers are aligned, the mobile electrons can move smoothly between the layers, leading to relatively low resistance. However, when the M of the FM layers are anti-aligned, there is a mismatch between the densities of states for spin-up and spin-down electrons between anti-aligned layers. The result is enhanced scattering of spin-polarized electrons at the interfaces between the normal and FM layers. (Crudely, a spin-down electron that comes from being the majority spin in one FM layer goes through the normal metal and runs into the anti-aligned FM layer, where that spin orientation is now the minority spin - there are too few empty states available for that electron in the new FM layer, so it is likely to be reflected from the interface.) More scattering = higher resistance. The resulting GMR effect can be 10x larger than AMR, meaning that read heads based on GMR multilayers could read much smaller bits (with smaller fringing fields) for the same signal-to-noise ratio.

Challenges in measurement

This post is only going to be relevant directly for those people working on the same kind of stuff that my group does. Still, it gives a flavor of the challenges that can pop up unexpectedly in doing experimental work.

Often we are interested in measuring the electronic conductance of some nanodevice. One approach to doing this is to apply a small AC voltage to one end of the device, and connect the other end to something called a current preamplifier (or a current-to-voltage converter, or a glorified ammeter) to measure the amount of current that flows. It's possible to build your own current preamp, but many nanodevice labs have a couple of general purpose ones lying around. A common one is the SR570, made by Stanford Research. This gadget is pretty nice - it has up to a 1 MHz bandwidth, it has built-in filter stages, it is remotely programmable, and it has various different gain settings depending on whether you want to measure microamps or picoamps of current.

Here's the problem, though. One of my students observed that his devices seemed to fail at a surprisingly high rate when using the SR570, while the failure rate was dramatically lower when using a different (though more expensive) preamp, the Keithley 428. After careful testing he found that when the SR570 changes gain ranges (there is an audible click of an internal relay when this happens, as the input stage of the amplifier is switched), spikes of > 1V (!) lasting tens of microseconds show up on the input of the amplifier (the part directly connected to the device), at least when hooked up to an oscilloscope. Our nanoscale junctions are very fragile, and these spikes irreversibly damage the devices. The Keithley, on the other hand, doesn't do this and is very quiet. Talking to SRS, this appears to be an unavoidable trait of the SR570. We're working to mitigate this problem, but it's probably good for people out there in the community using these things to know about this.

This week in cond-mat

Two recent papers in cond-mat this time, both rather thermodynamics-related. That's appropriate, since I'm teaching undergrad stat mech these days.

arxiv:0709.4181 - Kubala et al., Violation of Wiedemann-Franz law in a single-electron transistor
The Wiedemann-Franz law is one of those things taught in nearly every undergraduate solid-state physics class. It also happens to be extremely useful for doing cryogenic engineering, as I learned during my grad school days. The idea is simple: simple kinetic theory arguments (and dimensional analysis) imply that the conductivity of some parameter via some excitations is given by the product (carrying capacity of that parameter per excitation)*(speed of excitation carrying that parameter)*(mean free path of that excitation), with some geometric factor out in front (e.g., 1/3 for three dimensional diffusive motion of the excitation). For example, the electrical conductivity in a 3d, diffusive, ordinary metal is (1/3)(e)(v_F)(\ell), where e is the electronic charge, v_F is the Fermi velocity for conduction electrons, and \ell is the mean free path for those electrons (at low T, \ell is set by impurity scattering or boundary scattering). However, in a normal metal electrons can also carry thermal energy with some heat capacity c_v per electron that scales like T, while the speed and mean free path of the electrons are as above. This implies the Wiedemann-Franz law, that the ratio of the thermal conductivity to the (electrical conductivity*T) in an ordinary metal should be a constant (the Lorenz number, ~25 nanoOhms W/K^2). Deviations from the W-F law are indicators of interesting physics - basically that simple metal electrons either aren't the dominant carriers of the electrical current, or that the charge carriers don't carry thermal energy as normal. This paper is a theory piece by the Helsinki group showing that the W-F law fails badly for single-electron transistors. In particular, in the co-tunneling regime, when current is carried via quantum coherent processes, the Lorenz number is predicted to be renormalized upward by a factor of 9/5. This will be challenging to measure in experiments, but exquisite thermal conductivity measurements have been performed in similar systems in the past.

