Sunday, October 04, 2015

Annual Nobel speculation

It's getting to be that time of year again.  The 2015 Nobel Prize in Physics will be announced this coming Tuesday morning (EU/US time).  Based on past patterns, it looks like it could well be an astro prize.  Dark matter/galaxy rotation curves anyone?  Or extrasolar planets?  (I still like Aharonov + Berry for geometric phases, perhaps with Thouless as well.  However, it's unlikely that condensed matter will come around this year.)

On Wednesday, the chemistry prize will be awarded.  There, I have no clue.  Curious Wavefunction has a great write-up that you should read, though.

Speculate away!

Wednesday, September 30, 2015

DOE Experimental Condensed Matter Physics PI Meeting 2015 - Day 3

Things I learned from the last (half)day of the DOE PI meeting:
  • "vortex explosion" would be a good name for a 1980s metal band.
  • Pulsed high fields make possible some really amazing measurements in both high \(T_{\mathrm{C}}\) materials and more exotic things like SmB6.
  • Looking at structural defects (twinning) and magnetic structural issues (spin orientation domain walls) can give insights into complicated issues in pnictide superconductors.
  • Excitons can be a nice system for looking at coherence phenomena ordinarily seen in cold atom systems.  See here and here.  Theory proposes that you could play with these at room temperature with the right material system.
  • Thermal gradients can drive spin currents even in insulating paramagnets, and these can be measured with techniques that could be performed down to small length scales. 
  • Very general symmetry considerations when discussing competing ordered states (superconductivity, charge density wave order, spin density wave order) can lead to testable predictions.
  • Hybrid, monocrystalline nanoparticles combining metals and semiconductors are very pretty and can let you drive physical processes based on the properties of both material systems.

Tuesday, September 29, 2015

DOE Experimental Condensed Matter Physics PI Meeting 2015 - Day 2

Among the things I learned at the second day of the meeting:

  • In relatively wide quantum wells, and high fields, you can enter the quantum Hall insulating state.  Using microwave measurements, you can see signatures of phase transitions within the insulating state - there are different flavors of insulator in there.  See here.
  • As I'd alluded to a while ago, you can make "artificial" quantum systems with graphene-like energetic properties (for example).  
  • In 2d hole gasses at the interface between Ge and overlying SiGe, you can get really huge anisotropy of the electrical resistivity in magnetic fields, with the "hard" axis along the direction of the in-plane magnetic field.
  • In single-layer thick InN quantum wells with GaN above and below, you can have a situation where there is basically zero magnetoresistance.  That's really weird.
  • In clever tunneling spectroscopy experiments (technique here) on 2d hole gasses, you can see sharp inelastic features that look like inelastic excitation of phonons.  
  • Tunneling measurements through individual magnetic nanoparticles can show spin-orbit-coupling-induced level spacings, and cranking up the voltage bias can permit spin processes that are otherwise blockaded.  See here.
  • Niobium islands on a gold film are a great tunable system for studying the motion of vortices in superconductors, and even though the field is a mature one, new and surprising insights come out when you have a clean, controlled system and measurement techniques. 
  • Scanning Josephson microscopy (requiring a superconducting STM tip, a superconducting sample, and great temperature and positional control) is going to be very powerful for examining the superconducting order parameter on atomic scales.
  • In magnetoelectric systems (e.g., ferroelectrics coupled to magnetic materials), combinations of nonlinear optics and electronic measurements are required to unravel which of the various possible mechanisms (charge vs strain mediated) generates the magnetoelectric coupling.
  • Strongly coupling light in a cavity with Rydberg atoms should be a great technique for generating many body physics for photons (e.g., the equivalent of quantum Hall).
  • Carbon nanotube devices can be great systems for looking at quantum phase transitions and quantum critical scaling, in certain cases.
  • Controlling vortex pinning and creep is hugely important in practical superconductors.  Arrays of ferromagnetic particles as in artificial spin ice systems can control and manipulate vortices.  Thermal fluctuations in high temperature superconductors could end up limiting performance badly, even if the transition temperature is at room temperature or more, and the situation is worse if the material is more anisotropic in terms of effective mass.
  • "Oxides are like people; it is their defects that make them interesting."

