# Tage Erlander Award Conference "Frontiers of Condensed Matter Physics"

Europe/Stockholm
FB54

,
Description

### Venue

Nordita, Stockholm, January 3-8, 2011
Fysikum building, room FB 54 (Roslagstullsbacken 21)

### Organizers

Egor Babaev and Eddy Ardonne

### Tage Erlander's award of the Royal Swedish Academy of Sciences

Swedish Reasearch Council and Nordic Institute for Theoretical Physics

### Preliminary list of speakers

• Ehud Altman, Weizman (Israel)
• Boris Altshuler, Columbia University (USA)
• Daniel Arovas, Univesity of California San Diego (USA)
• Ophir Auslaender, Technion (Israel)
• Egor Babaev, KTH Sweden
• Alexander Balatsky, Los Alamos National Laboratory (USA)
• Aurel Bulgac, University of Washington (USA)
• Victor Galitski, University of Maryland (USA)
• Thierry Giamarchi, Geneva University (Switzerland)
• Paul Goldbart, University of Illinois (USA)
• Victor Gurarie, University of Colorado (USA)
• Alex Gurevich, National High Magnetic Field Laboratory (USA)
• Walter Hofstetter Frankfurt University (Germany)
• Beena Kalisky Stanford University (USA)
• Anatoly Kuklov, City University of New York (USA)
• W. Vincent Liu, Pittsburg University (USA)
• Yoshiteru Maeno, Kyoto University(Japan)
• Joel Moore, Berkeley University (USA)
• Victor Moshchalkov, KU-Leuven (Belgium)
• Olexei Motrunich, Caltech (USA)
• Arun Paramekanti, University of Toronto (Canada)
• Gil Refael, Caltech (USA)
• Charles Reichhardt, Los Alamos National Laboratory (USA)
• James Sauls, Northwestern University (USA)
• Edouard Sonin, Hebrew University of Jerusalem (Israel)
• Boris Spivak, University of Washington (USA)
• Boris Svistunov, University of Massachusetts Amherst (USA)
• Asle Sudbø, NTNU Trondheim (Norway)
• Oskar Vafek, Florida State University (USA)
• Matthias Troyer, ETH (Switzerland)
• Martin Zwierlein, MIT (USA)
• Monday, January 3
• 8:00 AM 9:00 AM
ARRIVAL DAY
• Tuesday, January 4
• 9:30 AM 10:00 AM
• 10:00 AM 10:50 AM
TBA 50m
Speaker: Boris Altschuler (Columbia U.) (Columbia U.)
• 10:50 AM 11:40 AM
Spin Dimers: from BEC to Luttinger liquids 50m
Localized spin systems, and in particular dimer systems, provide a fantastic laboratory to study the interplay between quantum effects and the interaction between excitations. Magnetic field and temperature allow an excellent control on the density of excitations and various very efficient probes such as neutrons and NMR are available. They can thus be used as quantum simulators'' to tackle with great success questions that one would normally search in itinerant interacting quantum systems. In particular they have provided excellent realizations of Bose-Einstein condensates [1,2]. This allowed not only to probe the properties of interacting bosons in a variety of dimensions but also to study in a controlled way additional effects such as disorder. If the dimensionality is reduced they also allow to test in a quantitative way Luttinger liquid physics [3,4,5]. I will discuss these various cases, and show that we have now good theoretical tools [6] to make quantitative comparisons with the experiments. Finally, how to go from this low dimensional case where the spins behave essentially as fermions, to the higher dimensional case where they behave as (essentially free) bosons, is a very challenging, and experimentally relevant issue. {\small \noindent [1] T. Giamarchi and A. Tsvelik, Phys. Rev. B {\bf 59} 11398 (1999).\\ [2] T. Giamarchi, C. R\"uegg and O. Tchernyshyov, Nat. Phys. {\bf 4} 198 (2008).\\ [3] M. Klanjsek et al., Phys. Rev. Lett. {\bf 101} 137207 (2008).\\ [4] C. R\"uegg et al., Phys. Rev. Lett. {\bf 101} 247202 (2008).\\ [5] B. Thielemann et al., Phys. Rev. B {\bf 79} 020408(R) (2009).\\ [6] P. Bouillot et al., arXiv:1009.0840 (2010).}
Speaker: Thierry Giamarchi (U. of Geneva)
• 11:40 AM 11:50 AM
Coffee break 10m
• 11:50 AM 12:40 PM
What can we learn from quantum entanglement spectra? 50m
