Spin-valley blockade in carbon nanotube double quantum dots

We present a theoretical study of the Pauli or spin-valley blockade for double quantum dots in semiconducting carbon nanotubes. In our model we take into account the following characteristic features of carbon nanotubes: (i) fourfold (spin and valley) degeneracy of the quantum dot levels, (ii) the intrinsic spin-orbit interaction which is enhanced by the tube curvature, and (iii) valley-mixing due to short-range disorder, i.e., substitutional atoms, adatoms, etc. We find that the spin-valley blockade can be lifted in the presence of short-range disorder, which induces two independent random (in magnitude and direction) valley-Zeeman-fields in the two dots, and hence acts similarly to hyperfine interaction in conventional semiconductor quantum dots. In the case of strong spin-orbit interaction, we identify a parameter regime where the current as the function of an applied axial magnetic field shows a zero-field dip with a width controlled by the interdot tunneling amplitude, in agreement with recent experiments.Comment: 15 pages, 6 figures, 2 tables; v2: published versio

Electrical conduction and magnetic properties of nanoconstrictions and nanowires created by focused electron/ion beam and of Fe3O4 thin films

La tesis se centra en la fabricación y estudio de las propiedades eléctricas y magnéticas de nanoestructuras con aplicaciones potenciales en nanoelectrónica. Se estudian las propiedades de magnetotransporte de películas epitaxiales de Fe3O4. Se desarrolla un método para fabricar constricciones de tamaño atómico en metales usando un haz focalizado de iones. Y se examinan diferentes tipos de nanohílos fabricados mediante deposición usando un haz focalizado de electrones/iones: los de Pt presentan una transición metal-aislante, los de W son superconductores por debajo de 5 K y los de Co son fuertemente magnético

Interaction-driven spin precession in quantum-dot spin valves

We analyze spin-dependent transport through spin valves composed of an interacting quantum dot coupled to two ferromagnetic leads. The spin on the quantum dot and the linear conductance as a function of the relative angle $\theta$ of the leads' magnetization directions is derived to lowest order in the dot-lead coupling strength. Due to the applied bias voltage spin accumulates on the quantum dot, which for finite charging energy experiences a torque, resulting in spin precession. The latter leads to a non-trivial, interaction-dependent, $\theta$-dependence of the conductance. In particular, we find that the spin-valve effect is reduced for all $\theta \neq \pi$.Comment: 5 pages, 3 figures, version to be published in Phys. Rev. Let

Quantum transport in nanostructures: From the effects of decoherence on localization to magnetotransport in two-dimensional electron systems

In this thesis, quantum transport in nanostructures is studied theoretically by means of the nonequilibrium Green's function (NEGF) method. Starting with coherent systems, we discuss ballistic transport and conductance quantization in homogeneous tight-binding lattices. We show that disorder gives rise to transmission resonances. A short introduction to Anderson localization is given and a compact analytical formula for the disorder averaged resistance is derived by means of generating functions. Transport in nanostructures generally takes place in an intermediate regime between quantum and classical transport due to decoherence. We study the effects of decoherence on electron transport by a statistical model. The essential idea of our model is to distribute spatially over the system decoherence regions, where phase and momentum of the electrons are randomized completely. The transport in between these regions is assumed as phase coherent. Afterwards, the transport quantity of interest is ensemble averaged over spatial decoherence configurations, which are generated according to a given distribution function. We discuss how homogeneous tight-binding lattices are driven by decoherence from the quantum-ballistic to the classical-Ohmic regime. We show that the transport through disordered tight-binding lattices is affected significantly by the spatial distribution of the decoherence regions. If the decoherence is homogeneously distributed, Ohmic conduction is found for any finite degree of decoherence. In contrast, for randomly distributed decoherence, we find an insulator-metal transition from the localized to the Ohmic regime at a critical degree of decoherence, which corresponds to a critical phase coherence length. We also discuss how transport in disordered tight-binding lattices can be enhanced by decoherence. The decoherence model is extended to obtain pure dephasing. We show that transmission resonances are suppressed by pure dephasing, but the average transmission is conserved. The insulator-metal transition is independent of whether phase randomization goes along with momentum randomization or not. Magnetotransport in two-dimensional electron systems is considered. We study how electrons, coherently injected at one point on the boundary of a two-dimensional electron gas (2DEG), are focused by a perpendicular magnetic field onto another point of that boundary. At weak magnetic field, the generalized 4-point Hall resistance shows equidistant peaks, which can be explained by classical cyclotron motion. When the magnetic field is increased, we observe anomalous resistance oscillations superimposed upon the quantum Hall plateaus. We show that all resistance oscillations can be explained by the interference of the occupied edge channels. The anomalous oscillations are beatings, which appear when only some few edge channels are occupied. By introducing decoherence and partially diffusive boundary scattering, we show that this effect is quite robust. The resistance oscillations can be observed not only in a nonrelativistic 2DEG, but also in the relativistic 2DEG found in graphene. We also report a finite current at armchair edges of graphene ribbons, which is not present at zigzag edges. This edge current can be traced back to the fact that at armchair edges carbon atoms of both graphene sublattices are present, whereas at zigzag edges only atoms of one sublattice appear. The thesis is concluded with some notes on Hofstadter's butterfly shown on the cover page

From narrow-gap and semimagnetic semiconductors to spintronics and topological matter: a life with spins

The abundance of semiconductors in our smartphones, computers, fiber optic junctions, cars, light sources, photovoltaic and thermoelectric cells results from the possibilities of controlling their properties through doping, lighting, and applying various fields. This paper, a part of the volume celebrating 100 years of the Polish Physical Society, presents a biased selection of worthwhile results obtained by researchers at the Institute of Physics, Polish Academy of Sciences relevant, as seen today, to topological matter and spintronics. Comprehensive studies, combining materials development, experimental investigations, and theoretical description of narrow-gap and dilute-magnetic semiconductors have been especially significant in this context. This survey also emphasizes, in an autobiographical tone, a half of a century of the author's intellectual emotions accompanying the rise of ideas and quantitative theories, allowing identifying the physics behind ongoing and future observations.Comment: some typos corrected in version