一维化学势调制的p波超导体中的拓扑量子相变

本文研究了一维公度势和非公度势调制下的p波超导量子线系统的拓扑相变.在公度势调制下,通过计算Z2拓扑不变量确定系统的相图,指出系统的拓扑相变强烈地依赖于调制参数α和相移δ.在非公度势调制下,以α=(√5-1)/2,δ=0为例,计算系统的低能激发谱、Z2拓扑不变量以及逆参与率等,发现p波配对强度△∈(0,0.33)时,系统存在拓扑非平庸超导相,拓扑平庸超导相和拓扑平庸局域相的转变.而当p波配对强度△>0.33时,系统存在拓扑非平庸超导相和拓扑平庸局域相的转变.     

Pairing in one-dimensional antiferromagnetic superconductors in the presence of a homogeneous magnetic field

Assuming a one-dimensional like electron band and considering the free energy difference δF(= F_s?F_N) near the second order phase transition temperature, it is shown that for an antiferromagnetic superconductor in a homogeneous magnetic field (howsoever weak) the minimum of δF occurs when the state with spatially varying order parameter (having same period as the antiferromagnetic ordering) coexists with the usual BCS state.     

Fluctuation theory of the one-dimensional “mixed” state in first kind superconductors

When superconductivity of the first kind in a cylinder is strongly suppressed by a current, only superconducting fluctuations exist near the sample axis. This one-dimensional “mixed” state is destroyed near some critical current. The width of the transition region is the smaller the purer the sample.     

Interplay between a homogeneous magnetic field and an antiferromagnetic field in one-dimensional superconductors

Mean-field theory applied to superconductors with one-dimensional band in the presence of both the homogeneous magnetic field H₀ and the antiferromagnetic field H_Q, the second-order phase transition temperature is investigated for the arbitrary angle θ between H₀ and H_Q. It is found that the remarkable superconducting region in the case of θ = 0 is retained only for small θ and that the spatially dependent order parameter coexists with the spatially uniform order parameter except for θ = τ/2.     

Transients in optical bistability

In this paper we report on both a theoretical model for the transients in optical bistability in a Fabry-Perot cavity and the corresponding experimental results. An analytical calculation permits to predict the shape of the transient curves and the order of magnitude of their time duration in the case of a slow sweep of the length of the cavity. The experiment, which uses the nonlinearity arising from a two-photon transition in rubidium, yields results in good agreement with the theoretical calculation.Work supported by the D.R.E.T.     

Transients in sheared granular matter

As dense granular materials are sheared, a shear band and an anisotropic force network form. The approach to steady-state behavior depends on the history of the packing and the existing force and contact network. We present experiments on shearing of dense granular matter in a 2D Couette geometry in which we probe the history and evolution of shear bands by measuring particle trajectories and stresses during transients. We find that when shearing is stopped and restarted in the same direction, steady-state behavior is immediately reached, in agreement with the typical assumption that the system is quasistatic. Although some relaxation of the force network is observed when shearing is stopped, quasistatic behavior is maintained because the contact network remains essentially unchanged. When the direction of shear is reversed, a transient occurs in which stresses initially decrease, changes in the force network reach further into the bulk, and particles far from the wheel become more mobile. This occurs because the force network is fragile to changes transverse to the force network established under previous shear; particles must rearrange before becoming jammed again, thereby providing resistance to shear in the reversed direction. The strong force network is re-established after displacing the shearing surface , where d is the mean grain diameter. Steady-state velocity profiles are reached after a shear of . Particles immediately outside of the shear band move on average less than 1 diameter before becoming jammed again. We also examine particle rotation during this transient and find that mean particle spin decreases during the transient, which is related to the fact that grains are not interlocked as strongly.Received: 5 March 2004, Published online: 24 August 2004PACS: 45.70.-n Granular systems - 83.80.Fg Granular solids     

Transients in thin-base semiconductor diodes

Transients for thin-base diodes following a step in the forward bias or current are discussed. The recombination rate at an ohmic contact is taken as arbitrary. The results enable this recombination rate to be determined, given the bulk lifetime of the minority carriers in the base.I am indebted to V. A. Chaldyshev, A. P. Vyatkin, and A. A. Sirotkin for valuable advice and discussions.     

Transients and asymptotics in granular phase space

The dynamics of dissipative dynamical systems can be described by the sequential appearance of two different regimes. From a given initial condition, one first observes transient behavior characterized by a high degree of contraction of volumes in phase space. This is followed by an asymptotic regime with one or several attractors into which trajectories inject after long times. There is however, no sharp crossover between these two regimes and the identification of either one depends on the precision of measurement. In order to investigate these issues, we studied the dynamics of contracting integer maps. We found out that for the cases which in the continuum limit correspond to bifurcations, transients consists of two regimes sharply separated by a crossover point which displays universal scaling with the size of the set. Moreover, their average lengths display power law dependence on the accuracy of their measurement. This behavior persists away from bifurcation but with a different scaling law. In addition, we studied deterministic diffusion on finite sets and obtained analytic expressions for the mean square displacement in the long time limit.     

Transients in semiconductor diodes at high injection levels

A theoretical calculation is presented of the duration of the first phase of the switching process from forward to reverse direction for a diode with a finite base, allowing for the finite surface-recombination velocity at the back contact at high injection levels. Formulas are obtained suitable for practical calculation of minority-carrier lifetime and surface recombination rate for junction diodes.     

Pair-breaking in kondo superconductors

An earlier theory for superconductors with magnetic impurities is extended by taking into account explicitly the energy dependence of pair breaking. Results on the transition temperature and the specific heat jumps are in very good agreement with experimental data on (La_1-cCe_c)Al₂.     

Bound-pair excitations in superconductors

Properties of bound-pair excitations as correlated states of two Fermi quasiparticles in superconductors are calculated within the BCS theory of superconductivity. The partition function of the interacting electron system is expressed as a product of the partition function of a free Fermi gas and the partition function of a free Bose gas. The Bose gas partition function determines energy levels of the bound-pair excitations and their degeneracies. The calculated properties of the bound-pair excitations perfectly respect generally proven theorems concerning the BCS theory of superconductivity. This work received financial support from the Grant Agency of the Slovak Republic, Grant No V2 F20-G and 2/4109/97.     

Domains in multiband superconductors

Multiband superconductors can have several types of domains that are inhibited in conventional single-band superconductors. These domains are phase domains and chiral domains and their domain wall are an interband phase difference soliton. In a superconductor with an odd number of electronic bands (five or more) and with positive interband Josephson interactions, we find other types of domains with different interband phase differences. We call these domains configuration domains because pseudo-order parameters for each band are dispersed in the complex plain and several configurations, which have several local minima. Fractional vortices serve as hubs for phase difference solitons (configuration domain walls). The divergence of the number of configurations with local minima would pose a serious problem for the stability of superconductivity.     

Transitions in magnetic superconductors

Ginzburg-Landau and RNG analyses show a first order transition in ferromagnetic superconductors with RKKY interaction. The phase diagram in anisotropic antiferromagnetic superconductors is discussed and shown to display different possible transitions as a function of the coupling parameters.