SYKness makes a metal incoherent


Sometimes ideas come from surprising places.  Recently Alexei Kitaev made a splash by pointing out that a problem studied decades ago by Subir Sachdev and Jinwu Ye, who were studying quantum spin glasses, has bearing on the problem of quantum gravity.  What is now called the Sachdev-Ye-Kitaev (SYK) model became the subject of intense study as a toy model in which gravity and black hole physics emerges from totally different microphysics: quantum mechanics of many fermions.  It has a very simple Hamiltonian:

H_{SYK} = \sum_{ijkl} U_{ijkl} c_i^\dagger c_j^\dagger c_k^{\vphantom\dagger} c_l^{\vphantom\dagger} ,

where the coefficients U_{ijkl} are totally random.  Here I’ve written what is called a “complex” SYK model, slightly different from what Kitaev studied, which was written for Majorana fermions.  The difference is not crucial for the gravity connection, but it is helpful for us.  The key feature is that according to this Hamiltonian electrons move collectively, transitioning from two “orbitals” (kl) to another two (ij) in pairs, never alone.  You could visualize it like this:


This is a “toy model” because what emerges (in the limit of a large number of orbitals N) is not the gravity of our world, but a slightly different, and lower-dimensional version.  But still it is remarkable and beautiful.

The SYK model also describes the physics of strongly interacting electrons, forming an unconventional metallic state.  This metal is what condensed matter physicists refer to as a “non-Fermi liquid”, which behaves differently from typical metals like copper, in which electrons act almost independently.  In the SYK metal, you cannot separate motions of single electrons, but instead only see a collective dance of the whole electron fluid. The problem of understanding non-Fermi liquids is a forefront one in the field of quantum materials, but there are very few solvable theoretical models for them.  As such, the SYK model is quite valuable.

The drawback of the SYK model is that it is “zero dimensional” — every electron in it interacts with every other one, and so there is no built-in locality to the problem.  Because of this we cannot imagine driving a flow from one side of the system to another, and hence cannot discuss conductivity.  We should view the SYK model as describing a strongly-interacting “quantum dot”: a little chunk of non-Fermi liquid metal.

A natural thing to do is to try to string together these quantum dots to build up a higher dimensional system.  Already a couple of very nice papers along these lines have appeared: this paper, and this one which is a generalization.  In both these works, coupling between the SYK dots is chosen to preserve the scaling of the SYK model: electrons must hop in pairs between the dots.  Consequently the whole system remains a non-Fermi liquid and no new energy scale is introduced by the coupling.

We just posted a paper in which we make a different choice — just ordinary one-electron hopping between the dots:

H = \sum_x H_{SYK}(x) - \sum_{\langle x,x'\rangle} t_{ij}(x,x') c_i^\dagger(x) c_j^{\vphantom\dagger}(x') + {\rm h.c.} .

In this model, there is a competition between the SYK interaction U and the hopping t.  For t<<U, we have a strongly correlated system.  The header image is suppose to show this: electrons zipping between the correlated fluid dots.  Fortunately, it remains soluble in the large N limit and one can obtain a rich structure.  Briefly, at low temperatures and low energy, the system is a Fermi liquid.  We can extract the parameters of the Fermi liquid theory: effective mass, Fermi liquid interactions, quasiparticle residue, etc.  However, at higher temperatures it is an incoherent metal, and displays properties that are a mixture of those of the pure SYK model and a regular metal.  Together, it is striking how many of the properties are similar to those of experimental correlated metals:

  • Small coherence scale Ec = t2/U
  • Large effective mass (Sommerfeld coefficient) γ ~ m*/m ~ U/t
  • Small quasiparticle weight Z ~ t/U
  • T2 low-temperature resistivity with Kadowaki-Woods ratio A/γ2 = constant
  • linear in temperature resistivity at high temperature
  • linear in temperature thermal “resistivity” T/κ at high temperature
  • Fermi liquid Lorenz number L = κ/(T σ) = π2/3 at low T
  • non-Fermi liquid Lorenz number L = π2/8 at high T

Anyway, I’m pretty pleased with how it all works out.  Maybe the most remarkable part of this story is that the person who really spearheaded all the calculations, Xue-Yang Song, is still an undergraduate.  Pretty impressive.  You can read the preprint here.

