# Condensed Matter Theory Seminars

Thursdays, 1:30 pm

Room SCI 352

Metcalf Science Center

590 Commonwealth Ave.

Boston, MA 02215

(google map
| satellite)

## Upcoming Talks

### Destruction of Neel order in square lattice anti-ferromagnets

*Ribhu Kaul, Harvard University*

February 21, 2008

It has been long believed in the study of the cuprates that a thorough understanding of the possible insulating paramagnetic phases that are proximate to Neel ordered parent materials may hold the key to the mysterious properties that appear with the introduction of charge carriers. The parent materials are well described by S=1/2 square lattice anti-ferromagnetic models. Hence the destruction of Neel order in such spin Hamiltonians, which is a subject of great theoretical interest in itself, might also shed light on one of the most exciting experimental puzzles of our time -- high-temperature superconductivity. In this seminar, I will report numerical results on the first model S=1/2 square-lattice quantum anti-ferromagnet in which Neel order can be destroyed and which is nevertheless amenable to unbiased sign-problem free quantum-Monte Carlo simulations. Our numerical study which is carried out on lattices that contain in excess of 10,000 spins indicate that the paramagnetic state that emerges on the destruction of Neel order in this model has valence-bond solid order (VBS). Close to the transition there is evidence for quantum criticality and I will show how the observed scaling behavior may be understood by a comparison with large-N computations on a particular continuum field theory that is expected to describe the Neel-VBS transition.

#### Recent Talks

### Superconducting-to-resistive switching in nanowires: thermal runaway by phase-slips

*David Pekker, Harvard university*

February 07, 2008

Hysteresis in the current-voltage characteristic of a superconducting nanowire reflects an underlying bistability. As the current is ramped up the state switches from a superconductive to a resistive one,doing so at random current values below the equilibrium critical current. Can a single phase-slip event somewhere along the wire—during which the order-parameter fluctuates to zero—induce such switching, via the local heating it causes? Can multiple phase-slips events? Are phase-slips thermal or quantum in nature? We address this and related issues by constructing a stochastic model for the time-evolution of the temperature in a nanowire that is heated by random-in-time phase-slips and cooled by thermal contact with leads.

### BCS and beyond: new insights into fermionic superfluids from ultracold gases

*Wilhelm Zwerger, Technical University of Munich*

January 31, 2008

Ultracold Fermi gases near a Feshbach resonance allow one to realize a crossover from a BCS-type superfluid to a Bose-Einstein-Condensate of strongly bound pairs. I will discuss the thermodynamics of this crossover, emphasizing the universal behavior at infinite scattering length. A microscopic signature of pairing is provided by radiofrequency-spectroscopy. The average clock-shift is a measure of the pair-interaction energy which, however, hardly changes between the superfluid and a normal state reached beyond a critical spin-imbalance. Understanding the phase diagram and excitation spectrum of imbalanced gases still provides a challenge for both theory and experiment.

### Eigenstate thermalization hypothesis and foundations of quantum thermodynamics

*Maxim Olshanii, University of Massachusetts at Boston*

December 13, 2007

One of the open questions in quantum thermodynamics is: how can linear quantum dynamics provide chaos necessary for thermalization of an isolated quantum system? To this end, we perform an ab initio numerical analysis of a system of hard-core bosons on a lattice and show that the above controversy can be resolved via the Eigenstate Thermalization Hypothesis [Deutsch (1991), Srednicki (1994)]. According to this hypothesis, in quantum systems thermalization happens at the level of individual eigenstates, but hidden initially by coherences between them. In course of time evolution the thermal properties become revealed through (linear) decoherence that need not to be chaotic.

### Coherence dynamics induced by Atom-Optics kicked rotor

*Alexey Tonyushkin, Harvard University*

December 06, 2007

We experimentally study the impact of an atomic kicked rotor on the dynamics of a de Broglie wave interferometer. The dynamics of the matter-wave's coherence is investigated by looking at the “echo” signal of the interferometer. The overlap of a reference interferometer signal and its perturbed version is analogous to a quantum fidelity amplitude measurement. Fidelity of a quantum system also known as the Loschmidt echo proved to be the most direct way to study the so-called “quantum chaos” in the system with a classically chaotic perturbation. There have been several theoretical studies of fidelity decay for a spin-echo as well as some experimental attempts. In our implementation of the idea a phase-displacement operator acts on the atomic external states and so it defines an instantaneous phase-space displacement perturbation. We have observed that at quantum resonances of a quantum kicked rotor the dynamics of atomic coherences reflects a fidelity decay freeze at a finite value after just a small number of kicks. This is a direct confirmation of predicted distinction between quantum and classically chaotic systems. I will discuss the applications of the implemented scheme to precision measurements and quantum simulations.

