S. Vieira

Universidad Autónoma de Madrid, Madrid, Madrid, Spain

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Publications (163)398.98 Total impact

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    ABSTRACT: Scanning tunneling microscopy (STM) has been by now made in many pnictide layered superconductors. In AFe2As2 (A=Ca, Sr, Ba), surface reconstructions dominate the STM topographic images and no surface termination with the bulk atomic lattice has been reported. Here, we present STM measurements on the surface of Ca(Fe0.965Co0.035)2As2 cleaved in-situ below liquid helium temperatures. We observe the ubiquitous surface reconstruction consisting of 1D stripes that can adopt two degenerate orientations with respect to the crystal lattice. By searching over many different scanning windows, we find that low temperature cleaving also exposes surfaces showing the As unreconstructed lattice of the bulk tetragonal crystal, over areas of some tens of nm lateral size.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
  • I Guillamon · J G Rodrigo · S Vieira · H Suderow
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    ABSTRACT: During past decades, an increasing number of laboratories is using cryogenic scanning tunneling microscopy and spectroscopy (STM/S) to probe different kinds of electronic systems. Measurements in a dilution refrigerator are particularly useful to study superconductors, because temperatures of order of 100 mK are well below most critical temperatures and effectively reduce thermally excited quasiparticles. The local electronic density of states is then obtained at atomic level with a resolution in energy of some tens of μeV. Visualizing spatial variations of the local density of states allows characterizing vortex cores and the vortex lattice. Vortex core electronic features provide the anisotropy of the superconducting properties, and help understanding the influence of competing orders such as charge density waves. Here we will review results in dichalcogenide superconductors, in the magnetic borocarbide TmNi2B2C and in thin films, discussing in some detail a few relevant aspects of thermal depinning and melting in thin films.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
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    ABSTRACT: We report in this paper the atomic level imaging in the polycrystalline film of titanium nitride. The crystalline boundaries are resolved at several places on the film surface, and the atomic positions are clearly seen with different orientations. We also observed presence of electronic charge density modulations, with their wave vectors varying over a large range, and in several directions. We associate the presence of charge density modulations to the disorder scattering of electronic waves at the low angle crystalline boundaries. We further consider the effect of Charge density modulations on the superconducting phase. In the STS measurement, we find that the conductance fluctuations at energies close to the superconducting gap are strongly influenced by the charge modulation patterns. At quasi-particle peak position and near zero bias, the conductance fluctuations are relatively suppressed.
    Full-text · Article · Feb 2014
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    ABSTRACT: 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.
    Full-text · Article · Feb 2014
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    ABSTRACT: 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.
    Full-text · Article · Jan 2014
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    ABSTRACT: We present resistivity, magnetization and specific heat measurements on flux grown single crystals of NiBi3. We find typical behavior of a type-II superconductor, with, however, a sizable magnetic signal in the superconducting phase. There is a hysteretic magnetization characteristic of a ferromagnetic compound. By following the magnetization as a function of temperature, we find a drop at temperatures corresponding to the Curie temperature of ferromagnetic amorphous Ni. Thus, we assign the magnetism in NiBi$_3$ crystals to amorphous Ni impurities.
    Full-text · Article · Oct 2013 · Physical Review B
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    ABSTRACT: We present resistivity, specific heat and magnetization measurements in high quality single crystals of HoBi, with a residual resistivity ratio of 126. We find, from the temperature and field dependence of the magnetization, an antiferromagnetic transition at 5.7 K, which evolves, under magnetic fields, into a series of up to five metamagnetic phases.
    Full-text · Article · Sep 2013 · Solid State Communications
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    A. Maldonado · S. Vieira · H. Suderow
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    ABSTRACT: 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.
    Full-text · Article · Sep 2013 · Physical Review B
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    ABSTRACT: We present very low temperature (0.15 K) scanning tunneling microscopy and spectroscopy experiments in the layered superconductor LaSb$_2$. We obtain topographic microscopy images with surfaces showing hexagonal and square atomic size patterns, and observe in the tunneling conductance a superconducting gap. We find well defined quasiparticle peaks located at a bias voltage comparable to the weak coupling s-wave BCS expected gap value (0.17 meV). The amount of states at the Fermi level is however large and the curves are significantly broadened. We find T$_c$ of 1.2 K by following the tunneling conductance with temperature.
    Full-text · Article · Jun 2013 · Physical review. B, Condensed matter
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    J G Rodrigo · V Crespo · H Suderow · S Vieira · F Guinea
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    ABSTRACT: We report on several low-temperature experiments supporting the presence of Majorana fermions in superconducting lead nanowires fabricated with a scanning tunneling microscope (STM). These nanowires are the connecting bridges between the STM tip and the sample resulting from indentation–retraction processes. We show here that by a controlled tuning 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 than 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.
    Full-text · Article · May 2013 · New Journal of Physics
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    ABSTRACT: We present measurements of the superconducting critical temperature Tc and upper critical field Hc2 as a function of pressure in the transition metal dichalcogenide 2H-NbS2 up to 20 GPa. We observe that Tc increases smoothly from 6K at ambient pressure to about 8.9K at 20GPa. This range of increase is comparable to the one found previously in 2H-NbSe2. The temperature dependence of the upper critical field Hc2(T) of 2H-NbS2 varies considerably when increasing the pressure. At low pressures, Hc2(0) decreases, and at higher pressures both Tc and Hc2(0) increase simultaneously. This points out that there are pressure induced changes of the Fermi surface, which we analyze in terms of a simplified two band approach.
