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We investigated the pressure dependence of electric transport in a superconducting sample, Ba0.77Na0.23Ti2Sb2O, to complete the phase diagram of superconducting transition temperature (Tc) against pressure (p). This superconducting sample exhibits a Tc value of 5.8 K at ambient pressure. Here, the superconductivity of the recently reported sample was investigated over a wide pressure range. The Tc value monotonously decreased with pressure below 8 GPa. Interestingly, the Tc value rapidly increased above 8 GPa and slowly declined with pressure above 11 GPa. Thus, a new superconducting phase was discovered above ∼9 GPa. The crystal structure of Ba0.77Na0.23Ti2Sb2O was also elucidated at 0-22.0 GPa with synchrotron X-ray powder diffraction. Consequently, an evident relation between the crystal structure and the superconductivity was revealed, namely, a clear structural phase transition was observed at 8-11 GPa, where the Tc value rapidly increased against pressure. This study provides detailed information on the superconductivity of Ba0.77Na0.23Ti2Sb2O under pressure, which will lead to a comprehensive understanding of pressure-driven superconductivity.

(Sm1/3Ca2/3)2.75C60 is a member of the family of non-stoichiometric strongly-correlated rare-earth fullerides, (Sm1−xCax)2.75C60 (0 ≤ x ≤ 1), in which an orthorhombic 2 × 2 × 2 supercell of the face-centred cubic (fcc) unit cell of stoichiometric A3C60 (A = alkali metal) fullerides is stabilized by the long-range ordering of partially-occupied metal sites. At ambient temperature and pressure, it is a mixed valence compound with an average Sm valence of +2.33(2) implying a formal charge of −5.78 for the C60 anions. Here we study its electronic response to the application of pressure in the range 0–9 GPa. Synchrotron X-ray absorption measurements in the high-resolution partial fluorescence yield mode (PFY-XAS) at ambient temperature show the onset of an abrupt strongly hysteretic (width ∼2.5 GPa) first-order reversible phase transition at ∼4 GPa, accompanied by a drastic increase in the bulk Sm valence by ∼20% to +2.71(3). This is coincident with the huge lattice contraction and concomitant insulator-to-metal transition encountered before for Sm2.75C60 in the same pressure range and provides a possible explanation of the physical properties in terms of the strong coupling between the lattice and electronic degrees of freedom. The behaviour is reminiscent of the electronic and lattice response to pressure of highly correlated Kondo insulators like SmS and its ternary derivatives, Sm1−xRxS (R = Ca, Y, etc.). However, a distinguishing feature of the fulleride systems is that the C60 anionic sublattice can act as an electron reservoir due to the availability of a close-lying band derived by the t1g orbitals and can accept excess charge as the 4f-electron occupation number decreases. The observed electronic response as a function of pressure is thus opening new possibilities for accessing metallic fullerides at elevated pressures.

Chemical reduction of the Li + @C 60 cation by decamethylferrocene was carried out to obtain neutral Li + @C 60 •– (simply denoted as Li@C 60 ). The method is scalable and does not demand long reaction times unlike electrolytic reduction routes. Powder X-ray diffraction and Raman and EPR spectroscopic measurements of the Li@C 60 solid sample are consistent with the presence mainly of (Li@C 60 ) 2 dimers together with remaining Li + @C 60 •– monomer species due to lack of crystallization time in the course of formation and precipitation.

The [10]phenacene and [11]phenacene molecules have been synthesized using a simple repetition of Wittig reactions followed by photocyclization. Sufficient amounts of [10]phenacene and [11]phenacene were obtained, and thin-film FETs using these molecules have been fabricated with SiO2 and ionic liquid gate dielectrics. These FETs operated in p-channel. The averaged measurements of field-effect mobility, <μ>, were 3.1(7) × 10−2 and 1.11(4) × 10−1 cm2 V−1 s−1, respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO2 gate dielectrics. Furthermore, [10]phenacene and [11]phenacene thin-film electric-double-layer (EDL) FETs with ionic liquid showed low-voltage p-channel FET properties, with <μ> values of 3(1) and 1(1) cm2 V−1 s−1, respectively. This study also discusses the future utility of the extremely extended π-network molecules [10]phenacene and [11]phenacene as the active layer of FET devices, based on the experimental results obtained.

