Four-fold structure of vortex core states in Bi2Sr2CaCu2O8 (Bi2212)

We present a detailed study of vortex core spectroscopy in slightly overdoped Bi2Sr2CaCu2O8 using a low temperature scanning tunneling microscope. Inside the vortex core we observe a four-fold symmetric modulation of the local density of states with an energy-independent period of (4.3\pm 0.3)a0. Furthermore we demonstrate that this square modulation is related to the vortex core states which are located at ~6 meV. Since the core-state energy is proportional to the superconducting gap magnitude, our results strongly suggest the existence of a direct relation between the superconducting state and the local electronic modulations in the vortex core.Comment: 5 pages, 4 figures. Submitted to Physical Review Letter

3D Finite Element Simulations of strip lines of a YBCO/Au Fault Current Limiter

Geometrical aspects of the design of fault current limiters (FCL) have a great impact on their performance. Recently, the University of Geneva have presented optimized geometries obtained by splitting the FCL into many small dissipative lengths in order to distribute the power along the device. We have performed 3D finite element method (FEM) simulations for studying the behavior of strip lines of a YBCO/Au FCL in AC nominal use (sinusoidal current at industrial frequency) up to 3 Ic. Particular attention has been paid to the mesh, due to the very large involved aspect ratios. The numerical results show a concentration of the electric field in the sharp corners. This results in very large power dissipation, which has been experimentally confirmed by wafer cracks during over-Ic tests. A new geometry, taking into account the length of the connecting path and the corner optimization, has been proposed. Finally, simulations coupling electromagnetic and thermal equations show the behavior of the device when a default occurs on the electrical network. This work is supported by the Swiss National Science Foundation through the National Center of Competence in Research “Materials with Novel Electronic Properties – MaNEP

Atlantic Salmon Reovirus Infection Causes a CD8 T Cell Myocarditis in Atlantic Salmon (Salmo salar L.)

Heart and skeletal inflammation (HSMI) of farmed Atlantic salmon (Salmo salar L.) is a disease characterized by a chronic myocarditis involving the epicardium and the compact and spongious part of the heart ventricle. Chronic myositis of the red skeletal muscle is also a typical finding of HSMI. Piscine reovirus (PRV) has been detected by real-time PCR from farmed and wild salmon with and without typical changes of HSMI and thus the causal relationship between presence of virus and the disease has not been fully determined [1]. In this study we show that the Atlantic salmon reovirus (ASRV), identical to PRV, can be passaged in GF-1 cells and experimental challenge of naïve Atlantic salmon with cell culture passaged reovirus results in cardiac and skeletal muscle pathology typical of HSMI with onset of pathology from 6 weeks, peaking by 9 weeks post challenge. ASRV replicates in heart tissue and the peak level of virus replication coincides with peak of heart lesions. We further demonstrate mRNA transcript assessment and in situ characterization that challenged fish develop a CD8+ T cell myocarditis

Vacuum tunneling spectroscopy and asymmetric density of states of Bi₂Sr₂CaCu₂O_8+δ

This paper reports very detailed low-temperature vacuum tunneling spectroscopy investigations of Bi2Sr2CaCu2O8+δ (BSCCO) single crystals. For clean vacuum junctions formed between a cleaved BSCCO single crystal and the normal-metal tip of a scanning tunneling microscope, we obtain stable c-axis vacuum tunneling conditions that allow very reproducible low-temperature electron tunneling spectroscopy. A vacuum junction is identified by tunneling spectra which neither depend on tip/sample spacing nor change as a function of time and position on the sample in the Meissner state. In contrast to the frequently reported linear or parabolic increase with increasing bias voltage, the background conductance of such spectra is largely constant with a slight decrease up to ±300 meV bias. The normal-state conductance inferred from this background has a local maximum at negative sample bias, indicating a pileup below the Fermi level in the (ab)-plane normal-state density of states of BSCCO. Vacuum junctions at 4.8 K show a well developed superconducting gap with large peaks at the gap edges and a finite density of quasiparticle excitations filling the gap. These characteristics are not consistent with an isotropic BCS-like gap parameter. Outside the superconducting gap, the differential conductance curves are very asymmetric, with a striking dip which appears at negative sample bias only. This dip contributes a substantial amount of states to satisfy the conservation of states we find between normal and superconducting BSCCO

Tunneling Spectroscopy and STM Observation of Flux Lines

This paper reviews results obtained from low temperature scanning tunneling spectroscopy investigations on cleaved 2H-Nb1-xTaxSe2 and Bi2Sr2CaCu208 single crystals. Our study of the former crystals allowed us to image the vortex lattice, and to study how the peak in the density of states in the centre of the vortices is modified by the transition from the clean to the dirty limit. We have also been able to observe slow vortex motion with this technique. In our study of the Bi2Sr2CaCu208 compound, we have been able to obtain reproducible I-V characteristics while scanning. The characteristic features of the differential conductance spectra are sharp peaks at the superconducting gap edge, a weak dip beyond this energy at negative bias, and a finite conductance below the gap. The gap is found to have a value of 31±4 meV, but the experimental density of states shows the presence of states below the gap and cannot easily be fitted to a BCS like s-wave state

Non BCS IV characteristics of superconducting Bi₂Sr₂CaCu₂O_8+δ single crystals

We present vacuum tunneling characteristics obtained on Bi2Sr2CaCu2O8+δ single crystals using a low temperature scanning tunneling microscope. We demonstrate very reproducible tunneling spectroscopy where the IV characteristics do not depend on modifications of the tunneling barrier obtained either by scanning the tip along the sample surface or by varying the tip/sample spacing. The IV curves and the differential conductance spectra show significant features which are not consistent with single gap BCS-like superconductivity. In particular the peaks at the gap edges and the low bias excitations in the experimental density of states are too large compared to the predictions of an isotropic BCS gap. Furthermore, we observe a dip in the differential conductance spectra at about -70meV sample bias, a structure which does not appear at positive sample bias

Vacuum tunneling spectroscopy of superconducting Bi₂Sr₂CaCu₂O₈ using scanning tunneling microscopy

We report STM spectroscopy measurements of in-situ cleaved Bi₂Sr₂CaCu₂O₈ single crystals at 4.8 Kelvin, where we achieved strong evidences for true vacuum tunneling. These careful experiments result in very reproducible spectroscopy as a function of position on the surface, and as a function of tip/sample spacing. The characteristic features of the tunneling spectra are a significant filling of the gap region, a large density of states at the gap edges and a weak dip about 70 meV below the Fermi level. Such IV characteristics are not compatible with a single gap BCS-like s-wave theory. Furthermore, we report spatially resolved spectroscopy where we observe regions with two distinct gap values. A double gap structure appears in the tunneling spectra acquired in the vicinity of the boundary between these regions. We believe the double gap structure we observe in this case does not reflect an intrinsic gap anisotropy, but seems rather related to crystalline inhomogeneities. This demonstrates the potential of the STM's spatial resolution to shed some light on the controversy among the tunneling spectroscopy of high temperature superconductors published so far

Enhanced field-emission investigation of aluminum

Enhanced field emission on high-purity aluminum has been investigated. The current/voltage characteristic (Fowler-Nordheim plot), the chemical composition as determined by Auger electron spectroscopy and X-ray microprobe analysis, and the topography of field-emitting sites were studied locally. In addition, results on the evolution of these sites under heat treatment, Ar/sup +/ sputtering, and exposure to atmosphere are presented. The field-emitting sites on aluminum are all related to particles approximately 30 mu m in size standing on the surface. The average emission over 1-cm/sup 2/ areas is little affected by the abovementioned surface treatments