### Electron Fractionalization in 2D Dirac Fermions

*Chang-Yu Hou, Boston University*

November 1, 2007

Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. We will show that quasiparticle excitations with irrational charge and irrational exchange statistics exist in tight-biding systems described, in the continuum approximation, by the Dirac equation in (2+1)-dimensional space and time. These excitations can be deconfined at zero temperature, but when they are, the charge re-rationalizes to the value 1/2 and the exchange statistics to that of ``quartons'' (half-semions).

### Synchronous dissipation in the superflow of He-II through nanoaperture arrays

*Roman Barankov, Boston University*

October 23, 2007

An array of nano-apertures in superfluid He^4 exhibits a transition from a regime in which various apertures dissipate the energy of superflow via phase-slips independently to one in which the dissipation is synchronous across the array [1]. We argue that the behavior of the system is controlled by the competition between the disorder in the critical velocities of individual apertures and the inter-aperture coupling mediated by the bulk superfluid [2]. At low disorder, fluctuations in the critical velocities are negligible, compared to the strong coupling between the apertures, and this leads to avalanching behavior of phase slips across the array. Stronger disorder, on the other hand, prevents the avalanche from spreading across the array. A simple model of superfluid phase dynamics [2] accounts for this transition in behavior, and yields predictions that compare favorably with experimental results. [1] Y. Sato, E. Hoskinson, and R. E. Packard, Phys. Rev. B 74, 144502 (2006) [2] David Pekker, Roman Barankov, and Paul M. Goldbart, Phys. Rev. Lett. 98, 175301 (2007)

### Quantum Hall Effect in Graphene p-n Junctions

*Dmitry Abanin, Massachusetts Institute of Technology*

October 18, 2007

In the first part of the talk, I will focus on Quantum Hall Effect (QHE) in graphene p-n junctions, which has been recently demonstrated experimentally. I will explain the observed conductance quantization which is fractional in the bipolar regime and integer in the unipolar regime in terms of QH edge modes propagating along and across the p-n interface. In the bipolar regime the electron and hole modes can mix at the p-n boundary, leading to current partition and quantized shot noise plateaus similar to those of conductance, while in the unipolar regime transport is noiseless. In the second part of the talk, I will discuss unusual nature of nu=0 QHE state in graphene and show that electron transport in this regime is dominated by counter-propagating edge states. Such states, intrinsic to massless Dirac quasiparticles, manifest themselves in a large longitudinal resistivity rho_xx h/e2, in striking contrast to rho_xx behavior in the standard QHE. If time allows, I will also discuss the effect of Coulomb interaction on the counterpropagating edge states, and show that it leads to Luttinger liquid behavior.

### Superconductivity and antiferromagnetism in the two-dimensional Hubbard model: a variational study

*David Eichenberger, L'Universit de Fribourg, Switzerland*

September 20, 2007

A variational ground state of the repulsive Hubbard model on a square lattice is investigated numerically for an intermediate coupling strength (U = 8t) and for moderate sizes (from 6 x 6 to 10 x 10). Our ansatz is clearly superior to other widely used variational wave functions. The results for order parameters and correlation functions provide new insight for the antiferromagnetic state at half filling as well as strong evidence for a superconducting phase away from half filling. I will also comment on the relevance of our findings for layered cuprates.

### Non-analytic behavior of Fermi liquids

*Dimitri Maslov, University of Florida*

May 24, 2007

The Fermi-liquid theory states that the low-energy properties of an interacting Fermi system are similar to that of the Fermi gas. However, in contrast to the Fermi gas, where all thermodynamic quantities are analytic functions of the approprite energy scales (temperature, Zeeman energy, etc.), the next-to-leading corrections to the Fermi liquid behavior are nonanalytic. Some of these non-analyticities, such as the non-analytic behavior of the specific heat in 2D, are due the collinear scattering events, which are rare for D>1 but icreasingly more probable as the dimensionality is reduced. The perturbation theory happens to be ill-defined even in 2D, as the self-energy exhibits a mass-shell singularity at every order of the perturbation theory. The resulting spectal function exhibits a non-Fermi-liquid feature a kink in the spectral function. The non-analytic behavior of the spin susceptibility signals a breakown of continuous ferromagnetic quantum phase transition.

