arXiv subjects

Hossein Dehghani

Publications and source records attributed to Hossein Dehghani.

23 records · Page 2Linked to original sources

arXiv2016

Floquet topological systems in the vicinity of band crossings: Reservoir induced coherence and steady-state entropy production

Results are presented for an open Floquet topological system represented by Dirac fermions coupled to a circularly polarized laser and an external reservoir. It is shown that when the separation between quasi-energy bands becomes small, and comparable to the coupling strength to the reservoir, the reduced density matrix in the Floquet basis, even at steady-state, has non-zero off-diagonal elements, with the magnitude of the off-diagonal elements increasing with the strength of the coupling to the reservoir. In contrast, the coupling to the reservoir only weakly affects the diagonal elements, hence inducing an effective coherence. The steady-state reduced density matrix synchronizes with the periodic drive, and a Fourier analysis allows the extraction of the occupation probabilities of the Floquet quasi-energy levels. The lack of detailed balance at steady-state is quantified in terms of an entropy production rate, and it is shown that this equals the heat current flowing out of the system, and into the reservoir. It is also shown that the entropy production rate mainly depends on the off-diagonal components of the Floquet density matrix. Thus a stronger coupling to the reservoir leads to an enhanced entropy production rate, implying a more efficient removal of heat from the system, which in turn helps the system maintain coherence. Analytic expressions in the vicinity of the Dirac point are derived which highlights these results, and also indicates how the reservoir may be engineered to enhance the coherence of the system.

cond-mat.mes-hall↗

arXiv2016

Occupation probabilities and current densities of bulk and edge states of a Floquet topological insulator

Results are presented for the occupation probabilities and current densities of bulk and edge states of half-filled graphene in a cylindrical geometry, and irradiated by a circularly polarized laser. It is assumed that the system is closed, and that the laser has been switched on as a quench. Laser parameters corresponding to some representative topological phases are studied: one where the Chern number of the Floquet bands equals the number of chiral edge modes, a second where anomalous edge states appear in the Floquet Brillouin zone boundaries, and a third where the Chern number is zero, yet topological edge states appear at the center and boundaries of the Floquet Brillouin zone. Qualitative differences are found for the high frequency off-resonant and low frequency on-resonant laser with edge states arising due to resonant processes occupied with a high effective temperature on the one hand, while edge states arising due to off-resonant processes occupied with a low effective temperature on the other. For an ideal half-filled system where only one of the bands in the Floquet Brillouin zone is occupied and the other empty, particle-hole and inversion symmetry of the Floquet Hamiltonian implies zero current density. However the laser switch-on protocol breaks the inversion symmetry, resulting in a net cylindrical sheet of current density at steady-state. Due to the underlying chirality of the system, this current density profile is associated with a net charge imbalance between the top and bottom of the cylinders.

cond-mat.mes-hall↗

arXiv2015

Optical Hall conductivity of a Floquet Topological Insulator

Results are presented for the optical Hall conductivity of a Floquet topological insulator (FTI) for an ideal closed quantum system, as well as an open system in a nonequilibrium steady-state with a reservoir. The steady-state, even for the open system, is strongly dependent on the topological phase of the FTI, with certain phases showing a remarkable near-cancellation from pockets of Berry-curvature of opposite signs, leading to a suppressed low-frequency Hall conductivity, that also shows an anomalous temperature dependence, by increasing as the temperature of the reservoir is increased. Such a behavior is in complete contrast to heating, and arises because of a strong modification of the effective system-reservoir coupling by the laser. The Berry curvature of the Floquet modes is time-dependent, and its frequency components are found to control the main features of the high-frequency Hall conductivity.

cond-mat.mes-hall↗

arXiv2015

Out of equilibrium electrons and the Hall conductance of a Floquet topological insulator

Graphene irradiated by a circularly polarized laser has been predicted to be a Floquet topological insulator showing a laser-induced quantum Hall effect. A circularly polarized laser also drives the system out of equilibrium resulting in non-thermal electron distribution functions that strongly affect transport properties. Results are presented for the Hall conductance for two different cases. One is for a closed system such as a cold-atomic gas where transverse drift due to non-zero Berry curvature can be measured in time of flight measurements. For this case the effect of a circularly polarized laser that has been switched on suddenly is studied. The second is for an open system coupled to an external reservoir of phonons. While for the former, the Hall conductance is far from the quantized limit, for the latter, coupling to a sufficiently low temperature reservoir of phonons is found to produce effective cooling, and thus an approach to the quantum limit, provided the frequency of the laser is large as compared to the band-width. For laser frequencies comparable to the band-width, strong deviations from the quantum limit of conductance is found even for a very low temperature reservoir, with the precise value of the Hall conductance determined by a competition between reservoir induced cooling and the excitation of photo-carriers by the laser. For the closed system, the electron distribution function is determined by the overlap between the initial wavefunction and the Floquet states which can result in a Hall conductance which is opposite in sign to that of the open system.

cond-mat.mes-hall↗

arXiv2014

Dissipative Floquet Topological Systems

Motivated by recent pump-probe spectroscopies, we study the effect of phonon dissipation and potential cooling on the nonequilibrium distribution function in a Floquet topological state. To this end, we apply a Floquet kinetic equation approach to study two dimensional Dirac fermions irradiated by a circularly polarized laser, a system which is predicted to be in a laser induced quantum Hall state. We find that the initial electron distribution shows an anisotropy with momentum dependent spin textures whose properties are controlled by the switching-on protocol of the laser. The phonons then smoothen this out leading to a non-trivial isotropic nonequilibrium distribution which has no memory of the initial state and initial switch-on protocol, and yet is distinct from a thermal state. An analytical expression for the distribution at the Dirac point is obtained that is relevant for observing quantized transport.

cond-mat.mes-hall↗