Showing 1–50 of 50 results for author: Yip, S -

Vortex nucleations in spinor Bose condensates under localized synthetic magnetic fields

Authors: L. -R. Liu, S. -C. Wu, T. -W. Liu, H. -Y. Hsu, T. -K. Shen, S. -K. Yip, Y. Kawaguchi, Y. -J. Lin

Abstract: Gauge fields are ubiquitous in modern quantum physics. In superfluids, quantized vortices can be induced by gauge fields. Here we demonstrate the first experimental observation of vortex nucleations in light-dressed spinor Bose-Einstein condensates under radially-localized synthetic magnetic fields. The light-induced spin-orbital-angular-momentum coupling creates azimuthal gauge potentials… ▽ More Gauge fields are ubiquitous in modern quantum physics. In superfluids, quantized vortices can be induced by gauge fields. Here we demonstrate the first experimental observation of vortex nucleations in light-dressed spinor Bose-Einstein condensates under radially-localized synthetic magnetic fields. The light-induced spin-orbital-angular-momentum coupling creates azimuthal gauge potentials $\vec{A}$ for the lowest-energy spinor branch dressed eigenstate. The observation of the atomic wave function in the lowest-energy dressed eigenstate reveals that vortices nucleate from the cloud center of a vortex-free state with canonical momentum $\vec{p} = 0$. This is because a large circulating azimuthal velocity field $\propto \vec{p}-\vec{A}$ at the condensate center results in a dynamically unstable localized excitation that initiates vortex nucleations. Furthermore, the long-time dynamics to reach the ground state stops in a metastable state when $|\vec{A}|$ is not sufficiently large. Our observation has reasonable agreement with the time-dependent Gross-Pitaevskii simulations. △ Less

Submitted 1 September, 2025; v1 submitted 26 March, 2024; originally announced March 2024.

Journal ref: Phys. Rev. Lett. 135, 093401 (2025)

Coupling of acoustic phonon to a spin-orbit entangled pseudospin

Authors: S. -K. Yip

Abstract: We consider coupling of acoustic phonon to pseudospins consisting of electronic spins locked to orbital angular momentum states. We show that a Berry phase term arises from projection onto the time-dependent lowest energy manifold. We examine consequences on the phonon modes, in particular mode splitting, induced chirality and Berry curvatures under an external magnetic field which Zeeman couples… ▽ More We consider coupling of acoustic phonon to pseudospins consisting of electronic spins locked to orbital angular momentum states. We show that a Berry phase term arises from projection onto the time-dependent lowest energy manifold. We examine consequences on the phonon modes, in particular mode splitting, induced chirality and Berry curvatures under an external magnetic field which Zeeman couples to the pseudospin. △ Less

Submitted 6 September, 2023; v1 submitted 30 June, 2023; originally announced July 2023.

Comments: 11 pages, 1 figure

Journal ref: Phys. Rev. B 108, 165116 (2023)

Visible stripe phases in spin-orbital-angular-momentum coupled Bose-Einstein condensates

Authors: N. -C. Chiu, Y. Kawaguchi, S. -K. Yip, Y. -J. Lin

Abstract: Recently, stripe phases in spin-orbit coupled Bose-Einstein condensates (BECs) have attracted much attention since they are identified as supersolid phases. In this paper, we exploit experimentally reachable parameters and show theoretically that annular stripe phases with large stripe spacing and high stripe contrast can be achieved in spin-orbital-angular-momentum coupled (SOAMC) BECs. In additi… ▽ More Recently, stripe phases in spin-orbit coupled Bose-Einstein condensates (BECs) have attracted much attention since they are identified as supersolid phases. In this paper, we exploit experimentally reachable parameters and show theoretically that annular stripe phases with large stripe spacing and high stripe contrast can be achieved in spin-orbital-angular-momentum coupled (SOAMC) BECs. In addition to using Gross-Pitaevskii numerical simulations, we develop a variational ansatz that captures the essential interaction effects to first order, which are not present in the ansatz employed in previous literature. Our work should open the possibility toward directly observing stripe phases in SOAMC BECs in experiments. △ Less

Submitted 25 May, 2020; originally announced May 2020.

Rotating atomic quantum gases with light-induced azimuthal gauge potentials and the observation of Hess-Fairbank effect

Authors: P. -K. Chen, L. -R. Liu, M. -J. Tsai, N. -C. Chiu, Y. Kawaguchi, S. -K. Yip, M. -S. Chang, Y. -J. Lin

Abstract: We demonstrate synthetic azimuthal gauge potentials for Bose-Einstein condensates from engineering atom-light couplings. The gauge potential is created by adiabatically loading the condensate into the lowest energy Raman-dressed state, achieving a coreless vortex state. The azimuthal gauge potentials act as effective rotations and are tunable by the Raman coupling and detuning. We characterize the… ▽ More We demonstrate synthetic azimuthal gauge potentials for Bose-Einstein condensates from engineering atom-light couplings. The gauge potential is created by adiabatically loading the condensate into the lowest energy Raman-dressed state, achieving a coreless vortex state. The azimuthal gauge potentials act as effective rotations and are tunable by the Raman coupling and detuning. We characterize the spin textures of the dressed states, in agreements with the theory. The lowest energy dressed state is stable with a 4.5-s half-atom-number-fraction lifetime. In addition, we exploit the azimuthal gauge potential to demonstrate the Hess-Fairbank effect, the analogue of Meissner effect in superconductors. The atoms in the absolute ground state has a zero quasi-angular momentum and transits into a polar-core vortex when the synthetic magnetic flux is tuned to exceed a critical value. Our demonstration serves as a paradigm to create topological excitations by tailoring atom-light interactions where both types of SO(3) vortices in the $|\langle \vec{F}\rangle|=1$ manifold, coreless vortices and polar-core vortices, are created in our experiment. The gauge field in the stationary Hamiltonian opens a path to investigating rotation properties of atomic superfluids under thermal equilibrium. △ Less

Submitted 17 December, 2018; v1 submitted 2 August, 2018; originally announced August 2018.

