Quantum Solids, Liquids, and Gases

19 Jul 2010, 08:00 → 27 Aug 2010, 18:00 Europe/Stockholm

Nordita

Christopher Pethick (Nordita), Egor Babaev (University of Massachussets Amherst), Emil Lundh (Umeå University), Jani-Petri Martikainen (Nordita), Mats Wallin (KTH)

This program will focus on frontiers in physics of quantum solids, liquids and gases (defined in a broad sense). The preliminary list of international participants: N.W. Ashcroft (Cornell) D. Agterberg (Wisconsin) A. Balatsky (LANL) Leon Balents (KITP UCSP) Eugene Demler (Harvard) M.P.A. Fisher (Microsoft Station Q) Victor Gurarie (Colorado) Tin-Lun Ho (Ohio State) David Huse (Princeton) Anatoly Kuklov (CUNY) W. Vincent Liu (Pittsburg) Joel Moore (Berkeley) Nikolay Prokof'ev (UMass) Leo Radzihovsky (Colorado) Sankar Das Sarma (Maryland) Gora Shlyapnikov (Paris) D.T. Son (U Washington) Boris Svistunov (UMass) Zlatko Tesanovic (Johns Hopkins U) Senthil Todadri (MIT) Mathias Troyer (ETH) Masahito Ueda (Tokyo) Ashvin Vishwanath (Berkeley). Daniel Agterberg (Wisconsin)

pictures nordita_logo_277x277.jpg

Slides Kjall_Norditatalkonline.pdf

Monday 19 July

09:15 → 14:00 Opening

14:00 → 14:15 Coffee break

14:15 → 15:15 From ultracold atoms to realization of strongly correlated condensed matter models

Progress in ultracold atomic physics allows to engineer and probe analogs of condensed matter systems. Using Feshbach resonance and optical lattices experimentalist are able to realize regimes where interactions between atoms play a crucial role. I will describe how dimensional reduction, in particular to 1D, is achieved by turning on an optical lattice and how interactions can be tuned using Feshbach resonance. For fermonic atoms and repulsive interactions, regimes such as the spin-coherent Luttinger liquid and the spin-incoherent Luttinger liquid can be realized by tuning the inter-atomic interaction strength and trap parameters. On the other hand, for attractive interactions, we study pairing in a spin-imbalanced ultracold atomic system of fermions to identify exotic states such as the 1D analog of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. In addition, I will also describe possibilities to study interplay between interaction and disorder in ultracold atomic systems. In particular, we investigate the damped hydrodynamic transport of a trapped Bose-Einstein condensate of bosonic atoms through a disordered potential created by an optical-speckle. Finally, I will summarize and address the concrete prospects for realizing and probing these phenomena experimentally using Feshbach resonances, optical lattices and optical speckles.

Speaker: Dr Paata Kakashivili (NORDITA)

Slides Kakashvili_19_07_2010.pdf

Tuesday 20 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Discrete reduced-symmetry solitons and second-band vortices in two-dimensional nonlinear waveguide arrays

Considering a two-dimensional lattice of weakly coupled waveguides,where each waveguide may carry two orthogonal modes of dipolar character, we present a nonlinear discrete vector model for the study of Kerr optical solitons with profiles having a reduced symmetry relative to the underlying lattice. We describe analytically and numerically existence and stability properties of such states in square and triangular lattices and also reveal directional mobility properties of two-dimensional gap solitons which were recently observed in experiment. The model also describes one-site peaked discrete vortices corresponding to experimentally observed second-band vortex lattice solitons, for which oscillatory instabilities are predicted. We also introduce a concept of rotational Peierls-Nabarro barrier characterizing the minimum energy needed for rotation of stable dipole modes and compare numerically translational and rotational energy barriers in regimes of good mobility. A similar model has also recently been proposed as a mean-field model for p-band bosons in optical lattices, describing quantum states in the superfluid regime.

Speaker: Dr Magnus Johansson (Linköping University)

Slides JohanssonMagnus.pdf

14:00 → 14:15 Coffee break

14:15 → 15:15 Superfluidity in Quantum Hall Bilayers

In this talk I will review some of the ideas and experiments suggesting that quantum Hall bilayers near filling factor 1 may form an exciton condensate state, leading to the possibility of superflow for counterflowing currents in the two layers. In real experiments, dissipationless transport appears to emerge, it at all, only at zero temperature, so that the observed superfluidity is at best imperfect. This appears likely to be due to disorder, which can flood the system with merons -- vortex-like excitations -- even at zero temperature. I will discuss a "coherence network" model which incorporates the presence of these objects in the groundstate, and qualitatively captures much of the physics observed in experiment. Finally I will discuss the results of drag experiments in this system, where interlayer coherence and quantum Hall physics conspire in such a way that one can probe different types of merons with measurements on different layers.

