arXiv subjects

D. N. Sheng

Publications and source records attributed to D. N. Sheng.

At least 181 records · Page 10Linked to original sources

arXiv2001

Spectral function of the electron in a superconducting RVB state

We present a model calculation of the spectral function of an electron in a superconducting resonating valence bond (RVB) state. The RVB state, described by the phase-string mean field theory is characterized by three important features: (i) spin-charge separation, (ii) short range antiferromagnetic correlations, and (iii) holon condensation. The results of our calculation are in good agreement with data obtained from Angle Resolved Photoemission Spectroscopy (ARPES) in superconducting Bi 2212 at optimal doping concentration.

cond-mat.supr-con↗

arXiv2001

Spin-charge separation: From one hole to finite doping

In the presence of nonlocal phase shift effects, a quasiparticle can remain topologically stable even in a spin-charge separation state due to the confinement effect introduced by the phase shifts at finite doping. True deconfinement only happens in the zero-doping limit where a bare hole can lose its integrity and decay into holon and spinon elementary excitations. The Fermi surface structure is completely different in these two cases, from a large band-structure-like one to four Fermi points in one-hole case, and we argue that the so-called underdoped regime actually corresponds to a situation in between.

cond-mat.str-el↗

arXiv2000

Quasiparticle Localization in Disordered d-Wave Superconductors

An extensive numerical study is reported on disorder effect in two-dimensional d-wave superconductors with random impurities in the unitary limit. It is found that a sharp resonant peak shows up in the density of states at zero energy and correspondingly the finite-size spin conductance is strongly enhanced which results in a non-universal feature in one-parameter scaling. However, all quasiparticle states remain localized, indicating that the resonant density peak alone is not sufficient to induce delocalization. In the weak disorder limit, the localization length is so long that the spin conductance at small sample size is close to the universal value predicted by Lee (Phys. Rev. Lett. {\bf 71}, 1887 (1993)).

cond-mat.supr-con↗

arXiv2000

Spin-charge separation in the single hole doped Mott antiferromagnet

The motion of a single hole in a Mott antiferromagnet is investigated based on the t-J model. An exact expression of the energy spectrum is obtained, in which the irreparable phase string effect [Phys. Rev. Lett. 77, 5102 (1996)] is explicitly present. By identifying the phase string effect with spin backflow, we point out that spin-charge separation must exist in such a system: the doped hole has to decay into a neutral spinon and a spinless holon, together with the phase string. We show that while the spinon remains coherent, the holon motion is deterred by the phase string, resulting in its localization in space. We calculate the electron spectral function which explains the line shape of the spectral function as well as the ``quasiparticle'' spectrum observed in angle-resolved photoemission experiments. Other analytic and numerical approaches are discussed based on the present framework.

cond-mat.str-el↗

arXiv2000

Understanding high-Tc cuprates based on the phase string theory of doped antiferromagnet

We present a self-consistent RVB theory which unifies the metallic (superconducting) phase with the half-filling antiferromagnetic (AF) phase. Two crucial factors in this theory include the RVB condensation which controls short-range AF spin correlations and the phase string effect introduced by hole hopping as a key doping effect. We discuss both the uniform and non-uniform mean-field solutions and show the unique features of the characteristic spin energy scale, superconducting transition temperature, and the phase diagram, which are all consistent with the experimental measurements of high-$T_c$ cuprates.

cond-mat.str-el↗

arXiv2000

Quantum Hall effect at weak magnetic field: New float-up picture

The fate of integer quantum Hall effect (IQHE) at weak magnetic field is studied numerically in the presence of {\it correlated} disorders. We find a systematic {\it float-up} and {\it merging} picture for extended levels on the low-energy side which results in direct transitions from higher-plateau IQHE states to the insulator. Such direct transitions are controlled by a quantum critical point with a {\it universal} scaling form of conductance. The phase diagram is in good agreement with recent experiments. The issue of continuum vs. lattice model is also discussed.