Journal ref: Phys. Rev. Lett. 121, 250401 (2018)

Spin-incoherent Luttinger liquid of one-dimensional SU($κ$) fermions

Authors: H. H. Jen, S. -K. Yip

Abstract: We theoretically investigate one-dimensional (1D) SU($κ$) fermions in the regime of spin-incoherent Luttinger liquid. We specifically focus on the Tonks-Girardeau gas limit where its density is sufficiently low that effective repulsions between atoms become infinite. In such case, spin exchange energy of 1D SU($κ$) fermions vanishes and all spin configurations are degenerate, which automatically p… ▽ More We theoretically investigate one-dimensional (1D) SU($κ$) fermions in the regime of spin-incoherent Luttinger liquid. We specifically focus on the Tonks-Girardeau gas limit where its density is sufficiently low that effective repulsions between atoms become infinite. In such case, spin exchange energy of 1D SU($κ$) fermions vanishes and all spin configurations are degenerate, which automatically puts them into spin-incoherent regime. In this limit, we are able to express the single-particle density matrices in terms of those of anyons. This allows us to numerically simulate the number of particles up to $N=32$. We numerically calculate single-particle density matrices in two cases: (1) equal populations for each spin components (balanced) and (2) all $S_z$ manifolds included. In contrast to noninteracting multi-component fermions, the momentum distributions are broadened due to strong interactions. As $κ$ increases, the momentum distributions are less broadened for fixed $N$, while they are more broadened for fixed number of particle per spin component. We then compare numerically calculated high momentum tails with analytical predictions which are proportional to $1/p^4$, in good agreement. Thus, our theoretical study provides a comparison with the experiments of repulsive multicomponent alkaline-earth fermions with a tunable SU($κ$) spin-symmetry in the spin-incoherent regime. △ Less

Submitted 7 May, 2018; originally announced May 2018.

Comments: 8 pages and 5 figures

Journal ref: Phys. Rev. A 98, 013623 (2018)

Non-Analytic Crossover Behavior of SU($\mathcal{N}_c$) Fermi Liquid

Authors: Pye Ton How, S. -K. Yip

Abstract: We consider the thermodynamic potential of a dilute Fermi gas with a contact interaction, at both finite temperature $T$ and non-zero effective magnetic fields $\mathbf{H}$, and derive the equation of state analytically using second order perturbation theory. Special attention is paid to the non-analytic dependence of $Ω$ on temperature $T$ and (effective) magnetic field $\mathbf{H}$, which exhibi… ▽ More We consider the thermodynamic potential of a dilute Fermi gas with a contact interaction, at both finite temperature $T$ and non-zero effective magnetic fields $\mathbf{H}$, and derive the equation of state analytically using second order perturbation theory. Special attention is paid to the non-analytic dependence of $Ω$ on temperature $T$ and (effective) magnetic field $\mathbf{H}$, which exhibits a crossover behavior as the ratio of the two is continuously varied. This non-analyticity is due to the particle-hole pair excitation being always gapless and long-ranged. The non-analytic crossover found in this paper can therefore be understood as an analog of the Ginzberg-Landau critical scaling, albeit only at the sub-leading order. We extend our results to an $\mathcal{N}_c$- component Fermi gas with an $\mathrm{SU}(\mathcal{N}_c)$-symmetric interaction, and point out possible enhancement of the crossover behavior by a large $\mathcal{N}_c$. △ Less

Submitted 17 April, 2018; originally announced April 2018.

Journal ref: Phys. Rev. A 97, 063623 (2018)

Spin-orbital-angular-momentum coupled Bose-Einstein condensates

Authors: H. -R. Chen, K. -Y. Lin, P. -K. Chen, N. -C. Chiu, J. -B. Wang, C. -A. Chen, P. -P. Huang, S. -K. Yip, Yuki Kawaguchi, Y. -J. Lin

Abstract: We demonstrate coupling between the atomic spin and orbital-angular-momentum (OAM) of the atom's center-of-mass motion in a Bose-Einstein condensate (BEC). The coupling is induced by Raman-dressing lasers with a Laguerre-Gaussian beam, and creates coreless vortices in a $F=1$ $^{87}$Rb spinor BEC. We observe correlations between spin and OAM in the dressed state and characterize the spin texture;… ▽ More We demonstrate coupling between the atomic spin and orbital-angular-momentum (OAM) of the atom's center-of-mass motion in a Bose-Einstein condensate (BEC). The coupling is induced by Raman-dressing lasers with a Laguerre-Gaussian beam, and creates coreless vortices in a $F=1$ $^{87}$Rb spinor BEC. We observe correlations between spin and OAM in the dressed state and characterize the spin texture; the result is in good agreement with the theory. In the presence of the Raman field our dressed state is stable for 0.1~s or longer, and it decays due to collision-induced relaxation. As we turn off the Raman beams, the vortex cores in the bare spin $|m_F=1\rangle$ and $|-1\rangle$ split. These spin-OAM coupled systems with the Raman-dressing approach have great potential for exploring new topological textures and quantum states. △ Less

Submitted 6 August, 2018; v1 submitted 21 March, 2018; originally announced March 2018.

