Date: April 25, 2018

Place: RIKEN, main building, room 435-437 (4th floor)

On the occasion of the visit to Tokyo of Meera Parish and Jesper Levinsen (Monash University), a one-day workshop is organised by Pascal Naidon (RIKEN), Yusuke Nishida (Tokyo Institute of Technology), Emiko Hiyama (RIKEN-Kyushu University) and Masahito Ueda (RIKEN - The University of Tokyo) to discuss the recent developments in ultra-cold atom theory done in Melbourne and Tokyo.

• 10:30-11:30 Meera Parish (Monash University)
  
  “Quantum dynamics of impurities coupled to a Fermi sea”
  Abstract
  The behaviour of a mobile impurity particle interacting with a quantum-mechanical medium is of fundamental importance in physics. Ultracold atomic gases have greatly improved our understanding of the impurity problem owing to the high degree of control over experimental parameters such as interactions and atom population. I will discuss recent theoretical and experimental progress in exploring the properties of impurities interacting with fermonic mediums. In particular, I will introduce a new theoretical approach for describing the coherent non-perturbative quantum evolution following a quench of the impurity-medium interactions [1,2].
  
  [1] M. Cetina et al., Science 354, 96 (2016).
  [2] M. M. Parish and J. Levinsen, Phys. Rev. B 94, 184303 (2016).

(lunch)

• 13:00-13:30 Takaaki Anzai (Tokyo Institute of Technology)
  
  “Two-dimensional Fermi gas in antiparallel magnetic fields”
• 13:30-14:00 Sho Higashikawa (The University of Tokyo)
  
  “Floquet chiral magnetic effect”
  Abstract
  We show that a single Weyl fermion can be realized as quasi-energy spectrum of a periodically driven three-dimensional lattice system, although it is prohibited in static systems by the Nielsen-Ninomiya theorem. In our driving protocol, reflecting the spin-momentum-locking nature of a Weyl fermion, a spin-polarized wave packet moves parallel to its spin direction. Furthermore, if a synthetic magnetic field is applied, a wave packet moves parallel to the magnetic field, leading to a Floquet realization of the chiral magnetic effect beyond the no-go theorem in equilibrium systems.

(break)

• 14:30-15:30 Jesper Levinsen (Monash University)
  
  “Universality of an impurity in a Bose-Einstein condensate”
  Abstract
  We consider the ground-state properties of an impurity particle (“polaron”) resonantly interacting with a Bose-Einstein condensate (BEC). Focusing on the equal-mass system, we use a variational wave function for the polaron that goes beyond previous work and includes up to three Bogoliubov excitations of the BEC, thus allowing us to capture both Efimov trimers and associated tetramers. We find that the length scale associated with Efimov trimers (i.e., the three-body parameter) can strongly affect the polaron’s behavior, even at densities where there are no well-defined Efimov states. However, by comparing our results with recent quantum Monte Carlo calculations, we argue that the polaron energy is a universal function of the Efimov three-body parameter for sufficiently low boson densities. We further support this conclusion by showing that the energies of the deepest bound Efimov trimers and tetramers at unitarity are universally related to one another, regardless of the microscopic model. On the other hand, we find that the quasiparticle residue and effective mass sensitively depend on the coherence length of the BEC, with the residue tending to zero as the coherence length diverges, in a manner akin to the orthogonality catastrophe.
  
  [1] S. M. Yoshida, S. Endo, J. Levinsen, and M. M. Parish, Phys. Rev. X 8, 011024 (2018)

(break)

• 16:00-16:30 Yuta Sekino (Tokyo Institute of Technology)
  
  “Universal properties of resonantly interacting systems in one dimension”
• 16:30-17:00 Yuto Ashida (The University of Tokyo)
  
  “Solving quantum spin-impurity problems in and out of equilibrium with variational approach”
  Abstract
  Quantum impurity problem is one of the most important paradigms in many-body physics. In this talk, I will explain how we can efficiently solve quantum spin-impurity problems in and out of equilibrium with variational approach. Employing a discrete symmetry in spin-impurity models, I will present a new canonical transformation that completely decouples the impurity and the bath. Combining it with fermonic Gaussian states, I will explain how we can use them to study ground-state and dynamical properties of a generic impurity model, and show benchmark results in Kondo-type models compared with MPS results. Finally, I will present a new type of nonequilibrium dynamics which should be challenging to obtain in previous approaches.
  
  Refs: YA, T. Shi, M. C. Bañuls, J. I. Cirac and E. Demler, arXiv:1801.05825; arXiv:1802.03861

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