arXiv subjects

S. Vieira

Publications and source records attributed to S. Vieira.

59 records · Page 4Linked to original sources

arXiv2000

Nonequilibrium effects in superconducting necks of nanoscopic dimensions

We have fabricated superconducting connecting necks of Pb with a scanning tunneling microscope (STM) and studied their properties under magnetic fields near the transition to the resistive state. A striking phenomenology is found with two well defined conduction regimes as a function of the magnetic field. We discuss the possible origin of this behavior in terms of the interplay between the field dependence of the quasiparticle charge imbalance length $Λ_{Q*}$ and the length of the neck which is superconducting under field.

cond-mat.mes-hall↗

arXiv1999

Andreev scattering in nanoscopic junctions at high magnetic fields

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.

cond-mat.mes-hall↗

arXiv1998

Nanosized superconducting constrictions

Nanowires of lead between macroscopic electrodes are produced by means of an STM. Magnetic fields may destroy the superconductivity in the electrodes, while the wire remains in the superconducting state. The properties of the resulting microscopic Josephson junctions are investigated.

cond-mat.mes-hall↗

arXiv1997

Fabrication and Characterization of Metallic Nanowires

The shape of metallic constrictions of nanoscopic dimensions (necks) formed using a scanning tunneling microscope (STM) is shown to depend on the fabrication procedure. Submitting the neck to repeated plastic deformation cycles makes possible to obtain long necks or nanowires. Point-contact spectroscopy results show that these long necks are quite crystalline, indicating that the repeated cycles of plastic deformation act as a "mechanical annealing" of the neck.

cond-mat.mes-hall↗

arXiv1996

Point-contact spectroscopy on URu$_2$Si$_2$

Tunnel and point contact experiments have been made in a URu$_2$Si$_2$ single crystal along the c-axis. The experiments were performed changing temperature and contact size in a low temperature scanning tunneling microscope. A resonance develops at the Fermi level at $T\sim 60$ K. This resonance splits and becomes asymmetric when the 17.5 K phase transition is crossed. These results are consistent with the existence of Kondo like bound states of the U$^{4+}$ ionic configurations and the conduction electrons. Below the transition, these configurations are split by the development of quadrupolar ordering. The peak separation can be interpreted as a direct measurement of the order parameter. Measurements on a policrystalline UAu_2Si_2$ sample are also reported, with a comparative study of the behavior of both materials.

cond-mat↗