Laser‐Induced Cooperative Transition in Molecular Electronic Crystal

Abstract

The competing and non‐equilibrium phase transitions, involving dynamic tunability of cooperative electronic and magnetic states in strongly correlated materials, show great promise in quantum sensing and information technology. To date, the stabilization of transient states is still in the preliminary stage, particularly with respect to molecular electronic solids. Here, a dynamic and cooperative phase in potassium‐7,7,8,8‐tetracyanoquinodimethane (K‐TCNQ) with the control of pulsed electromagnetic excitation is demonstrated. Simultaneous dynamic and coherent lattice perturbation with 8 ns pulsed laser (532 nm, 15 MW cm−2, 10 Hz) in such a molecular electronic crystal initiates a stable long‐lived (over 400 days) conducting paramagnetic state (≈42 Ωcm), showing the charge–spin bistability over a broad temperature range from 2 to 360 K. Comprehensive noise spectroscopy, in situ high‐pressure measurements, electron spin resonance (ESR), theoretical model, and scanning tunneling microscopy/spectroscopy (STM/STS) studies provide further evidence that such a transition is cooperative, requiring a dedicated charge–spin–lattice decoupling to activate and subsequently stabilize nonequilibrium phase. The cooperativity triggered by ultrahigh‐strain‐rate (above 106 s−1) pulsed excitation offers a collective control toward the generation and stabilization of strongly correlated electronic and magnetic orders in molecular electronic solids and offers unique electro‐magnetic phases with technological promises.

Spatially Resolved Investigation of Mixed Valence and Insulator-to-Metal Transition in an Organic Salt

“…Considering that molecules within this region, ∼1.5 nm from the physical domain boundary, are as well packed as those in the deep interior of the TCNQ assembly, these continuum states that emerge inside the molecular band gap cannot be attributed to localized defects states associated with disorders in the TCNQ structure. As discussed in our recent work, adding a K + vacancy in the bulk K-TCNQ unit cell introduces DOS within the bulk gap . Thus, the continuum DOS observed in our experiment most likely originates from the abrupt valence change at the boundary, the electrostatic effects of which spread a couple of nanometers into the interior of the TCNQ assembly.…”

“…As discussed in our recent work, adding a K + vacancy in the bulk K-TCNQ unit cell introduces DOS within the bulk gap. 33 Thus, the continuum DOS observed in our experiment most likely originates from the abrupt valence change at the boundary, the electrostatic effects of which spread a couple of nanometers into the interior of the TCNQ assembly. Local electric fields are created as these states reach equilibrium with the rest of the system, including the HOPG substrate via charge transfer.…”

“…Previous studies on bulk K-TCNQ and related alkali-TCNQ systems have witnessed IMT induced by electric field, optical field, or pressure. ,,,, The mechanisms of the induced IMT had been largely attributed to the avalanche dielectric breakdown. , From the energetics perspective, the application of electric field decreases the energy barrier between the insulating Mott state and the metastable correlated metallic (CM) state. Once the density of CM state reaches the critical value, an avalanche-like process could be triggered, resulting in the formation of percolated conducting pathways/filaments .…”

High-Endurance Magneto-Electronic Switchable Molecular Electronic Crystal

“…The energy gap in K-TCNQ is estimated to be ∼1 eV whereas the applied electric field does not break the gap under dielectric breakdown . This further suggests the role of electronic correlation and collective charge dynamics in understanding the transition in K-TCNQ. , …”

“…Macroscopic functionality of emergent electronic phases change by means of external perturbations, − such as magneto-electric switching − and conducting phase transition, , which are promising in quantum sensing and information technology. ,,− Low-dimensional molecular electronic crystals are rapidly emerging due to their cooperative charge-spin–lattice coupling in conjunction with its remarkable synthetic control, solution processing, low-density, and mechanical flexibilities. ,− Precisely tuning the electronic order in these low-dimensional molecular materials could trigger the transition between multiple states involving changes in molecular identity, such as charge, spin, and lattice degrees of freedom (Figure a,b). , …”