Monday, September 28, 2015

DOE Experimental Condensed Matter Physics PI meeting 2015 - Day 1

Things I learned at today's session of the DOE ECMP PI meeting:
  • In the right not-too-thick, not-too-thin layers of the 3d topological insulator Bi1.5Sb0.5Te1.7Se1.3 (a cousin of Bi2Se3 that actually is reasonably insulating in the bulk), it is possible to use top and bottom gates to control the surface states on the upper and lower faces, independently.  See here.
  • In playing with suspended structures of different stackings of a few layers of graphene, you can get some dramatic effects, like the appearance of large, sharp energy gaps.  See here.
  • While carriers in graphene act in some ways like massless particles because their band energy depends linearly on their crystal momentum (like photon energy depends linearly on photon momentum in electrodynamics), they have a "dynamical" effective mass, \(m^* = \hbar (\pi n_{2d})^{1/2}/v_{\mathrm{F}}\), related to how the electronic states respond to an electric bias.  
  • PdCoO2 is a weird layered metal that can be made amazingly clean, so that its residual resistivity can be as small as 8 n\(\Omega\)-cm.  That's about 200 times smaller than the room temperature resistivity of gold or copper.  
  • By looking at how anisotropic the electrical resistivity is as a function of direction in the plane of layered materials, and how that anisotropy can vary with applied strain, you can define a "nematic susceptibility".  That susceptibility implies the existence of fluctuations in the anisotropy of the electronic properties (nematic fluctuations).  Those fluctuations seem to diverge at the structural phase transition in the iron pnictide superconductors.  See here.   Generically, these kinds of fluctuations seem to boost the transition temperature of superconductors.
  • YPtBi is a really bizarre material - nonmetallic temperature dependence, high resistivity, small carrier density, yet superconducts.  
  • Skyrmions (see here) can be nucleated in controlled ways in the right material systems.  Using the spin Hall effect, they can be pushed around.  They can also be moved by thermally driven spin currents, and interestingly skyrmions tend to flow from the cold side of a sample to the hot side.  
  • It's possible to pump angular momentum from an insulating ferromagnet, through an insulating antiferromagnet (NiO), and into a metal.  See here.
  • The APS Conferences for Undergraduate Women in Physics have been a big hit, using attendance as a metric.  Extrapolating, in a couple of years it looks like nearly all of the undergraduate women majoring in physics in the US will likely be attending one of these.
  • Making truly nanoscale clusters out of some materials (e.g., Co2Si, Mn5Si3) can turn them from weak ferromagnets or antiferromagnets in the bulk into strong ferromagnets in nanoparticle form.   See here.

Friday, September 25, 2015

DOE Experimental Condensed Matter Physics PI meeting 2015

As they did two years ago, the Basic Energy Sciences part of the US DOE is having a gathering of experimental condensed matter physics investigators at the beginning of next week.  The DOE likes to do this (see here for proceedings of past meetings), with the idea of getting people together to talk about the current and future state of the field and ideally seed some collaborations.  I will try to blog a bit about the meeting, as I did in 2013 (here and here).

Friday, September 18, 2015

CMP and materials in science fiction

Apologies for the slower posting frequency.  Other obligations (grants, research, teaching, service) are significant right now.

I thought it might be fun to survey people for examples of condensed matter and materials physics as they show up in science fiction (and/or comics, which are fantasy more than hard SF).  I don't mean examples where fiction gets science badly wrong or some magic rock acts as a macguffin (Infinity Stones, Sankara stones) - I mean cases where materials and their properties are thought-provoking.

A couple of my favorites:
  • scrith, the bizarre material used to construct the Ringworld.  It's some exotic material that has 40% opacity to neutrinos without being insanely dense like degenerate matter.
  • From the same author, shadow square wire, which is an absurdly strong material that also doubles as a high temperature superconductor.  (Science goof in there:  Niven says that this material is also a perfect (!) thermal conductor.  That's incompatible with superconductivity, though - the energy gap that gives you the superconductivity suppresses the electronic contribution to thermal conduction.  Ahh well.)
  • Even better, from the same author, the General Products Hull, a giant single-molecule structure that is transparent in the visible, opaque to all other wavelengths, and where the strength of the atomic bonds is greatly enhanced by a fusion power source.
  • Vibranium, the light, strong metal that somehow can dissipate kinetic energy very efficiently.  (Like many materials in SF, it has whatever properties it needs to for the sake of the plot.  Hard to reconcile the dissipative properties with Captain America's ability to bounce his shield off objects with apparently perfect restitution.)
  • Old school:  cavorite, the H. G. Wells wonder material that blocks the gravitational interaction.
What are your favorite examples?

Friday, September 11, 2015

Amazingly clear results: density gradient ultracentrifugation edition

Ernest Rutherford reportedly said something like, if your experiment needs statistics, you should have done a better experiment.  Sometimes this point is driven home by an experimental technique that gives results that are strikingly clear.  To the right is an example of this, from Zhu et al., Nano Lett. (in press), doi:  10.1021/acs.nanolett.5b03075.  The technique here is called "density gradient ultracentrifugation". 

You know that the earth's atmosphere is denser at ground level, with density decreasing as you go up in altitude.  If you ignore temperature variation in the atmosphere, you get a standard undergraduate statistical physics problem ("the exponential atmosphere") - the gravitational attraction to the earth pulls the air molecules down, but the air has a non-zero temperature (and therefore kinetic energy).  A density gradient develops so that the average gravitational "drift" downward is balanced on average by "diffusion" upward (from high density to low density). 

The idea of density gradient ultracentrifugation is to work with a solution instead of the atmosphere, and generate a vastly larger effective gravitational force (to produce a much sharper density gradient within the fluid) by using an extremely fast centrifuge.  If there are particles suspended within the solution, they end up sitting at a level in the test tube that corresponds to their average density.  In this particular paper, the particles in question are little suspended bits of hexagonal boron nitride, a quasi-2d material similar in structure to graphite.  The little hBN flakes have been treated with a surfactant to suspend them, and depending on how many layers are in each flake, they each have a different effective density in the fluid.  After appropriate dilution and repeated spinning (41000 RPM for 14 hours for the last step!), you can see clearly separated bands, corresponding to layers of suspension containing particular thickness hBN flakes.  This paper is from the Hersam group, and they have a long history with this general technique, especially involving nanotubes.  The results are eye-popping and seem nearly miraculous.  Very cool.