Cooling quasiparticles in A3C60 fullerides by excitonic mid-infrared absorption

Abstract

Long after its discovery superconductivity in alkali fullerides A 3 C 60 still challenges conventional wisdom. The freshest inroad in such ever-surprising physics is the behaviour under intense infrared (IR) excitation. Signatures attributable to a transient superconducting state extending up to temperatures ten times higher than the equilibrium T c ∼ 20 K have been discovered in K 3 C 60 after ultra-short pulsed IR irradiation -an effect which still appears as remarkable as mysterious. Motivated by the observation that the phenomenon is observed in a broad pumping frequency range that coincides with the mid-infrared electronic absorption peak still of unclear origin, rather than to TO phonons as has been proposed, we advance here a radically new mechanism. First, we argue that this broad absorption peak represents a "super-exciton" involving the promotion of one electron from the t 1u half-filled state to a higher-energy empty t 1g state, dramatically lowered in energy by the large dipole-dipole interaction acting in conjunction with Jahn Teller effect within the enormously degenerate manifold of t 1u 2 t 1g 1 states. Both long-lived and entropy-rich because they are triplets, the IR-induced excitons act as a sort of cooling mechanism that permits transient superconductive signals to persist up to much larger temperatures.Superconducting alkali doped fullerenes A 3 C 60 are molecular compounds where several actors play together to determine an intriguing physical behaviour. The high icosahedral symmetry of C 60 arXiv:1704.05613v1 [cond-mat.str-el] 19 Apr 2017 2 implies, prior to intermolecular hybridisation, a large degeneracy of the molecular orbitals, thus a strong electronic response to JT molecular distortions lowering that symmetry. In particular, the t 1u LUMO, which accommodates the three electrons donated by the alkali metals, is threefold degenerate and JT coupled to eight fivefold-degenerate molecular vibrations of H g symmetry, which mediate the pairing [1]. The JT effect, favouring low spin, is partly hindered by (Coulomb) Hund's rule exchange, which favours high spin. Therefore the overall singlet pairing strength g, though still sizeable, is way too small compared to the charging energy of each C 3− 60 to justify by simple arguments why A 3 C 60 are s-wave superconductors. The explanation of this puzzle proposed in [2,3] and vindicated by recent experiments emphasises the crucial role of a parent Mott insulating state where the JT coupling effectively inverts Hund's rules, the molecular ground state therefore turning to spin S = 1/2 rather than S = 3/2 [4]. A S = 1/2 antiferromagnetic insulating phase is indeed the ground state in over-expanded NH 3 K 3 C 60 [5,6] and in Cs 3 C 60 [7] at ambient pressure. In the metallic state, attained under pressure in Cs 3 C 60 and at ambient pressure in K 3 C 60 and Rb 3 C 60 , the incipient Mott localisation slows down the coherent motion of quasiparticles while undressing them from charge correlations. As a result, the singlet pairing strength g eventual...

Nonequilibrium steady states and transient dynamics of conventional superconductors under phonon driving

“…As for the relation to the K 3 C 60 experiment mentioned in the introduction [8], from our analysis it seems likely that the apparent enhancement of the SC originates from the static change of the Hamiltonian [14,18] or some other dynamical effects [13,16,55], which are not captured in the present simple model. Since alkali doped fullerides are multiorbital systems and an effective negative Hund's coupling from the Jahn-Teller screening plays an important role [56][57][58][59], we need to go beyond the Holstein model to fully understand the mechanism.…”

Non-equilibrium Phenomena in Superconductors Probed by Femtosecond Time-Domain Spectroscopy

“…E.g., experiments on K 3 C 60 performed at temperatures below T c show a photoinduced suppression of superconductivity [118] instead of the expected gap enhancement. Apart from the proposed exciton-cooling model [132] this observation seems at odds with majority of the proposed scenarios [118,130,[138][139][140]. Secondly, the excitation densities in most of these experiments are extremely high.…”

Dynamical Jahn‐Teller effect in the first excited

“…For example, JT effect in the first excited C 60 anion where t 1g next lowest unoccupied molecular orbitals (NLUMOs) are populated is of fundamental importance to interpret the absorption spectra of isolated C − 60 , [23,29,[36][37][38][39][40][41][42] electron transfer process of fullerene, [43,44] and excitation spectra of alkali-doped fullerides. [45][46][47] The importance is also suggested [48] in recently reported light-induced superconductivity of alkali-doped fullerides. [49,50] Moreover, JT effect involving the NLUMOs must be significant in highly alkali doped [45] and alkali-earth/rare-earth doped fullerides.…”