Tuesday, June 21, 2016

Short items

Here are a few items:

  • This is fantastic.  Eric Schlaepfer, a hardware engineer at Google, has built a "disintegrated circuit", making a 6502 processor (the CPU from the Apple II and also used in one of my favorite undergrad courses back when I took it) out of surface-mount transistors.  It can't run at MHz clock speeds because of the stray capacitance of the traces on the circuit board, but it's still amazing.  If you want a metric for modern processors, if you made a version of the processor for the iPad Air 2, it would cover 82000 m2.
  • This is a bit "meta", but here is Peter Woit's recent Quick Items link.  I've steered clear from the whole multiverse discussion, but wow, I find it very disturbing how much recent mass publicity has been given to an idea that is described, at best, as an extremely speculative notion.  It's like having Bayesian arguments about how many angels can dance on the head of a pin.
  • Speaking of absurdist speculative garbage, Michio Kaku in recent days has claimed that we will shortly be able to create avatars that will live after us based on uploaded memories, and that we are living in The Matrix, which proves the existence of God.   How has this person become one of the well-known faces of science popularization?
  • American Ninja Warrior really is a good way to illustrate some fun physics.
  • Geekwrapped has highlighted this blog as one of the 20 best science blogs out there.  Thanks!

Thursday, June 16, 2016

Frontiers in Quantum Materials and Devices 2016 - day 2

Continuing with my very brief (and necessarily incomplete) summary of the FQMD 2016 meeting at RIKEN at the beginning of this week:

  • Eric Heller of Harvard gave a very interesting and provocative talk about two topics, Raman scattering in graphene and then the onset of optical absorption in semiconductors.  Regarding the former (see here), he makes a strong case that the "double resonance" theoretical treatment of Raman scattering in graphene that has been highly cited since 2000 is not the right way to think about the problem.  Rather, one should use the Kramers-Heisenberg-Dirac theory of Raman scattering c. 1925-27, and keep in mind the important role played by (crystal) momentum conservation, as explained in the paper linked above.   Regarding the latter topic, he went on to argue (persuasively, in my view) that the textbook approach (literally - I described it in my own book) to the onset of optical absorption in direct-gap semiconductors as the photon energy exceeds the band gap is incomplete and gets the functional dependence on frequency wrong.  This work isn't published yet, and it wouldn't be appropriate for me to present his argument before he does, but I will definitely be keeping an eye out for this.
  • Denis Maryenko of RIKEN spoke about measurements of the anomalous Hall effect in the 2d electron gas that is present at the interface between ZnO and MnZnO.  This system is pretty impressive, with disorder so small that it supports very clean fractional quantum Hall effect, but with larger Coulomb and Zeeman energies than the more traditional GaAs/AlGaAs interface because of the different dielectric functions and g factors, respectively of the ZnO system.   Interesting (not yet published) magnetic physics appears to be taking place at the interface due apparently to point defects that support unpaired spins.
  • Pertti Hakonen from Aalto presented a nice talk about the quantum Hall effect in suspended graphene.  They have (not yet published) measurements in suspended structures made in the Corbino geometry, where there is an electrode in the center of a disk, and a second contact around the disk's perimeter.  As you might imagine, making a structure like that where the graphene disk is suspended in space, yet there is a nice contact to the central electrode without disrupting the disk, is quite a fabrication tour de force, based on an approach from here.
  • Vincent Bouchiat from CNRS, Grenoble talked about using tin-decorated graphene as a system to explore the nature of the superconductor-insulator transition.  It's a flexible material system, in that you can control the coverage of the tin (the size and distribution of tin islands), the disorder in the graphene via damage, and the carrier density in the graphene via electrostatic gating.   An earlier paper is here, and a more recent one is here.
  • Steven Richardson of Howard University spoke about the challenges of trying to make germanene, the germanium analog to graphene.  One approach that has been used in graphene growth has been to start with small, polycyclic carbon ring molecules as seeds.  Doing this in germanium has proven difficult, and Prof. Richardson's group does quantum chemistry calculations with DFT to establish the relative energetic stability and properties of candidate molecules.  From his talk I learned something I had not appreciated, that treating dispersion forces (van der Waals interactions) in DFT is really nontrivial.  
  • James Analytis of Berkeley gave a very nice talk about Weyl fermions, where I actually felt like I had a grasp of this for a few minutes.  Up to now, most of the experiments on materials that are supposed to support Weyl-like band structure have been based on photoemission, rather than actual transport.  Prof. Analytis showed particular transport signatures (quantum oscillations of resistance as a function of magnetic field) that are consistent with what one would expect from electrons actually tracing out Weyl-expected trajectories (in both real space and reciprocal space).  This work relies on impressive nanofabrication, where a focused ion beam is used to carve Cd3As2 into nanostructures + leads without killing the material quality.
  • Yoshinori Tokura from RIKEN surveyed his group's results looking at the interplay of magnetism, the quantum Hall effect, and the quantum anomalous Hall effect, built on high quality epitaxial structures based on a topological insulator (Bi1-xSbx)2Te3 and its Cr-doped relative.  Relevant papers are here, here, and here.   This is a great example of how much scientific activity can spring forth when it becomes possible to grow a new material system with very high quality.
  • Jagadeesh Moodera from MIT presented work that is similar in spirit, involving Cr doping of Bi2Se3, and then V doping of Sb2Te3.  In systems like this it is possible to see robust, ballistic transport via chiral edge states over millimeters.  Again, excellent material quality + interesting choices of materials = impressive science.
  • Joe Checkelsky of MIT spoke about exploring electronic materials with magnetically frustrated lattices.  Many systems with magnetic frustration (where magnetic moments at different lattice sites have competing interactions so that it's not possible to satisfy all of them) are insulators.  In conducting versions of these systems, there can be really funky effects where the magnetic states interact with the electrons through mechanisms like Berry curvature.  This work is in press right now and I will come back and update this once it's available online.
  • Hajime Okamoto from NTT gave a neat talk about optomechanical effects (see here for a review) - where photogenerated carriers in an AlGaAs/GaAs cantilever can couple (via the piezoelectric properties of the material) to the mechanical oscillations of the cantilever.  This makes it possible to do an interesting kind of optical driving and optical cooling of such structures.   See here and here, for example.
Whew.  Overall, a fun, interesting, and dense two days!

