Semiclassical theory of the interaction correction to the conductance of antidot arrays

Electron-electron interactions are responsible for a correction to the conductance of a diffusive metal, the "Altshuler-Aronov correction" $\delta G_{AA}$. Here we study the counterpart of this correction for a ballistic conductor, in which the electron motion is governed by chaotic classical dynamics. In the ballistic conductance, the Ehrenfest time $\tau_{E}$ enters as an additional time scale that determines the magnitude of quantum interference effects. The Ehrenfest time effectively poses a short-time threshold for the trajectories contributing to the interaction correction. As a consequence, $\delta G_{AA}$ becomes exponentially suppressed if the Ehrenfest time is larger than the dwell time or the inverse temperature. We discuss the explicit dependence on Ehrenfest time in quasi-one and two-dimensional antidot arrays. For strong interactions, the sign of $\delta G_{AA}$ may change as a function of temperature for temperatures in the vicinity of $\hbar/\tau_{E}$.Comment: 20 pages, 10 figures, published versio

Fluctuation conductivity of disordered superconductors in magnetic fields

We calculate fluctuation corrections to the longitudinal conductivity of disordered superconductors subject to an external magnetic field. We derive analytic expressions that are valid in the entire metallic part of the temperature–magnetic field phase diagram as long as the effect of the magnetic field on the spin degrees of freedom of the electrons may be neglected. Our calculations are based on a kinetic equation approach. For the special case of superconducting films and wires in parallel magnetic fields, we perform a detailed comparison with results that were previously obtained with diagrammatic perturbation theory in the imaginary-time formalism. As an application, we study the fluctuation conductivity of films in tilted magnetic fields with a special focus on the low-temperature regime. We present a detailed discussion of the phenomenon of the nonmonotonic magnetoresistance and find that it displays a pronounced dependence on the tilting angle

Phenomenology of current-induced skyrmion motion in antiferromagnets

We study current-driven skyrmion motion in uniaxial thin film antiferromagnets in the presence of the Dzyaloshinskii-Moriya interactions and in an external magnetic field. We phenomenologically include relaxation and current-induced torques due to both spin-orbit coupling and spatially inhomogeneous magnetic textures in the equation for the N\'eel vector of the antiferromagnet. Using the collective coordinate approach we apply the theory to a two-dimensional antiferromagnetic skyrmion and estimate the skyrmion velocity under an applied DC electric current.Comment: 14 pages, 3 figures, 1 tabl

Persistent current in small superconducting rings

We study theoretically the contribution of fluctuating Cooper pairs to the persistent current in superconducting rings threaded by a magnetic flux. For sufficiently small rings, in which the coherence length $\xi$ exceeds the radius $R$, mean field theory predicts a full reduction of the transition temperature to zero near half-integer flux. We find that nevertheless a very large current is expected to persist in the ring as a consequence of Cooper pair fluctuations that do not condense. For larger rings with $R\gg \xi$ we calculate analytically the susceptibility in the critical region of strong fluctuations and show that it reflects competition of two interacting complex order parameters.Comment: 4+ pages, 3 figure

Fluctuation conductivity in disordered superconducting films

We study the effect of superconducting fluctuations on the longitudinal and the transverse (Hall) conductivity in homogeneously disordered films. Our calculation is based on the Usadel equation in the real-time formulation. We adjust this approach to derive analytic expressions for the fluctuation corrections in the entire metallic part of the temperature-magnetic field phase diagram, including the effects of both classical and quantum fluctuations. This method allows us to obtain fluctuation corrections in a compact and effective way, establishing a direct connection between phenomenological and microscopic calculations

Supermatrix models for disordered, chaotic and interacting electron systems

In dieser Arbeit geht es um die theoretische Beschreibung von Quanteninterferenzphänomenen und Wechselwirkungseffekten in ungeordneten Elektronensystemen. Es werden Methoden der statistischen Feldtheorie verwendet, basierend auf der Supersymmetrie-Technik. Die Arbeit gliedert sich in zwei Teile. Im ersten Teil wird ein Supermatrix-Modell zur Beschreibung von ungeordneten und chaotischen Systemen eingeführt und zudem der Einfluss der Quanteninterferenz auf den Magnetotransport in einem zweidimensionalen Elektronengas mit zusätzlicher periodischer Modulation des Potentials untersucht. Im Mittelpunkt des zweiten Teils steht die Konstruktion eines supersymmetrischen sigma-Modells für schwach wechselwirkende ungeordnete Systeme. Außerdem wird im Rahmen eines Modells für granulare Metalle die Zustandsdichte bei nicht sehr tiefen Temperaturen beschrieben