arXiv subjects

P. Ao

Publications and source records attributed to P. Ao.

At least 55 records · Page 3Linked to original sources

arXiv1998

Origin of Hall Anomaly in the Mixed State

There has been a corporative absence of understanding of Hall anomaly data in the mixed state in terms of vortex many-body effect and pinning, because of the dominant theoretical influence. Now D'Anna et al. [ Phys. Rev. Lett. 81, 2530 (1998) (cond-mat/9808164)] are brave enough to announce the prominent role played by vortex many-body effect and pinning in their interpretation of their own data. Here I wish to point out: (1) Indeed the data of D'Anna et al. can be explained within an existing Hall anomaly theory based on vortex many-body considerations; (2) It is not surprising that their data are not consistent with available microscopic Hall anomaly theories, because those theories are mathematically incorrect; and (3) The courage of D'Anna et al. should be appreciated.

cond-mat↗

arXiv1998

Theory of Hall Anomaly in the Mixed State

Analogous to Peierls' arguments for the `anomalous' Hall in metals I demonstrate that the Hall anomaly in the mixed state of superconductors, the sign change of the Hall resistivity, is a property of a vortex many-body correlation, and show that the anomaly is due to the competition between vortex vacancies and interstitials. Within this vortex many-body effect picture, many features of the complicated Hall effect can be understood in quantitative terms. For example, the expression for the vacancy formation energy is obtained, the scaling relation between the Hall and longitudinal resistivities with the power varying between 1 and 2 is found to depend on sample details, and near the superconducting transition temperature and for small magnetic fields the Hall conductivity is proportional to the inverse of the magnetic field and to the quadratic of the difference between the measured and the transition temperatures.

cond-mat↗

arXiv1998

Friction on a Quantized Vortex in a Superfluid

We obtain the explicit expression of the friction on a moving quantized vortex, following the formulation of Thouless, Ao and Niu. It is shown that the friction on a moving vortex is sensitive to details but does not change the transverse force. We provide a general thermodynamic interpretation for the mutual independence of the transverse force and the friction. The friction is evaluated for the case of quasiparticle contributions in a clean fermionic superfluid, showing a new feature of logarithmic divergence.

cond-mat↗

arXiv1998

Friction Coefficient and Berry Phase for a Topological Singularity in a Superfluid

The coexistence of the universal transverse force and the non-universal friction force on a topological singularity in a superfluid is shown here. Based on the BCS type microscopic theory, we explicitly evaluate the quasiparticle contribution to the friction coefficient in a clean fermionic superfluid, showing a new feature of logarithmic divergence.

cond-mat↗

arXiv1998

Invalidity of Classes of Approximated Hall Effect Calculations

In this comment, I point out a number of approximated derivations for the effective equation of motion, now been applied to d-wave superconductors by Kopnin and Volovik are invalid. The major error in those approximated derivations is the inappropriate use of the relaxation time approximation in force-force correlation functions, or in force balance equations, or in similar variations. This approximation is wrong and unnecessary.

cond-mat↗

arXiv1998

Aharonov-Bohm and Aharonov-Casher Effects: Connections to Dynamics of Topological Singularities

We analyze the physical processes involved in the Aharonov-Bohm (A-B) and the Aharonov-Casher (A-C) effects, showing that an incomplete A-B effect knowledge can lead a totally wrong conclusion on the A-C effect. Based on this we demonstrate that the Magnus force, the net force, is the only transverse force on a moving vortex, in analogous to the net charge in A-C effect. This conclusion has been arrived both theoretically and experimentally.

cond-mat↗

arXiv1998

Effects of Geometric Phases in Josephson Junction Arrays

We show that the en route vortex velocity dependent part of the Magnus force in a Josephson junction array is effectively zero, and predict zero Hall effect in the classical limit. However, geometric phases due to the finite superfluid density at superconductor grains have a profound influence on the quantum dynamics of vortices. Subsequently we find rich and complex Hall behaviors analogous to the Thouless-Kohmoto-Nightingale-den Nijs effect in the quantum regime.