cond-mat.mes-hall↗

arXiv2000

Nature of spin-charge separation

Quasiparticle properties are explored in an effective theory of the $t-J$ model which includes two important components: spin-charge separation and unrenormalizable phase shift. We show that the phase shift effect indeed causes the system to be a non-Fermi liquid as conjectured by Anderson on a general ground. But this phase shift also drastically changes a conventional perception of quasiparticles in a spin-charge separation state: an injected hole will remain {\em stable} due to the confinement of spinon and holon by the phase shift field despite the background is a spinon-holon sea. True {\em deconfinement} only happens in the {\em zero-doping} limit where a bare hole will lose its integrity and decay into holon and spinon elementary excitations. The Fermi surface structure is completely different in these two cases, from a large band-structure-like one to four Fermi points in one-hole case, and we argue that the so-called underdoped regime actually corresponds to a situation in between, where the ``gap-like'' effect is amplified further by a microscopic phase separation at low temperature. Unique properties of the single-electron propagator in both normal and superconducting states are studied by using the equation of motion method. We also comment on some of influential ideas proposed in literature related to the Mott-Hubbard insulator and offer a unified view based on the present consistent theory.

cond-mat.str-el↗

arXiv1999

Isotropic dispersion, line shape, and remnant Fermi surface in one hole problem

It is shown that the isotropic dispersion relation, line shape of the spectral function, and remnant Fermi surface found in recent photoemission experiment in insulating Ca_2CuO_2Cl_2 by Ronning et al.(Science, 282, 2067 (1998)) can be consistently explained by a direct holon hopping process which fundamentally undermines the conventional self-consistent Born approximation approach to the t-J model.

cond-mat.str-el↗

arXiv1999

Phase Diagram of Integer Quantum Hall Effect

The phase diagram of integer quantum Hall effect is numerically determined in the tight-binding model, which can account for overall features of recently obtained experimental phase diagram. In particular, the quantum Hall plateaus are terminated by two distinct insulating phases, characterized by the Hall resistance with classic and quantized values, respectively, which is also in good agreement with experiments.

cond-mat.mes-hall↗

arXiv1999

Universal scaling, beta function, and metal-insulator transitions

We demonstrate a universal scaling form of longitudinal resistance in the quantum critical region of metal-insulator transitions, based on numerical results of three-dimensional Anderson transitions (with and without magnetic field), two-dimensional quantum Hall plateau to insulator transition, as well as experimental data of the recently discovered two-dimensional metal-insulator transition. The associated reflection symmetry and a peculiar logarithmic form of the beta function exist over a wide range in which the resistance can change by more than one order of magnitude. Interesting implications for the two-dimensional metal-insulator transition are discussed.

cond-mat.dis-nn↗

arXiv1999

Metal-insulator transition in spatially-correlated random magnetic field system

We reexamine the problem of delocalization of two-dimensional electrons in the presence of random magnetic field. By introducing spatial correlations among random fluxes, a well-defined metal-insulator transition characterized by a two-branch scaling of conductance has been demonstrated numerically. Critical conductance is found non-universal with a value around $e^2/h$. Interesting connections of this system with the recently observed B=0 two-dimensional metallic phase (Kravchenko et al., Phys. Rev. B {\bf 50}, 8039 (1994)) are also discussed.

cond-mat.dis-nn↗

arXiv1998

Theory of Ferromagnetic Metal to Paramagnetic Insulator Transition i in R_{1-x}A_{x}MnO_{3}

The double-exchange model for the Mn oxides with orbital degeneracy is studied with including on-site Coulomb repulsion, Jahn-Teller (J-T) coupling and doping-induced disorder. In the strong interaction limit, it is mapped onto a single-band Anderson model, in which all scattering mechanisms can be treated on an equal footing. A sharp rise in the mean square fluctuation of lattice distortions is found near the Curie temperature $T_c$, in agreement with experiments. We show that the spin and J-T disorders lead to a metal-insulator transition (MIT) only at low carrier density. The MIT observed in samples with $0.2\leq x<0.5$ can be explained by further including the disorder effect of cation size mismatch.

cond-mat.str-el↗

arXiv1998

Reflection Symmetry and Quantized Hall Resistivity near Quantum Hall Transition

We present a direct numerical evidence for reflection symmetry of longitudinal resistivity $ρ_{xx}$ and quantized Hall resistivity $ρ_{xy}$ near the transition between $ν=1$ quantum Hall state and insulator, in accord with the recent experiments. Our results show that a universal scaling behavior of conductances, $σ_{xx}$ and $σ_{xy}$, in the transition regime decide the reflection symmetry of $ρ_{xx}$ and quantization of $ρ_{xy}$, independent of particle-hole symmetry. We also find that in insulating phase away from the transition region $ρ_{xy}$ deviates from the quantization and diverges with $ρ_{xx}$.

