Biexciton in one-dimensional Mott insulators

Abstract

The ultrafast dynamics of correlated electron systems after photoexcitation are now attracting considerable attention. This is based upon recent developments in femtosecond laser technology, which enabled us to detect ultrafast electronic responses to a light pulse in solids [1][2][3][4] . Applications of femtosecond pump-probe spectroscopy to correlated electron systems enable us to observe exotic photoinduced phase transitions 5-18 as represented by a photoinduced Mott-insulator-to-metal transition 6,12,[16][17][18] and also to derive detailed information about the interplays between the charge, spin, and lattice degrees of freedom from the transient responses of each degree to a light pulse [5][6][7][8][9][10][11][13][14][15] . Such information can hardly be obtained from the steady-state transport and magnetic measurements. The growing interest in the ultrafast dynamics of correlated electron systems synchronizes to the development of a new field called 'non-equilibrium quantum physics in solids'. In fact, new theoretical approaches have recently been explored to analyse the charge, spin, and lattice dynamics of non-equilibrium states after photoirradiation, as exemplified by the dynamical mean field theory 19 and the time-dependent density-matrix renormalization group method 20,21 .In the non-equilibrium quantum physics of correlated electron systems, the charge dynamics of photoexcited Mott insulators is the most fundamental subject to be studied from both the experimental 6,12,16-18 and theoretical viewpoints [22][23][24][25][26][27][28] . Recent studies have focussed on Mott insulator states realized not only in solids such as transition metal compounds 6,16-18 and organic molecular materials 12 but also in ultra-cold atoms on an optical lattice 29,30,31 . In fact, in the ultra-cold atoms, non-equilibrium dynamics can be investigated by tuning the intersite interaction using a Feshbach resonance 29 . Among various Mott insulators, a one-dimensional (1D) Mott insulator with large on-site Coulomb repulsion energy U is particularly important since the charge and spin degrees 4 of freedom are decoupled 32,33 , In the system, we can obtain clear information about the effects of Coulomb interactions on the charge dynamics. When the electronic structure and low-energy excitations in a 1D Mott insulator are theoretically analysed, the Hubbard model, which includes U and the transfer energy t as the important parameters, is generally used. In the photoexcited states, on the other hand, electron wavefunctions are more delocalized, and the effects of long-range Coulomb interactions will become important in the charge dynamics. However, it is difficult to evaluate these effects experimentally.Based upon these backgrounds, in the present study, we investigated the role of longrange Coulomb interactions in photoexcited states by focusing on the excitons and biexcitons in 1D Mott insulators. The long-range Coulomb interactions can stabilize not only the bound state of a doublon and a holon (that is, an exciton 34-36 ) b...

Ionic charge distributions in silicon atomic surface wires

“…Continuous structures, such as the DB wires examined in this work, are of particular interest as they have been proposed in prior studies to exhibit more exotic beha-viors through enhanced DB-DB coupling, including spin or ionic ordering [26][27][28][29] similar to that seen in low dimensional Mott insulators. [30][31][32] Experimental exploration of such behaviours has so far been limited to scanning tunneling microscopy (STM) measurements at energies outside the band-gap of the material, where the expected ground state ordering of DB wire structures is convoluted with higher energy features associated with the perturbative nature of a necessary tunneling current. [33][34][35][36][37] In this work, we compliment these former studies with the use of the less perturbative AFM.…”

Excitonic optical spectra and energy structures in a one-dimensional Mott insulator demonstrated by applying a many-body Wannier functions method to a charge model

“…H 0 is a conventional Hubbard model with a transfer integral T and on-site Coulomb interaction strength U. According to previous works on ET-F 2 TCNQ [15,43,46], we set U/T = 10. H V describes a long-range Coulomb interaction and V α corresponds to the αth nearest neighbor Coulomb interaction strength.…”

“…Regarding this, the excitonic energy structures of including information of both the odd-and even-parity low-energy one-HD-pair excited states can be estimated by electroreflectance spectroscopy with teraheltz (THz) electric fields. So far, several experiments have reported such excitonic energy structures in Mott insulators deduced from fitting parameters of measured third-order nonlinear susceptibility, χ (3) (−ω; 0, 0, ω) [40][41][42][43].…”

“…As one of such experiments, we choose the recent experiment of ET-F 2 TCNQ at 294 K [43] and reproduce its excitonic energy structure for evaluating the completeness of our evenparity effective model in this paper. In this experiment, χ (3) (−ω; 0, 0, ω) was obtained from a nonlinear polarization P (ω) ∝ χ (3) (−ω; 0, 0, ω)E 2 (ω ∼ 0)E(ω), where E(ω ∼ 0) can be regarded as a static electric field created by a THz pulse and E(ω) is an electric field of optical probe pulse, respectively.…”

“…Using the MBWFs, we also demonstrate the scheme of theoretically calculating σ(ω) with and without a modulated electric field for large system size. All the theoretical spectra and excitonic energy structures compared with corresponding experimental data of ET-F 2 TCNQ at 294 K [43] are shown and discussed in Sect.III. Throughout this paper, we treat absolute zero temperature and set = e = c = a = 1, where a is a lattice constant.…”