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 Tl2_2Ba2_2CaCu2_2O8_8 Probed by the Josephson Plasma Resonance

We measured the Josephson plasma resonance (JPR) in optimally doped Tl$_2$Ba$_2$CaCu$_2$O$_{8+\delta}$ 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 Tl$_2$Ba$_2$CaCu$_2$O$_{8+\delta}$ films, withanisotropy parameter $\gamma\approx 150$, are similar to the less anisotropic YBa$_2$Cu$_3$O$_{7-\delta}$ $(\gamma\approx 8)$ rather than to the most anisotropic Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals $\gamma\geq 500$).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. Bi$_2$Sr$_2$CaCu$_2$O$_x$) 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

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

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.