arxiv:0709.4125 - Allahverdyan et al., Work extremum principle: structure and function of quantum heat engines
Marlan Scully (also here) caused a bit of a flurry of excitement a few years ago by proposing a form of heat engine that uses quantum coherence and its destruction to do work, in addition to the conventional approach of using two thermal baths at different temperatures. This paper is a theoretical analysis of some such quantum heat engines. Carnot can sleep easy - in the end you can't violate the Carnot efficiency even with quantum heat engines, if you dot all the "i"s and cross all the "t"s. Neat to think about, though, and of some experimental relevance to the cold atom community, who can prepare highly coherent atomic gases at very low temperatures. This paper is long and detailed and I don't claim to have read it in depth, but it looks interesting.

Revised: Primer on faculty searches, part I

It's that time of year again, with Chad Orzel and the Incoherent Ponderer both posting about the faculty job market and job hunting. So, I'm recycling a post of mine from last year describing the search process, at least the way it's done at Rice. I'm going to insert some revisions that are essentially tips to would-be candidates, though I think the IP has already done a good job on this, and some are basically common sense. An obvious disclaimer: this is based on my experience, and may not generalize well to other departments with vastly differing cultures or circumstances.

Here are the main steps in a search:

• The search gets authorized. This is a big step - it determines what the position is, exactly: junior vs. junior or senior; a new faculty line vs. a replacement vs. a bridging position (i.e. we'll hire now, and when X retires in three years, we won't look for a replacement then).
• The search committee gets put together. In my dept., the chair asks people to serve. If the search is in condensed matter, for example, there will be several condensed matter people on the committee, as well as representation from the other major groups in the department, and one knowledgeable person from outside the department (in chemistry or ECE, for example). The chairperson or chairpeople of the committee meet with the committee or at least those in the focus area, and come up with draft text for the ad.
• The ad gets placed, and canvassing begins of lots of people who might know promising candidates. A special effort is made to make sure that all qualified women and underrepresented minority candidates know about the position and are asked to apply (the APS has mailing lists to help with this, and direct recommendations are always appreciated - this is in the search plan). Generally, the ad really does list what the department is interested in. It's a huge waste of everyone's time to have an ad that draws a large number of inappropriate (i.e. don't fit the dept.'s needs) applicants. The exception to this is the generic ad typically placed by MIT and Berkeley: "We are looking for smart folks. Doing good stuff. In some area." They run the same ad every year, trolling for talent. They seem to do ok. The other exception is when a university already knows who they want to get for a senior position, and writes an ad so narrow that only one person is really qualified. I've never seen this personally, but I've heard anecdotes.
  
• In the meantime, a search plan is formulated and approved by the dean. The plan details how the search will work, what the timeline is, etc. This plan is largely a checklist to make sure that we follow all the right procedures and don't screw anything up. It also brings to the fore the importance of "beating the bushes" - see above. A couple of people on the search committee will be particularly in charge of oversight on affirmative action/equal opportunity issues.
  
