Friday, May 27, 2011

Recently in the arxiv

As I get ready to head to Germany for my first ever experience lecturing at a Max Planck summer school, I wanted to point out very briefly three of a number of interesting papers that came through the arxiv this week.

arxiv:1105.4055 - Janssen et al., Graphene, universality of the quantum Hall effect and re-definition of the SI
This paper compares the quantization of the Hall resistance in two different two-dimensional electronic systems: a conventional 2d electron gas in a GaAs/AlGaAs structure, and graphene. The authors find that the Hall resistance is quantized in units of h/e2 identically in the two systems to parts in 1011. On the one hand, this is really amazing, since you're seeing essentially exact quantization in two different systems, and the whole basis for the quantum Hall effect relies in part on dirt - without disorder, you wouldn't see the quantum Hall physics. And yet, even though the materials differ and dirt plays an important role, you get precise quantization in terms of fundamental constants. This is the kind of emergent, exact phenomenon that shows the profound character of condensed matter physics.

arxiv:1105.4642 - Barends et al., Loss and decoherence due to stray infrared light in superconducting quantum circuits
As someone who struggled mightily in grad school to avoid the effects of infinitesimal amounts of rf noise leaking into his ultracold sample, this impressed me. The authors demonstrate that infrared radiation from the surroundings, even when those surroundings are at 4.2 K, can have marked, detectable impact on the coherence properties of superconducting quantum bits. They compare results with and without an absorbing radiation shield in the way, and the effects aren't small. Wild. Time to break out those 50 mK shields from our old nuclear demag cryostat....

arxiv:1105.4652 - Paik et al., How coherent are Josephson junctions?
Along these same lines, these authors have been able to demonstrate coherence times in superconducting qubits that stretching into the tens of microseconds scale. They do this via a new kind of cavity, essentially controlling the environmental dissipation. This isn't really my area, but I know enough to be impressed, and also to be surprised at the apparent lack of the usually ubiquitous 1/f noise problems (in the critical current) that often limit coherence in these kinds of devices. As they point out, these numbers are encouragingly close to the thresholds needed for quantum error correction to be realistic.

Friday, May 20, 2011

Nano for batteries

Improved batteries would be of enormous benefit and utility in many sectors of technology.  A factor of 10 improvement in battery capacity (with good charging rate, safety, etc.) would mean electric cars that get 1000 miles per charge, laptops that run for days w/o charging, electrical storage to help with the use of renewable energy, and a host of other changes.  This rate of performance enhancement is completely commonplace in semiconductor electronics and magnetic data storage, yet batteries have lagged far, far behind.

There is real hope that nanostructured materials can help in this area.  Three examples illustrate this well.  Conventional lithium ion batteries have an anode (usually graphitic carbon, into which lithium ions may be intercalated) and a cathode (such as cobalt oxide), with an intervening electrolyte, and a separator barrier to prevent the two sides from shorting together.  A reasonable figure of merit is the capacity of the electrodes, in units of mA-h/g.  The materials described above, anode and cathode, have capacities on the order of 200-300 mA-h/g.  It is known that silicon can take up even more lithium than carbon, with a possible capacity of more than 3000 mA-h/g (!).  Complicating matters, Si swells dramatically when taking in Li, meaning that bulk single-crystal Si cracks and self-pulverizes when taken through a few charge/discharge cycles.  However, Si nanowires have been observed to be much better behaved - they have large surface specific surface area, and have enough free surface to swell and shrink without destroying themselves - see here.  Very recently, this paper has spectacular electron micrographs of the swelling of such nanowires.

A second example:  nanostructured cobalt oxide particles, self-assembled using selectively modified virus proteins, have been put forward as high capacity Li ion battery cathodes.  This approach has also been extended to iron phosphate cathode material.

A third example:  dramatically improved charging rates may be possible using nanostructured electrode geometries, such as these inverse-opal shapes.

There is real hope that nanostructured materials may enable true breakthroughs in battery technology, even though batteries have been studied exhaustively for many decades.  The ability to engineer materials at previously inaccessible scales may bear fruit soon.

Monday, May 16, 2011

Rice University clean room manager needed.

Just in case anyone out there has or is a promising candidate, I wanted to point out that Rice University is looking for a new clean room facilities manager.  (This is not a soft money position.)  Here is the text of the advertisement:

Rice University is seeking a technical manager to oversee the operations of its clean room user facility and associated characterization equipment.  This Class 100/1000 facility contains a suite of instruments, including a photolithography mask maker, a contact mask aligner, an e-beam evaporator, an RIE/PECVD system, and a collection of characterization tools.  The manager’s responsibilities include oversight of this facility, training of undergraduate and graduate students and other users, and maintenance and upkeep of the equipment.  Applicants must have a BS degree in a science or engineering discipline (PhD preferred but not required), and extensive experience with several of the relevant instruments or a related technical degree or diploma with an additional 2 years of the related experience (for a total of 7 years of related experience working with clean room instruments).  Salary will commensurate with experience.  The need to fill this position is immediate, and resumes will be examined as they arrive.  Please visit to apply for this listing.  Rice University is an equal opportunity, affirmative action employer.