Journal ref: Phys. Rev. Lett. 121, 113204 (2018)

Spin-incoherent Luttinger liquid of one-dimensional spin-1 Tonks-Girardeau Bose gas: Spin-dependent properties

Authors: H. H. Jen, S. -K. Yip

Abstract: Spin-incoherent Luttinger liquid (SILL) is a different universal class from the Luttinger liquid.\ This difference results from the spin incoherence of the system when the thermal energy of the system is higher than the spin excitation energy.\ We consider one-dimensional spin-$1$ Bose gas in the SILL regime and investigate its spin-dependent many-body properties.\ In Tonks-Girardeau limit, we are… ▽ More Spin-incoherent Luttinger liquid (SILL) is a different universal class from the Luttinger liquid.\ This difference results from the spin incoherence of the system when the thermal energy of the system is higher than the spin excitation energy.\ We consider one-dimensional spin-$1$ Bose gas in the SILL regime and investigate its spin-dependent many-body properties.\ In Tonks-Girardeau limit, we are able to write down the general wave functions in a harmonic trap.\ We numerically calculate the spin-dependent (spin-plus, minus, and $0$) momentum distributions in the sector of zero magnetization which allows to demonstrate the most significant spin-incoherent feature compared to the spinless or spin-polarized case.\ In contrast to the spinless Bose gas, the momentum distributions are broadened and in the large momentum limit follow the same asymptotic $1/p^4$ dependence but with reduced coefficients.\ While the density matrices and momentum distributions differ between different spin components for small $N$, at large $N$ they approach each other.\ We show these by analytic arguments and numerical calculations up to $N$ $=$ $16$. △ Less

Submitted 27 March, 2017; originally announced March 2017.

Comments: 9 figures, 12 pages

Journal ref: Phys. Rev. A 95, 053631 (2017)

SU(N) Fermi liquid at finite temperature

Authors: Chi-Ho Cheng, S. -K. Yip

Abstract: We consider the thermodynamic potential $Ω$ of an N component Fermi gas with a short range interaction obeying SU(N) symmetry. We analyze especially the part of $Ω$ that depends on the temperature T non-analytically for small T . We examine the temperature range where one can observe this $T^4 ln T$ contribution and discuss how it can be extracted experimentally. We consider the thermodynamic potential $Ω$ of an N component Fermi gas with a short range interaction obeying SU(N) symmetry. We analyze especially the part of $Ω$ that depends on the temperature T non-analytically for small T . We examine the temperature range where one can observe this $T^4 ln T$ contribution and discuss how it can be extracted experimentally. △ Less

Submitted 1 March, 2017; v1 submitted 23 December, 2016; originally announced December 2016.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. A 95, 033619 (2017)

Spin-incoherent one-dimensional spin-1 Bose Luttinger liquid

Authors: H. H. Jen, S. -K. Yip

Abstract: We investigate spin-incoherent Luttinger liquid of a one-dimensional spin-1 Bose gas in a harmonic trap. In this regime highly degenerate spin configurations emerge since the spin exchange energy is much less than the thermal energy of the system, while the temperature is low enough that the lowest energetic orbitals are occupied. As an example we numerically study the momentum distribution of a o… ▽ More We investigate spin-incoherent Luttinger liquid of a one-dimensional spin-1 Bose gas in a harmonic trap. In this regime highly degenerate spin configurations emerge since the spin exchange energy is much less than the thermal energy of the system, while the temperature is low enough that the lowest energetic orbitals are occupied. As an example we numerically study the momentum distribution of a one-dimensional spin-1 Bose gas in Tonks- Girardeau gas limit and in the sector of zero magnetization.We find that the momentum distributions broaden as the number of atoms increase due to the averaging of spin function overlaps. Large momentum ($p$) asymptotic is analytically derived, showing the universal $1/p^4$ dependence. We demonstrate that the spin-incoherent Luttinger liquid has a momentum distribution also distinct from spinless bosons at finite temperature. △ Less

Submitted 30 June, 2016; v1 submitted 2 May, 2016; originally announced May 2016.

Comments: 5 figures

Journal ref: Phys. Rev. A 94, 033601 (2016)

Kinetic equation and magneto-conductance for Weyl metal in the clean limit

Authors: S. -K. Yip

Abstract: We discuss the semi-classical kinetic equation in the clean limit, with the presence of Berry curvatures and magnetic field B, with the aim of applying to Weyl semi-metals. Special attention is given to the conservation laws for the collision integrals. It is found that the magneto-resistance second order in B is in general negative, with or without Weyl points, though in the later case it is in g… ▽ More We discuss the semi-classical kinetic equation in the clean limit, with the presence of Berry curvatures and magnetic field B, with the aim of applying to Weyl semi-metals. Special attention is given to the conservation laws for the collision integrals. It is found that the magneto-resistance second order in B is in general negative, with or without Weyl points, though in the later case it is in general much smaller. △ Less

Submitted 5 August, 2015; originally announced August 2015.

Comments: 10 pages

Current response of a topological insulator to a static Zeeman field

Authors: Bor-Luen Huang, S. -K. Yip

Abstract: We study the magnetoelectric coupling at the surface of a topological insulator. We are in particular interested in the surface current induced by a static Zeeman/exchange field. This surface current can be related to the orbital magnetization of the system. For an insulator with zero Chern number, the orbital magnetization is independent of the details at the boundary. With the appearance of surf… ▽ More We study the magnetoelectric coupling at the surface of a topological insulator. We are in particular interested in the surface current induced by a static Zeeman/exchange field. This surface current can be related to the orbital magnetization of the system. For an insulator with zero Chern number, the orbital magnetization is independent of the details at the boundary. With the appearance of surface states in the topological insulator, it is not immediately obvious if the response is not affected by the conditions at the surface. We investigate this question using exact diagonalization to a lattice model. By applying a time-reversal symmetry-breaking term near the boundary, no matter if the surface states are gapped out, we still find no change in the surface current. This arises from cancelations between Pauli and Van-Vleck contributions between surface and bulk scattering states. We also show that the surface current response is independent of the chemical potential when it is within the bulk gap. Our results are consistent with the claim that orbital magnetization is a bulk property. △ Less

Submitted 30 December, 2015; v1 submitted 16 April, 2015; originally announced April 2015.