• The dean meets with the committee and we go over the plan, including a refresher for everyone on what is or is not appropriate for discussion in an interview (for an obvious example, you can't ask about someone's religion.).
• Applications come in and are sorted; rec letters are collated. Each candidate has a folder.
• The committee begins to review the applications. Generally the members of the committee who are from the target discipline do a first pass, to at least wean out the inevitable applications from people who are not qualified according to the ad (i.e. no PhD; senior people wanting a senior position even though the ad is explicitly for a junior slot; people with research interests or expertise in the wrong area). Applications are roughly rated by everyone into a top, middle, and bottom category. Each committee member comes up with their own ratings, so there is naturally some variability from person to person. Some people are "harsh graders". Some value high impact publications more than numbers of papers. Others place more of an emphasis on the research plan, the teaching statement, or the rec letters. Yes, people do value the teaching statement - we wouldn't waste everyone's time with it if we didn't care. Interestingly, often (not always) the people who are the strongest researchers also have very good ideas and actually care about teaching. This shouldn't be that surprising. As a friend of mine at a large state school once half-joked to me: 15% of the faculty in any department do the best research; 15% do the best teaching; 15% do the most service and committee work; and it's often the same 15%.
• Once all the folders have been reviewed and rated, a relatively short list (say 20-25 or so out of 120 applications) is arrived at, and the committee meets to hash that down to, in the end, five or so to invite for interviews. In my experience, this happens by consensus, with the target discipline members having a bit more sway in practice since they know the area and can appreciate subtleties - the feasibility and originality of the proposed research, the calibration of the letter writers (are they first-rate folks? Do they always claim every candidate is the best postdoc they've ever seen?). I'm not kidding about consensus; I can't recall a case where there really was a big, hard argument within the committee. I know I've been lucky in this respect, and that other institutions can be much more fiesty. The best, meaning most useful, letters, by the way, are the ones who say things like "This candidate is very much like CCC and DDD were at this stage in their careers." Real comparisons like that are much more helpful than "The candidate is bright, creative, and a good communicator." Regarding research plans, the best ones (for me, anyway) give a good sense of near-term plans, medium-term ideas, and the long-term big picture, all while being relatively brief and written so that a general committee member can understand much of it (why the work is important, what is new) without being an expert in the target field. It's also good to know that, at least at my university, if we come across an applicant that doesn't really fit our needs, but meshes well with an open search in another department, we send over the file. This, like the consensus stuff above, is a benefit of good, nonpathological communication within the department and between departments.

That's pretty much it up to the interview stage. No big secrets. No automated ranking schemes based exclusively on h numbers or citation counts.

Tips for candidates:

• Don't wrap your self-worth up in this any more than is unavoidable. It's a game of small numbers, and who gets interviewed where can easily be dominated by factors extrinsic to the candidates - what a department's pressing needs are, what the demographics of a subdiscipline are like, etc. Every candidate takes job searches personally to some degree because of our culture, but don't feel like this is some evaluation of you as a human being.
  
• Don't automatically limit your job search because of geography unless you have some overwhelming personal reasons. I almost didn't apply to Rice because neither my wife nor I were particularly thrilled about Texas, despite the fact that neither of us had ever actually visited the place. Limiting my search that way would've been a really poor decision.
• Really read the ads carefully and make sure that you don't leave anything out. If a place asks for a teaching statement, put some real thought into what you say - they want to see that you have actually given this some thought, or they wouldn't have asked for it.
• Research statements are challenging because you need to appeal to both the specialists on the committee and the people who are way outside your area. My own research statement back in the day was around three pages. If you want to write a lot more, I recommend having a brief (2-3 page) summary at the beginning followed by more details for the specialists. It's good to identify near-term, mid-range, and long-term goals - you need to think about those timescales anyway. Don't get bogged down in specific technique details unless they're essential. You need committee members to come away from the proposal knowing "These are the Scientific Questions I'm trying to answer", not just "These are the kinds of techniques I know".
  
• Be realistic about what undergrads, grad students, and postdocs are each capable of doing. If you're applying for a job at a four-year college, don't propose to do work that would require an experienced grad student putting in 60 hours a week.
  
• Even if they don't ask for it, you need to think about what resources you'll need to accomplish your research goals. This includes equipment for your lab as well as space and shared facilities. Talk to colleagues and get a sense of what the going rate is for start-up in your area. Remember that four-year colleges do not have the resources of major research universities. Start-up packages at a four-year college are likely to be 1/4 of what they would be at a big research school (though there are occasional exceptions). Don't shave pennies - this is the one prime chance you get to ask for stuff! On the other hand, don't make unreasonable requests. No one is going to give a junior person a start-up package comparable to a mid-career scientist.
• Pick letter-writers intelligently. Actually check with them that they're willing to write you a nice letter - it's polite and it's common sense. Beyond the obvious two (thesis advisor, postdoctoral mentor), it can sometimes be tough finding an additional person who can really say something about your research or teaching abilities. Sometimes you can ask those two for advice about this. Make sure your letter-writers know the deadlines and the addresses.

