arXiv subjects

S. Vieira

Publications and source records attributed to S. Vieira.

At least 19 recordsLinked to original sources

arXiv2017

Tilted vortex cores and superconducting gap anisotropy in 2H-NbSe2

Superconducting vortex cores have been extensively studied for magnetic fields applied perpendicular to the surface by mapping the density of states (DOS) through Scanning Tunneling Microscopy (STM). Vortex core shapes are often linked to the superconducting gap anisotropy---quasiparticle states inside vortex cores extend along directions where the superconducting gap is smallest. The superconductor 2H-NbSe$_2$ crystallizes in a hexagonal structure and vortices give DOS maps with a sixfold star shape for magnetic fields perpendicular to the surface and the hexagonal plane. This has been associated to a hexagonal gap anisotropy located on quasi two-dimensional Fermi surface tubes oriented along the $c$ axis. The gap anisotropy in another, three-dimensional, pocket is unknown. However, the latter dominates the STM tunneling conductance. Here we measure DOS in magnetic fields parallel to the surface and perpendicular to the $c$ axis. We find patterns of stripes due to in-plane vortex cores running nearly parallel to the surface. The patterns change with the in-plane direction of the magnetic field, suggesting that the sixfold gap anisotropy is present over the whole Fermi surface. Due to a slight misalignment between the vector of the magnetic field and the surface, our images also show outgoing vortices. Their shape is successfully compared to detailed calculations of vortex cores in tilted fields. Their features merge with the patterns due to in plane vortices, suggesting that they exit at an angle with the surface. Measuring the DOS of vortex cores in highly tilted magnetic fields with STM can thus be used to study the superconducting gap structure.

cond-mat.supr-con↗

arXiv2017

Subsurface bending and reorientation of tilted vortex lattices in the bulk due to Coulomb-like repulsion at the surface

We study vortex lattices (VLs) in superconducting weak-pinning platelet-like crystals of $β$-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.

cond-mat.supr-con↗

arXiv2015

Disorder driven inhomogeneous phase in the 2D-superconducting film of titanium nitride

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.

cond-mat.supr-con↗

arXiv2015

Stellar parameters for stars of the CoRoT exoplanet field

Aims:To support the computation and evolutionary interpretation of periods associated with the rotational modulation, oscillations, and variability of stars located in the CoRoT fields, we are conducting a spectroscopic survey for stars located in the fields already observed by the satellite. These observations allow us to compute physical and chemical parameters for our stellar sample. Method: Using spectroscopic observations obtained with UVES/VLT and Hydra/Blanco, and based on standard analysis techniques, we computed physical and chemical parameters ($T_{\rm{eff}}$, $\log \,(g)$, $\rm{[Fe/H]}$, $v_{\rm{mic}}$, $v_{\rm{rad}}$, $v \sin \,(i)$, and $A(\rm{Li})$) for a large sample of CoRoT targets. Results: We provide physical and chemical parameters for a sample comprised of 138 CoRoT targets. Our analysis shows the stars in our sample are located in different evolutionary stages, ranging from the main sequence to the red giant branch, and range in spectral type from F to K. The physical and chemical properties for the stellar sample are in agreement with typical values reported for FGK stars. However, we report three stars presenting abnormal lithium behavior in the CoRoT fields. These parameters allow us to properly characterize the intrinsic properties of the stars in these fields. Our results reveal important differences in the distributions of metallicity, $T_{\rm eff}$, and evolutionary status for stars belonging to different CoRoT fields, in agreement with results obtained independently from ground-based photometric surveys. Conclusions: Our spectroscopic catalog, by providing much-needed spectroscopic information for a large sample of CoRoT targets, will be of key importance for the successful accomplishment of several different programs related to the CoRoT mission, thus it will help further boost the scientific return associated with this space mission.

astro-ph.SR↗

arXiv2015

Magnetic field dependence of the density of states in the multiband superconductor $β$-Bi$_2$Pd

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $β$-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 $β$-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.

cond-mat.supr-con↗

arXiv2014

Enhancement of long range correlations in a 2D vortex lattice by incommensurate 1D disorder potential

Long range correlations in two-dimensional (2D) systems are significantly altered by disorder potentials. Theory has predicted the existence of disorder induced phenomena such as Anderson localization and the emergence of novel glass and insulating phases as the Bose glass. More recently, it has been shown that disorder breaking the 2D continuous symmetry, such as a one dimensional (1D) modulation, can enhance long range correlations. Experimentally, developments in quantum gases have allowed the observation of a wealth of phenomena induced by the competition between interaction and disorder. However, there are no experiments exploring the effect of symmetry-breaking disorder. Here, we create a 2D vortex lattice at 0.1 K in a superconducting thin film with a well-defined 1D thickness modulation and track the field induced modification using scanning tunneling microscopy. We find that the 1D modulation becomes incommensurate to the vortex lattice and drives an order-disorder transition, behaving as a scale-invariant disorder potential. We show that the transition occurs in two steps and is mediated by the proliferation of topological defects. We find that critical exponents determining the loss of positional and orientational order are far above theoretical expectations for scale-invariant disorder and follow instead the critical behaviour which describes dislocation unbinding melting. Our data show for the first time that randomness disorders a 2D crystal, and evidence enhanced long range correlations in presence of a 1D modulation demonstrating the transformation induced by symmetry breaking disorder in interactions and the critical behaviour of the transition.

