Hatem Mehrez

Carbon Nanotube Based Magnetic Tunnel Junctions

Spin-coherent quantum transport in carbon nanotube magnetic tunnel junctions is investigated theoretically. A spin-valve effect is found for metallic, armchair tubes, with a magnetoconductance ratio ranging up to 20%. Because of the finite length of the nanotube junctions, transport is dominated by resonant transmission. The magnetic tunnel junctions are found to have distinctly different transport behavior depending on whether or not the length of the tubes is commensurate with a 3N+1 rule, with N the number of basic carbon repeat units along the nanotube length.

I-V characteristics and differential conductance fluctuations of Au nanowires

Electronic transport properties of the Au nanostructure are investigated using both experimental and theoretical analysis. Experimentally, stable Au nanowires were created using a mechanically controllable break junction in air, and simultaneous current-voltage (I-V) and differential conductance δI/δV data were measured. The atomic device scale structures are mechanically very stable up to bias voltage V b ∼0.6 V and have a lifetime of a few minutes. Facilitated by a shape function data analysis technique which finger prints electronic properties of the atomic device, our data show clearly differential conductance fluctuations with an amplitude > 1 % at room temperature and a nonlinear I-V characteristics. To understand the transport features of these atomic scale conductors, we carried out ah initio calculations on various Au atomic wires. The theoretical results demonstrate that transport properties of these systems crucially depend on the electronic properties of the scattering region, the leads, and most importantly the interaction of the scattering region with the leads. For ideal, clean Au contacts, the theoretical results indicate a linear I-V behavior for bias voltage V b <0.5 V. When sulfur impurities exist at the contact junction, nonlinear I-V curves emerge due to a tunneling barrier established in the presence of the S atom. The most striking observation is that even a single S atom can cause a qualitative change of the I-V curve from linear to nonlinear. A quantitatively favorable comparison between experimental data and theoretical results is obtained. We also report other results concerning quantum transport through Au atomic contacts.

Resonant Andreev reflections in superconductor–carbon-nanotube devices

Resonant Andreev reflection through superconductor-carbon-nanotube devices was investigated theoretically with a focus on the superconducting proximity effect. Consistent with a recent experiment, we find that for high transparency devices on-resonance, the Andreev current is characterized by a large value and a resistance dip; low-transparency off-resonance devices give the opposite result. We also give evidence that the observed low-temperature transport anomaly may be a natural result of Andreev reflection process.

THEORETICAL INVESTIGATIONS OF QUANTUM TRANSPORT THROUGH CARBON NANOTUBE DEVICES

By combining a nonequilibrium Greens function analysis with a standard tight-binding model, we have investigated quantum transport through carbon nanotube devices. For finite-sized nanotubes, transport is dominated by resonant tunneling, with the conductance being strongly dependent on the length of the nanotubes. Turning to nanotube devices, we have investigated spin-coherent transport in ferromagnetic–nanotube–ferromagnetic devices and nanotube-superconducting devices. The former shows a significant spin valve effect, while the latter is dominated by resonant Andreev reflections. In addition, we discuss AC transport through carbon nanotubes and the role of photon-assisted tunneling.