Unconventional quantum Hall effect in Floquet topological insulators

Abstract

We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity σyx = 0 at zero Fermi energy, to a Hall insulator state with σyx = e 2 /2h. These findings open new possibilities for experimentally realizing nontrivial quantum states and unusual quantum Hall plateaux at (±1/2, ±3/2, ±5/2, ...)e 2 /h. In the regime of the integral quantum Hall effect (IQHE) for conventional, two-dimensional (2D) systems, e.g., in a GaAs/AlGaAs heterostructure, the Hall conductivity takes the values 2(n+1)e 2 /h = (2, 4, 6, ...) e 2 /h, where h is the Planck constant, e the electron charge, and n an integer. In graphene though the IQHE plateaux appear at 4(n + 1/2)e 2 /h = (±2, ±6, ±10, ...) e 2 /h [1], and the "half integer" aspect is hidden under the 4-fold degeneracy associated with the spin and valley degrees of freedom [2]. More recently, the IQHE has been assessed for silicene [3] and MoS 2 [4] in which the spinorbit interaction rearranges, as in 2D systems [5], the Landau levels (LLs) in two groups and the plateaux appear at integer values of e 2 /h due to a double degeneracy (±0, ±1, ±2, ±4, ±6, ...). In topological insulators (TIs) electrons on both top and bottom surfaces contribute to the Hall conductivity and its plateaux, due to the surface degeneracy, have heights 2(n + 1/2) e 2 /h = (±0, ±1, ±3, ±5, ...)e 2 /h [6-8]. Though the quest for a genuine "half-integer" QHE is long, a QHE like (n + 1/2) e 2 /h = (±0, ±1/2, ±3/2, ±5/2, ...) e 2 /h without any degeneracy prefactor, has not been observed. Then one wonders whether a "half-integer" QHE is possible in TIs by breaking their surface degeneracy.TIs More recently, the surface states of TIs driven by circularly polarized off-resonant light have become a subject of strong interest [17][18][19][20]. TIs driven by external timeperiodic perturbations are known as Floquet TIs (FTIs). * udo.schwingenschlogl@kaust.edu.sa For such systems it is convenient to use the Floquet theory [18]. In the appropriate frequency regime the offresonant light cannot generate real photon absorption or emission due to energy conservation. Accordingly, it does not directly excite electrons but instead modifies the electron band structure through second-order virtual-photon absorption processes. Averaged over time these processes result in an effective static alteration of the band structure. Illuminating, e.g., graphene or silicene with offresonant light generates a Haldane-type gap [21].Floquet bands were first realized in photonic crystals [22] and have been verified by recent experiments on the surface states of...

Room temperature nanoparticulate interfacial layers for perovskite solar cells via solvothermal synthesis

“…Both the binding energy of Cu 2p3/2 and the fine structure of the satellite are characteristic of CuO and are in agreement with literature. [36][37] This is an experimental evidence that high quality CuO layers are formed on top of ITO without any annealing treatment. Another important requirement for the bottom electrode of perovskite solar cells is its surface wetting properties.…”