First principles investigation of transition-metal doped group-IV semiconductors: Rx{_x}Y1−x_{1-x} (R=Cr, Mn, Fe; Y=Si, Ge)

A number of transition-metal (TM) doped group-IV semiconductors, R$_{x}$Y$_{1-x}$ (R=Cr, Mn and Fe; Y=Si, Ge), have been studied by the first principles calculations. The obtained results show that antiferromagnetic (AFM) order is energetically more favored than ferromagnetic (FM) order in Cr-doped Ge and Si with $x$=0.03125 and 0.0625. In 6.25% Fe-doped Ge, FM interaction dominates in all range of the R-R distances while for Fe-doped Ge at 3.125% and Fe-doped Si at both concentrations of 3.125% and 6.25%, only in a short R-R range can the FM states exist. In the Mn-doped case, the RKKY-like mechanism seems to be suitable for the Ge host matrix, while for the Mn-doped Si, the short-range AFM interaction competes with the long-range FM interaction. The different origin of the magnetic orders in these diluted magnetic semiconductors (DMSs) makes the microscopic mechanism of the ferromagnetism in the DMSs more complex and attractive.Comment: 14 pages, 2 figures, 6 table

Internal Transitions of Two-Dimensional Charged Magneto-Excitons X-: Theory and Experiment

Internal spin-singlet and spin-triplet transitions of charged excitons X- in magnetic fields in quantum wells have been studied experimentally and theoretically. The allowed X- transitions are photoionizing and exhibit a characteristic double-peak structure, which reflects the rich structure of the magnetoexciton continua in higher Landau levels (LL's). We discuss a novel exact selection rule, a hidden manifestation of translational invariance, that governs transitions of charged mobile complexes in a magnetic field.Comment: 4 pages, 2 figures, submitted to Physica

Epitaxial Growth of the Diluted Magnetic Semiconductors CryGe1-y and CryMnxGe1-x-y

We report the epitaxial growth of CryGe1-y and CryMnxGe1-x-y(001) thin films on GaAs(001), describe the structural and transport properties, and compare the measured magnetic properties with those predicted by theory. The samples are strongly p-type, and hole densities increase with Cr concentration. The CryGe1-y system remains paramagnetic for the growth conditions and low Cr concentrations employed (y < 0.04), consistent with density functional theory predictions. Addition of Cr into the ferromagnetic semiconductor MnxGe1-x host systematically reduces the Curie temperature and total magnetization.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter

Biaxial strain tuning of exciton energy and polarization in monolayer WS2

We perform micro-photoluminescence and Raman experiments to examine the impact of biaxial tensile strain on the optical properties of WS2 monolayers. A strong shift on the order of -130 meV per % of strain is observed in the neutral exciton emission at room temperature. Under near-resonant excitation we measure a monotonic decrease in the circular polarization degree under applied strain. We experimentally separate the effect of the strain-induced energy detuning and evaluate the pure effect coming from biaxial strain. The analysis shows that the suppression of the circular polarization degree under biaxial strain is related to an interplay of energy and polarization relaxation channels as well as to variations in the exciton oscillator strength affecting the long-range exchange interaction.Comment: 29 pages, 11 figure