arXiv subjects

Xiao-Gang Wen

Publications and source records attributed to Xiao-Gang Wen.

At least 235 records · Page 13Linked to original sources

arXiv1999

Continuous topological phase transitions between clean quantum Hall states

Continuous transitions between states with the {\em same} symmetry but different topological orders are studied. Clean quantum Hall (QH) liquids with neutral quasiparticles are shown to have such transitions. For clean bilayer (nnm) states, a continous transition to other QH states (including non-Abelian states) can be driven by increasing interlayer repulsion/tunneling. The effective theories describing the critical points at some transitions are derived.

cond-mat.mes-hall↗

arXiv1999

Staggered-vorticity correlations in a lightly doped t-J model: a variational approach

We report staggered vorticity correlations of current in the d-wave variational wave function for the lightly-doped t-J model. Such correlations are explained from the SU(2) symmetry relating d-wave and staggered-flux mean-field phases. The correlation functions computed by the variational Monte Carlo method suggest that pairs are formed of holes circulating in opposite directions.

cond-mat.str-el↗

arXiv1999

Does quasi-long-range order in the two-dimensional XY model really survive weak random phase fluctuations?

Effective theories for random critical points are usually non-unitary, and thus may contain relevant operators with negative scaling dimensions. To study the consequences of the existence of negative dimensional operators, we consider the random-bond XY model. It has been argued that the XY model on a square lattice, when weakly perturbed by random phases, has a quasi-long-range ordered phase (the random spin wave phase) at sufficiently low temperatures. We show that infinitely many relevant perturbations to the proposed critical action for the random spin wave phase were omitted in all previous treatments. The physical origin of these perturbations is intimately related to the existence of broadly distributed correlation functions. We find that those relevant perturbations do enter the Renormalization Group equations, and affect critical behavior. This raises the possibility that the random XY model has no quasi-long-range ordered phase and no Kosterlitz-Thouless (KT) phase transition.

cond-mat.dis-nn↗

arXiv1999

withdrawn

This paper has been withdrawn by the author.

cond-mat.str-el↗

arXiv1999

Non-equilibrium 2-Channel Kondo model for quantum dots

We find that, under certain condition, a quantum dot with odd number of electron and coupled to two leads can be described by a non-equilibrium 2-channel Kondo model, when the two leads has a large voltage bias between them. The model is exactly soluble and can be mapped into a free fermion system, even in the presence of external magnetic field and other relevant perturbations. All (dynamical) correlation functions can be calculated. The fixed point of the 2-channel Kondo model is a non-equilibrium fixed point and is different from the usual 1-channel and 2-channel Kondo fixed point.

cond-mat.mes-hall↗

arXiv1999

Binding Transition in Quantum Hall Edge States

We study a class of Abelian quantum Hall (QH) states which are topologically unstable (T-unstable). We find that the T-unstable QH states can have a phase transition on the edge which causes a binding between electrons and reduces the number of gapless edge branches. After the binding transition, the single-electron tunneling into the edge gains a finite energy gap, and only certain multi-electron co-tunneling (such as three-electron co-tunneling for $ν=9/5$ edges) can be gapless. Similar phenomenon also appear for edge state on the boundary between certain QH states. For example edge on the boundary between $ν=2$ and $ν=1/5$ states only allow three-electron co-tunneling at low energies after the binding transition.

cond-mat.mes-hall↗

arXiv1998

Preformed Pairs, SU(2) Slave-boson Theory, and High T_c Superconductors

A preformed-pair model was considered. The quantum disordered phase of the d-wave superconducting state was obtained by turning on an on-site repulsion. The low energy effective theory of the quantum disordered phase was derived, which was found to be qualitatively equivalent to the low energy effective theory of the SU(2) slave-boson theory in the staggered-flux phase. Many physical properties of the disordered phase at low energies were obtained, such as the Mott insulator property at zero doping, spin gap and low superfluid density at low doping.

cond-mat.supr-con↗

arXiv1998

Projective Construction of Non-Abelian Quantum Hall Liquids

Using projective construction, a generalized parton construction, we construct many non-Abelian quantum Hall (QH) states, which include the Pfaffian state at filling fraction $ν=1/2$. The projective construction allows us to calculate the bulk and the edge effective theory for the constructed QH state. We illustrate how to use the bulk effective theory to calculate the ground state degeneracy of non-Abelian QH liquids on torus. We point out that the full description of the effective theory requires both the effective Lagrangian and the definition of electron operators. The latter generates all physical states and defines the gauge structure of the theory.

cond-mat.mes-hall↗

arXiv1998

Edge tunneling in fractional quantum Hall regime

We address the issue of an apparent disagreement between theory and experiment on the I-V characteristic of electron tunneling from a metal to the edge of a two-dimensional electron gas in the fractional quantum Hall regime. A part of our result is a theory for the edge of the half-filled Landau level.

