Sungkit Yip

Two-dimensional superconductivity with strong spin-orbit interaction

We consider superconductivity confined at a two-dimensional interface with a strong surface spin-orbit (Rashba) interaction. Some peculiar properties of this system are investigated. In particular, we show that an in-plane Zeeman field can induce a supercurrent flow.

Dimer state of spin-1 bosons in an optical lattice.

We consider spin-1 bosons, such as 23Na, trapped in an optical lattice, in the regime of one particle per site. We argue that the ground state is expected to be the dimer phase in one, two, or three dimensions, thus realizing a state that has so far been studied only theoretically.

Superfluid stability in the BEC-BCS crossover

We consider a dilute atomic gas of two species of fermions with unequal concentrations under a Feshbach resonance. We find that the system can have distinct properties due to the unbound fermions. The uniform state is stable only when either (a) beyond a critical coupling strength, where it is a gapless superfluid, or (b) when the coupling strength is sufficiently weak, where it is a normal Fermi gas mixture. Phase transition(s) must therefore occur when the resonance is crossed, in contrast to the equal population case where a smooth crossover takes place.

Anisotropic Fermi superfluid via p-wave Feshbach resonance.

We investigate theoretically fermionic superfluidity induced by Feshbach resonance in the orbital p-wave channel and determine the general phase diagram. In contrast with superfluid (3)He, due to the dipole interaction, the pairing is extremely anisotropic. When this dipole interaction is relatively strong, the pairing has symmetry k(z). When it is relatively weak, it is of symmetry k(z) + ibetak(y) (up to a rotation about z;, here beta < 1). A phase transition between these two states can occur under a change in the magnetic field or the density of the gas.

Superfluidity in the interior-gap states

We investigate superfluidity in the interior-gap states proposed by Liu and Wilczek. At weak coupling, we find the gapless interior-gap state unstable in physically accessible regimes of the parameter space, where the superfluid density is shown to be always negative. We therefore conclude that the spatially uniform interior-gap phase is extremely unstable unless it is fully gapped; in this case, however, the state is rather similar to conventional BCS states.

Resonant pairing between fermions with unequal masses

We study via mean-field theory the pairing between fermions of different masses, especially at the unitary limit. At equal populations, the thermodynamic properties are identical with the equal mass case provided an appropriate rescaling is made. At unequal populations, for sufficiently light majority species, the system does not phase separate. For sufficiently heavy majority species, the phase separated normal phase have a density larger than that of the superfluid. For atoms in harmonic traps, the density profiles for unequal mass fermions can be drastically different from their equal-mass counterparts.

Spin current and spin accumulation near a Josephson junction between a singlet and triplet superconductor

We consider a Josephson junction with an arbitrary transmission coefficientD between a singlet and a triplet superconductor with the latter order parameter characterized by a d-vector of the form (kxˆ y −kyˆ x). Various quantities such as the tunnelling current, spin accumulation, and spin current are calculated via the quasiclassical Green’s functions. We also present a symmetry argument on the existence of these quantities and their dependencies on the phase difference across the junction. A physical picture is also given in terms of the Andreev states near the junction.

Larkin-Ovchinnikov state in resonant Fermi gas

We construct the phase diagram of a homogeneous two-component Fermi gas with population imbalance under a Feshbach resonance. In particular, we study the physics and stability of the Larkin-Ovchinnikov phase. We show that this phase is stable over a much larger parameter range than has been previously reported by other authors.

Magnetic Properties of a Superconductor with no Inversion Symmetry

AbstractWe study the magnetic properties of a superconductor in a crystal without