Speaker: Prof. Herbert Fertig (Indiana University)

Slides Fertig_nordita10.pdf

Wednesday 21 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Topological creation of vortices and monopoles in spin-1 Bose-Einstein condensates

I will give a review of the theoretical and experimental studies on the so-called topological vortex formation, in which a vortex-free spin-1 BEC is manipulated adiabatically using external magnetic fields into a state which contains a multiply quantized vortex. Even vortex pumping can be obtained to create deterministically vortices with high quantum numbers. Very recently, we extended this idea for a non-trivial three-dimensional case, in which Dirac monopoles can be created. This observation takes BEC to have high potential for the first experimental observation of Dirac monopoles, analogs of magnetic monopoles, in any quantum field.

Speaker: Dr Mikko Möttönen (Aalto University)

Slides Mottonen_Nordita_2010_p.pdf

14:00 → 14:15 Coffee break

14:15 → 15:15 Polarized atomic Fermi gases in optical lattices with elongated traps : a real-space dynamical mean-field theory

We study fermionic superfluidity of ultracold atom gases in optical lattices with elongated traps using the real-space dynamical mean-field theory (DMFT). First, the overview of DMFT and some details of our implementation of its real-space version will be given in the talk. Second, we will outline our recent progress in understanding the competition between the superfluidity and the spin polarization using our real-space DMFT code. We will discuss about the existence of critical polarization, the possibility of an exotic superfluid phase and the effect of the anisotropy of trapping potentials in three and two dimensional systems.

Speaker: Dr Dong-Hee Kim (Aalto University)

Slides Kim_nordita10_dmft.pdf

Thursday 22 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Magnetic Richtmyer-Meshkov instability in a two-component Bose-Einstein condensate

The magnetically induced Richtmyer-Meshkov instability in a two-component Bose-Einstein condensate is investigated. We construct and study analytical models describing the development of the instability at both the linear and nonlinear stages. The models indicate new features of the instability: the influence of quantum capillary waves and the separation of droplets, which are qualitatively different from the classical case. We perform numerical simulations of the instability in a trapped Bose-Einstein condensate using the Gross-Pitaevskii equation and compare the simulation results to the model predictions.

Speaker: Dr Alice Bezett (Umeå University)

14:00 → 14:15 Coffee break

14:15 → 15:15 p-orbital ultracold particles and Bose-Einstein crystal

The p-orbital band of optical lattices is a topic that emerged in recent few years of theoretical studies. Experimental observations of p-band BEC (reported by T. Mueller et al 2007 and G. Wirth et al 2010) make this topic an exciting one. In this talk, I will first try to explain why the system of ultracold particles in the lattice p-bands is conceptually intriguing and unique to cold atomic gases. Then I will report some interesting new quantum orbital phases that we have found, such as finite-momentum px+ipy Bose-Einstein condensates, staggered orbital currents, stripes of angular momenta, and incommensurate super-current density wave, as well as the realization of some condensed matter orbital models such as the quantum 120 degree model and the quantum dimer model. Work done in collaboration with X. Li, C. Lin, J. Moore, S. Das Sarma, K. Sun, V. M. Stojanovic,C. Wu, and E. Zhao.

Speaker: Prof. Vincent Liu (University of Pittsburgh)

Slides p-orbital_NORDITA2010-Liu.pdf

16:00 → 19:00 Barbeque

Barbeque outside NORDITA building

Friday 23 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Vortex Dynamics and Hall Conductivity of Hard Core Bosons

Speaker: Prof. Assa Auerbach (Technion (Haifa))

Slides Auerbach_Assa_Nordita2010.pptx

13:30 → 13:45 Coffee break

13:45 → 14:45 Recent Theory on FFLO and pair density wave superconductivity

With the groundbreaking work of Fulde, Ferrell, Larkin and Ovchinnikov (FFLO), it was realized that superconducting/superfluid order can also break translational invariance, leading to a phase in which the Cooper pairs develop a coherent periodic spatially oscillating structure. Such pair density wave (PDW) superconductivity/superfluidity has become relevant in a diverse range of systems, including cuprates, organic superconductors, heavy-fermion superconductors, cold atoms, and high-density quark matter. In this talk I discuss recent theoretical developments on PDW/FFLO phases. This will include both a discussion of the microscopic origin of such phases in materials lacking parity symmetry and a phenomenological description of these phases highlighting the role of fractional vortices and dislocations.

Speaker: Prof. Daniel Agterberg (University of Wisconsin - Milwaukee)

Slides Agterberg-nordita.ppt

Monday 26 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Ultracold gases in optical resonators

After a motivation why it is interesting to study cold atomic gases confined inside optical cavities, I will present a short summary of the experimental achievements up to date. I proceed by outlining the effective equations of motions, being nonlinear due to the intrinsic atom-field interaction, and discuss theoretical predictions that derive from them. I will focus on works that I have been involved in; optical bistability, Mott insulator states, collapse-revivals, and nonlinear dispersions.