cond-mat.str-el↗

arXiv1998

Classification of Disordered Phases of Quantum Hall Edge States

The effects of impurity scattering on a general Abelian fractional quantum Hall (FQH) edge state are analyzed within the chiral-Luttinger-liquid model of low-energy edge dynamics. We find that some disordered edges can have several different phases characterized by different symmetries. The stable impurity edge phases are in general more symmetric than the original clean system and demonstrate the phenomenon of dynamical symmetry restoration at low energies and long length scales. The phase transitions between different disordered phases are characterized by broken symmetries and obey Landau's symmetry breaking principle for continuous phase transitions. Phase diagrams for various edges are found using a new system of coordinates for the interactions between modes in a quantum Hall edge. The temperature dependence of tunneling through a point contact is calculated and is found to be able to distinguish different impurity edge phases of the same FQH state.

cond-mat.mes-hall↗

arXiv1997

Theory of Quasi-Particles in the Underdoped High Tc Superconducting State

The microscopic theory of superconducting (SC) state in the SU(2) slave-boson model is developed. We show how the pseudogap and Fermi surface (FS) segments in the normal state develop into a d-wave gap in the superconducting state. Even though the superfluid density is of order x (the doping concentration), the physical properties of the low lying quasiparticles are found to resemble those in BCS theory. Thus the microscopic theory lay the foundation for our earlier phenomenological discussion of the unusual SC properties in the underdoped cuprates.

cond-mat.str-el↗

arXiv1997

Electron and Quasiparticle Exponents of Haldane-Rezayi state in Non-abelian Fractional Quantum Hall Theory

The quasiparticle propagator of Haldane-Rezayi(HR) fractional quantum Hall (FQH) state is calculated, based on a chiral fermion model (or a Weyl fermion model) equipped with a hidden spin SU(2) symmetry. The spectrum of the chiral fermion model for each total spin and total momentum is shown to be identical to that of the SU(2) c = -2 model introduced to describe the edge spectrum of the HR state.

cond-mat.mes-hall↗

arXiv1997

Massless Dirac Fermions, Gauge Fields, and Underdoped Cuprates

We study 2+1 dimensional massless Dirac fermions and bosons coupled to a U(1) gauge field as a model for underdoped cuprates. We find that the uniform susceptibility and the specific heat coefficient are logarithmically enhanced (compared to linear-in-T behavior) due to the fluctuation of transverse gauge field which is the only massless mode at finite boson density. We analyze existing data, and find good agreement in the spin gap phase. Within our picture, the drop of the susceptibility below the superconducting T_c arises from the suppression of gauge fluctuations.

cond-mat.str-el↗

arXiv1997

The Unusual Superconducting State of Underdoped Cuprates

There is increasing experimental evidence that the superconducting energy gap $Δ_0$ in the underdoped cuprates is independent of doping concentration $x$ while the superfluid density is linear in $x$. We show that under these conditions, thermal excitation of the quasiparticles is very effective in destroying the superconducting state, so that $T_c$ is proportional to $xΔ_0$ and part of the gap structure remains in the normal state. We then estimate $H_{c2}$ and predict it to be proportional to $x^2$. We also discuss to what extent the assumptions that go into the quasiparticle description can be derived in the U(1) and SU(2) formulations of the t-J model.

cond-mat.supr-con↗

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

Disordered Critical Wave functions in Random Bond Models in Two Dimensions -- Random Lattice Fermions at $E=0$ without Doubling

Random bond Hamiltonians of the $π$ flux state on the square lattice are investigated. It has a special symmetry and all states are paired except the ones with zero energy. Because of this, there are always zero-modes. The states near $E=0$ are described by massless Dirac fermions. For the zero-mode, we can construct a random lattice fermion without a doubling and quite large systems ( up to $801 \times 801$) are treated numerically. We clearly demonstrate that the zero-mode is given by a critical wave function. Its multifractal behavior is also compared with the effective field theory.

cond-mat↗

arXiv1995

Gaussian field theories, random Cantor sets and multifractality

The computation of multifractal scaling properties associated with a critical field theory involves non-local operators and remains an open problem using conventional techniques of field theory. We propose a new description of Gaussian field theories in terms of random Cantor sets and show how universal multifractal scaling exponents can be calculated. We use this approach to characterize the multifractal critical wave function of Dirac fermions interacting with a random vector potential in two spatial dimensions. We show that the multifractal scaling exponents are self-averaging.

cond-mat↗

arXiv1995

Topological orders and Edge excitations in FQH states

Fractional quantum Hall (FQH) liquids contain extremely rich internal structures which represent a whole new kind of ordering. We discuss characterization and classification of the new orders (which is called topological orders). We also discuss the edge excitations in FQH liquids, which form the so-called chiral Luttinger liquids. The chiral Luttinger liquids at the edges also have very rich structures as a reflection of the rich topological orders in the bulk. Thus, edge excitations provide us a practical way to measure topological orders in experiments.

cond-mat↗