arXiv subjects

Tai-Kai Ng

Publications and source records attributed to Tai-Kai Ng.

At least 37 records · Page 2Linked to original sources

arXiv2010

Fermionic theory for quantum antiferromagnets with spin S > 1/2

The fermion representation for S = 1/2 spins is generalized to spins with arbitrary magnitudes. The symmetry properties of the representation is analyzed where we find that the particle-hole symmetry in the spinon Hilbert space of S =1/2 fermion representation is absent for S > 1/2. As a result, different path integral representations and mean field theories can be formulated for spin models. In particular, we construct a Lagrangian with restored particle-hole symmetry, and apply the corresponding mean field theory to one dimensional (1D) S = 1 and S = 3/2 antiferromagnetic Heisenberg models, with results that agree with Haldane's conjecture. For a S = 1 open chain, we show that Majorana fermion edge states exist in our mean field theory. The generalization to spins with arbitrary magnitude S is discussed. Our approach can be applied to higher dimensional spin systems. As an example, we study the geometrically frustrated S = 1 AFM on triangular lattice. Two spin liquids with different pairing symmetries are discussed: the gapped px + ipy-wave spin liquid and the gapless f-wave spin liquid. We compare our mean field result with the experiment on NiGa2S4, which remains disordered at low temperature and was proposed to be in a spin liquid state. Our fermionic mean field theory provide a framework to study S > 1/2 spin liquids with fermionic spinon excitations.

cond-mat.str-el↗

arXiv2010

Possibility of S=1 spin liquids with fermionic spinons on triangular lattices

In this paper we generalize the fermionic representation for $S=1/2$ spins to arbitrary spins. Within a mean field theory we obtain several spin liquid states for spin $S=1$ antiferromagnets on triangular lattices, including gapless f-wave spin liquid and topologically nontrivial $p_x+ip_y$ spin liquid. After considering different competing orders, we construct a phase diagram for the $J_1$-$J_3$-$K$ model. The application to recently discovered material $\mathrm{NiGa_2S_4}$ is discussed.

cond-mat.str-el↗

arXiv2010

Photonic analogue of Josephson effect in a dual-species optical-lattice cavity

We extend the idea of quantum phase transitions of light in the photonic Bose-Hubbard model with interactions to two atomic species by a self-consistent mean field theory. The excitation of two-level atoms interacting with coherent photon fields is analyzed with a finite temperature dependence of the order parameters. Four ground states of the system are found, including an isolated Mott-insulator phase and three different superfluid phases. Like two weakly coupled superconductors, our proposed dual-species lattice system shows a photonic analogue of Josephson effect. The dynamics of the proposed two species model provides a promising quantum simulator for possible quantum information processes.

quant-ph↗

arXiv2010

Raman signature of the U(1) Dirac spin-liquid state in spin-1/2 kagome system

We followed the Shastry--Shraiman formulation of Raman scattering in Hubbard systems and considered the Raman intensity profile in the spin-1/2 "perfect" kagome lattice herbertsmithite ZnCu_3(OH)_6Cl_2, assuming the ground state is well-described by the U(1) Dirac spin-liquid state. In the derivation of the Raman T-matrix, we found that the spin chirality term appears in the A_{2g} channel in the kagome lattice at the t^4/(ω_i-U)^3 order, but (contrary to the claims by Shastry and Shraiman) vanishes in the square lattice to that order. In the ensuing calculations on the spin-1/2 kagome lattice, we found that the Raman intensity profile in the E_g channel is invariant under an arbitrary rotation in the kagome plane, and that in all (A_{1g}, E_g, and A_{2g}) symmetry channels the Raman intensity profile contains broad continua that display power-law behaviors at low energy, with exponent approximately equal to 1 in the A_{2g} channel and exponent approximately equal to 3 in the E_g and the A_{1g} channels. For the A_{2g} channel, the Raman profile also contains a characteristic 1/ωsingularity, which arose in our model from an excitation of the emergent U(1) gauge field.

cond-mat.str-el↗

arXiv2009

Topologically distinct classes of valence bond solid states with their parent Hamiltonians

