58 research outputs found
Crystal structure and phase transitions across the metal-superconductor boundary in the SmFeAsO1-xFx (0 < x < 0.20) family
The fluorine-doped rare-earth iron oxyarsenides, REFeAsO1-xFx (RE =rare
earth) have recently emerged as a new family of high-temperature
superconductors with transition temperatures (Tc) as high as 55 K (refs 1-4).
Early work has provided compelling evidence that the undoped parent materials
exhibit spin-density-wave (SDW) antiferromagnetic order and undergo a
structural phase transition from tetragonal to orthorhombic crystal symmetry
upon cooling.5 Both the magnetic and structural instabilities are suppressed
upon doping with fluoride ions before the appearance of superconductivity.6,7
Here we use high-resolution synchrotron X-ray diffraction to study the
structural properties of SmFeAsO1-xFx (0 < x < 0.20) in which superconductivity
emerges near x ~ 0.07 and Tc increases monotonically with doping up to x ~
0.20.8 We find that orthorhombic symmetry survives through the
metal-superconductor boundary well into the superconducting regime 2 and the
structural distortion is only suppressed at doping levels, x > 0.15 when the
superconducting phase becomes metrically tetragonal. Remarkably this crystal
symmetry crossover coincides with reported drastic anomalies in the resistivity
and the Hall coefficient8 and a switch of the pressure coefficient of Tc from
positive to negative,9 thereby implying that the low-temperature structure
plays a key role in defining the electronic properties of these
superconductors
Low Temperature Magnetic Instabilities in Triply Charged Fulleride Polymers
The electronic properties of the C603- polymer in Na2Rb0.3Cs0.7C60 were studied by X-band and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On further cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below TN≈15K, as confirmed by the observation of an antiferromagnetic resonance mode. The Na2Rb1-xCsxC60 family of polymers offers a unique way to chemically control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive function of the interchain separation
Mott localization in the correlated superconductor Cs3C60 resulting from the molecular Jahn-Teller effect
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
ROTATIONAL-DYNAMICS OF SOLID C-70 - A NEUTRON-SCATTERING STUDY
PMID: 10011126PMID: 10011126 This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K.PMID: 10011126 This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K.PMID: 10011126 This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K.We report the results of neutron-diffraction and low-energy neutron-inelastic-scattering experiments on high-purity solid C-70 between 10 and 640 K. Thermal hysteresis effects are found to accompany structural changes both on cooling and on heating. The observed diffuse scattering intensity does not change with temperature. At 10 K broad librational peaks are observed at 1.82(16) meV [full width at half maximum=1.8(5) meV]. The peaks soften and broaden further with increasing temperature. At and above room temperature, they collapse into a single quasielastic line. At 300 K, the diffusive reorientational motion appears to be somewhat anisotropic, becoming less so with increasing temperature. An isotropic rotational diffusion model, in which the motions of adjacent molecules are uncorrelated, describes well the results at 525 K. The temperature dependence of the rotational diffusion constants is consistent with a thermally activated process having an activation energy of 32(7) meV.This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K
Optimized unconventional superconductivity in a molecular Jahn-Teller metal
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
Crystal structure of the new FeSe1-x superconductor
The newly discovered superconductor FeSe1-x (x=0.08, Tconset=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 REFeAsO1-xFx (RE = rare earth) superconductorsComment: Chem. Commun. (2008
Structural and electronic response upon hole-doping of rare-earth iron oxyarsenides Nd1-xSrxFeAsO (0 < x < 0.2)
Hole-doping of NdFeAsO via partial replacement of Nd3+ by Sr2+ is a
successful route to obtain superconducting phases (Tc = 13.5 K for a Sr2+
content of 20%); however, the structural and electronic response with doping is
different from and non-symmetric to that in the electron-doped side of the
phase diagram.Comment: 4 pages, 4 figure
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