This study demonstrates the feasibility to investigate the electrolyte-side of the Li/electrolyte interface using an X-ray total reflection phenomenon. By means of a specially designed cell and syn...

We investigated the pressure dependence of electric transport in a superconducting sample, Ba

Rb0.5Cs2.5C60 is a member of the family of face-centred-cubic (fcc)–structured alkali fullerides, A3C60 (A = alkali metal) with a highly expanded lattice size. At ambient temperature and pressure, it is a Mott–Jahn–Teller insulator. However, upon cooling it exhibits first a crossover to an anomalous metallic state (Jahn–Teller metal) in the vicinity of 90 K followed next by a transition to a bulk superconductor with a transition temperature, Tc of 29.4 K. Here we study its structural and electronic response to the application of pressure. Synchrotron X-ray powder diffraction at ambient temperature shows that the same Mott-insulator–metal electronic state crossover can be induced through pressure application under isothermal conditions, as evidenced by a distinct broad symmetry-preserving compressibility anomaly, which sets in at ∼0.4 GPa and extends to ∼0.75 GPa. Complementary magnetic susceptibility measurements also reveal that the crossover temperature, T′ tunable at ambient pressure through adjusting the dopant ratio to vary the unit cell volume, can be controlled through pressure application and be mapped onto the same global electronic phase diagram. The observed electronic response, as a function of both physical and ‘chemical’ pressure, is thus of the same electronic origin, namely the control of the bandwidth, W via outer wave function overlap of the constituent fulleride ions.

Molecular solids with cooperative electronic properties based purely on π electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of high-temperature superconductivity in materials obtained by the reaction of alkali metals with polyaromatic hydrocarbons, such as phenanthrene and picene, but the composition and structure of any compound in this family remained unknown. Here we isolate the binary caesium salts of phenanthrene, Cs(C

Molecular solids whose cooperative electronic properties are based purely on π-electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. In our search for novel conducting and magnetic functionalities, we have been exploring the electronic properties of such materials in which the active components of the extended structures are π-electron open-shell molecular units. These systems crystallise in architectures of varied dimensionality ranging from one- to three-dimensions and encompass molecular metals and superconductors such as the alkali fullerides, which provide the highest Tc (38 K) [1] and highest Hc2 (>90 T) [2] together with 3D Mott insulating quantum spin liquids (QSL) in alkali-doped polyaromatic hydrocarbons [3, 4]. In this contribution, we focus on some recent work on the structures and physical properties of selected mixed valence rare-earth fullerene- based architectures with stoichiometry RE2.75C60. These materials constitute an intriguing, essentially unique class of strongly correlated electron systems because, in addition to the highly correlated cation (rare-earth) sublattice [5], they also accommodate the electronically active anion (C60) sublattice [6]. The occurrence of temperature- and pressure-driven valence transitions and giant negative thermal expansion effects and the systematic tuning of the electronic response by both isovalent and aliovalent metal substitution are being systematically mapped out by a combination of structural (synchrotron X-ray diffraction) and spectroscopic (Raman spectroscopy, synchrotron X-ray absorption spectroscopy, resonant inelastic X-ray scattering) techniques at both ambient and elevated pressures (Fig. 1). In addition, the isolation and characterization of binary superconductors with nominal composition Ca6C60 (Fig. 2) have led to the prospect of coexisting superconductivity and Kondo phenomena by co- intercalation of alkaline earth anions in rare- earth fullerides. [1] A. Y. Ganin et al., Nature Mater. 7, 367 (2008). [2] Y. Kasahara et al., Nature Commun. 8, 14467 (2017). [3] Y. Takabayashi et al., Nature Chem. 9, 635 (2017). [4] F. D. Romero et al., Nature Chem. 9, 644 (2017). [5] J. Arvanitidis et al., Nature 425, 599 (2003). [6] R. H. Zadik et al., Science Adv. 1, e1500059 (2015).

