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Sungkit Yip

Publications and source records attributed to Sungkit Yip.

At least 19 recordsLinked to original sources
arXiv2026

Exact Organization of Density Matrices and Entanglement Structure in the Kitaev Spin Liquid

We give an exact form of the density matrix of the spin-1/2 Kitaev spin liquid represented in terms of spin operators and study the entanglement structures of the Kitaev honeycomb model within the spin framework. We show that the density matrix is naturally organized by equivalence classes of string operators associated with the underlying gauge structure of the model. With the explicit form of the density matrix, plus the exact Gauss law of the emergent gauge theory and the exact 1-form Wilson symmetry in the Kitaev model, we demonstrate the existence of the underlying symmetry-resolved block-diagonal structure of the reduced density matrix, which gives rise to the extensive degeneracy in the entanglement spectrum. The block-diagonal structure is then proven to be responsible for the separability of the entanglement entropy into the gauge and matter parts. Furthermore, we extend the formalism to subsystems with an odd number of lattice sites, revealing a relation between the entanglement spectrum and the fermion parity that is seldom mentioned in the literature.

cond-mat.str-el↗
arXiv2025

The gauge theory dual of the bilayer XY model with second order Josephson coupling

We formulate a duality transformation for a bilayer XY model where the layers are coupled by second order Josephson effect, which favors inter-layer phase difference of either $0$ or $\pi$. The model may represent a bilayer superconductor or a spin-1 ferromagnetic Bose gas in the easy-plane limit. The second order Josephson term is mapped to a U(1) gauge field, known to be trivially confining in two dimensions, and we argue that a Coulomb-gas analysis is not applicable to the dual theory. Instead, we appeal to the vast knowledge of gauge theory and infer that the only phase transition out of low-temperature ordered phase is an Ising transition driven by condensation of $\mathbb{Z}_2$ domain wall loops. The domain wall loops can be seen as a surviving vestige of single-layer vortex-anti-vortex pair, heavily deformed by the second order Josephson coupling. A theoretical or computational method that concentrates on point defects would most likely miss out on these excitations and reach erroneous results. Our dual theory offers a clear, intuitive picture of how the second order Josephson coupling induces confinement of vortices and drastically changes the physics.

cond-mat.supr-con↗
arXiv2025

Quantum Spin Liquid phases in Kitaev Materials

We develop a gauge-invariant renormalized mean-field theory (RMFT) to reliably find the quantum spin liquid (QSL) states and their field response for realistic Kitaev materials under strong magnetic fields and described by the generalized Kitaev $J$-$K$-$\Gamma$-$\Gamma'$ model. Remarkably, while our RMFT reproduces previous results based on using more complicated numerical methods, it also predicts several new stable QSL states. In particular, since Kitaev spin liquid (KSL) is no longer a saddle point solution, a new exotic 2-cone state distinct from the KSL, is found to describe experimental observations well, and hence should be the candidate state realized in the Kitaev material, $\alpha$-RuCl$_3$. We further explore the mechanism for the suppression of the observed thermal Hall conductivity at low temperatures within the fermionic framework, and show that the polar-angle dependence of the fermionic gap can distinguish the found 2-cone state from the KSL state in further experiments.

cond-mat.str-el↗
arXiv2024

Superfluid transition of a ferromagnetic Bose gas

The strongly ferromagnetic spin-1 Bose-Einstein condensate (BEC) has recently been realized with atomic $^{7}$Li. It was predicted that a strong ferromagnetic interaction can drive the normal gas into a magnetized phase at a temperature above the superfluid transition, and $^{7}$Li likely satisfies the criterion. We re-examine this theoretical proposal employing the two-particle-irreducible (2PI) effective potential, and conclude that there exists no stable normal magnetized phase for a dilute ferromagnetic Bose gas. For $^{7}$Li, we predict that the normal gas undergoes a joint first order transition and jump directly into a state with finite condensate density and magnetization. We estimate the size of the first order jump, and examine how a partial spin polarization in the initial sample affects the first order transition. We propose a qualitative phase diagram at fixed temperature for the trapped gas.

cond-mat.quant-gas↗
arXiv2019

Spontaneous thermal Hall conductance in superconductors with broken time-reversal symmetry