Tuesday, June 14, 2016

Frontiers in Quantum Materials and Devices 2016 - day 1

There were a number of really interesting talks at the Harvard/MIT sponsored, RIKEN-co-sponsored FQMD workshop this week.   I'm very grateful for the invitation to come and present.  It was a very dense two days!  I have to be a bit careful in what I write, given that some of the work is not yet published.  Here are some highlights.  I'll try to use links to the arxiv versions of the papers so that people without paid access can see them.

  • Ania Bleszynski-Jayich of UCSB spoke about her group's impressive nanoscale magnetic imaging using single nitrogen-vacancy centers in diamond AFM tips.   The N-V centers are defects in the diamond lattice, where a N atom is substituted for a C atom, directly adjacent to a C-atom vacancy.  These defects play host to a single unpaired electronic spin and can be probed through optically detected magnetic resonance.  Brendan Shields at Basel gave a talk later in the day on this technique as well - impressive imaging of domains in antiferromagnetic (!) structures.
  • Naoto Nagaosa of RIKEN gave an overview of his group's work on nonlinear and nonreciprocal electronic and optical responses in special (topological) materials - see here, here, and here for examples.  The last of these is an example where because of funky topological band structure, you can have a material that is rectifying (resistance \( R(I) \neq R(-I)\) ) where the rectification is controlled by a magnetic field.
  • Dylan Maher of Bristol, most recently in the spotlight for cool quantum optics work with Aephraim Steinberg, gave a great overview of the impressive integrated photonics capabilities at Bristol - see herehere, and here
  • Satoshi Iwamoto of Tokyo showed some neat results involving 3d chiral photonic materials (that is, materials with optical helicity built into their structure).  The wild thing here is that these materials in particular are constructed by manually stacking (!) individual nanoscale-thickness layers, using manipulation within an electron microscope - see here for an example.
  • Jason Petta from Princeton presented some really technically beautiful work involving SiGe quantum dots coupled to (and via) superconducting resonators.  These are gate-defined dots, where metal electrodes are used as capacitor electrodes to "suck in" and confine electrons.  It's hard to explain to a non-expert just how technically impressive the multiple gate structures are that they've developed.  See here.   Figure 1 just doesn't do it justice.
  • Makoto Kohda of Tohoku spoke very clearly about spin-orbit effects in GaAs 2d electron gas and in the layered semiconductor GaSe.  He showed very cool stuff - this paper showing coherent motion and precession of spin over long distances, and gate-controlled switching between weak localization and weak antilocalization in tape-exfoliated GaSe.
  • Bill Wilson, executive director of Harvard's CNS, gave an overview of their nanofab facility.  Truly, it is amazing how much internal investment Harvard has made in that facility, and I'm not even talking about the construction of the building itself.  It's very hard not to be jealous.  As often comes up when talking about Harvard, we again see that having a $40B endowment simply makes many problems faced by mere mortals simply evaporate.

