Cond-mat journal club on AdS/CFT

Here's a cond-mat journal club thing on AdS/CFT and applications to cond-mat, e.g. explaining the strange metal phase. The commentary is written by cond-mat theorists incl. Abrahams who's most famous for being one of the Gang of Four.

Giant SPIDERs etc.

Some good recent stuff in PRL:

Giant Surface-Plasmon-Induced Drag Effect in Metal Nanowires
Maxim Durach,1 Anastasia Rusina,1 and Mark I. Stockman1,2,3
Phys. Rev. Lett. 103, 186801 (2009)

Here, for the first time we predict a giant surface-plasmon-induced drag-effect rectification (SPIDER), which exists under conditions of the extreme nanoplasmonic confinement. In nanowires, this giant SPIDER generates rectified THz potential differences up to 10 V and extremely strong electric fields up to ~105–106 V/cm. The giant SPIDER is an ultrafast effect whose bandwidth for nanometric wires is ~20 THz. It opens up a new field of ultraintense THz nanooptics with wide potential applications in nanotechnology and nanoscience, including microelectronics, nanoplasmonics, and biomedicine.

[sg] I haven't read this paper and I'm not sure I want to, I'm just awed by the brilliance of "giant SPIDER effect." This is obviously going to collect oodles of citations, mostly from me.

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Holstein Polarons Near Surfaces
Glen L. Goodvin, Lucian Covaci, and Mona Berciu
Phys. Rev. Lett. 103, 176402 (2009)

We study the effects of a nearby surface on the spectral weight of a Holstein polaron, using the inhomogeneous momentum average approximation which is accurate over the entire range of electron-phonon (e-ph) coupling strengths. The broken translational symmetry is taken into account exactly. We find that the e-ph coupling gives rise to a large additional surface potential, with strong retardation effects, which may bind surface states even when they are not normally expected. The surface, therefore, has a significant effect and bulk properties are recovered only very far away from it. These results demonstrate that interpretation in terms of bulk quantities of spectroscopic data sensitive only to a few surface layers is not always appropriate.

[sg] The recent work on surface polarons is pretty interesting and it's something I'd like to get into at some point. It's primarily motivated by the recent glut of exptal work on heterostructures of strongly correlated materials; the interfaces often undergo substantial electronic and/or lattice reconstruction. Naturally these not-terribly-stable surfaces are morbidly sensitive to, e.g., the electrostatic force exerted by a moving electron.
I'm also curious about whether the authors are right about the interpretation of surface-based spectroscopic data.

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Phys. Rev. Lett. 103, 177002 (2009)
Evidence for Two Time Scales in Long SNS Junctions
F. Chiodi, M. Aprili, and B. Reulet

We use microwave excitation to elucidate the dynamics of long superconductor–normal metal–superconductor Josephson junctions. By varying the excitation frequency in the range 10 MHz–40 GHz, we observe that the critical and retrapping currents, deduced from the dc voltage versus dc current characteristics of the junction, are set by two different time scales. The critical current increases when the ac frequency is larger than the inverse diffusion time in the normal metal, whereas the retrapping current is strongly modified when the excitation frequency is above the electron-phonon rate in the normal metal. Therefore the critical and retrapping currents are associated with elastic and inelastic scattering, respectively.

[sg] In the traditional Josephson junction model (i.e. tilted washboard + friction), switching happens when the washboard's wells cease to be metastable and retrapping happens when the terminal velocity of a particle going down the washboard goes to zero. Retrapping depends on friction and switching doesn't, so you have a clean separation of energy scales. In long junctions things are generally messier because switching depends on heat transfer from a locally normal area to the rest of the wire, etc.; however, this paper manages to retrieve a rather nice separation of scales.

