1,171 research outputs found
Photo-induced spin filtering in a double quantum dot
We investigate the spin-resolved electron dynamics in a double quantum dot
driven by ultrafast asymmetric electromagnetic pulses. Using a analytical model
we show that applying an appropriate pulse sequence allows to control
coherently the spin degree of freedom on the femtosecond time scale. It can be
achieved that the spin-up state is localized in a selected quantum dot while
the spin-down state remains in the other dot. We show that this photo-induced
spin-dependent separation can be maintained for a desired period of time.Comment: shortened, revised version 2 article published at Appl. Phys. Let
Confinement-induced Berry phase and helicity-dependent photocurrents
The photocurrent in an optically active metal is known to contain a component
that switches sign with the helicity of the incident radiation. At low
frequencies, this current depends on the orbital Berry phase of the Bloch
electrons via the "anomalous velocity" of Karplus and Luttinger. We consider
quantum wells in which the parent material, such as GaAs, is not optically
active and the relevant Berry phase only arises as a result of quantum
confinement. Using an envelope approximation that is supported by numerical
tight-binding results, it is shown that the Berry phase contribution is
determined for realistic wells by a cubic Berry phase intrinsic to the bulk
material, the well width, and the well direction. These results for the
magnitude of the Berry-phase effect suggest that it may already have been
observed in quantum well experiments.Comment: 4 pages, 2 figure
Interface roughness, valley-orbit coupling and valley manipulation in quantum dots
We present a systematic study of interface roughness and its effect on
coherent dynamical processes in quantum dots. The potential due to a sharp,
flat interface lifts the degeneracy of the lowest energy valleys and yields a
set of valley eigenstates. Interface roughness is characterized by fluctuations
in the location of the interface and in the magnitude of the potential step.
Variations in the position of the interface, which are expected to occur on the
length scale of the lattice constant, reduce the magnitude of the valley-orbit
coupling. Variations in the size of the interface potential step alter the
magnitude of the valley-orbit coupling and induce transitions between different
valley eigenstates in dynamics involving two (or more) dots. Such transitions
can be studied experimentally by manipulating the bias between two dots and can
be detected by charge sensing. However, if the random variable characterizing
the position of the interface is correlated over distances of the order of a
quantum dot, which is unlikely but possible, the phase of the valley-orbit
coupling may be different in adjacent dots. In this case tunneling between like
and opposite valley eigenstates is in effect a random variable and cannot be
controlled. We suggest a resonant tunneling experiment that can identify the
matrix elements for tunneling between like and opposite valley eigenstates.Comment: 18 pages, 6 figure
Dirac and Klein-Gordon particles in one-dimensional periodic potentials
We evaluate the dispersion relation for massless fermions, described by the
Dirac equation, and for zero-spin bosons, described by the Klein-Gordon
equation, moving in two dimensions and in the presence of a one-dimensional
periodic potential. For massless fermions the dispersion relation shows a zero
gap for carriers with zero momentum in the direction parallel to the barriers
in agreement with the well-known "Klein paradox". Numerical results for the
energy spectrum and the density of states are presented. Those for fermions are
appropriate to graphene in which carriers behave relativistically with the
"light speed" replaced by the Fermi velocity. In addition, we evaluate the
transmission through a finite number of barriers for fermions and zero-spin
bosons and relate it with that through a superlattice.Comment: 9 pages, 12 figure
Choosing a basis that eliminates spurious solutions in k.p theory
A small change of basis in k.p theory yields a Kane-like Hamiltonian for the
conduction and valence bands of narrow-gap semiconductors that has no spurious
solutions, yet provides an accurate fit to all effective masses. The theory is
shown to work in superlattices by direct comparison with first-principles
density-functional calculations of the valence subband structure. A
reinterpretation of the standard data-fitting procedures used in k.p theory is
also proposed.Comment: 15 pages, 2 figures; v3: expanded with much new materia
Static polarizability of two-dimensional hole gases
We have calculated the density-density (Lindhard) response function of a
homogeneous two-dimensional (2D) hole gas in the static (omega=0) limit. The
bulk valence-band structure comprising heavy-hole (HH) and light-hole (LH)
states is modeled using Luttinger's kdotp approach within the axial
approximation. We elucidate how, in contrast to the case of conduction
electrons, the Lindhard function of 2D holes exhibits unique features