I'll revise more later if I have the time.

2007 Nobel Prize in Physics

Time for pointless speculation. I suggest Michael Berry and Yakir Aharonov for the 2007 physics Nobel, because of their seminal work on nonclassical phase factors in quantum mechanics. Thoughts?

Two seminars this past week

I've been remiss by not posting more interesting physics, either arxiv or published. I'll try to be better about that, though usually those aren't the posts that actually seem to generate comments. For starters, I'll write a little about two interesting condensed matter seminars that we had this week. (We actually ended up with three in one week, which is highly unusual, but I was only able to go to two.)

First, my old friend Mike Manfra from Bell Labs came and gave a talk about the interesting things that one sees in two-dimensional hole systems (2dhs) on GaAs (100). Over the last 25 years, practically a whole subdiscipline (including two Nobel prizes) has sprung up out of our ability to make high quality two-dimensional electron systems (2des). If you have a single interface between GaAs below and AlxGa(1-x)As above, and you put silicon dopants in the AlGaAs close to the interface, charge transfer plus band alignment plus band bending combine to give you a layer of mobile electrons confined in a roughly triangular potential well at the interface. Those electrons are free to move within the plane of the interface, but they typically have no ability to move out of the plane. (That is, the energy to excite momentum in the z direction is greater than their Fermi energy.) Now it's become possible to grow extremely high quality 2dhs, using carbon as a dopant rather than silicon. The physics of these systems is more complicated than the electron case, because holes live in the valence band and experience strong spin-orbit effects (in contrast to electrons in the conduction band). In the electron system, it's known that at relatively low densities, low temperatures, and moderate magnetic fields, there is a competition between different possible ground states, including ones where the electron density is spatially complicated ("stripes", "bubbles", "nematics"). Manfra presented some nice work on the analogous case with holes, where the spin-orbit complications make things even more rich.

Then yesterday we had a talk by Satoru Nakatsuji from the ISSP at the University of Tokyo. He was talking about an extremely cool material, Pr2Ir2O7. This material is a metal, but because of its structure it has very complicated low temperature properties. For example, the Pr ions live on a pyrochlore lattice, which consists of corner-sharing tetrahedra. The ions are ferromagnetically coupled (they want to align their spins), but the lattice structure is a problem because it results in geometric frustration - not all the spins can be satisfied. As a result, the spins never order at nonzero temperature (at least, down to the milliKelvin range) despite having relatively strong couplings. This kind of frustration is important in things like water ice, too. In water ice, the hydrogens can be thought of as being at the corners of such tetrahedra, but the O-H bond lengths can't all be the same. For each tetrahedron, two are short (the covalent O-H bonds) and two are long (hydrogen bonds). The result is a ground state for water ice that is highly degenerate, leading to an unusual "extra" residual entropy at T = 0 of R/2 ln 3/2 per mole (in contrast to the classical third law of thermodynamics that says entropy goes to zero at T = 0. The same kind of thing happens in Pr2I2O7 - the spins on the tetrahedron corners have to be "two-in" and "two-out" (see the link above), leading to the same kind of residual entropy as in water ice. This frustration physics is just the tip of the iceberg (sorry.) of what Nakatsuji discussed. Very neat.

The secret joys of running a lab II: equipment

The good news is that we're getting a cool new piece of equipment to be installed here next week. The bad news (apart from the fact that it uses liquid helium - see previous post) is that I've been spending my morning shifting through US import tariff codes trying to come up with a number that will make the shipping agent happy. You might think that the tariff code supplied by the vendor would be good enough. Apparently you'd be wrong. You might think that this would be the job of a customs broker. Again, apparently you'd be wrong. As the Incoherent Ponderer pointed out, there are many aspects of our jobs for which we never receive formal training. Customs agent is one. By the way: can anyone explain to me why US tariff codes are maintained by the US Census Bureau? Ok, so they're part of the Department of Commerce, but this is just odd.

