Name: Pushing the Boundaries with Cold Atoms
Start: 2013-01-21T08:00:00+01:00
End: 2013-02-15T18:00:00+01:00
Location: Nordita

Monday, 21 January 2013

10:30 Welcome!
Welcome!
10:30 - 10:50
Room: 132:028
10:50 Parity violating superfluidity in ultra-cold fermions under the influence of artificial non-Abelian gauge fields - Carlos Sa de Melo (Georgia Tech (USA))
Parity violating superfluidity in ultra-cold fermions under the influence of artificial non-Abelian gauge fields
- Carlos Sa de Melo (Georgia Tech (USA))
10:50 - 11:35
Room: 132:028 I discuss the creation of parity violating Fermi superfluids in the presence of non-Abelian gauge fields involving spin-orbit coupling and crossed Zeeman fields. I focus on spin-orbit coupling with equal Rashba and Dresselhaus (ERD) strengths which has been realized experimentally in ultra- cold atoms, but also discuss the case of arbitrary mixing of Rashba and Dresselhaus (RD) and of Rashba-only (RO) spin- orbit coupling. To illustrate the emergence of parity violation in the superfluid, I analyse first the excitation spectrum in the normal state and show that the generalized helicity bands do not have inversion symmetry in momentum space when crossed Zeeman fields are present. This is also reflected in the superfluid phase, where the order parameter tensor in the generalized helicity basis violates parity. However, the pairing fields in singlet and triplet channels of the generalized helicity basis are still parity even and odd, respectively. Parity violation is further reflected on ground state properties such as the spin-resolved momentum distribution, and in excitation properties such as the spin-dependent spectral function and density of states.
13:45 History of NORDITA - Chris Pethick (NORDITA)
History of NORDITA
- Chris Pethick (NORDITA)
13:45 - 14:00
Room: 132:028
14:00 Correlated Topological Phases: From Condensed-Matter Systems to Artificial Gauge Fields - Karyn Le Hur (Ecole Polytechnique Palaiseau)
Correlated Topological Phases: From Condensed-Matter Systems to Artificial Gauge Fields
- Karyn Le Hur (Ecole Polytechnique Palaiseau)
14:00 - 14:45
Room: 132:028 During the last decade, experiments have established the existence of unconventional states of matter in a variety of low-dimensional quantum systems. This includes equilibrium states characterized by topological properties as well as stationary states in and out of equilibrium situations. In this Talk, we focus on topological phases of matter, their experimental signatures, and possible ways of utilizing them as platforms for topologically protected quantum computation. With the important progress on the quantum control of light-matter interaction, one can now also engineer very tunable artificial complex quantum networks. We pedagogically introduce novel topological phases in correlated materials and artificial quantum networks, such as in cavity/circuit QED systems and cold atoms.
15:00 Coffee break
Coffee break
15:00 - 16:00
Room: 132:028
Tuesday, 22 January 2013
10:00 Long-lived repulsive and attractive polarons in a strongly interacting Fermi gas. - Georg Bruun (Aarhus University)
Long-lived repulsive and attractive polarons in a strongly interacting Fermi gas.
- Georg Bruun (Aarhus University)
10:00 - 10:45
Room: 132:028 I discuss the existence of long-lived repulsive as well as attractive polarons in a strongly interacting Fermi gas. The energy, lifetime, and quasiparticle residue of the polarons are calculated, and I show show how they accurately describe experimental data. Finally, I discuss possible consequences of these results regarding observing itinerant ferromagnetism in atomic gases
14:00 Finite temperature phase diagram of a spin-polarized Fermi gas in a dimensional crossover - Miikka Heikkinen (Aalto University)
Finite temperature phase diagram of a spin-polarized Fermi gas in a dimensional crossover
- Miikka Heikkinen (Aalto University)
14:00 - 14:30
Room: 132:028 We investigate exotic paired states of spin-polarized Fermi gases in the dimensional crossover between onedimensional and three-dimensional optical lattices. We compute the finite temperature phase diagram of the system along the dimensional crossover using real-space dynamical mean-field theory in combination with the continuous-time auxiliary field quantum Monte Carlo method. We find that the Fulde-Ferrell–Larkin-Ovchinnikov (FFLO) state can be realized for a broad range of parameters throughout the dimensional crossover. We study the melting of the FFLO state which involves the formation of a shell structure with a strong dependence on the dimensionality. Moreover, we investigate how the spectral function of the system is modified by the non-trivial spatial structure of the FFLO state.
