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Dung-Hai Lee

Publications and source records attributed to Dung-Hai Lee.

At least 145 records · Page 8Linked to original sources
arXiv2002

Quasiparticle Scattering Interference in High Temperature Superconductors

We propose that the energy-dependent spatial modulation of the local density of states seen by Hoffman, et al [hoff2] is due to the scattering interference of quasiparticles. In this paper we present the general theoretical basis for such an interpretation and lay out the underlying assumptions. As an example, we perform exact T-matrix calculation for the scattering due to a single impurity. The results of this calculation is used to check the assumptions, and demonstrate that quasiparticle scattering interference can indeed produce patterns similar to those observed in Ref. [hoff2].

cond-mat.supr-con↗
arXiv2002

Competing Order in the Mixed State of High Temperature Superconductors

We examine the low temperature behavior of the mixed state of a layered superconductor in the vicinity of a quantum critical point separating a pure superconducting phase from a phase in which a competing order coexists with superconductivity. At zero temperature, we find that there is an avoided critical point in the sense that the phase boundary in the limit $B\to 0$ does not connect to the B=0 critical point. Consequently, there exists a quasi-1D regime of the phase diagram, in which the competing order is largely confined to 1D ``halos'' about each vortex core, and in which interactions between neighboring vortices, although relevant at low temperature, are relatively weak.

cond-mat.supr-con↗
arXiv2002

Transport in Inhomogeneous Strongly Correlated Systems

We consider an inhomogeneous strongly correlated system where external disorder divides it into mesoscopic cells.Strong inter-particle repulsion suppresses the quantum tunneling between cells and open a wide temperature range for incoherent transport.For a particular class of such inhomogeneous systems we find the low-energy transport to be free-fermion like when the constituent particles are spinless fermions.

cond-mat.str-el↗
arXiv2001

Pairing in the spin sector

The nanometer-scale gap inhomogeneity revealed by recent STM images of BSCCO surface suggests that the ``gap coherence length'' is of that order. Thus a robust pairing gap can develop despite the poorly screened Coulomb interaction. This can be taken as an evidence that the pairing in high $T_c$ materials occurs primarily in the neutral (spin) sector. We provide theoretical support for this point of view.

cond-mat.supr-con↗
arXiv2001

The Chern-Simons Invariant in the Berry Phase of a Two by Two Hamiltonian

The positive (negaive)-energy eigen vectors of the two by two Hamiltonian $H=ř\cdot\vec{\s}$ where $\vec{\s}$ are the Pauli matrices and $ř$ is a 3-vector, form a U(1) fiber bundle when $ř$ sweeps over a manifold $\cM$ in the three dimensional parameter space of $ř$ . For appropriately chosen base space $\cM$ the resulting fiber bundle can have non-trivial topology. For example when $\cM=S^2\equiv\{ř; |ř|=1\}$ the corresponding bundle has a non-zero Chern number, which is the indicator that it is topologically non-trivial. In this paper we construct a two by two Hamiltonian whose eigen bundle shows a more subtle topological non-triviality over $\cM=R^3\bigcup\{\infty\}$, the stereographic projection of $S^3$. This non-triviality is characterized by a non-zero Chern-Simons invariant.

cond-mat.str-el↗
arXiv2001

Staggered currents in the mixed state

The current pattern in the mixed state of high-$T_{c}$ superconductors is studied in the U(1) mean field theory of the t-J model. Our findings are the following. 1) In the absence of antiferromagnetism a robust staggered current pattern exists in the core of vortices if the doping is not too high. 2) At a fixed doping and with increasing magnetic field, the size of the staggered current core expands, and eventually percolates. 3) The polarity of the staggered current is pinned by the direction of the magnetic field. 4) Vortex cores locally modify the hole density - in a staggered (non-staggered) core, the excess charge is slightly negative (positive). 5) Gutzwiller projection does not wash out the staggered current. Finally we present two experimental predictions concerning neutron scattering and STM spectra that capture the signature of the staggered current induced by the vortices.

cond-mat.supr-con↗
arXiv2000

Staggered Currents in the Vortex Core

We study the electronic structure of the vortex core in the cuprates using the U(1) slave-boson mean-field wavefunctions and their Gutzwiller projection. We conclude that there exists local orbital antiferromagnetic order in the core near optimal doping. We compare the results with that of BCS theory and analyze the spatial dependence of the local tunneling density of states.