I will summarize some recent developments in the study of quantum entanglement spectra. I will also discuss work performed in collaboration with R. Thomale and A. Bernevig on entanglement spectra in spin chains. Typically, bipartite entanglement entropy and spectra have been studied in the case of spatial partitions, i.e. A denotes the left half of a spin chain and B the right half, and the eigenvalues of the reduced density matrix of the A component comprise the entanglement spectrum (ES). We find that for the spin-half Heisenberg model that a remarkable structure in the ES is revealed if the partition is performed in momentum space, i.e. A = left-movers and B = right-movers. Further classifying the entanglement eigenstates by total crystal momentum, we observe a universal low-lying portion of the ES with specific multiplicities separated from a higher-lying nonuniversal set of levels by an entanglement gap, similar to what was observed by Li and Haldane (2008) for the fractional quantum Hall effect. Indeed, the momentum space ES for the Heisenberg chain is understood in terms of the proximity of the Haldane-Shastry model, which corresponds to a fixed point with no nonuniversal corrections, and whose ground state is related to the Laughlin state. We further explore the behavior of the ES as one tunes through the dimerization transition in a model with next-nearest-neighbor exchange.
Speaker: Daniel Arovas (UC San Diego) (UC San Diego)
• 2:30 PM 3:20 PM
Quantized pumping and topological Bose insulators 50m
Bosons in a one dimensional chain can form two gapped phases at integer filling, the Mott and Haldane insulators. The critical point separating these two phases is gapped out by a perturbation breaking lattice inversion symmetry. I will show that encircling the critical point adiabatically in the plane of the tuning parameter and the inversion symmetry breaking perturbation, entails pumping of exactly one boson across the Bose insulator. When multiple chains are coupled, the two insulating phases are no longer sharply distinct, but the pumping property survives and allows to define a topological flux associated with gapless regions in the phase diagram. This leads to strict constraints on the topology of the phase diagram of systems of quasi-one dimensional interacting bosons. Finally I will use the pumping property to elucidate the topological invariant underlying the Haldane phase and to discuss possible extensions to interacting topological phases in higher dimensions.
Speaker: Ehud Altman (Weizman Institute)
• 3:20 PM 4:10 PM
Lie-algebraic Approach to Quantum Dynamics and Quantum-to-Classical Correspondence 50m
In this talk, I will review our recent work on a Lie-algebraic approach to various quantum-mechanical problems. The first part will be devoted to non-equilibrium driven dynamics of closed quantum systems. It will be emphasized that mathematically a non-equilibrium Hamiltonian represents a trajectory in a Lie algebra, while the evolution operator is a trajectory in a Lie group generated by the underlying algebra via exponentiation. This turns out to be a constructive statement that establishes in particular the fact that classical and quantum unitary evolutions are two sides of the same coin determined uniquely by the same dynamic generators in the group. An equation for the generators, dubbed dual Schrodinger-Bloch equation, will be derived and analyzed. In the second part of the talk, I will extend the Lie algebraic approach to many-particle quantum systems in equilibrium, in particular to Hubbard-like models and quantum spin models. The canonical problem of determining the partition function of these models will be put in the context of classification of the Lie algebras. Based on these observations, examples of statistical-transmutation and new spin-to-fermion transforms will be constructed.
