arXiv subjects

F. C. Zhang

Publications and source records attributed to F. C. Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2006

Experimental Observation of Kondo-Fano Resonant Tunneling in Silicon-Doped GaAs/AlAs Multiple Quantum Wells

We report experimental observation of Kondo-Fano resonant tunneling in Si-doped GaAs/AlAs multiple quantum wells. The spectrum of differential tunneling conductance at low bias shows a strong temperature dependent resonance, whose peak is split in the presence of a magnetic field, characteristic of a Kondo resonance. The data is well explained as a resonant tunneling of conduction electrons at the Fermi level through the doped silicon impurity states inside the wells by using an impurity Anderson model. The Coulomb blockade resonance of the impurity states induces a Fano dip above the Kondo resonance.

cond-mat.mtrl-sci↗

arXiv2005

Exploring Quantum Phase Transitions with a Novel Sublattice Entanglement Scenario

We introduce a new measure called reduced entropy of sublattice to quantify entanglement in spin, electron and boson systems. By analyzing this quantity, we reveal an intriguing connection between quantum entanglement and quantum phase transitions in various strongly correlated systems: the local extremes of reduced entropy and its first derivative as functions of the coupling constant coincide respectively with the first and second order transition points. Exact numerical studies merely for small lattices reproduce several well-known results, demonstrating that our scenario is quite promising for exploring quantum phase transitions.

quant-ph↗

arXiv2005

Entanglement and Quantum Phase Transition in Low Dimensional Spin Systems

Entanglement of the ground states in $XXZ$ and dimerized Heisenberg spin chains as well as in a two-leg spin ladder is analyzed by using the spin-spin concurrence and the entanglement entropy between a selected sublattice of spins and the rest of the system. In particular, we reveal that quantum phase transition points/boundaries may be identified based on the analysis on the local extreme of this entanglement entropy, which is illustrated to be superior over the concurrence scenario and may enable us to explore quantum phase transitions in many other systems including higher dimensional ones.

quant-ph↗

arXiv2004

Gossamer Superconductivity near Antiferromagnetic Mott Insulator in Layered Organic Conductors

Layered organic superconductors are on the verge of the Mott insulator. We use Gutzwiller variational method to study a Hubbard model including a spin exchange coupling term. The ground state is found to be a Gossamer superconductor at small on-site Coulomb repulsion U and an antiferromagnetic Mott insulator at large U, separated by a first order phase transition. Our theory is qualitatively consistent with major experiments reported in organic superconductors.

cond-mat.supr-con↗

arXiv2004

The Physics Behind High-Temperature Superconducting Cuprates: The "Plain Vanilla" Version Of RVB

One of the first theoretical proposals for understanding high temperature superconductivity in the cuprates was Anderson's RVB theory using a Gutzwiller projected BCS wave function as an approximate ground state. Recent work by Paramekanti, Randeria and Trivedi has shown that this variational approach gives a semi-quantitative understanding of the doping dependences of a variety of experimental observables in the superconducting state of the cuprates. In this paper we revisit these issues using the ``renormalized mean field theory'' of Zhang, Gros, Rice and Shiba based on the Gutzwiller approximation in which the kinetic and superexchange energies are renormalized by different doping-dependent factors $g_{t}$ and $g_{S}$ respectively. We point out a number of consequences of this early mean field theory for experimental measurements which were not available when it was first explored, and observe that it is able to explain the existence of the pseudogap, properties of nodal quasiparticles and approximate spin-charge separation, the latter leading to large renormalizations of the Drude weight and superfluid density. We use the Lee-Wen theory of the phase transition as caused by thermal excitation of nodal quasiparticles, and also obtain a number of further experimental confirmations. Finally, we remark that superexchange, and not phonons, are responsible for d-wave superconductivity in the cuprates.

cond-mat.str-el↗

arXiv2003

A renormalised hamiltonian approach for a resonant valence bond wavefunction

The effective Hamiltonian of strongly correlated electrons on a square lattice is replaced by a renormalised Hamiltonian and the factors that renormalise the kinetic energy of holes and the Heisenberg spin-spin coupling are calculated using a Gutzwiller approximation scheme. The accuracy of this renormalisation procedure is tested numerically and found to be qualitatively excellent. Within the scheme a resonant valence bond (RVB) wavefunction is found at half-filling to be lower in energy than the antiferromagnetic state. If the wavefunction is expressed in fermion operators, local SU(2) and U(l) invariance leads to a redundancy in the representation. The introduction of holes removes these local invariances and we find that a d-wave RVB state is lowest in energy. This state has a superconducting order parameter whose amplitude is linear in the density of holes.

cond-mat.str-el↗

arXiv2003

Theory for Gossamer and Resonating Valence Bond Superconductivity

We use an effective Hamiltonian for two-dimensional Hubbard model including an antiferromagnetic spin-spin coupling term to study recently proposed gossamer superconductivity. We formulate a renormalized mean field theory to approximately take into account the strong correlation effect in the partially projected Gutzwiller wavefucntions. At the half filled, there is a first order phase transition to separate a Mott insulator at large Coulomb repulsion U from a gossamer superconductor at small U. Away from the half filled,the Mott insulator is evolved into an resonating valence bond state, which is adiabatically connected to the gossamer superconductor.

cond-mat.str-el↗

arXiv2002

Spin wave theory for antiferromagnetic XXZ spin model on a triangle lattice in the presence of an external magnetic field

Spin wave theory is applied to a quantum antiferromagnetic XXZ model on a triangle lattice in the presence of an in-plane magnetic field. The effect of the field is found to enhance the quantum fluctuation and to reduce the sublattice magnetization at the intermediate field strength in the anisotropic case. The possible implication to the field driven quantum phase transition from a spin solid to a spin liquid is discussed.

