arXiv subjects

S. K. Yip

Publications and source records attributed to S. K. Yip.

12 recordsLinked to original sources

arXiv2024

Broken time reversal symmetry vestigial state for a two-component superconductor in two spatial dimensions

We consider the vestigial phase with broken time-reversal symmetry above the superconducting transition temperature of a two-component superconductor in two spatial dimensions. We show that, in contrast to 3D, a vestigial phase is in general allowed within Ginzburg-Landau theory. The vestigial phase occupies an increasing temperature region if the parameters in the Ginzburg-Landau theory gives a larger energy difference between the broken time reversal symmetry phase and the other ordered phase.

cond-mat.supr-con↗

arXiv2022

Criterion for Vestigial Order above a Nematic Superconductor

A nematic superconductor can in principle support a vestigial order phase above its superconducting transition temperature, with rotational symmetry spontaneously broken while remain nonsuperconducting. We examine the condition for this vestigial nematic order to occur, within a Ginzburg-Landau theory with order parameter fluctuations included. Contrary to prior theoretical results, we found that this vestigial order actually requires very stringent conditions to be met: the material must be sufficiently deep in the nematic regime (i.e. far away from the boundary separating the nematic and chiral superconducting phases) to possibly exhibit a vestigial nematic order.

cond-mat.supr-con↗

arXiv2022

Phase Diagrams of Kitaev Models for Arbitrary Magnetic-Field Orientations

The Kitaev model is an exactly solvable quantum spin model within the language of the constrained real fermions. In spite of numerous studies along special magnetic-field orientations, there is a limited amount of knowledge on the complete field-angle characterization, which can provide valuable information on the existence of fractionalized excitations. For this purpose, we first extend previous studies on the field-angle response of the ferromagnetic Kitaev model to its antiferromagnetic version. Yet, the realistic description of the candidate Kitaev materials, within the edge-sharing octahedra paradigm, require additional coupling terms, including a large off-diagonal term $Γ$ along with possible anisotropic corrections $Γ_p$. It is therefore not sufficient to depend on the topological properties of the bare Kitaev model as the only source for the observed thermal Hall conductivity signals and an understanding of these extended Kitaev models with a complete field response is demanded. Starting from the zero-field phase diagram of realistic K-$Γ$-$Γ_p$ models, we identify antiferromagnetic zig-zag and (partially) polarized phases as well as two unusual Kitaev(-$Γ$) spin-liquid phases. The magnetic field response of these phases for arbitrary field orientations provides a remarkably rich phase diagram. A partially polarized phase is revealed between two ordered phases with a suppressed magnetization, finite fractionalization and finite Chern number. This phase is characterized as an extended Kitaev-$Γ$ spin-liquid. To comply our findings with experiments, we reproduce the asymmetry in the extent of the intermediate phases specifically for the two different field directions $θ= \pm 60^o$.

cond-mat.str-el↗

arXiv2003

Exact Results for a Spin-${\bf 1}$ lattice

We consider a lattice of spin-1 particles with a general pairwise interaction $ [ {\rm cos} γ({\bf S}_{l} \cdot {\bf S}_{l+1}) \ + {\rm sin} γ({\bf S}_{l} \cdot {\bf S}_{l+1})^2 \ ]$. We show that, for a large class of lattices with even number of sites, the ground state for the region $ - {3 π\over 4} < γ< - {π\over 2}$ belongs to total spin $S_{\rm tot} = 0$, whereas the state of minimum excited energy but with finite $S_{\rm tot}$ belongs to $S_{\rm tot} = 2$. These results are constrasted with the generalized Marshall theorems, applicable to a bipartite lattice and $ - {π\over 2} < γ\le 0$.

cond-mat.supr-con↗

arXiv2003

Dimer state of spin-1 Bosons in an optical lattice

In this paper we consider spin-1 Bosons, such as $^{23}$Na, 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.

physics.atom-ph↗

arXiv2003

Supercurrent and noise in point contact between two different superconductors

We show that, for a quantum point contact between two superconductors of different gap magnitudes, Andreev bound states do not exist for certain phase differences and gap ratios. Continuum states may dominate the transport, and the supercurrent noise is qualitatively different from the case of equal gaps.

cond-mat.supr-con↗

arXiv2001

2D 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.

cond-mat.supr-con↗

arXiv1999

The Phase Diagrams of F=2 Spinor Bose Condensates

We show that there are three possible phases for a spin-2 spinor Bose condensate, one more compared to the spin-1 case. The order parameters of these phases are the spontaneous magnetization and the singlet pair amplitude. Current estimates of scattering lengths show that all three phases have realizations in optically trapped alkali atoms. There is also a one-to-one correspondence between the structure of a spin-2 spinor Bose condensate and that of a d-wave BCS superfluid.

cond-mat↗

arXiv1997

Models for Superfluid $^3$He in Aerogel

Several recent experiments find evidence of superfluidity of 3He in 98%-porous aerogel. The primary effect of the aerogel is that it scatters the quasiparticles of 3He. We find that many experimental findings are quantitatively understood by a relatively simple model that takes into account strong inhomogeneity of the scattering on a length scale of 100 nm.

cond-mat↗

arXiv1995

The Nonlinear Meissner Effect in Unconventional Superconductors

We examine the long-wavelength current response in anisotropic superconductors and show how the field-dependence of the Meissner penetration length can be used to detect the structure of the order parameter. Nodes in the excitation gap lead to a nonlinear current-velocity constitutive equation at low temperatures which is distinct for each symmetry class of the order parameter. The effective Meissner penetration length is linear in $H$ and exhibits a characteristic anisotropy for fields in the $ab$-plane that is determined by the positions of the nodes in momentum space. The nonlinear current-velocity relation also leads to an intrinsic magnetic torque for in-plane fields that are not parallel to a nodal or antinodal direction. The torque scales as $H^3$ for $T\rightarrow 0$ and has a characteristic angular dependence. We analyze the effects of thermal excitations, impurity scattering and geometry on the current response of a $d_{x^2-y^2}$ superconductor, and discuss our results in light of recent measurements of the low-temperature penetration length and in-plane magnetization of single-crystals of $YBa_2Cu_3O_{7-δ}$ and $LuBa_2Cu_3O_{7-δ}$.

cond-mat↗

arXiv1993

Thin Films of 3He -- Implications on the Identification of 3 He -A

Recently the identification of 3He-A with the axial state has been questioned. It is suggested that the A-phase can actually be in the axiplanar state. We point out in the present paper that experiments in a film geometry may be useful to distinguish the above two possibilities. In particular a second order phase transition between an axial and an axiplanar state would occur as a function of thickness or temperature.

cond-mat↗

arXiv1992

Nonlinear Meissner Effect in CuO Superconductors

Recent theories of the NMR in the CuO superconductors are based on a spin-singlet $d_{x^2-y^2}$ order parameter. Since this state has nodal lines on the Fermi surface, nonlinear effects associated with low-energy quasiparticles become important, particularly at low temperatures. We show that the field-dependence of the supercurrent, below the nucleation field for vortices, can be used to locate the positions of the nodal lines of an unconventional gap in momentum space, and hence test the proposed $d_{x^2-y^2}$ state.

cond-mat↗