Vortex state and dynamics of a d-wave superconductor: Finite-element analysis

The finite-element method is extended to simulate the d-wave time-dependent Ginzburg-Landau equations. By utilizing this method and in the context of the (s+d)-wave pairing, we discuss the nature of a single vortex, the structure of equilibrium vortex lattices in bulk samples, the nature of vortices in finite-size samples, and most importantly the transport of the vortices. In particular, the low-field free-flux-flow resistivity turns out to obey the law of corresponding states discovered in conventional superconductors, while the high-field resistivity reveals a noticeable effect of the s-wave coupling on lifting the effective upper critical field. The flux flow near and above the depinning current in the presence of a twin boundary or random impurities also assumes a conventional behavior: The current dependence of the flux-flow resistivity can be well described by an overdamped model for a particle subject to driving and pinning forces. However, our results show a noticeable difference between the flux-flow resistivities at large currents in the presence and absence of pinning.published_or_final_versio

Firing and signal transduction associated with an intrinsic oscillation in neuronal systems

We study the nonlinear firing and signal transduction of a neuron subject to both a constant stimulus and a weak periodic signal, each of which is too small to, separately, fire spikes for the neuron. The subthreshold constant stimulus, regarded as the total input to the neuron from other neurons and from the external world, is found to give rise to only an intrinsic subthreshold oscillation. This oscillation can lead to the transduction of the weak periodic signal via a mechanism similar to stochastic resonance. This may enable us to understand reported experimental results of oscillation associated signal transduction and of finite signal-to-noise ratio in the absence of external noise. In addition, the most sensitive frequency range for signal transduction is also found.published_or_final_versio

Interface roughness and proximity effect on a c-axis Josephson junction between s-wave and d-wave superconductors

The scanning superconducting quantum interference device microscope on tri-crystal high-temperature superconductor (HTSC) samples unambiguously identifies the d-wave pairing symmetry as a predominant component. This fact was also seen clearly from the current phase relation (CPR) for an in-plane junction between HTSC's, where both π periodicity and 2π periodicity are observed, depending on the relative crystal orientation. However, for c-axis junctions between HTSC's and conventional superconductor, ac Josephson effect shows that the main Shapino steps occur at V=nhf/2e (n is integer) and thus a significant s-wave component is indicated. To understand the experimental measurements, we have studied interface roughness and proximity effect on CPR of such junctions. The order parameter profiles and current phase relation are computed self-consistently using the quasiclassical theory and rough interface model. Our results suggest that the existence of a minor surface s-wave component stemming from a repulsive s-channel pairing potential in the d-wave superconductor is able to give a coherent picture.published_or_final_versio

Spontaneous Aharonov-Casher effect of neutral hard-core bosons in one-dimensional mesoscopic rings

Using a tight-binding model Hamiltonian and applying the Jordan-Wigner transformation, we have investigated the Aharonov-Casher (AC) effect for many neutral hard-core bosons in one-dimensional (1D) mesoscopic rings with the self-induced AC phase included. The total energy and the persistent fluxon current are analytically derived for 1D perfect lattices. More importantly, it is suggested that, in the absence of the external AC flux, the self-sustained AC flux state could be the ground state of a system with weak disorder. The possibility of experimenal observations is briefly discussed. © 1995 The American Physical Society.published_or_final_versio

Transverse resistivity and Hall effect of d-wave superconductors with twin boundaries: Numerical solutions of time-dependent Ginzburg-Landau equations in the presence of thermal noise

Taking into account thermal fluctuations, we solve numerically the time-dependent Ginzburg-Landau equations to study the role of twin boundaries on the transverse resistivity as well as Hall effect for a d-wave superconductor. In the presence of an external current parallel to the twin boundary, we observe that the twin boundary (TB) not only behaves as a pinning center, but also induces a negative component of transverse resistivity. A sign reversal may occur for the transverse resistivity and Hall signal, changing from negative to positive as the magnetic field increases. The increase of the strength of the TB can enhance the negative transverse resistivity, but will soon saturate at higher twin-boundary strengths. Only the antisymmetric part of the transverse resistivity is significantly affected by the normal-state off-diagonal conductivity, while the symmetric part may reflect a key role of the TB on the anisotropy.published_or_final_versio

Supercurrent determined from the Aharonov-Bohm effect in mesoscopic superconducting rings

We have solved the Bogoliubov-de Gennes equation for a clean, one-dimensional mesoscopic superconducting ring threaded by a magnetic flux Φ. We show that the superfluid velocity is driven directly by Φ while the relative motion of the pair of electrons is independent of Φ. Meanwhile, the fluxoid quantization is obtained straightforwardly. More importantly, we have also calculated the supercurrent numerically and self-consistently and find it is periodic in Φ with the period Φshc/2e for Φs≤Φd=(mvdL/ Latin small letter h with strokeπ)Φs and with the period Φ0hc/e for Φd<Φs, which arises from mesoscopic effects. © 1994 The American Physical Society.published_or_final_versio

Novel Z2 Topological Metals and Semimetals

© 2016 American Physical Society.We report two theoretical discoveries for Z2 topological metals and semimetals. It is shown first that any dimensional Z2 Fermi surface is topologically equivalent to a Fermi point. Then the famous conventional no-go theorem, which was merely proven before for Z Fermi points in a periodic system without any discrete symmetry, is generalized so that the total topological charge is zero for all cases. Most remarkably, we find and prove an unconventional strong no-go theorem: all Z2 Fermi points have the same topological charge νZ2=1 or 0 for periodic systems. Moreover, we also establish all six topological types of Z2 models for realistic physical dimensions.postprin

Symmetry origin of the phase transitions and phase separation in manganites at low doping

We analyze the symmetry changes of paramagnetic to A-type antiferromagnetic and to ferromagnetic phase transitions in undoped and moderately doped LaMnO 3, respectively. We show that in orthorhombic-distorted perovskite manganites the phase separation at low doping is associated with the noncollinear nature of the magnetic orders permitted by symmetry. A simple model for the competition between the two phase transitions is put forward within the framework of the Landau theory of phase transitions. ©1999 The American Physical Society.published_or_final_versio

Nonadiabatic noncyclic geometric phase and persistent current in one-dimensional rings

The total geometric phase is composed of the nonadiabatic noncyclic Pancharatnam phase, the usual Aharonov-Bohm (AB) phase, and the effective AB phase. It is found that the persistent current in one-dimensional rings is determined from this phase. As applications, we address first the geometric phase and the persistent current in a ring subject to a cylindrically symmetric electromagnetic field. We show that the Pancharatnam phase recovers the Aharanov-Anandan phase in the case of cyclic evolution, as well as the Berry phase in the adiabatic evolution. Moreover, we discuss the persistent current induced by the spin-orbit-induced geometric phase in the presence of a local magnetic field. Generalization to many-body cases is also addressed. ©1999 The American Physical Society.published_or_final_versio

Symmetry analysis of the magnetic structures of bilayered manganites La 2-2xSr 1+2xMn 2O 7 near x = 0.3: Phase separations and percolation

We analyze the symmetry of the magnetic structures of tetragonal bilayered manganites La 2-2xSr 1+2xMn 2O 7 with doping near x=0.3 and formulate a corresponding Landau theory of the phase transitions involved. It is shown that a phase with a single magnetic order cannot be canting though with a mixture of different magnetic orders can, as is possibly the case near x=0.4. Accordingly, a schematic magnetic phase diagram near x=0.3 is constructed which may consistently account for the controversial experimental observations. Possible phase separations and a percolation mechanism of the colossal magnetoresistance are discussed.published_or_final_versio