Controlled access to the border of the Mott insulating state by variation of control parameters offers exotic electronic states such as anomalous and possibly high-transition-temperature (Tc) superconductivity. The alkali-doped fullerides show a transition from a Mott insulator to a superconductor for the first time in three-dimensional materials, but the impact of dimensionality and electron correlation on superconducting properties has remained unclear. Here we show that, near the Mott insulating phase, the upper critical field Hc2 of the fulleride superconductors reaches values as high as ∼90 T—the highest among cubic crystals. This is accompanied by a crossover from weak- to strong-coupling superconductivity and appears upon entering the metallic state with the dynamical Jahn–Teller effect as the Mott transition is approached. These results suggest that the cooperative interplay between molecular electronic structure and strong electron correlations plays a key role in realizing robust superconductivity with high-Tc and high-Hc2., Alkali-doped fullerides are superconductors but the impact of dimensionality and electron correlation remains unclear. Here, Kasahara et al. report an upper critical field about 90 T, suggesting cooperative interplay between molecular electronic structure and strong electron correlations.

The most expanded fcc-structured alkali fulleride, Cs3C60, is a Mott insulator at ambient pressure because of the weak overlap between the frontier t1u molecular orbitals of the C603− anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical temperature, Tc of 35 K upon compression. The effect of the localised t1u3 electronic configuration on the properties of the material is not well-understood. Here we study the relationship between the intrinsic crystallographic C603− orientational disorder and the molecular Jahn–Teller (JT) effect dynamics in the Mott insulating state. The high-resolution 13C magic-angle-spinning (MAS) NMR spectrum at room temperature comprises three peaks in the intensity ratio 1:2:2 consistent with the presence of three crystallographically-inequivalent carbon sites in the fcc unit cell and revealing that the JT-effect dynamics are fast on the NMR time-scale of 10−5 s despite the presence of the frozen-in C603− merohedral disorder disclosed by the 133Cs MAS NMR fine splitting of the tetrahedral and octahedral 133Cs resonances. Cooling to sub-liquid-nitrogen temperatures leads to severe broadening of both the 13C and 133Cs MAS NMR multiplets, which provides the signature of an increased number of inequivalent 13C and 133Cs sites. This is attributed to the freezing out of the C603− JT dynamics and the development of a t1u electronic orbital glass state guided by the merohedral disorder of the fcc structure. The observation of the dynamic and static JT effect in the Mott insulating state of the metrically cubic but merohedrally disordered Cs3C60 fulleride in different temperature ranges reveals the intimate relation between charge localization, magnetic ground state, lifting of electronic degeneracy, and orientational disorder in these strongly-correlated systems.

A 3 C 60 molecular superconductors share a common electronic phase diagram with unconventional high-temperature superconductors such as the cuprates: superconductivity emerges from an antiferromagnetic strongly correlated Mott-insulating state upon tuning a parameter such as pressure (bandwidth control) accompanied by a dome-shaped dependence of the critical temperature, T c . However, unlike atom-based superconductors, the parent state from which superconductivity emerges solely by changing an electronic parameter—the overlap between the outer wave functions of the constituent molecules—is controlled by the C 60 3− molecular electronic structure via the on-molecule Jahn–Teller effect influence of molecular geometry and spin state. Destruction of the parent Mott–Jahn–Teller state through chemical or physical pressurization yields an unconventional Jahn–Teller metal, where quasi-localized and itinerant electron behaviours coexist. Localized features gradually disappear with lattice contraction and conventional Fermi liquid behaviour is recovered. The nature of the underlying (correlated versus weak-coupling Bardeen–Cooper–Schrieffer theory) s-wave superconducting states mirrors the unconventional/conventional metal dichotomy: the highest superconducting critical temperature occurs at the crossover between Jahn–Teller and Fermi liquid metal when the Jahn–Teller distortion melts. This article is part of the themed issue ‘Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene’.

Ordering of the tetrahedral site vacancies in two crystals of refined compositions K0.93(1)Fe1.52(1)Se2 and K0.862(3)Fe1.563(4)Se2 produces a fivefold expansion of the parent ThCr2Si2 unit cell in the ab plane which can accommodate 20% vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, producing a potential doping mechanism which can operate at both average and local structure levels., Comment: 5 pages 3 figures accepted for publication in Chemical Science Chem. Sci., DOI:10.1039/C1SC00070E