associated with (i) the confinement-induced HH-LH energy splitting and (ii) the
HH-LH mixing arising from the charge carriers' in-plane motion. Implications
for the dielectric response and related physical observables are discussed.Comment: 11 pages, 3 figures, IOP latex style, v2: minor changes, to appear in
NJ
Binding energy of shallow donors in a quantum well in the presence of a tilted magnetic field
We present results of variational calculations of the binding energy of a
neutral donor in a quantum well in the presence of a magnetic field tilted
relative to the QW plane. Assuming that the donor is located in the center of
the QW, we perform calculations for parameters typical of a II-VI wide-gap
semiconductor heterostructure, using as an example the case of a rectangular
CdTe quantum well with CdMgTe barriers. We present the dependence of the
binding energy of a neutral donor on the tilt angle and on the magnitude of the
applied magnetic filed. As a key result, we show that measurement of the
binding energy of a donor at two angles of the magnetic field with respect to
the quantum well plane can be used to unambiguously determined the conduction
band offset of the materials building up heterostructure.Comment: 6 pages, 5 figure
Energy spectra for quantum wires and 2DEGs in magnetic fields with Rashba and Dresselhaus spin-orbit interactions
We introduce an analytical approximation scheme to diagonalize parabolically
confined two dimensional electron systems with both the Rashba and Dresselhaus
spin-orbit interactions. The starting point of our perturbative expansion is a
zeroth-order Hamiltonian for an electron confined in a quantum wire with an
effective spin-orbit induced magnetic field along the wire, obtained by
properly rotating the usual spin-orbit Hamiltonian. We find that the
spin-orbit-related transverse coupling terms can be recast into two parts W and
V, which couple crossing and non-crossing adjacent transverse modes,
respectively. Interestingly, the zeroth-order Hamiltonian together with W can
be solved exactly, as it maps onto the Jaynes-Cummings model of quantum optics.
We treat the V coupling by performing a Schrieffer-Wolff transformation. This
allows us to obtain an effective Hamiltonian to third order in the coupling
strength k_Rl of V, which can be straightforwardly diagonalized via an
additional unitary transformation. We also apply our approach to other types of
effective parabolic confinement, e.g., 2D electrons in a perpendicular magnetic
field. To demonstrate the usefulness of our approximate eigensolutions, we
obtain analytical expressions for the n^th Landau-level g_n-factors in the
presence of both Rashba and Dresselhaus couplings. For small values of the bulk
g-factors, we find that spin-orbit effects cancel out entirely for particular
values of the spin-orbit couplings. By solving simple transcendental equations
we also obtain the band minima of a Rashba-coupled quantum wire as a function
of an external magnetic field. These can be used to describe Shubnikov-de Haas
oscillations. This procedure makes it easier to extract the strength of the
spin-orbit interaction in these systems via proper fitting of the data.Comment: 13 pages, 11 figure
Theory of valley-orbit coupling in a Si/SiGe quantum dot
Electron states are studied for quantum dots in a strained Si quantum well,
taking into account both valley and orbital physics. Realistic geometries are
considered, including circular and elliptical dot shapes, parallel and
perpendicular magnetic fields, and (most importantly for valley coupling) the
small local tilt of the quantum well interface away from the crystallographic
axes. In absence of a tilt, valley splitting occurs only between pairs of
states with the same orbital quantum numbers. However, tilting is ubiquitous in
conventional silicon heterostructures, leading to valley-orbit coupling. In
this context, "valley splitting" is no longer a well defined concept, and the
quantity of merit for qubit applications becomes the ground state gap. For
typical dots used as qubits, a rich energy spectrum emerges, as a function of
magnetic field, tilt angle, and orbital quantum number. Numerical and
analytical solutions are obtained for the ground state gap and for the mixing
fraction between the ground and excited states. This mixing can lead to valley
scattering, decoherence, and leakage for Si spin qubits.Comment: 18 pages, including 4 figure
Lande-like formula for the g factors of hole-nanowire subband edges
We have analyzed theoretically the Zeeman splitting of hole-quantum-wire
subband edges. As is typical for any bound state, their g factor depends on
both an intrinsic g factor of the material and an additional contribution
arising from a finite bound-state orbital angular momentum. We discuss the
quantum-confinement-induced interplay between bulk-material and orbital
effects, which is nontrivial due to the presence of strong spin-orbit coupling.
A compact analytical formula is provided that elucidates this interplay and can
be useful for predicting Zeeman splitting in generic hole-wire geometries.Comment: 4 pages, 2 figure
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