Thursday, October 17, 2019

More items of interest

This continues to be a very very busy time, but here are a few interesting things to read:

Monday, October 07, 2019

"Phase of matter" is a pretty amazing emergent concept

As we await the announcement of this year's physics Nobel tomorrow morning (last chance for predictions in the comments), a brief note:

I think it's worth taking a moment to appreciate just how amazing it is that matter has distinct thermodynamic phases or states.

We teach elementary school kids that there are solids, liquids, and gases, and those are easy to identify because they have manifestly different properties.  Once we know more about microscopic details that are hard to see with unaided senses, we realize that there are many more macroscopic states - different structural arrangements of solids; liquid crystals; magnetic states; charge ordered states; etc.

When we take statistical physics, we learn descriptively what happens.  When you get a large number of particles (say atoms for now) together, the macroscopic state that they take on in thermal equilibrium is the one that corresponds to the largest number of microscopic arrangements of the constituents under the given conditions.  So, the air in my office is a gas because, at 298 K and 101 kPa, there are many many more microscopic arrangements of the molecules with that temperature and pressure that look like a gas than there are microscopic arrangements of the molecules that correspond to a puddle of N2/O2 mixture on the floor. 

Still, there is something special going on.  It's not obvious that there should have to be distinct phases at all, and such a small number of them.  There is real universality about solids - their rigidity, resistance to shear, high packing density of atoms - independent of details.  Likewise, liquids with their flow under shear, comparative incompressibility, and general lack of spatial structure.  Yes, there are detailed differences, but any kid can recognize that water, oil, and lava all have some shared "liquidity".  Why does matter end up in those configurations, and not end up being a homogeneous mush over huge ranges of pressure and temperature?  This is called emergence, because while it's technically true that the standard model of particle physics undergirds all of this, it is not obvious in the slightest how to deduce the properties of snowflakes, raindrops, or water vapor from there.    Like much of condensed matter physics, this stuff is remarkable (when you think about it), but so ubiquitous that it slides past everyone's notice pretty much of the time.

Saturday, September 28, 2019

Items of interest

As I struggle with being swamped this semester, some news items:
  • Scott Aaronson has a great summary/discussion about the forthcoming google/John Martinis result about quantum supremacy.  The super short version:  There is a problem called "random circuit sampling", where a sequence of quantum gate operations is applied to some number of quantum bits, and one would like to know the probability distribution of the outcomes.  Simulating this classically becomes very very hard as the number of qubits grows.  The google team apparently just implemented the actual problem directly using their 53-qubit machine, and could infer the probability distribution by directly sampling a large number of outcomes.   They could get the answer this way in 3 min 20 sec for a number of qubits where it would take the best classical supercomputer 10000 years to simulate.  Very impressive and certainly a milestone (though the paper is not yet published or officially released).  This has led to some fascinating semantic discussions with colleagues of mine about what we mean by computation.  For example, this particular situation feels a bit to me like comparing the numerical solution to a complicated differential equation (i.e. some Runge-Kutta method) on a classical computer with an analog computer using op-amps and R/L/C components.  Is the quantum computer here really solving a computational problem, or is it being used as an experimental platform to simulate a quantum system?  And what is the difference, and does it matter?  Either way, a remarkable achievement.  (I'm also a bit jealous that Scott routinely has 100+ comment conversations on his blog.)
  • Speaking of computational solutions to complex problems.... Many people have heard about chaotic systems and why numerical solutions to differential equations can be fraught with peril due to, e.g., rounding errors.  However, I've seen two papers this week that show just how bad this can be.  This very good news release pointed me to this paper, where it shows that even using 64 bit precision doesn't save you from issues in some systems.  Also this blog post points to this paper, which shows that n-body gravitational simulations have all sorts of problems along these lines.  Yeow.
  • SpaceX has assembled their mammoth sub-orbital prototype down in Boca Chica.  This is going to be used for test flights up to 22 km altitude, and landings.  I swear, it looks like something out of Tintin or The Conquest of Space.  Awesome.
  • Time to start thinking about Nobel speculation.  Anyone?