Speaker: Victor Galitski (Joint Quantum Institute and Physics Department, U. of Maryland) (U. of Maryland)
• 4:10 PM 4:20 PM
cofee break 10m
• 4:20 PM 5:10 PM
Ultracold spin-imbalanced Fermi gases in low dimensions 50m
In this talk, I will present some theoretical results motivated by the ongoing experiment of a lattice array of one-dimensional spin imbalanced Fermi gases with strong interaction. The system is suitable for studying dimensional crossover of the FFLO phase. I will first report for exact one dimension a breakthrough in analytically reducing the infinite coupled thermodynamic Bethe ansatz equations, whose physics is exact but notoriously difficult to understand, to a powerful set of four algebraic equations. The new formulation reveals universal finite-temperature crossovers between exotic quantum phases at zero temperature. Then, for quasi-1D, I will report a model of coupled Luttinger liquids obtained as effective field theory by treating spin-charge mixing through the method of Bethe ansatz and conformal invariance. Finally, I will show that, when the system is tuned to 2D, the Kosterlitz-Thouless transition of the FFLO phase is described by a U(1)xU(1)/Z2 field theory with fractional topological defects. Work done in collaboration with M. T. Batchelor, X.-W. Guan, X. Li, C. Lin, M. Oshikawa, and E. Zhao. Supported by ARO-DARPA-OLE and ARO.
Speaker: Vincent Liu (Pittsburg U)
• 5:10 PM 6:00 PM
Dirac materials 50m
Recently a new single-layer material—graphene—has been discovered. This is a material where Dirac points in the fermionic spectrum lead to very unusual properties, including transport and impurity states. I will argue that these properties are not unique to graphene and in fact are a direct consequence of the Dirac spectrum in the fermionic excitation sector. Strong similarities with d-wave superconductors, superfluid 3He, p-wave superconductors and topological insulators with similar energy spectra offfer a unifying perspective. Among the similarities are response to defects, suppressed backscattering and unusual transport properties. I will discuss some observable effects, such as Kondo effect and impurity resonances, that differentiate Dirac Materials from other classes.
Speaker: Alexander Balatsky (LANL)
• 6:00 PM 6:15 PM
Coffee break 15m
• 6:15 PM 7:05 PM
Models and ground states of interacting anyons 50m
In quasi-two-dimensional systems non-abelian anyons can appear as quasiparticle excitations next to fermions and bosons. They have been proposed to appear, among other potential realizations, in certain fractional quantum Hall states. These non-Abelian anyons have non-trivial braiding statistics and can be used to realize a universal topological quantum computer. In this talk I will focus on interactions between such anyons and the plethora of possible ground states in systems of interacting anyons. For one-dimensional arrangements of anyons there are close connections to the minimal models of conformal field theory which can be viewed as generalized Heisenberg models. In two dimensions we find that unlike for localized fermions, where a gapless symmetry-broken Neel state is formed, the ground state of an array of localized anyons is another chiral anyonic quantum Hall liquid
Speaker: Matthias Troyer (ETH)
• Wednesday, January 5
• 10:00 AM 10:50 AM
Diagrammatic Monte Carlo for Fermionic Systems 50m
Monte Carlo sampling of the Feynman diagrammatic series can be used for tackling hard fermionic quantum many-body problems in the thermodynamic limit. I will introduce the technique and present illustrative results for the repulsive Hubbard model in the correlated Fermi liquid regime, as well as the results for the equation of state for the system of resonant fermions in the regime of BCS-BEC crossover.