Speaker: Dr Jonas Larson (Stockholm University)

13:30 → 13:45 Coffee break

13:45 → 14:45 Superconductivity at very high magnetic fields in ferropnictides. The effect of pairing symmetry and impurity scattering

An overview of recent results on the unusual behavior of ferropnictide superconductors under strong magnetic fields is given. The talk focuses on the extremely high Hc2 values, well above the BCS paramagnetic limit, anisotropy of vortex properties and manifestations of the anomalous temperature dependencies of Hc2(T) along different crystallographic directions, multiband superconductivity, and the effect of impurity scattering in different pairing scenarios. The role of anisotropic vortex fluctuations and their effect on the resistive transition and the irreversibility field in pnictides will be addressed. We also discuss our recent results on the Kondo effect induced by -particle irradiation of Nd(FeAs))OF single crystals and the resulting anomalously weak suppression of Tc by magnetic defects inconsistent with current theoretical models.

Speaker: Prof. Alex Gurevich (NHMFL, Florida State University)

Tuesday 27 July

10:40 → 11:00 Coffee break

13:30 → 13:45 Coffee break

13:45 → 14:45 Generation and dynamics of vortices in a superfluid unitary Fermi gas

The unitary Fermi gas has emerged as one of the most fascinating objects of study in the last decade with an wide impact on various physics sub-fields, from nuclear physics and astrophysics, AdS/CFT, to condensed matter physics. A unitary Fermi gas has remarkable properties, among them: the highest critical temperature known of a superfluid system (in appropriate units), the highest critical Landau velocity, a pseudogap phase and a FFLO phase. I will first describe briefly the nature and main properties of unitary gas. This will be followed by a description of an extension of the Density Functional Theory to superfluid systems, both in its static and time-dependent from, its validation and verification, and finally a number of simulations of vortex generation and dynamics in such systems, which reveal some unexpected features, among them the possibility of superflow with supercritical Landau velocity.

Speaker: Prof. Aurel Bulgac (University of Washington)

Slides Bulgac_Stockholm_2010.pdf

15:00 → 16:15 Pedagogical introduction to multicomponent unconventional superconductors

This talk will discuss symmetry based Ginzburg-Landau theories for unconventional superconductors with an emphasis on multicomponent order parameters. This will begin with an introduction to the group theory needed for Ginzburg Landau theory. This will be followed by an overview of the derivation of the Ginzburg Landau phenomenological coefficients from weak-coupling BCS theory. The example of a p+ip spin-triplet superconductor will be discussed in detail.

Speaker: Prof. Daniel Agterberg (University of Wisconsin - Milwaukee)

Slides Agterberg-Lectures.pdf

Wednesday 28 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Dissipative dynamics of magnetic solitons in metallic ferromagnets

We develop the hydrodynamic theory of collinear spin currents coupled to magnetization dynamics in spin-textured, metallic ferromagnets. Our semi-phenomenological theory captures a wide range of magneto-electric phenomena, including a dissipative force generated by magnetization dynamics and the Onsager reciprocal "beta" spin torque. Furthermore, we find that electronic dynamics gives rise to a non-local Gilbert damping tensor in the Landau-Lifshitz-Gilbert equation for the magnetization. Applying our hydrodynamic equations to soliton dynamics, we find that soliton motion generate electrical currents, which produce back-action through spin torques. As an example, we consider vortex gyration in a point-contact spin valve, and find modifications to orbit radius, frequency, and dissipation power.

Speaker: Dr Clement Wong (UCLA)

Slides Wong_Clement_nordita.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Complex Phases for Systems with Competing Repulsive and Attractive Interactions: Implications for Vortex Matter and Charge Ordering Systems

We examine the properties of systems of particles which have competing repulsive and attractive interactions as a function of density and temperature. Such interactions can arise in type-1.5 superconductors, magnetic superconductors, stripes in 2D electron gas systems, high temperature superconductors, colloidal particles, and even dense nuclear matter. For systems with long range repulsion and short range attraction as a function of increasing density, we find transitions between a low density clump phase, an intermediate stripe phase, an anticlump phase, and a high density uniform phase. These phases can exhibit a multistep melting process in which a state appears which has liquidlike structure at short length scales but at long length scales the solidlike stripe or clump stuctures remain intact. At higher temperatures the larger scale structures also melt. We examine the effects of quenched disorder on these systems and find isotropic and anisotropic responses under an applied drive. I will also show results for systems of vortex matter with competing interactions and will discuss the static and dynamic phases that occur in this system with and without a pinning substrate.

Speaker: Prof. Charles Reichhardt

15:00 → 16:00 Pedagogical introduction to multicomponent unconventional superconductors PART II

This talk will discuss symmetry based Ginzburg-Landau theories for unconventional superconductors with an emphasis on multicomponent order parameters. This will begin with an introduction to the group theory needed for Ginzburg Landau theory. This will be followed by an overview of the derivation of the Ginzburg Landau phenomenological coefficients from weak-coupling BCS theory. The example of a p+ip spin-triplet superconductor will be discussed in detail.

