With both spin and valley degrees of freedom, the low-lying excitonic spectra of photo-excited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intra-valley bright exciton states as well as various intra-and inter-valley dark ones. The latter states can be classified as those of the spin-and momentum-forbidden dark excitons according to the violated optical selection rules. Because of their optical invisibility, these two types of the dark states are in principle hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have 1 arXiv:1811.06728v3 [cond-mat.mes-hall] 9 Apr 2019 been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photo-luminescence spectra of strained tungsten diselenide monolayers (WSe 2 -MLs) where the intra-valley spin-forbidden dark exciton lies in the lowest exciton states and other momentumforbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe 2 -ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin-and momentum-forbidden exciton states of strained WSe 2 -MLs in the temperature-dependent photo-luminescences and present the guiding principle to infer the relative energetic locations of the two types of dark excitons.

Twisted light carries a well-defined orbital angular momentum (OAM) of ℏ per photon. The quantum number of its OAM can be arbitrarily set, making it an excellent light source to realize high-dimensional quantum entanglement and ultrawide bandwidth optical communication structures. In spite of its interesting properties, twisted light interaction with solid state materials, particularly two-dimensional materials, is yet to be extensively studied via experiments. In this work, photoluminescence (PL) spectroscopy studies of monolayer molybdenum disulfide (MoS 2 ), a material with ultrastrong light−matter interaction due to reduced dimensionality, are carried out under photoexcitation of twisted light. It is observed that the measured spectral peak energy increases for every increment of of the incident light. The nonlinear -dependence of the spectral blue shifts is well accounted for by the analysis and computational simulation of this work. More excitingly, the twisted light excitation revealed the unusual lightlike exciton band dispersion of valley excitons in monolayer transition metal dichalcogenides. This linear exciton band dispersion is predicted by previous theoretical studies and evidenced via this work's experimental setup.

The intrinsic fine structure splittings (FSSs) of the exciton states of semiconductor quantum dots (QDs) are known to be the major obstacle for realizing the QD-based entangled photon pair emitters. In this study, we present a theoretical and computational investigation of the excitonic fine structures of droplet-epitaxial (DE) GaAs/AlGaAs QDs under the electro-mechanical control of micro-machined piezoelectricity actuators. From the group theory analysis with numerical confirmation based on the developed exciton theory, we reveal the general principle for the optimal design of micro-machined actuators whose application on to an elongated QD can certainly suppress its FSS. We show that the use of two independently tuning stresses is sufficient to achieve the FSS-elimination but is not always necessary as widely deemed. The use of a single tuning stress to eliminate the FSS of an elongated QD is possible as long as the crystal structure of the actuator material is in coincidence with that of the QD. As a feasible example, we show that a single symmetric bi-axial stress naturally generated from the (001) PMN-PT actuator can be used as a single tuning knob to make the full FSS-elimination for elongated DE GaAs QDs.

The thermal stability of the uniaxial anisotropies in Co86 Zr14 and Co91 Zr9 amorphous films prepared by rf sputtering are investigated through isothermal annealing in a static magnetic field. It is found that the stability is improved with increasing content of Zr. The relaxation curves have been fitted with calculated ones assuming the lognormal distribution of activation energy. The mean value of activation energy has been estimated at 2.5 eV for Co86 Zr14 and 1.7 eV for Co91 Zr9 films, respectively. The anisotropy increases with decreasing metal content, that can be explained by the precipitation caused by the nonmagnetic interaction in amorphous alloys.

Some previous experimental results about the concentration dependence of magnetic ani-sotropy for metalmetal amorphous alloys were analyzed. By introducing a new parameter, a model of tetrahedral directional order was proposed. The comparison with some experimental data shows good agreement.