arXiv subjects

Tai-Kai Ng

Publications and source records attributed to Tai-Kai Ng.

51 records · Page 3Linked to original sources

arXiv2005

Transport equation for 2D electron liquid under microwave radiation plus magnetic field and the Zero Resistance State

A general transport equation for the center of mass motion is constructed to study transports of electronic system under uniform magnetic field and microwave radiation. The equation is applied to study 2D electron system in the limit of weak disorder where negative resistance instability is observed when the radiation field is strong enough. A solution of the transport equation with spontaneous AC current is proposed to explain the experimentally observed Radiation-Induced Zero Resistance State.

cond-mat.mes-hall↗

arXiv2004

Spinon-Holon binding in $t-J$ model

Using a phenomenological model, we discuss the consequences of spinon-holon binding in the U(1) slave-boson approach to $t-J$ model. Within a small $x$ ($x=$ hole concentration) expansion, we show that spinon-holon binding produces a pseudo-gap normal state with a segmented Fermi surface and the superconducting state is formed by opening an "additional" d-wave gap on the segmented Fermi surface. The d-wave gap merge with the pseudo-gap smoothly as temperature $T\to0$. The quasi-particles in the superconducting state are coupled to external electromagnetic field with a coupling constant of order $x^γ$ where $0\leqγ\leq1/2$, depending on the strength of the effective spinon-holon binding potential.

cond-mat.supr-con↗

arXiv2003

Landau Transport equations in slave-boson mean-field theory of t-J model

In this paper we generalize slave-boson mean-field theory for $t-J$ model to the time-dependent regime, and derive transport equations for $t-J$ model, both in the normal and superconducting states. By eliminating the boson and constraint fields exactly in the equations of motion we obtain a set of transport equations for fermions which have the same form as Landau transport equations for normal Fermi liquid and Fermi liquid superconductor, respectively with all Landau parameters explicity given. Our theory can be viewed as a refined version of U(1) Gauge theory where all lattice effects are retained and strong correlation effects are reflected as strong Fermi-liquid interactions in the transport equation. Some experimental consequences are discussed.

cond-mat.str-el↗

arXiv2003

"quasi-particles" in bosonization theory of interacting fermion liquids at arbitrary dimensions

Within bosonization theory we introduce in this paper a new definition of "quasi-particles" for interacting fermions at arbitrary space dimenions. In dimensions higher than one we show that the constructed quasi-particles are consistent with quasi-particle descriptions in Landau Fermi liquid theory whereas in one-dimension the quasi-particles" are non-perturbative objects (spinons and holons) obeying fractional statistics. The more general situation of Fermi liquids with singular Landau interaction is discussed.

cond-mat.str-el↗

arXiv2000

Dynamics of fermions coupling to a U(1) gauge field in the limit $e^2\to\infty$

We study in this paper the properties of a gas of fermions coupling to a U(1) gauge field at wavevectors $q<Λ<<k_F$ at dimensions larger than one, where $Λ<<k_F$ is a high momentum cutoff and $k_F$ is the fermi wave vector. In particular, we shall consider the $e^2\to\infty$ limit where charge and current fluctuations at wave vectors $q<Λ$ are forbidden. Within a bosonization approximation, effective actions describing the low energy physics of the system are constructed, where we show that the system can be described as a fermion liquid formed by chargeless quasi-particles which has vanishing wavefunction overlap with the bare fermions in the system.

cond-mat.str-el↗

arXiv2000

Properties of a magnetic superconductor with weak magnetization - application to $ErNi_2B_2C$

Using a Ginsburg-Landau free energy functional, we study the $H-T$ phase diagram of a weak magnetic superconductor, where the magnetization from the magnetic component is marginal in supporting a spontaneous vortex phase in absence of external magnetic field. In particular, the competition between the spiral state and spontaneous vortex phase is analysed. Our theory is applied to understand the magnetic properties of $ErNi_2B_2C$.

cond-mat.supr-con↗

arXiv2000

Topological effects at short antiferromagnetic Heisenberg chains

The manifestations of topological effects in finite antiferromagnetic Heisenberg chains is examined by density matrix renormalization group technique in this paper. We find that difference between integer and half-integer spin chains shows up in ground state energy per site when length of spin chain is longer than $\simξ$, where $ξ\sim\exp(πS)$ is a spin-spin correlation length, for spin magnitude S up to 5/2. For open chains with spin magnitudes $S=5/2$ to S=5, we verify that end states with fractional spin quantum numbers $S'$ exist and are visible even when the chain length is much smaller than the correlation length $ξ$. The end states manifest themselves in the structure of the low energy excitation spectrum.

cond-mat.str-el↗

arXiv2000

Probing superconducting phase fluctuations from the current noise spectrum of pseudogaped metal-superconductor tunnel junctions