Journal ref: J. Phys.: Condens. Matter 28 (2016) 026002

Fragmented Many-body States of Spin-2 Bose Gas

Authors: H. H. Jen, S. -K. Yip

Abstract: We investigate the fragmented many-body ground states of a spin-2 Bose gas in zero magnetic field.\ We point out that the exact ground state is not simply an average over rotationally-invariant mean-field states, in contrast to the spin-1 case with even number of particles N.\ We construct the exact ground states and compare them with the angular-averaged polar and cyclic states.\ The angular-aver… ▽ More We investigate the fragmented many-body ground states of a spin-2 Bose gas in zero magnetic field.\ We point out that the exact ground state is not simply an average over rotationally-invariant mean-field states, in contrast to the spin-1 case with even number of particles N.\ We construct the exact ground states and compare them with the angular-averaged polar and cyclic states.\ The angular-averaged polar states fail to retrieve the exact eigenstate at $N$ $\ge$ $6$ while angular-averaged cyclic states sustain only for N with a multiple of $3$.\ We calculate the density matrices and two-particle density matrices to show how deviant the angular-averaged state is from the exact one. △ Less

Submitted 16 April, 2015; v1 submitted 16 February, 2015; originally announced February 2015.

Comments: 2 figures and supplemental materials

Journal ref: Phys. Rev. A 91, 063603 (2015)

Quantum Critical Spin-2 Chain with Emergent SU(3) Symmetry

Authors: Pochung Chen, Zhi-Long Xue, I. P. McCulloch, Ming-Chiang Chung, Chao-Chun Huang, S. -K. Yip

Abstract: We study the quantum critical phase of a SU(2) symmetric spin-2 chain obtained from spin-2 bosons in a one-dimensional lattice. We obtain the scaling of the entanglement entropy and finite-size energies by exact diagonalization and density-matrix renormalization group methods. From the numerical results of the energy spectrum, central charge, and scaling dimension we identify the conformal field t… ▽ More We study the quantum critical phase of a SU(2) symmetric spin-2 chain obtained from spin-2 bosons in a one-dimensional lattice. We obtain the scaling of the entanglement entropy and finite-size energies by exact diagonalization and density-matrix renormalization group methods. From the numerical results of the energy spectrum, central charge, and scaling dimension we identify the conformal field theory describing the whole critical phase to be the SU(3)$_1$ Wess-Zumino-Witten model. We find that while in the whole critical phase the Hamiltonian is only SU(2) invariant, there is an emergent SU(3) symmetry in the thermodynamic limit. △ Less

Submitted 17 March, 2015; v1 submitted 23 February, 2014; originally announced February 2014.

Journal ref: Phys. Rev. Lett. 114, 145301 (2015)

Theory of SU(N) Fermi liquid

Authors: S. -K. Yip, Bor-Luen Huang, Jung-Shen Kao

Abstract: We generalized the Fermi liquid theory to N component systems with SU(N) symmetry. We emphasize the important role of fluctuations when N is large. These fluctuations dramatically modifies the properties for repulsive Fermi gases, in particular the spin susceptibility. We generalized the Fermi liquid theory to N component systems with SU(N) symmetry. We emphasize the important role of fluctuations when N is large. These fluctuations dramatically modifies the properties for repulsive Fermi gases, in particular the spin susceptibility. △ Less

Submitted 24 December, 2013; originally announced December 2013.

Comments: 7 pages, 2 figures; with Supplemental information

Entanglement entropy scaling of the XXZ chain

Authors: Pochung Chen, Zhi-long Xue, I. P. McCulloch, Ming-Chiang Chung, Miguel Cazalilla, S. -K. Yip

Abstract: We study the entanglement entropy scaling of the XXZ chain. While in the critical XY phase of the XXZ chain the entanglement entropy scales logarithmically with a coefficient that is determined by the associated conformal field theory, at the ferromagnetic point, however, the system is not conformally invariant yet the entanglement entropy still scales logarithmically albeit with a different coeff… ▽ More We study the entanglement entropy scaling of the XXZ chain. While in the critical XY phase of the XXZ chain the entanglement entropy scales logarithmically with a coefficient that is determined by the associated conformal field theory, at the ferromagnetic point, however, the system is not conformally invariant yet the entanglement entropy still scales logarithmically albeit with a different coefficient. We investigate how such an nontrivial scaling at the ferromagnetic point influences the estimation of the central charge $c$ in the critical XY phase. In particular we use the entanglement scaling of the finite or infinite system, as well as the finite-size scaling of the ground state energy to estimate the value of $c$. In addition, the spin-wave velocity and the scaling dimension are also estimated. We show that in all methods the evaluations are influenced by the nearby ferromagnetic point and result in crossover behavior. Finally we discuss how to determine whether the central charge estimation is strongly influenced by the crossover behavior and how to properly evaluate the central charge. △ Less

Submitted 14 September, 2013; v1 submitted 24 June, 2013; originally announced June 2013.

Journal ref: J. Stat. Mech. (2013) P10007

Models of Superconducting Cu:Bi2Se3: single versus two-band description

Authors: S. -K. Yip

Abstract: Starting from a model Hamiltonian for the normal state of the topological insulator Bi2Se3, we construct a pseudospin basis for the single-particle wavefunctions. Considering weak superconducting pairing near the Fermi surface, we express the recently proposed superconducting order parameters for Cu doped Bi2Se3 in this basis. For the odd parity states, the d-vectors specifying the order parameter… ▽ More Starting from a model Hamiltonian for the normal state of the topological insulator Bi2Se3, we construct a pseudospin basis for the single-particle wavefunctions. Considering weak superconducting pairing near the Fermi surface, we express the recently proposed superconducting order parameters for Cu doped Bi2Se3 in this basis. For the odd parity states, the d-vectors specifying the order parameter can have unusual momentum dependence for certain parameter regimes. Some peculiar results in the literature for surface states are discussed in light of the forms of these d(k)'s. Properties of the even parity states are also illuminated using this pseudospin basis. Results from this single-band description are compared with those from the full two-band model. △ Less

Submitted 4 January, 2013; originally announced January 2013.