cond-mat.str-el↗

arXiv2002

Gossamer Superconductor, Mott Insulator, and Resonating Valence Bond State in Correlated Electron Systems

Gutzwiller variational method is applied to an effective two-dimensional Hubbard model to examine the recently proposed gossamer superconductor by Laughlin. The ground state at half filled electron density is a gossamer superconductor for smaller intra-site Coulomb repulsion U and a Mott insulator for larger U. The gossamer superconducting state is similar to the resonant valence bond superconducting state, except that the chemical potential is approximately pinned at the mid of the two Hubbard bands away from the half filled.

cond-mat.str-el↗

arXiv2002

Antiferromagnetic Heisenberg model on anisotropic triangular lattice in the presence of magnetic field

We use Schwinger boson mean field theory to study the antiferromagnetic spin-1/2 Heisenberg model on an anisotropic triangular lattice in the presence of a uniform external magnetic field. We calculate the field dependence of the spin incommensurability in the ordered spin spiral phase, and compare the results to the recent experiments in Cs$_{2}$CuCl$_{4}$ by Coldea et al. (Phys. Rev. Lett. 86, 1335 (2001)).

cond-mat.str-el↗

arXiv2001

Theory for high spin systems with orbital degeneracy

High-spin systems with orbital degeneracy are studied in the large spin limit. In the absence of Hund's coupling, the classical spin model is mapped onto disconnected orbital systems with spins up and down, respectively. The ground state of the isotropic model is an orbital valence bond state where each bond is an orbital singlet with parallel spins, and neighbouring bonds interact antiferromagnetically. The possible relevance to the transition metal oxides are discussed.

cond-mat.str-el↗

arXiv2001

Theory for Phase Transitions in Insulating Vanadium Oxide

We show that the recently proposed S=2 bond model with orbital degrees of freedom for insulating V$_{2}$O$_{3}$ not only explains the anomalous magnetic ordering, but also other mysteries of the magnetic phase transition. The model contains an additional orbital degree of freedom that exhibits a zero temperature quantum phase transtion in the Ising universality class.

cond-mat.str-el↗

arXiv2000

Metal-insulator transition in CMR materials

We report on resistivity measurements in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ and Nd$_{0.7}$Sr$_{0.3}$MnO$_{3}$ thin films in order to elucidate the underlying mechanism for the CMR behavior. The experimental results are analyzed in terms of quantum phase transition ideas to study the nature of the metal-insulator transition in manganese oxides. Resistivity curves as functions of magnetization for various temperatures show the absence of scaling behavior expected in a continuous quantum phase transition, which leads us to conclude that the observed metal-insulator transition is most likely a finite temperature crossover phenomenon.

cond-mat.str-el↗

arXiv1999

Biquadratic interactions and spin-Peierls transition in the spin 1 chain LiVGe2O6

The magnetic susceptibility of a new one-dimensional, S=1 system, the vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is consistent with a gapless spectrum above this temperature. We propose that this behaviour is due to the presence of a significant biquadratic exchange interaction, a suggestion supported by quantum chemistry calculations that take into account the quasi-degeneracy of the t2g levels.

cond-mat.str-el↗

arXiv1998

Elementary excitations of magnetically ordered systems with orbital degeneracy

The Holstein-Primakoff transformation is generalized to develop a quantum flavor wave theory for spin systems with orbital degeneracy. Elementary excitations consist of spin, orbital, and spin-orbital waves. Spin and spin-orbital waves couple each other due to orbital anisotropy and Hund's rule, resulting in new modes observable by inelastic neutron scattering. In the SU(4) limit, flavor waves are dispersionless along one or more directions, and give rise to quantum fluctuations of reduced dimensionality.

cond-mat.str-el↗

arXiv1998

Theory for Metal Hydrides with Switchable Optical Properties

Recently it has been discovered that lanthanum, yttrium, and other metal hydride films show dramatic changes in the optical properties at the metal-insulator transition. Such changes on a high energy scale suggest the electronic structure is best described by a local model based on negatively charged hydrogen (H$^-$) ions. We develop a many-body theory for the strong correlation in a H$^-$ ion lattice. The metal hydride is described by a large $U$-limit of an Anderson lattice model. We use lanthanum hydride as a prototype of these compounds, and find LaH$_3$ is an insulator with a substantial gap consistent with experiments. It may be viewed either as a Kondo insulator or a band insulator due to strong electron correlation. A H vacancy state in LaH$_3$ is found to be highly localized due to the strong bonding between the electron orbitals of hydrogen and metal atoms. Unlike the impurity states in the usual semiconductors, there is only weak internal optical transitions within the vacancy. The metal-insulator transition takes place in a band of these vacancy states.

cond-mat.str-el↗

arXiv1998

SU(4) Theory for Spin Systems with Orbital Degeneracy

The isotropic limit of spin systems with orbital degeneracy is shown to have global SU(4) symmetry. On many 2D lattices, the ground state does not posses long range order, which may explain the observed spin liquid properties of $LiNiO_2$. In the SU(4) Neel ordered state, spin-spin correlations can be antiferromagneitc between two neighboring sites with parallel magnetic moments.

cond-mat.str-el↗

arXiv1996

Electronic Structure of Lanthanum Hydrides with Switchable Optical Properties

Recent dramatic changes in the optical properties of LaH_{2+x} and YH_{2+x} films discovered by Huiberts et al. suggest their electronic structure is described best by a local model. Electron correlation is important in H^- -centers and in explaining the transparent insulating behavior of LaH_3. The metal-insulator transition at $x\sim 0.8$ takes place in a band of highly localized states centered on the $H$-vacancies in the LaH_3 structure.

cond-mat↗