Magnetisation of type-II superconductors in the mixed state

The variational method, used in a previous work to study an isolated vortex tube, has been extended to calculate the magnetisation of type-II superconductors for arbitrary values of Ginzburg-Landau parameter κ and over the whole range of the magnetic field from hc₁ to hc₂. The results are discussed.     

Terahertz electromagnetic wave emission by using intrinsic Josephson junctions of high-Tc superconductors

High-Tc cuprate superconductors like Bi₂Sr₂CaCu₂O_8+y (Bi-2212) show very large anisotropy in the electrical conduction. Inside the CuO₂ plan the conduction looks like that in good metals, while the block layers located between CuO₂ planes behave like insulating barriers. Thus, neighboring CuO₂ layers are linked by the Josephson coupling in superconducting states, and the Bi-2212 single crystal can be regarded as a natural serial array of Josephson junctions along the c-axis. It has been found out that in such a system a unique excitation mode called the collective Josephson plasma exists. The mode collectively oscillates along the c-axis, while it propagates along any direction ranged from the ab-plane to the c-axis. If the mode can be coherently excited, the system should be very useful as a device generating strong electromagnetic wave emission. In this paper we review a theoretical framework describing the Josephson plasma modes. The Josephson vortex penetrates into the sample under the magnetic field parallel to the CuO₂ plane and it moves along the ab-plane under an external current parallel to the c-axis. In the vortex flow state, the vortex speed can reach and exceed the propagating velocities of the Josephson plasma modes due to very weak dissipation. Then, the vortices strongly couple with the plasma modes and change their flowing lattice configurations into patterns dependent on the interacting plasma mode profiles. Among many expected flow patterns, we pay attention to a special state resonating with a plasma mode in-phase along the c-axis because all junctions synchronize in such a state. Large-scale numerical simulations by Machida et al. confirmed that the special state characterized by the rectangular lattice stably appears in a wide range of I–V characteristics. In this paper, we report their recent simulation results.     

A new understanding of the resistive state and the V-I characteristic of high-Tc superconductors

Bardeen-Stephen dissipation in the regime of thermally activated flux flow (TAFF) and flux creep has been considered with respect to its contribution to the activation energy for thermally initiated flux movement. The inclusion of this flux flow dissipation term leads to a new expression for the voltage-current (V-I, or E-J) characteristics. The extended model describes the flux dynamics in the three regimes of flux movement: TAFF, creep, and flux flow, in a satisfactory way and explains the main experimental observations.     

Transport coefficients in the mixed state of dirty superconductors

We present a general formalism for the calculation of electronic transport properties in inhomogeneous superconducting alloys, neglecting fluctuations of the self-consistent potentials. All previously known results can easily be rederived as limiting cases and for the first time the ultrasonic absorption and thermal conductivity are determined numerically throughout the whole mixed state regime.