Excitonic effect on the optical response in the one-dimensional two-band Hubbard model

Charles Allen Stafford

Hengyue Li, Junya Otsuki, M. Naka +1 more

Optical responses in an excitonic insulating (EI) system with strong electron correlation are studied. We adopt the two-orbital Hubbard model with a finite energy difference between the two orbitals where the spin state degree of freedom exists. This model is analyzed by the variational cluster approach. In order to include the local electron correlation effect, the vertex correction is taken into account in the formulation of the optical conductivity spectra. We calculate a finite-temperature phase diagram in which an EI phase appears between a low-spin band insulating state and a high-spin Mott insulating state. Characteristic components of the optical conductivity spectra consisting of a sharp peak and continuum appear in the EI phase. Integrated intensity almost follows the order parameter of the EI state, suggesting that this component is available to identify the EI phases and transitions.

Enrique Conejero Jarque, Viktor A. Malyshev

A theoretical study of the resonance optical response of assemblies of oriented short (as compared to an optical wavelength) linear Frenkel chains is carried out. Despite the fact that the energy spectrum of a single chain is composed of the bands of Frenkel exciton states, a two-level model is used to describe the optical response of a single linear chain. We account for only the (on-resonance) optical transition between the ground state and the state of the one-exciton band bottom as having the dominating oscillator strength as compared to the other states of the one-exciton manifold. The (off-resonance) process of creation of two excitons per chain is neglected because it requires a higher excitation frequency due to the quasi-fermionic nature of one-dimensional Frenkel excitons. A distribution of linear chains over length resulting in fluctuations of all exciton optical parameters, such as the transition frequency and dipole moment as well as the radiative rate, are taken explicitly into account. We show that both transmittivity and reflectivity of the film may behave in a bistable fashion, originating from saturation of the nonlinear refraction index, and analyze how the effects found depend on the film thickness and on the inhomogeneous width of the exciton optical transition. Estimates of the driving parameters show that films of oriented J-aggregates of polymethine dyes at low temperatures seem to be suitable species for the experimental verification of the behavior found.

W. Brenig

A. Niyazi, Dominique Geffroy, J. Kuneš

We present a dynamical mean-field study of two-particle dynamical response functions in a two-band Hubbard model across several phase transitions. We observe that the transition between the excitonic condensate and spin-state ordered state is continuous with a narrow strip of supersolid phase separating the two. Approaching transition from the excitonic condensate is announced by softening of the excitonic mode at the $M$ point of the Brillouin zone. Inside the spin-state ordered phase there is a magnetically ordered state with $2\ifmmode\times\else\texttimes\fi{}2$ periodicity, which has no precursor in the normal phase.

K. A. Al-Hassanieh, Adrian E. Feiguin, Fernando Agustín Reboredo +2 more