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Biomedical subjects

Silvano De Franceschi

Publications and source records attributed to Silvano De Franceschi.

6 recordsLinked to original sources
PubMed2006

Supercurrent reversal in quantum dots.

When two superconductors are electrically connected by a weak link--such as a tunnel barrier--a zero-resistance supercurrent can flow. This supercurrent is carried by Cooper pairs of electrons with a combined charge of twice the elementary charge, e. The 2e charge quantum is clearly visible in the height of voltage steps in Josephson junctions under microwave irradiation, and in the magnetic flux periodicity of h/2e (where h is Planck's constant) in superconducting quantum interference devices. Here we study supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Owing to strong Coulomb interaction, electrons only tunnel one-by-one through the discrete energy levels of the quantum dot. This nevertheless can yield a supercurrent when subsequent tunnel events are coherent. These quantum coherent tunnelling processes can result in either a positive or a negative supercurrent, that is, in a normal or a pi-junction, respectively. We demonstrate that the supercurrent reverses sign by adding a single electron spin to the quantum dot. When excited states of the quantum dot are involved in transport, the supercurrent sign also depends on the character of the orbital wavefunctions.

Journal Article↗
PubMed2006

Manganese-induced growth of GaAs nanowires.

GaAs nanowires have been grown on SiO2 and GaAs by molecular beam epitaxy using manganese as growth catalyst. Transmission electron microscopy shows that the wires have a wurtzite-type lattice and that alpha-Mn particles are found at the free end of the wires. X-ray absorption fine structure measurements reveal the presence of a significant fraction of Mn-As bonds, suggesting Mn diffusion and incorporation during wire growth. Transport measurements indicate that the wires are p-type, as expected from doping of GaAs with Mn.

Arsenicals↗
PubMed2005

Tunable supercurrent through semiconductor nanowires.

Nanoscale superconductor/semiconductor hybrid devices are assembled from indium arsenide semiconductor nanowires individually contacted by aluminum-based superconductor electrodes. Below 1 kelvin, the high transparency of the contacts gives rise to proximity-induced superconductivity. The nanowires form superconducting weak links operating as mesoscopic Josephson junctions with electrically tunable coupling. The supercurrent can be switched on/off by a gate voltage acting on the electron density in the nanowire. A variation in gate voltage induces universal fluctuations in the normal-state conductance, which are clearly correlated to critical current fluctuations. The alternating-current Josephson effect gives rise to Shapiro steps in the voltage-current characteristic under microwave irradiation.

Journal Article↗
PubMed2005

Orbital Kondo effect in carbon nanotubes.

Progress in the fabrication of nanometre-scale electronic devices is opening new opportunities to uncover deeper aspects of the Kondo effect--a characteristic phenomenon in the physics of strongly correlated electrons. Artificial single-impurity Kondo systems have been realized in various nanostructures, including semiconductor quantum dots, carbon nanotubes and individual molecules. The Kondo effect is usually regarded as a spin-related phenomenon, namely the coherent exchange of the spin between a localized state and a Fermi sea of delocalized electrons. In principle, however, the role of the spin could be replaced by other degrees of freedom, such as an orbital quantum number. Here we show that the unique electronic structure of carbon nanotubes enables the observation of a purely orbital Kondo effect. We use a magnetic field to tune spin-polarized states into orbital degeneracy and conclude that the orbital quantum number is conserved during tunnelling. When orbital and spin degeneracies are present simultaneously, we observe a strongly enhanced Kondo effect, with a multiple splitting of the Kondo resonance at finite field and predicted to obey a so-called SU4 symmetry.

Journal Article↗
PubMed2004

Epitaxial growth of InP nanowires on germanium.

The growth of III-V semiconductors on silicon would allow the integration of their superior (opto-)electronic properties with silicon technology. But fundamental issues such as lattice and thermal expansion mismatch and the formation of antiphase domains have prevented the epitaxial integration of III-V with group IV semiconductors. Here we demonstrate the principle of epitaxial growth of III-V nanowires on a group IV substrate. We have grown InP nanowires on germanium substrates by a vapour-liquid-solid method. Although the crystal lattice mismatch is large (3.7%), the as-grown wires are monocrystalline and virtually free of dislocations. X-ray diffraction unambiguously demonstrates the heteroepitaxial growth of the nanowires. In addition, we show that a low-resistance electrical contact can be obtained between the wires and the substrate.

Germanium↗