Friday, May 13, 2011

A university selling its soul

I'll get back to physics shortly.  These two articles (here and here) explain how, in exchange for $1.5M in donations, the Florida State economics department agreed to give the donors veto power over faculty hiring for the donor-supported positions.  Moreover, the donors can withdraw the positions if they aren't happy with annual performance reviews of the professors.  Wow.  I know times are tight, but FSU has clearly decided that they're up for bid.  I don't care whether the donors are right-wing or left-wing (hint:  they're right wing) or centerist - a university that allows donors direct control over faculty hiring and evaluation is out of its mind.  Gee, you think those professors are going to be free to do whatever research they want?  Do you think there's going to be pressure on all of the faculty within the department to toe the line rather than risk angering the donors?  What a mess.  Well, at least it confirms that Texas doesn't have a monopoly on idiocy.

Update:  blogger ate this post, and I had to reconstitute it from the cached version on bing (google blew this one all the way around).  Clearly the Koch brothers are responsible :-)

Thursday, May 05, 2011

Gravity Probe B

Finally, after only 45 years from conception to publication of results, Gravity Probe B has announced (dramatic pause) that Einstein's General Theory of Relativity is consistent with their data.  I had mentioned GPB ("The Project that Ate Stanford") once before.  It was a fascinating, complex, multidisciplinary project that, thanks to its experimental design and extraordinarily long duration, had great impact on a large number of physics, materials science, and engineering careers.  Still, I think they were in a bit of a no-win scenario, particularly once it became clear that there were problems with interpreting the data.  Either they support general relativity, or people just wouldn't trust the results, given how much other evidence there is out there that GR is right, at least in the relatively weak field limit.

Nano for solar

Sorry about the delay in this posting.  Real life has been busy.

Solar energy is an obvious candidate for a long-term solution to many of our energy problems.  The amount of power reaching the surface of the earth is on the order of 350 W/m2.  We could meet the world's projected energy needs in 2030 by covering around 250 km by 250 km with 10% efficient solar cells.  Unfortunately, the total surface area of all photovoltaics ever manufactured is less than 0.1% of that.  (This is why being able to produce photovoltaic cells by printing processes would be great.  Hint:  estimate the total area printed by the New York Times in a month.)  There are a number of challenges involved in solar.  Why might "nano" broadly defined be a big help?  Let me give three examples from the large wealth of ideas out there.

1) Semiconductor nanocrystals as absorbers.  Because of the beauty of quantum confinement, it is possible to make semiconductor nanocrystals out of a single material, and use different sizes to capture different parts of the solar spectrum.  Moreover, there is evidence (after some controversy) that nanocrystals may enhance "multiexciton generation" (e.g., here and here).  In a traditional solar cell, a photon with energy twice as large as the semiconductor band gap will generate an electron-hole pair (which must be ripped apart somehow), and inelastic processes will lead to the excess (above the band gap) energy being lost as heat.  However, at some rate, instead you can generate two band-gap-energy pairs.  The idea is that the rate of that process can be enhanced in nanocrystals, since conservation of "crystal momentum" can be relaxed in materials that are so surface-dominated.

2) Nanostructured materials for photoelectrochemical cells.  There are a number of proposals for using electrolytes in solar applications, including dye-sensitized solar cells.  In this case, one would like to use a high surface area anode, such as nanostructured TiO2 or some similar nanostructured material.  Moreover, instead of using organic dyes as the absorbers and sources of photoexcited electrons, one could imagine again using semiconductor nanocrystals.

3) Plasmon-enhanced photovoltaics.  One way to try to boost the efficiency of solar cells is to get the light to hang around the absorber material for longer.  One compact way to do so is to use plasmonically active metal nanoparticles or nanostructures as optical antennas.  The local fields near these structures can enhance scattering and local intensity in ways that tend to boost performance, though resistive losses in the metal may limit their effectiveness.  It's worth pointing out that one can also use plasmonic antennas as sources of hot electrons, also interesting from the photovoltaic angle.

There are many more ideas out there - I haven't even mentioned anything about nanotubes or graphene.  While the odds of any individual idea being a truly transformative breakthrough are small, there are probably more clever things being proposed in this area now that at any time ever before, thanks to our ability to manipulate matter on very small scales.