Thursday 29 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Bergman, Doron (California Institute of Technology): Metallic twist on topological insulators

TBA

13:30 → 13:45 Coffee break

13:45 → 14:45 Quantum criticality, high Tc superconductivity and the AdS/CFT correspondence of string theory

The central mystery in quantum matter is the general nature of matter formed from fermions. The methods of many body quantum physics fail and one can only rely on the phenomenological Fermi-liquid and BCS theories. However, in heavy fermion systems and cuprates one deals with non Fermi-liquid quantum critical metals, and to understand the superconductivity one needs to understand these normal states first. Remarkably, it might well be that the mathematics of string theory is capable of describing such states of fermion matter. The AdS/CFT correspondence translates this problem into an equivalent general-relativity problem involving the propagation of classical fields in an Anti-de-Sitter space-time with a black hole in its center. This development started with the demonstration that AdS/CFT predicts correctly the low viscosity of the quark-gluon plasma of the Brookhaven heavy ion collider. In 2007 it was realized that it could have relevance to high Tc superconductors [1] but only last year the focus shifted to the way AdS/CFT processes fermions, creating much excitement: it appears that both emergent heavy Fermi-liquids [2] and non Fermi-liquids can be gravitationally encoded, as well as ‘holographic’ superconductors having suggestive traits in common with the real life high Tc variety [3]. 1) J. Zaanen, Nature 448, 1000 (2007).,2) M. Cubrovic, J. Zaanen and K. Schalm , Science 325, 439 (2009).,3) J. Zaanen, Nature 462, 15 (2009).

Speaker: Prof. Jan Zaanen (Institituut Lorentz for Theoretical Physics, Leiden University)

16:00 → 19:00 Reception: something to eat and drink

Thursdays get-together

Friday 30 July

10:40 → 11:00 Coffee break

11:00 → 12:00 Nernst effect as a probe of broken symmetries in the normal state of cuprate materials

Experiments on underdoped cuprate superconductors suggest an intricate relation between the normal-state Nernst effect and stripe order: The Nernst signal appears enhanced near 1/8 hole doping and its onset temperature scales with the stripe-ordering temperature over some range of doping. Here, we analyze the thermoelectric response inlayered metals with spontaneously broken rotation ortranslation symmetry. For broken rotation symmetry we identify the anisotropy of the quasiparticle Nernst signal as an extremely sensitive probe of Fermi surface distortions characteristic of the ordered state. Applied to recent experiments, our results reinforce the proposal that the underdoped cuprate superconductor YBCO displays such "electron-nematic" order in the pseudogap regime. Furthermore, we find that Fermi pockets caused by translational symmetry breaking lead to a strongly enhanced Nernst signal with a sign depending on the modulation period of the ordered state and other details of the Fermi surface. We compare our findings with recent data from Nd-LSCO and YBCO.

Speaker: Dr Andreas Hackl (California Institute of Technology)

Slides Hackl_Stockholm.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Spin-orbit-coupled Bose-Einstein Condensates

TBA

Speaker: Prof. Victor Galitski (Joint Quantum Insitute, University of Maryland)

Monday 2 August

10:40 → 11:00 Coffee break

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Kuklov, Anatoly: Superclimb of dislocations in supersolids

TBA

Tuesday 3 August

10:40 → 11:00 Coffee break

11:00 → 12:00 Watanabe, Gentaro: Superfluid unitary Fermi gases in a 1D optical lattice --- thermodynamic properties and critical velocity

A recent experiment with Fermi superfluids in one-dimensional (1D) optical lattices has revealed that superfluidity is particularly robust at unitarity [1]. Motivated by this experiment, we study the effects of a 1D optical lattice on the thermodynamic properties [2] and on the critical velocity for the Landau (energetic) instability [3] of superfluid unitary Fermi gases. We show that, the inclusion of a 1D optical lattice, by favoring the formation of molecular configurations and by inducing a band structure in the quasiparticle spectrum, has profound consequences on the thermodynamic quantities, the density profile, and the collective oscillations of the unitary Fermi gas [2]. Regarding the Landau critical velocity, we have derived an analytical expression, which is applicable for both bosonic and fermionic superfluids in hydrodynamic regime flowing through any shape of the external potential [3]. We also find that the behavior of the critical velocity in the presence of a periodic potential is very different from that of a single barrier [3]. In the last part of this presentation, our recent results about the loop structure of the energy band of superfluid Fermi gases are also given. References: [1] D. E. Miller et al., Phys. Rev. Lett. 99, 070402 (2007). [2] GW, Orso, Dalfovo, Pitaevskii, and Stringari, Phys. Rev. A 78, 063619 (2008). [3] GW, Dalfovo, Piazza, Pitaevskii, and Stringari, Phys. Rev. A 80, 053602 (2009).

Slides gentaro_watanabe.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Kuklov, Anatoly: Quantum phases of flexible quasi-molecular chains of dipolar molecules in optical lattice

We consider bosonic dipolar molecules in layered optical lattice and demonstrate possibility of formation of quantum rough flexible chains (perpendicular to the layers) by ab initio MC simulations. BKT transition into a stiff state for a single chain is observed. Results of preliminary J-current-type model simulations as well as of the bosonization analysis of multi-chain ensemble are discussed.