Wednesday, September 18, 2019

DOE Experimental Condensed Matter PI Meeting, day 3 and wrapup

On the closing day of the PI meeting, some further points and wrap-up:

  • I had previously missed work that shows that electric field can modulate magnetic exchange in ultrathin iron (overview).
  • Ferroelectric layers can modulate transport in spin valves by altering the electronic energetic alignment at interfaces.  This can result in some unusual response (e.g., the sign of the magnetoresistance can flip with the sign of the current, implying spin-diode-like properties).
  • Artificial spin ices are still cool model systems.  With photoelectron emission microscopy (PEEM), it's possible to image ultrathin, single-domain structures to reveal their mangetization noninvasively.  This means movies can be made showing thermal fluctuations of the spin ice constituents, revealing the topological character of the magnetic excitations in these systems.  
  • Ultrathin oxide membranes mm in extent can be grown, detached from their growth substrates, and transferred or stacked.  When these membranes are really thin, it becomes difficult to nucleate cracks, allowing the membranes to withstand large strains (several percent!), opening up the study of strain effects on a variety of oxide systems.
  • Controlled growth of stacked phthalocyanines containing transition metals can generate nice model systems for studying 1d magnetism, even using conventional (large-area) methods like vibrating sample magnetometry.
  • In situ oxide MBE and ARPES, plus either vacuum annealing or ozone annealing, has allowed the investigation of the BSCCO superconducting phase diagram over the whole range of dopings, from severely underdoped to so overdoped that superconductivity is completely suppressed.  In the overdoped limit, analyzing the kink found in the band dispersion near the antinode, it seems superconductivity is suppressed at high doping because the coupling (to the mode that causes the kink) goes to zero at large doping.  
  • It's possible to grow nice films of C60 molecules on Bi2Se3 substrates, and use ARPES to see the complicated multiple valence bands at work in this system.  Moreover, by doing measurements as a function of the polarization of the incoming light, the particular molecular orbitals contributing to those bands can be identified.
  • Through careful control of conditions during vacuum filtration, it's possible to produce dense, locally crystalline films of aligned carbon nanotubes.  These have remarkable optical properties, and with the anisotropy of their electronic structure plus ultraconfined character, it's possible to get exciton polaritons in these into the ultrastrong coupling regime.
Overall this was a very strong meeting - the variety of topics in the program is impressive, and the work shown in the talks and posters was uniformly interesting and of high quality. 

Tuesday, September 17, 2019

DOE Experimental Condensed Matter PI Meeting, Day 2

Among the things I heard about today, as I wondered whether newly formed Tropical Storm Imelda would make my trip home a challenge:

  • In "B20" magnetic compounds, where the crystal structure is chiral but lacks mirror or inversion symmetry, a phase can form under some circumstances that is a spontaneous lattice of skyrmions.  By adding disorder through doping, it is possible to un-pin that lattice.
  • Amorphous cousins of those materials still show anomalous Hall effect (AHE), even though the usual interpretation these days of the AHE is as a consequence of Berry phase in momentum space that is deeply connected to having a lattice.  It's neat to see that some Berry physics survives even when the lattice does not.
  • There is a lot of interest in coupling surface states of topological insulators to ferromagnets, including using spin-orbit torque to switch the magnetization direction of a ferromagnetic insulator.
  • You could also try to switch the magnetization of \(\alpha-Fe_{2}O_{3}\) using spin-orbit torques, but watch out when you try to cram too much current through a 2 nm thick Pt film.
  • The interlayer magnetic exchange in van der Waals magnets continues to be interesting and rich.
  • Heck, you could look at several 2D materials with various kinds of reduced symmetry, to see what kinds of spin-orbit torques are possible.
  • It's always fun to find a material where there are oscillations in magnetization with applied field even though the bulk is an insulator.
  • Two-terminal devices made using (Weyl superconducting) MoTe2 show clear magnetoresistance signatures, indicating supercurrents carried along the material edges.
  • By side-gating graphene structures hooked up to superconductors, you can also make a superconducting quantum intereference device using edge states of the fractional quantum Hall effect.
  • In similar spirit, coupling a 2D topological insulator (1T'-WTe2) to a superconductor (NbSe2) means it's possible to use scanning tunneling spectroscopy to see induced superconducting properties in the edge state.
  • Just in time, another possible p-wave superconductor.
  • In a special stack sandwiching a TI between two magnetic TI layers, it's possible to gate the system to break inversion symmetry, and thus tune between quantum anomalous Hall and "topological Hall" response.
  • Via a typo on a slide, I learned of the existence of the Ohion, apparently the smallest quantized amount of Ohio.


