120 research outputs found
Circular dichroism of magneto-phonon resonance in doped graphene
Polarization resolved, Raman scattering response due to E phonon in
monolayer graphene has been investigated in magnetic fields up to 29 T. The
hybridization of the E phonon with only the fundamental inter Landau
level excitation (involving the n=0 Landau level) is observed and only in one
of the two configurations of the circularly crossed polarized excitation and
scattered light. This polarization anisotropy of the magneto-phonon resonance
is shown to be inherent to relatively strongly doped graphene samples, with
carrier concentration typical for graphene deposited on SiO
Fine structure of -excitons in multilayers of transition metal dichalcogenides
Reflectance and magneto-reflectance experiments together with theoretical
modelling based on the approach have been employed to study
the evolution of direct bandgap excitons in MoS layers with a thickness
ranging from mono- to trilayer. The extra excitonic resonances observed in
MoS multilayers emerge as a result of the hybridization of Bloch states of
each sub-layer due to the interlayer coupling. The properties of such excitons
in bi- and trilayers are classified by the symmetry of corresponding crystals.
The inter- and intralayer character of the reported excitonic resonances is
fingerprinted with the magneto-optical measurements: the excitonic -factors
of opposite sign and of different amplitude are revealed for these two types of
resonances. The parameters describing the strength of the spin-orbit
interaction are estimated for bi- and trilayer MoS.Comment: 14 pages, 10 figure
Light and electric field control of ferromagnetism in magnetic quantum structures
A strong influence of illumination and electric bias on the Curie temperature
and saturation value of the magnetization is demonstrated for semiconductor
structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well
placed in various built-in electric fields. It is shown that both light beam
and bias voltage generate an isothermal and reversible cross-over between the
paramagnetic and ferromagnetic phases, in the way that is predetermined by the
structure design. The observed behavior is in quantitative agreement with the
expectations for systems, in which ferromagnetic interactions are mediated by
the weakly disordered two-dimensional hole liquid.Comment: 4 pages and 3 figure
Probing and manipulating valley coherence of dark excitons in monolayer WSe
Monolayers of semiconducting transition metal dichalcogenides are
two-dimensional direct-gap systems which host tightly-bound excitons with an
internal degree of freedom corresponding to the valley of the constituting
carriers. Strong spin-orbit interaction and the resulting ordering of the
spin-split subbands in the valence and conduction bands makes the lowest-lying
excitons in WX (X~being S or Se) spin-forbidden and optically dark. With
polarization-resolved photoluminescence experiments performed on a WSe
monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic
exchange interaction in combination with the applied in-plane and/or
out-of-plane magnetic fields enables one to probe and manipulate the valley
degree of freedom of the dark excitons.Comment: Manuscript: 6 pages, 3 figures; SM: 6 pages, 5 figure
Single spin optical read-out in CdTe/ZnTe quantum dot studied by photon correlation spectroscopy
Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe
quantum dot is investigated by polarization-resolved correlation spectroscopy.
Spin memory effects extending over at least a few tens of nanoseconds have been
directly observed in magnetic field and described quantitatively in terms of a
simple rate equation model. We demonstrate an effective (68%) all-optical
read-out of the single carrier spin state through probing the degree of
circular polarization of exciton emission after capture of an oppositely
charged carrier. The perturbation introduced by the pulsed optical excitation
serving to study the spin dynamics has been found to be the main source of the
polarization loss in the read-out process. In the limit of low laser power the
read-out efficiency extrapolates to a value close to 100%. The measurements
allowed us as well to determine neutral exciton spin relaxation time ranging
from 3.4 +/- 0.1 ns at B = 0 T to 16 +/- 3 ns at B = 5 T.Comment: to appear in Phys. Rev.
Neutral and charged dark excitons in monolayer WS
Low temperature and polarization resolved magneto-photoluminescence
experiments are used to investigate the properties of dark excitons and dark
trions in a monolayer of WS encapsulated in hexagonal BN (hBN). We find
that this system is an -type doped semiconductor and that dark trions
dominate the emission spectrum. In line with previous studies on WSe, we
identify the Coulomb exchange interaction coupled neutral dark and grey
excitons through their polarization properties, while an analogous effect is
not observed for dark trions. Applying the magnetic field in both perpendicular
and parallel configurations with respect to the monolayer plane, we determine
the g-factor of dark trions to be -8.6. Their decay rate is close to 0.5
ns, more than 2 orders of magnitude longer than that of bright excitons.Comment: 6 pages, 6 figures, supplemental materia
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