Wednesday 4 August

10:40 → 11:00 Coffee break

11:00 → 11:30 Cetoli, Alberto: Correlations and superfluidity of a one-dimensional Bose gas in a quasiperiodic potential

TBA

Slides Cetoli_slides.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Pitaevskii, Lev: On the general theory of solitons

TBA

Thursday 5 August

10:40 → 11:00 Coffee break

11:00 → 12:00 Burkov, Anton: Incompressible vortex liquids and Mott insulators in two dimensions

TBA

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Polkovnikov, Anatoli: Phase space representation of quantum dynamics

In this talk I will discuss representation of quantum dynamics in classical phase space. This representation is based on the perturbative expansion of dynamics in the powers of the effective Planck's constant (saddle point parameter). I will explicitly discuss dynamics in the coordinate-momentum and the coherent state (number-phase) representations. I will show how such concepts as Wigner function, Weyl quantization, Moyal brackets, Bopp operators and others autmatically follow from the Feynmann's path integral representation of quantum evolution.

Slides Polkovnikov_nordita_wigner.pptx

16:00 → 19:00 Reception: something to eat and drink

Friday 6 August

10:40 → 11:00 Coffee break

13:30 → 13:45 Coffee break

13:45 → 14:45 Dr. Turner, Ari: Vortex Clusters in Condensates

Superfluids made from atoms with spins can form many different types of vortices, unlike superfluid helium, which can support only ordinary vortices. Ordinary vortices are surrounded by a stable flow of atoms; the additional types of vortices also are wreathed by spin patterns. The more symmetry the condensate has, the greater the variety of vortices. An imperfection in the symmetry causes some of types vortices to form molecules, according to the laws of ``vortex chemistry." I will explain two applications: one application predict metastable vortex molecules; the other predicts Newtonian physics (instead of Magnus force dynamics) for a spin-current vortex.

Monday 9 August

10:40 → 11:00 Coffee break

13:30 → 13:45 Coffee break

13:45 → 14:45 Dr. Barnett, Ryan: Quantum dynamics in ferromagnetic and antiferromagnetic condensates

In this talk I will first give an overview of the field spinor condensates, explaining how they exhibit many phenomena not found in scalar condensates. I will then discuss dynamics in spin-one 23Na and 87Rb condensates, addressing recent experiments at NIST and Berkeley respectively. For the antiferromagnetic 23Na system, we consider tight traps so that the dynamics occur only in the spin degrees of freedom. I will describe an exact mapping of the system onto a quantum rotor model, and show how this sheds light on its dynamics. I will also describe how this system is a natural candidate to observe collapse and revival phenomena. I will then move on to discuss ferromagnetic condensates in larger traps. For this, we applied the truncated Wigner approximation to the spinor system with all spatial and spin degrees of freedom. We will use this as a tool to address the possibility of thermalization at long times.

Slides Barnett_Nordita2010.pdf

Tuesday 10 August

10:40 → 11:00 Coffee break

11:00 → 12:00 Prof. Dorsey, Alan: Low temperature properties of solid 4He: Supersolidity or quantum "metallurgy"?

A "supersolid" is a putative phase of matter possessing the distinguishing property of a solid--a nonzero shear modulus--together with Bose condensation. Numerous experiments over the last six years have yielded hints of supersolid behavior in solid 4^He, but the threads of these investigations have not produced a consistent interpretation. I'll briefly review some of the history of the subject, the recent experimental and theoretical work, and conclude with an overview of my own work on phenomenological modeling of defects in solid 4He.

Slides Dorsey_nordita_seminar_august_2010.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Törmä, Päivi: Imbalanced Fermi gases: the FFLO state, polarons, and the Josephson effect

In this talk, I will discuss three topics. First, the FFLO phase in one dimensional optical lattice and a direct way to observe it as narrowing of the hopping modulation spectrum. Second, I discuss our work on polaron-type physics in one dimension where we can explain the results of exact simulations by a polaron ansatz in one the weakly interacting and by a spinless Fermion solution given by the Bethe ansatz in the strongly interacting limit. This corresponds to the polaron-molecule crossover in three dimensions. Finally, I present a novel type of Josephson effect where the components of the Cooper pair feel a different potential (voltage), which is possible to realize in ultracold gases. We show that this leads to spin-asymmetric Josephson oscillations and provide an explanation of this intriguing phenomenon which also gives new information about the traditional Josephson effect.

Slides Nordita10PTorma.pdf

Wednesday 11 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Dr. Parish, Meera: Polarons, molecules and trimers in polarized atomic Fermi gases

In this talk, I will consider an atomic Fermi gas in the limit of extreme spin imbalance, where one has a single spin-down impurity atom interacting attractively with a spin-up atomic Fermi gas. By constructing variational wave functions for polarons, molecules and trimers, I will explore the quantum phase transitions between each of these bound states as a function of mass ratio and interaction strength. I will show that Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing is mostly superceded by the formation of a p-wave trimer, which can be viewed as a FFLO molecule that has bound an additional majority atom. When the mass of impurity atom is sufficiently light, I find that these transitions lie outside the region of superfluid-normal phase separation in spin-imbalanced Fermi gases and should thus be observable in experiment, unlike the well-studied equal-mass case.

