Equilibration of quantum Hall edge states by an Ohmic contact

Ohmic contacts are crucial elements of electron optics that have not received a clear theoretical description yet. We propose a model of an Ohmic contact as a piece of metal of the finite capacitance $C$ attached to a quantum Hall edge. It is shown that charged quantum Hall edge states may have weak coupling to neutral excitations in an Ohmic contact. Consequently, despite being a reservoir of neutral excitations, an Ohmic contact is not able to efficiently equilibrate edge states if its temperature is smaller than $\hbar\Omega_c$, where $\Omega_c$ is the inverse RC time of the contact. This energy scale for a floating contact may become as large as the single-electron charging energy $e^2/ C$.Comment: 5 pages, 4 figures; revised versio

Synthesizing and multiplexing autonomous quantum coherences

Quantum coherence is a crucial prerequisite for quantum technologies. Therefore, the robust generation, as autonomous as possible, of quantum coherence remains the essential problem for developing this field. We consider a method of synthesizing and multiplexing quantum coherence from spin systems without any direct drives only coupled to bosonic baths. The previous studies in this field have demonstrated that a back-action of the bath to the spin subsystem is important to generate it, however, it simultaneously gives significant limits to the generated coherence. We propose a viable approach with the bosonic bath that allows overcoming these limits by avoiding the destructive effect of the back-action processes. Using this approach, we suggest an advanced synthesis of the quantum coherence non-perturbatively in the spin-boson coupling parameters of multiple bosonic baths to increase and multiplex it for upcoming proof-of-principle experiments.Comment: 28 pages, 6 figure

Zeeman spectroscopy of excitons and hybridization of electronic states in few-layer WSe2_2, MoSe2_2 and MoTe2_2

Monolayers and multilayers of semiconducting transition metal dichalcogenides (TMDCs) offer an ideal platform to explore valley-selective physics with promising applications in valleytronics and information processing. Here we manipulate the energetic degeneracy of the $\mathrm{K}^+$ and $\mathrm{K}^-$ valleys in few-layer TMDCs. We perform high-field magneto-reflectance spectroscopy on WSe$_2$, MoSe$_2$, and MoTe$_2$ crystals of thickness from monolayer to the bulk limit under magnetic fields up to 30 T applied perpendicular to the sample plane. Because of a strong spin-layer locking, the ground state A excitons exhibit a monolayer-like valley Zeeman splitting with a negative $g$-factor, whose magnitude increases monotonically when thinning the crystal down from bulk to a monolayer. Using the $\mathbf{k\cdot p}$ calculation, we demonstrate that the observed evolution of $g$-factors for different materials is well accounted for by hybridization of electronic states in the $\mathrm{K}^+$ and $\mathrm{K}^-$ valleys. The mixing of the valence and conduction band states induced by the interlayer interaction decreases the $g$-factor magnitude with an increasing layer number. The effect is the largest for MoTe$_2$, followed by MoSe$_2$, and smallest for WSe$_2$. Keywords: MoSe$_2$, WSe$_2$, MoTe$_2$, valley Zeeman splitting, transition metal dichalcogenides, excitons, magneto optics.Comment: 14 pages, 5 figure

Analogy and dissimilarity of excitons in monolayer and bilayer of MoSe2_2

Excitons in thin layers of semiconducting transition metal dichalcogenides are highly subject to the strongly modified Coulomb electron-hole interaction in these materials. Therefore, they do not follow the model system of a two-dimensional hydrogen atom. We investigate experimentally and theoretically excitonic properties in both the monolayer (ML) and the bilayer (BL) of MoSe$_2$ encapsulated in hexagonal BN. The measured magnetic field evolutions of the reflectance contrast spectra of the MoSe$_2$ ML and BL allow us to determine $g$-factors of intralayer A and B excitons, as well as the $g$-factor of the interlayer exciton. We explain the dependence of $g$-factors on the number of layers and excitation state using first principles calculations. Furthermore, we demonstrate that the experimentally measured ladder of excitonic $s$ states in the ML can be reproduced using the $\mathbf{k\cdot p}$ approach with the Rytova-Keldysh potential that describes the electron-hole interaction. In contrast, the analogous calculation for the BL case requires taking into account the out-of-plane dielectric response of the MoSe$_2$ BL.Comment: 10 pages, 4 figures, + S

Rydberg series of dark excitons and the conduction band spin-orbit splitting in monolayer WSe2

Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction are at the heart of studies of the rich physics of excitons in monolayers of transition metal dichalcogenides (TMD). Those archetypes of two-dimensional systems promise a design of new optoelectronic devices. In intrinsic TMD monolayers the basic, intravalley excitons, are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is dark, although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in tungsten diselenide monolayer, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band. Excitonic physics dominates the optical response of semiconductor monolayers but single particle band structure parameters are hard to probe experimentally. Here, spin-orbit splitting in the conduction band of monolayer WSe2 is revealed by the identification of the Rydberg series of dark excitons

Ultrafast Dynamics of Valley-Polarized Excitons in WSe₂ Monolayer Studied by Few-Cycle Laser Pulses

We report on the experimental investigation of the ultrafast dynamics of valley-polarized excitons in monolayer WSe2 using transient reflection spectroscopy with few-cycle laser pulses with 7 fs duration. We observe that at room temperature, the anisotropic valley population of excitons decays on two different timescales. The shorter decay time of approximately 120 fs is related to the initial hot exciton relaxation related to the fast direct recombination of excitons from the radiative zone, while the slower picosecond dynamics corresponds to valley depolarization induced by Coloumb exchange-driven transitions of excitons between two inequivalent valleys

The optical response of monolayer, few-layer and bulk tungsten disulfide

International audienceWe present a comprehensive optical study of thin flakes of tungsten disulfide (WS2) with thickness ranging from mono-to octalayer and in the bulk limit. It is shown that the optical band-gap absorption of monolayer WS2 is governed by competing resonances arising from one neutral and two distinct negatively charged excitons whose contributions to the overall absorption of light vary as a function of temperature and carrier concentration. The photoluminescence response of monolayer WS2 is found to be largely dominated by disorder/impurity-and/or phonon-assisted recombination processes. The indirect band-gap luminescence in multilayer WS2 turns out to be a phonon-mediated process whose energy evolution with the number of layers surprisingly follows a simple model of a two-dimensional confinement. The energy position of the direct band-gap response (A and B resonances) is only weakly dependent on the layer thickness, which underlines an approximate compensation of the effect of the reduction of the exciton binding energy by the shrinkage of the apparent band gap. The A-exciton absorption-type spectra in multilayer WS2 display a non-trivial fine structure which results from the specific hybridization of the electronic states in the vicinity of the K-point of the Brillouin zone. The effects of temperature on the absorption-like and photoluminescence spectra of various WS2 layers are also quantified