Efficient Mott Insulator-Metal Transition by an Intense Terahertz Electric Field Pulse Via Quantum Tunneling

When a semiconductor is subjected to a strong electric field, carriers are generated via quantum tunneling; thisis termed as dielectric breakdown. Thus, using a terahertz pulse to drive the dielectric breakdown in Mott insula-tors, which exhibit variations in their electronic structures under carrier doping, a filling-controlled transition canbe induced in the subpicosecond time scale. However, to generate carriers via quantum tunneling in a materialwith a band gap in the visible or near-infrared regions, an electric field pulse significantly exceeding 1 MV cm-1is necessary. In this paper, using an organic molecular compound, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane, which is a typical one-dimensional (1D) Mott insulator with a Mott gapof 0.7 eV, we aimed at realizing carrier generation and metallization via a strong electric field component of aterahertz pulse enhanced with an organic nonlinear optical crystal up to 2.8 MV cm-1. Even after the terahertzelectric field decays, the reflectivity change caused by the terahertz pulse remains; this is different from the caseinvolving the use of weaker electric fields. More importantly, this response indicates a threshold behavior againstthe electric field amplitude, which is characteristic of carrier generation via the quantum tunneling process.Furthermore, transient reflectivity spectra across the mid-infrared region could be reproduced well by numericalsimulations using the Drude model, in which inhomogeneous carrier distributions are considered. The observedDrude response of the doublons and holons was ascribed to the spin-charge separation characteristic of 1Dstrongly correlated electron systems. We also demonstrate that the energy efficiency of such carrier generationby the terahertz pulse excitation is at least five times greater than that when using photoexcitation beyond theMott gap. This indicates that excitation with the strong terahertz pulse is more effective for carrier doping insolids; thus, the proposed method is expected to be widely applicable for the electronic-state control of variouscorrelated electron materials in which chemical carrier doping is currently difficult