Saturday, December 16, 2017

Finding a quantum phase transition, part 2

See here for part 1.   Recall, we had been studying electrical conduction in V5S8, a funky material that is metallic, but on one type of vanadium site has local magnetic moments that order in a form of antiferromagnetism (AFM) below around 32 K.  We had found a surprising hysteresis in the electrical resistance as a function of applied magnetic field.  That is, at a given temperature, over some magnetic field range, the resistance takes different values depending on whether the magnitude of H is being swept up or back down. 

One possibility that springs to mind when seeing hysteresis in a magnetic material is domains - the idea that the magnetic order in the material has broken up into regions, and that the hysteresis is due to the domains rearranging themselves.  What speaks against that in this case is the fact that the hysteresis happens over the same field range when the field is in the plane of the layered material as when the field is perpendicular to the layers.   That'd be very weird for domain motion, but makes much more sense if the hysteresis is actually a signature of a first-order metamagnetic transition, a field-driven change from one kind of magnetic order to another.   First order phase transitions are the ones that have hysteresis, like when water can be supercooled below zero Celsius.

That's also consistent with the fact that the field scale for the hysteresis starts at low fields just below the onset of antiferromagnetism, and very rapidly goes to higher fields as the temperature falls and the antiferromagnetic state is increasingly stable.   Just at the ordering transition, when the AFM state is just barely favored over the paramagnetic state, it doesn't necessarily take much of a push to destabilize AFM order.... 

There was one more clue lingering in the literature.  In 2000, a paper reported a mysterious hysteresis in the magnetization as a function of H down at 4.2 K and way out near 17-18 T.  Could this be connected to our hysteresis?  Well, in the figure here at each temperature we plot a dot for the field that is at the middle of our hysteresis, and a horizontal bar to show the width of the hysteresis, including data for multiple samples.  The red data point is from the magnetization data of that 2000 paper.  

A couple of things are interesting here.   Notice that the magnetic field apparently required to kill the AFM state extrapolates to a finite value, around 18 T, as T goes to zero.  That means that this system has a quantum phase transition (as promised in the post title).  Moreover, in our experiments we found that the hysteresis seemed to get suppressed as the crystal thickness was reduced toward the few-layer limit.  That may suggest that the transition trends toward second order in thin crystals, though that would require further study.  That would be interesting, if true, since second order quantum phase transitions are the ones that can show quantum criticality.  It would be fun to do more work on this system, looking out there at high fields and thin samples for signatures of quantum fluctuations....

The bottom line:  There is almost certainly a lot of interesting physics to be done with magnetic materials approaching the 2d limit, and there are likely other phases and transitions lurking out there waiting to be found.

Saturday, December 09, 2017

Finding a quantum phase transition, part 1

I am going to try to get the post frequency back up now that some tasks are getting off the to-do list....

Last year, we found what seems to be a previously undiscovered quantum phase transition, and I think it's kind of a fun example of how this kind of science gets done, with a few take-away lessons for students.  The paper itself is here.

My colleague Jun Lou and I had been interested in low-dimensional materials with interesting magnetic properties for a while (back before it was cool, as the hipsters say).  The 2d materials craze continues, and a number of these are expected to have magnetic ordering of various kinds.  For example, even down to atomically thin single layers, Cr2Ge2Te6 is a ferromagnetic insulator (see here), as is CrI3 (see here).  The 2d material VS2 had been predicted to be a ferromagnet in the single-layer limit.  

In the pursuit of VS2, Prof. Lou's student Jiangtan Yuan found that the vanadium-sulphur phase diagram is rather finicky, and we ended up with a variety of crystals of V5S8 with thicknesses down to about 10 nm (a few unit cells).  

[Lesson 1:  Just because they're not the samples you want doesn't mean that they're uninteresting.]   