The secret joys of running a lab: helium.

In my lab, and in many condensed matter physics labs around the world, we use liquid helium to run many of our experiments. At low temperatures, many complicating effects in condensed matter systems are "frozen out", and it becomes easier to understand the effects that remain. Often we are interested in the ground state of some system and want to reduce thermal excitations. Quantum effects are usually more apparent at low temperatures because the inelastic processes that lead to decoherence are suppressed as T approaches zero. For example, the quantum coherence length (the distance scale over which the phase of an electron's wavefunction is well defined before it gets messed up due to inelastic effects of the environment) of an electron in a metal like silver at room temperature is on the order of 1 nm, while that length can be thousands of times longer at 4.2 K, the boiling point of liquid helium at atmospheric pressure. Those kinds of temperatures are also necessary for running good superconducting magnet systems.

The downside of liquid helium is that it's damned expensive, and getting more so by the minute. Running at full capacity I could blow through several thousand liters in a year, and at several dollars a liter minimum plus overhead, that's real money. As a bonus, lately our supplier of helium has become incredibly unreliable, missing orders and generally flaking out, while simultaneously raising prices because of actual production shortages. I just had to read the sales guy the riot act, and if service doesn't improve darn fast, we'll take our business elsewhere, as will the other users on campus. (Helium comes from the radioactive decay of uranium and other alpha emitters deep in the earth, and comes out of natural gas wells.) The long-term solutions are (a) set up as many cryogen-free systems as possible, and (b) get a helium liquifier to recycle the helium that we do use. Unfortunately, (a) requires an upfront cost comparable to about 8 years of a system's helium consumption per system, and (b) also necessitates big capital expenses as well as an ongoing maintenance issue. Of course none of these kinds of costs are the sort of thing that it's easy to convince a funding agency to support. Too boring and pedestrian.

Fortunately, when you work at really nanometer scales, interesting physics often happens at higher temperatures. I've been lucky that two major things going on in my lab right now don't require helium at all. Still, it's bad enough worrying about paying students without the added fun of helium concerns.

UPDATE: See here.

Other Packard meeting highlights

I'm back from California, and the remainder of the Packard meeting was just as much intellectual fun as the first day. It's great to see so much good science and engineering outside my own discipline. Some fun things I learned:

• Plants really can communicate by smell (that is, by giving off and detecting volatile compounds).
• Many flying insects have evolutionarily found wing flap patterns that optimize for minimum energy consumption when hovering.
• Most of the huge number of insect species in tropical rainforests (at least in New Guinea) are specialist feeders, preferring to eat only one type of plant.
• When you split a molecular ion (say I2-) into a neutral atom and an atomic ion, the coherent superposition (in this case, 1/\sqrt(2) [(I + I-) + (I- + I)]) can persist even when the atom and ion are separated by more than 10 atomic diameters.
• Super fancy mass spec plus amazing statistical capabilities can let you do serious proteomics.
• There may have been as many as four supercontinent phases and two "snowball earth" phases in the last three billion years.
• If you come up with a computationally efficient way to model viscoelastic materials (e.g. jello, human skin), you can develop virtual surgery tools for reconstructive surgeons, and win an Oscar for special effects by modeling Davey Jones for POTC II.
• If you develop a DNA microarray chip that lets you cheaply and reliably identify any known virus or the nearest relative of an unknown virus, and you want to use this clinically, the established medical testing companies will react in a very negative way (because they're afraid that if you're successful, they won't be able to keep chargin insurers $3K per possibly unnecessary blood test). The fact that you can save lives won't be of interest to them.
• Comparing different measurement techniques can really tell you a lot about how cells sense and respond to touch.
• You can design a Si photonic crystal to act as a superprism and show negative refraction and negative diffraction, all at the same time, over a useful bandwidth near 1.55 microns wavelength (the standard telecommunications band).
  

