Search

We investigate the energy levels and persistent currents of MoS 2 quantum rings having different shapes and edge types in the presence of a perpendicular magnetic field by means of the tight-binding approach. We find states localized at the inner and outer boundaries of the ring. These energy levels exhibit different magnetic field dependences for the inner and outer ring states due to their different localization properties. They both exhibit the usual Aharanov–Bohm oscillations but with different oscillation periods. In the presence of spin–orbit coupling, we show distinct spin and charge persistent currents for inner and outer ring states. We find well-defined spin currents with negligibly small charge currents. This is because the local currents of spin-up and -down states flow in opposite directions. [ABSTRACT FROM AUTHOR]

In this study, we generalize the impulse response approach and Poisson statistics proposed by Smith [Phys. Rev. B 64, 155106 (2001)] to evaluate the longitudinal and transverse magneto-optical conductivities in an electron gas system in Faraday geometry. Comparing with the standard Drude model, the coefficients an are introduced in the Drude-Smith formula to describe the backscattering or localization effect for the nth electronic scattering event. Such a formula can also be applied to study the elements of the dielectric function matrix in the presence of magnetic and radiation fields in electron gas systems. This theoretical work is primely motivated by recent experimental activities in measuring the real and imaginary parts of longitudinal and transverse magneto-optical conductivities in condensed matter materials and electronic devices using terahertz time-domain spectroscopy. We believe that the results obtained from this study can provide an appropriate theoretical tool in reproducing the experimental findings and in fitting with experimental data to determine the important sample and material parameters. [ABSTRACT FROM AUTHOR]

We present a theoretical study on plasmon-polariton modes in graphene placed inside an optical cavity and driven by a source-to-drain electric field. The electron velocity and electron temperature are determined by solving self-consistently the momentum- and energy-balance equations in which electron interactions with impurities, acoustic-, and optic-phonons are included. Based on manybody self-consistent field theory, we develop a tractable approach to study plasmon-polariton in an electron gas system. We find that when graphene is placed inside a Fabry-Pérot cavity, two branches of the plasmon-polariton modes can be observed and these modes are very much optic- or plasmon-like. The frequencies of these modes depend markedly on driving electric field especially at higher resonant frequency regime. Moreover, the plasmon-polariton frequency in graphene is in terahertz (THz) bandwidth and can be tuned by changing the cavity length, gate voltage, and driving electric field. This work is pertinent to the application of graphene-based structures as tunable THz plasmonic devices [ABSTRACT FROM AUTHOR]

We present a detailed theoretical study on the optoelectronic properties of topological insulator thin film (TITFs). The k . p approach is employed to calculate the energy spectra and wave functions for both the bulk and surface states in the TITF. With these obtained results, the optical conductivities induced by different electronic transitions among the bulk and surface states are evaluated using the energy-balance equation derived from the Boltzmann equation. We find that for Bi2Se3-based TITFs, three characteristic regimes for the optical absorption can be observed. (i) In the low radiation frequency regime (photon energy ℏω < 200 meV), the free-carrier absorption takes place due to intraband electronic transitions. An optical absorption window can be observed. (ii) In the intermediate radiation frequency regime (200 < ℏω < 300 meV), the optical absorption is induced mainly by interband electronic transitions from surface states in the valance band to surface states in the conduction band and an universal value σ0 = e2=(8ℏ) for the optical conductivity can be obtained. (iii) In the high radiation frequency regime (ℏω > 300 meV), the optical absorption can be achieved via interband electronic transitions from bulk and surface states in the valance band to bulk and surface states in the conduction band. A strong absorption peak can be observed. These interesting findings indicate that optical measurements can be applied to identify the energy regimes of bulk and surface states in the TITF. [ABSTRACT FROM AUTHOR]

We present a theoretical study on the thermoelectric properties of two-dimensional topological insulators (2DTIs) doped with nonmagnetic impurities. We develop a tractable model to calculate the electronic band structure without additional input parameters and to evaluate the thermoelectric properties of 2DTIs based on CdTe/HgTe quantum wells. We find that with increasing the doping concentration of nonmagnetic impurity, the edge states dominate the thermoelectric transport and the bulk-state conduction is largely suppressed. For typical sample parameters, the thermoelectric figure of merit ZT (a quantity used to characterize the conversion efficiency of a thermoelectric device between the heat and electricity) can be much larger than 1, which is a great advance over conventional thermoelectric materials. Furthermore, we show that with decreasing the 2DTI ribbon width or the Hall-bar width, ZT can be considerably further improved. These results indicate that the CdTe/HgTe 2DTIs doped with nonmagnetic impurities can be potentially applied as high-efficiency thermoelectric materials and devices. [ABSTRACT FROM AUTHOR]

The Sr3(Ti0.95R0.05)2O7 (R = Ta, Nb, W) polycrystalline compounds were fabricated, and their transport and thermoelectric properties were investigated. The results indicate that at T > 300 K electrical resistivity ρ for all the doped compounds increases monotonically with temperature, and basically can be described by a relation ρ ∝ TM at T > ∼650 K, with M = 1.39, 1.66, and 1.77 for R = Ta, Nb, and W, respectively, implying that at the high temperatures the acoustic phonon scattering dominates the scattering process. Although the resistivity ρ of Sr3(Ti0.95Ta0.05)2O7 exhibits a metallic-like behavior at the temperature as low as 5 K, a transition from metallic state (dρ/dT > 0) to semiconductor-like state (dρ/dT < 0) was observed at a critical low temperature ∼41 K and ∼79 K for R = Nb and W, respectively. At T < ∼22 K, ∼57 K, and ∼80 K, a relation of σ ∝ T1/2 (here conductivity σ = 1/ρ) holds for the doped compounds with R = Nb, Ta, and W, respectively, suggesting that at the low temperatures the main transport mechanism is electron-electron interaction due to the presence of disorder induced by the dopants. The thermoelectric figure of merit (ZT) for Ta-doped compound increases more steeply with increasing temperature among the three compounds and reaches 0.066 at 1000 K. [ABSTRACT FROM AUTHOR]

We present a systematic theoretical study on optical properties of short-period InAs/GaSb type-II superlattices (SLs) which can serve for midinfrared (MIR) detection. Using the standard Kronig–Penney model we calculate the miniband structure of such SLs. The obtained band-gap energies are in line with those realized experimentally. On the basis of the energy-balance equation derived from the Boltzmann equation we develop a simple approach to calculate the optical absorption coefficient for the corresponding SL systems. Our results agree with recent experimental findings. Moreover, the dependence of MIR absorption in InAs/GaSb type-II SLs on temperature and well width are examined. This study is pertinent to the application of InAs/GaSb type-II SLs as MIR photodetectors working in the ambient condition. [ABSTRACT FROM AUTHOR]