cond-mat.supr-con↗
arXiv2000

The Superfluid Density in the D-density-wave Scenario

Recently Chakravarty, Laughlin, Morr and Nayak made an interesting proposal that the cuprate superconductors possess a hidden ``d-density wave'' order. We study the implication of this proposal for the superfluid density ρ_s. We find that it predicts a temperature gradient |dρ_s/dT|_{T=0} that is strongly doping dependent near the critical doping (\approx 5%) at which the superconducting transition temperature vanishes.

cond-mat.supr-con↗
arXiv2000

Antiferromagnetism, Stripes, and Superconductivity in the t-J Model with Coulomb Interaction

We study mean-field phases of the t-J model with long-range Coulomb interaction. In the order of increasing doping density we find a classical antiferromagnet, charge and spin stripes, and a uniform d-wave superconductor, at the realistic doping parameters. Both in-phase and anti-phase stripes exist as metastable configurations, but the in-phase stripes have a slightly lower energy. The dependence of the stripe width and the inter-stripe spacing on the doping is examined. Effects of fluctuations around the mean-field states are discussed.

cond-mat.supr-con↗
arXiv2000

The Structure of a Vortex in the t-J Model

We study the single-vortex solution of the t-J model within resonating-valence-bond (RVB) mean-field theory. We find two types of vortex cores, insulating and metallic, depending on the parameters of the model. The pairing order parameter near both cores have $d_{x^2 -y^2}+iηd_{xy}$ symmetry. For some range of $t/J$ the calculated tunneling spectrum of the metallic vortex core agrees qualitatively with the STM tunneling data for BSCCO.

cond-mat.supr-con↗
arXiv1999

Superconductivity in a doped Mott insulator

Starting from the d-wave RVB mean-field theory of Kotliar and Liu, we present a new, long-wavelength/low-energy exact, treatment of gauge fluctuations. The result is a theory of gapless fermion quasiparticles coupled to superconducting phase fluctuations. We will discuss the physical implications, and the similarity/differences with a theory of BCS pairing with phase fluctuations.

cond-mat.str-el↗
arXiv1999

Pairing via Index theorem

This work is motivated by a specific point of view: at short distances and high energies the undoped and underdoped cuprates resemble the $π$-flux phase of the t-J model. The purpose of this paper is to present a mechanism by which pairing grows out of the doped $π$-flux phase. According to this mechanism pairing symmetry is determined by a parameter controlling the quantum tunneling of gauge flux quanta. For zero tunneling the symmetry is $d_{x^2-y^2}+id_{xy}$, while for large tunneling it is $d_{x^2-y^2}$. A zero-temperature critical point separates these two limits.

cond-mat.str-el↗
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

Neutral Fermions at $ν=1/2$

We present a particle-hole symmetric theory for a two-dimensional electron gas at filling factor one half. In this theory, elementary excitations are dipole-like fermions floating on top of the $ν=1/2$ boson quantum Hall liquid. In the absence of disorder these dipoles can form a Fermi-liquid. Disorder can break them apart and drive the ground state into a quantum critical state.

cond-mat.mes-hall↗
arXiv1996

Critical Conductance and Its Fluctuations at Integer Hall Plateau Transitions

Under periodic boundary condition in the transverse direction, we calculate the averaged zero-temperature two-terminal conductance ($ $) and its statistical fluctuations ($<(\dg)^{2n}>$ for $n\le 4$) at the critical point of integer quantum Hall plateau transitions. We find {\it universal} values for $ =(0.58\pm0.03){e^2\over h}$, and $<(\dg)^{2n}>=({e^2\over h})^{2n}A_{2n}$, where $A_{2,4,6,8}=0.081\pm0.005$; $0.013\pm0.003$; $0.0026\pm0.005$; and $(8\pm2)\times10^{-4}$ respectively. We also determine the leading finite size scaling corrections to these observables. Comparisons with experiments will be made.

cond-mat.mes-hall↗
arXiv1996

The Effects of Electron-Electron Interactions on the Integer Quantum Hall Transitions

We study the effects of electron-electron interaction on the critical properties of the plateau transitions in the {\it integer} quantum Hall effect. We find the renormalization group dimension associated with short-range interactions to be $-0.66\pm0.04$. Thus the non-interacting fixed point (characterized $z=2$ and $ν\approx 2.3$) is stable. For the Coulomb interaction, we find the correlation effect is a marginal perturbation at a Hartree-Fock fixed point ($z=1$, $ν\approx 2.3$) by dimension counting. Further calculations are needed to determine its stability upon loop corrections.

cond-mat↗