Generalized rotating-wave approximation for the quantum Rabi model with optomechanical interaction

We investigate the spectrum of energy and eigenstates of a hybrid cavity optomechanical system, where a cavity field mode interacts with a mechanical mode of a vibrating end mirror via radiation pressure and with a two level atom via electric dipole interaction. In the spirit of approximations developed for the quantum Rabi model beyond rotating-wave approximation (RWA), we implement the so-called generalized RWA (GRWA) to diagonalize the tripartite Hamiltonian for arbitrary large couplings. Notably, the GRWA approach still allows to rewrite the hybrid Hamiltonian in a bipartite form, like a Rabi model with dressed atom-field states (polaritons) coupled to mechanical modes through reparametrized coupling strenght and Rabi frequency. We found a more accurate energy spectrum for a wide range of values of the atom-photon and photon-phonon couplings, when compared to the RWA results. The fidelity between the numerical eigenstates and its approximated counterparts is also calculated. The degree of polariton-phonon entanglement of the eigenstates presents a non-monotonic behavior as the atom-photon coupling varies, in contrast to the characteristic monotonic increase in the RWA treatment

Spin and charge optical conductivities in spin-orbit coupled systems

We study the frequency dependent spin- and charge- conductivity tensors of a two-dimensional electron gas (2DEG) with Rashba and Dresselhaus spin-orbit interaction. We show that the angular anisotropy of the spin-splitting energy induced by the interplay between the Rashba and Dresselhaus couplings gives rise to a characteristic spectral behavior of the spin and charge response which is significantly different from that of pure Rashba or Dresselhaus case. Such new spectral structures open the possibility for control of the optical response by applying an external bias and/or by adjusting the light frequency. In addition, it is shown that the relative strength of the spin-orbit coupling parameters can be obtained through optical probing.Comment: 13 pages, 4 figures. Revised versio

Linear and nonlinear spin current response in anisotropic spin-orbit coupled systems

We calculate the linear and the second harmonic (SH) spin current response of two anisotropic systems with spin orbit (SO) interaction. The first system is a two-dimensional (2D) electron gas in the presence of Rashba and k-linear Dresselhaus SO couplings. The dependence of the anisotropic spin splitting on the sample growth direction introduces an additional path to modify the linear and nonlinear spectra. In particular, vanishing linear and second order spin conductivity tensors are achievable under SU(2) symmetry conditions, characterized by a collinear SO vector field. Additional conditions under which specific tensor components vanish are posible, without having such collinearity. Thus, a proper choice of the growth direction and SO strengths allows to select the polarization of the linear and SH spin currents according to the direction of flowing. The second system is an anisotropic 2D free electron gas with anisotropic Rashba interaction, which has been employed to study the optical conductivity of 2D puckered structures with anisotropic energy bands. The presence of mass anisotropy and an energy gap open several distinct scenarios for the allowed optical interband transitions, which manifest in the linear and SH response contrastingly. The linear response displays only out-of-plane spin polarized currents, while the SH spin currents flow with spin orientation lying parallel to the plane of the system strictly. The models illustrate the possibility of the nonlinear spin Hall effect in systems with SO interaction, under the presence or absence of time-reversal symmetry. The results suggest different ways to manipulate the linea

Thermal difference reflectivity of tilted 2D Dirac materials

Deviation from perfect conical dispersion in Dirac materials, such as the presence of mass or tilting, enhances control and directionality of electronic transport. To identify these signatures, we analyze the thermal derivative spectra of optical reflectivity in doped massive tilted Dirac systems. The density of states and chemical potential are determined as preliminary steps to calculate the optical conductivity tensor at finite temperature using thermal convolution. Changes in reflection caused by temperature variations enable clear identification of critical frequencies in the optical response. By measuring these spectral features in the thermoderivative spectrum, energy gaps and band structure tilting can be determined. A comparison is presented between the spectra of various low-energy Dirac Hamiltonians. Our findings suggest that thermal difference spectroscopy holds promise as a valuable technique for probing interband transitions of 2D Dirac fermion

Spin torque contribution to the a.c. spin Hall conductivity

Using the recently proposed definition of a conserved spin-current operator [J. Shi et al., Phys. Rev. Lett. 96, 076604 (2006)] we explore the frequency dependent spin Hall conductivity for a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit interaction in response to an oscillating electric field. We show that the optical spectrum of the spin Hall conductivity exhibit remarkable changes when the new definition of spin current is applied. Such behavior is mainly due to a significant contribution of the spin torque term which is absent in the conventional form of the spin current. In addition, it is observed that the magnitude and direction of the dynamic spin Hall current strongly depends on the electric field frequency as with the interplay of the spin-orbit coupling strengths.Comment: 8 pages, 4 figures, pape