Speaker: Boris Svistunov (U.Mass Amherst)
• 10:50 AM 11:50 AM
Type-1.5 superconductivity and non-Meissner electrodynamics in multicomponent superconductors (overview talk) 1h
In the recent years the interest in multicomponent superconductivity is growing due to discoveries of multiband superconducting materials and renewed experimental pursuit of the projected superconducting states of metallic hydrogen and deuterium as well as some applications in the physics of pulsars. In this talk I will overview two topics in multicomponent superconductivity. (i) Within the standard paradigm the magnetic properties of superconductors are dictated by the mass of the gauge field acquired via the Andersson-Higgs effect. I will discuss that in multicomponent superconductors the fluctuating gauge field can alter this physics by generation of Faddeev-Skyrme-like terms which makes free energy of a multicomponent superconductors mappable onto a version of ostensibly unrelated Faddeev-Skyrme model. I will give a number of examples how this leads to entirely novel physics in multicomponent theories: non-exponential localization and nonmonotonoc behavior of magnetic field inside a superconductor with local electrodynamics, topological excitations in the form of vortex loops which energy is a non-monotonic function of the size of the defect etc. I will argue that it should lead to new physics of post-quench relaxation, new kind of superfluid turbulence and other phenomena which have no counterparts in single-component systems. (partially based on [1]) (ii) In contrast to single-component superconductors, which are described at the level of Ginzburg-Landau theory by a single Ginzburg-Landau parameter and are divided in type-I and type-II classes, two-component systems in general possesses three fundamental length scales and have been shown to possess a separate "type-1.5" superconducting state [2]. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges, and therefore should form clusters in low magnetic fields. In such clusters one can define a negative interface energy inside a cluster and at the same there one can define a positive interface energy associated with the cluster's boundary. In particular I will discuss that, in some temperature range, the type-1.5 superconductivity can exist in multiband materians even when there is a substantial interband couplings such as intrinsic Josephson coupling. Based on: [1] Egor Babaev Phys. Rev. B 79, 104506 (2009) Egor Babaev, Juha Jäykkä, Martin Speight Phys. Rev. Lett 103, 237002 (2009) [2] E. Babaev & J.M. Speight Phys.Rev. B 72 180502 (2005) Johan Carlstrom, Egor Babaev, Martin Speight arXiv:1009.2196 E. Babaev, J. Carlstrom, J. M. Speight Phys. Rev. Lett. 105, 067003 (2010)
Speaker: Prof. Egor Babaev
• 11:50 AM 12:00 PM
break 10m
• 12:00 PM 12:50 PM
Phase transitions in a three dimensional U(1)xU(1) lattice London superconductor: Metallic superfluid and charge-4e superconducting states 50m
We consider a three-dimensional lattice $U(1) \times U(1)$ and $[U(1)]^N$ superconductors in the London limit, with individually conserved condensates. The $U(1) \times U(1)$ problem, generically, has two types of intercomponent interactions of different characters. First, the condensates are interacting via a minimal coupling to the same fluctuating gauge field. A second type of coupling is the direct dissipationless drag represented by a local intercomponent current-current coupling term in the free energy functional. In this work, we present a study of the phase diagram of a $U(1) \times U(1)$ superconductor which includes both of these interactions. We study phase transitions and two types of competing paired phases which occur in this general model: (i) a metallic superfluid phase (where there is order only in the gauge invariant phase difference of the order parameters), (ii) a composite superconducting phase where there is order in the phase sum of the order parameters which has many properties of a single-component superconductor but with a doubled value of electric charge. We investigate the phase diagram with particular focus on what we call preemptive phase transitions.'' These are phase transitions {\it unique to multicomponent condensates with competing topological objects}. A sudden proliferation of one kind of topological defects may come about due to a fluctuating background of topological defects in other sectors of the theory. For $U(1) \times U(1)$ theory with unequal bare stiffnesses where components are coupled by a non-compact gauge field only, we study how this scenario leads to a merger of two $U(1)$ transitions into a single $U(1) \times U(1)$ discontinuous phase transition. We also report a general form of vortex-vortex bare interaction potential and possible phase transitions in an $N$-component London superconductor with individually conserved condensates.
Speaker: Asle Sudbo (NTNU Trondheim) (NTNU Trondheim)
• 2:40 PM 3:30 PM
Vortex Matter in Type-1.5 Superconductors 50m
Speaker: Victor Moshchalkov (KU Leuven) (KU Leuven)
• 3:30 PM 4:20 PM
Topological Competition of Superconductivity in Sr_2 RuO_4 Junctions 50m
We devise a new proximity junction configuration where an /s/-wave superconductivity and the superconductivity of Sr_2 RuO_4 interfere with each other. In order to explain the observed extraordinary temperature dependence of the critical current in a Pb/Ru/Sr_2 RuO_4 junction, we propose a competition effect involving topologically distinct superconducting phases around Ru inclusions. This work is done in collaboration with Taketomo Nakamura, R. Nakagawa, T. Yamagishi, S. Yonezawa, T. Terashima, and M. Sigsrist.
Speaker: Yoshiteru Maeno (Kyoto U) (Kyoto U.)
