The newly discovered superconductor FeSe(1-x) (x approximately 0.08, T(c)(onset) approximately 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure-the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO(1-x)F(x) (RE = rare earth) superconductors.

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.

to negative, 9 thereby implying that the low-temperature structure plays a key role in defining the electronic properties of these superconductors.The possible mechanism of superconductivity in the REFeAsO 1-x F x and related REFeAsO 1-δ materials is currently unknown. The rapidly developing structural and electronic phenomenology points to considerable similarities with the well-established behaviour of high-T c cuprate superconductors and early theoretical work has suggested that conventional electron-phonon coupling mechanisms are not able to account for the high T c , implying non-BCS origin of the pairing interactions. [10][11][12][13] The parent REFeAsO phases exhibit both a structural and a magnetic phase transition on cooling in a similar fashion to the parent cuprate phase, La 2 CuO 4 . 5,14 Upon doping with fluoride ions, again much like La 2-x Sr x CuO 4 , both the crystallographic and magnetic transitions are suppressed in the superconducting compositions, 6,7 while T c first increases smoothly before passing over a maximum value at an optimal level of doping. Detailed experimental mapping of the structural and electronic phase diagrams as the doping level varies is necessary before we achieve a fundamental understanding of the superconductivity mechanism.Here we probed the temperature evolution of the structural properties of the However, the structural behaviour of the SmFeAsO 1-x F x compositions is very different on cooling. No reflections violating tetragonal extinction rules are evident for the heavily-doped compositions with x = 0.15 and 0.20 ( Fig. 1e and 1f), in which both lattice constants, a and c decrease smoothly with their crystal structure remaining strictly tetragonal down to 20 K ( Fig. 2e and 2f). The rate of contraction, dlna/dT and dlnc/dT at ~5 and ~18 ppm K -1 for the a and c lattice constants, respectively is considerably anisotropic and leads to a gradual decrease of the (c/a) ratio with decreasing temperature. This behaviour is in sharp contrast to the observed thermal structural response of the SmFeAsO 1-x F x (x = 0, 0.05, 0.10, and 0.12) compositions. In these systems, the tetragonal structure is initially robust upon cooling showing a normal contraction of the lattice parameters and interatomic distances. However, as the samples are cooled further, all hkl (h, k ≠ 0) reflections in the diffraction profiles begin first to 4 broaden before splitting at a characteristic temperature, T s (Fig. 1a-1d Supplementary Table S1.The most prominent point arising from the results of the present structural refinements as a function of both temperature and composition is the survival of the orthorhombic crystal symmetry in SmFeAsO 1-x F x well beyond the onset of superconductivity. Crossing the metal-to-superconductor boundary at x ~ 0.07 is not accompanied by the complete suppression of the orthorhombic-to-tetragonal structural phase transition and, as for both x = 0.10 and 0.12 compositions studied here T s > T c , both superconducting phases are orthorhombically distorted (Fig. 3). A...

The 'expanded fulleride' Cs(3)C(60) is an antiferromagnetic insulator in its normal state and becomes a molecular superconductor with T(c) as high as 38 K under pressure. There is mounting evidence that superconductivity is not of the conventional BCS type and electron-electron interactions are essential for its explanation. Here we present evidence for the dynamic Jahn-Teller effect as the source of the dramatic change in electronic structure occurring during the transition from the metallic to the localized state. We apply infrared spectroscopy, which can detect subtle changes in the shape of the C(60)3- ion due to the Jahn-Teller distortion. The temperature dependence of the spectra in the insulating phase can be explained by the gradual transformation from two temperature-dependent solid-state conformers to a single one, typical and unique for Jahn-Teller systems. These results unequivocally establish the relevance of the dynamic Jahn-Teller effect to overcoming Hund's rule and forming a low-spin state, leading to a magnetic Mott-Jahn-Teller insulator.

The superconducting transition temperature of the high-Tc SmO1-xFxFeAs superconductors increases monotonically as the F-doping level x increases to 0.20. High-pressure magnetization experiments reveal a strong sensitivity of Tc to interatomic distances in the underdoped regime (x

We have investigated the absorption of cyclohexane vapor into polystyrene films and the resulting polymer interdiffusion that is enabled by this process. The swelling of films due to solvent absorption was relatively insensitive to the film thickness or polymer molecular weight but increased with increasing temperature. The interdiffusion enabled by absorbed solvent was examined in multilayered films comprising alternating layers of deuteriopolystyrene (dPS) and hydrogenous polystyrene (hPS). Deuterium concentration gradients were measured in dry films following exposure to cyclohexane vapor at controlled temperatures and times to establish the relationship between the coefficient of solvent accelerated interdiffusion, D*, and (i) M w of hPS, (ii) Mw of both hPS and dPS, (iii) temperature, and (iv) film thickness. Scaling relationships were established for D* as a function of molecular weight. D* ∼ Mw -1 for increasing hPS molecular weight for Mw(hPS) < Mw(dPS) and was independent of Mw(hPS) for Mw(hPS) > Mw(dPS). This indicates that the lower molecular weight polymer, in keeping with predictions of "fast-mode" diffusion theory, dominates the rate of interdiffusion. When both dPS and hPS molecular weights were increased simultaneously, the interdiffusion coefficient scaled with Mw -1.8 . A marked increase in D* with increasing solvent temperature was observed, consistent with the temperature dependence of film swelling. A strong and unexpected correlation between D* and total film thickness was found. This effect was attributed to prolonged solvent retention in thicker films following exposure to solvent vapor.

The most expanded fcc-structured alkali fulleride, Cs 3 C 60 is a Mott insulator at ambient pressure because of the weak overlap between the frontier t 1u molecular orbitals of the C 60 3-anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical temperature, T c of 35 K upon compression. The effect of the localised t 1u 3 electronic configuration on the properties of the material is not wellunderstood. Here we study the relationship between the intrinsic crystallographic C 60 3− orientational disorder and the molecular Jahn-Teller (JT) effect dynamics in the Mott insulating state. The high-resolution 13 C 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-