45 research outputs found
Determination of coordinate dependence of a pinning potential from a microwave experiment with vortices
The measurement of the complex impedance response accompanied by power absorption P(x) in
the radiofrequency and microwave ranges represents a most popular experimental method for the
investigation of pinning mechanisms and vortex dynamics in type-II superconductors. In the
theory, the pinning potential (PP) well for a vortex must be a priori specified in order to subsequently
analyze the measured data. We have theoretically solved the inverse problem at T¼0K
and exemplify how the coordinate dependence of a PP can be determined from a set of experimental
curves P(xjj0) measured at subcritical dc currents 0<j0<jc under a small microwave
excitation j1 jc with frequency x. We furthermore elucidate how and why the depinning frequency
xp, which separates the non-dissipative (quasi-adiabatic) and the dissipative (high-frequency)
regimes of small vortex oscillations in the PP, is reduced with increasing j0. The results
can be directly applied to a wide range of conventional superconductors with a PP subjected to
superimposed dc and small microwave ac currents at T Tc
Frequency-dependent ratchet effect in superconducting films with a tilted washboard pinning potential
The influence of an ac current of arbitrary amplitude and frequency on the
mixed-state dc-voltage-ac-drive ratchet response of a superconducting film with
a dc current-tilted uniaxial cosine pinning potential at finite temperature is
theoretically investigated. The results are obtained in the single-vortex
approximation, i.e., for non-interacting vortices, within the frame of an exact
solution of the appropriate Langevin equation in terms of a matrix continued
fraction. Formulas for the dc voltage ratchet response and absorbed power in ac
response are discussed as functions of ac current amplitude and frequency as
well as dc current induced tilt in a wide range of corresponding dimensionless
parameters. Special attention is paid to the physical interpretation of the
obtained results in adiabatic and high-frequency ratchet responses taking into
account both running and localized states of the (ac+dc)-driven vortex motion
in a washboard pinning potential. Our theoretical results are discussed in
comparison with recent experimental work on the high-frequency ratchet response
in nanostructured superconducting films [B. B. Jin et al., Phys. Rev. B 81
(2010) 174505].Comment: 13 pages, 11 figure
Microwave Absorption by Vortices in Superconductors with a Washboard Pinning Potential
let us compare the results presented in the chapter with the analogous but more
general results obtained by the authors [25] on the basis of a stochastic model for arbitrary
temperature T and densities j0 and j1. In that work, the Langevin equation (1), supplemented
with a thermofluctuation term, has been exactly solved for γ = 1 interms ofamatrix continued
fraction [52] and, depending on the WPP’s tilt caused by the dc current, two substantially
differentmodes in the vortexmotion have been predicted. Inmore detail, at low temperatures
and relatively high frequencies in a nontilted pinning potential each pinned vortex is confined
to its pinning potential well during the ac period. In the case of superimposed strong ac and
dc driving currents a running state of the vortex may appear when it can visit several (or
many) potential wells during the ac period. As a result, two branches of new findings have
been elucidated [25, 27]. First, the influence of an ac current on the usual E0(j0) and ratchet
E0(j1) CVCs has been analyzed. Second, the influence of a dc current on the ac nonlinear
impedance response and nonlinear power absorption has been investigated. In particular,
the appearance of Shapiro-like steps in the usual CVC and the appearance of phase-locking
regions in the ratchet CVC has been predicted. At the same time, it has been shown that
an anomalous power absorption in the ac response is expected at close-to critical currents
j0 jc and relatively low frequencies ω ωp. Figure 8 shows the main predictions of
these works. Namely, predicted are (i) an enhanced power absorption at low frequencies,
(ii) a temperature- and current-dependent minimum at intermediate frequencies. (iii) At
substantially low temperatures, the absorption can acquire negative values which physically
corresponds to the generation by vortices. However, a more general and formally precise
solution of the problem in terms of a matrix-continued fraction does not allow the main
physical results of the problemto be investigated in the formof explicit analytical functions of
the main physical quantities (j0, j1, ω, α, T,
, and γ) which, we believe, has helped us greatly
to elucidate the physics in the problem under consideration