Scanning tunneling microscopy and spectroscopy (STM/S) measurements in the superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density of states with two well defined features at 0.97 meV and 0.53 meV, located respectively above and below the value for the superconducting gap expected from single band s-wave BCS model (∆=1.76kBTc=0.9 meV). Both features have a continuous temperature evolution and disappear at Tc = 5.7 K. Moreover, we observe the hexagonal vortex lattice with radially symmetric vortices and a well developed localized state at the vortex cores. The sixfold star shape characteristic of the vortex lattice of the compound 2H-NbSe2 is, together with the charge density wave order (CDW), absent in 2H-NbS2.PACS numbers: 71.45. Lr, 74.25.Jb,74.50.+r,74.70.Ad The study of the coexistence of superconductivity with competing physical phenomena such as magnetic or charge order has historically produced great interest on the scientific community. Anisotropies or modulations of the superconducting properties (in real and/or reciprocal space) often appear as a consequence of competing orders within the same system [1,2]. In the compound 2H-NbSe 2 , superconductivity appears within a CDW state (T CDW =33K and T c =7.2 K) [3]. Low lying excitations measured deep in the superconducting state long time ago by specific heat [4,5] have been explained by recent experiments and theoretical calculations with a multiband superconductivity and a peculiar anisotropy of the superconducting gap [6,7]. Recent angular resolved photoemission spectroscopy measurements demonstrate that the superconducting gap has, close to T c (at 5.7 K), largest values at k-space positions connected with CDW wavevectors [8,9]. Hess et al. [10,11,12] found that the local superconducting density of states (LDOS) at the center of the vortex core shows a high peak close to the Fermi level highlighting the lowest quasiparticle state bound within the vortex core well [13]. Around the vortex core, the LDOS is far from respecting in-plane symmetry and intriguing vortex lattice images with patterns showing strong in-plane LDOS modulations are obtained [10,11,12]. 2H-NbSe 2 belongs to the transition-metal dichalcogenides (2H-MX 2 with M = Ta, Nb and X = Se, S), a family of systems which is unique to study the interplay between CDW order and superconductivity. The 2H-MX 2 compounds share a double layered structure made of two hexagonal X sheets with an intercalated M sheet (X-M-X), connected through very weak van der Waals bonds [14]. This produces highly anisotropic, quasi two dimensional electronic properties. The features of the Fermi Surface (FS) expected to be common in all systems of the series are two concentric cylindrical FS sheets centered on both Γ and K points, derived from the transition-metal d bands [15,16,17,18]. When going over the series from 2H-TaSe 2 and 2H-TaS 2 to 2H-NbSe 2 and 2H-NbS 2 , the ratio of the intralayer lattice constant with the interlayer distance a/c increases, as well as T c , whereas T CDW decreas...

Topological defects such as dislocations and disclinations are predicted to determine the twodimensional (2-D) melting transition [1][2][3] . In 2-D superconducting vortex lattices, macroscopic measurements evidence melting close to the transition to the normal state. However, the direct observation at the scale of individual vortices of the melting sequence has never been performed. Here we provide step by step imaging through scanning tunneling spectroscopy of a 2-D system of vortices up to the melting transition in a focused-ion-beam nanodeposited W-based superconducting thin film. We show directly the transition into an isotropic liquid below the superconducting critical temperature. Before that, we find a hexatic phase, 1 characterized by the appearance of free dislocations, and a smectic-like phase, possibly originated through partial disclination unbinding. These results represent a significant step in the understanding of melting of 2-D systems, with impact across several research fields, such as liquid crystal molecules, or lipids in membranes [4][5][6][7] .The seminal ideas of Kosterlitz and Thouless , have been much discussed in connection with melting of many different 2-D hexagonal crystals. These include crystals formed from elongated rod-like entities such as superfluid vortices, lipids or liquid crystal molecules, as well as from more isotropic constituents, as electrons Wigner crystals, particles or bubbles, or electronic charge density arrangements [8][9][10] . Theoretically, it is found that the unbinding of dislocation pairs creates an intermediate phase in a 2-D hexagonal crystal, named the hexatic phase, which retains sixfold orientational order but has no long range translational order [1][2][3] . The hexatic phase, which appears between the 2-D crystalline phase and the isotropic liquid, has been since then subject of much research. Vortex lattices in superconducting thin films appear as an ideal system where the mechanism of the 2-D melting process can be investigated. Until now, work on the 2-D solid-liquid superconducting vortex lattice transition has focused on macroscopic studies of thermal properties or of critical currents around the melting transition [11][12][13][14][15] . The transition seems to be consistent with the theory of 2-D dislocation-unbinding melting behavior [1][2][3][16][17][18] . However, no local direct observation of melting has been provided. Here we track the modifications induced by temperature on a 2-D vortex system, from its formation up to the isotropic liquid state. Increasing temperature we first observe thermal de-pinning of vortices, which produces a more ordered hexagonal lattice. Melting process is then initiated with 2 the appearance of free dislocations, corresponding to the hexatic phase, from which a smectic-like phase emerges, before the isotropic liquid is formed.Our sample is an amorphous W-based thin film nano-deposited using focused-ion-beam, with a superconducting critical temperature at zero field T c = 4. 15 K 19, 20 . It is an extrem...

