31 research outputs found
Magnetically tunable exciton valley coherence in monolayer WS mediated by the electron-hole exchange and exciton-phonon interactions
We develop a model, which incorporates both intra- and intervalley
scatterings to master equation, to explore exciton valley coherence in
monolayer WS subjected to magnetic field. For linearly polarized (LP)
excitation accompanied with an initial coherence, our determined valley
dynamics manifests the coherence decay being faster than the exciton population
relaxation, and agrees with experimental data by Hao et al.[Nat. Phys. 12, 677
(2016)]. Further, we reveal that magnetic field may quench the electron-hole
(e-h) exchange induced pure dephasing -- a crucial decoherence source -- as a
result of lifting of valley degeneracy, allowing to magnetically regulate
valley coherence. In particular, at low temperatures for which the
exciton-phonon (ex-ph) interaction is weak, we find that the coherence time is
expected to attain ps, facilitating full control
of qubits based on the valley pseudospin. For dark excitons, we demonstrate an
emerging coherence even in the absence of initial coherent state, which has a
long coherence time ( ps) at low temperature. Our work provides an
insight into tunable valley coherence and coherent valley control based on dark
excitons.Comment: 7 pages, 4 figure
Electronic Structure and Topological Quantum Phase Transitions in Strained Graphene Nanoribbons
In this chapter, we discuss the new classes of matter, such as the quantum spin Hall (QSH) and quantum anomalous Hall (QAH) states, that have been theoretically predicted and experimentally observed in graphene and beyond graphene systems. We further demonstrate how to manipulate these states using mechanical strain, internal exchange field, and spin‐orbit couplings (SOC). Spin‐charge transport in strained graphene nanoribbons is also discussed assuming the system in the QAH phase, exploring the prospects of topological devices with dissipationless edge currents. A remarkable zero‐field topological quantum phase transition between the time‐reversal‐symmetry‐broken QSH and quantum anomalous Hall states is predicted, which was previously thought to take place only in the presence of external magnetic field. In our proposal, we show as the intrinsic SOC is tuned, how it is possible to two different helicity edge states located in the opposite edges of the graphene nanoribbons exchange their locations. Our results indicate that the strain‐induced pseudomagnetic field could be coupled to the spin degrees of freedom through the SOC responsible for the stability of a QSH state. The controllability of this zero‐field phase transition with strength and direction of the strain is also explored as additional phase‐tuning parameter. Our results present prospect of strain, electric and magnetic manipulation of the QSH, and QAH effect in these novel two‐dimensional (2D) materials
Anomalous shift of the recombination energy in single asymmetric quantum wells
Self-consistent numerical calculation and photoluminescence (PL) measurements have been used to investigate the temperature dependence of the optical Stark effect in n-doped GaAs/AlGaAs single asymmetric quantum wells (SAQWs), grown by molecular beam epitaxy. In the low-temperature regime (5 to 40 K) a remarkable blue shift (9.8 meV) is observed in the PL peak energy, as the optical excitation intensity increases from 0.03 to 90 W/cm2. The blue shift is well explained by the reduction of the two-dimensional electron gas (2DEG) density, due to a charge-transfer mechanism. At about 80 K, however, an anomalous behavior of the PL peak energy was found, i.e. a red shift has been observed as the optical excitation intensity increases. This anomalous behavior has been explained by combining the effects of band gap renormalization, band bending, temperature dependence of the band gap, temperature dependence of the 2DEG density, and temperature dependence of the fundamental energy position
Electron-phonon induced spin relaxation in InAs quantum dots
We have calculated spin relaxation rates in parabolic quantum dots due to the
phonon modulation of the spin-orbit interaction in presence of an external
magnetic field. Both, deformation potential and piezoelectric electron-phonon
coupling mechanisms are included within the Pavlov-Firsov spin-phonon
Hamiltonian. Our results have demonstrated that, in narrow gap materials, the
electron-phonon deformation potential and piezoelectric coupling give
comparable contributions as spin relaxation processes. For large dots, the
deformation potential interaction becomes dominant. This behavior is not
observed in wide or intermediate gap semiconductors, where the piezoelectric
coupling, in general, governs the spin relaxation processes. We also have
demonstrated that spin relaxation rates are particularly sensitive to the
Land\'e -factor.Comment: 4 pages, 2 figures, to be appear in Physica E: Proceedings of the 11
International Conference on Narrow Gap Semiconductor
Second-order-like cluster-monomer transition within magnetic fluids and its impact upon the magnetic susceptibility
The low-field (below 5 Oe) ac and dc magnetic response of a magnetic fluid [MF] sample in the range of 305 to 360 K and 410 to 455 K was experimentally and theoretically investigated. We found a systematic deviation of Curie's law, which predicts a linear temperature dependence of inverse initial susceptibility in the range of our investigation. This finding, as we hypothesized, is due to the onset of a second-order-like cluster-to-monomer transition with a critical exponent which is equal to 0.50. The susceptibility data were well fitted by a modified Langevin function, in which cluster dissociation into monomers, at the critical temperature [T*], was included. In the ac experiments, we found that T* was reducing from 381.8 to 380.4 K as the frequency of the applied field increases from 123 to 173 Hz. In addition, our ac experiments confirm that only monomers respond for the magnetic behavior of the MF sample above T*. Furthermore, our Monte Carlo simulation and analytical results support the hypothesis of a thermal-assisted dissociation of chain-like structures