Unconventional high-T c superconductivity, defined both in terms of the magnitude of the superconducting transition temperature, T c, and the key role played by electronic correlations, not only is the realm of atom-based low-dimensional layered systems such as the cuprates or the iron pnictides but is also accessible in molecular systems such as the cubic alkali fullerides with stoichiometry A3C60 (A=alkali metal). In fulleride superconductors, isotropic high-T c superconductivity occurs in competition with electronic ground states resulting from a fine balance between electron correlations and electron–phonon coupling in an electronic phase diagram strikingly similar to those of unconventional superconductors such as the cuprates and the heavy fermions. Superconductivity at the highest T c (38 K) known for any molecular material emerges from the antiferromagnetic insulating state solely by changing an electronic parameter – the overlap between the outer wave functions of the constituent molecules – and T c scales universally in a structure-independent dome-like relationship with proximity to the Mott metal–insulator transition (quantified by V, the volume/C60, or equivalently by (U/W), the ratio of the on-site Coulomb energy, U, to the electronic bandwidth, W), a hallmark of electron correlations characteristic of high-T c superconductors other than fullerides. The C60 molecular electronic structure plays a key role in the Mott–Jahn–Teller (MJT) insulator formed at large V, with the on-molecule dynamic Jahn–Teller (JT) effect distorting the C60 3– units and quenching the t 1u orbital degeneracy responsible for metallicity. As V decreases, the MJT insulator transforms first into an unconventional correlated JT metal (where localised electrons coexist with metallicity and the on-molecule distortion persists) and then into a Fermi liquid with a less prominent molecular electronic signature. This normal state crossover is mirrored in the evolution of the superconducting state, with the highest T c found at the boundary between unconventional correlated and conventional weak-coupling BCS superconductivity, where the interplay between extended and molecular aspects of the electronic structure is optimised to create the superconductivity dome.

The nanocrystalline valency-delocalized λ-phase of the binary oxide Ti(3)O(5) has recently emerged as a promising phase-change material that exhibits rapid photo-reversible optical and resistance changes at ambient temperature. Nanoscaling caused the monoclinic-λ to monoclinic charge-ordered β-phase structural instability to shift to considerably lower temperatures compared to the bulk material, and led to a broad thermal hysteresis. The structural transformation was accompanied by a large change in volume and large lattice relaxations, which imply the presence of strong electron-phonon coupling. We attribute the suppression of the phase transition to the enhanced surface energy on the nanoscale.

The ternary fulleride, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text], is the most expanded member of the family of face-centered cubic (fcc) structured superconducting fullerides ever accessed with superconductivity surviving at ambient pressure closest to the Mott insulator boundary. Here, we study the evolution of its structural and electronic properties with temperature. At ambient temperature, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text] lies in the Mott–Jahn–Teller (MJT) insulating part of the phase diagram. High-resolution synchrotron X-ray diffraction shows that its structure remains strictly cubic at all temperatures, but the transition to the metallic state at [Formula: see text] 50 K — evident in the evolution of the magnetic susceptibility with temperature — is accompanied by a lattice collapse, [Formula: see text]V/V0 of [Formula: see text]. Bulk superconductivity then emerges on further cooling with a T[Formula: see text] of 25.9 K. The results permit the extension of the electronic phase diagram of A3C[Formula: see text] fullerides as close as possible to the metal–insulator (M–I) crossover.

The crystal structure of a solid controls the interactions between the electronically active units and thus its electronic properties. In the high-temperature superconducting copper oxides, only one spatial arrangement of the electronically active Cu(2+) units-a two-dimensional square lattice-is available to study the competition between the cooperative electronic states of magnetic order and superconductivity. Crystals of the spherical molecular C(60)(3-) anion support both superconductivity and magnetism but can consist of fundamentally distinct three-dimensional arrangements of the anions. Superconductivity in the A(3)C(60) (A = alkali metal) fullerides has been exclusively associated with face-centred cubic (f.c.c.) packing of C(60)(3-) (refs 2, 3), but recently the most expanded (and thus having the highest superconducting transition temperature, T(c); ref. 4) composition Cs(3)C(60) has been isolated as a body-centred cubic (b.c.c.) packing, which supports both superconductivity and magnetic order. Here we isolate the f.c.c. polymorph of Cs(3)C(60) to show how the spatial arrangement of the electronically active units controls the competing superconducting and magnetic electronic ground states. Unlike all the other f.c.c. A(3)C(60) fullerides, f.c.c. Cs(3)C(60) is not a superconductor but a magnetic insulator at ambient pressure, and becomes superconducting under pressure. The magnetic ordering occurs at an order of magnitude lower temperature in the geometrically frustrated f.c.c. polymorph (Néel temperature T(N) = 2.2 K) than in the b.c.c.-based packing (T(N) = 46 K). The different lattice packings of C(60)(3-) change T(c) from 38 K in b.c.c. Cs(3)C(60) to 35 K in f.c.c. Cs(3)C(60) (the highest found in the f.c.c. A(3)C(60) family). The existence of two superconducting packings of the same electronically active unit reveals that T(c) scales universally in a structure-independent dome-like relationship with proximity to the Mott metal-insulator transition, which is governed by the role of electron correlations characteristic of high-temperature superconducting materials other than fullerides.

