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.

We discuss the emergence of zero-energy Majorana modes in a disordered finite-length p-wave one-dimensional superconducting ring, pierced by a magnetic flux Φ tuned at an appropriate value Φ = Φ * . In the absence of fermion parity conservation, we evidence the emergence of the Majorana modes by looking at the discontinuities in the persistent current I[Φ] at Φ = Φ * . By monitoring the discontinuities in I[Φ], we map out the region in parameter space characterized by the emergence of Majorana modes in the disordered ring.PACS numbers: 73.23. Ra, 74.81.g, 74.45.+c, I. INTRODUCTIONMajorana fermions, particles coinciding with their own antiparticles, were proposed by Majorana in 1937 1 . While, so far, they have never been detected in a particle physics experiment in the last years, after Kitaev's proposal that Majorana fermions may appear as zero-energy real fermionic modes ["Majorana modes" -MMs] localized at the interface between a p-wave one-dimensional superconductor and a normal metal 2 , the search for Majorana fermions in such systems has become one of the most relevant and promising areas in condensed matter physics 3 .Besides Kitaev's proposal, the emergence of MMs in condensed matter systems has been predicted in a number of systems such as superconductor-topological insulator interfaces 4-7 , in proximity-induced superconducting quantum wires with strong spin-orbit interaction 8-11 , in helical magnets 12 , in ferromagnetic atoms in proximity to superconductors 13,14 . In this context, interesting phases with unconventional properties have been predicted at junctions between topological superconductors, hosting MMs at their endpoints, and interacting one-dimensional electronic systems (Luttinger liquids) 15-17 . In addition, MMs emerging at junctions of one-dimensional interacting quantum wires 18-21 , or of systems formally described as interacting electronic systems, such as quantum Ising spin chains 22-25 , one-dimensional XX-26 , or XY-27 models, or pertinently designed Josephson junction networks 28 , have been predicted to give rise to the so-called "Topological Kondo Effect", a remarkable realization of Kondo Effect in which the impurity spin, determined by the MMs, is nonlocal in the wire indices and, thus, cannot be expressed as a functional of local operators 18,19 . Finally, it is worth mentioning that, besides being of remarkable interest for fundamental physics, MMs are also of great interest for quantum computation since, due to their nonabelian statistics 29 , they appear to be among the most natural candidates to work as robust qubits 30 .The proliferation of theoretical literature about Majorana fermions in condensed matter systems has triggered a number of experimental attempts to probe MMs in pertinently designed devices. The main route followed in the experiments consists in measuring the effects in the transport across junctions between topological superconductors and normal metals possibly due to the presence of localized MMs at the interfaces 31-33 . Unfortunately, despite the ex...

We investigate the dissipative dynamics yielded by the Lindblad equation within the coexistence region around a first order phase transition. In particular, we consider an exactly-solvable fullyconnected quantum Ising model with n-spin exchange (n > 2) -the prototype of quantum first order phase transitions -and several variants of the Lindblad equations. We show that physically sound results, including exotic non-equilibrium phenomena like the Mpemba effect, can be obtained only when the Lindblad equation involves jump operators defined for each of the coexisting phases, whether stable or metastable.

A 3D metal-organic framework (MOF) having single-molecule magnet (SMM) linkers was prepared in crystalline form by using a tetrairon(III) complex functionalised with two divergent pyridyl groups, namely [Fe4 (pPy)2 (dpm)6 ] (1; H3 pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane). Reaction of 1 with silver(I) perchlorate afforded {[Fe4 (pPy)2 (dpm)6 ]2 Ag}ClO4 (2), which crystallises in a cubic face-centred lattice and exhibits two interlocked diamondoid networks. In 2, the SMMs act as linear ditopic synthons, and silver(I) ions as tetrahedral nodes coordinated by four pyridyl nitrogen atoms. The magnetic properties of 1 (S=5 and D≈-0.4 cm(-1) in the ground spin state) are largely preserved in 2, which shows slow magnetic relaxation with an anisotropy barrier of Ueff /kB =11.46(10) K in zero field and 14.25(8) K in an applied field of 1 kOe. However, crystal symmetry triggers highly noncollinear magnetic anisotropy contributions oriented at 109.47° from each other along the threefold axes of AgN4 tetrahedra, a unique scenario fully confirmed by a single-crystal cantilever torque magnetometry investigation. Magnetisation curves down to 0.03 K demonstrated the occurrence of a wide hysteresis loop when the magnetic field was swept along one of the four Ag-N bonds. By symmetry, the crystalline compound can then be persistently magnetised parallel or antiparallel to the four main diagonals of the unit cell, although the crystals have no overall second-order anisotropy.

Using the properties of the transfer matrix of one-dimensional quantum mechanical systems, we derive an exact formula for the persistent current across a quantum mechanical ring pierced by a magnetic flux Φ as a single integral of a known function of the system's parameters. Our approach provides exact results at zero temperature, which can be readily extended to a finite temperature T . We apply our technique to exactly compute the persistent current through p-wave and s-wave superconducting-normal hybrid rings, deriving full plots of the current as a function of the applied flux at various system's scales. Doing so, we recover at once a number of effects such as the crossover in the current periodicity on increasing the size of the ring and the signature of the topological phase transition in the p-wave case. In the limit of a large ring size, resorting to a systematic expansion in inverse powers of the ring length, we derive exact analytic closed-form formulas, applicable to a number of cases of physical interest.