217 research outputs found
Quadexciton cascade and fine structure splitting of the triexciton in a single quantum dot
We report the properties of emission lines associated with the cascaded
recombination of a quadexciton in single GaAlAs/AlAs quantum dots, studied by
means of polarization-resolved photoluminescence and single-photon correlation
experiments. It is found that photons which are emitted in a double-step 4X-3X
process preserve their linear polarization, similarly to the case of conserved
polarization of correlated photons in the 2X-X cascade. In contrast, an
emission of either co-linear or cross-linear pairs of photons is observed for
the 3X-2X cascade. Each emission line associated with the quadexciton cascade
shows doublet structure in the polarization-resolved photoluminescence
experiment. The maximum splitting is seen when the polarization axis is chosen
along and perpendicular to the [110] crystallographic direction. This effect is
ascribed to the fine structure splitting of the exciton and triexciton states
in the presence of an anisotropic confining potential of ae dot. We also show
that the splitting in the triexciton state surpasses that in the exciton state
by a factor up to eight and their ratio scales with the energy distance between
the 3X and X emission lines, thus, very likely, with a lateral size and/or a
composition of the dot.Comment: submitted to Physical Review
2s exciton-polariton revealed in an external magnetic field
We demonstrate the existence of the excited state of an exciton-polariton in
a semiconductor microcavity. The strong coupling of the quantum well heavy-hole
exciton in an excited 2s state to the cavity photon is observed in non-zero
magnetic field due to surprisingly fast increase of Rabi energy of the 2s
exciton-polariton in magnetic field. This effect is explained by a strong
modification of the wave-function of the relative electron-hole motion for the
2s exciton state.Comment: 5 pages, 5 figure
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
Experimental approval of the extended flat bands and gapped subbands in rhombohedral multilayer graphene
Graphene layers are known to stack in two stable configurations, namely ABA
or ABC stacking, with drastically distinct electronic properties. Unlike the
ABA stacking, little has been done to experimentally investigate the electronic
properties of ABC graphene multilayers. Here, we report the first magneto
optical study of a large ABC domain in a graphene multilayers flake, with ABC
sequences exceeding 17 graphene sheets. The ABC-stacked multilayers can be
fingerprinted with a characteristic electronic Raman scattering response, which
persists even at room temperatures. Tracing the magnetic field evolution of the
inter Landau level excitations from this domain gives strong evidence to the
existence of a dispersionless electronic band near the Fermi level,
characteristic of such stacking. Our findings present a simple yet powerful
approach to probe ABC stacking in graphene multilayer flakes, where this highly
degenerated band appears as an appealing candidate to host strongly correlated
states.Comment: 8 pages, 4 figure
Breathing modes in few-layer MoTe activated by h-BN encapsulation
The encapsulation of few-layer transition metal dichalcogenides (TMDs) in
hexagonal boron nitride (h-BN) is known to improve significantly their optical
and electronic properties. However, it may be expected that the h-BN
encapsulation may affect also vibration properties of TMDs due to an atomically
flat surface of h-BN layers. In order to study its effect on interlayer
interactions in few-layer TMDs, we investigate low-energy Raman scattering
spectra of bi- and trilayer MoTe. Surprisingly, three breathing modes are
observed in the Raman spectra of the structures deposited on or encapsulated in
h-BN as compared to a single breathing mode for the flakes deposited on a
SiO/Si substrate. The shear mode is not affected by changing the MoTe
environment. The emerged structure of breathing modes is ascribed to the
apparent interaction between the MoTe layer and the bottom h-BN flake. The
structure becomes visible due to a high-quality surface of the former flake.
Consequently, the observed triple structure of breathing modes originates from
the combination modes due to interlayer and layer-substrate interactions. Our
results confirm that the h-BN encapsulation affects substantially vibration
properties of layered materials.Comment: 10 pages, 9 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
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