Monday, June 13, 2016

Quantum materials workshop followup and preview

At the beginning of last month, the Rice Center for Quantum Materials hosted a workshop "Interacting Quantum Systems Driven Out of Equilibrium", which I reported here and here.  As promised, the slides from the talks are now available here if you click on the names of the speakers.

I am currently attending this workshop at RIKEN, sponsored by the Harvard/MIT NSF-supported Center for Integrated Quantum Materials.  I will be posting a limited summary of this workshop as well, once I recover from jet lag.

Sunday, June 12, 2016

The 2016 Kavli Prize in Nanoscience

Every two years the Kavli Foundation awards three large scientific prizes, in astrophysics, neuroscience, and nanoscience.  This year's nanoscience prize goes to Gerd Binnig, Christoph Gerber, and Cal Quate, for the invention and development of the atomic force microscope (AFM).

The AFM is a great example of one of those inventions that seems elegant and simple, yet could only come into being after the stage had been set through the development of several other enabling technologies.  (My former faculty colleague Prof. Cyrus Mody does an excellent job telling this story in his book, which I heartily recommend.)

The atomic force microscope idea is very simple in concept.  Take a very sharp stylus on a flexible cantilevered arm, and scan it in a controlled way over a surface.  If the stylus tip is actually in contact with the sample surface, changes in surface topography will be detectable through the deflection of the cantilever, which can be measured optically (e.g. deflection of a laser) or by other means (e.g., changes in the electrical resistance of the cantilever as it is strained).  This is basically an extrapolation to the very small scale of the profilometer.  Alternately, you don't need the tip to be in hard contact with the surface - it just needs to get close enough to detect the short-range forces between the tip atoms and the surface.  Oscillating the cantilever/tip up and down at or near its tuning-fork-like mechanical resonance can give you benefits in terms of detection sensitivity.  Unlike STM, AFM has the benefit of working on insulating surfaces.

To implement this requires a number of building blocks:  fabrication of tips with nm-scale sharpness; precise (nm-scale or better) control at the nanoscale of the tip position relative to the sample; computerized data acquisition to map out the tip response as a function of tip position.  These are similar to the necessary requirements for scanning tunneling microscopy, and it is no coincidence that Binnig was associated with STM as well.  Widespread adoption of AFM (as discussed in Mody's book) required these building blocks to be widely available.

AFM has turned out to be incredibly versatile.  These devices can be used to measure extremely tiny local forces.  Once you know the topography, you can withdraw the tip a little, scan back over the surface and measure longer-ranged forces (electrostatics, magnetic forces if you have a magnetic tip).  Lateral deflection of the tip can tell you about frictional interactions between the tip and the sample.  A conducting tip may be used as a local potentiometer, or as a scanning "gate" electrode.  Functionalizing the tip and high frequency techniques have enabled AFM to image surfaces and even molecular orbitals with better-than-atomic resolution.  AFM has been an incredible enabling technology with utility far beyond the original vision of its pioneers.  That's exactly the kind of achievement that big prizes are meant to recognize.

Sunday, June 05, 2016

Journal costs - what's the answer?

Sorry for the brief break in posting - real life obligations sometimes make it tough to blog as frequently as I would like.

The Nature Publishing Group is going to launch another five journals this year.  University library subscription costs for each of these are going to be around $5K/yr.   Other journal publishers are making similar moves - the ACS has launched three new journals this year, including an open access journal that sounds like it's meant to be a direct competitor to NPG's Scientific Reports.