March for Science

marchI went down to Los Angeles yesterday for the March for Science.  Although there was also a march in Santa Barbara, I wanted to see a larger scale event.  In LA there were 50,000 people registered, and I could believe there were that many there!  It was packed, and the atmosphere was great.  Amazing to see so many people care about science!  It was a lot of fun to see the creative signs people had put together.

My impression was that actual scientists were a pretty small fraction of the participants. Not being a scientist has some advantages.  As a non-scientist, you can feel comfortable with the sign on the left, which is a good sign, but I can’t help but be bothered by the fact that there are in reality no elements “T”, “M”, or “Es”.  The sign on the far right is definitely from a scientist: it takes a physicist to recognize a Hamiltonian!  Wish I had thought of that myself.

The beat of the drum

The hottest off-shoot of topological insulators these days is the study of nodal electronic structures.  By this I mean the situation in which two different bands touch along some locus in momentum space.  This started a few years ago with the re-discovery of Weyl points – I wrote a commentary some time ago with the poetic title, “Weyl electrons kiss”, using kissing as a metaphor for band touching.  The actual contact for Weyl fermions is linear in energy versus momentum, so a bit angular compared to lips.  The kissing image seems more apt for quadratic band touching, which also occurs and is interesting in its own right.

The contact locus between bands can be extended, and form for example a loop in momentum space.  This is often called a “nodal loop”.  An exotic phenomena that happens in such cases is that there can be a branch of surface bound states which exists only for surface momenta which project inside the nodal loop.  If one plots the energy of the surface state versus the two dimensional momentum of the surface, it forms a “drumhead” that stretches across the projection of the nodal loop into the 2d surface Brillouin zone.  I think this drumhead surface state was first pointed out by Volovik, though I do not recall the reference at the moment.

To a good approximation such a drumhead state is “flat”, i.e. its energy is nearly constant. A flat band has no kinetic energy, so one might think it should be susceptible to interactions.  With Jianpeng Liu, a postdoc at KITP, we studied this and indeed found various instabilities due to electron-electron forces.   Here is a phase diagram from our paper, for a simple limit of our simple model:


You can see in the inset the charge density wave pattern that forms in our simple model – it looks a bit like the drumhead surface state is “ringing”, though this is really oscillation of the charge density as a function of distance into the material.

This sort of tendency to form ordered states means that nodal loop semimetals should be an interesting place to study two-dimensional electronic surface phase transitions.    A neat feature of this situation is that the nodal loop means there are gapless states in the bulk, and that the surface bound states become bulk-like as they approach the momenta of the loop.  These attributes mean that surface quantum criticality in this situation is different from the criticality of a purely 2d system, and is instead inextricably tied to the 3d nature of the system.

You can read our paper if you want to know the details.

Strange days

I feel a certain sense of unreality lately.  Science in general, and my own scientific situation, seems in a strange state.  The administration just released its proposed budget, and it is awfully bad news for science.  The huge cuts to the EPA were expected, and get the most press, but the situation is uniformly grim for basic science.  A 20 percent cut to the NIH and 17 percent to the DOE office of science are also massive, and the latter, if it happens, is likely to be devastating to physics.  The National Science Foundation (NSF) is not even mentioned in the budget document, but probably it is considered one of a set of “other agencies” slated for a 10 percent cut.  Those percentages might not seem large, but they are bound to be distributed unevenly: these agencies have various commitments that have already been made, and need to be prioritized.  This level of cut could easily translate into many programs not being renewed, complete cancellation of any new grants, etc.. I wonder how many jobs will be lost at national labs and universities, and how many students and postdocs will lose their support.   I can only hope that congress does not follow these recommendations.

It is interesting to make some comparisons internationally, but hard to find very recent data.  According to wikipedia as of 2014, the US ranked 11th in the world in its research and development spending as a fraction of GDP.   Anyone in my field has had the experience of traveling to Germany, Switzerland, South Korea, Japan, etc., and being overwhelmed by the availability and level of research funding there.  American science is still strong on any scale, but for how long can it remain so under these terms?  It is hard to imagine a great country making these decisions.

