Friday, March 29, 2013

Vanadium dioxide and ionic liquids

Before posting about the APS topics I'd mentioned, I wanted to write a bit about a recent paper that has received quite a bit of attentionStuart Parkin's group at IBM Research in Almaden reported in Science last week that they had used an ionic liquid to alter the properties of the strongly correlated oxide VO2.  As I'd written previously, VO2 has a metal-insulator transition at 67 C in the bulk, and there has been much interest in using some kind of electrical means to trigger the transition from the insulating to the metallic state in such correlated oxides.  If the insulating state results from Mott physics (i.e., the on-site electron-electron repulsion in the half-filled d band is so strong that the electrons cannot doubly occupy any of the transition metal sites, and the system acts like an insulator even though it would be a metal in the absence of the Coulomb repulsion), then electronically tuning the system away from half-filling (e.g., with a gate using the field effect) could switch the system into the metallic state.  Such a MottFET would be a switch operating on principles very different from those used in conventional Si transistors, and could in principle have superior operating characteristics (for the experts, things like subthreshold swing).   In the Parkin group's work, however, they report pretty compelling evidence that the switching in their system is driven by electrochemistry rather than simple electrostatic doping.  They argue that the metallic state that they produce results from chemical reactions between the VO2 and the gate-biased ionic liquid that pull oxygen atoms out of the VO2 film.  The evidence for this is that a film treated this way recovers the insulating state when properly exposed to oxygen.  Moreover, they can expose the device to a specific isotope, 18O.  They can then sputter material off the film and use mass spectrometry to "weigh" the fragments (secondary ion mass spectrometry), and they find evidence that 18O makes it into the film to a depth of tens of nanometers (many unit cells). 

I find three things interesting about this paper.  First, the actual science is very nice, and I like the isotope tagging/SIMS quite a bit.  Second, I found the perspective put on this by IBM (and the resulting media coverage) a bit surprising - that using liquids and ion motion was a major advance because it would allow chips that operate more like the brain (history dependence = learning, + nonvolatile state retention).   I think that's a surprising spin to put on this.  That brings me to my third point, the true significance of the paper in (part of) the CM community:  This shows that you have to be very very careful when playing with ionic liquids to avoid electrochemistry!  There are previous papers out there that show very modest response of VO2 in some forms to ionic liquid gating (here and here, for example), and a high profile Nature paper from last summer that reports a huge response.  The present work places these prior publications in an important context, calling into question the relative importance of electrostatics vs. electrochemistry. 

Friday, March 22, 2013

APS March meeting, day 4 (and wrapup for me)

Due to the constraints of real life, the March Meeting was over for me yesterday afternoon, but I did see a few more interesting talks.

In the morning, I went to the session about valley polarization in transition metal dichalcogenides That's quite a mouthful, and demands some explanation.  In a number of materials (including dichalcogenides like MoS2), the conduction band has more than one energy minimum (or valley) as a function of electron momentum.  In MoS2, there are two energetically equivalent valleys.  Achieving "valley polarization" refers to exciting electrons in only one of those valleys.  Why would you care?  Well, any way of labeling your charge carriers is potentially a means of storing and manipulating information.  In materials like these but possessing broken inversion symmetry (that is, the material has a built-in directionality due to its structure), it is possible to do clever things with circularly polarized light to populate a valley preferentially.  In materials with strong spin-orbit coupling, it is then possible to manipulate spin through valley selection, etc.  The talk by Wang Yao did a very clear job of explaining all this pedagogically, and later talks in the session were also good.

I tried to check out an invited talk on resistive memories, but the chair had let the session fall 15 minutes behind schedule in the first hour.  Note:  there is a reason for timers, and if you're going to be a session chair, you have to hold people to their allotted slots.

I did make it to John Martinis' talk about whether materials are good enough to build a superconducting quantum computer.   It sounds like there is cause for cautious optimism, but wow is it going to be a difficult engineering task.  I need to look up how surface coding is supposed to work.

Finally, I finished off my time at the meeting by going to a session on science and public policy.  Unfortunately this was a depressing way to leave things, since the general message in the end was that Congress is truly dysfunctional, with little hope for any bipartisan support for science - in part because of reflexive opposition, and in part because a significant fraction of the Republican base literally does not believe that science is a valid tool for shaping policy.

One final note for the APS planners in future years:  Please make sure that the APS webserver for the meeting site can actually handle the load.  Still, all in all, a good meeting.

Thursday, March 21, 2013

APS March meeting, day 3

Yesterday was again a mix of talks (including three from my group), chatting with friends and colleagues, looking around the vendor show (nice toys, and books, including the new edition of Purcell (and Morin) in SI units, and an intriguing graduate E&M text by Zangwill intended as a replacement for Jackson that focuses more on physics than on special functions), and answering email.

One session that was particularly fun was dedicated to artificial quantum matter. This topic is again worthy of a dedicated blog post that I will write sometime soon. The basic idea for the first few talks is a simple one: we know a number of different ways to impose spatially dependent potential energies on electrons constrained to move in 2d. (Note that while it is often convenient to act like the electrons in 2d electron gas or 2d surface states are free, as always this is shorthand for the true situation, where the single particle states are really Bloch-like states that exist due to the underlying periodic potential from the atoms). For example, if you impose a hexagonal lattice of potential wells on the free electrons, you get an effective band structure that looks like that of graphene. This has been done by etching on top of semiconductor structures, and by arranging molecules on the surface of Cu (as I'd mentioned here a year ago, work by Hari Manoharan). Making deeper potential wells gives you the chance to try to create an engineered system analogous to Mott insulators.