Comments: 13 pages,1 figure

Journal ref: Physical Review B, 87, 104505 (2013)

Mesoscopic p-wave superconductor near the phase transition temperature

Authors: Bor-Luen Huang, S. -K. Yip

Abstract: We study the finite-size and boundary effects on a p-wave superconductor in a mesoscopic rectangular sample using Ginzburg-Landau (GL) and quasi-classical (QC) Green's function theory. Except for a square sample with parameters far away from the isotropic weak-coupling limit, the ground state near the critical temperature always prefers a time-reversal symmetric state, where the order parameter ca… ▽ More We study the finite-size and boundary effects on a p-wave superconductor in a mesoscopic rectangular sample using Ginzburg-Landau (GL) and quasi-classical (QC) Green's function theory. Except for a square sample with parameters far away from the isotropic weak-coupling limit, the ground state near the critical temperature always prefers a time-reversal symmetric state, where the order parameter can be represented by a real vector. For large aspect ratio, this vector is parallel to the long side of the rectangle. Within a critical aspect ratio, it has instead a vortex-like structure, vanishing at the sample center. △ Less

Submitted 8 July, 2013; v1 submitted 15 November, 2012; originally announced November 2012.

Comments: Revised version published in Physical Review B

Journal ref: Phys. Rev. B 87, 064507 (2013)

Preparation of two-particle total hyperfine spin singlet states via spin-changing dynamics

Authors: Chao-Chun Huang, M. -S. Chang, S. -K. Yip

Abstract: We present our proposals for generating total hyperfine spin zero state for two f=1 or two f=2 particles, starting from initial unentangled states. We show that our goal can be achieved by exploiting spin changing dynamics and quadratic Zeeman shifts with realistic choices of external magnetic fields and evolution time intervals. We present our proposals for generating total hyperfine spin zero state for two f=1 or two f=2 particles, starting from initial unentangled states. We show that our goal can be achieved by exploiting spin changing dynamics and quadratic Zeeman shifts with realistic choices of external magnetic fields and evolution time intervals. △ Less

Submitted 11 June, 2012; originally announced June 2012.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. A 86, 013403 (2012)

Phase diagrams of a p-Wave superconductor inside a mesoscopic disc-shaped sample

Authors: Bor-Luen Huang, S. -K. Yip

Abstract: We study the finite-size and boundary effects on a time-reversal-symmetry breaking p-wave superconducting state in a mesoscopic disc geometry using Ginzburg-Landau theory. We show that, for a large parameter range, the system exhibits multiple phase transitions. The superconducting transition from the normal state can also be reentrant as a function of external magnetic field. We study the finite-size and boundary effects on a time-reversal-symmetry breaking p-wave superconducting state in a mesoscopic disc geometry using Ginzburg-Landau theory. We show that, for a large parameter range, the system exhibits multiple phase transitions. The superconducting transition from the normal state can also be reentrant as a function of external magnetic field. △ Less

Submitted 18 October, 2012; v1 submitted 21 March, 2012; originally announced March 2012.

Comments: Revised version published in Physical Review B

Journal ref: Phys. Rev. B 86, 064506 (2012)

Dimerized and trimerized phases for spin-2 Bosons in a one-dimensional optical lattice

Authors: Pochung Chen, Zhi-Long Xue, I. P. McCulloch, Ming-Chiang Chung, S. -K. Yip

Abstract: We study the phase diagram for spin-2 bosons loaded in a one-dimensional optical lattice. By using non-Abelian density matrix renormalization group (DMRG) method we identify three possible phases: ferromagnetic, dimerized, and trimerized phases. We sketch the phase boundaries based on DMRG. We illustrate two methods for identifying the phases. The first method is based on the spin-spin correlation… ▽ More We study the phase diagram for spin-2 bosons loaded in a one-dimensional optical lattice. By using non-Abelian density matrix renormalization group (DMRG) method we identify three possible phases: ferromagnetic, dimerized, and trimerized phases. We sketch the phase boundaries based on DMRG. We illustrate two methods for identifying the phases. The first method is based on the spin-spin correlation function while in the second method one observes the excitation gap as a dimerization or a trimerization superlattice is imposed. The advantage of the second method is that it can also be easily implemented in experiments. By using the scattering lengths in the literature we estimate that $^{83}$Rb, $^{23}$Na, and $^{87}$Rb be ferromagnetic, dimerized, and trimerized respectively. △ Less

Submitted 20 May, 2011; v1 submitted 18 May, 2011; originally announced May 2011.

Comments: 4 pages, 3 figures. Add acknowledgement

Journal ref: Phys. Rev. A 85, 011601(R) (2012)

Theory of Fermionic superfluid with SU(2)xSU(6) symmetry

Authors: S. -K. Yip

Abstract: We study theoretically interspecies Cooper pairing in a fermionic system with SU(2)xSU(6) sym- metry. We show that, with suitable unitary transformations, the order parameter for the ground state can be reduced to only two non-vanishing complex components. The ground state has a large degeneracy. We find that while some Goldstone modes have linear dispersions, others are quadratic at low frequenci… ▽ More We study theoretically interspecies Cooper pairing in a fermionic system with SU(2)xSU(6) sym- metry. We show that, with suitable unitary transformations, the order parameter for the ground state can be reduced to only two non-vanishing complex components. The ground state has a large degeneracy. We find that while some Goldstone modes have linear dispersions, others are quadratic at low frequencies. We compare our results with the case of SU(N). △ Less

Submitted 14 March, 2011; v1 submitted 10 January, 2011; originally announced January 2011.

