259 research outputs found
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
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
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
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
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
ERA Critical Issues Report: Report of the Forward Visions on the European Research Area (VERA) project
Identification of fracture toughness parameters to understand the fracture resistance of advanced high strength sheet steels
The fracture toughness of four advanced high strength steel (AHSS) thin sheets is evaluated through different characterization methodologies, with the aim of identifying the most relevant toughness parameters to describe their fracture resistance. The investigated steels are: a Complex Phase steel, a Dual Phase steel, a Trip-Aided Bainitic Ferritic steel and a Quenching and Partitioning steel. Their crack initiation and propagation resistance is assessed by means of J-integral measurements, essential work of fracture tests and Kahn-type tear tests. The results obtained from the different methodologies are compared and discussed, and the influence of different parameters such as specimen geometry or notch radius is investigated. Crack initiation resistance parameters are shown to be independent of the specimen geometry and the testing method. However, significant differences are found in the crack propagation resistance values. The results show that, when there is a significant energetic contribution from necking during crack propagation, the specific essential work of fracture (we) better describes the overall fracture resistance of thin AHSS sheets than JC. In contrast, energy values obtained from tear tests overestimate the crack propagation resistance and provide a poor estimation of AHSS fracture performance. we is concluded to be the most suitable parameter to describe the global fracture behaviour of AHSS sheets and it is presented as a key property for new material design and optimization.Peer ReviewedPostprint (author's final draft
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