cond-mat.supr-con↗

arXiv2014

Zero-bias conductance peak in detached flakes of superconducting 2H-TaS$_2$ probed by scanning tunneling spectroscopy

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 $Γ$ 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$_2$.

cond-mat.supr-con↗

arXiv2014

Imaging superconducting vortex core and lattice with the scanning tunneling microscope

The observation of vortices in superconductors was a major breakthrough in developing the conceptual background for superconducting applications. Each vortex carries a flux quantum, and the magnetic field radially decreases from the center. Techniques used to make magnetic field maps, such as magnetic decoration, give vortex lattice images in a variety of systems. However, strong type II superconductors allow penetration of the magnetic field over large distances, of order of the magnetic penetration depth λ. Superconductivity survives up to magnetic fields where, for imaging purposes, there is nearly no magnetic contrast. Static and dynamic properties of vortices are largely unknown at such high magnetic fields. Reciprocal space studies using neutron scattering give insight into the collective behavior. But the microscopic details of vortex arrangements and their motion remain difficult to obtain. Direct real space visualization can be made using scanning tunneling microscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the electronic density of states describes spatial variations of the quasiparticle and pair wavefunction properties. These are of order of the superconducting coherence length ξ, which is much smaller than λ. In principle, individual vortices can be imaged using STM up to the upper critical field where vortex cores, of size ξ, overlap. In this review, we describe recent advances in vortex imaging made with scanning tunneling microscopy and spectroscopy. We introduce the technique and discuss vortex images which reveal the influence of the Fermi surface distribution of the superconducting gap on the internal structure of vortices, the collective behavior of the lattice in different materials and conditions, and the observation of vortex lattice melting.

cond-mat.supr-con↗

arXiv2014

Conductance behavior with temperature and magnetic field in the disordered films of titanium nitride

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.

cond-mat.supr-con↗

arXiv2014

Local electronic properties in the superconducting and the normal phase in the disordered film of titanium nitride

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.

cond-mat.supr-con↗

arXiv2013

Superconductivity and magnetism on flux grown single crystals of NiBi3

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.

cond-mat.supr-con↗

arXiv2013

Low temperature magnetic transitions of single crystal HoBi

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.

cond-mat.mtrl-sci↗

arXiv2013

Supercurrent on a vortex core in 2H-NbSe$_2$: current driven scanning tunneling spectroscopy

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.

cond-mat.supr-con↗

arXiv2013

Scanning Tunneling Microscopy in the superconductor LaSb2

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.

cond-mat.supr-con↗

arXiv2013

Overview of semi-sinusoidal stellar variability with the CoRoT satellite

To date, the CoRoT space mission has produced more than 124,471 light curves. Classifying these curves in terms of unambiguous variability behavior is mandatory for obtaining an unbiased statistical view on their controlling root-causes. The present study provides an overview of semi-sinusoidal light curves observed by the CoRoT exo-field CCDs. We selected a sample of 4,206 light curves presenting well-defined semi-sinusoidal signatures. The variability periods were computed based on Lomb-Scargle periodograms, harmonic fits, and visual inspection. Color-period diagrams for the present sample show the trend of an increase of the variability periods as long as the stars evolve. This evolutionary behavior is also noticed when comparing the period distribution in the Galactic center and anti-center directions. These aspects indicate a compatibility with stellar rotation, although more information is needed to confirm their root-causes. Considering this possibility, we identified a subset of three Sun-like candidates by their photometric period. Finally, the variability period versus color diagram behavior was found to be highly dependent on the reddening correction.

astro-ph.SR↗

arXiv2013

Pressure dependence of superconducting critical temperature and upper critical field of 2H-NbS2

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.

cond-mat.supr-con↗

arXiv2013

Scanning tunneling spectroscopy of layers of superconducting 2H-TaSe$_\textbf{2}$: Evidence for a zero bias anomaly in single layers

We report a characterization of surfaces of the dichalcogenide TaSe$_2$ using scanning tunneling microscopy and spectroscopy (STM/S) 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 $3a_0 \times 3a_0$ charge density wave of 2H-TaSe$_2$. Multilayers of 2H-TaSe$_2$ 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.

cond-mat.supr-con↗

arXiv2013

Majorana Fermions in Topological Superconducting Metallic Nanowires probed by a Superconducting Condensate

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.

cond-mat.supr-con↗