The ternary iron chalcogenide, Fe(1.03)Se(0.57)Te(0.43) is a member of the recently discovered family of Fe-based superconductors with an ambient pressure T(c) of 13.9 K and a simple structure comprising layers of edge-sharing distorted Fe(Se/Te)(4) tetrahedra separated by a van der Waals gap. Here we study the relationship between its structural and electronic responses to the application of pressure. T(c) depends sensitively on applied pressure attaining a broad maximum of 23.3 K at approximately 3 GPa. Further compression to 12 GPa leads to a metallic but nonsuperconducting ground state. High-resolution synchrotron X-ray diffraction shows that the superconducting phase is metrically orthorhombic at ambient pressure but pressurization to approximately 3 GPa leads to a structural transformation to a more distorted structure with monoclinic symmetry. The exact coincidence of the crystal symmetry crossover pressure with that at which T(c) is maximum reveals an intimate link between crystal and electronic structures of the iron chalcogenide superconductors.

The hyperexpanded (CH3NH2)K3C60 fulleride and its perdeuterated analogue were investigated by 1H and 2H NMR spectroscopy between room temperature and 4 K. Two phase transitions at TS = 220 K and TN = 11 K clearly were detected in the temperature dependence of the NMR line shapes, the spectral first (M1) and second (M2) moments, and the spin lattice relaxation times, T1. From 2H NMR line shape analysis, we found that the structural phase transition at TS was driven by the freezing out of the discrete Markovian-type jump motion of the entire K+-ND2CD3 unit that is characterized by an activation energy, Ea = 236(51) meV. Below TS, a sudden change in the temperature dependence of M1 suggests the appearance of a small but nonzero spin density on the K+-NH2CH3 unit. This could influence the electronic properties of the fulleride phase by modulating the strength of the exchange interactions between C603− anions and controlling the width, W, of the t1u-derived band. The 1H NMR spectra below TN show significant li...

The photoion yield spectra of an endohedral metallofullerene Pr@C 82 were measured in the photon energy range of 100–150 eV by using time-of-flight mass spectrometry. Parent ions Pr@C 82 + , Pr@C 82 2+ and Pr@C 82 3+ were observed in the mass spectra. The photoion yield spectra of Pr@C 82 2+ showed a broad peak at 120–140 eV that was assigned to the 4 d –4 f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C 82 were evaluated from the photoion yield spectra to be 37±12 Mb at 110 eV (off-resonance) and 52±13 Mb at 130 eV (on-resonance). These cross sections of Pr@C 82 were compared with the results of Ce@C 82 , the only metallofullerene whose photoionization properties have ever been studied near the 4 d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C 82 and Ce@C 82 , but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms.

C(60)-based solids are archetypal molecular superconductors with transition temperatures (Tc) as high as 33 K (refs 2-4). Tc of face-centred-cubic (f.c.c.) A(3)C(60) (A=alkali metal) increases monotonically with inter C(60) separation, which is controlled by the A(+) cation size. As Cs(+) is the largest such ion, Cs(3)C(60) is a key material in this family. Previous studies revealing trace superconductivity in Cs(x)C(60) materials have not identified the structure or composition of the superconducting phase owing to extremely small shielding fractions and low crystallinity. Here, we show that superconducting Cs(3)C(60) can be reproducibly isolated by solvent-controlled synthesis and has the highest Tc of any molecular material at 38 K. In contrast to other A(3)C(60) materials, two distinct cubic Cs(3)C(60) structures are accessible. Although f.c.c. Cs(3)C(60) can be synthesized, the superconducting phase has the A15 structure based uniquely among fullerides on body-centred-cubic packing. Application of hydrostatic pressure controllably tunes A15 Cs(3)C(60) from insulating at ambient pressure to superconducting without crystal structure change and reveals a broad maximum in Tc at approximately 7 kbar. We attribute the observed Tc maximum as a function of inter C(60)separation--unprecedented in fullerides but reminiscent of the atom-based cuprate superconductors--to the role of strong electronic correlations near the metal-insulator transition onset.