We report on tunneling spectroscopy experiments in small grains of the new binary intermetallic superconductor MgB(2). Experiments have been performed at 2.5 K using a low temperature scanning tunneling microscope. Good fit to the BCS model is obtained, with a gap value of 2 meV. In the framework of this model, this value should correspond to a surface critical temperature of 13.2 K. No evidence of gap anisotropy has been found.

A superconductor in a magnetic field acquires a finite electrical resistance caused by vortex motion. A quest to immobilize vortices and recover zero resistance at high fields made intense studies of vortex pinning one of the mainstreams of superconducting research. Yet, the decades of efforts resulted in a realization that even promising nanostructures, utilizing vortex matching, cannot withstand high vortex density at large magnetic fields. Here, we report a giant reentrance of vortex pinning induced by increasing magnetic field in a W-based nanowire and a TiN-perforated film densely populated with vortices. We find an extended range of zero resistance with vortex motion arrested by self-induced collective traps. The latter emerge due to order parameter suppression by vortices confined in narrow constrictions by surface superconductivity. Our findings show that geometric restrictions can radically change magnetic properties of superconductors and reverse detrimental effects of magnetic field.

We present scanning tunneling microscopy and spectroscopy measurements at 100mK in the superconducting material 2H-NbSe2 that show well defined features in the superconducting density of states changing in a pattern closely following atomic periodicity. Our experiment demonstrates that the intrinsic superconducting density of states can show atomic size modulations, which reflect the reciprocal space structure of the superconducting gap. In particular we obtain that the superconducting gap of 2H-NbSe2 has six fold modulated components at 0.75 mV and 1.2 mV. Moreover, we also find related atomic size modulations inside vortices, demonstrating that the much discussed star shape vortex structure produced by localized states inside the vortex cores, has a, hitherto undetected, superposed atomic size modulation. The tip substrate interaction in an anisotropic superconductor has been calculated, giving position dependent changes related to the observed gap anisotropy.

The pressure dependence of the critical temperature T c and upper critical field H c2 (T ) has been measured up to 19 GPa in the layered superconducting material 2H-NbSe 2 . T c (P ) has a maximum at 10.5 GPa, well above the pressure for the suppression of the CDW order. Using an effective two band model to fit H c2 (T ), we obtain the pressure dependence of the anisotropy in the electron phonon coupling and Fermi velocities, which reveals the peculiar interplay between CDW order, Fermi surface complexity and superconductivity in this system.

We report on high quality local tunnel spectroscopy measurements in superconductors using in-situ fabricated superconducting tips as counterelectrode. The experiments were made at very low temperatures using a dilution refrigerator and a 3 He cryostat. Spectra obtained with superconducting tip and sample of Al show that the spectroscopic resolution of our set-up is of 15µeV . Following the observation of Josephson current in tunneling regime (with tips of Pb and of Al), we discuss the feasibility of Scanning Josephson Spectroscopy with atomic size resolution. Experiments showing new applications of these superconducting tips under applied external magnetic fields are also reported.

We present new measurements of the thermal conductivity (x)