14:30 Stability of spin liquid phases of alkaline earth atoms at Finite temperature - Gergely Szirmai (Wigner Research Centre of the Hungarian Academy of Sciences, Budapest)
Stability of spin liquid phases of alkaline earth atoms at Finite temperature
- Gergely Szirmai (Wigner Research Centre of the Hungarian Academy of Sciences, Budapest)
14:30 - 15:00
Room: 132:028 We describe the spin dynamics of the antiferromagnetic Mott insulator ground state of high spin fermions on a 2- dimensional hexagonal lattice. It was pointed out that such multicomponent systems in 1 and 2 dimensions can realize states without breaking the spin rotation symmetry when the number of components is large enough [1-3]. The low energy fluctuations on top of these so called spin liquid states are described by various gauge theories whose character depend on the symmetries of the mean-field solution [4]. Therefore high spin, ultracold, fermionic alkaline earth metal atoms loaded into optical lattices can serve as simulators of quantum gauge theories. Since in experiments with ultracold atoms it is a hard task to go to sufficiently low temperatures it becomes important to study the effects of finite temperature. We carry out the stability analysis of the mean-field solution and calculate the free energy at Finite temperature to determine the phase diagram relevant for experiments. References: 1. J. B. Marston, and I. Affleck, Phys. Rev. B 39, 11538, (1989). 2. M. Hermele, V. Gurarie, and A. M. Rey, Phys. Rev. Lett. 103, 135301 (2009). 3. G. Szirmai, E. Szirmai, A. Zamora, and M. Lewenstein, Phys. Rev. A 84, 011611 (2011). 4. X.-G. Wen, Quantum Field Theory of Many-Body Systems (Oxford University Press, 2004).
15:00 Coffee break
Coffee break
15:00 - 16:00
Room: 132:028
Wednesday, 23 January 2013
10:00 Ultracold atoms and neutron-rich matter in nuclei and in astrophysics - Achim Scwenk (TU Darmstadt/EMMI)
Ultracold atoms and neutron-rich matter in nuclei and in astrophysics
- Achim Scwenk (TU Darmstadt/EMMI)
10:00 - 10:45
Room: 132:028
14:00 Mobile impurities within a many-body quantum system - Adrian Kantian (University of Geneva)
Mobile impurities within a many-body quantum system
- Adrian Kantian (University of Geneva)
14:00 - 14:45
Room: 132:028 Advances in cold gases physics are enabling experiments involving the direct manipulation and observation of single- or few-atom mobile impurities [1,2] within a many-body quantum system, a topic of longstanding interest for condensed matter theory, where it is related to studies of e.g. conductivity and the X-ray edge problem. In light of these developments we study the dynamics of single mobile impurities in 1D quantum liquids, using analytical and DMRG techniques. We address the question of whether the recently proposed subdiffusive regime of impurity motion [3] constitutes a novel universality class of single particle excitations, one that is very different from that of the standard Tomonaga- Luttinger liquid excitations. We study the conditions for observing this regime and its' crossover to the ballistic regime. We furthermore examine the possibilities to observe the intermediate diffusive motion of impurities in these systems as well as various types of polaronic dynamics [2,4]. [1] J. Catani, G. Lamporesi, D. Naik et. al., Phys. Rev. A 85, 023623 (2012) [2[ T. Fukuhara, A. Kantian, M. Endres et. al., arXiv:1209.6468 [3] M. B. Zvonarev, V. V. Cheianov, T. Giamarchi, PRL 99, 240404 (2007); PRL 103, 110401 (2009) [4] F. Massel, A. Kantian, A. J. Daley et. al., arXiv:1210.4270
15:00 Coffee break
Coffee break
15:00 - 15:30
Room: 132:028

17:30 Reception
Reception
17:30 - 19:30
Room: 132:028
Thursday, 24 January 2013
10:00 Effects of spin-orbit coupling on the BCS-BEC crossover - Luca Salasnich (Department of Physics, University of Padova)
Effects of spin-orbit coupling on the BCS-BEC crossover
- Luca Salasnich (Department of Physics, University of Padova)
10:00 - 10:30
Room: 132:028 We study theoretically the effects of spin-orbit coupling on a two-spin-component ultracold atomic Fermi gas along the BCS-BEC crossover of a Feshbach resonance. We find that the condensate fraction of Cooper pairs characterizes the crossover better than other quantities, like the chemical potential or the pairing gap. We also find that, due to the spin-orbit coupling, in addition to singlet pairing, there is a finite triplet pairing. We predict that a large enough spin-orbit interaction enhances the singlet condensate fraction with respect to the triplet one in the BCS side while suppressing it on the BEC side. [1] L. Dell'Anna, G. Mazzarella, L. Salasnich, Phys. Rev. A 84, 033633 (2011). [2] L. Dell'Anna, G. Mazzarella, L. Salasnich, Phys. Rev. A 86, 053632 (2012).