cond-mat↗

arXiv1998

Double Counting of `Chiral Anomaly' in a Model Study of Primordial Baryogenesis

In this comment to the Nature paper of Bevan et al I point out that their interpretation of experimental data is based on a double counting of the `chiral anomaly' due to a vortex motion: using the calculation far away from the vortex core (Berry phase) to cancel the equivalent calculation at the core (spectral flow). The relaxation time approximation in force or momentum balance equation involved in their theory is also wrong, which has been rigorous proved in transport theory since 60's. Hence their affirmative conclusion is premature.

hep-ph↗

arXiv1997

Vortex Dynamics within the BCS Theory

We outline a conventional path integral derivation of the transverse force and the friction for a vortex in a superconductor based on the BCS theory. The derivation is valid in both clean and dirty limits at both zero and finite temperatures. The transverse force is found to be precisely as what has been obtained by Ao and Thouless using the Berry's phase method. The friction is essentially the same as the Bardeen and Stephen's result. Errors in some previous representive microscopic derivations are discussed.

cond-mat.supr-con↗

arXiv1996

Nernst Effect, Seebeck Effect, and Vortex Dynamics in the Mixed State of Superconductors

Based on the vortex dynamics with the Magnus force and a two-fluid model we have derived a set of explicit expressions between the Nernst and Seebeck coefficients, and the Hall and longitudinal resistivities in the linear response regime of the mixed state of superconductors. Effects of vortex pinning are included. The expressions are found to be in agreement with available experimental data. Present results are valid for large as well as for small Hall angle samples, and if Hall angle terms are dropped we recover previous theoretical results. On the other hand, the similar expressions based on an alternative vortex dynamics with various normal current drag forces on vortices are in qualitative disagreement with experiments, which implies the absence of those normal current drag forces.

cond-mat↗

arXiv1996

Derivation of the transverse force on a moving vortex in a superfluid

We describe an exact derivation of 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. The 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.

cond-mat↗

arXiv1996

Hall effect and geometric phases in Josephson junction arrays

Since effectively the local contact vortex velocity dependent part of the Magnus force in a Josephson junction array is zero in the classical limit, we predict zero classical Hall effect. In the quantum limit because of the geometric phases due to the finite superfluid density at superconductor grains, rich and complex Hall effect is found in this quantum regime due to the Thouless-Kohmoto-Nightingale-den-Nijs effect.

cond-mat↗

arXiv1996

Vortex Dynamics in Superfluid Systems: Cyclotron Type Motion

Vortex dynamics in superfluids is investigated in the framework of the nonlinear Schrödinger equation. The natural motion of the vortex is of cyclotron type, whose frequency is found to be on the order of phonon velocity divided by the coherence length, and may be heavily damped due to phonon radiation. Trapping foreign particles into the vortex core can reduce the cyclotron frequency and make the cyclotron motion underdamped. The density fluctuations can follow the vortex motion adiabatically within the phonon wave length at the cyclotron frequency, which results in a further downward renormalization of the cyclotron frequency. We have also discussed applications on the dynamics of vortices in superconducting films.

cond-mat↗

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↗

arXiv1996

Spectral Flow, the Magnus Force, and the Josephson-Anderson Relation

Based on a recent work (Thouless, Ao, and Niu, Transverse force on a quantized vortex in a superfluid) I show that the spectral or momentum flow due to a moving vortex is identical to the phase slippage process discussed by Josephson and Anderson. I also show that the force due to the spectral flow is the reaction force of the Magnus force, and that the two forces act on different objects. Therefore one can conclude that the evaluation of the spepctral flow confirms the results by the Berry's phase calculation of the Magnus force, not the cancelation of the Magnus force as claimed by some authors. A by-product of the present proof is that localized core states do not contribute to the spectral flow, as also noted in the work of Thouless, Ao, and Niu on the Magnus force.

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

Effective Lagrangians for BCS Superconductors at T=0

We show that the low frequency, long wavelength dynamics of the phase of the pair field for a BCS-type s-wave superconductor at T=0 is equivalent to that of a time-dependent non-linear Schrödinger Lagrangian (TDNLSL), when terms required by Galilean invariance are included. If the modulus of the pair field is also allowed to vary, the system is equivalent to two coupled TDNLSL's. We also refer the interested reader to our earlier paper, `Nonlinear Schrodinger equation for superconductors' [cond-mat/9312099], for a different line of derivation

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↗