• 4:20 PM 4:30 PM
cofee break 10m
• 4:30 PM 5:10 PM
Local variations of superfluid density in twinned high temperature superconductors 40m
We use scanning SQUID microscopy to investigate local variations in the diamagnetic susceptibility in twinned samples of two families of high temperature superconductors. In Ba(Fe1-xCox)2As2 of the pnictides we observe increased diamagnetic susceptibility on the twin boundaries in underdoped, but not overdoped, single crystals. Vortex behavior near the twin boundaries reveals that vortices avoid pinning on the twins, and prefer to travel parallel to them. These results indicate a relationship between superfluid density, local strain, and frustrated magnetism, and demonstrate two mechanisms for enhancing critical currents. In YBa2Cu3O7-x of the cuprates vortices have been observed to line up on the twin boundaries. I will show our preliminary data on direct measurement of the susceptibility near twin boundaries in YBa2Cu3O7-x . I will also show and discuss SQUID imaging of vortices at very low fields in both systems.
Speaker: Beena Kalisky (Stanford U.) (Stanford)
• 5:10 PM 5:50 PM
Magnetic force microscopy of superconductors: vortex manipulation and measuring the penetration depth 40m
We use a low temperature magnetic force microscope (MFM) to study superconductors. The interaction between the magnetic tip and individual vortices allows us to both image vortices and to manipulate them. The manipulation results depend on sample thickness and on the superconducting properties. Here I concentrate on YBCO samples and on an underdoped pnictide sample. In thin films, if the force exerted by the tip is strong enough to overcome the pinning potential a vortex jumps as a whole to a new pinning site. The behavior in thick YBCO single crystals depends on the doping level. In a slightly overdoped sample vortices stretch rather than jump when we perturb them strongly. The dragging distance in this crystal is anisotropic: it is easier to drag vortices along the Cu-O chains than across them, consistent with the tilt modulus and the pinning potential being weaker along the chains. We also find that when we wiggle'' the top of a vortex we can drag it significantly farther than when we do not, giving rise to a striking dynamic anisotropy between the fast and the slow directions of the scan pattern. In underdoped YBCO single crystals, where superconductivity is so anisotropic that a vortex should be viewed as a stack of two dimensional pancakes, we show that vortices kink rather than tilt when we perturb them. I will also present preliminary data on the behavior of pancake vortices near a twin boundary (TB). Our measurements indicate that it is easier to drag a vortex along a TB than across it. Surprisingly, we find a difference between the two opposite directions normal to a TB in terms of how hard it is to drag a vortex. Since the discovery of the pnictides, a new family of high temperature superconductors, we have also been developing ways to determine the absolute value of the magnetic penetration depth, which is notoriously difficult to measure, as well as its dependence on temperature. For that we either use the Meissner repulsion of the magnetic MFM tip from the sample or the magnetic interaction between the tip and the magnetic field from a vortex. The temperature dependence that we find allows us to comment on the symmetry of the order parameter.
Speaker: Ophir Auslaender (Technion - Israel Institute of Technology)
• 6:30 PM 9:30 PM
CONFEERENCE DINNER 3h
• Thursday, January 6
• 10:00 AM 10:50 AM
Universal Spin Transport in Strongly Interacting Fermi Gases 50m
We study spin transport in a two-state mixture of ultracold Fermi gases near a Feshbach resonance. The strong interactions lead to extremely slow relaxation of spin currents. The relevant transport coefficients, the spin conductivity and the spin diffusion coefficient, are observed to attain universal minimal values, with the spin diffusion being given by hbar/m, where m is the atomic mass. The ratio of spin conductivity and spin diffusion coefficient yields the spin susceptibility in these gases, showing the Curie law at high temperatures and a departure from the compressibility at low temperatures, that we interpret as a signature for entering the Fermi liquid regime.