10:30 BCS-BEC crossover in a quasi-2D Fermi gas - Andrea Fischer (University of Cambridge, Cavendish Laboratory)
BCS-BEC crossover in a quasi-2D Fermi gas
- Andrea Fischer (University of Cambridge, Cavendish Laboratory)
10:30 - 11:00
Room: 132:028 We consider a gas of fermionic atoms conned to a quasi- 2D geometry by a strong harmonic confinement potential in the transverse direction. For a two-component population balanced system, we construct a mean field theory for the BCS-BEC crossover, which correctly renormalises the s-wave contact interaction and allows infinitely many harmonic oscillator bands to be taken into account. The two band calculation can be done analytically giving the first order correction to the 2D results. However, we note that pairing is strongly modified by the presence of higher harmonic oscillator bands, even for weak interactions and Fermi energies much smaller than the confinement energy. We argue that recent experiments on pairing in quasi-2D Fermi gases [Y. Zhang et al., Phys. Rev. Lett. 108, 235302 (2012)] have already observed the expects of higher transverse levels.
14:00 Integrable pairing models in cold atom physics - Jorge Dukelsky (Instituto de Estructura de la Materia. CSIC.)
Integrable pairing models in cold atom physics
- Jorge Dukelsky (Instituto de Estructura de la Materia. CSIC.)
14:00 - 14:45
Room: 132:028 The exact solution of the SU(2) pairing Hamiltonian with non-degenerate single particle orbits was introduced by Richardson in the early sixties, although it was recovered in the last decade in an effort to describe the disappearance of superconductivity in ultrasmall grains. Since then it has been widely applied to mesoscopic systems where finite size effects play an important role. Lately we have extended this family of exactly solvable models to higher rank algebras to describe pairing between multi-component fermion systems like three and four color atomic gases. In this talk I will review some of the achievements in the application of the exactly solvable Richardson models to cold atomic gases. I will start with the description of the BCS-BEC crossover view from the exact Richardson wavefunction which defines uniquely the structure of the Cooper pairs from extended resonances in the BCS regime to tightly bound molecules in the BEC regime. Another recent development is associated to the implementation of the hyperbolic family of the SU(2) Richardson model to describe p-wave pairing. Using this new tool we study the quantum phase diagram of a spinless Fermi gas in a 2D optical lattice with px + i py pairing interaction symmetry. Unlike the case of s-wave pairing, which has a smooth a crossover between BCS and BEC, p-wave pairing displays a quantum phase transition separating two gapped superfluid phases known as weak-pairing and strong-pairing. We use the exact solution as well as mean-field to characterize the quantum phase transition and the properties of the two phases. Finally, as an example of the higher rank Richardson models, I will introduce the exact solution of an SO(6) Richardson model that describes pairing between three component atomic fermions. The study of this new exactly solvable model allowed us to unveil a complex structure of breached pairing phases. We find two competing superfluid phases at weak and intermediate couplings, each with two-color pair condensates that can be distinguished with density profile measurements.
