We investigate the basic properties of the thermopower (Seebeck coefficient) of phase-coherent conductors under the influence of dephasing and inelastic processes. Transport across the system is caused by a voltage bias or a thermal gradient applied between two terminals. Inelastic scattering is modeled with the aid of an additional probe acting as an ideal potentiometer and thermometer. We find that inelastic scattering reduces the conductor's thermopower and, more unexpectedly, generates a magnetic field asymmetry in the Seebeck coefficient. The latter effect is shown to be a higher-order effect in the Sommerfeld expansion. We discuss our result by using two illustrative examples. First, we consider a generic mesoscopic system described within random matrix theory and demonstrate that thermopower fluctuations disappear quickly as the number of probe modes increases. Second, the asymmetry is explicitly calculated in the quantum limit of a ballistic microjunction. We find that asymmetric scattering strongly enhances the effect and discuss its dependence on temperature and Fermi energy.

We predict the occurrence of Fano line shapes in a semiconductor quantum wire with local spin-orbit (Rashba) coupling. We show that the Rashba interaction acts in a strictly one-dimensional channel as an attractive impurity, leading to the formation of purely bound states. In a quasi-one-dimensional system these bound states couple to the conduction ones through Rashba intersubband mixing, giving rise to pronounced dips in the linear conductance plateaus. We give exact numerical results and propose an approximate model capturing the main ingredients of the effect.

We discuss the thermoelectrical properties of nanowires hosting Majorana states. For a Majorana nanowire directly coupled to two normal reservoirs the thermopower always vanishes regardless of the value of the Majorana overlap parameter ${\varepsilon }_M$, due to the particle-hole symmetric nature of the Majorana states. This situation changes drastically if a quantum dot (QD) is inserted to break the particle-hole symmetry. Then, the Majorana-side-coupled QD system exhibits a different behavior for the thermopower depending on the value of ${\varepsilon }_M$. The thermopower globally reverses its sign when the half-fermionic nature of the Majorana state is relevant, i.e., for ${\varepsilon }_M\lesssim k_B{T}_b$, where $T_b$ is the background temperature. For large overlap, ${\varepsilon }_M\gtrsim k_B{T}_b$, on the other hand, the Seebeck coefficient behaves similarly to that in a resonant level system. The sign change of the thermopower and the fact that both the electrical and thermal conductances reach their half-fermionic value when Majorana physics are maximal could serve as a proof of the existence of Majorana states in nanowires.

We investigate the occurrence of Majorana modes in semiconductor quantum wires in close proximity with a superconductor and when both Rashba interaction and magnetic field are present. We consider long, but finite, multiband wires (namely, planar wires with dimensions $L_x\gg L_y$). Our results demonstrate that interband mixing coming from the Rashba spin-orbit term hybridizes Majorana pairs originating from different transverse modes while simultaneously closing the effective gap. Consequently, multiple Majorana modes do not coexist in general. On the contrary, Majorana physics is robust provided that only one single transverse mode contributes to a Majorana pair. Finally, we analyze the robustness of Majorana physics with respect to magnetic orbital effects. DOI: http://dx.doi.org/10.1103/PhysRevB.86.121103 ©2012 American Physical Society

The electromagnetic response of the fullerene C₆₀ in the energy range 0<or=E<or=35 eV, exhausting 84% of the Thomas-Reiche-Kuhn sum rule, has been worked out using the local density and random phase approximations. A detailed correspondence between the calculated pi -plasmons and the peaks determined in optical/UV studies is found. The damping width ( approximately=12 eV) of the Mie resonance observed at approximately=20 eV is well reproduced, and is due to Landau damping. This resonance is found to be determined by sigma to sigma and, to a large extent, also by sigma to pi , delta particle-hole transitions.