Speaker: Martin Zwierlein (MIT)
• 10:50 AM 11:40 AM
Exciton Bose Liquid in a Hard-Core Boson Ring Mod 50m
I will present our Quantum Monte Carlo studies of hard-core boson Hamiltonians with ring-only interactions on the 2D square lattice searching for a so-called Excitonic Bose Liquid (EBL). This phase, which can be viewed as a special type of "Bose metal", had eluded numerical realization since the first proposal a decade ago, as the original model showed either charge-order at half-filling or phase separation at generic density. By a minimal modification of the model by including more extended ring exchanges, we are able to stabilize the EBL both at half-filling and away from half-filling and start confronting the EBL theory with unbiased numerical results. I will also describe a parton-gauge perspective on the EBL and some lessons for Gutzwiller-type wavefunctions
Speaker: Olexei Motrunich (Caltech) (Caltech)
• 11:40 AM 11:50 AM
Coffee break 10m
• 11:50 AM 12:50 PM
Transport in strongly correlated two dimensional electron fluids (overview talk) 1h
An overview of the measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities is presented as well as some of the theories that have been proposed to account for them. Many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium. Rather, they reflect new physics associated with strong correlation effects, which warrant further study.
Speaker: Boris Spivak (U. of Washington) (University of Washington)
• 2:40 PM 3:30 PM
The Weakly Coupled Pfaffian as a Type I Quantum Hall Liquid 50m
he Pfaffian phase of electrons in the proximity of a half-filled Landau level is understood to be a p+ip superconductor of composite fermions. We consider the properties of this paired quantum Hall phase when the pairing scale is small, i.e. in the weak-coupling, BCS, limit, where the coherence length is much larger than the charge screening length. We find that, as in a Type I superconductor, the vortices attract so that, upon varying the magnetic field from its magic value at \nu=5/2, the system exhibits Coulomb frustrated phase separation. We propose that the weakly and strongly coupled Pfaffian states exemplify a general dichotomy between Type I and Type II quantum Hall fluids.
Speaker: Boris Spivak (U. of Washington) (University of Washington)
• 3:30 PM 4:30 PM
Excitations,Topological Defects and Collective Modes in Unconventional Superconductors and Superfluid 3He (overview talk) 1h
Broken symmetries in bulk condensed matter systems have implications for the spec- trum of Fermionic excitations bound to surfaces and topological defects. I discuss the relationship between the broken symmetry of the ground state and the topolog- ical nature of bound states at surfaces, domain walls and topological line defects. In addition to the Fermionic spectrum, strong interactions lead to Bosonic excita- tions in unconventional superconductors and superfluids. These excitations provide unique signatures for broken symmetries of the ground state. I discuss examples and applications to the superfluid phases of 3He, Sr2RuO4, cuprate and heavy fermion superconductors.
Speaker: James Sauls (Northwestern University) (Northwestern University)
• 4:30 PM 5:20 PM
Emergent co-crystallization of atoms and light in multimode cavities 50m
The self-organization of a Bose-Einstein condensate in a transversely pumped optical cavity is a process akin to crystallization: when pumped by a laser of sufficient intensity, the coupled matter and light fields evolve, spontaneously, into a spatially modulated pattern, or crystal, whose lattice structure is dictated by the geometry of the cavity. In cavities having multiple degenerate modes, the quasi-continuum of possible lattice arrangements, and the continuous symmetry breaking associated with the adoption of a particular one, give rise to phenomena such as phonons, defects, and frustration. A nonequilibrium field-theoretic approach enables the exploration of the self-organization of a Bose-Einstein condensate in a pumped, lossy optical cavity. At nonzero temperatures, this organization occurs via a fluctuation-driven first-order phase transition of the Brazovskii class; the transition persists to zero temperature and crosses over into a quantum phase transition. The field-theoretic approach also enables the investigation of the role of nonequilibrium fluctuations in the self-organization transition, as well as the nucleation of ordered-phase droplets, the nature and energetics of topological defects, supersolidity in the ordered phase, and the possibility of frustration effects controlled by the cavity geometry.
