16 research outputs found
Comment to the paper : Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned YBaCuO, by Matl \QTR{em}{et al.}
In a recent paper, Matl et al present a high-frequency study of the complex
resistivity of a pinned vortex lattice in YBaCuO . They focus on the
inductive-to-resistive transition which is investigated as a function of
temperature at a constant field T, so that the transition is associated
with the vanishing of vortex pinning strength. To our view, their conclusions
rely on a rather brittle experimental body and the collapse of C66 results from
an involved analysis of the finite frequency corrections to .
These corrections are not necessary since the complex frequency spectrum has
been previously interpreted by the two modes model, first proposed for low Tc
materials. We think that it is more adequate to interpret the present data and
should be at least considered.Comment: 4pages tex. submitted to PR
rf-studies of vortex dynamics in isotropic type-II superconductors
We have measured the surface impedance of thick superconductors in the mixed
state over a broad 2 kHz - 20 MHz frequency range. The depinning cross-over is
observed; but it is much broader than expected from classical theories of
pinning. A striking result is the existence of size effects which invalidate
the common interpretation of the low-frequency surface inductance in terms of a
single penetration depth. Instead, a two-mode description of vortex dynamics,
assuming free vortex flow in the bulk and surface pinning, accounts
quantitatively for the spectrum of the complex apparent penetration depth.Comment: 20 pages, 6 figures, 28 reference
Depinning transition in type-II superconductors
The surface impedance Z(f) of conventional isotropic materials has been
carefully measured for frequencies f ranging from 1 kHz to 3 MHz, allowing a
detailed investigation of the depinning transition. Our results exhibit the
irrelevance of classical ideas to the dynamics of vortex pinning. We propose a
new picture, where the linear ac response is entirely governed by disordered
boundary conditions of a rough surface, whereas in the bulk vortices respond
freely. The universal law for Z(f) thus predicted is in remarkable agreement
with experiment, and tentatively applies to microwave data in YBaCuO films.Comment: 4 pages, 4 figures, 14 reference
Phase-Sensitive Impurity Effects in Vortex Core of Moderately Clean Chiral Superconductors
We study impurity effects in vortex core of two-dimensional moderately clean
su perconductors within the quasiclassical theory. The impurity scattering rate
\G amma(E) of the Andreev bound states in vortex core with +1 vorticity of
p-wav e superconductors with {\mib d}=\hat{\mib z}(p_x+\iu p_y) is suppre
ssed, compared to the normal state scattering rate in the
energ y region \Gamma_{\rm n}^3/E_\delta^2\ll E\ll E_\delta\equiv
|\delta_0|\Delta_\i nfty with scattering phase shift
and the pair-po tential in bulk . Further we
find that for p-wave superconductors with {\mib
d}=\hat{\mib z}(p_x-\iu p_y) is at most {\cal O}(E/\Delta_\i nfty). These
results are in marked contrast to the even-parity case (s,d-wave), where
is known to be proportional to \ln(\Delta_\i
nfty/E) . Parity- and chirality-dependences of impurity effects are attributed
to the Andr eev reflections involved in the impurity-induced scattering between
bound states . Implications for the flux flow conductivity is also discussed.
Novel enhanceme nt of flux flow conductivity is expected to occur at for {\mib d}=\hat{\mib z}(p_x+\iu p_y) and at
for {\mib d}=\hat{\mib z}(p_x-\iu p_y).Comment: 9 pages, No figures, To appear in JPSJ Vol. 69, No. 10 (2000
Ancrage des vortex dans les supraconducteurs.
Ce travail porte sur l'étude de l'ancrage des vortex par
la réponse linéaire haute fréquence. Nous présentons une
série de mesures de la profondeur de pénétration complexe dans l'état
mixte sur une variété d'échantillons qui va des
supraconducteurs classiques (Nb, V, PbIn), aux composés de fermions lourds
non-conventionnels (UPt3), en passant par les cuprates à haute température
critique (YBaCuO). La large gamme des fréquences explorées
(1 kHz–10 MHz) permet de couvrir le changement de régime entre la
réponse quasistatique dominée par l'interaction élastique des vortex
avec les défauts, et la réponse haute fréquence amortie par la
friction visqueuse du réseau de vortex au cristal ionique.
Pour décrire quantitativement le spectre de fréquence, nous nous
appuyons sur une théorie phénoménologique qui fait une distinction
explicite et rigoureuse entre lignes de vortex, décrites par un champ omega,
et lignes de champ magnétique B. On
prédit ainsi l'existence d'un second mode électrodynamique, évanescent
et de courte portée, lié à la tension de ligne des vortex. On montre
que la réponse aux petits mouvements, amplitude et phase, est réglée
par une condition limite supplémentaire sur le réseau de vortex à la
surface ; elle prend la forme d'une condition de glissement avec une
longueur phénoménologique contrôlée par la rugosité de
l'échantillon. Le spectre de fréquence associé à ce mécanisme se
distingue nettement du spectre de Campbell, générique des modèles
d'ancrage en volume.
Nos mesures sur des échantillons PbIn, Nb, V et YBaCuO confirment
entièrement notre modèle, y compris des effets de taille peu intuitifs
qui se produisent à basse fréquence quand l'échantillon devient
transparent au mode flux flow. En revanche, l'étude des
vortex dans les phases B et C d'UPt3 montre une
contribution importante du volume à l'ancrage des vortex
Peak effect and surface crystal-glass transition for surface-pinned vortex array
We present a theoretical and experimental study of the peak effect
in the surface pinning of vortices. It is associated with a sharp
transition in the vortex slippage length which we relate to a
crossover from a weakly disordered crystal to a surface glass
state. Experiments are performed on ion-beam–etched \chem{Nb}
crystals. The slippage length is deduced from
1\un{kHz}–1\un{MHz} linear AC penetration depth measurements