Comments: extended version; include comparison with SU(N)

Journal ref: Phys. Rev. A 83, 063607 (2011)

Bose-Einstein Condensation in the presence of an artificial spin-orbit interaction

Authors: S. -K. Yip

Abstract: Bose-Einstein condensation in the presence of a synthetic spin-momentum interaction is considered, focusing on the case where a Dirac or Rashba potential is generated via a tripod scheme. We found that the ground states can be either plane wave states or superpositions of them, each characterized by their unique density distributions. Bose-Einstein condensation in the presence of a synthetic spin-momentum interaction is considered, focusing on the case where a Dirac or Rashba potential is generated via a tripod scheme. We found that the ground states can be either plane wave states or superpositions of them, each characterized by their unique density distributions. △ Less

Submitted 11 January, 2011; v1 submitted 13 August, 2010; originally announced August 2010.

Comments: 5 pages, no figures

Wavefunction topology of two-dimensional time-reversal symmetric superconductors

Authors: S. -K. Yip

Abstract: We discuss the topology of the wavefunctions of two-dimensional time-reversal symmetric superconductors. We consider (a) the planar state, (b) a system with broken up-down reflection symmetry, and (c) a system with general spin-orbit interaction. We show explicitly how the relative sign of the order parameter on the two Fermi surfaces affects this topology, and clarify the meaning of the $Z_2$ c… ▽ More We discuss the topology of the wavefunctions of two-dimensional time-reversal symmetric superconductors. We consider (a) the planar state, (b) a system with broken up-down reflection symmetry, and (c) a system with general spin-orbit interaction. We show explicitly how the relative sign of the order parameter on the two Fermi surfaces affects this topology, and clarify the meaning of the $Z_2$ classification for these topological states. △ Less

Submitted 8 December, 2009; v1 submitted 5 October, 2009; originally announced October 2009.

Comments: only the Introduction has been modified from v1

Journal ref: J. Low Temp. Phys. , 160, 12 (2010)

Phase diagram of asymmetric Fermi gas across Feshbach resonance

Authors: C. -H. Pao, S. -K. Yip

Abstract: We study the phase diagram of the dilute two-component Fermi gas at zero temperature as a function of the polarization and coupling strength. We map out the detailed phase separations between superfluid and normal states near the Feshbach resonance. We show that there are three different coexistence of superfluid and normal phases corresponding to phase separated states between: (I) the partiall… ▽ More We study the phase diagram of the dilute two-component Fermi gas at zero temperature as a function of the polarization and coupling strength. We map out the detailed phase separations between superfluid and normal states near the Feshbach resonance. We show that there are three different coexistence of superfluid and normal phases corresponding to phase separated states between: (I) the partially polarized superfluid and the fully polarized normal phases, (II) the unpolarized superfluid and the fully polarized normal phases and (III) the unpolarized superfluid and the partially polarized normal phases from strong-coupling BEC side to weak-coupling BCS side. For pairing between two species, we found this phase separation regime gets wider and moves toward the BEC side for the majority species are heavier but shifts to BCS side and becomes narrow if they are lighter. △ Less

Submitted 16 September, 2008; originally announced September 2008.

Comments: 4 pages, 3 figures. Submitted to LT25 on June 2008

Journal ref: J. of Phys.: Conference Series 150 (2009) 032078

Fermions in Deep Optical Lattice under p-wave Feshbach Resonance

Authors: S. -K. Yip

Abstract: We investigate theoretically the energy levels of two identical Fermions in a harmonic potential well under p-wave Feshbach resonance. The magnetic fields needed to affect the energy levels differ strongly among the levels, and they can be at values far from the free space resonant field. We investigate theoretically the energy levels of two identical Fermions in a harmonic potential well under p-wave Feshbach resonance. The magnetic fields needed to affect the energy levels differ strongly among the levels, and they can be at values far from the free space resonant field. △ Less

Submitted 4 January, 2008; originally announced January 2008.

Comments: 2 figures

Journal ref: Phys. Rev. A 78, 013612 (2008)

Asymmetric Fermi superfluid with different atomic species in a harmonic trap

Authors: C. -H. Pao, shin-Tza Wu, S. -K. Yip

Abstract: We study the dilute fermion gas with pairing between two species and unequal concentrations in a harmonic trap using the mean field theory and the local density approximation. We found that the system can exhibit a superfluid shell structure sandwiched by the normal fermions. This superfluid shell structure occurs if the mass ratio is larger then certain critical value which increases from the w… ▽ More We study the dilute fermion gas with pairing between two species and unequal concentrations in a harmonic trap using the mean field theory and the local density approximation. We found that the system can exhibit a superfluid shell structure sandwiched by the normal fermions. This superfluid shell structure occurs if the mass ratio is larger then certain critical value which increases from the weak-coupling BCS region to the strong-coupling BEC side. In the strong coupling BEC regime, the radii of superfluid phase are less sensitive to the mass ratios and are similar to the case of pairing with equal masses. However, the lighter leftover fermions are easier to mix with the superfluid core than the heavier ones. A partially polarized superfluid can be found if the majority fermions are lighter, whereas phase separation is still found if they are heavier. △ Less

Submitted 23 August, 2007; originally announced August 2007.

Comments: 12 pages, 7 figures

Larkin-Ovchinnikov state in resonant Fermi gas

Authors: Nobukatsu Yoshida, S. -K. Yip

Abstract: 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 what has been previously reported by other authors. 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 what has been previously reported by other authors. △ Less

Submitted 14 March, 2007; v1 submitted 8 March, 2007; originally announced March 2007.

Comments: Typos corrected

Journal ref: Phys. Rev. A 75, 063601 (2007)

Symmetry and inert states of spin Bose Condensates

Authors: S. -K. Yip

Abstract: We construct the list of all possible inert states of spin Bose condensates for $S \le 4$. In doing so, we also obtain their symmetry properties. These results are applied to classify line defects of these spin condensates at zero magnetic field. We construct the list of all possible inert states of spin Bose condensates for $S \le 4$. In doing so, we also obtain their symmetry properties. These results are applied to classify line defects of these spin condensates at zero magnetic field. △ Less

Submitted 27 November, 2006; v1 submitted 7 November, 2006; originally announced November 2006.

