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...

Single-crystal torque magnetometry performed on weakly-coupled polynuclear systems provides access to a complete description of single-site anisotropy tensors. Variable-temperature, variable-field torque magnetometry was used to investigate triiron(III) complex [Fe3La(tea)2(dpm)6] (Fe3La), a lanthanum(III)-centred variant of tetrairon(III) single molecule magnets (Fe4) (H3tea = triethanolamine, Hdpm = dipivaloylmethane). Due to the presence of the diamagnetic lanthanoid, magnetic interactions among iron(III) ions (si = 5/2) are very weak (<0.1 cm(−1)) and the magnetic response of Fe3La is predominantly determined by single-site anisotropies. The local anisotropy tensors were found to have Di > 0 and to be quasi-axial with |Ei/Di| ~ 0.05. Their hard axes form an angle of approximately 70° with the threefold molecular axis, which therefore corresponds to an easy magnetic direction for the molecule. The resulting picture was supported by a High Frequency EPR investigation and by DFT calculations. Our study confirms that the array of peripheral iron(III) centres provides substantially noncollinear anisotropy contributions to the ground state of Fe4 complexes, which are of current interest in molecular magnetism and spintronics.

Tetrairon(III) single-molecule magnets [Fe4(pPy)2(dpm)6] (1) (H3pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane) have been deliberately organized into supramolecular chains by reaction with Ru(II)Ru(II) or Ru(II)Ru(III) paddlewheel complexes. The products [Fe4(pPy)2(dpm)6][Ru2(OAc)4](BF4)x with x=0 (2 a) or x=1 (2 b) differ in the electron count on the paramagnetic diruthenium bridges and display hysteresis loops of substantially different shape. Owing to their large easy-plane anisotropy, the s=1 diruthenium(II,II) units in 2 a act as effective s(eff)=0 spins and lead to negligible intrachain communication. By contrast, the mixed-valent bridges (s=3/2, s(eff)=1/2) in 2 b introduce a significant exchange bias, with concomitant enhancement of the remnant magnetization. Our results suggest the possibility to use electron transfer to tune intermolecular communication in redox-responsive arrays of SMMs.

We study the local density of states (LDOS) in systems of Luttinger-liquid nanowires connected to a common mesoscopic superconducting island, in which Majorana bound states give rise to different types of topological Kondo effects. We show that electron interactions enhance the low-energy LDOS in the leads close to the island, with unusual exponents due to Kondo physics that can be probed in tunneling experiments.