The electron paramagnetic resonance (EPR) technique is used to study the temperature evolution of the electronic and magnetic properties of the hyperexpanded fulleride (CH 3 NH 2 )K 3 C 60 in the temperature range 4-294 K. Unambiguous signatures for the development of large internal magnetic fields and a transition to an antiferromagnetic ground state below T N = 11 K are provided by the temperature dependence of the spin susceptibility, the EPR line width, and the position of the EPR line. The EPR results in the paramagnetic regime are indicative of an insulating electronic state, with localized S = 1/2 moments on the C 60 3- ions. The accompanying complex evolution of the spin susceptibility, EPR line width, and g-factor anisotropy furthermore reveals that effects of electronic origin (t 1u orbital order-disorder transition, crossover from static to dynamic Jahn-Teller distortions) in the vicinity of the structural transformation are important for the understanding of the exchange interactions between the fulleride ions.

A superconductivity dome is created by the electronic structure of the molecular building block of an unconventional superconductor., Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above Tc is a major challenge for all unconventional superconductors. The molecular A3C60 fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C603– electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller–active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of Tc with interfulleride separation, demonstrating molecular electronic structure control of superconductivity.

We describe a new group of well-organized cubic mesostructured metal chalcogenides. They were obtained from the assembly of the tetrahedral [SbSe4]3- unit and the linking metal ions In3+, Zn2+, and Cd2+ in the presence of Cn (n = 12, 14, 16) pyridinium surfactants acting as structure-directing agents. The assembly of the cubic gyroid mesostructure is immediate upon mixing the reactants. The cubic structure, confirmed with X-ray diffraction and transmission electron microscopy, contains an infinitely extended M/Sb/Se network that defines a well-known periodic minimal surface known as the gyroid and possesses an Ia3d space group symmetry. The ion In3+ is a more versatile linking metal and can also form a hexagonal mesostructure by changing the surfactant concentration or carbon chain length. The pore−pore separations and pore sizes are a function of surfactant chain length in all cases. The cubic mesophases exhibit reversible ion-exchange properties, although the cubic symmetry cannot be retained. All meso...

The photoion yield curves for Dy@C 82 z + ( z = 1 and 2) from Dy@C 82 are measured by using synchrotron radiation in the photon energy range from 24.5 to 39.5 eV. Correction has been made to compensate the effect of transient change of the density of Dy@C 82 in the interaction region, with the help of the yield curve of C 60 z + produced from C 60 remaining as a trace impurity in the sample. The yield of Dy@C 82 + exhibits a gradually descending curve with a flat region at 30–33 eV, similarly to the yield curve of C 60 + from C 60 . The total photoabsorption cross section of Dy@C 82 was evaluated to be (1.2 ± 0.4) × 10 −20 m 2 at the photon energy of 39.5 eV.

X-ray diffractions and electronic transports for the Ce@C82 isomers I and II, which refer to major and minor isomers, respectively, are studied in a wide temperature region to clarify the structura...

Location of the Dy ion in the minor isomer of Dy@C 82 was determined by Dy L III -edge EXAFS. The Dy ion is found to lie near the fused bond between the two hexagon rings. This finding is consistent with that predicted by a quantum-chemical calculation [Chem. Phys. Lett. 282 (1998) 325].

A solution chemistry synthetic route yields Cs3C60 with a face-centred cubic structure. The described method uses well-established Schlenk techniques and THF as a solvent. The controlled addition of an organo-metallic salt reducing agent prevents the formation of C604− salts. The final product can be precipitated from the solution using hexane as an anti-solvent.

We have investigated the low-temperature magnetic state of face-centered-cubic (fcc) ${\mathrm{Cs}}_{3}{\mathrm{C}}_{60}$, a Mott insulator and the first molecular analog of a geometrically frustrated Heisenberg fcc antiferromagnet with $S=1/2$ spins. Specific heat studies reveal the presence of both long-range antiferromagnetic ordering and a magnetically disordered state below ${T}_{N}=2.2$ K, which is in agreement with local probe experiments. These results together with the strongly suppressed ${T}_{N}$ are unexpected for conventional atom-based fcc antiferromagnets, implying that the fulleride molecular degrees of freedom give rise to the unique magnetic ground state.