Title: Off-diagonal correlations in a one-dimensional gas of dipolar bosons

Authors: Tommaso Roscilde, Massimo Boninsegni

We present a quantum Monte Carlo study of the one-body density matrix (OBDM) and the momentum distribution of one-dimensional dipolar bosons, with dipole moments polarized perpendicular to the direction of confinement. We observe that the long-range nature of the dipole interaction has dramatic effects on the off-diagonal correlations: although the dipoles never crystallize, the system goes from a quasi-condensate regime at low interactions to a regime in which quasi-condensation is discarded, in favor of quasi-solidity. For all strengths of the dipolar interaction, the OBDM shows an oscillatory behavior coexisting with an overall algebraic decay; and the momentum distribution shows sharp kinks at the wavevectors of the oscillations, $Q = \pm 2\pi n$ (where $n$ is the atom density), beyond which it is strongly suppressed. This \emph{momentum filtering} effect introduces a characteristic scale in the momentum distribution, which can be arbitrarily squeezed by lowering the atom density. This shows that one-dimensional dipolar Bose gases, realized e.g. by trapped dipolar molecules, show strong signatures of the dipolar interaction in time-of-flight measurements.

http://arxiv.org/abs/0910.4165

Supersolid Helium and anamolous transport

Underlying Mechanism for the Giant Isochoric Compressibility of Solid 4he: Superclimb of Dislocations

authors:
S. G. Söyler,1 A. B. Kuklov,2 L. Pollet,3 N. V. Prokof'ev,1,4 and B. V. Svistunov1,4

In the experiment on superfluid transport in solid 4He [Phys. Rev. Lett. 100, 235301 (2008)], Ray and Hallock observed an anomalously large isochoric compressibility: the supersolid samples demonstrated a significant and apparently spatially uniform response of density and pressure to chemical potential, applied locally through Vycor “electrodes.” We propose that the effect is due to superclimb: edge dislocations can climb because of mass transport along superfluid cores. We corroborate the scenario by ab initio simulations of an edge dislocation in solid 4He at T=0.5 K. We argue that at low temperature the effect must be suppressed due to a crossover to the smooth dislocation.



http://link.aps.org/doi/10.1103/PhysRevLett.103.175301

QMC and Scaling

This letter addresses the case without a sign problem. I think eqn. 3 can be trivially applied to a DMC calculation. I should look at it further though.

Quantum Monte Carlo Simulations of Fidelity at Magnetic Quantum Phase Transitions

Authors:
David Schwandt,1,2 Fabien Alet,1,2 and Sylvain Capponi1,2
1Laboratoire de Physique Théorique, Université de Toulouse, UPS, (IRSAMC), F-31062 Toulouse, France
2CNRS, LPT (IRSAMC), F-31062 Toulouse, France

Abstract:
When a system undergoes a quantum phase transition, the ground-state wave function shows a change of nature, which can be monitored using the fidelity concept. We introduce two quantum Monte Carlo schemes that allow the computation of fidelity and its susceptibility for large interacting many-body systems. These methods are illustrated on a two-dimensional Heisenberg model, where fidelity estimators show marked behavior at two successive quantum phase transitions. We also develop a scaling theory which relates the divergence of the fidelity susceptibility to the critical exponent of the correlation length. A good agreement is found with the numerical results.

http://link.aps.org/doi/10.1103/PhysRevLett.103.170501

Gowers on Complexity Bounds

Timothy Gowers has apparently been thinking about the P = NP problem. I've enjoyed his series of posts on this so far, even if the details are often a little over my head. Here's a link to the first post; the others can be accessed by conventional means.

New preprint

I have a new preprint on the arxiv with Siddhartha Lal and Paul Goldbart. It's about N-channel Kondo physics with interacting 1D bosons (e.g. cold atoms) as the leads.

Quenches as probes of quantum phase transitions

This numerical paper has some fascinating results, and the germ of a very clever idea (I heard about this work via Andy Millis):

Dynamical phase transition in correlated fermionic lattice systems
Martin Eckstein, Marcus Kollar, Philipp Werner
http://arxiv.org/abs/0904.0976

Abstract: We use non-equilibrium dynamical mean-field theory to demonstrate the existence of a critical interaction in the real-time dynamics of the Hubbard model after an interaction quench. The critical point is characterized by fast thermalization and separates weak-coupling and strong-coupling regimes in which the relaxation is delayed due to prethermalization on intermediate timescales. This dynamical phase transition should be observable in experiments on trapped fermionic atoms.