On the one hand, these journals wouldn't be launched if publishers didn't think they could at least break even, meaning that someone somewhere has done a marketing study suggesting that there is sufficient demand out there both from authors and would-be subscribers.  On the other hand, it's hard for me to believe that the market can really sustain continuous growth in the number of journals, especially when this implies a similar growth in the number of requests to review papers (for free of course) from all of these editorial boards.  

What is the endpoint of this proliferation of journals, especially when many university library budgets simply make it impossible for those schools to pay for institutional subscriptions, and the pool of qualified reviewers is not similarly expanding?  In the long term, it seems like services like the arxiv have to win, perhaps with some kind of post-publication review/commentary.  However, the reward structures in place (i.e., the emphasis on particular "high impact" journal publications in hiring and promotion) put in place a huge barrier to change in that direction.  This is another area where I worry about the inevitability of a greater bifurcation into "have" and "have not" institutions, something that has a certain internal consistency but is probably long-term bad for creativity in research.

Monday, May 23, 2016

Research blogging: Magnetism in layered materials

Following on from graphene, there has been enormous interest in other layered materials for the last few years, such as transition metal dichalcogenides (TMDs) like MoS2.   Depending on the constituents and particular structure, these materials can be semiconductors, superconductors, charge density wave compounds, etc., and can have properties that vary strongly as the number of layers in the material is reduced toward one.  You can expand the palette further by substitutionally doping different elements into the chalcogenide layers, or you can intercalate other atoms between the layers.  There are a huge number of possible compounds and variations.  (Fun note:  TMDs have been studied intensely before.  See here for a review from almost 50 years ago!  And magnetism in intercalated TMDs was examined by people like Stuart Parkin and Richard Friend almost 40 years ago.   The resurgence now is due to a combination of improved growth and characterization techniques, interest in low-dimensionality materials, and theoretical appreciation for the richness of possible states in these systems.)

Recently, collaborating with my colleague Jun Lou, we had some fun examining a related material, V5S8, which you can also think of as (V0.25)VS2.  There are vanadium disulfide layers, and intercalated between them are additional vanadium atoms in an ordered pattern.  The bulk version of this material was found in the 1970s to be an antiferromagnet - below the Neel temperature TN ~ 32 K, the spins of the unpaired electrons on the intercalated vanadium atoms spontaneously order into the arrangement shown in the upper panel at right.   If an external magnetic field bigger than about 4 T is applied perpendicular to the planes of the material, the spins flop over into the arrangement shown in the bottom panel - this is called a spin flop transition. 

Prof. Lou's group has figured out how to grow V5S8 by chemical vapor deposition, so that we were able to make measurements on single crystals of a variety of thicknesses, down to about 10 nm.  We found a couple of cool things, as reported here.   

First, we found a previously unreported first-order (in the thicker crystals) phase transition as a function of externally applied magnetic field.   The signature of this is hysteresis in the electrical resistance of the material as a function of the magnetic field, H.  Just below TN, the hysteresis appears near zero magnetic field.  As T is lowered, the magnetic field where the hysteresis takes place increases dramatically - in a thick crystal, it can go from basically 0 T to taking place at 9 T when the temperature is lowered by only three Kelvin!  Indeed, that's probably one reason why the transition was missed by previous investigators:  If you take data at only select temperatures, you could easily miss the whole thing.   This kind of a transition is called metamagnetic, and we think that large applied fields kill the antiferromagnetism (AFM), driving the material into a paramagnetic (PM) state.  We suggest a phase diagram shown in the table-of-contents figure shown here.  The transition extrapolates to a finite value of H at zero temperature.  That implies that it ends up as a quantum phase transition.

Second, we found that there are systematic changes in the magnetic properties as a function of the thickness of the crystals.  In thinner crystals, the antiferromagnetism appears to be weaker, with TN falling.  Moreover, the hysteresis in the field-driven transition vanishes in thinner crystals, suggesting that the metamagnetic transition goes from first-order to second order in the thin limit.   

This work was a lot of fun.  As far as I know, it's the first example of a systematic study of magnetic properties in one of these layered materials as a function of material thickness.  I think we've just scratched the surface in terms of what could be possible in terms of magnetism in this layered material platform.