The alkali fullerides, A(3)C(60) (A = alkali metal) are molecular superconductors that undergo a transition to a magnetic Mott-insulating state at large lattice parameters. However, although the size and the symmetry of the superconducting gap, Δ, are both crucial for the understanding of the pairing mechanism, they are currently unknown for superconducting fullerides close to the correlation-driven magnetic insulator. Here we report a comprehensive nuclear magnetic resonance (NMR) study of face-centred-cubic (f.c.c.) Cs(3)C(60) polymorph, which can be tuned continuously through the bandwidth-controlled Mott insulator-metal/superconductor transition by pressure. When superconductivity emerges from the insulating state at large interfullerene separations upon compression, we observe an isotropic (s-wave) Δ with a large gap-to-superconducting transition temperature ratio, 2Δ0/k(B)T(c) = 5.3(2) [Δ0 = Δ(0 K)]. 2Δ0/k(B)T(c) decreases continuously upon pressurization until it approaches a value of ~3.5, characteristic of weak-coupling BCS theory of superconductivity despite the dome-shaped dependence of Tc on interfullerene separation. The results indicate the importance of the electronic correlations for the pairing interaction as the metal/superconductor-insulator boundary is approached.

Two isomers of Dy@C 82 were separated by high performance liquid chromatography (HPLC), and their UV–VIS absorption spectra were measured to characterize these isomers. The crystalline powder of Dy@C 82 was obtained by removing the solvent (toluene) at 250°C under vacuum. The X-ray diffraction pattern can be indexed with fcc crystal lattice, as that in La@C 82 . The lattice constant a at 298 K, 15.86(1) A, is close to that of La@C 82 , 15.78 A. The distances between Dy and the first and second nearest C atoms are determined to be 2.52(2) and 2.86(2) A, respectively, on the basis of Dy L III -edge EXAFS. The XANES shows that the valence of the Dy atom in Dy@C 82 is +3.

The structure of La2@C80 is studied by La K-edge XAFS from 40 to 295 K. The distances between the La atom and the first nearest C atoms have been determined to be 2.42(1) A at 40 K and 2.44(2) A at 295 K, and those between the La atom and the second nearest C atoms to be 2.97(2) A at 40 K and 2.98(3) A at 295 K. The La–La distance has been determined to be 3.90(1) A at 40 K and 3.88(2) A at 295 K. The temperature dependence of the mean-square displacement of La–C is also studied to get an insight into the dynamical behavior of two La atoms in the C80 cage.

Eu endohedral C60, Eu@C60, has been extracted with aniline from soot prepared by arc-heating of a graphite/Eu2O3 composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@C60+. The UV–VIS absorption spectrum of this sample has a red-shift in the onset (>900 nm) in comparison with those for C60 and C70, as expected from electron transfer from the Eu atom to the C60 cage. The valence state of the Eu atom in Eu@C60 has been determined to be +2 by Eu LIII-edge XANES.

Temperature dependence of electron spin resonance in Na2C60 was studied in a temperature range from 2 to 350 K. It was shown that Na2C60 was metallic above 50 K and had a metal-insulator transition at 50 K. The center frequency for the Hg(2) Raman mode in Na2C60 at 298 K was close to those in the metallic Rb3C60, K3C60, and Cs3C60, while the linewidth was close to that in the metallic but nonsuperconducting Cs3C60. The Hg(2) mode showed a large blueshift and narrowing at 50 K. The center frequency and the linewidth in the low-temperature region from 50 K were almost the same as those in the insulating C-60 and Rb6C60, which showed the metal-insulator transition at 50 K in Na2C60. The origin of this metal-insulator transition was discussed in terms of the electron-phonon interaction (Jahn-Teller effect) and the electron-electron interaction (Mott-Hubbard picture). [S0163-1829(99)04123-5].

