62 research outputs found
The approach to a superconductor-to-Bose-insulator transition in disordered films
Through a detailed study of scaling near the magnetic field-tuned
superconductor-to-insulator transition in strongly disordered films, we find
that results for a variety of materials can be collapsed onto a single phase
diagram. The data display two clear branches, one with weak disorder and an
intervening metallic phase, the other with strong disorder. Along the strongly
disordered branch, the resistance at the critical point approaches and the scaling of the resistance is consistent with quantum
percolation, and therefore with the predictions of the dirty boson model.Comment: 4 pages, 4 figure
Pseudogap state in slightly doped by aluminium and praseodymium YBaCuO single crystals with a given topology of plane defects
In present work the conductivity in the basis plane of YBaCuO single crystals
slightly doped by Al and Pr with a pre-specified topology of twin boundaries
has been investigated. The excess conductivity for the analyzed samples shows
dependence like in wide
temperature range , where can be represents as mean field
temperature of superconducting transition. The temperature dependence of
pseudogap can be satisfactory described in terms of the BCS-BEC crossover
theoretical model.Comment: 3 pages, 2 figure
Anomalous temperature dependence of the supercurrent through a chaotic Josephson junction
We calculate the supercurrent through a Josephson junction consisting of a
phase-coherent metal particle (quantum dot), weakly coupled to two
superconductors. The classical motion in the quantum dot is assumed to be
chaotic on time scales greater than the ergodic time , which itself
is much smaller than the mean dwell time . The excitation
spectrum of the Josephson junction has a gap , which can be less than
the gap in the bulk superconductors. The average supercurrent is
computed in the ergodic regime , using
random-matrix theory, and in the non-ergodic regime , using a semiclassical relation between the supercurrent and
dwell-time distribution. In contrast to conventional Josephson junctions,
raising the temperature above the excitation gap does not necessarily lead to
an exponential suppression of the supercurrent. Instead, we find a temperature
regime between and where the supercurrent decreases
logarithmically with temperature. This anomalously weak temperature dependence
is caused by long-range correlations in the excitation spectrum, which extend
over an energy range greater than . A similar logarithmic temperature dependence of the
supercurrent was discovered by Aslamazov, Larkin, and Ovchinnikov, in a
Josephson junction consisting of a disordered metal between two tunnel
barriers.Comment: 14 pages with 2 figures; the revision corrects the published version
in Eqs. 8, 15, and 21d (with thanks to Marlies Goorden
Origin of Orthorhombic Transition, Magnetic Transition, and Shear Modulus Softening in Iron Pnictide Superconductors: Analysis based on the Orbital Fluctuation Theory
The main features in iron-pnictide superconductors are summarized as (i) the
orthorhombic transition accompanied by remarkable softening of shear modulus,
(ii) high-Tc superconductivity close to the orthorhombic phase, and (iii)
stripe-type magnetic order induced by orthorhombicity. To present a unified
explanation for them, we analyze the multiorbital Hubbard-Holstein model with
Fe-ion optical phonons based on the orbital fluctuation theory. In the
random-phase-approximation (RPA), a small electron-phonon coupling constant
() is enough to produce large orbital (=charge quadrupole)
fluctuations. The most divergent susceptibility is the
-antiferro-quadrupole (AFQ) susceptibility, which causes the s-wave
superconductivity without sign reversal (s_{++}-wave state). At the same time,
divergent development of -ferro-quadrupole (FQ) susceptibility is
brought by the "two-orbiton process" with respect to the AFQ fluctuations,
which is absent in the RPA. The derived FQ fluctuations cause the softening of
shear modulus, and its long-range-order not only triggers the
orthorhombic structure transition, but also induces the instability of
stripe-type antiferro-magnetic state. In other words, the condensation of
composite bosons made of two orbitons gives rise to the FQ order and structure
transition. The theoretically predicted multi-orbital-criticality presents a
unified explanation for abovementioned features of iron pnictide
superconductors.Comment: 19 pages, 15 figure
Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films
We have patterned a hexagonal array of nano-scale holes into a series of
ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K
5 K. These regular perforations give the films a phase-sensitive
periodic response to an applied magnetic field. By measuring this response in
their resistive transitions, , we are able to distinguish regimes in
