Research Project: QUANTUM PHASES OF FERMI-FERMI, BOSE-BOSE AND BOSE-FERMI MIXTURES OF ATOMIC GASES AT ULTRACOLD TEMPERATURES
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Contributors
Funders
ID
EC.00022
Authors
Işkın, Menderes
Faculty Member
Publications
Stability of spin-orbit coupled Fermi gases with population imbalance
(American Physical Society (APS), 2011) Işkın, Menderes; Subaşı, A. L.; Department of Physics; Yes; College of Sciences
We use the self-consistent mean-field theory to analyze the effects of Rashba-type spin-orbit coupling (SOC) on the ground-state phase diagram of population-imbalanced Fermi gases throughout the BCS–Bose-Einstein condensate evolution. We find that the SOC and population imbalance are counteracting, and that this competition tends to stabilize the uniform superfluid phase against the phase separation. However, we also show that the SOC stabilizes (destabilizes) the uniform superfluid phase against the normal phase for low (high) population imbalances. In addition, we find topological quantum phase transitions associated with the appearance of momentum-space regions with zero quasiparticle energies, and study their signatures in the momentum distribution.
Vortex line in spin-orbit coupled atomic Fermi gases
(American Physical Society (APS), 2012) Işkın, Menderes; Department of Physics; Yes; College of Sciences
It has recently been shown that the spin-orbit coupling gives rise to topologically nontrivial and thermodynamically stable gapless superfluid phases when the pseudospin populations of an atomic Fermi gas are imbalanced, with the possibility of featuring Majorana zero-energy quasiparticles. In this paper, we consider a Rashba-type spin-orbit coupling, and use the Bogoliubov-de Gennes formalism to analyze a single vortex line along a finite cylinder with a periodic boundary condition. We show that the signatures for the appearance of core-and edge-bound states can be directly found in the density of single-particle states and particle-current density. In particular, we find that the pseudospin components counterflow near the edge of the cylinder, the strength of which increases with increasing spin-orbit coupling.
Cooper pairing and BCS-BEC evolution in mixed-dimensional Fermi gases
(American Physical Society (APS), 2010) Işkın, Menderes; Department of Physics; Yes; College of Sciences
Similar to what has recently been achieved with Bose-Bose mixtures [G. Lamporesi, J. Catani, G. Barontini, Y. Nishida, M. Inguscio, and F. Minardi, Phys. Rev. Lett. 104, 153202 (2010)], mixed-dimensional Fermi-Fermi mixtures can be created by applying a species-selective one-dimensional optical lattice to a two-species Fermi gas (σ ≡ {↑,↓}), in such a way that both species are confined to quasi–two-dimensional geometries determined by their hoppings along the lattice direction. We investigated the ground-state phase diagram of superfluidity for such mixtures in the BCS-BEC evolution, and found normal, gapped superfluid, gapless superfluid, and phase separated regions. In particular, we found a stable gapless superfluid phase where the unpaired ↑ and ↓ fermions coexist with the paired (or superfluid) ones in different momentum space regions. This phase is in some ways similar to the Sarma state found in mixtures with densities, but in our case, the gapless superfluid phase is unpolarized and most importantly it is stable against phase separation.
Superfluid-Mott-insulator transition in the spin-orbit-coupled Bose-Hubbard model
(American Physical Society (APS), 2014) Işkın, Menderes; Bölükbaşı, Ahmet Tuna; Department of Physics; Yes; College of Sciences
We consider a square optical lattice in two dimensions and study the effects of both the strength and symmetry of spin-orbit coupling and Zeeman field on the ground-state, i.e., Mott-insulator (MI) and superfluid (SF), phases and phase diagram, i.e., MI-SF phase-transition boundary, of the two-component Bose-Hubbard model. In particular, based on a variational Gutzwiller ansatz, our numerical calculations show that the spin-orbit-coupled SF phase is a nonuniform (twisted) one, with its phase (but not the magnitude) of the order parameter modulating from site to site. Fully analytical insights into the numerical results are also given.
Superfluid phases of ultracold Fermi gases on a checkerboard superlattice
(American Physical Society (APS), 2013) Işkın, Menderes; Department of Physics; Yes; College of Sciences
We analyze the ground-state phase diagram of two-component Fermi gases loaded into a two-dimensional checkerboard superlattice, i.e., a double-well optical lattice, potential within the BCS mean-field theory. We show that, by coupling the two s-wave sublattice superfluid order parameters, a checkerboard potential gives rise to an effectively two-band model with three (two intraband and an interband) nonlocal order parameters. We study the evolution of these order parameters as a function of particle filling, interaction strength, and checkerboard potential and find that the system always prefers the 0-phase solutions, i.e., where the phase difference between sublattice order parameters is 0, but never the pi-phase one. In addition, we find that the ground state of the system undergoes a superfluid-normal quantum phase transition at half fillings beyond a critical checkerboard potential C, the threshold of which is precisely determined by the magnitude of the order parameter at C = 0, and that the normal state rapidly turns into a checkerboard insulator as C increases.
