Non-adiabatic perturbations in multi-component perfect fluids

The evolution of non-adiabatic perturbations in models with multiple coupled perfect fluids with non-adiabatic sound speed is considered. Instead of splitting the entropy perturbation into relative and intrinsic parts, we introduce a set of symmetric quantities, which also govern the non-adiabatic pressure perturbation in models with energy transfer. We write the gauge invariant equations for the variables that determine on a large scale the non-adiabatic pressure perturbation and the rate of changes of the comoving curvature perturbation. The analysis of evolution of the non-adiabatic pressure perturbation has been made for several particular models.Comment: 11 pages, v2, major revision, some clarifications added, English is improved, results unchange

Shunt-capacitor-assisted synchronization of oscillations in intrinsic Josephson junctions stack

We show that shunt capacitor stabilizes synchronized oscillations in intrinsic Josephson junction stacks biased by DC current. This synchronization mechanism has an effect similar to the previously discussed radiative coupling between junctions, however, it is not defined by the geometry of the stack. It is particularly important in crystals with smaller number of junctions, where radiation coupling is week, and is comparable with the effect of strong super-radiation in crystal with many junctions. The shunt also helps to enter the phase-locked regime in the beginning of oscillations, after switching on the bias current. Shunt may be used to tune radiation power, which drops as shunt capacitance increases.Comment: 9 pages, 1 fi

Electrodynamics of Josephson vortex lattice in high-temperature superconductors

We studied response of the Josephson vortex lattice in layered superconductors to the high-frequency c-axis electric field. We found a simple relation connecting the dynamic dielectric constant with the perturbation of the superconducting phase, induced by oscillating electric field. Numerically solving equations for the oscillating phases, we computed the frequency dependences of the loss function at different magnetic fields, including regions of both dilute and dense Josephson vortex lattices. The overall behavior is mainly determined by the c-axis and in-plane dissipation parameters, which is inversely proportional to the anisotropy. The cases of weak and strong dissipation are realized in $\mathrm{Bi_{2}Sr_{2}CaCu_{2}O_{x}}$ and underdoped $\mathrm{YBa_{2}Cu_{3} O_{x}}$ correspondingly. The main feature of the response is the Josephson-plasma-resonance peak. In the weak-dissipation case additional satellites appear in the dilute regime mostly in the higher-frequency region due to excitation of the plasma modes with the wave vectors set by the lattice structure. In the dense-lattice limit the plasma peak moves to higher frequency and its intensity rapidly decreases, in agreement with experiment and analytical theory. Behavior of the loss function at low frequencies is well described by the phenomenological theory of vortex oscillations. In the case of very strong in-plane dissipation an additional peak in the loss function appears below the plasma frequency. Such peak has been observed experimentally in underdoped $\mathrm{YBa_{2}Cu_{3} O_{x}}$. It is caused by frequency dependence of in-plane contribution to losses rather then a definite mode of phase oscillations.Comment: 10 pages, 7 figures, to be published in Phys.Rev.B, supplementary animations of oscillating local electric field can be found at http://mti.msd.anl.gov/homepages/koshelev/projects/JPRinJVL/Nz2vc0_32vab6_0Anim.ht

Radiation Due to Josephson Oscillations in Layered Superconductors

We derive the power of direct radiation into free space induced by Josephson oscillations in intrinsic Josephson junctions of highly anisotropic layered superconductors. We consider the super-radiation regime for a crystal cut in the form of a thin slice parallel to the c-axis. We find that the radiation correction to the current-voltage characteristic in this regime depends only on crystal shape. We show that at large enough number of junctions oscillations are synchronized providing high radiation power and efficiency in the THz frequency range. We discuss crystal parameters and bias current optimal for radiation power and crystal cooling.Comment: 4 pages, 1 figure, to be published in Phys. Rev. Let

Plasma resonance at low magnetic fields as a probe of vortex line meandering in layered superconductors

We consider the magnetic field dependence of the plasma resonance frequency in pristine and in irradiated Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals near $T_c$. At low magnetic fields we relate linear in field corrections to the plasma frequency to the average distance between the pancake vortices in the neighboring layers (wandering length). We calculate the wandering length in the case of thermal wiggling of vortex lines, taking into account both Josephson and magnetic interlayer coupling of pancakes. Analyzing experimental data, we found that (i) the wandering length becomes comparable with the London penetration depth near T$_{c}$ and (ii) at small melting fields ($< 20$ G) the wandering length does not change much at the melting transition. This shows existence of the line liquid phase in this field range. We also found that pinning by columnar defects affects weakly the field dependence of the plasma resonance frequency near $T_c$.Comment: RevTex, 4 pages, 2 PS figures, Submitted to Phys. Rev.

Slow-roll Inflation for Generalized Two-Field Lagrangians

We study the slow-roll regime of two field inflation, in which the two fields are also coupled through their kinetic terms. Such Lagrangians are motivated by particle physics and by scalar-tensor theories studied in the Einstein frame. We compute the power spectra of adiabatic and isocurvature perturbations on large scales to first order in the slow-roll parameters. We discuss the relevance of the extra coupling terms for the amplitude and indexes of the power spectra. Beyond the consistency condition which involves the amplitude of gravitational waves, additional relations may be found in particular models based on such Lagrangians: as an example, we find an additional general consistency condition in implicit form for Brans-Dicke theory in the Einstein frame.Comment: 17 pages, 1 figure, accepted for publication in Phys. Rev.