9 research outputs found
Coherent spin rotations in open driven double quantum dots
We analyze the charge and spin dynamics in a DC biased double quantum dot
driven by crossed DC and AC magnetic fields. In this configuration, spatial
delocalization due to inter-dot tunnel competes with intra-dot spin rotations
induced by the time dependent magnetic field, giving rise to a complicated time
dependent behavior of the tunnelling current. When the Zeeman splitting has the
same value in both dots and spin flip is negligible, the electrons remain in
the triplet subspace (dark subspace) performing coherent spin rotations and the
current does not flow. This electronic trapping is removed either by finite
spin relaxation or when the Zeeman splitting is different in each quantum dot.
In the first case, our results show that measuring the current will allow to
get information on the spin relaxation time. In the last case, we will show
that applying a resonant bichromatic magnetic field, the electrons become
trapped in a coherent superposition of states and electronic transport is
blocked. Then, manipulating AC magnetic fields, electrons are driven to perform
coherent spin rotations which can be unambiguously detected by direct
measurement of the tunneling current.Comment: 7 pages, 6 figures. Extended published versio
Temperature-dependent dynamical nuclear polarization bistabilities in double quantum dots in the spin-blockade regime
The interplay of dynamical nuclear polarization (DNP) and leakage current
through a double quantum dot in the spin-blockade regime is analyzed. A finite
DNP is built up due to a competition between hyperfine (HF) spin-flip
transitions and another inelastic escape mechanism from the triplets, which
block transport. We focus on the temperature dependence of the DNP for zero
energy-detuning (i.e. equal electrostatic energy of one electron in each dot
and a singlet in the right dot). Our main result is the existence of a
transition temperature, below which the DNP is bistable, so a hysteretic
leakage current versus external magnetic field B appears. This is studied in
two cases: (i) Close to the crossing of the three triplet energy levels near
B=0, where spin-blockade is lifted due to the inhomogeneity of the effective
magnetic field from the nuclei. (ii) At higher B-fields, where the two
spin-polarized triplets simultaneously cross two different singlet energy
levels. We develop simplified models leading to different transition
temperatures T_TT and T_ST for the crossing of the triplet levels and the
singlet-triplet level crossings, respectively. We find T_TT analytically to be
given solely by the HF couplings, whereas T_ST depends on various parameters
and T_ST>T_TT. The key idea behind the existence of the transition temperatures
at zero energy-detuning is the suppression of energy absorption compared to
emission in the inelastic HF transitions. Finally, by comparing the rate
equation results with Monte Carlo simulations, we discuss the importance of
having both HF interaction and another escape mechanism from the triplets to
induce a finite DNP.Comment: 26 pages, 17 figure
Spin dependent electron transport through single and double quantum dot systems
Tesis doctoral in茅dita. Universidad Aut贸noma de Madrid, Facultad de Ciencias, Departamento de F铆sica de la Materia Condensada. Fecha de lectura: 14-11-201
Dynamical nuclear spin polarization induced by electronic current through double quantum dots
We analyze electron spin relaxation in electronic transport through
coherently coupled double quantum dots in the spin blockade regime. In
particular, we focus on hyperfine interaction as the spin relaxation mechanism.
We pay special attention to the effect of the dynamical nuclear spin
polarization induced by the electronic current on the nuclear environment. We
discuss the behaviour of the electronic current and the induced nuclear spin
polarization versus an external magnetic field for different hyperfine coupling
intensities and interdot tunnelling strengths. We take into account, for each
magnetic field, all hyperfine mediated spin relaxation processes coming from
the different opposite spin levels approaches. We find that the current as a
function of the external magnetic field shows a peak or a dip, and that the
transition from a current dip to a current peak behaviour is obtained by
decreasing the hyperfine coupling or by increasing the interdot tunnelling
strength. We give a physical picture in terms of the interplay between the
electrons tunnelling out of the double quantum dot and the spin flip processes
due to the nuclear environment.Comment: 25 pages and 8 figures. To be published in New Journal of Physic