The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are 'doped' away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers ('holes') into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of approximately 14 A. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

We report on low temperature scanning tunneling microscopy (STM) studies of the electronic structure of vortex cores in Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO). At the vortex core center, an enhanced densityof-states (DOS) is observed at energies near Ω = ± 7 meV. Spectroscopic imaging at these energies reveals an exponential decay of these 'core states' with a decay length of 22 ± 3 Å. The four-fold symmetry sometimes predicted for d-wave vortices is not seen in spectroscopic vortex images. A locally nodeless order parameter induced by the magnetic field may be consistent with these measurements.PACS# 74.72.Hs , 74.50.+r , 75.30.Hx , 61.16.Ch In conventional type II superconductors, the order parameter (OP) is suppressed in the cores of quantized magnetic vortices [1] and recovers over a distance of about one coherence length ξ. Bound quasi-particle states can exist inside these cores [2] with lowest energy given approximately by E ~ ∆ 2 /2E F , where E F is the Fermi energy and ∆ is the superconducting energy gap. Such 'core' states were first imaged by Hess et al. using low temperature scanning tunneling microscopy [3]. In the high temperature superconductors (HTSC) the zero magnetic field OP is believed to have mainly a d x 2 -y 2 symmetry [4][5][6][7] and is therefore not uniform in k-space but instead has four nodes [8]. The electronic structure of vortex cores in a superconductor with such an unconventional OP has attracted much attention, both theoretical [ 9-27] and experimental [28][29][30][31], since it serves as a coherence-length scale (~10 Å) probe of the condensate structure.Evidence for low energy vortex core states in YBCO was first provided by infrared absorption experiments [28].Pioneering STM experiments on YBCO by Maggio-Aprile et al. directly identified vortex core states at energies of ± 5.5 meV by tunneling spectroscopy [29]. Similar states were not however observed by STM in BSCCO vortex cores [30][31].Here we report new STM experiments which demonstrate the existence of low energy states associated with the vortex cores in BSCCO. Two different types of as-grown BSCCO single crystals were used. The first set, grown by the directional solidification technique, has a transition temperature T c = 87 K, a transition width of 5 K and contains scattering centers at a density of 0.3%, while the second set, grown by the floating zone technique, has a very dilute (~0.3%) doping concentration of Zn atoms acting as impurity scatterers, a T c of 84 K and a transition width of 4 K. All samples are cleaved in cryogenic ultrahigh vacuum at 4.2 K and immediately inserted into the STM head [32].In addition to topographic imaging, STM can be used to map the electronic DOS on the surface at energy E = eV by measuring the differential tunneling conductance G at sample bias V as a function of position. When studied in zero magnetic field, images of only the impurity scattering resonances [33][34][35] appear in a zero-bias conductance map. Figure 1a is such an image and shows the impurity resonances as dark feature...

Low-temperature scanning tunneling spectroscopy of the high transition temperature (high-Tc) cuprate Bi2Sr2CaCu2O8+delta reveals the existence of large numbers of identical regions with diameters of about 3 nanometers that have a relatively high density of low-energy quasi-particle states. Their spatial and spectroscopic characteristics are consistent with theories of strong quasi-particle scattering from atomic-scale impurities in a d-wave superconductor. These characteristics include breaking of local particle-hole symmetry, a diameter near twice the superconducting coherence length, and an inverse square dependence of their local density-of-states on distance from the scattering center. In addition to the validation of d-wave quasi-particle scattering theories, these observations identify a source for the anomalously high levels of low-energy quasi-particles in Bi2Sr2CaCu2O8+delta at low temperatures.

Thermal hysteresis in a micron-size Superconducting Quantum Interference Device (µ-SQUID), with weak links as Josephson junctions, is an obstacle for improving its performance for magnetometery. Following the "hot-spot" model of Skocpol et al. [J. Appl. Phys. 45, 4054 (1974)] and by incorporating the temperature dependence of thermal conductivity of superconductor using a linear approximation, we find a much better agreement with the observed temperature dependence of the retrapping current in short superconducting Nb-based weak links and µ-SQUIDs. In addition, using the temperature dependence of the critical current, we find that above a certain temperature hysteresis disappears. We analyze the current-voltage characteristics and the weak link temperature variation in both the hysteretic and non-hysteretic regimes. We also discuss the effect of the weak link geometry in order to widen the temperature range of hysteresis-free operation.

We demonstrate the role of the proximity effect in the thermal hysteresis of superconducting constrictions. From the analysis of successive thermal instabilities in the transport characteristics of micron-size superconducting quantum interference devices with a well-controlled geometry, we obtain a complete picture of the different thermal regimes. These determine whether or not the junctions are hysteretic. Below the superconductor critical temperature, the critical current switches from a classical weak-link behavior to one driven by the proximity effect. The associated small amplitude of the critical current makes it robust with respect to the heat generation by phase slips, leading to a nonhysteretic behavior.