Sebastian Vieira

Mechanical Properties and Formation Mechanisms of a Wire of Single Gold Atoms

A scanning tunneling microscope supplemented with a force sensor is used to study the mechanical properties of a novel metallic nanostructure: a freely suspended chain of single gold atoms. We find that the bond strength of the nanowire is about twice that of a bulk metallic bond. We perform ab initio calculations of the force at chain fracture and compare quantitatively with experimental measurements. The observed mechanical failure and nanoelastic processes involved during atomic wire fabrication are investigated using molecular dynamics simulations, and we find that the total effective stiffness of the nanostructure is strongly affected by the detailed local atomic arrangement at the chain bases.

Onset of energy dissipation in ballistic atomic wires.

Electronic transport at finite voltages in free-standing gold atomic chains of up to seven atoms in length is studied at low temperatures using a scanning tunneling microscope. The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is nondissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain.

Tunneling Spectroscopy in Small Grains of Superconducting MgB 2

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.

Superconducting Density of States and Vortex Cores of 2H-NbS2

Scanning tunneling microscopy and spectroscopy measurements in the superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density of states with two well-defined features at 0.97 and 0.53 meV, located, respectively, above and below the value for the superconducting gap expected from the single band s-wave BCS model (Delta=1.76k_(B)T_(c)=0.9 meV). Both features have a continuous temperature evolution and disappear at T_(c)=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, absent in 2H-NbS2.

Electron transport and phonons in atomic wires

Abstract A scanning tunnelling microscope (STM) at low temperatures is used to fabricate and measure the electronic transport properties of atomic wires up to 7 atoms in length. We observe that the conductance of these wires, which at zero bias voltage is 2 e 2 / h , as corresponds to ballistic electron transport through a single conductance quantum channel, drops sharply at a well-defined voltage due to the inelastic scattering of electrons with phonons. This behaviour is characteristic of one-dimensional systems, where an electron can interact only with a phonon of a well-defined wavenumber. We find that the frequency of this phonon and the magnitude of the interaction depend strongly on the state of strain of the wire. We also explore the energy exchange mechanism between electrons and phonons in the wire itself which causes heating in these nanostructures.

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

This work was supported by Spanish MINECO (Consolider-Ingenio CSD2007-00010 and CSD2009-00013 programs, SAB2009-0057, FIS2011-23488, and MAT2011-27470-C02-02), by the Comunidad de Madrid through program Nanobiomagnet, and by NanoSc-COST program

Scanning tunneling microscopy and spectroscopy at very low temperatures

Abstract We discuss recent results and advances in very low temperature scanning tunneling microscopy in superconductors, focusing on the experimental developments that have permitted to extend the present applications of this technique to new materials. We detail the experimental apparatus used and discuss the results and the future possibilities of S–I–S tunneling using a superconducting tip and sample. Further on, we present results on the superconductivity of single grains of MgB 2 and compare them with other works. We also discuss recent data on the borocarbide material TmNi 2 B 2 C.

Andreev scattering in nanoscopic junctions in a magnetic field

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.

Chiral charge order in the superconductor 2H-TaS2

We observed chiral charge order in the superconductor 2H-TaS2 using scanning tunneling microscopy and spectroscopy at 0.1 K. Topographic images show hexagonal atomic lattice and charge density wave (CDW) with clockwise and counterclockwise charge modulations. Tunneling spectroscopy reveals the superconducting density of states, disappearing at Tc = 1.75 K and showing a wide distribution of values of the superconducting gap, centered around Δ = 0.28 meV.

Anisotropy of the upper critical field near Tc and the properties of URu2Si2 and UBe13 in the normal state

Simultaneous measurements of the electrical resistivity ρ Seebeck coefficientS, and thermal conductivity k of URu2Si2 and UBe13 are carried out. For URu2Si2 a strong anomaly in the k(T) dependence near the temperature of the antiferromagnetic transition was observed. The thermal conductivity of UBe13 exhibits only a small (about 30%) change of the derivativedk/dT near 8–9 K, which may be related to an increase of the characteristic length of antiferromagnetic correlations below this temperature. Studying STM spectra in the normal and magnetic phase of a URu2Si2 single crystal we observed that the magnetic gap, partially opened at the Fermi surface below the Néel temperature,TN=17.5 K, is strongly anisotropic: gapped states mainly correspond to the tetragonalab-plane. Anisotropy of the upper critical field nearTc(H<4kOe), studied for URu2Si2 and UBe13 single crystals, was found to be in a good correspondence with the character of the antiferromagnetic transition.