Spin and charge transport in thermally and ac driven nanodevices

Alomar, M. I. (supervisor: Sánchez, David)

PhD Thesis (2017)

PhD Thesis (2017)

This thesis deals with electronic transport in nanodevices driven by temperature gradients or time-dependent potentials. Our emphasis is on both spintronic effects arising from the influence of inhomogeneous spin-orbit couplings and charging effects originated from strong electron-electron interactions in quantum dots.

Chapter 1 is a broad introduction aimed at nonspecialists. We discuss the history of the subject paying attention to the latest developments. We explain the general concepts employed in the rest of this thesis with the objective of offering a self-contained presentation of our research.

In Chapter 2 we investigate the transport properties of a graphene layer in the presence of Rashba spin-orbit interaction. We calculate within the scattering approach the linear electric and thermoelectric responses of a clean sample when the Rashba coupling is localized around a finite region. We find that the thermoelectric conductance, unlike its electric counterpart, is quite sensitive to external modulations of the Fermi energy. Furthermore, we find that the junction thermopower is largely dominated by an intrinsic term independently of the spin-orbit potential scattering.

In order to investigate whether the previous results are similar for a semiconductor heterostructure two-dimensional electron gas (2DEG), in Chapter 3 we consider a spin-orbit-coupled 2DEG under the influence of a thermal gradient externally applied to two attached reservoirs. We discuss the charge, spin and magneto-Seebeck effects also in the ballistic regime of transport at linear response. We find that the charge thermopower (S) is an oscillating function of both the spin-orbit strength and the quantum well width. We also observe that S is always negative for normal leads. When the contacts are ferromagnetic, we calculate the spin-resolved Seebeck coefficient and investigate its sign changes by tuning the Fermi energy. Additionally, we determine the magneto-Seebeck ratio, which shows dramatic changes in the presence of the Rashba potential.

Because the spin-injection efficiency between dissimilar materials tends to be low, in Chapter 4 we investigate the transmission properties of a spin transistor coupled to two quantum point contacts acting as spin injector and detector. Interestingly, the Rashba interaction can be tuned in such a way that nonuniform spin-orbit fields can point along distinct directions in different points of the sample. We discuss both spin-conserving and spin-flipping transitions as the spin-orbit angle of orientation varies from parallel to antiparallel configuration. Spin precession oscillations are clearly seen as a function of the length of the central channel. Remarkably, we find that these oscillations combine with the Fabry-Perot motion giving rise to quasiperiodic transmissions in the purely one-dimensional case. Furthermore, we consider the more realistic case of a finite width in the transverse direction and find that the coherent oscillations become deteriorated for moderate values of the spin-orbit strength.

In Chapter 5 we consider an interacting quantum dot working as a coherent source of single electrons. The dot is tunnel coupled to a reservoir and capacitively coupled to a gate terminal with an applied ac potential. We investigate the quantized dynamics as a consequence of ac pulses with large amplitude. Within a Keldysh-Green function formalism we derive the time dependent current in the Coulomb blockade regime. We prove that the electron emission and absorption resonances undergo a splitting when the charging energy is larger than the tunnel broadening. Quantization of the charge emitted by the capacitor is reduced due to Coulomb repulsion and additional plateaus arise.

Finally, a summary and outlook of our results are included in Chapter 6.

Chapter 1 is a broad introduction aimed at nonspecialists. We discuss the history of the subject paying attention to the latest developments. We explain the general concepts employed in the rest of this thesis with the objective of offering a self-contained presentation of our research.

In Chapter 2 we investigate the transport properties of a graphene layer in the presence of Rashba spin-orbit interaction. We calculate within the scattering approach the linear electric and thermoelectric responses of a clean sample when the Rashba coupling is localized around a finite region. We find that the thermoelectric conductance, unlike its electric counterpart, is quite sensitive to external modulations of the Fermi energy. Furthermore, we find that the junction thermopower is largely dominated by an intrinsic term independently of the spin-orbit potential scattering.

In order to investigate whether the previous results are similar for a semiconductor heterostructure two-dimensional electron gas (2DEG), in Chapter 3 we consider a spin-orbit-coupled 2DEG under the influence of a thermal gradient externally applied to two attached reservoirs. We discuss the charge, spin and magneto-Seebeck effects also in the ballistic regime of transport at linear response. We find that the charge thermopower (S) is an oscillating function of both the spin-orbit strength and the quantum well width. We also observe that S is always negative for normal leads. When the contacts are ferromagnetic, we calculate the spin-resolved Seebeck coefficient and investigate its sign changes by tuning the Fermi energy. Additionally, we determine the magneto-Seebeck ratio, which shows dramatic changes in the presence of the Rashba potential.

Because the spin-injection efficiency between dissimilar materials tends to be low, in Chapter 4 we investigate the transmission properties of a spin transistor coupled to two quantum point contacts acting as spin injector and detector. Interestingly, the Rashba interaction can be tuned in such a way that nonuniform spin-orbit fields can point along distinct directions in different points of the sample. We discuss both spin-conserving and spin-flipping transitions as the spin-orbit angle of orientation varies from parallel to antiparallel configuration. Spin precession oscillations are clearly seen as a function of the length of the central channel. Remarkably, we find that these oscillations combine with the Fabry-Perot motion giving rise to quasiperiodic transmissions in the purely one-dimensional case. Furthermore, we consider the more realistic case of a finite width in the transverse direction and find that the coherent oscillations become deteriorated for moderate values of the spin-orbit strength.

In Chapter 5 we consider an interacting quantum dot working as a coherent source of single electrons. The dot is tunnel coupled to a reservoir and capacitively coupled to a gate terminal with an applied ac potential. We investigate the quantized dynamics as a consequence of ac pulses with large amplitude. Within a Keldysh-Green function formalism we derive the time dependent current in the Coulomb blockade regime. We prove that the electron emission and absorption resonances undergo a splitting when the charging energy is larger than the tunnel broadening. Quantization of the charge emitted by the capacitor is reduced due to Coulomb repulsion and additional plateaus arise.

Finally, a summary and outlook of our results are included in Chapter 6.