There’s a personal side to it for me.  Our KITP grant is up for renewal this year from the NSF.  The KITP is without a doubt the most successful visiting center for theoretical physics in the world.  Among other things, the KITP’s postdoctoral program has trained some huge fraction of the leaders in theoretical physics, who have gone on to become faculty at all the top institutions.  The KITP’s scientific programs chart the future of physics.  It has become a model for other institutes worldwide, though the competition doesn’t come close to its level of productivity despite in many cases significantly more financial support in these imitators.  The KITP’s budget has been stagnant for many years, even before the change of administration.  I’ll be attending a “reverse site visit” in D.C. to present the case for renewal of the KITP’s grant to the NSF in May.

The KITP’s NSF grant does not support my own group’s research.  For that I rely on my own grants.  This year my DOE grant from the office of Basic Energy Sciences is up for renewal.  A 17% cut  does not bode well for that.  This DOE grant supported nearly all of our group’s work on quantum spin ice and pyrochlores, which has been probably the most influential part of my research in the past decade.  It supported my (former) graduate student Dr. Lucile Savary, who received a permanent CNRS research position in France, and has been winning a lot of prizes lately.

In mid-April, I’ll be traveling to New York City to present a proposal to the Simons Foundation, a private philanthropic organization that funds mathematics and science.  My team is competing for what they call a “Simons Collaboration”: the goal being to bring a group of researchers together to attack some major scientific challenge that needs the combined efforts of a team, and which cannot be supported by governmental means.  With the proposed budget changes, the latter criteria should not be too hard to satisfy.  It seems that private support for science may become especially crucial given the governmental trends.  Already nearly all of our postdocs in condensed matter at the KITP are funded by the Betty and Gordon Moore Foundation, another fantastic philanthropic foundation.

In order not to end on a low note, let me say that these thoughts about funding are the backdrop for my main concern, which is the science itself.  That is flourishing!  It’s an exciting time to do physics.  This has been a great year for the recognition of the topological revolution.  I’m amazed by the talent of young minds in the field: I’ve been working with an undergraduate on non-Fermi liquids, hydrodynamics, and disorder – research that crosses the boundaries between condensed matter physics and quantum gravity, which I myself would not have been close to understanding at that age!  At the Boulder summer school for condensed matter and materials physics last July, the students were simply incredible.  Our KITP postdocs are doing breakthrough work on their own at the cutting edge of topological matter. Experiments on quantum materials are rich and varied (though increasingly the ones I care about are done outside of the US), and it seems likely we will see increasing technological impacts of these basic research areas soon.  I don’t think my own research has ever been as diverse as it is now.  So it is a great time to do physics, even though I worry about being able to afford it.






Recently I’ve spent some time thinking about domains in some magnetic materials, trying to understand how they influence and interact with topological electronic structure and Hall transport.  In the process, I played around a bit with simple models of the domain formation and dynamics.  This is a kind of classic problem in statistical mechanics.  You have a system which in equilibrium spontaneously orders below some temperature, for example a ferromagnet.  There are several possible directions to the magnetization, all with the same energy.  If you suddenly lower the temperature from above the ordering temperature, where there is no magnetization, to below it, magnetization starts to form.  This is called a “quench”.  But at different points in space, the orientation is different, and random.  Then over time after the quench, nearby regions begin to align with one another, and larger regions, called domains, develop, within which the magnetization is uniform.  These grow with time, and this process is called “coarsening”.

The simplest example is an Ising magnet, in which there are only two types of domains.  But there are more complex situations.  Here I was considering a case in which the magnetization orients within a plane, and has 6 preferred directions, say multiples of 60 degrees.  This is modeled by a simple Langevin dynamics

\gamma \partial_t \theta_i = -\frac{\partial H}{\partial \theta_i} + \eta_i(t)

where \gamma>0 is a damping constant, \eta_i(t) is a stochastic Gaussian delta-function correlated “white” noise, and H is a Hamilonian

H = -J \sum_{\langle ij\rangle} \cos(\theta_i-\theta_j) - \lambda \sum_i \cos 6\theta_i,

where J gives an energy for neighboring spins to align, and \lambda >0 is a six-fold anisotropy that favors the 6 domains.  This is what would be called “model A” dynamics of an XY model with 6-fold anisotropy.