The off-diagonal components of thermal conductance tensor, thermal Hall conductivities (THCs), have extensively been studied in recent condensed matter experiments to investigate fractionalized quantum spin liquids, and quantum Hall systems. Under zero magnetic field, THCs spontaneously become non-zero for time-reversal symmetry (TRS) broken systems, and can have contributions from topologically protected edge states. Here we focus on an additional bulk effect, the impurity mechanism in TRS broken superconductors. Inspired by $Sr_2 Ru O_4$, the low temperature THC was calculated [Sup. Sci. and Tech. 29, 085006 (2016)] for the chiral p-wave superconductors induced by point impurities. Compared to topological part of THC, this contribution can be orders of magnitude larger as it scales with the density of states at the Fermi level. Motivated by TRS broken superconductors, URu$_2$Si$_2$ and SrPtAs and Sr$_2$RuO$_4$ as recently also been suggested as d-wave possibly, we calculate the THCs to $i.$ finite temperatures $ii.$ d-wave pairing states, $iii.$ finite size impurities. For this study, the non-equilibrium quasi-classical Keldysh Green's function formalism is utilized. The THCs are calculated by the systematic expansion of the quasiclassical transport equation in the center of mass gradients, self-consistently. $κ_{ij}$ are obtained analytically at low temperatures ($T \to 0$) and numerically at finite temperatures. We find that the impurity mechanism is dominant in $κ_{yx}$ at finite temperatures when compared to the topological part except at very low temperatures.There are two experimental signatures of IM on $κ_{yx}$: A non-monotonic temperature dependence and a sign change as a function of temperature depending on the scattering process.

cond-mat.supr-con↗
arXiv2016

Pseudospin bases for a model of Cu:Bi$_2$Se$_3$

We consider the construction of pseudospin bases for a time-reversal and inversion symmetric system, illustrated by a model for Cu:Bi$_2$Se$_3$. Different methods and bases are compared.

cond-mat.supr-con↗
arXiv2016

Low Temperature Thermal Hall Conductivity of a Nodal Chiral Superconductor

Motivated by Sr2RuO4, we consider a chiral superconductor where the gap is strongly suppressed along certain momentum directions. We evaluate the thermal Hall conductivity in the gapless regime, i.e., at temperature small compared with the impurity band width γ, taking the simplest model of isotropic impurity scattering. We find that, under favorable circumstances, this thermal Hall conductivity can be quite significant, and is smaller than the diagonal component (the universal thermal conductivity) only by a factor of 1/ ln(2 Δ_M/γ), where Δ_M is the maximum gap.

cond-mat.supr-con↗
arXiv2011

Edge State, Entanglement Entropy Spectra and Critical Hopping Coupling of Anisotropic Honeycomb Lattice

For a bipartite honeycomb lattice, we show that the Berry phase depends not only on the shape of the system but also on the hopping couplings. Using the entanglement entropy spectra obtained by diagonalizing the block Green's function matrices, the maximal entangled state with the eigenvalue $λ_m=1/2$ of the reduced density matrix is shown to have one-to-one correspondence to the zero energy states of the lattice with open boundaries, which depends on the Berry phase. For the systems with finite bearded edges along $x$-direction we find critical hopping couplings: the maximal entangled states (zero-energy states) appear pair by pair if one increases the hopping coupling $h$ over the critical couplings $h_c$s.

cond-mat.other↗
arXiv2010

Spin current in topologically trivial and nontrivial noncentrosymmetric superconductors

We study theoretically the surface of time-reversal-symmetric, noncentrosymmetric superconductor with mixed singlet and triplet order parameters. A pair of counterpropagating subgap quasiparticle surface bound states with opposite spin projections are obtained in the nontrivial Z$_2$ case where the triplet component is larger than the singlet one, contributing to a spin current. In contrast to the pure p-wave cases, these subgap states do not have a fixed spin projections but depend on the momenta along the surface. In the trivial Z$_2$ case where the singlet order parameter is larger, no subgap surface bound states show up. In both cases, there is also a finite contribution to the spin current from the continuum states with energies between the two gaps. The method for obtaining the quasiclassical Green's functions associated with the noncentrosymmetric superconductors is also presented.

cond-mat.supr-con↗
arXiv2009

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

We consider a Josephson junction with an arbitrary transmission coefficient $\mathcal{D}$ between a singlet and a triplet superconductor with the latter order parameter characterized by a d-vector of the form ($k_x\hat{y}-k_y\hat{x}$). 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.

cond-mat.supr-con↗
arXiv2009

Phase Diagrams for Spin-1 Bosons in an Optical Lattice

In this paper, the phase diagrams of a polar spin-1 Bose gas in a three-dimensional optical lattice with linear and quadratic Zeeman effects both at zero and finite temperatures are obtained within mean-field theory. The phase diagrams can be regrouped to two different parameter regimes depending on the magnitude of the quadratic Zeeman effect $Q$. For large $Q$, only a first-order phase transition from the nematic (NM) phase to the fully magnetic (FM) phase is found, while in the case of small $Q$, a first-order phase transition from the nematic phase to the partially magnetic (PM) phase, plus a second-order phase transition from the PM phase to the FM phase is obtained. If a net magnetization in the system exists, the first-order phase transition causes a coexistence of two phases and phase separation: for large $Q$, NM and FM phases and for small $Q$, NM and PM phases. The phase diagrams in terms of net magnetization are also obtained.

