arXiv subjects

Q. Niu

Publications and source records attributed to Q. Niu.

29 records · Page 2Linked to original sources

arXiv1996

Transverse force on a quantized vortex in a superfluid

We have derived an exact expression for the total nondissipative transverse force acting on a quantized vortex moving in a uniform background. The derivation is valid for neutral boson or fermion superfluids, provided the order parameter is a complex scalar quantity. This force is determined by the one-particle density matrix far away from the vortex core, and is found to be the Magnus force proportional to the superfluid density. We conclude that contributions of the localized core states do not change this force.

cond-mat↗

arXiv1995

Berry phase, hyperorbits, and the Hofstadter spectrum: semiclassical dynamics in magnetic Bloch bands

We have derived a new set of semiclassical equations for electrons in magnetic Bloch bands. The velocity and energy of magnetic Bloch electrons are found to be modified by the Berry phase and magnetization. This semiclassical approach is used to study general electron transport in a DC or AC electric field. We also find a close connection between the cyclotron orbits in magnetic Bloch bands and the energy subbands in the Hofstadter spectrum. Based on this formalism, the pattern of band splitting, the distribution of Hall conduct- ivities, and the positions of energy subbands in the Hofstadter spectrum can be understood in a simple and unified picture.

cond-mat↗

arXiv1995

Berry phase, hyperorbits, and the Hofstadter spectrum

We develop a semiclassical theory for the dynamics of electrons in a magnetic Bloch band, where the Berry phase plays an important role. This theory, together with the Boltzmann equation, provides a framework for studying transport problems in high magnetic fields. We also derive an Onsager-like formula for the quantization of cyclotron orbits, and we find a connection between the number of orbits and Hall conductivity. This connection is employed to explain the clustering structure of the Hofstadter spectrum. The advantage of this theory is its generality and conceptual simplicity.

cond-mat↗

arXiv1995

Weak Field Phase Diagram for an Integer Quantum Hall Liquid

We study the localization properties in the transition from a two-dimensional electron gas at zero magnetic field into an integer quantum Hall (QH) liquid. By carrying out a direct calculation of the localization length for a finite size sample using a transfer matrix technique, we systematically investigate the field and disorder dependences of the metal-insulator transition in the weak field QH regime. We obtain a different phase diagram from the one conjectured in previous theoretical studies. In particular, we find that: (1) the extended state energy $E_{c}$ for each Landau level (LL) is {\it always} linear in magnetic field; (2) for a given Landau level and disorder configuration there exists a critical magnetic field $B_{c}$ below which the extended state disappears; (3) the lower LLs are more robust to the metal-insulator transition with smaller $B_{c}$. We attribute the above results to strong LL coupling effect. Experimental implications of our work are discussed.

cond-mat↗

arXiv1994

Double-tip STM for Surface Analysis

We explore the possibility of using a double-tip STM to probe the single electron Green function of a sample surface, and describe a few important applications: (1) Probing constant energy surfaces in $\k$-space by ballistic transport; (2) Measuring scattering phase shifts of defects; (3) Observing the transition from ballistic to diffusive transport to localization; and (4) Measuring inelastic mean free paths.

cond-mat↗

arXiv1994

Interactions of Collective Excitations with Vortices in Superfluid Systems

We investigate the interactions of collective excitations with vortices in superfluid systems, including $~^4$He and superconductors. The dynamical equations are obtained by the aid of the many-body wavefunction and the density-density correlation function. The scattering cross-section of collective excitations with a vortex is calculated in the Born approximation (valid at long wavelengths), and is expressed in terms of the collective excitation spectrum $ ω(q) $ by the simple and general formula $ σ= \fracπ{2} \frac{q}{[ω'(q)]^2} \cot^2\fracθ{2} $, where $ q $ is the wave number of the excitation, and $θ$ is the scattering angle. At short wave lengths, the classical equations of motion are derived.

cond-mat↗

arXiv1994

Dynamic Fractional Stark Ladders in DC--AC Fields

A single band in a spatially periodic system splits into a series of quasienergy subbands under the action of DC--AC electric fields. These dynamic fractional Stark ladders should be observable in an investigation of optical absorption.

cond-mat↗

arXiv1994

Electron Band Structure in a Two Dimensional Periodic Magnetic Field

In this paper we study the energy spectrum of a two dimensional electron gas (2DEG) in a two dimensional periodic magnetic field. Both a square magnetic lattice and a triangular one are considered. We consider the general case where the magnetic field in a cell can be of any shape. A general feature of the band structure is bandwidth oscillation as a function of the Landau index. A triangular magnetic lattice on a 2DEG can be realized by the vortex lattice of a superconductor film coated on top of a heterojunction. Our calculation indicates a way of relating the energy spectrum of the 2DEG to the vortex structure. We have also derived conditions under which the effects of a weak magnetic modulation, periodic or not, may be reproduced by an electric potential modulation, and vice versa.

cond-mat↗

arXiv1994

Quantized Density Response in Insulators

The response of particle density to a dilation of a periodic potential in an insulator, with or without a fixed background potential or a magnetic field, is shown to be quantized. A similar phenomenon occurs in a quantum Hall system, where the derivative of the electron density with respect to the magnetic field is quantized in units of $e/h$, the inverse flux quantum. A number of other interesting results may be unified under the notion of quantized density response, including the quantization of adiabatic particle transport, the gap labeling theorem and its generalization in the presence of a magnetic field.

cond-mat↗

arXiv1994

From Feynman's Wave Function to the Effective Theory of Vortex Dynamics

We calculate the overlap between two many-body wave functions for a superfluid film containing a vortex at shifted positions. Comparing the results to phenomenological theories, which treat vortices as point particles, we find that the results are consistent if the point-particle vortices are considered as under the action of the Magnus force and in weak interaction with sound waves of the superfluid. We are then able to resolve the disagreement concerning the effective mass of vortices, showing it is finite.

cond-mat↗

arXiv1993

Local Density of States and Level Width for Wannier-Stark Ladders

The local density of states ρ(x,E) is calculated for a Bloch electron in an electric field. Depending on the system size, we can see one or more sequences of Wannier-Stark ladders in ρ(x,E), with Lorentz type level widths and apparent spatial localization of the states. Our model is a chain of delta function potential barriers plus a step-like electric potential, with open boundary condition at both ends of the system. Using a wave tunneling picture, we find that the level widths shrink to zero as an inverse power as the system size approaches infinity, confirming an earlier result.

cond-mat↗