It is surprisingly easy to simulate this thing – I did it with Julia’s DifferentialEquations package. The animation shows the evolution with time starting from a completely random initial condition, on a 90×90 square lattice.  It is beautiful how the domains develop.  You also see some “vortices”, which are points around which the angle winds by \pm 2\pi, which due to the anisotropy become the intersection point of 6 domain walls.  Over time the domains grow – you can see the “walls” between domains moving – and the vortices slowly meander and eventually will find a way to annihilate one another.  The size of the domains, the distance between vortices, etc., all show power-law dependence on time in this type of problem.  Can you spot the different kinds of vortices (clockwise versus counter-clockwise winding), and see some of the annihilation events?

In a real material eventually the domain motions will be affected by disorder, which slows and finally stops them in some frozen or nearly stationary state. Lots of other effects can also come in, like extra forces at the boundaries of the sample, applied fields, strains, etc., that can make the eventual structure quite complex and hard to predict.  But still it is cool.

A new quantum spin liquid?

I’ve been feeling I’ve not been posting enough.  I’ll try to get back into it…


Inelastic neutron scattering intensity of YbMgGaO4, from two recent papers:  arXiv:1607.02615 (left) and arXiv:1607.03231 (right).

I’ve been studying quantum spin liquids for many years now.  Or maybe I should say looking for them?  Quantum spin liquids have both theoretical and experimental definitions.  In theory, they are exotic phases of quantum matter whose ground states are intrinsically and inextricably in quantum superposition.  From this quantum entanglement of the ground state they obtain the ability to support exotic excitations that behave like particles different from the elementary ones in our world: emergent non-local “quasiparticles”.  In experiment, they are materials containing spins that avoid order at the lowest temperature, for which the conventional magnetic excitations – magnons – are absent, and which show some anomalous properties, e.g. unusual thermodynamics.  Somehow it has been hard to bring these two meanings together.  Either this is a failing of theory, a lack of the right measurements, or maybe we just haven’t found the right material yet.

The last option would make things easy – maybe we will just find a new system where the agreement is obvious.  So I get excited when a new material with some promise appears.  About a year ago, I heard from Gang Chen, my former student, now a professor at Fudan university in China, of YbMgGaO4.  This is a material with a triangular lattice of Yb ions, each of which hosts an effective S=1/2 quantum spin.  Typical of 4f rare earth ion spins, it has very short-range but highly anisotropic interactions with its neighbors.  Guided by Gang, the experimentalists identified it as a spin liquid: at least the spins remain correlated but disordered down to millikelvin temperatures.  How and why, remains to be understood.

One intriguing thing is some similarity to the thermodynamic properties observed in the more famous organic spin liquid materials, which have been studied for many years.  And both share the triangular lattice structure.  The reason the new YbMgGaO4 material is of particular interest, though, is a practical one.  The Yb spins have large magnetic moments, and they are dense, so it is a good material for inelastic neutron scattering studies.   This is the most powerful (i.e. data-rich) technique to study magnetism we know today, so this is a big deal.  Moreover, single crystals big enough for such measurements are now available.  The above images are from papers this year by two different groups, arXiv:1607.02615 (left), and arXiv:1607.03231 (right), which carried out the first such measurements, and show a smooth continuum of excitations, i.e. an apparent absence of conventional magnons.  Nice pictures.

For now I won’t speculate (at least in writing!) about what’s going on, but it is certainly interesting and I’m sure it will be an active research subject for the near future.  Let me also mention that in a material like this, the chemistry is largely independent of the rare earth element, so Yb can be substituted for almost any other 4f atom.  This means a wide variety of magnetic systems can be found with identical geometry.  So YbMgGaO4 is just the first of many new materials we can probably expect to see studied soon.  Something to look forward to!



Congratulations to David Thouless, Duncan Haldane, and Mike Kosterlitz for the 2016 Nobel prize in physics!  They pioneered applications of topology to condensed matter physics.  It took a while, but now the use of topology in condensed matter is commonplace.

These guys did foundational work, and the field has developed a lot since their seminal contributions.  Given this prize, it will be interesting to see how the work of Charlie Kane and others on topological insulators fares with the Nobel committee in the future!