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Zwierlein, Martin: TBA

TBA

Thursday 12 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Nikolic, Predrag:Unitarity in periodic potentials: a renormalization group analysis

We explore the universal properties of interacting fermionic lattice systems, mostly focusing on the development of pairing correlations from attractive interactions. Using renormalization group we identify a large number of fixed points and show that they correspond to resonant scattering in multiple channels. Pairing resonances in finite-density band insulators occur between quasiparticles and quasiholes living at different symmetry-related wavevectors in the Brillouin zone. This allows a BCS-BEC crossover interpretation of both Cooper and particle-hole pairing. We show that in two dimensions the run-away flows of relevant attractive interactions lead to charged-boson-dominated low energy dynamics in the insulating states, and superfluid transitions in bosonic mean-field or XY universality classes. Analogous phenomena in higher dimensions are restricted to the strong coupling limit, while at weak couplings the transition is in the pair-breaking BCS class. The models discussed here can be realized with ultra-cold gases of alkali atoms tuned to a broad Feshbach resonance in an optical lattice, enabling experimental studies of pairing correlations in insulators, especially in their universal regimes. In turn, these simple and tractable models capture the emergence of fluctuation-driven superconducting transitions in fermionic systems, which is of interest in the context of high temperature superconductors.

Slides Predrag_cpins.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Todadri, Senthil: Quantum spin liquids and the Mott transition

TBA

17:00 → 20:00 Dinner at Fågelängen

Fågelängen is inside the Albanova building (next to Nordita)

Friday 13 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Ran, Ying:TBA

TBA

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Gurarie, Victor:SU(N) magnetism with cold atoms and chiral spin liquids

Certain cold atoms, namely the alkaline earth-like atoms whose electronic degrees of freedom are decoupled from their nuclear spin, can be thought of as quantum particles with an SU(N)-symmetric spin. These have recently been cooled to quantum degeneracy in the laboratories around the world. A new world of SU(N) physics has thus become accessible to experiment, including that described by the SU(N) Hubbard model in various dimensions as well as many others. We show that the Mott insulator of such cold atoms is a SU(N) symmetric antiferromagnet of the type not commonly studied in the literature. We further show that in 2 dimensions, this antiferromagnet is a chiral spin liquid, a long sought-after topological state of magnets, with fractional and non-Abelian excitations.

Slides Gurarie_Nordita_2010.pdf

Monday 16 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Radzihovsky, Leo: Fluctuations, stability, and phase transitions of Larkin-Ovchinnikov states: quantum liquid crystals

Motivated by polarized Feshbach-resonant atomic gases, I will discuss the nature of low-energy fluctuations in the putative Larkin-Ovchinnikov (LO) state. Because the underlying rotational and translational symmetries are broken spontaneously, this gapless superfluid is a quantum smectic liquid crystal, that exhibits fluctuations that are qualitatively stronger than in a conventional superfluid, thus requiring a fully nonlinear description of its Goldstone modes. Consequently, at nonzero temperature the LO superfluid is an algebraic phase even in 3d. It exhibits half-integer vortex-dislocation defects, whose unbinding leads to transitions to a superfluid nematic and other phases. In 2d at nonzero temperature, the LO state is always unstable to a charge-4 (paired Cooper-pairs) nematic superfluid. I expect this superfluid liquid-crystal phenomenology to be realizable in imbalanced resonant Fermi gases trapped isotropically.

Slides RadzihovskyffloNordita.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Dr. Chung, Suk Bum: Half-quantum vortices in p_x + ip_y superconductors

Half-quantum vortices, each with flux of h/4e, are needed to realize topological quantum computation in a p+ip superconductor. However, until recently, there had not been any clear experimental observation of such vortices. We point out, although the magnetic energy is reduced by breaking full vortices into half-quantum vortices, there is an energy cost (which diverges with system size) due to the unscreened spin current and the spin state locking. The recent observation of half-quantum vortices by the Budakian group can be best explained by the fact that the magnetic energy savings can dominate over the spin energy cost in a mesoscopic setting. A finite density vortex lattice may have similar energetics, leading to a lattice of half-quantum vortices. Lastly we show that there can be entropy driven dissociation of a full vortex into two half-quantum vortices.

15:15 → 16:15 Mr. Mross, David:TBA

TBA

Tuesday 17 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Raghu Srinivas: Superconductivity in the repulsive Hubbard model: an asymptotically exact weak coupling solution

We study the phase diagram of the Hubbard model in the limit where U, the onsite repulsive interaction, is much smaller than the bandwidth. We present an asymptotically exact expression for T$_c$, the superconducting transition temperature, in terms of the correlation functions of the non-interacting system which is valid for arbitrary densities so long as the interactions are sufficiently small. Our strategy for computing T$_c$ involves first integrating out all degrees of freedom having energy higher than an unphysical initial cutoff $\Omega_0$. Then, the renormalization group (RG) flows of the resulting effective action are computed and T$_c$ is obtained by determining the scale below which the RG flows in the Cooper channel diverge. We prove that T$_c$ is independent of $\Omega_0$.

