405 research outputs found
Tunable g factor and phonon-mediated hole spin relaxation in Ge/Si nanowire quantum dots
We theoretically consider g factor and spin lifetimes of holes in a
longitudinal Ge/Si core/shell nanowire quantum dot that is exposed to external
magnetic and electric fields. For the ground states, we find a large anisotropy
of the g factor which is highly tunable by applying electric fields. This
tunability depends strongly on the direction of the electric field with respect
to the magnetic field. We calculate the single-phonon hole spin relaxation
times T1 for zero and small electric fields and propose an optimal setup in
which very large T1 of the order of tens of milliseconds can be reached.
Increasing the relative shell thickness or the longitudinal confinement length
further prolongs T1. In the absence of electric fields, the dephasing vanishes
and the decoherence time T2 is determined by T2 = 2 T1.Comment: 7 pages, 4 figure
Anisotropic g factor in InAs self-assembled quantum dots
We investigate the wave functions, spectrum, and g-factor anisotropy of
low-energy electrons confined to self-assembled, pyramidal InAs quantum dots
(QDs) subject to external magnetic and electric fields. We present the
construction of trial wave functions for a pyramidal geometry with hard-wall
confinement. We explicitly find the ground and first excited states and show
the associated probability distributions and energies. Subsequently, we use
these wave functions and 8-band theory to derive a Hamiltonian
describing the QD states close to the valence band edge. Using a perturbative
approach, we find an effective conduction band Hamiltonian describing
low-energy electronic states in the QD. From this, we further extract the
magnetic field dependent eigenenergies and associated g factors. We examine the
g factors regarding anisotropy and behavior under small electric fields. In
particular, we find strong anisotropies, with the specific shape depending
strongly on the considered QD level. Our results are in good agreement with
recent measurements [Takahashi et al., Phys. Rev. B 87, 161302 (2013)] and
support the possibility to control a spin qubit by means of g-tensor
modulation.Comment: 9 pages, 9 figure
Majorana Fermions in Ge/Si Hole Nanowires
We consider Ge/Si core/shell nanowires with hole states coupled to an
-wave superconductor in the presence of electric and magnetic fields. We
employ a microscopic model that takes into account material-specific details of
the band structure such as strong and electrically tunable Rashba-type
spin-orbit interaction and factor anisotropy for the holes. In addition,
the proximity-induced superconductivity Hamiltonian is derived starting from a
microscopic model. In the topological phase, the nanowires host Majorana
fermions with localization lengths that depend strongly on both the magnetic
and electric fields. We identify the optimal regime in terms of the directions
and magnitudes of the fields in which the Majorana fermions are the most
localized at the nanowire ends. In short nanowires, the Majorana fermions
hybridize and form a subgap fermion whose energy is split away from zero and
oscillates as a function of the applied fields. The period of these
oscillations could be used to measure the dependence of the spin-orbit
interaction on the applied electric field and the factor anisotropy.Comment: 11 pages, 7 figure
Strongly Interacting Holes in Ge/Si Nanowires
We consider holes confined to Ge/Si core/shell nanowires subject to strong
Rashba spin-orbit interaction and screened Coulomb interaction. Such wires can,
for instance, serve as host systems for Majorana bound states. Starting from a
microscopic model, we find that the Coulomb interaction strongly influences the
properties of experimentally realistic wires. To show this, a Luttinger liquid
description is derived based on a renormalization group analysis. This
description in turn allows to calculate the scaling exponents of various
correlation functions as a function of the microscopic system parameters. It
furthermore permits to investigate the effect of Coulomb interaction on a small
magnetic field, which opens a strongly anisotropic partial gap
Effect of strain on hyperfine-induced hole-spin decoherence in quantum dots
We theoretically consider the effect of strain on the spin dynamics of a
single heavy-hole (HH) confined to a self-assembled quantum dot and interacting
with the surrounding nuclei via hyperfine interaction. Confinement and strain
hybridize the HH states, which show an exponential decay for a narrowed nuclear
spin bath. For different strain configurations within the dot, the dependence
of the spin decoherence time on external parameters is shifted and the
non-monotonic dependence of the peak is altered. Application of external strain
yields considerable shifts in the dependence of on external parameters.
We find that external strain affects mostly the effective hyperfine coupling
strength of the conduction band (CB), indicating that the CB admixture of the
hybridized HH states plays a crucial role in the sensitivity of on
strain
When you isn't <i>you</i>:The attraction of selfÂ-ascription in children’s interpretation of pronouns in reported speech
In language comprehension, 'you 'is a 'de se 'pronoun, which means that its interpretation is guided by a simple 'de se 'rule ('you '= self-ascription by addressee), while the interpretation of other pronouns requires more complicated reasoning. This predicts that 'you 'should be easier to process than 'I 'or 'he', especially for children. But not all occurrences of 'you 'can be correctly interpreted via self-ascription. We consider two cases where 'you 'does not indicate self-ascription: interpretation as an eavesdropper and direct speech. In our experiment, we compare children’s interpretation of the pronouns 'I', 'you 'and 'he', in both direct and indirect reported speech, and in both addressee and eavesdropping situations. We tested 71 five-year-olds, 63 nine-year-olds, and 52 adults in a referent-selection task and found a clear 'de se 'effect for children when directly addressed: they performed better with 'you 'than with 'I'/'he 'in indirect speech, but worse with 'you 'than with 'I'/'he 'in direct speech. We explain the latter finding in terms of the attraction of the 'de se 'interpretation strategy, which leads addressees to automatically self-ascribe 'you 'even in a direct speech report
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