Christophe Renner

Critical currents approaching the depairing limit at a twin boundary in YBa2Cu3O7-δ

Interest in vortex matter has risen considerably since the discovery of the high-temperature superconductors, which exhibit magnetic vortex states that are especially rich and complex. The global behaviour of magnetic vortices in nearly perfect crystals—such as melting of the vortex lattice—has been much studied, but of more technological relevance is the influence on the vortex states of the various structural defects present in most practical superconductors. An important example of such a defect is the twin boundary present in twinned orthorhombic crystals of YBa2Cu3O7−δ (YBCO). Studies of such samples using magnetic-field-sensitive probes have suggested that the twin boundary plays an important role in pinning the vortices and so enhancing the currents that YBCO can support while remaining superconducting. But the low spatial resolution of these techniques does not permit these effects to be studied at the scale of the vortices or boundaries themselves. Scanning tunnelling spectroscopy offers a means of circumventing these problems of resolution, as it directly probes the superconducting order parameter at nanometre length scales. Here we use this technique to investigate the importance of twin boundaries in YBCO. In particular, we observe an unexpectedly large pinning strength for perpendicular vortex flux across the boundary, which implies that the critical current that can be supported along the boundary approaches the theoretical ‘depairing’ limit.

Specific heat of high temperature superconductors in high fields at Tc: from BCS to the Bose–Einstein condensation

We consider the experimental trends in a database of specific heat measurements near T c in high magnetic fields for type-II superconductors with a large value of κ = λ/ξ, including mostly high-temperature superconductors. Whereas the BCS limiting case is well established in low-T c superconductors, the exact 3D-XY behavior illustrated by the λ-transition of 4 He applies only in the particular case of optimally doped YBa 2 Cu 3 O x . Otherwise, transitions are intermediate either between the BCS and the 3D-XY models (e.g., YBa 2 Cu 3 O 7.00 ), or between the 3D-XY model and the Bose-Einstein condensation (BEC) (e.g., Bi 2 Sr 2 CaCu 2 O 8 ). The key parameter in ordering this sequence appears to be the product k F ξ of the Fermi wave number by the coherence length, as evaluated from tunneling spectra in the vortex cores. Such a trend, which is consistent with theoretical descriptions of the strong coupling limit, is visible in the thermodynamics of the phase transition. Implications on the effective mass, the density of pairs just above T c , the pseudo-gap behavior, etc., are discussed.

SCANNING TUNNELING SPECTROSCOPY ON HIGH TEMPERATURE SUPERCONDUCTORS

The discovery of the high temperature superconductors in 1986 radically changed the situation in the field of superconductivity. Not only did the critical temperature increase drastically, but it becomes more and more clear that the nature of the superconducting state in the high temperature superconductors is radically different from the one found in the well known low temperature superconductors. In spite of a considerable world wide effort to study these new superconductors we still do not know the origin and the nature of the pairing interaction. It remains one of the central challenges of solid state physics today to elucidate this mechanism. One essential ingredient in the quest for an understanding of the superconducting state is the quasiparticle density of states or in other words the density of single particle excitations above the superconducting ground state. In 1960 Ivar Giaever 2 demonstrated that tunneling between a superconductor and a normal metal allowed a direct measurement of the quasiparticle density of states in the superconductor. After this path breaking work, tunneling became a main tool for investigating the various low temperature superconducting materials known at the time. In particular the inversion scheme developed by Rowell and McMillan to extract the electron phonon coupling from the tunneling spectra made a quantitative verification of the BCS model for these low temperature materials possible. Since the discovery of the high temperature superconductors, numerous investigations have been carried out with the hope that such tunneling studies will finally allow to unravel the mechanisms leading to high

STM Vortex Core Spectroscopy and Non-BCS Pairing in High Temperature Superconductors

We present scanning tunneling spectroscopy (STS) investigations of high temperature superconductors illustrating the very unusual characteristics of the superconducting gap, and the intimate relation existing between the superconducting and the normal-state gap (pseudogap). A unique feature of STS is to probe the local density of quasiparticle states bound to individual vortices. The vortex core spectroscopy sheds new light on the pseudogap and the microscopic nature of HTS. The experiments reviewed here suggest that HTS are in a regime very different from BCS, approaching the crossover to Bose-Einstein condensation.

Electronic structure of individual flux lines in YBa2Cu3O7-delta

We present images of the flux line lattice and spectroscopic measurements of the vortex core region on the as grown (001) surface of an YBa2Cu3O7-(delta ) single crystal at 4.2 Kelvin. These measurements were done in a magnetic field of 6 Tesla applied parallel to the c-axis using scanning tunneling microscopy. We find an oblique vortex lattice with nearly equal primitive vectors at an angle of 77 degrees. The vortex cores are ellipsoidal, consistent with the ab-plane anisotropy of YBCO. These characteristics can be fit to a distorted square lattice rotated 45 degrees with respect to the YBCO crystal lattice. The tunneling spectroscopy into the vortex core region reveals two bound quasiparticle energy levels with a gap of 11 meV at the Fermi level. Away from the vortex core, the tunneling characteristics are very similar to the zero-field spectra and show a multiple gap-like structure and a large zero-bias conductance.

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 STMs spatial resolution to shed some light on the controversy among the tunneling spectroscopy of high temperature superconductors published so far.

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 Bi2Sr2CaCu2O8 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 Bi2Sr2CaCu2O8 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.