20 research outputs found
Dynamical fluctuations in mode locking experiments on vortices moving through mesoscopic channels
We have studied the flow properties of vortices driven through easy flow
mesoscopic channels by means of the mode locking (ML) technique. We observe a
ML jump with large voltage broadening in the real part of the rf-impedance.
Upon approaching the pure dc flow by reducing the rf amplitude, the ML jump is
smeared out via a divergence of the voltage width. This indicates a large
spread in internal frequencies and lack of temporal coherence in the dc-driven
state.Comment: 2 pages, 2 figures, contribution to M2S-HTSC 2003, Ri
In-plane field-induced vortex liquid correlations in underdoped Bi_2Sr_2CaCu_2O_8+\delta
The effect of a magnetic field component parallel to the superconducting
layers on longitudinal Josephson plasma oscillations in the layered high
temperature superconductor BiSrCaCuO is shown to
depend on the thermodynamic state of the underlying vortex lattice. Whereas the
parallel magnetic field component depresses the Josephson Plasma Resonance
(JPR) frequency in the vortex solid phase, it may enhance it in the vortex
liquid. There is a close correlation between the behavior of microwave
absorption near the JPR frequency and the effectiveness of pancake vortex
pinning, with the enhancement of the plasma resonance frequency occurring in
the absence of pinning, at high temperature close to the vortex melting line.
An interpretation is proposed in terms of the attraction between pancake
vortices and Josephson vortices, apparently also present in the vortex liquid
state.Comment: 8 pages, 7 Figures, submitted to Phys. Rev.
Inverse melting of the vortex lattice
Inverse melting, in which a crystal reversibly transforms into a liquid or
amorphous phase upon decreasing the temperature, is considered to be very rare
in nature. The search for such an unusual equilibrium phenomenon is often
hampered by the formation of nonequilibrium states which conceal the
thermodynamic phase transition, or by intermediate phases, as was recently
shown in a polymeric system. Here we report a first-order inverse melting of
the magnetic flux line lattice in Bi2Sr2CaCu2O8 superconductor. At low
temperatures, the material disorder causes significant pinning of the vortices,
which prevents observation of their equilibrium properties. Using a newly
introduced 'vortex dithering' technique we were able to equilibrate the vortex
lattice. As a result, direct thermodynamic evidence of inverse melting
transition is found, at which a disordered vortex phase transforms into an
ordered lattice with increasing temperature. Paradoxically, the structurally
ordered lattice has larger entropy than the disordered phase. This finding
shows that the destruction of the ordered vortex lattice occurs along a unified
first-order transition line that gradually changes its character from
thermally-induced melting at high temperatures to a disorder-induced transition
at low temperatures.Comment: 13 pages, 4 figures, Nature, In pres
Vortex fluctuations in underdoped Bi2Sr2CaCu2O8+d crystals
Vortex thermal fluctuations in heavily underdoped Bi2Sr2CaCu2O8+d (Tc=69.4 K)
are studied using Josephson plasma resonance (JPR). From the data in zero
magnetic field, we obtain the penetration depth along the c-axis,
lambda_{L,c}(0) = 229 micrometers and the anisotropy ratio gamma(0) = 600. The
low plasma frequency allows us to study phase correlations over the whole
vortex solid (Bragg-glass) state. The JPR results yield a wandering length
r_{w} of vortex pancakes. The temperature dependence of r_{w} as well as its
increase with applied dc magnetic field can only be explained by the
renormalization of the tilt modulus by thermal fluctuations, and suggest the
latter is responsible for the dissociation of the vortices at the first order
transition.Comment: 4 pages, 5 figures. Submitted to Phys. Rev. Let
Nature of c-axis coupling in underdoped Bi2Sr2CaCu2O8 with varying degrees of disorder
The dependence of the Josephson Plasma Resonance (JPR) frequency in heavily
underdoped Bi2Sr2CaCu2O8+\delta on temperature and controlled pointlike
disorder, introduced by high-energy electron irradiation, is cross-correlated
and compared to the behavior of the ab-plane penetration depth. It is found
that the zero temperature plasma frequency, representative of the superfluid
component of the c-axis spectral weight, decreases proportionally with T_c when
the disorder is increased. The temperature dependence of the JPR frequency is
the same for all disorder levels, including pristine crystals. The reduction of
the c-axis superfluid density as function of disorder is accounted for by
pair-breaking induced by impurity scattering in the CuO2 planes, rather than by
quantum fluctuations of the superconducting phase. The reduction of the c-axis
superfluid density as function of temperature follows a T^{2}--law and is
accounted for by quasi-particle hopping through impurity induced interlayer
states.Comment: 10 pages, 9 Figure
Shear viscosity measurements at the vortex melting transition in confined geometry in optimally doped Bi2Sr2CaCu2O8
International audienceIn order to probe the vortex shear viscosity in the vortex liquid phase, we have introduced two types of vortex-confining structures in optimally doped Bi2Sr2CaCu2O8 single crystals. First, walls of strong vortex pinning separated by weakly pinning channels are fashioned by heavy ion irradiation through 25 um-thick Ni masks. Second, a low density of homogeneously distributed amorphous columnar defects is known to impose a polycrystalline structure to the vortex lattice. Resistivity measurements show that the inclusion of confining structures impede vortex flow in the liquid. The resistivity is remarkably well described by the Halperin-Nelson theory for the viscosity due to free two-dimensional vortex lattice dislocations
c-axis coupling in underdoped Bi(2)Sr(2)CaCu(2)O(8+δ) with varying degrees of disorder
c-axis coupling in underdoped Bi(2)Sr(2)CaCu(2)O(8+δ) with varying degrees of disorde
Hall-conductivity sign change and fluctuations in amorphous NbGe films
The sign change in the Hall conductivity has been studied in thin amorphous
NbGe0.3) films. By changing the film thickness it is
shown that the field at which the sign reversal occurs shifts to lower values
(from above to below the mean-field transition field ) with increasing
film thickness. This effect can be understood in terms of a competition between
a positive normal and a negative fluctuation contribution to the Hall
conductivity.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Disorder and -axis quasiparticle dynamics in underdoped Bi2Sr2CaCu2O8
Contribution à LT25International audienceWe present measurements of the Josephson plasma frequency and the in-plane penetration depth of underdoped single crystalline Bi2Sr2CaCu2O8 with varying degrees of disorder introduced by irradiation with 2.3 MeV electrons. Increasing disorder drives T_c down, in agreement with in all model descriptions of high T_c superconductivity. However, the manner in which the JPR frequency, the square of which represents the zero-frequency spectral weight of the c-axis conductivity in the superconducting state, is driven down by disorder depends more strongly on the model description. We show that only the model of impurity assisted quasiparticle hopping in a d-wave superconductor, together with strongly scattering point defects in the superconducting layers, can explain the disorder dependence of the c-axis plasma frequency, the in-plane penetration depth, and T_c consistently. From the data, we extract the energy scale governing nodal quasiparticle excitations, Delta_0 ~ 2.5 k_BT_c
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