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So what they do is quench the interaction strength and see how long it takes the system to thermalize. In general when you quench into a gapped state, thermalization can take a really long time because particles trapped on the wrong side of the gap don't have anything to decay into. (See here.) On the other hand, e.g. near a quantum phase transition, the gap is small and thermalization should be easy. So by looking at how long a system takes to approach equilibrium as you quench from a reference parameter value to some value U_f, you can essentially map out a quantum phase diagram and maybe get some information about the exponents. [I should note that this is all somewhat speculative.] What's particularly interesting about the Kollar/Eckstein case is that they didn't even know there was a phase transition there until they did their simulations.

There's some recent work on (possibly) similar ideas that I haven't read yet:

Quench dynamics as a probe of quantum criticality
Authors: R. A. Barankov
http://arxiv.org/abs/0910.0255

Abstract: Quantum critical points of many-body systems can be characterized by studying response of the ground-state wave function to the change of the external parameter, encoded in the ground-state fidelity susceptibility. This quantity characterizes the quench dynamics induced by sudden change of the parameter. In this framework, I analyze scaling relations concerning the probability of excitation and the excitation energy, with the quench amplitude of this parameter. These results are illustrated in the case of one-dimensional sine-Gordon model.

My paper in Nature Phys.

I guess I should link to my paper with Benjamin Lev and Paul Goldbart on BECs in multimode cavities now that it's finally been published.

He-3B revisited

What happens to an anisotropic superconductor (e.g. helium-3) when you shrink the Cooper pairs? In the case of a normal (BCS) superconductor, nothing terribly interesting takes place: the system goes smoothly from a BCS-like condensate, in which the distance between an electron and its partner is larger than the spacing between them, to a regular condensate of diatomic molecules for which the reverse is true.

This result generalizes, e.g., to the case of p-wave pairing in a completely spin-polarized Fermi gas. (See Leggett, J. Phys. Colloques 41 (1980) C7-19-C7-26.) The situation of s-wave pairing in a spin-imbalanced Fermi gas is somewhat more interesting: at the BEC end the molecules don't really care about the presence of additional fermions and one therefore has a BEC with free fermions floating on top of it; at the BCS end, however, the spin-imbalance frustrates Cooper pairing -- which requires the spin-up and -down Fermi seas to be degenerate. For finite spin-imbalance, one therefore has a quantum phase transition (I believe this is also a thermal phase transition) instead of a crossover.

I was interested in generalizing these ideas to the case of equal-spin p-wave superconductors (or Fermi superfluids) like He-3A and He-3B. The basic reason one might expect something interesting to happen is that the superfluid states are extremely fancy: much fancier, in particular, than anything one expects from tightly bound diatomic molecules. (This is particularly true for the A phase, which is stable only because of a subtle feedback mechanism.) The question is whether there's a quantum phase transition between the BEC and BCS ends; the answer -- which, rather disappointingly, Volovik got to -- is that there is. Volovik constructs a topological invariant that's zero in the BEC limit and nonzero in the BCS limit; it follows, of course, that these limits cannot be adiabatically connected.

Topological invariant for superfluid 3He-B and quantum phase transitions
Authors: G.E. Volovik
http://arxiv.org/abs/0909.3084

Abstract: We consider topological invariant describing the vacuum states of superfluid 3He-B, which belongs to the special class of time-reversal invariant topological insulators and superfluids. Discrete symmetries important for classification of the topologically distinct vacuum states are discussed. One of them leads to the additional subclasses of 3He-B states and is responsible for the finite density of states of Majorana fermions living on the diffusive wall. Integer valued topological invariant is expressed in terms of the Green's function, which allows us to consider systems with interaction.

Polariton BEC

The cond-mat Journal Club has a thing on polariton BEC with a commentary by Peter Littlewood who's a relatively important figure in the field. I'm not super-familiar with the literature but there are two basic reasons to be interested in polariton BEC: (1) it's pretty high-temperature and might actually be useful, (2) it's intrinsically very non-equilibrium as the polaritons owe their existence to the pump laser -- this has some rather neat implications, at least if you like far-from-eqm physics, like the observation that the Goldstone mode goes diffusive at very long wavelengths, and therefore these condensates aren't unambiguously "superfluid."