Comments: an error in Sec III C corrected

Journal ref: Phys. Rev. A 75, 023625 (2007)

Phase diagram of a dilute fermion gas with density imbalance

Authors: C. -H. Pao, S. -T. Wu, S. -K. Yip

Abstract: We map out the phase diagram of a dilute two-component atomic fermion gas with unequal populations and masses under a Feshbach resonance. As in the case of equal masses, no uniform phase is stable for an intermediate coupling regime. For majority component heavier, the unstable region moves towards the BEC side. When the coupling strength is increased from the normal phase, there is an increased… ▽ More We map out the phase diagram of a dilute two-component atomic fermion gas with unequal populations and masses under a Feshbach resonance. As in the case of equal masses, no uniform phase is stable for an intermediate coupling regime. For majority component heavier, the unstable region moves towards the BEC side. When the coupling strength is increased from the normal phase, there is an increased parameter space where the transition is into the FFLO state. The converse is true if the majority is light. △ Less

Submitted 27 September, 2006; originally announced September 2006.

Comments: Proceeding for M$^2$S-HTSC VIII meeting, July 9-14 2006, Dresden; To appear in Physica C

Local stability conditions for asymmetric Fermi superfluid

Authors: S. -K. Yip

Abstract: Two different local stability conditions for an asymmetric superfluid has been discussed in the literature. We here consider the relations between them. Two different local stability conditions for an asymmetric superfluid has been discussed in the literature. We here consider the relations between them. △ Less

Submitted 7 September, 2006; originally announced September 2006.

Reply to Comment on "Superfluid stability in the BEC-BCS crossover" by Sheehy and Radzihovsky

Authors: C. -H. Pao, S. -T. Wu, S. -K. Yip

Abstract: The reason behind the discrepancy between the phase diagrams of our earlier work [1] and the comment of Sheehy and Radzihovsky [2] is discussed. We show that, in contrast to what is claimed in [2], the requirement of positive susceptibility is sufficient to rule out states that are local maximum of the free energy (as a function of the order parameter $Δ$ The reason behind the discrepancy between the phase diagrams of our earlier work [1] and the comment of Sheehy and Radzihovsky [2] is discussed. We show that, in contrast to what is claimed in [2], the requirement of positive susceptibility is sufficient to rule out states that are local maximum of the free energy (as a function of the order parameter $Δ$ △ Less

Submitted 23 August, 2006; originally announced August 2006.

Comments: 6 pages, 1 figure

Resonant pairing between Fermions with unequal masses

Authors: S. -T. Wu, C. -H. Pao, S. -K. Yip

Abstract: We study 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 phas… ▽ More We study 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. △ Less

Submitted 28 August, 2006; v1 submitted 25 April, 2006; originally announced April 2006.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. B 74, 224504 (2006)

Asymmetric Fermi superfluid in a harmonic trap

Authors: C. -H. Pao, S. -K. Yip

Abstract: We consider a dilute two-component atomic fermion gas with unequal populations in a harmonic trap potential using the mean field theory and the local density approximation. We show that the system is phase separated into concentric shells with the superfluid in the core surrounded by the normal fermion gas in both the weak-coupling BCS side and near the Feshbach resonance. In the strong-coupling… ▽ More We consider a dilute two-component atomic fermion gas with unequal populations in a harmonic trap potential using the mean field theory and the local density approximation. We show that the system is phase separated into concentric shells with the superfluid in the core surrounded by the normal fermion gas in both the weak-coupling BCS side and near the Feshbach resonance. In the strong-coupling BEC side, the composite bosons and left-over fermions can be mixed. We calculate the cloud radii and compare axial density profiles systemically for the BCS, near resonance and BEC regimes. △ Less

Submitted 23 April, 2006; originally announced April 2006.

Comments: 15 pages, 5 figures

Journal ref: J. Phys.: Condens. Matter 18 (2006) 5567-5577

Effects of voltage fluctuations on the current correlations in mesoscopic Y-shaped conductors

Authors: Shin-Tza Wu, S. -K. Yip

Abstract: We study current fluctuations in a phase coherent Y-shaped conductor connected to external leads and voltage probes. The voltage probes are taken to have finite impedances and thus can cause voltage fluctuations in the circuit. Applying the Keldysh formulation and a saddle point approximation appropriate for slow fluctuations, we examine at zero temperature the feedback effects on the current fl… ▽ More We study current fluctuations in a phase coherent Y-shaped conductor connected to external leads and voltage probes. The voltage probes are taken to have finite impedances and thus can cause voltage fluctuations in the circuit. Applying the Keldysh formulation and a saddle point approximation appropriate for slow fluctuations, we examine at zero temperature the feedback effects on the current fluctuations due to the fluctuating voltages. We consider mesoscopic Y-shaped conductors made of tunnel junctions and of diffusive wires. Unlike two-terminal conductors, we find that for the Y-shaped conductors the current moments in the presence of external impedances cannot be obtained from simple rescaling of the bare moments already in the second moments. As a direct consequence, we find that the cross correlation between the output terminals can become positive due to the impedances in the circuit. We provide formulas for the range of parameters that can cause positive cross correlations. △ Less

Submitted 11 March, 2006; originally announced March 2006.

Comments: Figures 2(b)(c) left out due to file size constraints (which, however, can be requested from authors). Submitted to Phys. Rev. B

Superfluid stability in BEC-BCS crossover

Authors: C. -H. Pao, Shin-Tza Wu, S. -K. Yip

Abstract: 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 Ferm… ▽ More 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. △ Less

Submitted 17 June, 2005; originally announced June 2005.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. B 73, 132506 (2006)

Anisotropic Fermi Superfluid via p-wave Feshbach Resonance

Authors: Chi-Ho Cheng, S. -K. Yip

Abstract: 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 t… ▽ More 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. △ Less

Submitted 12 April, 2005; originally announced April 2005.