cond-mat.mes-hall↗

arXiv1998

Magnetic Incommensurability in Doped Mott Insulator

In this paper we explore the incommensurate spatial modulation of spin-spin correlations as the intrinsic property of the doped Mott insulator, described by the $t-J$ model. We show that such an incommensurability is a direct manifestation of the phase string effect introduced by doped holes in both one- and two-dimensional cases. The magnetic incommensurate peaks of dynamic spin susceptibility in momentum space are in agreement with the neutron-scattering measurement of cuprate superconductors in both position and doping dependence. In particular, this incommensurate structure can naturally reconcile the neutron-scattering and NMR experiments of cuprates.

cond-mat.str-el↗

arXiv1998

Mean-Field Description of Phase String Effect in the $t-J$ Model

A mean-field treatment of the phase string effect in the $t-J$ model is presented. Such a theory is able to unite the antiferromagnetic (AF) phase at half-filling and metallic phase at finite doping within a single theoretical framework. We find that the low-temperature occurrence of the AF long range ordering (AFLRO) at half-filling and superconducting condensation in metallic phase are all due to Bose condensations of spinons and holons, respectively, on the top of a spin background described by bosonic resonating-valence-bond (RVB) pairing. The fact that both spinon and holon here are bosonic objects, as the result of the phase string effect, represents a crucial difference from the conventional slave-boson and slave-fermion approaches. This theory also allows an underdoped metallic regime where the Bose condensation of spinons can still exist. Even though the AFLRO is gone here, such a regime corresponds to a microscopic charge inhomogeneity with short-ranged spin ordering. We discuss some characteristic experimental consequences for those different metallic regimes. A perspective on broader issues based on the phase string theory is also discussed.

cond-mat.str-el↗

arXiv1997

A bosonic RVB description of doped antiferromagnet

We propose a theory for doped antiferromagnet based on a bosonic resonating-valence-bond (RVB) state with incorporating the phase string effect. Both antiferromagnetic (AF) and superconducting phase transitions occur naturally within such a bosonic RVB phase. Two distinct metallic regions -- underdoping and optimum-doping -- are also found to be a logic consequence, whose unique features explain the recent neutron-scattering measurements in cuprates.

cond-mat.str-el↗

arXiv1997

New universality of metal-insulator transition in integer quantum Hall effect system

A new universality of metal-insulator transition in integer quantum Hall effect (IQHE) system is studied based on a lattice model, where the IQHE states only exist within a finite range of Fermi energy in the presence of disorders. A two-parameter scaling law is found at the high-energy boundary where direct transitions from high IQHE states to insulator occur. We find $ρ_{xx}= ρ_{xy}$ at the critical point whose value can continuously vary as a single function of the Landau-level filling number $n_ν$. Such a new universality well explains recent experiment by Song et al. (Phys. Rev. Lett. 78, 2200 (1997)).

cond-mat.mes-hall↗

arXiv1996

Phase String Effect in the t-J Model: General Theory

We reexamine the problem of a hole moving in an antiferromagnetic spin background and find that the injected hole will always pick up a sequence of nontrivial phases from the spin degrees of freedom. Previously unnoticed, such a string-like phase originates from the hidden Marshall signs which are scrambled by the hopping of the hole. We can rigorously show that this phase string is non-repairable at low energy and give a general proof that the spectral weight Z must vanish at the ground-state energy due to the phase string effect. Thus, the quasiparticle description fails here and the quantum interference effect of the phase string dramatically affects the long-distance behavior of the injected hole. We introduce a so-called phase-string formulation of the t-J model for a general number of holes in which the phase string effect can be explicitly tracked. As an example, by applying this new mathematical formulation in one dimension, we reproduce the well-known Luttinger-liquid behaviors of the asymptotic single-electron Green's function and the spin-spin correlation function. We can also use the present phase string theory to justify previously developed spin-charge separation theory in two dimensions, which offers a systematic explanation for the transport and magnetic anomalies in the high-T_c cuprates.

cond-mat.str-el↗