DOE experimental condensed matter PI meeting, day 1

The first day of the DOE ECMP PI meeting was very full, including two poster sessions.  Here are a few fun items:

  • Transition metal dichalcogenides (TMDs) can have very strongly bound excitons, and if two different TMDs are stacked, you can have interlayer excitons, where the electron and hole reside in different TMD layers, perhaps separated by a layer or two of insulating hBN.  Those interlayer excitons can have long lifetimes, undergo Bose condensation, and have interesting optical properties.  See here, for example.   
  • Heterojunctions of different TMDs can produce moire lattices even with zero relative twist, and the moire coupling between the layers can strongly affect the optical properties via the excitons.
  • Propagating plasmons in graphene can have surprisingly high quality factors (~ 750), and combined with their strong confinement have interesting potential.
  • You can add AlAs quantum wells to the list of materials systems in which it is possible to have very clean electronic transport and see fractional quantum Hall physics, which is a bit different because of the valley degeneracy in the AlAs conduction band (that can be tuned by strain).
  • And you can toss in WSe2 in there, too - after building on this and improving material quality even further.
  • There continues to be progress in trying to interface quantum Hall edge states with superconductors, with the end goal of possible topological quantum computing.  A key question is understanding how the edge states undergo Andreev processes at superconducting contacts.
  • Application of pressure can take paired quantum Hall states (like those at \(\nu = 5/2, 7/2\)) and turn them into unpaired nematic states, a kind of quantum phase transition.  
  • With clever (and rather involved) designs, it is possible to make high quality interferometers for fractional quantum Hall edge states, setting the stage for detailed studies of exotic anyons.

Sunday, September 15, 2019

DOE Experimental Condensed Matter PI meeting, 2019

The US Department of Energy's Basic Energy Sciences component of the Office of Science funds a lot of basic scientific research, and for the last decade or so had a tradition of regular gatherings of their funded principal investigators for a number of programs.  Every two years there has been a PI meeting for the Experimental Condensed Matter Physics program, and this year's meeting starts tomorrow.

These meetings are very educational (at least for me) and, because of their modest size, a much better networking setting than large national conferences.  In past years I've tried to write up brief highlights of the meetings (for 2017, see a, b, c; for 2015 see a, b, c; for 2013 see a, b).   I will try to do this again; the format of the meeting has changed to include more poster sessions, which makes summarizing trickier, but we'll see.

update:  Here are my write-ups for day 1, day 2, and day 3.

Tuesday, September 10, 2019

Faculty position at Rice - Astronomy

Faculty position in Astronomy at Rice University

The Department of Physics and Astronomy at Rice University invites applications for a tenure-track faculty position in astronomy in the general field of galactic star formation and planet formation, including exoplanet characterization. We seek an outstanding theoretical, observational, or computational astronomer whose research will complement and extend existing activities in these areas within the department. In addition to developing an independent and vigorous research program, the successful applicant will be expected to teach, on average, one undergraduate or graduate course each semester, and contribute to the service missions of the department and university. The department expects to make the appointment at the assistant professor level. A Ph.D. in astronomy/astrophysics or related field is required.

Applications for this position must be submitted electronically at http://jobs.rice.edu/postings/21236. Applicants will be required to submit the following: (1) cover letter; (2) curriculum vitae; (3) statement of research; (4) teaching statement; (5) PDF copies of up to three publications; and (6) the names, affiliations, and email addresses of three professional references. We will begin reviewing applications December 1, 2019. To receive full consideration, all application materials must be received by January 10, 2020. The appointment is expected to begin in July, 2020.

Rice University is an Equal Opportunity Employer with a commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status. We encourage applicants from diverse backgrounds to apply.