Comment on `Strong Vortex Liquid Correlation' from Multiterminal Measurements on Untwinned YBa2_2Cu3_3O7−δ_{7-\delta} Single Crystals'

A.Rydh and \"O.Rapp [Phys. Rev. Lett. {\bf 86}, 1873 (2001).] claim that the vortex liquid in untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ crystals is correlated above the melting transition, in striking contrast to previous work [D.L\'opez {\it et al.}, Phys. Rev. Lett. {\bf 76}, 4034 (1996).]. In this Comment we present new measurements using the same experimental technique on twinned and untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ crystals with similar overall characteristics as those reported by Rydh and Rapp . The comparison of the vortex correlation response in both cases indicates that the central conclusion of their work is not correct. Our results reconfirm the work by L\'opez {\it et al.} and points on the origin of the misinterpretation in the work of Rydh and Rapp.Comment: comment on A.Rydh and \"O.Rapp, Phys. Rev. Lett. {\bf 86}, 1873 (2001). accepted in Phys. Rev. Let

Observation of superluminal geometrical resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions

We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux-flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the THz frequency range

Disparity of superconducting and pseudogap scales in low-Tc Bi-2201 cuprates

We experimentally study transport and intrinsic tunneling characteristics of a single-layer cuprate Bi(2+x)Sr(2-y)CuO(6+delta) with a low superconducting critical temperature Tc < 4 K. It is observed that the superconducting energy, critical field and fluctuation temperature range are scaling down with Tc, while the corresponding pseudogap characteristics have the same order of magnitude as for high-Tc cuprates with 20 to 30 times higher Tc. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.Comment: 5 page

Surface plasmons at single nanoholes in Au-films

The generation of surface plasmon polaritons (SPP's) at isolated nanoholes in 100 nm thick Au films is studied using near-field scanning optical microscopy (NSOM). Finite-difference time-domain calculations, some explicitly including a model of the NSOM tip, are used to interpret the results. We find the holes act as point-like sources of SPP's and demonstrate that interference between SPP's and a directly transmitted wave allows for determination of the wavelength, phase, and decay length of the SPP. The near-field intensity patterns can be manipulated by varying the angle and polarization of the incident beam.Comment: 12 pages, 3 figure

Anisotropy of the upper critical field in MgB2: the two-gap Ginzburg-Landau theory

The upper critical field in MgB2 is investigated in the framework of the two-gap Ginzburg-Landau theory. A variational solution of linearized Ginzburg-Landau equations agrees well with the Landau level expansion and demonstrates that spatial distributions of the gap functions are different in the two bands and change with temperature. The temperature variation of the ratio of two gaps is responsible for the upward temperature dependence of in-plane Hc2 as well as for the deviation of its out-of-plane behavior from the standard angular dependence. The hexagonal in-plane modulations of Hc2 can change sign with decreasing temperature.Comment: 6 pages, 6 figures, accepted in the European Physical Journal

Anomalies and Schwinger terms in NCG field theory models

We study the quantization of chiral fermions coupled to generalized Dirac operators arising in NCG Yang-Mills theory. The cocycles describing chiral symmetry breaking are calculated. In particular, we introduce a generalized locality principle for the cocycles. Local cocycles are by definition expressions which can be written as generalized traces of operator commutators. In the case of pseudodifferential operators, these traces lead in fact to integrals of ordinary local de Rham forms. As an application of the general ideas we discuss the case of noncommutative tori. We also develop a gerbe theoretic approach to the chiral anomaly in hamiltonian quantization of NCG field theory.Comment: 30 page