It turns out that V5S8  had been investigated in bulk form (that is, mm-cm sized crystals) rather heavily by several Japanese groups starting in the mid-1970s.  They discovered and figured out quite a bit.  Using typical x-ray methods they found the material's structure:  It's better to think of V5S8  as V0.25VS2.  There are VS2 layers with an ordered arrangement of vanadium atoms intercalated in the interlayer space.  By measuring electrical conduction, they found that the system as a whole is metallic.   Using neutron scattering, they showed that there are unpaired 3d electrons that are localized to those intercalated vanadium atoms, and that those local magnetic moments order antiferromagnetically below a Neel temperature of 32 K in the bulk.  The moments like to align (antialign) along a direction close to perpendicular to the VS2 layers, as shown in the top panel of the figure.   (Antiferromagnetism can be tough to detect, as it does not produce the big stray magnetic fields that we all associate with ferromagnetism. )

If a large magnetic field is applied perpendicular to the layers, the spins that are anti-aligned become very energetically unfavored.  It becomes energetically favorable for the spins to find some way to avoid antialignment but still keep the antiferromagnetism.  The result is a spin-flop transition, when the moments keep their antiferromagnetism but flop down toward the plane, as in the lower panel of the figure.  What's particularly nice in this system is that this ends up producing a kink in the electrical resistance vs. magnetic field that is a clear, unambiguous signature of the spin flop, and therefore a way of spotting antiferromagnetism electrically

My student Will Hardy figured out how to make reliable electrical contact to the little, thin V5S8 crystals (not a trivial task), and we found the physics described above.  However, we also stumbled on a mystery that I'll leave you as a cliff-hanger until the next post:  Just below the Neel temperature, we didn't just find the spin-flop kink.  Instead, we found hysteresis in the magnetoresistance, over an extremely narrow temperature range, as shown here.

[Lesson 2:  New kinds of samples can make "old" materials young again.]

[Lesson 3:  Don't explore too coarsely.  We could easily have missed that entire ~ 2.5 K temperature window when you can see the hysteresis with our magnetic field range.] 

Tune in next time for the rest of the story....

Tuesday, November 28, 2017

Very busy time....

Sorry for the light blogging - between departmental duties and deadline-motivated writing, it's been very difficult to squeeze in much blogging.  Hopefully things will lighten up again in the next week or two.   In the meantime, I suggest watching old episodes of the excellent show Scrapheap Challenge (episode 1 here).  Please feel free to put in suggestions of future blogging topics in the comments below.  I'm thinking hard about doing a series on phases and phase transitions.

Friday, November 17, 2017

Max the Demon and the Entropy of Doom

My readers know I've complained/bemoaned repeatedly how challenging it can be to explain condensed matter physics on a popular level in an engaging way, even though that's the branch of physics that arguably has the greatest impact on our everyday lives.  Trying to take such concepts and reach an audience of children is an even greater, more ambitious task, and teenagers might be the toughest crowd of all.  A graphic novel or comic format is one visually appealing approach that is a lot less dry and perhaps more nonthreatening than straight prose.   Look at the success of xkcd and Randall Munroe!   The APS has had some reasonable success with their comics about their superhero Spectra.  Prior to that, Larry Gonick had done a very nice job on the survey side with the Cartoon Guide to Physics.  (On the parody side, I highly recommend Science Made Stupid (pdf) by Tom Weller, a key text from my teen years.  I especially liked Weller's description of the scientific method, and his fictional periodic table.)

Max the Demon and the Entropy of Doom is a new entry in the field, by Assa Auerbach and Richard Codor.  Prof. Auerbach is a well-known condensed matter theorist who usually writes more weighty tomes, and Mr. Codor is a professional cartoonist and illustrator.  The book is an entertaining explanation of the laws of thermodynamics, with a particular emphasis on the Second Law, using a humanoid alien, Max (the Demon), as an effective superhero.  

The comic does a good job, with nicely illustrated examples, of getting the point across about entropy as counting how many (microscopic) ways there are to do things.  One of Max's powers is the ability to see and track microstates (like the detailed arrangement and trajectory of every air molecule in this room), when mere mortals can only see macrostates (like the average density and temperature).    It also illustrates what we mean by temperature and heat with nice examples (and a not very subtle at all environmental message).   There's history (through the plot device of time travel), action, adventure, and a Bad Guy who is appropriately not nice (and has a connection to history that I was irrationally pleased about guessing before it was revealed).   My kids thought it was good, though my sense is that some aspects were too conceptually detailed for 12 years old and others were a bit too cute for world-weary 15.  Still, a definite good review from a tough crowd, and efforts like this should be applauded - overall I was very impressed.