### Quantum transport of non-interacting 2D Dirac fermions

*Shinsei Ryu, University of California, Santa Barbara*

May 17, 2007

We discuss the quantum transport of the 2D non-interacting Dirac Hamiltonian, which, e.g., underlies theoretical descriptions of graphene and a surface state of 3D Z_2 topological insulators. For a random scalar potential type disorder, a Z_2 topological term is derived in the non-linear sigma model encoding the physics of Anderson localization in the symplectic symmetry class. Unlike the Pruisken term (Chern integer) in the IQHE, the Z_2 topological term cannot be expressed, in general, as an integral of a local quantity, but as a sign of the Pfaffian of a family of Dirac operators. The Z_2 topological term has a significant effect on the renomalization group flow of the conductance.

### Dirty bosons: twenty years later

*Peter B. Weichman, BAE Systems, Advanced Information Technologies*

May 10, 2007

I will review what we have learned about quantum phase transitions in disordered boson systems since the problem first became the subject of serious study about twenty years ago. With careful attention paid to the various underlying symmetries that may or may not be present, I will present some fairly simple, semi-rigorous arguments that go a long way toward defining the possible topologies of the phase diagram, and discuss their impact on the interpretation of various recent numerical simulations. I will also revisit the famous prediction that the dynamical exponent z should equal the spatial dimensionality d, showing that the original scaling arguments can break down. This is consistent with the recent quantum Monte Carlo result z = 1.4 ± 0.02 in d = 2 obtained by a group at Duke University.

### Hidden Charge 2e boson in doped Mott insulators: Field theory of Mottness

*Philip Phillips, University of Illinois at Urbana-Champaign*

May 03, 2007

We construct the low energy theory of a doped Mott insulator, such as the high-temperature superconductors, by explicitly integrating over the degrees of freedom far away from the chemical potential. For either hole or electron doping, integration of the high energy scale generates a charge 2e bosonic field that mediates dynamical spectral weight transfer across the Mott gap. The bosonic field represents non-projective physics, preserves a jump in the chemical potential across half-filling, and competes with d-wave superconductivity. The relevance of the boson to photoemission and transport measurements on the cuprates will be discussed.

### Electrostatic doping of new materials: challenges, opportunities and first accomplishments

*Dimitri N. Basov, University of California, San Diego*

April 26, 2007

One focus of contemporary condensed matter physics is on exploring novel electronic and magnetic effects prompted by introducing a high density of mobile charges in an insulating host usually via chemical doping. Electrostatic doping that can be realized by integrating a host material of interest in an architecture of a field effect transistor offers an appealing alternative. Indeed, the electrostatic modulation of carrier density is reversible and it does not introduce disorder inevitable in chemically doped substances. I will overview recent progress with this challenging endeavor and will describe advances in the understanding of intrinsic transport properties of organic semiconductors enabled by the electrostatic doping of high quality organic molecular crystals.

### Manipulating optical information with coherent matter-wave dynamics

*Sean R. Garner, Harvard University*

April 5, 2007

I will describe recent experiments [1] in which a laser pulse is ''stored'' in one Bose-Einstein condensate (BEC) and later revived from another, spatially separate BEC. The information in the light pulse is transferred to the second BEC as a traveling matter wave. During its flight coherent evolution causes changes to the atomic wavefunction, which can be read out onto the outgoing light pulse. This constitutes a new type of quantum-coherent optical processing. [1] Ginsberg, Garner, and Hau, Nature 445, 623 (2007).

### Double exchange, magnetic polarons and phase separation in the context of Eu-based hexaborides

*Vitor Pereira, Boston University*

March 29, 2007

In this presentation, important details of the Double Exchange model at low densities and their pertinence to the physics of ferromagnetic hexaborides will be addressed. After a brief survey of some key experiments and signatures of these compounds, it will be shown that in such systems, where itinerant electrons at extremely reduced densities interact with a dense local spin subsystem, effects of Anderson localization are paramount, providing a consistent picture of the remarkable and unusual response of materials like Eu_(1-x)Ca_(x)B_6 in magnetic, optical and transport measurements. In this context we will show how one can understand the blue shift in the plasma frequency, the enhancement of carrier density and the CMR effect upon entering the ferromagnetic phase of EuB_6 as effects stemming from Anderson localization mechanisms. We will provide an interpretation for the metal insulator transition that occurs upon doping, under the light of recent magneto-optical experiments.The region of stability of magnetic polarons in the low density Double Exchange Model is discussed, and shown to be consistent with experimental hints of a polaronic phase mediating the PM-FM transition in the vicinity of Tc. The connections between the polaronic phase and a phase separated regime will also be addressed.