Passion for Physics

I hold a weekly group meeting, Wednesday afternoons, with an extended group including some students, visiting KITP grad fellows, postdocs, and sometimes other visitors.  It runs an hour, and generally unstructured.  Yesterday our new postdoc, Xiao Chen, who just arrived from UIUC, talked about his work on “out of time ordered” correlations, which is a new-ish way to study dynamics of quantum systems, the transition to chaos and thermalization, and other related phenomena.  There are connections of this subject to condensed matter physics, ultra-cold atoms, quantum information science, and even black hole information (which is where this originated!).

It’s a pretty stimulating subject, and after an hour when the meeting ended, four of us stayed there and kept talking for about another hour – I didn’t keep track – chatting about all these different connected ideas, and what we did and didn’t understand.  None of us are really expert in this subject, and only Xiao has worked on it very seriously.  If you know me, you know my interests tend to lean towards “real world” topics – i.e. specific to some material or experiment.  This is pretty far from that!  But how can you not be fascinated by these concepts that are so far ranging that they touch both the foundations of statistical mechanics and the origins of gravity?  I don’t know how, but there were still only four of us that stayed.

In most professions, free-ranging discussions don’t seem like a part of “work”.  With all the many responsibilities of an academic position, it can be hard to take time for them even for us.   Yet I firmly believe that such dialogues are critical to learning, developing new ideas, and doing good research.    More than anything else, if you have a passion for physics, you should love this stuff!   It made my day yesterday – thanks people!


Deceptive advertising

I just saw the summary for a feature story on “Physics”, the online journal highlighting what the APS considers exciting developments

Researchers provide new evidence for the existence of type-II Weyl semimetals, which would be both conducting and insulating in different spatial directions. ” (my emphasis)

Weyl semimetals are definitely a hot subject lately.  These are electronic materials in which non-degenerate (i.e. spin-split) conduction and valence bands touch at a single point (called a Weyl point) in three dimensions.  Anton Burkov and I actually were one of the first groups to predict these things some years ago (see this paper), hot on the heels of Ashvin Vishwanath’s group (here).  Well full disclosure requires me to point out that Herring discovered them in 1937!  Anyway, like the related topological insulators, these things are not actually all that rare, and are pretty easy to find using very standard DFT methods, so the subject is booming.

One of the latest twists is the discovery of “type II” Weyl semimetals (if you buy into this, you now call the original ones type I).  I’m of II minds myself about this.  On the one hand, the type II Weyl point is no different from a type I Weyl point, the only distinction is that in the type II case,  the dispersion away from the point goes only in one direction of energy, in some directions in k space.  On the other hand, because of this type of slope, such a type II Weyl point cannot be isolated: there are always states at the same energy elsewhere in k space.  And those other states form a Fermi surface, which has much more density of states than the vicinity of the point itself.  So I would have thought type II Weyl points are mainly special in that they by construction are less able to influence electronic properties than the type I Weyl point (which can be isolated in energy and in that case would control all the physics).

But I’ve been wrong before about what becomes big.  Type II Weyl points…sure!

Nevertheless, I found the summary of this Physics piece crazy.  Type II Weyl semimetals would be (what does “would be” mean here, anyway?  In some hypothetical world?) both conducting and insulating in different directions?  Certainly not!  As cool as Weyl fermions are, whatever type they are, they are extremely well-described by non-interacting quasiparticle theory.  According to this cornerstone of the theory of solids, electrons travel in directions determined by their group velocity, and because they certainly disperse in three dimensions, they also move in all directions.  So the statement, at face value, is pretty much absurd.

When I read the actual feature, a few paragraphs down there is some discussion where a magnetic field is mentioned in this context, which might allow for some speck of truth in the summary.  Actually it has been known for decades that any low density electron system becomes, in high magnetic field, much more conducting along the field than normal to it.  This is a called the ultra-quantum limit. If conditions are right, ultra-quantum electrons can be conducting along the field and insulating normal to it.  This does not depend much upon the nature of the band dispersion.

So, well, I don’t know what the writer had in mind, and maybe there is some way it could make sense here. Yet…I doubt it.

I should say that there certainly are materials that are conducting in some directions and insulating in others.  Usually this is related to structure: they are built from linear chains or planes, that are weakly coupled.  The disparity between conducting in different directions can be magnified by magnetic fields, by strong interactions, by disorder, or all of the above.  I don’t think this is particularly rare.