18 research outputs found
Vortex Solid-Liquid Transition in BiSrCaCuO with a High Density of Strong Pins
The introduction of a large density of columnar defects in %underdoped
BiSrCaCuO crystals does not, at sufficiently low
vortex densities, increase the irreversibility line beyond the first order
transition (FOT) field of pristine crystals. At such low fields, the flux line
wandering length behaves as in pristine
%BiSrCaCuO crystals. Next, vortex positional
correlations along the --axis in the vortex Bose glass at fields above the
FOT are smaller than in the low--field vortex solid. Third, the
Bose-glass-to-vortex liquid transition is signaled by a rapid decrease in
c-axis phase correlations. These observations are understood in terms of the
``discrete superconductor'' model.Comment: 4 pages, 4 figures Submitted to Phys. Rev. B Rapid Comm. 16-1-2004
Revised version 18-3-200
Evidence for LineLike Vortex Liquid Phase in TlBaCaCuO Probed by the Josephson Plasma Resonance
We measured the Josephson plasma resonance (JPR) in optimally doped
TlBaCaCuO thin films using terahertz time-domain
spectroscopy in transmission. The temperature and magnetic field dependence of
the JPR frequency shows that the c-axis correlations of pancake vortices remain
intact at the transition from the vortex solid to the liquid phase. In this
respect TlBaCaCuO films, withanisotropy parameter
, are similar to the less anisotropic
YBaCuO rather than to the most
anisotropic BiSrCaCuO single crystals ).Comment: Submitted to Physical Review Letter
Microwave generation in synchronized semiconductor superlattices
We study high-frequency generation in a system of electromagnetically coupled semiconductor superlattices fabricated on the same doped substrate. Applying a bias voltage to a single superlattice generates high-frequency current oscillations. We demonstrate that within a certain range of the applied voltage, the current oscillations within the superlattices can be self-synchronized, which leads to a dramatic rise in the generated microwave power. These results, which are in good agreement with our numerical model, open a promising practical route towards the design of high-power miniature microwave generators
Potential and current distribution in strongly anisotropic Bi(2)Sr(2) CaCu(2)O(8) single crystals at current breakdown
Experiments on potential differences in the low-temperature vortex solid
phase of monocrystalline platelets of superconducting Bi(2)Sr(2)CaCu(2)O(8)
(BSCCO) subjected to currents driven either through an "ab" surface or from one
such surface to another show evidence of a resistive/nonresistive front moving
progressively out from the current contacts as the current increases. The depth
of the resistive region has been measured by a novel in-depth voltage probe
contact. The position of the front associated with an injection point appears
to depend only on the current magnitude and not on its withdrawal point. It is
argued that enhanced nonresistive superconducting anisotropy limits current
penetration to less than the London length and results in a flat rectangular
resistive region with simultaneous "ab" and "c" current breakdown which moves
progressively out from the injection point with increasing current.
Measurements in "ab" or "c" configurations are seen to give the same
information, involving both ab-plane and c-axis conduction properties.Comment: 9 pages, 13 figures, typo error corrected, last section was refine
Theory of c-axis Josephson tunneling in d-wave superconductors
The temperature and angular dependence of the c-axis Josephson current and
the superfluid density in layered d-wave superconductors are studied within the
framework of an extended Ambegaokar-Baratoff formalism. In particular, the
effects of angle-dependent tunneling matrix elements and Andreev scattering at
grain boundaries are taken into account. These lead to strong corrections of
the low-temperature behavior of the plasma frequency and the Josephson current.
Recent c-axis measurements on the cuprate high-temperature superconductors
HgBa_2CaCu_{1+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} can therefore be
interpreted to be consistent with a d-wave order parameter.Comment: Revtex, 4 pages with 4 eps figures, to appear in PRB R
Josephson Plasma in RuSr2GdCu2O8
Josephson plasma in RuSrGdCuO,
RuSrGdCuO (x = 0.3), and
RuSrEuCeCuO (x = 0.5) compounds is
investigated by the sphere resonance method. The Josephson plasma is observed
in a low-frequency region (around 8.5 cm at T ) for
ferromagnetic RuSrGdCuO, while it increases to 35 cm
for non-ferromagnetic RuSrGdCuO (x = 0.3), which
represents a large reduction in the Josephson coupling at ferromagnetic
RuO block layers. The temperature dependence of the plasma does not shift
to zero frequency ({\it i.e.} = 0) at low temperatures, indicating that
there is no transition from the 0-phase to the -phase in these compounds.
