Photomolecular High-Temperature Superconductivity

Abstract

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer saltκ−(BEDT−TTF)2Cu[N(CN)2]Brinduce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperatureT*≃50K, far higher than the equilibrium transition temperatureTC=12.5K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.

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Plain Language Summary

High-temperature superconductivity is found in a wide variety of organic conductors—synthetic metals composed of molecular building blocks whose nature and arrangement fully determine the material’s properties. Typically, these properties are altered by the use of chemical substitutions or by the application of external pressure. These approaches affect only the mutual arrangement of the molecular building blocks, leaving their intrinsic nature unchanged. Here, we use ultrafast midinfrared laser pulses to directly manipulate the molecular building blocks and induce transient superconductivity at a temperature much higher than that found at equilibrium.

In our experiment, we focus on the organic superconductorκ−(BEDT−TTF)2Cu[N(CN)2]Br. This compound has a superconducting transition temperature of 12.5 K, the highest for this family of superconductors. Using femtosecond midinfrared laser pulses tuned to excite specific molecular vibrational modes, we induce transient superconductivity at temperatures as high as 50 K. This change in behavior arises from how the laser pulses modulate the local molecular wave function and with it the electronic interactions in the solid.

This discovery provides a possible physical scenario for photomolecular superconductivity and a new way of tuning electronic ground states in solids.