I know I'm leaving some out, too. Very fun stuff.

Packard meeting

I'm currently in Monterey thanking the Packard Foundation for their generous support. They're fantastic, and their fellowship has been a godsend that's really given me the flexibility in my research that I've needed. The best part about their annual meetings is that it's a chance for me to listen to good talks pitched to a general audience on an enormously broad set of science and engineering subjects. Some things that I learned yesterday:

• It's possible to do successful astronomical planet-hunting surveys using 300mm camera lenses to make a telescope array.
• There are molecules and molecular ions in astronomical gas clouds that are extremely difficult to make and study on earth (e.g., CH5-; C6H7+).
• The human brain is 2% of the body's mass but uses 20% of the body's oxygen. It also has roughly 10x the concentration of iron, copper, and zinc as other soft tissues on the body.
• Chemical "noise" (e.g., concentration fluctuations) is essential for some kinds of cell differentiation.
• There are other photoactive parts in your eye besides rods and cones, and if those other parts are intact, your body clock can still re-set itself even in the absence of vision.
• Soft tissue can (pretty convincingly) survive inside fossil bones dating back tens of millions of years.
• Viral phylogeny shows convincingly that HIV did not start from contaminated polio vaccines grown in monkeys, and that HIV came from Africa first to Haiti, and then from Haiti to the US in the late 1960s.
• Lots of microbes live as biofilms on the ocean floor via chemical energy gained from the decomposition of basaltic rock.

Invited talk suggestions, APS March Meeting 2008

Along with Eric Isaacs, I am co-organizing a focus topic at the March Meeting of the APS this year on "Fundamental Challenges in Transport Properties of Nanostructures". The description is:

This focus topic will address the fundamental issues that are critical to our understanding, characterization and control of electronic transport in electronic, optical, or mechanical nanostructures. Contributions are solicited in areas that reflect recent advances in our ability to synthesize, characterize and calculate the transport properties of individual quantum dots, molecules and self-assembled functional systems. Resolving open questions regarding transport in nanostructures can have a huge impact on a broad range of future technologies, from quantum computation to light harvesting for energy. Specific topics of interest include: fabrication or synthesis of nanostructures involved with charge transport; nanoscale structural characterization of materials and interfaces related to transport properties; advances in the theoretical treatment of electronic transport at the nanoscale; and experimental studies of charge transport in electronic, optical, or mechanical nanostructures.

The sorting category is 13.6.2, if you would like to submit a contributed talk. Until Friday August 31, we're still soliciting suggestions for invited speakers for this topic, and I would like to hear what you out there would want to see. If you've got a suggestion, feel free either to post below in the comments, or to email me with it, including the name of the suggested speaker and a brief description of why you think they'd be appropriate. The main restriction is that suggested speakers can't have given an invited talk at the 2007 meeting. Beyond that, while talks by senior people can be illuminating, it's a great opportunity for postdocs or senior students to present their work to an audience. Obviously space is limited, and I can make no promises, but suggestions would be appreciated. Thanks.

Quantum impurities from Germany II

A recurring theme at the workshop in Dresden last week was quantum impurities driven out of equilibrium. In general this is an extremely difficult problem! One of the approaches discussed was that of Natan Andrei's group, presented here and here. I don't claim to understand the details, but schematically the idea is to remap the general problem into a scattering language. You set up the nonequilibrium aspect (in the case of a quantum dot under bias, this corresponds to setting the chemical potentials of the leads at unequal values) as a boundary condition. By recasting things this way, you can use a clever ansatz to find eigenstates of the scattering form of the problem, and if you're sufficiently clever you can do this for different initial conditions and map out the full nonequilibrium response. Entropy production and eventual relaxation of the charge carriers far from the dot happens "at infinity". Andrei gives a good (if dense) talk, and this formalism seems very promising, though it also seems like actually calculating anything for a realistic system requires really solving for many-body wavefunctions for a given system.