Abstract:
We show that the flux-flow transport of the Josephson vortex lattice (JVL) in layered high-temperature superconductors provides a convenient probe for both components of quasiparticle conductivity, $\sigma_{c}$ and $\sigma_{ab}$. We found that the JVL flux-flow resistivity, $\rho_{ff}$, in a wide range of magnetic fields is mainly determined by the in-plane dissipation. In the dense lattice regime ($B>1$ T) $\rho_{ff}(B)$ dependence is well fitted by the theoretical formula for that limit. That allows us to independently extract from the experimental data the values of $\sigma_{c}$ and of the ratio $\sigma _{ab}/(\sigma_{c}\gamma ^{4})$. The extracted temperature dependence $\sigma _{ab}(T)$ is consistent with microwave data. The shape of the current-voltage characteristics is also sensitive to the frequency dependence of $\sigma_{ab}$ and that allows us to estimate the quasiparticle relaxation time and relate it to the impurity bandwidth using data obtained for the same crystal.

Abstract:
We study theoretically and experimentally influence of pancake vortices on motion of the Josephson vortex lattice in layered high-temperature superconductors. Mobility of the Josephson vortices in layered superconductors is strongly suppressed by small amount of pancake-vortex stacks. Moving Josephson vortex lattice forces oscillating zigzag deformation of the pancake-vortex stacks contributing to damping. The salient feature of this contribution is its nonmonotonic dependence on the lattice velocity and the corresponding voltage. Maximum pancake effect is realized when the Josephson frequency matches the relaxation frequency of the stacks. The pancake-vortex damping is strongly suppressed by thermal fluctuations of the pancake vortices. This theoretical picture was qualitatively confirmed by experiments on two mesas prepared out of Bi_2Sr_2CaCu_2O_{8+d} whiskers. We found that the Josephson-vortex flux-flow voltage is very sensitive to small c-axis magnetic field. The pancake-vortex contribution to the current indeed nonmonotonically depends on voltage and decreases with increasing temperature and in-plane magnetic field. We also found that irradiation with heavy ions has no noticeable direct influence on motion of the Josephson vortices but dramatically reduces the pancake-vortex contribution to the damping of the Josephson vortex lattice at low temperatures.

Abstract:
We developed the focused ion beam (FIB) and ion milling techniques for a fabrication of the Bi_2Sr_2CaCu_2O_{8+\delta} (Bi-2212) stacked junctions with in-plane size L_{ab} from several microns down to the submicron scale without degradation of T_c. We found that behaviour of submicron junctions (L_{ab} < 1 {\mu}m) is quite different from the bigger ones. The critical current density is considerably suppressed, the hysteresis and multibranched structure of the IV characteristics are eliminated, the periodic structure of current peaks reproducibly appears on the IV curves at low temperatures. A period of the structure, {\Delta}V, is consistent with the Coulomb charging energy of a single pair, {\Delta}V = e/C with C the effective capacitance of the stack. We consider this behaviour to originate from the Coulomb blockade of the intrinsic Josephson tunneling in submicron Bi-2212 stacks.

Abstract:
We studied the commensurate semi-fluxon oscillations of Josephson flux-flow (JFF) in a Bi-2212 stacked structures near $T_c$ as a probe of melting of Josephson vortex lattice (JVL). We found that oscillations exist above 0.5T. The amplitude of oscillations is found to decrease gradually with temperature and to turn to zero without any jump at $T=T_0$, 3.5K below the resistive transition temperature $T_c$ indicating the phase transition of the second order. This characteristic temperature $T_0$ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, $T_{BKT}$, in elementary superconducting layers of Bi-2212 at zero magnetic field. On the base of these facts we infer that melting of triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and anti-vortices. The $B-T$ phase diagram of the BKT phase found out from our experiment is consistent with theoretical predictions.

Abstract:
Characteristic features of transverse transport along the $a^*$ axis in the NbSe$_3$ charge-density-wave conductor are studied. At low temperatures, the $I$-$V$ characteristics of both layered structures and NbSe$_3$-NbSe$_3$ point contacts exhibit a strong peak of dynamic conductivity at zero bias voltage. In addition, the $I$-$V$ characteristics of layered structures exhibit a series of peaks that occur at voltages equal to multiples of the double Peierls gap. The conductivity behavior observed in the experiment resembles that reported for the interlayer tunneling in Bi-2212 high-$T_c$ superconductors. The conductivity peak at zero bias is explained using the model of almost coherent interlayer tunneling of the charge carriers that are not condensed in the charge density wave.

Abstract:
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

Abstract:
A pronounced asymmetry of magnetoresistance with respect to the magnetic field direction is observed for NbSe$_3$ crystals placed in a magnetic field perpendicular to their conducting planes. It is shown that the effect persists in a wide temperature range and manifests itself starting from a certain magnetic induction value $B_0$, which at $T=4.2$ K corresponds to the transition to the quantum limit, i.to the state where the Landay level splitting exceeds the temperature.

Abstract:
Interlayer tunneling in graphite mesa-type structures is studied at a strong in-plane magnetic field $H$ up to 55 T and low temperature $T=1.4$ K. The tunneling spectrum $dI/dV$ vs. $V$ has a pronounced peak at a finite voltage $V_0$. The peak position $V_0$ increases linearly with $H$. To explain the experiment, we develop a theoretical model of graphite in the crossed electric $E$ and magnetic $H$ fields. When the fields satisfy the resonant condition $E=vH$, where $v$ is the velocity of the two-dimensional Dirac electrons in graphene, the wave functions delocalize and give rise to the peak in the tunneling spectrum observed in the experiment.

Abstract:
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.

Abstract:
We studied the c-axis transport of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) cross-whisker junctions formed by annealing ``naturally'' formed whisker junctions. These frequently appear during growth when the $ab$-faces of neighboring whiskers come in contact. We obtained Fraunhofer patterns of the cross-junction critical currents in a parallel magnetic field, and found a sharp increase in the quasiparticle tunneling conductance at $eV=50-60$ mV, indicating high junction quality. For our weak junctions, the interface critical current density is about 0.03 of the critical current density across the stack of bulk intrinsic junctions, as is the room temperature conductivity, and is independent of the twist angle, in contrast to most of the data reported on ``artificial'' cross-whisker junctions [Y. Takano {\it et al.}, Phys. Rev. B {\bf 65}, 140513(R) (2002)]. Our results provide strong evidence of incoherent interface tunneling and for at least a small s-wave order parameter component in the bulk of Bi2212 for $T\le T_c$. They are also consistent with the bicrystal twist experiments of Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)].