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Evidence for metastable photo-induced superconductivity in K3C60

Excitation of high-Tc cuprates and certain organic superconductors with intense far-infrared optical pulses has been shown to create non-equilibrium states with optical properties that are consistent with transient high-temperature superconductivity.
These non-equilibrium phases have been generated using femtosecond drives, and have been observed to disappear immediately after excitation, which is evidence of states that lack intrinsic rigidity. Here we make use of a new optical device to drive metallic K3C60 with mid-infrared pulses of tunable duration, ranging between one picosecond and one nanosecond. The same superconducting-like optical properties observed over short time windows for femtosecond excitation are shown here to become metastable under sustained optical driving, with lifetimes in excess of ten nanoseconds. Direct electrical probing, which becomes possible at these timescales, yields a vanishingly small resistance with the same relaxation time as that estimated by terahertz conductivity. We provide a theoretical description of the dynamics after excitation, and justify the observed
slow relaxation by considering randomization of the order-parameter phase as the rate-limiting process that determines the decay of the light-induced superconductor.

Reproduced with permission. Copyright 2021, Springer Nature

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Nickel addition to optimize the hydrogen storage performance of lithium intercalated fullerides

The addition of transition metals to alkali intercalated fullerides proved to enhance their already good hydrogen absorption properties. Herein we present a study based on two different synthetic strategies, allowing the addition of nickel as aggregates with different size to the lithium fulleride Li6C60: the former is based on the metathesis of nickel chloride, while the latter on the thermal decomposition of nickel carbonyl clusters. The hydrogen-storage properties of the obtained materials have been investigated with manometric and calorimetric measurements, which indicated a clear enhancement of the final absorption value and kinetics with respect to pristine Li6C60, as a consequence of nickel surface catalytic activity towards hydrogen molecules dissociation. We found up to 10 % increase of the total H2 weight % absorbed (5.5 wt% H2) in presence of Ni aggregates. Furthermore, the control of the transition metal particles size distribution allowed reducing the hydrogen desorption enthalpy of the systems.

Reproduced with permission. Copyright 2020, Elsevier

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Pressure tuning of light-induced superconductivity in K3C60

Optical excitation at terahertz frequencies has emerged as an effective means to dynamically manipulate complex materials. In the molecular solid K3C60, short mid-infrared pulses transform the high-temperature metal into a non-equilibrium state with the optical properties of a superconductor. Here we tune this effect with hydrostatic pressure and find that the superconducting-like features gradually disappear at around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth induced by pressure is also detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesized previously, and rather extends all the way to room temperature.

Reproduced with permission. Copyright 2018, Springer Nature.

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Electrochemical intercalation of fullerene and hydrofullerene with sodium

We report on the ability of fullerene C60 and hydrogenated fullerene C60Hx (x∼39) to operate as negative electrodes in novel Na-ion batteries. Building upon the known solubility of C60 in common organic electrolytes used in batteries, we developed a suitably optimized solid-state Na-(polyethylene oxide) electrolyte for this application. Electrochemical and structural properties of the fullerene electrodes were investigated through cyclic voltammetry, fixed-current charge/discharge of the electrodes, impedance spectroscopy and powder X-ray diffraction. Both C60 and hydrogenated C60 have been electrochemically intercalated with sodium. Specific capacities after the first cycle are 250 mAh g−1 and 230 mAh g−1 for C60 and C60Hx respectively. However, C60 electrode shows a strong irreversible character after the first discharge, probably due to the formation of stable polymeric NaxC60 phases, where Na+ ions diffusion is hindered. On the contrary, C60Hx displays better reversibility, suggesting that hydrogenation of the buckyball could be effective to preserve sufficiently large interstitial pathways for Na+ diffusion upon intercalation.

Reproduced with permission. Copyright 2018, Elsevier

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Mott Transition in the A15 Phase of Cs3C60: Absence of a Pseudogap and Charge Order

We present a detailed NMR study of the insulator-to-metal transition induced by an applied pressure p in the A15 phase of Cs3C60. We evidence that the insulating antiferromagnetic (AFM) and superconducting (SC) phases coexist only in a narrow p range. At fixed p, in the metallic state above the SC transition Tc, the 133Cs and 13C NMR spin-lattice relaxation data are seemingly governed by a pseudogaplike feature. We prove that this feature, also seen in the 133Cs NMR shift data, is rather a signature of the Mott transition which broadens and smears out progressively for increasing (p, T). The analysis of the variation of the quadrupole splitting νQ of the 133Cs NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of the lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three-dimensional system well beyond its critical point.

Reproduced with permission. Copyright 2017, American Physical Society