96 research outputs found
Josephson coupling and plasma resonance in vortex crystal
We investigate the magnetic field dependence of the plasma resonance
frequency in vortex crystal state. We find that low magnetic field induces a
small correction to the plasma frequency proportional to the field. The slope
of this linear field dependence is directly related to the average distance
between the pancake vortices in the neighboring layers, the wandering length.
This length is determined by both Josephson and magnetic couplings between
layers. At higher fields the plasma frequency is suppressed collectively and is
determined by elastic energy of the vortex lattice. Analyzing experimental
data, we find that (i) the wandering length becomes comparable with the London
penetration depth near T_{c}, (ii) at small melting fields (< 20G) the
wandering length does not change noticeably at the melting transition
demonstrating existence of the line liquid phase in this field range, and (iii)
the self consistent theory of pancake fluctuations describes very well the
field dependence of the Josephson plasma resonance frequency up to the melting
point.Comment: 4 pages, 2 Figers, Paper submitted to the conference proceedings of
M2S-2000 Houston, T
Defective Vortex Lattices in Layered Superconductors with Point Pins at the Extreme Type-II Limit
The mixed phase of layered superconductors with no magnetic screening is
studied through a partial duality analysis of the corresponding frustrated XY
model in the presence of weak random point pins. Isolated layers exhibit a
defective vortex lattice at low temperature that is phase coherent.
Sufficiently weak Josephson coupling between adjacent layers results in an
entangled vortex solid that exhibits weak superconductivity across layers. The
corresponding vortex liquid state shows an inverted specific heat anomaly that
we propose accounts for that seen in YBCO. A three-dimensional vortex lattice
with dislocations occurs at stronger coupling. This crossover sheds light on
the apparent discrepancy concerning the observation of a vortex-glass phase in
recent Monte Carlo simulations of the same XY model.Comment: 4 pages, 1 figure. To appear in PRB, rapid communicatio
Josephson vortices and solitons inside pancake vortex lattice in layered superconductors
In very anisotropic layered superconductors a tilted magnetic field generates
crossing vortex lattices of pancake and Josephson vortices (JVs). We study the
properties of an isolated JV in the lattice of pancake vortices. JV induces
deformations in the pancake vortex crystal, which, in turn, substantially
modify the JV structure. The phase field of the JV is composed of two types of
phase deformations: the regular phase and vortex phase. The phase deformations
with smaller stiffness dominate. The contribution from the vortex phase
smoothly takes over with increasing magnetic field. We find that the structure
of the cores experiences a smooth yet qualitative evolution with decrease of
the anisotropy. At large anisotropies pancakes have only small deformations
with respect to position of the ideal crystal while at smaller anisotropies the
pancake stacks in the central row smoothly transfer between the neighboring
lattice positions forming a solitonlike structure. We also find that even at
high anisotropies pancake vortices strongly pin JVs and strongly increase their
viscous friction.Comment: 22 pages, 11 figures, to appear in Phys. Rev.
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
Field Dependence of the Josephson Plasma Resonance in Layered Superconductors with Alternating Junctions
The Josephson plasma resonance in layered superconductors with alternating
critical current densities is investigated in a low perpendicular magnetic
field. In the vortex solid phase the current densities and the squared bare
plasma frequencies decrease linearly with the magnetic field. Taking into
account the coupling due to charge fluctuations on the layers, we extract from
recent optical data for SmLa_{1-x} Sr_x CuO_{4-delta} the Josephson penetration
length lambda_{ab} approximately 1100 A parallel to the layers at T=10 K.Comment: 5 pages, 6 eps-figures, final version with minor misprints correcte
Phase Transitions in Isolated Vortex Chains
In very anisotropic layered superconductors (e.g. BiSrCaCuO)
a tilted magnetic field can penetrate as two co-existing lattices of vortices
parallel and perpendicular to the layers. At low out-of-plane fields the
perpendicular vortices form a set of isolated vortex chains, which have
recently been observed in detail with scanning Hall-probe measurements. We
present calculations that show a very delicate stability of this isolated-chain
state. As the vortex density increases along the chain there is a first-order
transition to a buckled chain, and then the chain will expel vortices in a
continuous transition to a composite-chain state. At low densities there is an
instability towards clustering, due to a long-range attraction between the
vortices on the chain, and at very low densities it becomes energetically
favorable to form a tilted chain, which may explain the sudden disappearance of
vortices along the chains seen in recent experiments.Comment: 9 pages, 10 figure
Vortex lattice melting in multilayers with variable anisotropies
Quantum Matter and Optic
Stability of Driven Josephson Vortex Lattice in Layered Superconductors Revisited
We analytically study stability of sliding lattice of Josephson vortices
driven by a transport current in the stack direction in strong in-plane
magnetic field. In contrast to recent findings we obtain that there are no
diverse configurations of stable vortex lattices, and, hence, the stable
sliding vortex lattice can not be selected by boundary conditions. We find that
only the triangular (rhombic) lattice can be stable, its stability being
limited by a critical velocity value. At higher velocities there are no simple
stable lattices with single flux line per unit cell. Oblique sliding lattices
are found to be never stable. Instability of such lattices is revealed beyond
the linear approximation in perturbations of the lattice.Comment: 11 pages, 2 figures, RevTeX 4, Submitted to Phys. Rev.
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
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