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Zlatko Tesanovic: Recent Developments in High-Temperature Superconductivity: Pnictides versus Cuprates

Two years ago, the discovery of high-temperature superconductivity in iron-pnictides reshaped the landscape of condensed matter physics. Until that time, for more than two decades, the copper-oxide materials were the only game in town and their mysterious properties loomed large as perhaps the greatest intellectual challenge in our field. Cuprates are strongly interacting systems, near to the so-called Mott insulating limit, in which electrons are made motionless by strong correlations, and it is currently believed that much of their unusual behavior stems from such correlations. In contrast, the newly discovered iron-based high-temperature superconductors exhibit a more moderate degree of correlations and do not appear to be near the Mott limit. Consequently, some of their properties might be easier to understand. In this talk, the basic ideas in theory of iron- pnictides will be introduced and illustrated with experimentally-relevant examples. Particular attention will be paid to the interband resonant-pairing mechanism of multiband superconductivity and the renormalization group description of the underlying physics. This will be contrasted with strongly correlated cuprates, where a thousand fancy theoretical ideas bloom, from quantum fluctuations to Berry phases, from gauge field theory to AdS/CMT duality. But we will never lose touch with reality and promise to keep a watchful eye on recent and sometimes conflicting experiments.

15:15 → 16:15 Kjäll, Jonas: Bound states with E8 symmetry in quantum Ising-like chains

In a recent experiment on CoNb2O6, Coldea et. al. found for the first time experimental evidence of the exceptional Lie algebra E8. The emergence of this symmetry was theoretically predicted long ago for the transverse quantum Ising chain in the presence of a weak longitudinal field. We consider an accurate microscopic model of CoNb2O6 and calculate numerically the dynamical structure function using a recently developed matrix-product state based method. The excitation spectra contain bound states which are characteristic to the E8 symmetry. We furthermore compare the observed bound states to the ones found in the transverse Ising chain in a longitudinal field.

Wednesday 18 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Bruun, Georg: RF spectroscopy, polarons, and dipolar interactions in cold gases

TBA

Slides BruunNorditaTalk.pdf

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Ueda, Masahito:Topological excitations in Bose-Einstein Condensation

TBA

Thursday 19 August

10:45 → 11:00 Coffee break

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Svistunov, Boris: Superfluid turbulence

TBA

17:00 → 19:00 Dinner at Fågelängen

Dinner in the restaurant inside Albanova (next to Nordita)

Friday 20 August

10:45 → 11:00 Coffee break

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Shlyapnikov, Georgy:New phases of fermionic dipolar gases

TBA

Monday 23 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Nikolay Prokofiev: The solution of the dirty-boson problem

I will discuss the theorem of inclusions which makes important rigorous statements about phase transitions in disordered systems and how it applies to the phase diagram of the disordered three-dimensional Bose-Hubbard model at unity filling which has been controversial for many years. The theorem of inclusions states that transitions between fully gaped and superfluid phases are forbidden and there must exist an intermediate gapless insulating phase. The other result is that all transitions between gapfull and gapless phases have to be of the Griffiths type when the vanishing of the gap at the critical point is due to a zero concentration of rare regions where extreme fluctuations of disorder mimic a regular gapless system. I will also explain the vortex phase mechanism governing the shape of the phase diagram in the vicinity of the diagram tip in d=1,2. A highly non-trivial overall shape of the phase diagram in d=3 is revealed with the worm algorithm; it features a long superfluid finger at strong disorder and on-site interaction. Moreover, bosonic superfluidity is extremely robust against disorder in a broad range of interaction parameters; it persists in random potentials nearly 50 (!) times larger than the particle half-bandwidth. Finally, we comment on the feasibility of obtaining this phase diagram in cold-atom experiments, which work with trapped systems at finite temperature.

Slides Prokovief_Disorder.ppt

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Immanuel Bloch: Non-equilibrium Dynamics and Single Site Addressing in Strongly Interacting Quantum Gases in Optical Lattices

Ultracold quantum gases offer novel and intriguing possibilities to probe the dynamical evolution of strongly correlated quantum systems far from equilibrium. We report on recent experiments, in which we have analyzed the transport behaviour of fermionic quantum gases in an optical lattices. We find three distinct transport regimes for non-interacting, weakly- and strongly-interacting quantum gas mixtures for both attractive and repulsive interactions. In a second series of experiments we probe the quantum dynamics of 1D ladder systems far from equilibrium. Here we investigate generalized Landau-Zener transitions between two Luttinger liquids, for which we find striking effects of ground state phase transitions that manifest in the dynamical evolution of the system. Finally, we report on fluorescence imaging of strongly interacting bosonic Mott insulators in an optical lattice with single-atom and single-site resolution. From our images, we fully reconstruct the atom distribution on the lattice. Imaging lattice quantum gases with single site and single atom resolution offers unprecedented novel opportunities for the analysis of and the dynamics of strongly correlated quantum systems.

