257 research outputs found
Defect-unbinding and the Bose-glass transition in layered superconductors
The low-field Bose-glass transition temperature in heavy-ion irradiated
Bi_2Sr_2CaCu_2O_8+d increases progressively with increasing density of
irradiation-induced columnar defects, but saturates for densities in excess of
1.5 x10^9 cm^-2. The maximum Bose-glass temperature corresponds to that above
which diffusion of two-dimensional pancake vortices between different vortex
lines becomes possible, and above which the ``line-like'' character of vortices
is lost. We develop a description of the Bose-glass line that is in excellent
quantitative agreement with the experimental line obtained for widely different
values of track density and material parameters.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Broadband SNAIL parametric amplifier with microstrip impedance transformer
Josephson parametric amplifiers have emerged as a promising platform for
quantum information processing and squeezed quantum states generation.
Travelling wave and impedance-matched parametric amplifiers provide broad
bandwidth for high-fidelity single-shot readout of multiple qubit
superconducting circuits. Here, we present a quantum-limited 3-wave-mixing
parametric amplifier based on superconducting nonlinear asymmetric inductive
elements (SNAILs), whose useful bandwidth is enhanced with an on-chip
two-section impedance-matching circuit based on microstrip transmission lines.
The amplifier dynamic range is increased using an array of sixty-seven SNAILs
with 268 Josephson junctions, forming a nonlinear quarter-wave resonator.
Operating in a current-pumped mode, we experimentally demonstrate an average
gain of across bandwidth, along with an average saturation
power of , which can go as high as with quantum-limited
noise performance. Moreover, the amplifier can be fabricated using a simple
technology with just a one e-beam lithography step. Its central frequency is
tuned over a several hundred megahertz, which in turn broadens the effective
operational bandwidth to around .Comment: 7 pages, 3 figure
Possibility of the new type phase transition
The scalar field theory and the scalar electrodynamics quantized in the flat
gap are considered. The dynamical effects arising due to the boundary presence
with two types of boundary conditions (BC) satisfied by scalar fields are
studied. It is shown that while the Neumann BC lead to the usual scalar field
mass generation, the Dirichlet BC give rise to the dynamical mechanism of
spontaneous symmetry breaking. Due to the later, there arises the possibility
of the new type phase transition from the normal to spontaneously broken phase.
The decreasing in the characteristic size of the quantization region (the gap
size here) and increasing in the temperature compete with each other, tending
to transport the system in the spontaneously broken and in the normal phase,
respectively. The system evolves with a combined parameter, simultaneously
reflecting the change in temperature and in the size. As a result, at the
critical value of this parameter there occurs the phase transition from the
normal phase to the spontaneously broken one. In particular, the usual massless
scalar electrodynamics transforms to the Higgs model
Double sign reversal of the vortex Hall effect in YBa2Cu3O7-delta thin films in the strong pinning limit of low magnetic fields
Measurements of the Hall effect and the resistivity in twinned
YBa2Cu3O7-delta thin films in magnetic fields B oriented parallel to the
crystallographic c-axis and to the twin boundaries reveal a double sign
reversal of the Hall coefficient for B below 1 T. In high transport current
densities, or with B tilted off the twin boundaries by 5 degrees, the second
sign reversal vanishes. The power-law scaling of the Hall conductivity to the
longitudinal conductivity in the mixed state is strongly modified in the regime
of the second sign reversal. Our observations are interpreted as strong,
disorder-type dependent vortex pinning and confirm that the Hall conductivity
in high temperature superconductors is not independent of pinning.Comment: 4 pages, 4 figure
Nucleation of Stable Superconductivity in YBCO-Films
By means of the linear dynamic conductivity, inductively measured on
epitaxial films between 30mHz and 30 MHz, the transition line to
generic superconductivity is studied in fields between B=0 and 19T. It follows
closely the melting line described recently in terms of a blowout of
thermal vortex loops in clean materials. The critical exponents of the
correlation length and time near , however, seem to be dominated by
some intrinsic disorder. Columnar defects produced by heavy-ion irradiation up
to field-equivalent-doses of lead to a disappointing reduction
of while for the generic line of the pristine film
is recovered. These novel results are also discussed in terms of a loop-driven
destruction of generic superconductivity.Comment: 11 pages including 7 EPS figures, accepted for publication in the
Proceedings of the Spring Meeting of the German Physical Society, Muenster
1999,Festkoerperprobleme/Advances in Solid State Physics 199
Hydrodynamics and Nonlocal Conductivities in Vortex States of Type II Superconductors
A hydrodynamical description for vortex states in type II superconductors is
presented based on the time-dependent Ginzburg-Landau equation (TDGL). In
contrast to the familiar extension of a single vortex dynamics based on the
force balance, our description is consistent with the known hydrodynamics of a
rotating neutral superfluid and correctly includes informations on the
Goldstone mode. Further it enables one to examine nonlocal conductivities
perpendicular to the magnetic field in terms of Kubo formula. The nonlocal
conductivities deviate from the usual vortex flow expressions typically when
the nonlocality parallel to the field becomes weaker than the perpendicular one
measuring a degree of positional correlations, and, for instance, the
superconducting contribution of dc Hall conductivity nonlocal only in
directions perpendicular to the field becomes vanishingly small in the
situations with large shear viscosity, leading to an experimentally measurable
relation among the total resistivity components.
Other situations are also discussed on the basis of the resulting expressions.Comment: 12 pages, no figures, to appear in J. Phys. Soc. Jpn. in October,
199
Transport and Magnetic Properties of R1-xAxCoO3 (R=La, Pr and Nd; A=Ba, Sr and Ca)
Transport and magnetic measurements have been carried out on perovskite
Co-oxides R1-xAxCoO3 (R=La, Pr, and Nd; A=Ba, Sr and Ca; 0<x<0.5: All sets of
the R and A species except Nd1-xBaxCoO3 have been studied.). With increasing
the Sr- or Ba-concentration x, the system becomes metallic ferromagnet with
rather large magnetic moments. For R=Pr and Nd and A=Ca, the system approaches
the metal- insulator phase boundary but does not become metallic. The magnetic
moments of the Ca-doped systems measured with the magnetic field H=0.1 T are
much smaller than those of the Ba- and Sr-doped systems. The thermoelectric
powers of the Ba- and Sr-doped systems decrease from large positive values of
lightly doped samples to negative ones with increasing doping level, while
those of Ca-doped systems remain positive. These results can be understood by
considering the relationship between the average ionic radius of R1-xAx and the
energy difference between the low spin and intermediate spin states. We have
found the resistivity-anomaly in the measurements of Pr1-xCaxCoO3 under
pressure in the wide region of x, which indicates the existence of a phase
transition different from the one reported in the very restricted region of
x~0.5 at ambient pressure [Tsubouchi et al. Phys. Rev. B 66 (2002) 052418.]. No
indication of this kind of transition has been observed in other species of R.Comment: 9 pages, 8 figures. J. Phys. Soc. Jpn. 72 (2003) No.
Possible new vortex matter phases in BSCCO
The vortex matter phase diagram of BSCCO crystals is analyzed by
investigating vortex penetration through the surface barrier in the presence of
a transport current. The strength of the effective surface barrier, its
nonlinearity, and asymmetry are used to identify a possible new ordered phase
above the first-order transition. This technique also allows sensitive
determination of the depinning temperature. The solid phase below the
first-order transition is apparently subdivided into two phases by a vertical
line extending from the multicritical point.Comment: 11 pages, 3 figures, accepted for publication in PR
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