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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 study vortex lattices (VLs) in superconducting weak-pinning platelet-like crystals of $\beta$-Bi$_{2}$Pd in tilted fields with a Scanning Tunneling Microscope. We show that vortices exit the sample perpendicular to the surface and are thus bent beneath the surface. The structure and orientation of tilted VL in the bulk are, for large tilt angles, strongly affected by Coulomb-type intervortex repulsion at the surface due to stray fields.

Trabajo presentado en la conferencia Fuerzas y Túnel (FyT2016), celebrada en Girona del 5 al 7 de septiembre de 2016., Direct visualization and manipulation of magnetic structures in superconductors with high spatial resolution and sensitivity is at the aim of state-of-the-art research in fundamental and applied physics. The possibility of imaging the evolution of the vortex lattice provides valuable information on microscopic properties such as the symmetry of the superconducting order parameters or the pinning strength. Here we discuss the observation of superconducting vortices in β-Bi2Pd (Tc=5K[1]) up to fields of 600 Oe combining Magnetic Force Microscopy (MFM) and Squid On the Tip Microscopy (SOT). The combination of both techniques allows investigating the vortex lattice in two different regimes, not accessible with each microscopy alone: from very low fields (~1 Oe) up to higher fields (~600 Oe). We use a commercial MFM (Nanomagnetics Ltd.) in a home-made three axis superconducting coil system [2] that cools down to 2K. Experiments at Weizmann are made at 4.2K in a SOT [3]. We discuss in detail the pinning process of the vortex lattice, the evolution of the magnetic contrast with the temperature and the applied magnetic field. We find that the superconducting parameters of β-Bi2Pd are excellent to allow vortex lattice observation in a large field range. Furthermore, in MFM experiments, interesting new vortex shapes have been observed in the images, that will be discussed in terms of the magnetic interaction between tip and sample, and the different magnetic domain configuration of the MFM tip.

The layered rare-earth diantimonides RSb2 are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. LaSb2 is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in LaSb2 (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the La1−xCexSb2 series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for x ≈ 0.5. We observe a distortion of rare-earth–Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that LaSb2 has a CDW which is stabilized in the La1−xCexSb2 series due to substitutional disorder, This work was supported by the Spanish MINECO (FIS2014-54498-R, MAT2011-27470-C02-02, and CSD-2009-00013), by the European Union (Graphene Flagship Contract No. CNECT-ICT-604391 and COST MP1201 action), and by the Comunidad de Madrid through programs Nanofrontmag-CM (S2013/MIT-2850) and MAD2D-CM (S2013/MIT-3007).We acknowledge MINECO and CSIC for financial support and for provision of synchrotron radiation facilities and would like to thank the SpLine BM25 staff for assistance in using the beamline

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $\beta$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $\beta$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces., Comment: 8 pages, 7 figures

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $��$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $��$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces., 8 pages, 7 figures

We report an anomalous tunneling conductance with a zero bias peak in flakes of superconducting 2H-TaS$_2$ detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2H-TaS$_2$ with a simplified 2D Fermi surface and sign-changing Cooper pair wavefunction induced by Coulomb repulsion. Superconductivity is induced in the central $\Gamma$ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by non-perturbative coupling between tip and sample, gives Andreev states that lead to a zero bias peak in the tunneling conductance. We suggest that reducing dimensionality down to a few atom thick crystals could drive a crossover from conventional to sign changing pairing in the superconductor 2H-TaS$_2$., Comment: 8 pages, 4 figures

We report an anomalous tunneling conductance with a zero-bias peak in flakes of superconducting 2H-TaS2 detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2H-TaS2 with a simplified two-dimensional Fermi surface and a sign-changing Cooper-pair wave function induced by Coulomb repulsion. Superconductivity is induced in the central Γ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by nonperturbative coupling between tip and sample, gives Andreev states that lead to a zero-bias peak in the tunneling conductance. We suggest that reducing dimensionality down to crystals a few atoms thick could drive a crossover from conventional to sign-changing pairing in the superconductor 2H-TaS2., This work was supported by the Spanish MINECO (Consolider Ingenio Molecular Nanoscience CSD2007-00010 program and FIS2011-23488, ACI2009-0905, FIS2011-23713, and MAT2011-22785), by the European Union (ERC Advanced Grants No. 290846 and No. 247384, Graphene Flagship Contract No. CNECT-ICT-604391, Marie Curie Actions FP7-PEOPLE-2013-CIG-618321, and COST MP1201 action), by the Comunidad de Madrid through program Nanobiomagnet, and by the Generalidad de Valencia through programs PROMETEO and ISIC-NANO.