Another flavor of artificial quantum matter was discussed by Andrew Houck. If you make a little microwave resonator (a piece of stripline, superconducting to minimize loss), and then add in a superconducting quantum bit to act as a nonlinear element, you can have an effective photon-photon interaction in the cavity. Now consider wiring up a coupled network of such cavities, where the photons feel each other in each cavity and have some hopping from cavity to cavity. This raises the possibility of making "insulating" states of photons. As the speaker said, it's condensed matter without matter. Very thought provoking.


Wednesday, March 20, 2013

APS March Meeting, day 2

Yesterday I spent much of the meeting talking with collaborators and old friends, and seeing some invited talks at the sessions associated with some of the APS prizes. There were several really excellent talks.

The first that really stood out was Daniel Fisher's talk on the occasion of his winning the Onsager Prize. Fisher is a statistical mechanician, and he gave a very clear all about randomness, using domain walls in random magnets as an illustrative case for his ideas. In ferromagnets, there is an energetic cost associated with having a domain wall between regions of differently oriented magnetization. That acts like a surface tension, with the system tending to try to minimize the length of such a boundary, all other things being equal. Now if you allow the magnetic coupling between neighboring spins to have a random variation, the domain walls take on funny shapes, "finding" the lowest exchange locations because that also lowers the energy cost. Fisher talked about the statistical physics of this system, including the characteristic slow, history dependent kinetics of equilibration. The tails of the distribution of exchange values are really important here. Fisher then finished up talking about evolution as a statistical mechanics problem, where instead of minimizing an energy, the system tries to maximize "fitness", which is essentially the difference between birth and death rates.

The other talks that were exceptional were those in the Buckley Prize session. The prize this year was awarded to John Slonczewski, who predicted, quantitatively, the existence of the effects of spin transfer torque, which I've indirectly discussed before. Since spin really is angular momentum, flowing a spin polarized current into a magnetized material exerts a torque on the magnetization, if the flowing spins are not aligned with M. This is a way of using currents to cause magnetic domains to precess (ferromagnetic resonance) or flip altogether. Luc Berger gave a very good talk outlining the history of this field in a very pedagogical way, harkening all the way back to work done eighty years ago. Dan Ralph in the same session spoke about their incredibly beautiful results demonstrating all of these effects with quantitative agreement with theory. Further, Ralph showed how one can pump spin currents like this and drive such systems using the spin Hall effect rather than just direct current flow. That's worthy of a blog post all of its own, which I will do sometime soon.


Tuesday, March 19, 2013

APS March Meeting, Day 1

It's that time of year again, when I get together with thousands of my closest condensed matter physics friends to hear and give talks, swap gossip, and swill overpriced coffee. This year the action is in Baltimore, where one of my main observations after the first day is that someone needs to label the correct room lighting setting so that the project images aren't really washed out.

Real life has intruded in a couple of ways on the meeting this year for me, so my posting will likely be more brief than in past years - sorry.

Yesterday I spent most of my time in the sessions on nickelates and vanadates. The nickelates are a very interesting system, of the form RNiO3, where R is a rare earth atom. These form a family of strongly correlated oxides, where electron-electron interactions can be extremely important in determining the properties. The key is the partially filled d band from the Ni atoms, each of which is octahedrally coordinated by oxygens. Depending on the rare earth ion, the Ni-O bond angles change, and there can be two inequivalent Ni sites. Simple band structure without interactions says these should be metals, and LaNiO3 is a (correlated) metal. However, other members of the family are more complex, such as NdNiO3, which has a metal insulator transition in the bulk at around 200K, between a high T paramagnetic metal and a low T antiferromagnetic insulator - some flavor of the Mott transition. I heard a very interesting talk by Greg Fiete from UT about the possibility that one can use interactions in these materials to create new topological insulators, ones where the energy gap that makes them insulating is an interaction-based gap (as opposed to ordinary TI materials, where they are boring band insulators).

The vanadate sessions were also pretty compelling. VO2 also has a metal-insulator transition, this one at 340K, where interactions and lattice distortions both play very important roles. There were many good talks, including one about some extremely pretty work to figure out the triple point of the phase transitions between rutile and two monoclinic phases. There were also multiple talks about manipulating the transition via chemical doping and field effect approaches. Fun stuff.

Other observations so far: lots of sessions on topological materials, lots of sessions on experimental approaches to quantum bits, and lots of worried discussion of the sequester.


Tuesday, March 05, 2013

Interesting links

Time to play catch-up.  Here are some interesting links I've seen recently.

How to get a faculty offer - a lecture by John Guttag of MIT to their comp sci graduate students.  Not everything translates to the physics/chem/materials/nano communities, but much of this is great advice.  Thanks to Jen Rexford for bringing this to my attention.

A related post from the FSP about the faculty search process. 

An editorial/blog post at Scientific American about the importance of basic research and the painful choices being faced in the US right now.  It contains some choice quotes from Marc Kastner, a great physicist and current dean at MIT.

A very weird article from the Guardian, essentially taking some secular popularizers of science to task for trying to inspire a sense of wonder.  I had no idea that inspiring a sense of wonder was entirely the purview of the clergy.

A Swiftian editorial in the Journal of Cell Science, decrying blogging efforts to point out suspicious (at least to some) figures in scientific papers.  While it's sensible to have some concerns about how blogs are used in this way, I think this editorial is way off the mark.