We study the current noise spectra of a tunnel junction of a metal with strong pairing phase fluctuation and a superconductor. It is shown that there is a characteristic peak in the noise spectrum at the intrinsic Josephson frequency $ω_J=2eV$ when $ω_J$ is smaller than the pairing gap but larger than the pairing scattering rate. In the presence of an AC voltage, the tunnelling current noise shows a series of characteristic peaks with increasing DC voltage. Experimental observation of these peaks will give direct evidence of the pair fluctuation in the normal state of high-$T_c$ superconductors and from the half width of the peaks the pair decay rate can be estimated.

cond-mat.str-el↗

arXiv1999

Finite size spectrum, magnon interactions and magnetization of S=1 Heisenberg spin chains

We report our density matrix renormalization-group and analytical work on S=1 antiferromagnetic Heisenberg spin chains. We study the finite size behavior within the framework of the non-linear sigma model. We study the effect of magnon-magnon interactions on the finite size spectrum and on the magnetization curve close to the critical magnetic field, determine the magnon scattering length and compare it to the prediction from the non-linear $σ$ model.

cond-mat.str-el↗

arXiv1999

Topological spin excitations of Heisenberg antiferromagnets in two dimensions

In this paper we discuss the construction and the dynamics of vortex-like topological spin excitations in the Schwinger-boson description of Heisenberg antiferromagnets in two dimensions. The topological spin excitations are Dirac fermions (with gap) when spin value $S$ is a half-integer. Experimental and theoretical implications of these excitations are being investigated.

cond-mat.str-el↗

arXiv1997

Functional Integral approach to quasi-particles in Fermi Liquid theory

In this paper we propose a new way of organizing many-body perturbation theory in the Path-integral formulation where a set of quasi-particle wave-functionals $ψ$'s are introduced and are identified with quasi-particles in Landau Fermi liquid theory. We show how Fermi liquid theory can be obtained through $ψ$'s in this new framework of perturbation theory, where the only assumption is adiabaticity between non-interacting and interacting states and the quasi-particle renormalization factor z does not appear explicitly. Consequences of our new formulation are discussed.

cond-mat↗

arXiv1997

An SU(2) Formulation of the t-J model: Application to Underdoped Cuprates

We develop a slave-boson theory for the t-J model at finite doping which respect a SU(2) symmetry -- a symmetry previously known to be important at half filling. The mean field phase diagram is found to be consistent with the phases observed in the cuprate superconductors, which contains d-wave superconductor, spin gap, strange metal, and Fermi liquid phases. The spin gap phase is best understood as the staggered flux phase, which is nevertheless translationally invariant for physical quantities. The physical electron spectral function shows small Fermi segments at low doping which continuously evolve into the large Fermi surface at high doping concentrations. The close relation between the SU(2) and the U(1) slave-boson theory is discussed. The low energy effective theory for the low lying fluctuations is derived, and new lying modes (which were over looked in the U(1) theory) are identified.

cond-mat.str-el↗

arXiv1996

Non-magnetic impurities in S=1/2 spin-Peierls system and Heisenberg ladder systems

In this paper we study the effect of non-magnetic impurities in spin-Peierls system $CuGeO_3$ within the framework of non-linear sigma model plus topological term. We show that local moments are induced in both the $Zn$-doped and $Si$-doped $CuGeO_3$ compounds. Effective low energy theories for the impurity-induced local moments are derived in both cases where interesting differences between the two cases and between the $Zn$-doped Heisenberg two-ladder system are pointed out. The low-energy physics of the $Zn$-doped Heisenberg three-leg ladder system is also discussed.

cond-mat↗

arXiv1996

Non-magnetic impurities in two- and three- dimensional Heisenberg antiferromagnets

In this paper we study in a large-S expansion effects of substituting spins by non-magnetic impurities in two- and three- dimensional Heisenberg antiferromagnets in a weak magnetic field. In particular, we demonstrate a novel mechanism where magnetic moments are induced around non-magnetic impurities when magnetic field is present. As a result, Curie-type behaviour in magnetic susceptibility can be observed well below the Neel temperature, in agreement with what is being observed in $La_2Cu_{1-x}Zn_{x}O_4$ and $Sr(Cu_{1-x}Zn_x)_2O_3$ compounds.

cond-mat↗

arXiv1995

Edge states in Open Antiferromagnetic Heisenberg Chains

In this letter we report our results in investigating edge effects of open antiferromagnetic Heisenberg spin chains with spin magnitudes $S=1/2, 1,3/2,2$ using the density-matrix renormalization group (DMRG) method initiated by White. For integer spin chains, we find that edge states with spin magnitude $S_{edge}=S/2$ exist, in agreement with Valence-Bond-Solid model picture. For half-integer spin chains, we find that no edge states exist for $S=1/2$ spin chain, but edge state exists in $S=3/2$ spin chain with $S_{edge}=1/2$, in agreement with previous conjecture by Ng. Strong finite size effects associated with spin dimmerization in half-integer spin chains will also be discussed.

cond-mat↗