Suppression of decoherence in a graphene monolayer ring

The influence of high magnetic fields on coherent transport is investigated. A monolayer graphene quantum ring is fabricated and the Aharonov-Bohm effect is observed. For increased magnitude of the magnetic field higher harmonics appear. This phenomenon is attributed to an increase of the phase coherence length due to reduction of spin flip scattering

Spin Blockade in Capacitively Coupled Quantum Dots

We present transport measurements on a lateral double dot produced by combining local anodic oxidation and electron beam lithography. We investigate the tunability of our device and demonstrate, that we can switch between capacitive and tunnel coupling. In the regime of capacitive coupling we observe the phenomenon of spin blockade in a magnetic field and analyze the influence of capacitive interdot coupling on this effect.Comment: 4 pages, 3 figure

Non-invasive detection of molecular bonds in quantum dots

We performed charge detection on a lateral triple quantum dot with star-like geometry. The setup allows us to interpret the results in terms of two double dots with one common dot. One double dot features weak tunnel coupling and can be understood with atom-like electronic states, the other one is strongly coupled forming molecule-like states. In nonlinear measurements we identified patterns that can be analyzed in terms of the symmetry of tunneling rates. Those patterns strongly depend on the strength of interdot tunnel coupling and are completely different for atomic- or molecule-like coupled quantum dots allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure

Parasitic pumping currents in an interacting quantum dot

We analyze the charge and spin pumping in an interacting dot within the almost adiabatic limit. By using a non-equilibrium Green's function technique within the time-dependent slave boson approximation, we analyze the pumped current in terms of the dynamical constraints in the infinite-U regime. The results show the presence of parasitic pumping currents due to the additional phases of the constraints. The behavior of the pumped current through the quantum dot is illustrated in the spin-insensitive and in the spin-sensitive case relevant for spintronics applications

Electron spin relaxation in n-type InAs quantum wires

We investigate the electron spin relaxation of $n$-type InAs quantum wires by numerically solving the fully microscopic kinetic spin Bloch equations with the relevant scattering explicitly included. We find that the quantum-wire size and the growth direction influence the spin relaxation time by modulating the spin-orbit coupling. Due to inter-subband scattering in connection with the spin-orbit interaction, spin-relaxation in quantum wires can show different characteristics from those in bulk or quantum wells and can be effectively manipulated by various means.Comment: 8 pages, 6 figure

Berry Phase Transition in Twisted Bilayer Graphene

The electronic dispersion of a graphene bilayer is highly dependent on rotational mismatch between layers and can be further manipulated by electrical gating. This allows for an unprecedented control over electronic properties and opens up the possibility of flexible band structure engineering. Here we present novel magnetotransport data in a twisted bilayer, crossing the energetic border between decoupled monolayers and coupled bilayer. In addition a transition in Berry phase between pi and 2pi is observed at intermediate magnetic fields. Analysis of Fermi velocities and gate induced charge carrier densities suggests an important role of strong layer asymmetry for the observed phenomena.Comment: 20 pages main paper + 10 pages supporting informatio

Interaction-Induced Spin Polarization in Quantum Dots

The electronic states of lateral many electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb blockade peaks are measured. A zig-zag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure

Nonequilibrium Green's function theory for nonadiabatic effects in quantum electron transport

We develop nonequilibribrium Green's function based transport theory, which includes effects of nonadiabatic nuclear motion in the calculation of the electric current in molecular junctions. Our approach is based on the separation of slow and fast timescales in the equations of motion for the Green's functions by means of the Wigner representation. Time derivatives with respect to central time serves as a small parameter in the perturbative expansion enabling the computation of nonadiabatic corrections to molecular Green's functions. Consequently, we produce series of analytic expressions for non-adiabatic electronic Green's functions (up to the second order in the central time derivatives); which depend not solely on instantaneous molecular geometry but likewise on nuclear velocities and accelerations. Extended formula for electric current is derived which accounts for the non-adiabatic corrections. This theory is concisely illustrated by the calculations on a model molecular junction

Adiabatic pumping through a quantum dot in the Kondo regime: Exact results at the Toulouse limit

Transport properties of ultrasmall quantum dots with a single unpaired electron are commonly modeled by the nonequilibrium Kondo model, describing the exchange interaction of a spin-1/2 local moment with two leads of noninteracting electrons. Remarkably, the model possesses an exact solution when tuned to a special manifold in its parameter space known as the Toulouse limit. We use the Toulouse limit to exactly calculate the adiabatically pumped spin current in the Kondo regime. In the absence of both potential scattering and a voltage bias, the instantaneous charge current is strictly zero for a generic Kondo model. However, a nonzero spin current can be pumped through the system in the presence of a finite magnetic field, provided the spin couples asymmetrically to the two leads. Tunneling through a Kondo impurity thus offers a natural mechanism for generating a pure spin current. We show, in particular, that one can devise pumping cycles along which the average spin pumped per cycle is closely equal to $\hbar$. By analogy with Brouwer's formula for noninteracting systems with two driven parameters, the pumped spin current is expressed as a geometrical property of a scattering matrix. However, the relevant %Alex: I replaced topological with geometrical in the sentence above scattering matrix that enters the formulation pertains to the Majorana fermions that appear at the Toulouse limit rather than the physical electrons that carry the current. These results are obtained by combining the nonequilibrium Keldysh Green function technique with a systematic gradient expansion, explicitly exposing the small parameter controlling the adiabatic limit.Comment: 14 pages, 3 figures, revised versio