We report in this paper the temperature and mangetic field dependence of the conductance in the polycrystalline film of titanium nitride, before and after heating at ambient conditions. The difference between the two films is the room temperature sheet resistance which remains within 15 percent and both the films show superconducting transition at lower temperatures. The zero field and the high field data, respectively, corresponds to the superconducting and the normal states. Both the films display Atshuler-Aronov zero bias anamoly in their normal states, and the superconducting gap openeing up at low fields. However the heated film has a smaller gap owing to more pronounced zero bias suppression of the density of states. The normal states in both the films are similar to the quasi-2d-disordered metal and its behavior is studied with temperature. Our data suggests that the zero bias anamoly suppresses the superconducting gap with increase in the disorder., 5 pages, 7 figures

We present in this paper the conductance maps at 100 mK in the disordered polycrystalline film of titanium nitride (TiN). At 5 nm, the film is close to quasi-two dimensional limit and exhibits features pertaining to the superconductor to insulator transition. We measured conductance maps at zero field and at 4 T, which represent the superconducting and the normal phase, respectively. The conductance map at 4 T is uniform, in which the conductance behavior, with logarithmic variation, resembles to the disorder enhanced electron-electron interaction in the two dimensional metallic phase. At low fields we observe the spatial variations of the conductance in the superconducting phase. At several places the superconducting energy gap fluctuates to an extent that the quasi-particle peaks are absent in the conductance curves. The conductance map over a region encompassing only few crystallites suggests that the inhomogeneities in the superconducting phase related to the spatial variations of the electronic density are across the crystalline boundaries., 5 pages, 7 figures

Typically the superconducting phase weakens at several points with the increase in disorder before it is distroyed in the 2d-thin films. This may lead to an inhomogeneous superconducting state without a continuous phase. Here we present scanning tunneling spectroscopy measurements at 0.1 K in the disordered polycrystalline film of TiN describing the nanoscale size features of the superconducting state. The imaging shows imcommensurate charge density modulations, originating at the crystalline bounadries, and intercepted on large scale by the beat patterns in the regions of overlap. Electronic coherence is maintained over length scale minimum of crystalline sizes, and suffers scattering across low angle crystalline boundaries. The superconducting state fluctuates at the positions of the charge density modulations and zones of the weak phase appear in the vicinity of the beats. Our data shows that the BCS-like behavior evolves into the V-shaped density of states in such inhomogeneous regions as a result of the competition between the superconducting correlations with that of the strong electron-electron repulsive interactions assisted by the inelastic scattering at the crystalline boundaries., Comment: 38 pages, 8 figures

We report current driven scanning tunneling spectroscopy (CDSTS) measurements at very low temperatures on vortices in 2H-NbSe2. We find that a current produces an increase of the density of states at the Fermi level in between vortices, and a reduction of the zero bias peak at the vortex center. This occurs well below the de-pairing current. We conclude that a supercurrent affects the low energy part of the superconducting gap structure of 2H-NbSe2., Comment: 5 pages, 5 figures

We report on several low temperature experiments supporting the presence of Majorana fermions in superconducting lead nanowires fabricated with a scanning tunneling microscope. These nanowires are the connecting bridges between the STM tip and the sample resulting from indentation processes. We show here that by a controlled tuning of the geometry of the nanowire region, in which superconductivity is confined by applied magnetic fields, the conductance curves obtained in these situations are indicative of topological superconductivity and Majorana fermions. The most prominent feature of this behavior is the emergence of a zero bias peak in the conductance curves, superimposed on a background characteristic of the conductance between a normal metal and a superconductor in the Andreev regime. The zero bias peak emerges in some nanowires when a magnetic field larger that the lead bulk critical field is applied. This field drives one of the electrodes into the normal state while the other, the tip, remains superconducting on its apex. Meanwhile a topological superconducting state appears in the connecting nanowire of nanometric size., Comment: Contribution to a New Journal of Physics topical issue

