Conference on Frontiers in Quantum Gases Liquids and Solids

9 Aug 2010, 08:00 → 20 Aug 2010, 17:00 Europe/Stockholm

NORDITA

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

This conference is a part of the NORDITA program on "Quantum solids, liquids, and gases" and will focus on frontiers in physics of quantum solids, liquids and gases (defined in a broad sense). This conference is supported by NORDITA and the Swedish Research Council.

Monday 9 August

1 Dr. Barnett, Ryan (Joint Quantum Institute): Quantum dynamics in ferromagnetic and antiferromagnetic condensates

TBA

Slides Barnett_Nordita2010.pdf

Tuesday 10 August

2 Prof. Dorsey, Alan (University of Florida): 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

3 Prof. Törmä, Päivi (Aalto University): 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 Torma_Nordita10PTorma.pdf

Wednesday 11 August

4 Dr. Parish, Meera (University of Cambridge): 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.

5 Prof. Zwierlein, Martin (Massachusetts Institute of Technology): TBA

TBA

Thursday 12 August

6 Prof. Nikolic, Predrag (George Mason University):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 Nikolic_cpins.pdf

7 Prof. Todadri, Senthil (Massachusetts Institute of Technology): Quantum spin liquids and the Mott transition

TBA

Dinner at Fågelängen

Dinner at Fågelängen (inside Albanova building, next to NORDITA)

Friday 13 August

8 Prof. Ran, Ying (Boston College):TBA

TBA

9 Prof. Gurarie, Victor (University of Colorado at Boulder):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 Prof. Radzihovsky, Leo (University of Colorado): 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 Radzihovsky_ffloNordita.pdf

11 Dr. Chung, Suk Bum (Stanford University): 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.

12 Mr. Mross, David (Massachusetts Institute of Technology):TBA

TBA

Tuesday 17 August

13 Prof. Raghu Srinivas (Rice University/Stanford): 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 $\Omega_0$.

14 Prof. Zlatko Tesanovic (Johns Hopkins University, Bloomberg Center): 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 Kjäll, Jonas (UC Berkeley): 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.

Slides Kjall_Norditatalkonline.pdf

Wednesday 18 August

16 Prof. Bruun, Georg (University of Aarhus): RF spectroscopy, polarons, and dipolar interactions in cold gases

TBA

Slides Bruun_NorditaTalk.pdf

17 Prof. Ueda, Masahito (Universit of Tokyo):Topological excitations in Bose-Einstein Condensation

TBA

Thursday 19 August

18 Prof. Svistunov, Boris (University of Massachusetts): Superfluid turbulence

TBA

Dinner at Fågelängen

Dinner in the resturant inside Albanova (building next to
Nordita)

Friday 20 August

19 Prof. Shlyapnikov, Georgy (LPTMS, Universite Paris Sud):New phases of fermionic dipolar gases

TBA