Temperature dependence of Raman scattering in a pressure-induced superconductor Cs3C60 was studied under ambient pressure between 2 and 290 K. The contributions of the intramolecular vibration modes to electron–phonon coupling were evaluated by comparing the linewidths of the modes in Cs3C60 with those in pristine C60. The total electron–phonon coupling constant λ was ≈0.18, approximately one-third of the values for Rb3C60 and K3C60. Temperature-dependent Raman scattering in the superconductor Rb3C60 and a non-superconductor Rb6C60 was also measured for comparison.

NaC60 was prepared and identified by Raman spectroscopy. Differential scanning calorimetry showed the existence of two stable phases above and below 280 K. The high-temperature phase was supercooled down to 205 K and further transformed to a glass state below 205 K. X-ray diffraction at 295 K could be indexed by a face-centered cubic lattice. The 23 Na-NMR spectra below 252 K exhibited the second-order nuclear quadrupole effect accompanied by a descent in symmetry at the site occupied by the Na+ ion. An ESR study has shown that the fcc NaC60 becomes nonmagnetic below 180 K.

Alkali-metal-intercalated FeSe materials, (NH3)(y)M0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)(y)Cs0.4FeSe shows a superconducting transition temperature (T-c) as high as 31.2 K, which is higher by 3.8 K than the T-c of nonammoniated Cs0.4FeSe. The T(c)s of (NH3)(y)K0.4FeSe and (NH3)(y)Rb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The T-c of (NH3)(y)Cs0.4FeSe shows a negative pressure dependence. A clear correlation between T-c and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity.

Cs3C60 is a correlated superconductor under pressure, but an insulator under ambient conditions. The mechanism causing this insulating behavior is the combination of Mott localization and the dynamic Jahn-Teller effect. We show evidence from infrared spectroscopy for the dynamic Jahn-Teller distortion. The continuous change with temperature of the splitting of infrared lines is typical Jahn-Teller behavior, reflecting the change in population of solid-state conformers. We conclude that the electronic and magnetic solid-state properties of the insulating state are controlled by molecular phenomena. We estimate the time scale of the dynamic Jahn-Teller effect to be above 10^(-11) s and the energy difference between the conformers less than 20 cm-1., 6 pages, 4 figures, 1 supplementary movie; XXI International Symposium on the Jahn-Teller Effect, Tsukuba, Japan, August 26-31, 2012

We report the observation of zero resistivity (\ensuremath{\rho}) in a hydrocarbon superconductor, and describe the temperature dependence of \ensuremath{\rho} in metal-doped hydrocarbons. The resistivity of K-doped picene (K${}_{x}$picene) has been recorded from pellet samples in a four-terminal measurement mode. A drop in $\ensuremath{\rho}$ is observed below 7 K for K${}_{3.1}$picene and below 11 K for K${}_{3.5}$picene, which clearly displays zero resistivity. The resistivity drop at 7 K is consistent with the superconducting critical temperature (${T}_{\mathrm{c}}$) obtained from the magnetic susceptibility of the 7 K phase of K${}_{3}$picene, while the drop at 11 K is inconsistent with the ${T}_{\mathrm{c}}$'s of both the 7 and 18 K phases of K${}_{3}$picene reported previously. The temperature dependence of \ensuremath{\rho} for both samples exhibits granular-metal-like behavior in the normal state.

We report electronic structure and physical properties of metal doped picene as well as selective synthesis of the phase exhibiting 18 K superconducting transition. First, Raman scattering is used to characterize the number of electrons transferred from the dopants to picene molecules. The charge transfer leads to a softening of Raman scattering peaks, which enables us to determine the number of transferred electrons. From this we have identified that three electrons are transferred to each picene molecule in the superconducting doped-picene solids. Second, we report the pressure dependence of Tc in 7 and 18 K phases of K3picene. The 7 K phase shows a negative pressure-dependence, while the 18 K phase exhibits a positive pressure-dependence which cannot be understood with a simple phonon mechanism of BCS superconductivity. Third, we report a new synthesis method for superconducting K3picene by a solution process with monomethylamine, CH3NH2. This method enables one to prepare selectively the K3picene sample exhibiting 18 K superconducting transition. The discovery of suitable way for preparing K3picene with Tc = 18 K may facilitate clarification of the mechanism of superconductivity.

Expanded alkali fullerides are molecular superconductors with a Mott parent insulating state. Klupp et al. use infrared spectroscopy to detect the molecular Jahn–Teller distortion of fulleride ions and establish its relevance to strongly correlated superconductivity.