Tuesday, November 07, 2017

Taxes and grad student tuition

As has happened periodically over the last couple of decades (I remember a scare about this when Newt Gingrich's folks ran Congress in the mid-1990s), a tax bill has been put forward in the US House that would treat graduate student tuition waivers like taxable income (roughly speaking).   This is discussed a little bit here, and here.

Here's an example of why this is an ill-informed idea.  Suppose a first-year STEM grad student comes to a US university, and they are supported by, say, departmental fellowship funds or a TA position during that first year.  Their stipend is something like $30K.  These days the university waives their graduate tuition - that is, they do not expect the student to pony up tuition funds.  At Rice, that tuition is around $45K.  Under the proposed legislation, the student would end up getting taxed as if their income was $75K, when their actual gross pay is $30K.   

That would be extremely bad for both graduate students and research universities.  Right off the bat this would create unintended (I presume) economic incentives, for grad students to drop out of their programs, and/or for universities to play funny games with what they say is graduate tuition.   

This has been pitched multiple times before, and my hypothesis is that it's put forward by congressional staffers who do not understand graduate school (and/or think that this is the same kind of tuition waiver as when a faculty member's child gets a vastly reduced tuition for attending the parent's employing university).  Because it is glaringly dumb, it has been fixed whenever it's come up before.  In the present environment, the prudent thing to do would be to exercise caution and let legislators know that this is a problem that needs to be fixed.

Tuesday, October 31, 2017

Links + coming soon

Real life is a bit busy right now, but I wanted to point out a couple of links and talk about what's coming up.
  • I've been looking for ways to think about and discuss topological materials that might be more broadly accessible to non-experts, and I found this paper and videos like this one and this one.  Very cool, and I'm sorry I'd missed it back in '15 when it came out.
  • In the experimental literature talking about realizations of Majorana fermions in the solid state, a key signature is a peak in the conductance at zero voltage - that's an indicator that there is a "zero-energy mode" in the system.  There are other ways to get zero-bias peaks, though, and nailing down whether this has the expected properties (magnitude, response to magnetic fields) has been a lingering issue.  This seems to nail down the situation more firmly.
  • Discussions about "quantum supremacy" strictly in terms of how many qubits can be simulated on a classical computer right now seem a bit silly to me.  Ok, so IBM managed to simulate a handful of additional qubits (56 rather than 49).  It wouldn't shock me if they could get up to 58 - supercomputers are powerful and programmers can be very clever.  Are we going to get a flurry of news stories every time about how this somehow moves the goalposts for quantum computers?    
  • I'm hoping to put out a review of Max the Demon and the Entropy of Doom, since I received my beautifully printed copies this past weekend.

Wednesday, October 25, 2017

Thoughts after a NSF panel

I just returned from a NSF proposal review panel.  I had written about NSF panels back in the early days of this blog here, back when I may have been snarkier.

  • Some things have gotten better.  We can work from our own laptops, and I think we're finally to the point where everyone at these things is computer literate and can use the online review system.  The program officers do a good job making sure that the reviews get in on time (ahead of the meeting).
  • Some things remain the same.  I'm still mystified at how few people from top-ranked programs (e.g., Harvard, Stanford, MIT, Cornell, Cal Tech, Berkeley) I see at these.  Maybe I just don't move in the right circles.  
  • Best quote of the panel:  "When a review of one of my papers or proposals starts with 'Author says' rather than 'The author says', I know that the referee is Russian and I'm in trouble."
  • Why does the new NSF headquarters have tighter security screenings that Reagan National Airport?  
  • The growth of funding costs and eight years of numerically flat budgets has made this process more painful.  Sure looks like morale is not great at the agency.  Really not clear where this is all going to go over the next few years.  There was a lot of gallows humor about having "tax payer advocates" on panels.  (Everyone on the panel is a US taxpayer already, though apparently that doesn't count for anything because we are scientists.)
  • NSF is still the most community-driven of the research agencies. 
  • I cannot overstate the importance of younger scientists going to one of these and seeing how the system works, so you learn how proposals are evaluated.