17 research outputs found
Time relaxation of microwave second order response of superconductors in the critical state
Relaxation of the microwave second order response of YBa2Cu3O7 and
Ba0.6K0.4BiO3 crystals in the critical state is studied. The samples are
exposed to static and pulsed microwave magnetic fields. The second harmonic
signals decay during the time in which the microwave pulse endures. The decay
times depend on the supercon-ductor investigated and on the way the value of
the static field has been reached.Comment: 2 pages, 1 figur
Surface-Barrier Effects in the Microwave Second-Harmonic Response of Superconductors in the Mixed State
We report on transient effects in the microwave second-order response of
different type of superconductors in the mixed state. The samples have
contemporarily been exposed to a dc magnetic field, varying with a constant
rate of 60 Oe/s, and a pulsed microwave magnetic field. The time evolution of
the signal radiated at the second-harmonic frequency of the driving field has
been measured for about 500 s from the instant in which the dc-field sweep has
been stopped, with sampling time of about 0.3 s. We show that the
second-harmonic signal exhibits two relaxation regimes; an initial exponential
decay, which endures roughly 10 s, and a logarithmic decay in the time scale of
minutes. Evidence is given that the decay in the time scale of minutes is ruled
by magnetic relaxation over the surface barrier.Comment: 6 pages, 6 embedded figure
Time evolution of the microwave second-order response of YBaCuO powder
Transient effects in the microwave second-order response of YBaCuO powder are
investigated. The time evolution of the second harmonic signal has been
measured for about 300 s after the sample had been exposed to variations of the
DC magnetic field. We show that in different time scales the transient response
has different origin. In the time scale of milliseconds the transient response
of samples in the critical state is ascribable to processes of flux
redistribution induced by the switching on/off of the microwave field. At
longer times, the time evolution of the second harmonic signal can be ascribed
to motion of fluxons induced by the variation of the DC magnetic field. In
particular, diffusive motion of fluxons determines the response in the first 10
seconds after the stop of the magnetic field variation; magnetic relaxation
over the surface barrier determines the response in the time scale of minutes.Comment: 16 pages, 7 figures, submited to Physica
Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals
The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined
from magnetization curves for different field sweep rates dH/dt and
temperatures. The obtained results are consistent with theoretical reports in
the literature about vortex creep over surface and geometrical barriers. The
frequently observed low-temperature upturn of H_p is shown to be related to
metastable configurations due to barriers for vortex entry. Data of the true
lower critical field H_c1 are presented. The low-temperature dependence of H_c1
is consistent with a superconducting state with nodes in the gap function.
[PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs
Pressure Dependence of the Irreversibility Line in BiSrCaCuO:Role of Anisotropy in Flux-Line Formation
One of the important problems of high-temperature superconductivity is to
understand and ultimately to control fluxoid motion. We present the results of
a new technique for measuring the pressure dependence of the transition to
superconductivity in a diamond anvil cell. By measuring the third harmonic of
the {\it ac} susceptibility, we determine the onset of irreversible flux
motion. This enables us to study the effects of pressure on flux motion. The
application of pressure changes interplanar spacing, and hence the interplanar
coupling, without significantly disturbing the intraplanar superconductivity.
Thus we are able to separate the effects of coupling from other properties that
might affect the flux motion. Our results directly show the relationship
between lattice spacing, effective- mass anisotropy, and the irreversibility
line in BiSrCaCuO. Our results also demonstrate
that an application of 2.5 GPa pressure causes a dramatic increase in
interplanar coupling.Comment: 4 pages, 4 figure
First-order disorder-driven transition and inverse melting of the vortex lattice
Vortex matter phase transitions in the high-temperature superconductor
Bi2Sr2CaCu2O8 were studied using local magnetization measurements combined with
a vortex 'shaking' technique. The measurements revealed thermodynamic evidence
of a first-order transition along the second magnetization peak line, at
temperatures below the apparent critical point Tcp. We found that the
first-order transition line does not terminate at Tcp, but continues down to at
least 30 K. This observation suggests that the ordered vortex lattice phase is
destroyed through a unified first-order transition that changes its character
from thermally induced melting at high temperatures to a disorder-induced
transition at low temperatures. At intermediate temperatures the transition
line shows an upturn, which implies that the vortex matter displays 'inverse'
melting behavior.Comment: 9 pages, 6 figures, Physica C, in pres