Superconductors with an odd number of bands crossing the Fermi energy have topologically protected Andreev states at interfaces, including Majorana states in one dimensional geometries. Superconductivity, a low number of 1D channels, large spin orbit coupling, and a sizeable Zeeman energy, are present in lead nanowires produced by nanoindentation of a Pb tip on a Pb substrate, in magnetic fields higher than the Pb bulk critical field. A number of such devices have been analyzed. In some of them, the dependence of the critical current on magnetic field, and the Multiple Andreev Reflections observed at finite voltages, are compatible with the existence of topological superconductivity.

Resumen del trabajo presentado al: "International Workshop Science and Technology at High Magnetic Fields" celebrado en Miraflores de la Sierra (Madrid) del 6 al 9 de Noviembre de 2012.-- et al., Under the applied current magnetic vortices move and dissipate the energy driving a superconductor into a resistive state. We demonstrate experimentally and theoretically that confining vortices within the narrow superconducting constrictions with the characteristic dimensions comparable to the size of the vortex core, one can reverse the detrimental effect of the magnetic field and recover superconductivity. We investigate two different exemplary systems, the superconducting wire and the thin superconducting film patterned into a regular array of nanoholes, and observe the drop of the resistance by several orders of magnitude to immeasurably small in a wide range of magnetic fields and temperatures. We show that the mechanism behind the magnetic field-induced superconductivity is the combined action of merging of the densely packed constricted Abrikosov vortices into large immobile hypervortices and the effects of surface superconductivity. We develop a quantitative theory of reentrant superconductivity in superconducting strips in terms of the phase-slip concept and show that the experimentally observed non-monotonic phase slips related magnetoresistance results from the existence of the longitudinal order parameter instability near the transition between the vortex-free and vortex states in a superconducting strip. We propose a quantitative description of the resistance of the wires and films and demonstrate that theoretical results favorably compare with the experiment.

Resumen del trabajo presentado al: "International Workshop Science and Technology at High Magnetic Fields" celebrado en Miraflores de la Sierra (Madrid) del 6 al 9 de Noviembre de 2012., We have developed a method to make scanning tunneling microscopy/spectroscopy imaging at very low temperatures while driving a constant electric current up to some tens of mA through the sample. It gives a new local probe, which we term current driven scanning tunneling microscopy/spectroscopy (CDSTM/S) [1]. Applications include local vortex motion experiments and local density of states studies. In this poster, the possibilities of this technique will be introduced by showing CDSTM/S studies in NbSe2 and W-based amorphous superconducting thin films. Both are the prototype systems to study the electronic structure of superconducting vortex cores and the dynamics of a two dimensional superconducting vortex lattice, respectively. Perspectives of applications will be also presented.

We present measurements of the superconducting gap of URu 2Si 2 made with scanning tunneling microscopy using a superconducting tip of Al. We find tunneling conductance curves with a finite value at the Fermi level. The density of states is V shaped at low energies, and the quasiparticle peaks are located at values close to the expected superconducting gap from weak-coupling BCS theory. Our results point to rather opened gap structures and gap nodes on the Fermi surface, This work was supported by the Spanish MICINN and MEC (Consolider Ingenio Molecular Nanoscience CSD2007-00010 program, FIS2011-23488, ACI-2009-0905, and FPU grant), by the Comunidad de Madrid through program Nanobiomagnet, and by ERC (NewHeavyFermion) and French ANR projects (CORMAT, SINUS, DELICE)

Topological defects such as dislocations and disclinations are predicted to determine the twodimensional (2-D) melting transition. 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, 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., Comment: Submitted to Nature Physics

