Exciton recombination in one-dimensional organic Mott insulators

Abstract

We present a theory for the recombination of (charged) holons and doublons in one-dimensional organic Mott insulators, which is responsible for the decay of the photoexcited state. Due to the charge-spin separation, the dominant mechanism for recombination at low density of charges involves a multiphonon emission. We show that a reasonable coupling to phonons is sufficient to explain the fast recombination observed by pumpprobe experiments in ET-F 2 TCNQ, whereby we can also account for the measured pressure dependence of the recombination rate. Introduction. Femtosecond pump-probe spectroscopy is a powerful probe for the charge relaxation and thermalization phenomena in complex materials. These measurements can directly address and unveil the role of strong electron correlations, as well as the coupling to phonon degrees of freedom. Materials that behave as Mott insulators due to strong electron Coulomb repulsion contain all the latter physics, and are therefore of high theoretical and experimental interest. It has been observed that photoinduced charges decay within the picosecond range, i.e., well within the experimental resolution, but on the other hand orders of magnitude faster than in clean semiconductors with similar energy gaps.So far two classes of Mott insulators, investigated by pumpprobe spectroscopy, revealed similar behavior. These are the layered undoped cuprates La 2 CuO 4 and Nd 2 CuO 4 [1-3], and the quasi-one-dimensional (1D) organic Mott insulators of the tetracyanoquinodimethane (TCNQ) family [4], in particular, ET-F 2 TCNQ [4][5][6][7], which will be the focus of our study. Both undoped cuprates and ET-F 2 TCNQ reveal ultrafast picosecond charge recombination with some similarities: (a) The charged carriers created by the pump pulse above the Mott-Hubbard (MH) gap are holons and doublons, and their recombination requires the distribution of a large energy quantum (the MH gap ∼ 1 eV) into several final excitations with smaller energy 0 . At low density of charges candidates for recipient bosons can be spin or phonon excitations. (b) The decay is exponential in time. This excludes bi-and higher-molecular processes involving inelastic collision of several "free" charge carriers, and implicitly reveals the existence of an intermediate bound state of a holon and a doublon (the MH exciton). In this respect a different observation has been obtained on Ca 2 CuO 3 from the 1D cuprate family, which is known to have negligible excitonic effects [8] and thus shows a nonexponential decay [9].From a theoretical viewpoint the challenge of understanding the charge recombination has analogies with the decay of the double occupancy in ultracold bosons [10] and fermions [11,12] in optical lattices, where the decay rate exhibits an exponential dependence on the ratio of the Coulomb repulsion U and the typical excitation's energy scale 0 . In the latter case the system can be described by

Phonon-induced electronic relaxation in a strongly correlated system: The Sn/Si(111) (3×3) adlayer revisited

“…Optical excitation of the surface will generate electron-hole pairs. Recently, the various processes contributing to the decay of such an excitation have been studied analytically, employing an effective Mott-Hubbard (MH) Hamiltonian coupled linearly to a phonon bath [23]. To make contact to this study, it is helpful to cast our DFT results into the form of an effective MH Hamiltonian…”

“…Thus, the electronphonon coupling, expressed by the deformation potential λ, is the decisive parameter. Lenarčič and co-workers [23] have presented a MH model coupled to a phononic Hamiltonian that allows them to estimate the lifetime of the exciton due to decay into phonons by a Zener-type expression. In their approach, the basic time scale for the decay is set by the ratio of the phonon frequency and the band width, τ 0 .…”

Photoenhanced excitonic correlations in a Mott insulator with nonlocal interactions

“…Photoinduced excitonic features have been studied using exact diagonalization of small lattice systems [15,16]. These excitonic features can originate either from the non-local interactions [16][17][18] or the modification of the spin background [10]. Methods for infinite lattices based on single-site dynamical mean field theory (DMFT) [19], such as extended DMFT [20][21][22][23] or the combination of GW and extended DMFT [24][25][26], can capture the dynamical screening of the local Coulomb interaction resulting from nonlocal interactions, but they cannot describe exciton formation.…”

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

“…The former is just a conventional extended Hubbard model and several candidates of V for ET-F 2 TCNQ are reported [46][47][48][49]. In addition, the latter is treated in the recent paper trying to explain the experimental spectra of ET-F 2 TCNQ at 294 K with exact diagonalization analysis at V /T = 4.286 and N = 14 [43].…”