Pressure-Dependent Relaxation in the Photoexcited Mott InsulatorET–F2TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates

Abstract

We measure the ultrafast recombination of photoexcited quasiparticles (holon-doublon pairs) in the one dimensional Mott insulator ET-F 2 TCNQ as a function of external pressure, which is used to tune the electronic structure. At each pressure value, we first fit the static optical properties and extract the electronic bandwidth t and the intersite correlation energy V. We then measure the recombination times as a function of pressure, and we correlate them with the corresponding microscopic parameters. We find that the recombination times scale differently than for metals and semiconductors. A fit to our data based on the time-dependent extended Hubbard Hamiltonian suggests that the competition between local recombination and delocalization of the Mott-Hubbard exciton dictates the efficiency of the recombination. The recombination of hot carriers in solids is a fundamental process of interest to nonlinear optics and to device applications, as well as a spectroscopic tool that exposes the physics of interacting microscopic degrees of freedom. "Hot electron" spectroscopy has been applied extensively to metals and semiconductors, for which well-established models have been developed.For direct gap semiconductors recombination occurs at a rate that depends on the joint density of states between valence and conduction bands ∝ð∂E v =∂kÞð∂E c =∂kÞ, and is thus expected to slow down with the square of the bandwidth τ ∝ t 2 . On the other hand, in the case of metals, the dynamics are well captured by the two-temperature model [1,2], which considers the energy stored in the optically excited nonequilibrium electron distribution as flowing into the lattice at a rate determined by the electronphonon coupling strength and by the electronic and lattice heat capacities. As the relaxation of hot electrons accelerates with smaller electronic specific heat, and because c e v is proportional to the density of states at the Fermi level [3], for metals relaxation should accelerate linearly with the reciprocal of the bandwidth τ ∝ 1=t.For solids with strongly correlated electrons, the dependence of nonequilibrium quasiparticle recombination rates on the microscopic parameters has not been systematically investigated and it is not well understood. In this Letter, we study the recombination of impulsively excited quasiparticles in a one dimensional Mott insulator, in which we tune electronic bandwidth and intersite correlation energy by applying external pressure. We find that the recombination of quasiparticles accelerates for increasing bandwidth, as expected for a metal, but with a dependence on microscopic parameters that is unique to the physics of electronic insulators in one dimension and that descends from a competition between local decay and coherent delocalization of photoexcited holon-doublon pairs [4].We study bis-(ethylendithyo)-tetrathiafulvalenedifluorotetracyano-quinodimethane (ET-F 2 TCNQ), a half filled organic salt with quasi-1D electronic structure, negligible electron-phonon interaction [5], and with electronic pr...

THz-Frequency Modulation of the HubbardUin an Organic Mott Insulator

“…[27]. In the limit of large ≫ , it is possible to analyze the extended Hubbard model by means of a 1/ expansion [28].…”

“…As discussed in Refs. [26,27,28], the charge transfer band of ET-F2TCNQ in equilibrium is described by two dominant contributions to the optical conductivity (see Fig. 4c).…”

“…As this holon-doublon exciton is bound by the intersite Coulomb correlation , its energy is centered at − − 4 2 / [28]. A second weaker contribution to the conductivity spectrum results from the excitation of delocalized holon-doublon pairs not bound by the intersite Coulomb energy [15], which form a particle-hole (PH) continuum described by a band of width 8 centered at [27]. Figure 4c reports fit results of the optical conductivity, in which individually normalized contributions from bound HD pairs and and PH continuum are displayed in interval of ¼ of time period of the pump electric field.…”

“…We extract and , fitting the optical conductivity along the lines described in Ref. [27], as a function of pump-probe delay time. Assuming that the hopping amplitude does not oscillate with the driven local vibrational motion [29], this procedure yields the effective correlation term / and / .…”

Coexistence of excited polarons and metastable delocalized states in photoinduced metals

“…Signatures of strong electron-phonon coupling and polaronic effects in photo-excited systems have been found for the self-trapping of excitons [8][9][10][11], in Mott insulators [12,13], and organic materials [14][15][16]. A direct observation of the self-localization process was achieved by two-photon photoemission for electrons in surface states which couple to adsorbate layers [17][18][19][20].…”