RESUMEN
Band crossings observed in a wide range of condensed matter systems are recognized as a key to understand low-energy fermionic excitations that behave as massless Dirac particles. Despite rapid progress in this field, the exploration of non-equilibrium topological states remains scarce and it has potential ability of providing a new platform to create unexpected massless Dirac states. Here we show that in a semiconductor quantum-well driven by a cw-laser with linear polarization, the optical Stark effect conducts bulk-band crossing, and the resulting Floquet-Dirac semimetallic phase supports an unconventional edge state in the projected one-dimensional Brillouin zone under a boundary condition that an electron is confined in the direction perpendicular to that of the laser polarization. Further, we reveal that this edge state mediates a transition between topological and non-topological edge states that is caused by tuning the laser intensity. We also show that the properties of the edge states are strikingly changed under a different boundary condition. It is found that such difference originates from that nearly fourfold-degenerate points exist in a certain intermediate region of the bulk Brillouin zone between high-symmetry points.
RESUMEN
Quantum control of excitonic Floquet states in semiconductor superlattices driven by an intense monochromatic laser is examined from a theoretical point of view. High-resolution optical absorption spectra, calculated using multichannel scattering theory with the R-matrix propagation method, clarified that the excitonic Fano resonance structure is induced by the laser field. Each of the physical quantities related to this resonance-such as the spectral intensity, an asymmetry parameter (q-parameter), a resonance width, and so on-shows a characteristic extremum as a function of laser strength (F(ac)) in the vicinity of a critical value of F(ac) where dynamic localization is realized. It has also been shown that this F(ac)-dependence is caused by an ac-Zener coupling between two photon sidebands. Further, we have shown that these quantities are also controlled by changing the laser frequency (ω), as well as F(ac), and the underlying physics is explained on the basis of anticrossing behavior of the two photon sidebands.
RESUMEN
The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor (EDO-TTF)2PF6 has been examined by ultrafast reflective measurements and time-dependent model calculations incorporating both electron-electron and electron-phonon interactions. The transient optical conductivity spectrum over a wide probe photon-energy range revealed that photoexcitation induced a new type of charge-disproportionate state. Additionally, coherent and incoherent oscillations dependent on probe photon energies were found, as predicted by the calculation.
RESUMEN
We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long-range order.