### Unconventional insulators with time reversal symmetry

*Rahul Roy, University of Illinois at Urbana-Champaign*

March 13, 2007

Traditionally, band insulators which obey time reversal symmetry have only received limited attention. Recently it has been realized that due to spin-orbit coupling, novel spin transport phenomena such as the spin Hall effect can occur in these systems. Certain insulators can have localized gapless states at the edge, analogous to quantum Hall systems. An analog of the Hall conductance for these systems is the "Z_2 invariant." I describe a new approach to the Z_2 characterization of two dimensional band insulators which clarifies the connection with robust edge states. I also present a new characterization of three dimensional band insulators and other systems with time reversal symmetry. Systems which might realize the exotic phases of this characterization and the connection with the quantized spin Hall effect will also be discussed.

### Electronics with single-molecule magnets

*Eduardo Mucciolo, University of Central Florida*

February 22, 2007

In this seminar I will talk about combining molecular electronics and magnetism by using a special class of inorganic molecules, the so-called single-molecule magnets (SMMs). These molecules are typically formed by rings of transition metal ions surrounded by various ligands. SMMs have some very peculiar properties due to their large total spin and strong magnetic anisotropy. For instance, it is possible to control the magnetization through the manipulation of a topological (Berry) phase. I will show our recent theoretical proposal of novel forms of the Kondo and Coulomb blockade effects based on this topological phase. Experimental results on SMM electronic devices obtained by our group at UCF will also be presented.

### Quantum phase transitions beyond the dilute Bose gas limit

*Valeri Kotov, Boston University*

February 15, 2007

Conventional quantum transitions in Heisenberg antiferromagnets, such as transitions between dimerized (valence bond crystal) and magnetically ordered phases, are typically very well described in terms of condensation of triplet modes with low density. The fact that the density stays low means that the quantum fluctuations are not strong enough to destroy the dimer correlations even in the Neel phase.After a short overview I will discuss a model, proposed by Anders Sandvik, where the dimer order is ``spontaneous", and disappears at the critical point; this model therefore is in the ``de-confined criticality" universality class. I will describe our efforts to understand this unconventional transition, with the ultimate conclusion that strong departure from the dilute limit is necessary in order to have consistency with the recent Monte Carlo results.

### Spirals in the t-J model and structure of the spin-glass state of La(2-x)SrxCuO4

*Oleg P. Sushkov, University of New South Wales, Sydney*

February 1, 2007

There are numerous experimental data that indicate incommensurate spin structures in cuprates. The most systematic studies have been performed for La(2-x)SrxCuO4. A short review of the data for this compound will be presented. A spin spiral is the most natural and simple solution that arises in a weakly doped t-J model. In spite of simplicity the solution has fundamental problems. These problems have been resolved recently for the insulating "spin-glass" state of La(2-x)SrxCuO4. The theory allows us to explain quantitatively a number of effects that have not been understood before. This includes incommensurate neutron scattering, the anisotropy of in plane conductivity, negative magnetoresistance, etc.

### Nonequilibrium quantum criticality in open electronic systems

*Yong Baek Kim, University of Toronto*

January 25, 2007

Theory of equilibrium quantum phase transitions has been around for a while, but quantum phase transitions in systems driven out of equilibrium have not been studied much. Here I will present a theory of nonequilibrium quantum criticality in open (coupled to reservoirs) electronic systems. I will discuss the cases of itinerant Ising and Heisenberg magnets with nonequilibrium provided by current flow across the system. Both departures from equilibrium at conventional (equilibrium) quantum critical points and the physics of phase transitions induced by the nonequilibrium drive will be discussed.

### Cold bosonic atoms in rings, optical lattices and other traps: beyond Gross-Pitaevskii and Bose-Hubbard

*Ofir Alon, University of Heidelberg, Germany*

January 18, 2007

The necessity to go beyond Gross-Pitaevskii to properly describe strongly-interacting cold bosonic atoms, and even weakly-interacting atoms in double- and multi-well traps has by now become well-accepted by the community. Starting from the problem of attractive condensates on a ring and ending with that of repulsive bosons in general traps, we present new theoretical and computational approaches going beyond standard mean-field. These have enabled us to predict new physical phenomena. I discuss fragmentation of attractive and repulsive condensates, a zoo of Mott-insulator phases in optical lattices [thereby going beyond Bose-Hubbard model], and the pathway to fermionizion of trapped cold bosonic systems.