    No preview · Article · Apr 2013 · Physical review. B, Condensed matter
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    ABSTRACT: Heavy fermions offer a rich physical phenomenology at very low temperatures, exhibiting different phase transitions on cooling that determine their electronic properties. Their ground states cover many electronic interactions, such as Kondo effect, superconducting or long range magnetic ones and, eventually, their coexistence. Thus, exploring the local electronic properties of these systems using scanning tunneling microscopy/spectroscopy (STM/S) at different temperatures is essential. In this communication, tunneling spectroscopy measurements using a superconducting tip of Al in the superconducting phase of URu2Si2footnotetextA. Maldonado et al., Phys. Rev. B 85, 214512 (2012) and using one of Au in the paramagnetic and antiferromagnetic phases of, respectively, CeRu2Si2 and CeRh2Si2footnotetextA. Maldonado et al., Accepted in J. Phys.: Condens. Matter will be discussed. The features found in the tunneling spectroscopy of each compound at 0.15K and their respective thermal evolution reflect the formation of different electronic ground states.
    No preview · Article · Mar 2013
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    ABSTRACT: We report a characterization of surfaces of the dichalcogenide TaSe2 using scanning tunneling microscopy and spectroscopy at 150 mK. When the top layer has the 2H structure and the layer immediately below the 1T structure, we find a singular spatial dependence of the tunneling conductance below 1 K, changing from a zero-bias peak on top of Se atoms to a gap in between Se atoms. The zero-bias peak is additionally modulated by the commensurate 3a(0) x 3a(0) charge-density wave of 2H-TaSe2. Multilayers of 2H-TaSe2 show a spatially homogeneous superconducting gap with a critical temperature also of 1 K. We discuss possible origins for the peculiar tunneling conductance in single layers. DOI: 10.1103/PhysRevB.87.094502
    Full-text · Article · Mar 2013 · Physical Review B
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    ABSTRACT: 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.
    Full-text · Article · Feb 2013 · Nature Communications
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    ABSTRACT: 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.
    Full-text · Article · Feb 2013
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    ABSTRACT: Using a scanning tunnelling microscope or mechanically controllable break junction it has been shown that it is possible to control the formation of a wire made of single gold atoms. In these experiments an interatomic distance between atoms in the chain of ∼ 3.6 ˚A was reported which is not consistent with recent theoretical calculations. Here, using precise calibration procedures for both techniques, we measure the length of the atomic chains. Based on the distance between the peaks observed in the chain length histogram we find the mean value of the interatomic distance before chain rupture to be 2.5 ± 0.2 ˚A. This value agrees with the theoretical calculations for the bond length. The discrepancy with the previous experimental measurements was due to the presence of He gas, that was used to promote the thermal contact, and which affects the value of the work function that is commonly used to calibrate distances in scanning tunnelling microscopy and mechanically controllable break junctions at low temperatures.
    Full-text · Dataset · Dec 2012
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    J G Rodrigo · V Crespo · H Suderow · S Vieira · F Guinea
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    ABSTRACT: 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. We propose here that repeated indentation of a Pb tip on a Pb substrate can lead to nanowires such that the resulting superconducting system has novel topological properties. We have analyzed a number of conductance curves obtained in different nanowires, and observe, in a few cases, very peculiar dependence of the critical current on magnetic field. In these cases, the form of multiple Andreev reflections observed at finite voltages are compatible with topological superconductivity. The nanowires give a low number of 1D channels, large spin orbit coupling, and a sizable Zeeman energy, provided that the applied magnetic field is higher than the Pb bulk critical field.
    Full-text · Article · Dec 2012 · Physical Review Letters
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    A. Maldonado · H. Suderow · S. Vieira · D. Aoki · J. Flouquet
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    ABSTRACT: CeRu2Si2 and CeRh2Si2 are two similar heavy fermion stoichiometric compounds located on the two sides of the magnetic quantum critical phase transition. CeRh2Si2 is an antiferromagnet below T_N=36 K with moderate electronic masses whereas CeRu2Si2 is a paramagnetic metal with particularly heavy electrons. Here we present tunneling spectroscopy measurements as a function of temperature (from 0.15 K to 45 K). The tunneling conductance at 0.15 K reveals V-shaped dips around the Fermi level in both compounds, which disappear in CeRu2Si2 above the coherence temperature, and above the N\'eel temperature in CeRh2Si2. In the latter case, two different kinds of V-shaped tunneling conductance dips are found.
    Full-text · Article · Oct 2012 · Journal of Physics Condensed Matter
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    ABSTRACT: We present measurements of the superconducting upper critical field Hc2(T) and the magnetic phase diagram of the superconductor ErNi2B2C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi2B2C, extending between different magnetic transitions. A close correlation between magnetic transitions and Hc2(T) is found, showing that superconductivity is strongly linked to magnetism.
    Full-text · Article · Oct 2012 · Solid State Communications
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    Full-text · Article · Oct 2012

Publication Stats

4k Citations
398.98 Total Impact Points

Institutions

  • 1037-2013
    • Universidad Autónoma de Madrid
      • • Department of Condensed Matter Physics
      • • Facultad de Ciencias
      • • Department of Applied Physics
      Madrid, Madrid, Spain
  • 2002-2006
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
    • Leiden University
      • Kamerlingh Onnes Laboratory
      Leyden, South Holland, Netherlands
  • 1996-1997
    • University Carlos III de Madrid
      Getafe, Madrid, Spain
  • 1989
    • Michigan State University
      • Department of Physics and Astronomy
      East Lansing, MI, United States