Wrocław University of Technology

About: Wrocław University of Technology is a education organization based out in Wrocław, Poland. It is known for research contribution in the topics: Laser & Fuzzy logic. The organization has 13115 authors who have published 31279 publications receiving 338694 citations.

Probing of free and localized excitons and trions in atomically thin WSe2, WS2, MoSe2 and MoS2 in photoluminescence and reflectivity experiments.

Abstract: We report on detailed temperature dependent (T = 7-295 K) optical spectroscopy studies of WSe2, WS2, MoSe2 and MoS2 monolayers exfoliated onto the same SiO2/Si substrate. In the high energy region of absorption type (reflectivity contrast-RC) and emission (photo-luminescence-PL) spectra of all the monolayers resonances related to the neutral and charged excitons (X and T) are detected in the entire measured temperature range. The optical amplitudes of excitons and trions strongly depend on the temperature and two dimensional carrier gas (2DCG) concentration. In the low energy PL spectra of WSe2 and WS2 we detect a group of lines (L) which dominates the spectra at low temperatures but rapidly quenches with the increase in the temperature. Interestingly, in the same energy range of the RC spectra recorded for WS2, we observe an additional line (L 0), which behaves in the same way as the L lines in the PL spectra. The optical amplitude of L 0 and T resonances in the RC spectra strongly increases with the growth of the 2DCG concentration. On the base of these observations we identify the L 0 resonance in the RC spectra as arising from the fine structure of the trion. We also propose that the line interpreted previously in PL spectra of WSe2 and WS2 as related to the biexciton emission is a superposition of the biexciton, trion and localized exciton emission. We find that with the temperature increase from 7-295 K the total PL intensity decreases moderately in WSe2 and WS2, strongly in MoS2 and dramatically in MoSe2.

Charged excitons in a dilute two-dimensional electron gas in a high magnetic field

Abstract: A theory of charged excitons ${X}^{\ensuremath{-}}$ in a dilute two-dimensional (2D) electron gas in a high-magnetic field is presented. In contrast to previous calculations, three bound ${X}^{\ensuremath{-}}$ states (one singlet and two triplets) are found in a narrow and symmetric GaAs quantum well. The singlet and a ``bright'' triplet are the two optically active states observed in experiments. The bright triplet has the binding energy of about 1 meV, smaller than the singlet and a ``dark'' triplet. The interaction of bound ${X}^{\ensuremath{-}}\mathrm{'}\mathrm{s}$ with a dilute 2D electron gas is investigated using exact diagonalization techniques. It is found that the short-range character of the $e--{X}^{\ensuremath{-}}$ interactions effectively isolates bound ${X}^{\ensuremath{-}}$ states from a dilute $e--h$ plasma. This results in the insensitivity of the photoluminescence spectrum to the filling factor $\ensuremath{ u},$ and a rapid decrease of the oscillator strength of the dark triplet ${X}^{\ensuremath{-}}$ as a function of ${\ensuremath{ u}}^{\ensuremath{-}1}.$

Fractional Fokker-Planck equation with tempered α-stable waiting times: langevin picture and computer simulation.

Abstract: In this paper we introduce a Langevin-type model of subdiffusion with tempered α-stable waiting times. We consider the case of space-dependent external force fields. The model displays subdiffusive behavior for small times and it converges to standard Gaussian diffusion for large time scales. We derive general properties of tempered anomalous diffusion from the theory of tempered α-stable processes, in particular we find the form of the fractional Fokker-Planck equation corresponding to the tempered subdiffusion. We also construct an algorithm of simulation of sample paths of the introduced process. We apply the algorithm to approximate solutions of the fractional Fokker-Planck equation and to study statistical properties of the tempered subdiffusion via Monte Carlo methods.

Molecular Structures and Infrared Spectra of p-Chlorophenol and p-Bromophenol. Theoretical and Experimental Studies

Abstract: The molecular structures of p-chlorophenol and p-bromophenol have been calculated with the MP2, DFT(hybrid), and HF methods using the extended 6-311++G(df, pd) basis set. The geometrical parameters of p-ClPh and pBrPh in the gas phase have not been reported as yet. The results show that substitution of phenol with σ-electron-withdrawing groups (Br and Cl) leads to small shortening of the C−C and C−O bonds and small changes in the CCC angles. The structural changes of the phenol ring are governed mainly by the electronegativity of the para-substituent and, to a lesser extend, by resonance factors. The FT-IR spectra of p-ClPh, p-BrPh, and their OD counterparts were measured in CCl4 and cyclohexane solutions in the frequency range of 3700−400 cm-1, and the integrated infrared intensities were determined. The theoretical harmonic frequencies and infrared intensities were calculated for all the molecules using the DFT and HF methods. The best overall agreement between the calculated and experimental spectra ha...