The temperature dependence and the oscillator strength of the peak are
different from those of other non-magnetic cuprates, and the origins of these
anomalies are discussed.Comment: to appear in Phys. Rev.B Rapid Com
Absence of the zero bias peak in vortex tunneling spectra of high temperature superconductors?
The c-axis tunneling matrix of high-Tc superconductors is shown to depend
strongly on the in-plane momentum of electrons and vanish along the four nodal
lines of the d(x^2-y^2)-wave energy gap. This anisotropic tunneling matrix
suppresses completely the contribution of the most extended quasiparticles in
the vortex core to the c-axis tunneling current and leads to a spectrum similar
to that of a nodeless superconductor. Our results give a natural explanation of
the absence of the zero bias peak as well as other features observed in the
vortex tunneling spectra of high-Tc cuprates.Comment: 4 pages 3 figures, minor corrections, to appear in Phys Rev
Quasiparticle and Cooper Pair Tunneling in the Vortex State of Bi-2212
From measurements of the c-axis I-V characteristics of intrinsic Josephson
junctions in Bi_2Sr_2CaCu_2O_{8+delta} (Bi-2212) mesas we obtain the field
dependence (H || c) of the quasiparticle (QP) conductivity, sigma_q(H,T), and
of the Josephson critical current density, J_c(H,T). The quasiparticle
conductivity sigma_q(H) increases sharply with H and reaches a plateau at 0.05
T <H< 0.3 T. We explain such behavior by the dual effect of supercurrents
around vortices. First, they enhance the QP DOS, leading to an increase of
sigma_q with H at low H and, second, they enhance the scattering rate for
specular tunneling as pancakes become disordered along the c-axis at higher H,
leading to a plateau at moderate H.Comment: 4 pages, 4 figure
Crystallization of the ordered vortex phase in high temperature superconductors
The Landau-Khalatnikov time-dependent equation is applied to describe the
crystallization process of the ordered vortex lattice in high temperature
superconductors after a sudden application of a magnetic field. Dynamic
coexistence of a stable ordered phase and an unstable disordered phase, with a
sharp interface between them, is demonstrated. The transformation to the
equilibrium ordered state proceeds by movement of this interface from the
sample center toward its edge. The theoretical analysis dictates specific
conditions for the creation of a propagating interface, and provides the time
scale for this process.Comment: 8 pages and 3 figures; to be published in Phys. Rev. B (Rapid
Communications section
Novel features of Josephson flux-flow in Bi-2212: contribution of in-plane dissipation, coherent response to mm-wave radiation, size effect
We studied Josephson flux-flow (JFF) in Bi-2212 stacks fabricated from single
crystal whiskers by focused ion beam technique. For long junctions with the
in-plane sizes 30 x 2 (mu)m^2, we found considerable contribution of the
in-plane dissipation to the JFF resistivity, (rho)_(Jff), at low temperatures.
According to recent theory [A. Koshelev, Phys. Rev. B62, R3616 (2000)] that
results in quadratic type dependence of (rho)_(Jff)(B) with the following
saturation. The I-V characteristics in JFF regime also can be described
consistently by that theory. In JFF regime we found Shapiro-step response to
the external mm-wave radiation. The step position is proportional to the
frequency of applied microwaves and corresponds to the Josephson emission from
all the 60 intrinsic junctions of the stack being synchronized. That implies
the coherence of the JFF over the whole thickness of the stack and demonstrates
possibility of synchronization of intrinsic junctions by magnetic field. We
also found a threshold character of an appearance of the JFF branch on the I-V
characteristic with the increase of magnetic field, the threshold field B_t
being scaled with the junction size perpendicular to the field L (L = 30-1.4
(mu)m), as B_t = (Phi)_0/Ls, where (Phi)_0 is flux quantum, s is the interlayer
spacing. On the I-V characteristics of small stacks in the JFF regime we found
Fiske-step features associated with resonance of Josephson radiation with the
main resonance cavity mode in transmission line formed by stack.Comment: 20 pages including 8 figures, submitted to the proceedings of the
Superconducting Device Physics (SDP-2001) conference, Tokyo, June 200