Speaker: Paul Goldbart (U. of Illions Urbana Champaign) (University of Illions Urbana Champaign)
• Friday, January 7
• 10:00 AM 10:50 AM
Pattern Formation from Competing Interactions: Implications for Soft and Hard Condensed Matter Systems 50m
I will give an overview of how systems with competing interactions such as repulsion on one scale and attraction on another can generically give rise rise to bubble, stripe, clump, and other patterns. The same patterns can occur for systems with purely repulsive interactions provided there are two or more distinct length scales in the potential. I will show how these patterns can arise in soft matter systems, charge ordered systems, neutron stars in the form of pasta phases, and in the recently proposed type-1.5 superconductors. Under application of an external drive these systems can also exhibit numerous nonequilibrium phase transitions which produce pronounced changes in the transport properties. I will also discuss how pattern formation in these systems can be influenced by a patterned substrate such as a periodic array of two-dimensional pinning sites.
Speaker: Charles Reichhardt (Los Alamos National Laboratory) (Los Alamos National Laboratory)
• 10:50 AM 11:40 AM
Responses and effective theories in topological insulators 50m
The original definition of a topological insulator was as a time-reversal-symmetric insulator in which spin-orbit coupling leads to protected metallic edge or surface states. An alternate definition comes from considering the effect of a small perturbation that breaks the symmetry and gaps the surfaces; then the material can be viewed as having a quantized magnetoelectric effect. We discuss generalizations of this result to other materials and symmetry classes. In closing we discuss how a version of BF theory can capture both definitions of a topological insulator in either 2D or 3D, just as the Chern-Simons effective theory captures the universal features of quantum Hall states. Possible generalizations to "fractional" topological insulators are discussed.
Speaker: Joel Moore (Berkeley) (Berkeley)
• 11:40 AM 11:50 AM
break 10m
• 11:50 AM 12:40 PM
Single particle Green’s functions and interacting topological insulators 50m
We develop a method to characterize interacting topological insulators with single particle Green's functions. If the interactions are switched off, it reproduces the known behavior of noninteracting topological insulators, in particular the existence of the edge states at their boundary. At the same time, the method explains why topological insulators, once the interactions are turned on, can lose their edge states.
Speaker: Victor Gurarie (U of Colorado Boulder)
• 2:30 PM 3:20 PM
Generation and Dynamics of Quantized Vortices in a Unitary Fermi Superfluid 50m
Superfluidity and superconductivity are remarkable manifestations of quantum coherence at a macroscopic scale. The dynamics of superfluids has dominated the study of these systems for decades now, but a comprehensive theoretical framework is still lacking. We introduce a local extension of the time-dependent density functional theory to describe the dynamics of fermionic superfluids. Within this approach one can correctly represent vortex quantization, generation, and dynamics, the transition from a superfluid to a normal phase and a number of other large amplitude collective modes which are beyond the scope of two-fluid hydrodynamics, Ginzburg-Landau and/or Gross-Pitaevskii approaches. We illustrate the power of this approach by studying the generation of quantized vortices, vortex rings, vortex reconnection, and transition from a superfluid to a normal state in real time for a unitary Fermi gas. We predict the emergence of a new qualitative phenomenon in superfluid dynamics of gases, the existence of stable superfluidity when the systems are stirred with velocities significantly exceeding the nominal Landau critical velocity in these syste
Speaker: Aurel Bulgac (U. of Washington) (University of Washington)
• 3:20 PM 4:10 PM
Multiband superconductivity at high magnetic fields and the FFLO instability in ferropnictides. 50m
Low carrier densities and short coherence lengths in semi-metallic ferropnictide superconductors can result in exotic behaviors at strong magnetic fields due to the interplay of multiband superconductivity, unconventional pairing symmetry and Zeeman and orbital pairbreaking. In this talk I discuss how these effects manifest themselves in the anomalous temperature dependence of the upper critical field Hc2(T) and the Fulde-Ferrel- Larkin-Ovchinnikov (FFLO) transition in clean anisotropic multband superconductors. In particular, if the magnetic field H it is tilted away from the symmetry axis, the wave vector Q of the FFLO oscillations is not parallel to H, which may result in fractional vortices. The crystalline anisotropy and the s± pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase the orbitally-limited Hc2(T) and facilitate the FFLO transition in multiband ferropnictides. In these materials a small shift of the chemical potential upon doping can result in the Lifshits transition due to emerging pockets of the Fermi surface, which in turn trigger the FFLO instability even for moderate values of the Maki parameter in the main bands. It is argued that the FFLO state may be more common than is usually assumed based on the observed shapes of HC2(T), so specific heat or torque magnetometry may be required to reveal the FFLO state.
