Synthesis, crystal structure and chemical stability of the superconductor FeSe_{1-x}

We report on a comparative study of the crystal structure and the magnetic properties of FeSe1-x (x= 0.00 - 0.15) superconducting samples by neutron powder diffraction and magnetization measurements. The samples were synthesized by two different methods: a 'low-temperature' one using powders as a starting material at T =700 C and a "high-temperature' method using solid pieces of Fe and Se at T= 1070 C. The effect of a starting (nominal) stoichiometry on the phase purity of the obtained samples, the superconducting transition temperature Tc, as well as the chemical instability of FeSe1-x at ambient conditions were investigated. It was found that in the Fe-Se system a stable phase exhibiting superconductivity at Tc~8K exists in a narrow range of selenium concentration (FeSe0.974(2)).Comment: 7 pages, 7 figures, 1 tabl

Negative Oxygen Isotope Effect on the Static Spin Stripe Order in La_(2-x)Ba_xCuO_4 (x = 1/8)

Large negative oxygen-isotope (16O/18O) effects (OIE's) on the static spin-stripe ordering temperature T_so and the magnetic volume fraction V_m were observed in La_(2-x)Ba_xCuO_4 (x = 1/8) by means of muon spin rotation experiments. The corresponding OIE exponents were found to be alpha_(T_so) = -0.57(6) and alpha_(V_m) = -0.71(9), which are sign reversed to alpha_(T_c) = 0.46(6) measured for the superconducting transition temperature T_c. This indicates that the electron-lattice interaction is involved in the stripe formation and plays an important role in the competition between bulk superconductivity and static stripe order in the cuprates.Comment: 5 pages, 4 figure

Intrinsic and structural isotope effects in Fe-based superconductors

The currently available results of the isotope effect on the superconducting transition temperature T_c in Fe-based high-temperature superconductors (HTS) are highly controversial. The values of the Fe isotope effect (Fe-IE) exponent \alpha_Fe for various families of Fe-based HTS were found to be as well positive, as negative, or even be exceedingly larger than the BCS value \alpha_BCS=0.5. Here we demonstrate that the Fe isotope substitution causes small structural modifications which, in turn, affect T_c. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of \alpha~0.35-0.4 is observed for at least three different families of Fe-based HTS.Comment: 4 pages, 2 figure

Evolution of two-gap behavior of the superconductor FeSe_1-x

The superfluid density, \rho_s, of the iron chalcogenide superconductor, FeSe_1-x, was studied as a function of pressure by means of muon-spin rotation. The zero-temperature value of \rho_s increases with increasing transition temperature T_c (increasing pressure) following the tendency observed for various Fe-based and cuprate superconductors. The analysis of \rho_s(T) within the two-gap scheme reveals that the effect on both, T_c and \rho_s(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.Comment: 5 pages, 3 figure

Coherency of the superconducting state: the muon spin rotation and ARPES studies of (BiPb)_2(SrLa)_2CuO_{6+\delta}

The superfluid density \rho_s in underdoped (T_c\simeq23K), optimally doped (T_c\simeq35K) and overdoped (T_c\simeq29K) single crystalline (BiPb)_2(SrLa)_2CuO_{6+\delta} samples was studied by means of muon-spin rotation (\muSR). By combining the \muSR data with the results of ARPES measurements on similar samples [Nature 457, 296 (2009)] good self-consistent agreement is obtained between two techniques concerning the temperature and the doping evolution of \rho_s.Comment: 4 pages, 3 figures

Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)

We report the temperature dependent mid- and near-infrared spectra of K4C60, Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C604- which brings the fulleride anion from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K4C60 and Rb4C60. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high temperature. In K4C60 and Rb4C60 we suggest a transition from a static to a dynamic Jahn-Teller state without changing the average structure. We studied the librations of these two fullerides by temperature dependent inelastic neutron scattering and conclude that both pseudorotation and jump reorientation are present in the dynamic Jahn-Teller state.Comment: 13 pages, 10 figures, to be published in Phys. Rev.

Pressure Induced Static Magnetic Order in Superconducting FeSe_1-x

We report on a detailed investigation of the electronic phase diagram of FeSe_1-x under pressures up to 1.4GPa by means of AC magnetization and muon-spin rotation. At a pressure \simeq0.8GPa the non-magnetic and superconducting FeSe_1-x enters a region where long range static magnetic order is realized above T_c and bulk superconductivity coexists and competes on short length scales with the magnetic order below T_c. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.Comment: 5 pages, 3 figure

Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La2−x_{2-x}Srx_{x}CuO4_{4}

The oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{\ast}$ is investigated for the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure (XANES) studies. A strong $x$ dependent and sign reversed OIE on $T^{\ast}$ is observed. The OIE exponent $\alpha_{T^{\ast}}$ systematically decreases from $\alpha_{T^{\ast}} = - 0.6(1.3)$ for $x = 0.15$ to $\alpha_{T^{\ast}} = - 4.4(1.1)$ for $x = 0.06$, corresponding to increasing $T^{\ast}$ and decreasing superconducting transition temperature $T_{c}$. Both $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ exhibit a linear doping dependence with different slopes and critical end points (where $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ fall to zero) at $x_{c}(^{16}{\rm O}) = 0.201(4)$ and $x_{c}(^{18}{\rm O}) = 0.182(3)$, indicating a large positive OIE of $x_{c}$ with an exponent of $\alpha_{x_{c}} = 0.84(22)$. The remarkably large and strongly doping dependent OIE on $T^{\ast}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state

Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5

Iron-chalcogenide single crystals with the nominal composition FeSe$_{0.5}$Te$_{0.5}$ and a transition temperature of $T_{c}\simeq14.6$ K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe_1.045Se_0.406Te_0.594. Additionally, a minor hexagonal Fe_7Se_8 impurity phase was identified. The magnetic penetration depth \lambda at zero temperature was found to be 491(8) nm in the ab-plane and 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density \rho_s(0) \lambda^-2(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron-pnictides. The temperature dependences of both \lambda_ab and \lambda_c are well described by a two-gap s+s-wave model with the zero-temperature gap values of \Delta_S(0)=0.51(3) meV and \Delta_L(0)=2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter \gamma_\lambda(T)=\lambda_c(T)/\lambda_{ab}(T) increases with decreasing temperature, in agreement with \gamma_\lambda(T) observed in the iron-pnictide superconductors

Tuning the superconducting and magnetic properties in Fe_ySe_0.25Te_0.75 by varying the Fe-content

The superconducting and magnetic properties of Fe$_{y}$Se$_{0.25}$Te$_{0.75}$ single crystals ($0.9\leq y \leq1.1$) were studied by means of x-ray diffraction, SQUID magnetometry, muon spin rotation, and elastic neutron diffraction. The samples with $y<1$ exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for $y\geq 1$, but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe$_{y}$Se$_{1-x}$Te$_{x}$ can be tuned not only by varying the Se/Te ratio but also by changing the Fe content