A. E. Koshelev

Emission of Coherent THz Radiation from Superconductors

Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to ∼50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

Flux pinning and creep in very anistropic high temperature superconductors

Abstract The effect of thermal fluctuations on collective flux-pinning and creeps is investigated for thin-film superconductors and layered superconductors with weak Josephson coupling between the layers in a field normal to the layers and much less than B c2 . Temperature and field dependences of the critical current j c in a two-dimensional (2D) system are obtained. The activation barriers for 2D flux-creep are shown to grow infinitely as U ( j )∝ j -μ at j ⪡ j c , which is characteristics for the vortex-glass state. At very small currents this behaviour is cut off by the plastic motion of edge-dislocation pairs which are either induced by disorder or thermally created, leading to linear current-voltage behaviour inhibiting the existence of a vortex-glass state in 2D systems. The Josephson coupling in layered superconductors changes the dimensionality of the vortex lattice. It is shown that a sufficiently large field when the lattice constant a o becomes less than the characteristic length of the interlayer coupling a 0 r 3D =( R j a 0 ) 1 2 or B > B 2D =Ф 0 / R 2 J , ( R J =Г 1 2 s is the effective Josephson length, s is the interlayer spacing and Г= m z / m is the mass anisotropy), the fluctuations of the vortex lines become of 2D nature. This means in particular that 3D-lattice melting will take place at T = T m , T m is the dislocation-mediated melting temperature of a 2D vortex-lattice. The mixed state at B > B 2D is studied and the possibility of different regimes of pinning and creep is demonstrated. The crossover from 2D to 3D pinning is found when the pinning length R c exceeds r 3D . The crossover conditions are derived and displayed in a schematic phase diagram.

Two-band superconductivity in MgB{sub 2}.

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]].

Crossing Lattices, Vortex Chains, and Angular Dependence of Melting Line in Layered Superconductors

We investigate vortex structure in very anisotropic layered superconductors at fields tilted with respect to the c axis. We show that even a small in-plane field does not tilt the vortex lattice but, instead, penetrates inside the superconductor in the form of Josephson vortices (JVs). At high c -axis magnetic field the phase field of the JV is built up from the phase perturbations created by displacements of pancake vortices. The crossing-lattices ground state leads to linear dependence of the {ital c}-axis melting field on the in-plane field, in agreement with recent experimental observations. At small fields stacks of JVs accumulate additional pancake stacks creating vortex rows with enhanced density. This mechanism explains the mixed chains-lattice state observed by decorations. {copyright} {ital 1999} {ital The American Physical Society}We investigate vortex structure and melting transition in very anisotropic layered superconductors at fields tilted with respect to the c-axis. We show that even a small in-plane field does not homogeneously tilt the vortex lattice but, instead, penetrates inside the superconductor in the form of Josephson vortices (JVs) similar to the Meissner state. At high c-axis magnetic field the phase field of the JV is built up from the phase perturbations created by displacements of pancake vortices. The crossing-lattices ground state leads to linear dependencies of the melting field and melting temperature on the in-plane field, in agreement with recent experimental observations. At small fields stacks of JVs accumulate additional pancake strings creating vortex rows with enhanced density. This mechanism explains the mixed chains-lattice state observed by Bitter decorations.

Tunable terahertz emission from Bi2Sr2CaCu2O8+δ mesa devices

We have measured coherent terahertz emission spectra from Bi2Sr2CaCu2O8+\delta\ mesa devices, as a function of temperature and mesa bias voltage. The emission frequency is found to be tunable by up to 12% by varying the temperature and bias voltage. We attribute the appearance of tunability to asymmetric boundaries at the top and bottom, and the non-rectangular cross-section of the mesas. This interpretation is consistent with numerical simulations of the dynamics of intrinsic Josephson junctions in the mesa. Easily tunable emission frequency may have important implications for the design of terahertz devices based on stacked intrinsic Josephson junctions.

Mixed state of a dirty two-band superconductor: application to MgB2.

We investigate the vortex state in a two-band superconductor with strong intraband and weak interband electronic scattering rates. Coupled Usadel equations are solved numerically, and the distributions of the pair potentials and local densities of states are calculated for two bands at different values of magnetic fields. The existence of two distinct length scales corresponding to different bands is demonstrated. The results provide qualitative interpretation of recent scanning tunneling microscopy experiments on vortex structure imaging in MgB2.

Resonant electromagnetic emission from intrinsic Josephson-junction stacks with laterally modulated Josephson critical current.

Intrinsic Josephson-junction stacks realized in mesas fabricated out of high-temperature superconductors may be used as sources of coherent electromagnetic radiation in the terahertz range. The major challenge is to synchronize Josephson oscillations in all junctions in the stack to get significant radiation out of the crystal edge parallel to the c axis. We suggest a simple way to solve this problem via artificially prepared lateral modulation of the Josephson critical current identical in all junctions. In such a stack, phase oscillations excite the in-phase Fiske mode when the Josephson frequency matches the Fiske-resonance frequency which is set by the stack lateral size. The powerful, almost standing electromagnetic wave is excited inside the crystal in the resonance. This wave is homogeneous across the layers, meaning that the oscillations are synchronized in all junctions in the stack. We evaluate behavior of the I-V characteristics and radiated power near the resonance for arbitrary modulation and find exact solutions for several special cases corresponding to symmetric and asymmetric modulations of the critical current.

Plasma Resonance and Remaining Josephson Coupling in the "Decoupled Vortex Liquid Phase" in Layered Superconductors.

We relate the frequency of the Josephson plasma resonance in layered superconductors with the frequency dependent superconducting response. We demonstrate that the sharp resonance can persist even when the global superconducting coherence is broken provided the resonance frequency is larger than the frequency of interlayer phase slips. In this situation the plasma frequency is determined by the average Josephson energy, which can be calculated using the high temperature expansion. We also find the temperature dependence of the average Josephson energy from the Monte Carlo simulations and determine the applicability region of the high temperature expansion. {copyright} {ital 1996 The American Physical Society.}

Upper critical field in dirty two-band superconductors: breakdown of the anisotropic Ginzburg-Landau theory

We investigate the upper critical field in a dirty two-band superconductor within quasiclassical Usadel equations. The regime of very high anisotropy in the quasi-2D band, relevant for MgB

Powerful terahertz emission from Bi2Sr2CaCu2O8+δ mesa arrays

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