15:00 Coffee break
Coffee break
15:00 - 16:00
Room: 132:028
Friday, 25 January 2013
10:00 Three-body problem in a two-dimensional Fermi gas - Jesper Levinsen (University of Cambridge)
Three-body problem in a two-dimensional Fermi gas
- Jesper Levinsen (University of Cambridge)
10:00 - 10:30
Room: 132:028 We investigate the three-body properties of two identical "up" fermions and one distinguishable "down" atom interacting in a strongly confined two-dimensional geometry. We compute exactly the atom-dimer scattering properties and the three-body recombination rate as a function of collision energy and mass ratio m_up/m_down. We find that the recombination rate for fermions is strongly energy dependent, similarly to what was found for two-dimensional identical bosons. For m_up < m_down, the s-wave atom-dimer scattering below threshold is completely described by the scattering length. Furthermore, we examine the "up-up- down" bound states (trimers) appearing at large m_up/m_down and find that the energy spectrum for the deepest bound trimers resembles that of a hydrogen atom confined to two dimensions.
10:30 Three-body states in a many-body background - Nikolaj Zinner (University of Århus)
Three-body states in a many-body background
- Nikolaj Zinner (University of Århus)
10:30 - 11:00
Room: 132:028 The famous prediction of Efimov [1] that an infinitude of three-body bound states appear in shortrange interacting three-dimensional systems when there is a two-body bound state at zero energy has generated a large amount of interest in the cold atomic gas community after its initial observation in 133Cs [2]. The theoretical description of these experiments have thus far used the vacuum formalism. However, current experiments are in a regime where the background energy scale (such as the Fermi energy in degenerate Fermi systems) can play a significant role. We demonstrate that while Efimov states can be strongly perturbed by the background, the original scaling ideas play a crucial role for the manner in which the states change. In fact, we find that scaling laws related to the background parameters emerges that should be observable in current experiments [3]. References: [1] Efimov, V. Weakly bound states of three resonantly interacting particles. Yad. Fiz. 12, 1080-1091 (1970); Sov. J. Nucl. Phys. 12, 589-595 (1971); [2] T. Kraemer et al., Evidence for Efimov quantum states in an ultracold gas of caesium atoms, Nature 440, pp. 315-318 (2006). [3] N. G. Nygaard and N. T. Zinner, The Fate of the Efimov Effect in a Many-Body World, arXiv:1110.5854
14:00 Suppression of the quantum-mechanical collapse by repulsive interactions - Boris Malomed (Tel Aviv University, Faculty of Engineering, Dept. of Physical Electronics)
Suppression of the quantum-mechanical collapse by repulsive interactions
- Boris Malomed (Tel Aviv University, Faculty of Engineering, Dept. of Physical Electronics)
14:00 - 14:45
Room: 132:028 The quantum-mechanical collapse (alias "fall onto the center" of particles attracted by potential -1/r^2) is a well- known issue in the elementary quantum theory. It is closely related to the so-called "quantum anomaly", i.e., breaking of the scaling invariance of the respective Hamiltonian by the quantization. We demonstrate that, in a rarefied gas of quantum particles attracted by the above- mentioned potential, the mean-field repulsive nonlinearity induced by collisions between the particles prevents the collapse, and thus puts forward a solution to the quantum- anomaly problem different from that previously developed in the framework of the linear quantum-field theory. This solution may be realized in the 3D or 2D gas of dipolar bosons attracted by a central charge, and also in the 2D gas of magnetic dipoles attracted by a current filament. In lieu of the collapse, the cubic nonlinearity creates a 3D ground state (GS), which does not exist in the respective linear Schroedinger equation. The addition of the harmonic trapping potential gives rise to a tristability, in the case when the Schroedinger equation still does not lead to the collapse. In the 2D setting, the cubic nonlinearity is not strong enough to prevent the collapse; however, the quintic term does it. The analysis is also extended to the 3D anisotropic setting, with the dipoles polarized by an external uniform field. Publications: H. Sakaguchi and B. A. Malomed, Suppression of the quantum-mechanical collapse by repulsive interactions in a quantum gas, Phys. Rev. A 83, 013907 (2011); H. Sakaguchi and B. A. Malomed, Suppression of the quantum collapse in an anisotropic gas of dipolar bosons, Phys. Rev. A 84, 033616 (2011).
15:00 Coffee break
Coffee break
15:00 - 16:00
Room: 132:028
Saturday, 26 January 2013
Sunday, 27 January 2013