Comments: 4 pages, 2 figures

Journal ref: Physical Review Letters 95, 070404 (2005)

Magnetic Properties of a Superconductor with no Inversion Symmetry

Authors: S. -K. Yip

Abstract: We study the magnetic properties of a superconductor in a crystal without $z \to -z$ symmetry, in particular how the lack of this symmetry exhibits itself. We show that, though the penetration depth itself shows no such effect, for suitable orientation of magnetic field, there is a magnetic field discontinuity at the interface which shows this absence of symmetry. The magnetic field profile of a… ▽ More We study the magnetic properties of a superconductor in a crystal without $z \to -z$ symmetry, in particular how the lack of this symmetry exhibits itself. We show that, though the penetration depth itself shows no such effect, for suitable orientation of magnetic field, there is a magnetic field discontinuity at the interface which shows this absence of symmetry. The magnetic field profile of a vortex in the $x-y$ plane is shown to be identical to that of an ordinary anisotropic superconductor except for a shift in the $-z$ direction by ${\tilde κ} λ_x$ (see errata). For a vortex along $z$, there is an induced magnetization along the radial direction. △ Less

Submitted 11 November, 2008; v1 submitted 18 February, 2005; originally announced February 2005.

Comments: J. Low Temp. Physics, 140, 67 (2005); with Errata

A dilute mixture of atoms and molecules

Authors: S. -K. Yip

Abstract: 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 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. △ Less

Submitted 3 February, 2006; v1 submitted 28 March, 2002; originally announced March 2002.

Comments: errors removed, reference not updated

Comment on "Phonon Spectrum and Dynamical Stability of a Dilute Quantum Degenerate Bose-Fermi Mixture

Authors: S. -K. Yip

Abstract: We show that the conclusions of a recent PRL by Pu et al is incorrect. We show that the conclusions of a recent PRL by Pu et al is incorrect. △ Less

Submitted 4 March, 2002; originally announced March 2002.

Comments: latex

Identification of the Orbital Pairing Symmetry in UPt_3

Authors: Matthias J. Graf, S. -K. Yip, J. A. Sauls

Abstract: 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… ▽ More 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. △ Less

Submitted 11 June, 2000; originally announced June 2000.

Comments: 11 pages with 7 figures

Report number: LA-UR:99-4118

Journal ref: Physical Review B 62 (2000) 14393-14402

Magnetic Field Effect on the Supercurrent of an SNS junction

Authors: S. -K. Yip

Abstract: 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. 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. △ Less

Submitted 25 February, 2000; originally announced February 2000.

Comments: 4 figures in postscript

Transport in the Heavy Fermion Superconductor UPt3

Authors: M. J. Graf, S. -K. Yip, J. A. Sauls

Abstract: 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… ▽ More 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. △ Less

Submitted 19 July, 1999; originally announced July 1999.

Comments: 2 pages, contribution at Int. Conf. LT-22, Helsinki, Finland, 4-11 Aug. 1999

Report number: LA-UR:99-3419

Journal ref: Physica B 280 (2000) 176-177

pi states in Josephson Junction between He-3-B

Authors: S. -K. Yip

Abstract: 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 magn… ▽ More 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''. △ Less

Submitted 7 July, 1999; originally announced July 1999.

Comments: 5 figures in postscript

Energy Resolved Supercurrent between two superconductors

Authors: S. -K. Yip

Abstract: 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. 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. △ Less

Submitted 11 August, 1998; originally announced August 1998.

Comments: revtex, 7 pages, 8 figures. accepted by Phys. Rev. B

Thermal Conductivity of the Accidental Degeneracy and Enlarged Symmetry Group Models for Superconducting UPt₃

Authors: M. J. Graf, S. -K. Yip, J. A. Sauls

Abstract: 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-tem… ▽ More 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. △ Less

Submitted 28 July, 1998; originally announced July 1998.

Comments: 18 pages, 7 figures; submitted to JLTP

Report number: LA-UR-98-3139

Journal ref: Journal Of Low Temperature Physics, v114(#1-2) pp. 257-273 Jan 1999

Time Reversal Symmetry Breaking States Near Grain Boundaries Between d-wave Superconductors

Authors: M. Fogelstrom, S. -K. Yip

Abstract: 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. 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. △ Less

Submitted 2 March, 1998; originally announced March 1998.

Comments: 4 pages, RevTeX, 4 postscript figures

Universality in Transport Processes of Unconventional Superconductors

Authors: M. J. Graf, S. -K. Yip, J. A. Sauls

Abstract: 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 temperatu… ▽ More 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. △ Less

Submitted 5 September, 1996; originally announced September 1996.

Comments: 2 pages, including 1 table, 1 postscript figure; presented at LT-21

Journal ref: Czech. J. Phys. 46 (1996) S2 1005

Thermal Conductivity of Superconducting UPt$_3$ at Low Temperatures

Authors: M. J. Graf, S. -K. Yip, J. A. Sauls

Abstract: 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… ▽ More 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. △ Less

Submitted 10 December, 1995; v1 submitted 7 December, 1995; originally announced December 1995.

Comments: 11 pages PostScript file (670k). Replaced with earlier version, added one more citation

Journal ref: J. Low Temp. Phys. 102 (1996) 367

Electronic thermal conductivity and the Wiedemann-Franz law for unconventional superconductors

Authors: M. J. Graf, S. -K. Yip, J. A. Sauls, D. Rainer

Abstract: 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 temper… ▽ More 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. △ Less

Submitted 8 September, 1995; originally announced September 1995.

Comments: 17 pages, PostScript file compressed and uuencoded

Journal ref: Phys. Rev. B 53 (1996) 15147