Speaker: Alex Gurevich (Florida State U)
• 4:10 PM 4:20 PM
Coffee break 10m
• 4:20 PM 5:10 PM
Interacting fermions on the honeycomb bilayer: From weak to strong coupling 50m
Many-body instabilities of the half-filled honeycomb bilayer are studied using weak-coupling renormalization group (RG) as well as strong-coupling expansion [1,2]. For spinless fermions, there are 4 independent four-fermion contact couplings. Generally, we find runaway RG flows which we associate with ordering tendencies. The broken symmetry state is typically a gapped insulator with either broken inversion or broken time-reversal symmetry, with a quantized anomalous Hall effect. Additionally, a tight-binding model with nearest-neighbor hopping and nearest-neighbor repulsion is studied in weak and strong couplings and in each regime a gapped phase with inversion symmetry breaking is found. In the strong-coupling limit, the ground-state wave function is constructed for vanishing in-plane hopping but finite interplane hopping, which explicitly displays the broken inversion symmetry and a finite difference between the number of particles on the two layers. In the spin-1/2 case we use Fierz identities to show that there are 9 independent four-fermion contact couplings[2]. The 9 RG equations in this case reduce to the 3 found in Ref.[1] assuming that screened Coulomb interactions dominate and in this case lead to the electronic nematic as the leading instability. The 9 RG equations are also used to show that, just as in strong coupling, the most dominant weak-coupling instability of the repulsive Hubbard model (at half filling) is an antiferromagnet. [1] O. Vafek and K. Yang, PRB 81, 041401 (2010). (Physics 3, 1 (2010)) [2] O. Vafek, PRB 82, 205106 (2010)
Speaker: Oskar Vafek (Florida State U)
• 5:10 PM 6:00 PM
Magnetic Order and Spectroscopy of Strongly Correlated Quantum Gases 50m
Cold atoms in optical lattices offer a new laboratory for the study of strong correlation phenomena.I will focus on two recent developments: i) We report the first detection of the Higgs-type amplitude mode using Bragg spectroscopy in a strongly interacting Bose condensate in an optical lattice. By the comparison of our experimental data with a spatially resolved, dynamical Gutzwiller calculation, we obtain good quantitative agreement. This allows for a clear identification of the amplitude mode, showing that it can be detected with full momentum resolution by going beyond the linear response regime. A systematic shift of the sound and amplitude modes' resonance frequencies due to the finite Bragg beam intensity is observed. ii) We investigate antiferromagnetic ordering of trapped spin-1/2 fermions using large-scale dynamical mean-field theory simulations. We find a clear experimental signature - enhanced double occupancy - for the onset of magnetic order at low temperatures in current experiments. We also discuss the effect of spin imbalance leading to canted antiferromagnetism.
Speaker: Walter Hofstetter (Goethe-Universität Frankfurt)
• Saturday, January 8
• 10:00 AM 11:00 AM
Domain walls in superconductors with broken time-reversal symmetry 1h
Possible domain structure in p-wave superconductors with broken time-reversal symmetry is intensively discussed in literature in connection with failed experimental attempts to detect stray magnetic fields in Sr2RuO4, which were theoretical predicted for these materials. This puts in question the very idea of p-wave pairing. The lecture starts from the short overview of the domain-structure concept in normal ferromagnets, then switches to domains in systems with coexistence of ferromagnetism and superconductivity (superconducting ferromagnets), and finally addresses domain structure and stray magnetic fields in p-wave superconductors, in which ferromagnetism originates from the electron orbital moment but not from spin.
Speaker: Edouard Sonin (Racah Inst. Hebrew University of Jerusalem) (Racah Inst. Hebrew University of Jerusalem)
• 11:00 AM 11:50 AM
On type-1.5 superconductivity 50m
Speaker: Mihail Silaev (KTH)