We present local tunneling spectroscopy experiments in the superconducting and ferromagnetic phases of the reentrant superconductor ErRh$_4$B$_{4}$. The tunneling conductance curves jump from showing normal to superconducting features within a few mK close to the ferromagnetic transition temperature, with a clear hysteretic behavior. Within the ferromagnetic phase, we do not detect any superconducting correlations. Within the superconducting phase we find a peculiar V-shaped density of states at low energies, which is produced by the magnetically modulated phase that coexists with superconductivity just before ferromagnetism sets in., Comment: 5 pages, 4 figures

We report on tunneling spectroscopy measurements using a Scanning Tunneling Microscope (STM) on the spin triplet superconductor Sr2RuO4. We find a negligible density of states close to the Fermi level and a fully opened gap with a value of $\Delta$=0.28 meV, which disappears at T$_c$ = 1.5 K. $\Delta$ is close to the result expected from weak coupling BCS theory ($\Delta_0$=1.76kBT$_c$ = 0.229 meV). Odd parity superconductivity is associated with a fully isotropic gap without nodes over a significant part of the Fermi surface.

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., 9 pages, 5 figures. Discussion extended, references added, one more figure

We present local tunnelling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C. The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states., 9 pages, 3 figures

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$_2$. Relating the behavior of $H_{c2}(T)$ to Fermi surface parameters, we find that the electron phonon coupling of the 2D Nb 4d derived bands shows a peak at 5 GPa when the charge density wave (CDW) order is suppressed. On the other hand, $T_c(P)$ shows a bell shaped curve with a maximum at 10.5 GPa, well above the pressure for the suppression of the CDW order. Changes in the band structure produce this shift in the maximum of $T_c(P)$, demonstrating that 2H-NbSe$_2$ shows important differences with respect to other compounds where $T_c$ has a maximum in the temperature-density phase diagram shaped by the suppression of another, non-superconducting, ground state., Comment: 5 pages, 4 figures. Small changes in discussion. Typos corrected

The properties of nanoscopic superconducting structures fabricated with a scanning tunnelling microscope are reviewed, with emphasis on the effects of high magnetic fields. These systems include the smallest superconducting junctions which can be fabricated, and they are a unique laboratory where to study superconductivity under extreme conditions. The review covers a variety of recent experimental results on these systems, highlighting their unusual transport properties, and theoretical models developed for their understanding., Comment: Topical Review, 35 pages, 18 figures (with reduced quality to decrease file size)

We present measurements of H$_{c2}(T)$ under pressure in MgB$_2$ and in YNi$_2$B$_2$C. The changes in the shape of H$_{c2}(T)$ are interpreted within current models and show the evolution of the main Fermi surface velocities $v_F$ and electron-phonon coupling parameters $\lambda$ with pressure. In MgB$_2$ the electron-phonon coupling strength of the nearly two dimensional $\sigma$ band, responsible for the high critical temperature, is more affected by pressure than the $\pi$ band coupling, and the hole doping of the $\sigma$ band decreases. In YNi$_2$B$_2$C, the peculiar positive curvature of H$_{c2}(T)$ is weakened by pressure., Comment: 5 pages, 5 figures

We present scanning tunnelling spectroscopy measurements on the heavy fermion superconductor PrOs$_4$Sb$_{12}$. Our results show that the superconducting gap opens over a large part of the Fermi surface. The deviations from isotropic BCS s-wave behavior are discussed in terms of a finite distribution of values of the superconducting gap., Comment: 4 pages, 4 figures

We present new measurements about the tunneling conductance in the borocarbide superconductor TmNi$_2$B$_2$C. The results show a very good agreement with weak coupling BCS theory, without any lifetime broadening parameter, over the whole sample surface. We detect no particular change of the tunneling spectroscopy below 1.5K, when both the antiferromagnetic (AF) phase and the superconducting order coexist., Comment: Submitted to Phys. Rev. B, Rapid Communications

We report on the measurement of multiple Andreev resonances at atomic size point contacts between two superconducting nanostructures of Pb under magnetic fields higher than the bulk critical field, where superconductivity is restricted to a mesoscopic region near the contact. The small number of conduction channels in this type of contacts permits a quantitative comparison with theory through the whole field range. We discuss in detail the physical properties of our structure, in which the normal bulk electrodes induce a proximity effect into the mesoscopic superconducting part., Comment: 4 pages