We introduce a general method to construct one-dimensional translationally invariant valence bond solid states with a built-in Lie group $G$ and derive their matrix product representations. The general strategies to find their parent Hamiltonians are provided so that the valence bond solid states are their unique ground states. For quantum integer spin-$S$ chains, we discuss two topologically distinct classes of valence bond solid states: One consists of two virtual SU(2) spin-$J$ variables in each site and another is formed by using two $SO(2S+1)$ spinors. Among them, a new spin-1 fermionic valence bond solid state, its parent Hamiltonian, and its properties are discussed in detail. Moreover, two types of valence bond solid states with SO(5) symmetry are further generalized and their respective properties are analyzed as well.

cond-mat.str-el↗

arXiv2009

Ginzburg-Landau theory of dirty two band $s_{\pm}$ superconductors

In this paper we study the effect of non-magnetic impurities on two-band $s_{\pm}$ superconductors by deriving the corresponding Ginzburg-Landau (GL) equation. Depending on the strength of (impurity-induced) inter-band scattering we find that there are two distinctive regions where the superconductors behave very differently. In the strong impurity induced inter-band scattering regime $T_c<<τ^{-1}_t$, where $τ_t\sim$ mean-life time an electron stays in one band the two-band superconductor behaves as an effective one-band dirty superconductor. In the other limit $T_c\geqτ^{-1}_t$, the dirty two-band superconductor is described by a network of frustrated two-band superconductor grains connected by Josepshon tunnelling junctions. We argue that most pnictide superconductors are in the later regime.

cond-mat.supr-con↗

arXiv2009

In-gap bound states and tunneling conductance of multiband superconductors through a normal/superconductor/superconductor junction

The tunneling conductance between a metal and a multiband s-wave superconductor with a thin layer of single-band s-wave superconductor sandwiched in between is examined in this paper. We show that an in-gap peak in conductance curve is found as a result of the formation of in-gap bound state between the single-band and multiband superconductor junctions if the phases of the superconducting order parameters of the multiband superconductor are frustrated. The implication of this result in determining the gap symmetry of the iron-based superconductors is discussed.

cond-mat.supr-con↗

arXiv2009

Fermi-edge problem in the presence of AC electric field

We study in this paper a non-equilibrium Fermi-edge problem where the system under investigation is a single electron reservoir putting under an AC electric field. We show that the electron Green's function and other correlation functions in the problem can be solved and expressed exactly in terms of a well-defined integral. The qualitative behaviors of the solution is studied and compared with the situation where the impurity is coupled to more than one reservoirs at different chemical potentials.

cond-mat.mes-hall↗

arXiv2009

Topological glass states

In connection with recent discussion of topological order and topological phase transitions in quantum systems, we reexamine circumstances that lead to the appearance of a topological glass in certain classical lattice spin models. Local bonding enforces constraints on low energy states which organize themselves into topologically distinct classes that break ergodicity but not any apparent symmetry as in the usual Landau theory of phase transitions. Various properties of such a topological glass are demonstrated using two classical Ising-like models.

cond-mat.stat-mech↗

arXiv2008

Na4Ir3O8 as a 3D spin liquid with fermionic spinons

Spin liquid states for spin-1/2 antiferromagnetic Heisenberg model on a hyperkagome lattice are studied. We classify and study flux states according to symmetries. Applying this model to Na4Ir3O8, we propose that the high temperature state may be described by a spinon Fermi surface, which forms a paired state with line nodes below 20 K. The possible mixed spin singlet and spin triplet pairing states are discussed according to the lattice symmetry which breaks inversion.

cond-mat.str-el↗

arXiv2007

On the (circular) polarization-independence of microwave-induced resistance oscillations and zero resistance state

The immunity of microwave-induced magneto-resistance oscillations and corresponding zero resistance regions to the direction of (circular) polarization of microwave is studied in this paper. We propose that a spontaneous circular motion of the whole electron fluid would stabilize the system and minimize the polarization sensitivity of the oscillatory DC resistance. Results of a self-consistent calculation capture the qualitative features of the experimental observation.

cond-mat.mes-hall↗

arXiv2007

Comment to "Multipolar expansion of the electrostatic interaction between charged colloids at interfaces"

