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Register a new userThermal expansion of LaFeAsO: Evidence for high-temperature fluctuations
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We present measurements of the thermal expansion coefficient alpha of polycrystalline LaFeAsO1-xFx (x <= 0.1). The magnetic and structural transitions of the samples with x <= 0.04 give rise to large anomalies in alpha(T), while the onset of superconductivity in the crystals with x >= 0.05 is not resolved. Above the structural transition, the thermal expansion coefficient of LaFeAsO is significantly enhanced. This is attributed to fluctuations, which also affect the electrical transport properties of the compound. The complete absence of these fluctuations in the superconducting samples even for x = 0.05 is taken as evidence for an abrupt first-order type of suppression of the structural and magnetic transitions upon F doping.
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The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0 $leq x leq$ 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies, the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at T$rm{_{S}}$~=~130~K which is rapidly suppressed by $x simeq$ 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120~K. The temperature dependence of the thermal expansion coefficient $alpha_{V}$ reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering. At much lower temperatures than T$rm{_{N}}$, SmFeAsO exhibits a significant negative thermal expansion on the order of -40~ppm~$cdot$~K$^{-1}$ in contrast to the behavior of other rare-earth oxypnictides such as PrFeAsO and the actinide oxypnictide NpFeAsO where the onset of $alpha <$ 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements.
Single crystals of LaFeAsO were successfully grown out of KI flux. Temperature dependent electrical resistivity was measured with current flow along the basal plane, rho_perpend(T), as well as with current flow along the crystallographic c-axis, rho_parallel(T), the latter one utilizing electron beam lithography and argon ion beam milling. The anisotropy ratio was found to lie between rho_parallel/rho_perpend = 20 - 200. The measurement of rho_perpend(T) was performed with current flow along the tetragonal [1 0 0] direction and along the [1 1 0] direction and revealed a clear in-plane anisotropy already at T leq 175 K. This is significantly above the orthorhombic distortion at T_0 = 147 K and indicates the formation of an electron nematic phase. Magnetic susceptibility and electrical resistivity give evidence for a change of the magnetic structure of the iron atoms from antiferromagnetic to ferromagnetic arrangement along the c-axis at T^ast = 11 K.
This paper has been withdrawn by the authors due to errors in the X-ray diffraction data. Other measured data are not affected; however, the errors significantly change the interpretation and conclusions, and thus warrant withdrawal and later resubmission.
Within the phase fluctuation picture for the pseudogap state of a high-$T_{c}$ superconductor, we incorporate the phase fluctuations generated by the classical XY model with the Bogoliubov-de Gennes formalism utilizing a field-theoretical method. This picture delineates the inhomogeneous characteristics of local order parameters observed in high-$T_{c}$ superconductors above $T_{c}$. We also compute the local density of states near a non-magnetic impurity with a strong scattering potential. The resonance peak smoothly evolves as temperature increases through $T_{c}$ without showing any sudden broadening, which is consistent with recent experimental findings.
Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of Tc is 21 K at ~12 GPa. The pressure dependence of the Tc is similar to those of LaFeAsO1-xFx series reported previously.
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