1,328 research outputs found
Single exciton spectroscopy of single-Mn doped InAs quantum dots
The optical spectroscopy of a single InAs quantum dot doped with a single Mn
atom is studied using a model Hamiltonian that includes the exchange
interactions between the spins of the quantum dot electron-hole pair, the Mn
atom and the acceptor hole. Our model permits to link the photoluminescence
spectra to the Mn spin states after photon emission. We focus on the relation
between the charge state of the Mn, or , and the different spectra
which result through either band-to-band or band-to-acceptor transitions. We
consider both neutral and negatively charged dots. Our model is able to account
for recent experimental results on single Mn doped InAs PL spectra and can be
used to account for future experiments in GaAs quantum dots. Similarities and
differences with the case of single Mn doped CdTe quantum dots are discussed.Comment: 15 pages, 9 figure
Orbital Magnetization of Quantum Spin Hall Insulator Nanoparticles
Both spin and orbital degrees of freedom contribute to the magnetic moment of
isolated atoms. However, when inserted in crystals, atomic orbital moments are
quenched because of the lack of rotational symmetry that protects them when
isolated. Thus, the dominant contribution to the magnetization of magnetic
materials comes from electronic spin. Here we show that nanoislands of quantum
spin Hall insulators can host robust orbital edge magnetism whenever their
highest occupied Kramers doublet is singly occupied, upgrading the spin edge
current into a charge current. The resulting orbital magnetization scales
linearly with size, outweighing the spin contribution for islands of a few nm
in size. This linear scaling is specific of the Dirac edge states and very
different from Schrodinger electrons in quantum rings. Modelling Bi(111)
flakes, whose edge states have been recently observed, we show that orbital
magnetization is robust with respect to disorder, thermal agitation, shape of
the island and crystallographic direction of the edges, reflecting its
topological protection.Comment: 7 pages, 5 figures, + Supporting Informatio
Majorana Zero Modes in Graphene
A clear demonstration of topological superconductivity (TS) and Majorana zero
modes remains one of the major pending goal in the field of topological
materials. One common strategy to generate TS is through the coupling of an
s-wave superconductor to a helical half-metallic system. Numerous proposals for
the latter have been put forward in the literature, most of them based on
semiconductors or topological insulators with strong spin-orbit coupling. Here
we demonstrate an alternative approach for the creation of TS in
graphene/superconductor junctions without the need of spin-orbit coupling. Our
prediction stems from the helicity of graphene's zero Landau level edge states
in the presence of interactions, and on the possibility, experimentally
demonstrated, to tune their magnetic properties with in-plane magnetic fields.
We show how canted antiferromagnetic ordering in the graphene bulk close to
neutrality induces TS along the junction, and gives rise to isolated,
topologically protected Majorana bound states at either end. We also discuss
possible strategies to detect their presence in graphene Josephson junctions
through Fraunhofer pattern anomalies and Andreev spectroscopy. The latter in
particular exhibits strong unambiguous signatures of the presence of the
Majorana states in the form of universal zero bias anomalies. Remarkable
progress has recently been reported in the fabrication of the proposed type of
junctions, which offers a promising outlook for Majorana physics in graphene
systems.Comment: 14 pages, 8 figures. Included simulations of Andreev spectroscopy and
mor
Spin depolarization in the transport of holes across GaMnAs/GaAlAs/p-GaAs
We study the spin polarization of tunneling holes injected from ferromagnetic
GaMnAs into a p-doped semiconductor through a tunneling barrier. We obtain an
upper limit to the spin injection rate. We find that spin-orbit interaction
interaction in the barrier and in the drain limits severely spin injection.
Spin depolarization is stronger when the magnetization is parallel to the
current than when is perpendicular to it.Comment: Accepted in Phys. Rev. B. 4 pages, 4 figure
Coherently photo-induced ferromagnetism in diluted magnetic semiconductors
Ferromagnetism is predicted in undoped diluted magnetic semiconductors
illuminated by intense sub-bandgap laser radiation . The mechanism for
photo-induced ferromagnetism is coherence between conduction and valence bands
induced by the light which leads to an optical exchange interaction. The
ferromagnetic critical temperature T_C depends both on the properties of the
material and on the frequency and intensity of the laser and could be above 1
K.Comment: 11 pages, 2 figures, preprint styl
Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells
We have shown experimentally that an electric field decreases the energy
separation between the two components of a dense spin-polarized exciton gas in
a coupled double quantum well, from a maximum splitting of meV to
zero, at a field of 35 kV/cm. This decrease, due to the field-induced
deformation of the exciton wavefunction, is explained by an existing
calculation of the change in the spin-dependent exciton-exciton interaction
with the electron-hole separation. However, a new theory that considers the
modification of screening with that separation is needed to account for the
observed dependence on excitation power of the individual energies of the two
exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press
Anisotropic intrinsic spin relaxation in graphene due to flexural distortions
We propose an intrinsic spin scattering mechanism in graphene originated by
the interplay of atomic spin-orbit interaction and the local curvature induced
by flexural distortions of the atomic lattice. Starting from a multiorbital
tight-binding Hamiltonian with spin-orbit coupling considered
non-perturbatively, we derive an effective Hamiltonian for the spin scattering
of the Dirac electrons due to flexural distortions. We compute the spin
lifetime due to both flexural phonons and ripples and we find values in the
microsecond range at room temperature. Interestingly, this mechanism is
anisotropic on two counts. First, the relaxation rate is different for
off-plane and in-plane spin quantization axis. Second, the spin relaxation rate
depends on the angle formed by the crystal momentum with the carbon-carbon
bond. In addition, the spin lifetime is also valley dependent. The proposed
mechanism sets an upper limit for spin lifetimes in graphene and will be
relevant when samples of high quality can be fabricated free of extrinsic
sources of spin relaxation.Comment: extended version with 7 pages, 4 figures and several new results; a
numerical error has been corrected leading to longer spin lifetimes than in
the previous versio
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