14 research outputs found
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" . 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 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,
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 may change as a function
of temperature for temperatures in the vicinity of .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 exceeds the
radius , 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 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