arXiv subjects
Publications and source records attributed to S. -K. Yip.
We study a dilute gas with two species of Fermionic atoms of unequal concentrations, interacting via a short-range interaction with one deeply bound state. We study the properties of this system under the mean-field approximation. We obtain the effective interaction among the fermions and bosons, and discuss the collective modes of the system.
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.
We investigate theoretically Fermionic superfluidity induced by Feshbach resonance in the orbital p-wave channel. We show that, 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 $k_z + i βk_y$ (up to a rotation about $\hat z$, here $β< 1$). A phase transition between these two states can occur under a change in the magnetic field or the density of the gas.
We show that the conclusions of a recent PRL by Pu et al is incorrect.
We here develop a theory on optical absorption in a dilute Bose gas at low temperatures. This theory is motivated by the Bogoliubov theory of elementary excitations for this system, and takes into account explicitly the modification of the nature and dispersion of elementary excitations due to Bose-Einstein condensation. Our results show important differences from existing theories.
We here study the collective excitations of a dilute spin-polarized Bose-Fermi mixture at zero temperature, considering in particular the features arising from the interaction between the two species. We show that a propagating zero-sound mode is possible for the fermions even when they do not interact among themselves.
We discuss the line shape for an internal state transition for bosonic atoms confined as a one-dimensional gas. Typical line shape is an edge singularity due to the absence of Bose-Einstein Condensation in such systems.
This paper summarizes the results of a comprehensive analysis of the thermodynamic and transport data for the superconducting phases of UPt_3. Calculations of the transverse sound attenuation as a function of temperature, frequency, polarization, and disorder are presented for the leading models of the superconducting order parameter. Measurements of the specific heat, thermal conductivity, and transverse sound attenuation place strong constraints on the orbital symmetry of the superconducting order parameter. We show that the superconducting A and B phases are in excellent agreement with pairing states belonging to the odd-parity E_{2u} orbital representation.
In this paper we study the effect of a Zeeman field on the supercurrent of a mesoscopic SNS junction. It is shown that the supercurrent suppression is due to a redistribution of current-carrying states in energy space. A dramatic consequence is that (part of the) the suppressed supercurrent can be recovered with a suitable non-equilibrium distribution of quasiparticles.
The internal vortex structure of a trapped spin-1 Bose-Einstein condensate is investigated. It is shown that it has a variety of configurations depending on, in particular, the ratio of the relevant scattering lengths and the total magnetization.
We report new theoretical results and analysis for the transport properties of superconducting UPt3 based on the leading models for the pairing symmetry. We use Fermi surface data and the measured inelastic scattering rate to show that the low-temperature thermal conductivity and transverse sound attenuation in the A and B phase of UPt3 are in excellent agreement with pairing states belonging to the two-dimensional orbital E2u representation.
We investigate the dependence of the current-phase relationship on the orientation of the order parameter for a pinhole between two $^3$He-B reservoirs. We show that, due to the internal spin structure of the superfluid, the energy of the junction may have a relative minimum at phase difference equals $π$ at low temperatures. The dependence of the supercurrent on the direction of an applied magnetic field can be used to verify the present mechanism for the ``$π$-states''.
In this paper I study the energy resolved supercurrent of a junction consisting of a dirty normal metal between two superconductors. I also consider a cross geometry with two additional arms connecting the above mentioned junction with two normal reservoirs at equal and opposite voltages. The dependence of the supercurrent between the two superconductors on the applied voltages is studied.
We present theoretical calculations of the thermal conductivity for the accidental degeneracy and enlarged symmetry group models that have been proposed to explain the phase diagram of UPt3. The order parameters for these models possess point nodes or cross nodes, reflecting the broken symmetries of the ground state. These broken symmetries lead to robust predictions for the ratio of the low-temperature thermal conductivity for heat flow along the c axis and in the basal plane. The anisotropy of the heat current response at low temperatures is determined by the phase space for scattering by impurities. The measured anisotropy ratio, $κ_c/κ_b$, provides a strong constraint of theoretical models for the ground state order parameter. The accidental degeneracy and enlarged symmetry group models based on no spin-orbit coupling do not account for the thermal conductivity of UPt3. The models for the order parameter that fit the experimental data for the c and b directions of the heat current are the 2D E1g and E2u models, for which the order parameters possess line nodes in the ab-plane and point nodes along the c axis, and the A1g+E1g model of Zhitomirsky and Ueda. This model spontaneously breaks rotational symmetry in theab-plane below Tc2 and predicts a large anisotropy for the $ab$-plane heat current.
In this paper we study the order parameter and density of states near a grain boundary between two $d_{x^2-y^2}$ superconductors. We examine broken time reversal symmetry near the interface. In particular we show that, under suitable circumstances, time reversal symmetry must be broken even when the order parameter is purely $d_{x^2-y^2}$ everywhere in space.
We show that some of the low temperature transport coefficients (e.g., electrical and thermal conductivities, viscosity and sound attenuation) are {\it universal}, i.e., independent of the impurity concentration and phase shift for specific classes of unconventional superconductors. The existence of a universal limit depends on the symmetry of the order parameter and is achieved at low temperatures $k_B T \ll γ\ll Δ_0$, where $γ$ is the bandwidth of the impurity induced Andreev bound states. The density of states is finite at zero energy and leads to the re-appearance of the Wiedemann-Franz law deep in the superconducting phase for $k_B T \ll γ$. Our findings also show that impurity concentration studies at low temperatures can distinguish between different order parameter symmetries.
We study the thermal conductivity within the E$_{1g}$ and E$_{2u}$ models for superconductivity in UPt$_3$ and compare the theoretical results for electronic heat transport with recently measured results reported by Lussier, Ellman and Taillefer. The existing data down to $T/T_c\approx 0.1$ provides convincing evidence for the presence of both line and point nodes in the gap, but the data can be accounted for either by an E$_{1g}$ or E$_{2u}$ order parameter. We discuss the features of the pairing symmetry, Fermi surface, and excitation spectrum that are reflected in the thermal conductivity at very low temperatures. Significant differences between the E$_{1g}$ and E$_{2u}$ models are predicted to develop at excitation energies below the bandwidth of the impurity-induced Andreev bound states. The zero-temperature limit of the $\bf \hat c$ axis thermal conductivity, $\lim_{T\to 0} κ_c/T$, is \underline{universal} for the E$_{2u}$ model, but non-universal for the E$_{1g}$ model. Thus, impurity concentration studies at very low temperatures should differentiate between the nodal structures of the E$_{2u}$ and E$_{1g}$ models.
We use the quasiclassical theory of superconductivity to calculate the electronic contribution to the thermal conductivity. The theory is formulated for low temperatures when heat transport is limited by electron scattering from random defects and for superconductors with nodes in the order parameter. We show that certain eigenvalues of the thermal conductivity tensor are universal at low temperature, $k_B T\ll γ$, where $γ$ is the bandwidth of impurity bound states in the superconducting phase. The components of the electrical and thermal conductivity also obey a Wiedemann-Franz law with the Lorenz ratio, $L(T)=κ/σT$, given by the Sommerfeld value of $L_{\!S}=({π^2}/{3})(k_B/e)^2$ for $k_BT\llγ$. For intermediate temperatures the Lorenz ratio deviates significantly from $L_{\!S}$, and is strongly dependent on the scattering cross section, and qualitatively different for resonant vs.\ nonresonant scattering. We include comparisons with other theoretical calculations and the thermal conductivity data for the high $T_c$ cuprate and heavy fermion superconductors.