We present a comment to a recent paper by Dominguez et. al. They claimed that the electrostatic interaction between charged colloids trapped at an interface formed by a dielectric and a screening phase is always isotropic to order $d^-3$. Base on this result, they claimed that in-plane dipolar attraction of order $d^-3$ between colloids cannot exist, in contrast to previous proposals. We point out that their analysis is not completed and anisotropic (dipolar) interaction between colloids can exist up to order $d^-3$.

cond-mat.soft↗

arXiv2007

Power-law Conductivity inside the Mott gap: application to $κ-(BEDT-TTF)_2Cu_2(CN)_3$

The charge dynamics of spin-liquid states described by U(1) gauge theory coupling to fermionic spinons is discussed in this paper. We find that the gapless spinons give rise to a power law optical conductivity inside the charge gap. The theory is applied to explain the unusual optical conductivity observed recently in the organic compound $κ-(BEDT-TTF)_2Cu_2(CN)_3$. We also propose an optical experiment to search for the in-gap excitations in the Kagome spin liquid insulator.

cond-mat.str-el↗

arXiv2007

Origin of long-ranged attraction between like-charged particles at water-air interface

The nature of electrostatic interaction between like-charged particles at water-air interface is analyzed in this paper. We show that long-ranged electrostatic dipolar attraction between these objects generally exists. Our result provides a natural mechanism to explain the experimental observations of attraction between like-charge species trapped at water-air interface. We speculate that a similar (but presumably weaker) mechanism also exists for particles near water-solid interface.

cond-mat.soft↗

arXiv2007

Electromagnetic properties of a Fermi liquid superconductor with singular Landau parameter $F_{1s}\to-1$

The electromagnetic properties of a Fermi liquid superconductor with singular Landau parameter $F_{1s}\to-1$, corresponding to a state with marginal spin-charge separation (MSCS) is analyzed in this paper. We show that the MSCS state describes a strongly phase-disordered superconductor which is a diamagnetic metal with Drude-like optical conductivity. The phase diagram and electromagnetic properties of the MSCS state is found to be in qualitative agreement with what is observed in the pseudo-gap phase of (underdoped) High-$T_c$ cuprates. We predicted that vortices with {\em un-quantized} magnetic flux will be observed in the pseudo-gap phase.

cond-mat.str-el↗

arXiv2006

Ginzburg-Landau equation and vortex liquid phase of Fermi liquid superconductors

In this paper we study the Ginzburg-Landau (GL) equation for Fermi liquid superconductors with strong Landau interactions $F_0$ and $F_1$. We show that Landau interactions renormalize two parameters entering the GL equation leading to renormalization of superfluid density and separation of length and energy scales for amplitude and phase fluctuations. The separation of energy/length scales for amplitude and phase fluctuations in term leads to unconventional (2D) KT transition and vortex liquid phase. Application of the GL equation to describe underdoped high-$T_c$ cuprates is being discussed.

cond-mat.supr-con↗

arXiv2006

Josephson effect for superconductors lacking center of inversion

Due to the absence of a center of inversion in some superconducting compounds, a p-wave admixture to the dominant d-wave order parameter must exist. If time-reversal is also violated, an allowed invariant is the product of the d-wave, p-wave and an appropriately directed current. We show that this leads to an anomalous Josephson current for tunneling along the direction parallel to the axis of the p-wave component, where the current is the Meissner current and the Josephson loop current along the surface of the tunnel barrier. These ideas are applied to the superconducting state of the cuprates in the pseudogap region of the phase diagram where in the normal phase some experiments have detected a time-reversal and inversion symmetry broken phase. The effect is relevant also to heavy-fermion compounds which lack center of inversion due to crystalline symmetry.

cond-mat.supr-con↗

arXiv2006

Fermi liquid behavior and spin-charge separation in underdoped cuprates

The Gaussian fluctuations in slave-boson mean-field theory of $t-J$ model is analyzed in this paper in the low-doping regime where the superconducting to normal (pseudo-gap phase) transition is driven by vanishing of bose-condensation amplitude. By eliminating the boson and constraint fields exactly in the linear response regime we show that the Gaussian theory describes a Fermi liquid superconductor where the superconducting to normal transition is actually a spin-charge separation transition characterized by a change of Landau parameter from $F_1>-1$ in the superconducting phase to $F_1=-1$ in the pseudo-gap phase. Consequences of this proposal are discussed.

cond-mat.supr-con↗