which fluctuations of the amplitude, both the amplitude and phase, and the
phase of the superconducting order parameter dominate the transport. The
portion of dominated by amplitude fluctuations is larger in lower
films and thus, grows with proximity to the superconductor to
insulator transition.Comment: Revised title, abstract, text, figure
Dynamic fluctuations in the superconductivity of NbN films from microwave conductivity measurements
We have measured the frequency and temperature dependences of complex ac
conductivity, \sigma(\omega)=\sigma_1(\omega)-i\sigma_2(\omega), of NbN films
in zero magnetic field between 0.1 to 10 GHz using a microwave broadband
technique. In the vicinity of superconducting critical temperature, Tc, both
\sigma_1(\omega) and \sigma_2(\omega) showed a rapid increase in the low
frequency limit owing to the fluctuation effect of superconductivity. For the
films thinner than 300 nm, frequency and temperature dependences of fluctuation
conductivity, \sigma(\omega,T), were successfully scaled onto one scaling
function, which was consistent with the Aslamazov and Larkin model for two
dimensional (2D) cases. For thicker films, \sigma(\omega,T) data could not be
scaled, but indicated that the dimensional crossover from three dimensions (3D)
to 2D occurred as the temperature approached Tc from above. This provides a
good reference of ac fluctuation conductivity for more exotic superconductors
of current interest.Comment: 8 pages, 7 Figures, 1 Table, Accepted for publication in PR
Landau diamagnetism and magnetization of interacting diffusive conductors
We show how the orbital magnetization of an interacting disordered diffusive
electron gas can be simply related to the magnetization of the non-interacting
system having the same geometry. This result is applied to the persistent
current of a mesoscopic ring and to the relation between Landau diamagnetism
and the interaction correction to the magnetization of diffusive systems. The
field dependence of this interaction contribution can be deduced directly from
the de Haas-van Alphen oscillations of the free electron gas. Known results for
the free orbital magnetism of finite systems can be used to derive the
interaction contribution in the diffusive regime in various geometries.Comment: 4 pages, 2 figure
Fluctuation Conductivity in Unconventional Superconductors near Critical Disorder
The fluctuation conductivity in bulk superconductors with
non s-wave pairing and with nonmagnetic disorder of strength is studied at
low and within the Gaussian approximation. It is shown by assuming a quasi
two-dimensional (2D) electronic state that, only if the gap function
d_\mu({\p}) is, as in a 2D p-wave pairing state, linear in the in-plane
(relative) momentum {\p}_\perp, the in-plane fluctuation conductivity on the
line is weakly divergent in low limit. The present result may be
useful in clarifying the true gap function of spin-triplet
through resistivity measurements.Comment: 8 pages, 1 figure, to be published in J. Phys. Soc. Jpn. 70, No.10
(2001
Coulomb drag as a signature of the paired quantum Hall state
Motivated by the recent Coulomb drag experiment of M. P. Lilly et. al, we
study the Coulomb drag in a two-layer system with Landau level filling factor
. We find that the drag conductivity in the incompressible paired
quantum Hall state at zero temperature can be finite. The drag conductivity is
also greatly enhanced above , at which the transition between the weakly
coupled compressible liquids and the paired quantum Hall liquid takes place. We
discuss the implications of our results for the recent experiment.Comment: 4 pages, 1 figure included, replaced by the published versio
Resistivity study of the pseudogap phase for (Hg,Re) - 1223 superconductors
The pseudogap phase above the critical temperature of high
superconductors (HTSC) presents different energy scales and it is currently a
matter of intense study. The complexity of the HTSC normal state requires very
accurate measurements with the purpose of distinguishing different types of
phenomena. Here we have performed systematically studies through electrical
resistivity () measurements by several different current densities in
order to obtain an optimal current for each sample. This approach allows to
determine reliable values of the pseudogap temperature , the layer
coupling temperature between the superconductor layers , the
fluctuation temperature and the critical temperature as
function of the doping . The interpretation of these different temperature
scales allows to characterize possible scenarios for the (Hg,Re) - 1223 normal
state. This method, described in detail here, and used to derive the
(Hg,Re)-1223 phase diagram is general and can be applied to any HTSC.Comment: 31 pages, 12 figures, Latex; 25 pages, LaTeX; 11 figures; rewrited
section II and III; added 18 reference; rewrited title, added discussion
sectio
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