Tuesday 24 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Dr. Annica Black-Schaffer: Superconductivity and magnetism in graphene from electronic correlations

While not yet widely appreciated, electronic correlations appear to play an important role in graphene. In fact, already Pauling’s resonance valence bond theory established that nearest-neighbor spin-singlet bond (SB) correlations are important in pp-bonded planar organic molecules of which graphene is the infinite extension. Through the use of a phenomenological Hamiltonian, which includes SB correlations, we show that a superconducting time-reversal symmetry breaking d-wave state is possible at finite doping in graphene. DFT calculations are then used to study the charge transfer between graphene and sulfur, demonstrating that this d-wave superconducting state should be achievable in graphite-sulfur structures. We also show that in a d-wave contact SNS graphene Josephson junction the effects of the SB correlations are large even high above Tc. We therefore propose that these junctions will provide a promising experimental system for measuring the effective strength of the intrinsic SB correlations. In addition, we study the magnetic properties of the same phenomenological Hamiltonian in undoped graphene and we show that the SB correlations significantly enhance the RKKY coupling between two impurity magnetic moments. When matching our results to recent DFT calculations we not only establish that electronic correlations are essential to properly account for the behavior of the RKKY coupling but we also extract a surprisingly large value of the SB coupling constant, indicating that undoped graphene is possibly very close to an antiferromagnetic instability.

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Chris Foot: Ultracold atoms in a rotating optical lattice

We have observed vortex nucleation in a rotating optical lattice. A Bose-Einstein condensate, of Rb-87 atoms, was loaded into a static two-dimensional lattice and the rotation frequency of the lattice was then increased from zero. We have studied how vortex nucleation depends on the optical lattice depth and rotation frequency. For deep lattices above the chemical potential of the condensate we observed a linear dependence of the number of vortices created with the rotation frequency, even below the thermodynamic critical frequency required for vortex nucleation. At these lattice depths the system formed an array of Josephson-coupled condensates. The effective magnetic field produced by rotation introduced characteristic relative phases between neighbouring condensates, such that vortices were observed upon ramping down the lattice depth and recombining the condensates. Future work towards direct quantum simulation (DQS) of frustrated antiferromagnets, and the observation of strongly correlated states of bosons analogous to those of electrons in the Fractional Quantum Hall Efffect will also bediscussed.

15:00 → 15:30 Dr. Lundh, Emil: Mott insulator dynamics

I present some recent simulations of time-dependent phenomena in bosons in optical lattices. For a system with a Mott-superfluid interface, an instability reminiscent of the Kelvin-Helmholtz instability is seen to occur. In a trap, this instability is seen to be responsible for phase slip.

Speaker: Dr Emil Lundh (Umeå University)

Wednesday 25 August

11:00 → 12:00 Prof. Egor Babaev: Type 1.5 superconductors

TBA

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Yip, Sungkit: Bose-Einstein Condensation under an artificial spin-momentum interaction

TBA

Thursday 26 August

10:45 → 11:00 Coffee break

11:00 → 12:00 Prof. Zohar Nussinov: Orbital solids and liquids and orbital order driven quantum criticality

I will briefly review the rudiments of orbital order physics and then discuss several more recent results. Orbitals are described by SU(2) operators but unlike spins the orbitals live in real space and thus their interactions are highly frustrated. It will be shown that spatial orders of electronic orbital states in a crystal can be triggered by thermal fluctuations alone (contrary to earlier lore, no zero point quantum fluctuations are necessary to stabilize orders). It will be further shown how symmetry considerations alone give rise to dimensional reduction as well as topological order and selection rules that may be experimentally tested from scattering measurements. I will illustrate that in addition to charge and spin order driven quantum critical points, Orbital Order Driven Quantum Criticality, as a matter of principle, also occur in the simplest orbital Hamiltonians. New predicted orbital nematic orders will be demonstrated in some of the best known orbital dependent Hamiltonians.

13:30 → 13:45 Coffee break

13:45 → 14:45 Prof. Zohar Nussinov: Topological quantum orders and duality mappings via symmetry and algebraic considerations

We show how symmetries can lead to a dimensional reduction and topological quantum orders and discuss the effects of thermal fluctuations. We will further show how exact dualities as well as duality relations that appear only in a sector of certain theories ("emergent dualities") can be systematically derived. Our method relies on the use of bond algebras wherein the algebraic relations between all terms in the Hamiltonian are examined. This method enables us to solve exactly several quantum spin systems in high dimensions and to determine the exact self dual points of all $Z_{N}$ lattice gauge theories in 3+1 dimensions. Implications for quantum information will be discussed.

Friday 27 August

13:45 → 14:15 M. Silaev: Fractional vortices and a Bean-Livingston barrier in two-component superconductors

TBA

Slides Silaev_TalkNordita.pdf