cond-mat.quant-gas↗
arXiv2008

Cooling into the Spin-Nematic State for a Spin-1 Bose gase in an optical lattice

The possibility of adiabatically cooling a spin-1 polar Bose gas to a spin-nematic phase is theoretically discussed. The relation between the order parameter of the final spin-nematic phase and the starting temperature of the spinor Bose gas is obtained both using the mean-field approach for the high temperature and spin-wave approach for the low temperature. We find that there exists a good possibility to reach the spin-nematic ordering starting with spinor antiferromagnetic Bose gases.

cond-mat.other↗
arXiv2008

Tranverse magnetic field distribution in the vortex state of noncentrosymmetric superconductor with O symmetry

We investigate the magnetic field distribution inside a Type II superconductor which has point group symmetry $O$ such as Li$_2$Pt$_3$B. The absence of inversion symmetry as a departure from perfect cubic group $O_h$ causes a magnetization collinear with the phase gradient associated with the order parameter, and a component of current collinear with the local magnetic field. In the vortex state, we predict, by solving the Maxwell equation, a local magnetic field transverse to the vortex lines. The probability distribution of this transverse field is also obtained for the prospective muon-spin-rotation measurements.

cond-mat.supr-con↗
arXiv2007

Signature of superconducting states in cubic crystal without inversion symmetry

The effects of absence of inversion symmetry on superconducting states are investigated theoretically. In particular we focus on the noncentrosymmetric compounds which have the cubic symmetry $O$ like Li$_2$Pt$_3$B. An appropriate and isotropic spin-orbital interaction is added in the Hamiltonian and it acts like a magnetic monopole in the momentum space. The consequent pairing wavefunction has an additional triplet component in the pseudospin space, and a Zeeman magnetic field $\bf{B}$ can induce a collinear supercurrent $\bf{J}$ with a coefficient $κ(T)$. The effects of anisotropy embedded in the cubic symmetry and the nodal superconducting gap function on $κ(T)$ are also considered. From the macroscopic perspectives, the pair of mutually induced $\bf{J}$ and magnetization ${\bf{M}}$ can affect the distribution of magnetic field in such noncentrosymmetric superconductors, which is studied through solving the Maxwell equation in the Meissner geometry as well as the case of a single vortex line. In both cases, magnetic fields perpendicular to the external ones emerge as a signature of the broken symmetry.

cond-mat.supr-con↗
arXiv2006

Effect of Quadratic Zeeman Energy on the Vortex of Spinor Bose-Einstein Condensates

The spinor Bose-Einstein condensate of atomic gases has been experimentally realized by a number of groups. Further, theoretical proposals of the possible vortex states have been sugessted. This paper studies the effects of the quadratic Zeeman energy on the vortex states. This energy was ignored in previous theoretical studies, although it exists in experimental systems. We present phase diagrams of various vortex states taking into account the quadratic Zeeman energy. The vortex states are calculated by the Gross-Pitaevskii equations. Several new kinds of vortex states are found. It is also found that the quadratic Zeeman energy affects the direction of total magnetization and causes a significant change in the phase diagrams.

cond-mat.other↗
arXiv2005

Feedback effects on the current correlations in Y-shaped conductors

We study current fluctuations in a Y-shaped conductor connected to external leads with finite impedances. We show that, due to voltage fluctuations in the circuit, the moments of the transferred charges cannot be obtained from simple rescaling of the bare values already in the second moments. The cross-correlation between the output terminals can change from negative to positive under certain parameter regimes.

cond-mat.mes-hall↗
arXiv2004

ac Josephson effect in asymmetric superconducting quantum point contacts

We investigate ac Josephson effects between two superconductors connected by a single-mode quantum point contact, where the gap amplitudes in the two superconductors are unequal. In these systems, it was found in previous studies on the dc effects that, besides the Andreev bound-states, the continuum states can also contribute to the current. Using the quasiclassical formulation, we calculate the current-voltage characteristics for general transmission $D$ of the point contact. To emphasize bound versus continuum states, we examine in detail the low bias, ballistic (D=1) limit. It is shown that in this limit the current-voltage characteristics can be determined from the current-phase relation, if we pay particular attention to the different behaviors of these states under the bias voltage. For unequal gap configurations, the continuum states give rise to non-zero sine components. We also demonstrate that in this limit the temperature dependence of the dc component follows $\tanh(Δ_s/2T)$, where $Δ_s$ is the smaller gap, with the contribution coming entirely from the bound state.

cond-mat.supr-con↗