Abstract:We use midinfrared pulses with stable carrier-envelope phase offset to drive molecular vibrations in the charge transfer salt ET-F2TCNQ, a prototypical one-dimensional Mott insulator. We find that the Mott gap, which is probed resonantly with 10 fs laser pulses, oscillates with the pump field. This observation reveals that molecular excitations can coherently perturb the electronic on-site interactions (Hubbard U) by changing the local orbital wave function. The gap oscillates at twice the frequency of the vibrational mode, indicating that the molecular distortions couple quadratically to the local charge density.

Optical pulses at THz and mid-infrared frequencies tuned to specific vibrational resonances modulate the lattice along chosen normal mode coordinates. In this way, solids can be switched between competing electronic phases and new states are created. Here, we use vibrational modulation to make electronic interactions (Hubbard-U) in Mott-insulator time dependent. Mid-infrared optical pulses excite localized molecular vibrations in ET-F2TCNQ, a prototypical one-dimensional Mott-insulator. A broadband ultrafast probe interrogates the resulting optical spectrum between THz and visible frequencies. A red-shifted charge-transfer resonance is observed, consistent with a time-averaged reduction of the electronic correlation strength U. Secondly, a sideband manifold inside of the Mott-gap appears, resulting from a periodically modulated U. The response is compared to computations based on a quantum-modulated dynamic Hubbard model. Heuristic fitting suggests asymmetric holon-doublon coupling to the molecules and that electron double-occupancies strongly squeeze the vibrational mode.

We derive ab initio local Hubbard models for several optical lattice potentials of current interest, including the honeycomb and Kagomé lattices, verifying their accuracy on each occasion by comparing the interpolated band structures against the originals. To achieve this, we calculate the maximally-localized generalized Wannier basis by implementing the steepest-descent algorithm of Marzari and Vanderbilt [N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)] directly in one and two dimensions. To avoid local minima we develop an initialization procedure that is both robust and requires no prior knowledge of the optimal Wannier basis. The MATLAB code that implements our full procedure is freely available online at

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...