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Register a new userStructural transition and anisotropic properties of single crystalline SrFe2As2
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Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.
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The structural properties of the SrFe2As2 and CaFe2As2 compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20K. The experimental results demonstrate that the SrFe2As2 crystal, in consistence with previous x-ray data, has a tetragonal structure at room temperature and undergoes a tetragonal (T)-orthorhombic (O) phase transition at about 210K. Moreover, twinning lamella arising from T-O transition evidently appears in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudo-periodic structural modulation with the periodicity of around 40nm at room temperature. This modulation is likely in connection with the local structural distortions within the Ca layer. In-situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase.
Single crystalline, single phase CaKFe$_{4}$As$_{4}$ has been grown out of a high temperature, quaternary melt. Temperature dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization and specific heat, combined with field dependent measurements of electrical resistivity and field and pressure dependent measurements of magnetization indicate that CaKFe$_{4}$As$_{4}$ is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, $T_c$ = 35.0 $pm$ 0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8 K $leq T leq$ 300 K. All of these thermodynamic and transport data reveal striking similarities to that found for optimally- or slightly over-doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, suggesting that stoichiometric CaKFe$_4$As$_4$ is intrinsically close to what is referred to as optimal-doped on a generalized, Fe-based superconductor, phase diagram. The anisotropic superconducting upper critical field, $H_{ctext{2}}(T)$, of CaKFe$_{4}$As$_{4}$ was determined up to 630 kOe. The anisotropy parameter $gamma(T)=H_{ctext{2}}^{perp}/H_{ctext{2}}^{|}$, for $H$ applied perpendicular and parallel to the c-axis, decreases from $simeq 2.5$ at $T_c$ to $simeq 1.5$ at 25 K which can be explained by interplay of paramagnetic pairbreaking and orbital effects. The slopes of $dH_{ctext{2}}^{|}/dTsimeq-44$ kOe/K and $dH_{ctext{2}}^{perp}/dT simeq-109$ kOe/K at $T_c$ yield an electron mass anisotropy of $m_{perp}/m_{|}simeq 1/6$ and short Ginzburg-Landau coherence lengths $xi_{|}(0)simeq 5.8 text{AA}$ and $xi_{perp}(0)simeq 14.3 text{AA}$. The value of $H_{ctext{2}}^{perp}(0)$ can be extrapolated to $simeq 920$ kOe, well above the BCS paramagnetic limit.
Doped BaCoSO was recently predicted to be a high-temperature superconductor in a new class based on Co and Ni. Using a Co-S self flux method, we synthesized single crystals of the antiferromagnetic insulator BaCoSO. Our magnetic and specific heat measurements and neutron diffraction provide details of its magnetic anisotropy and order. Its band gap was determined to be about 1.3 eV by our measurements of its photoemission spectrum and infrared optical conductivity. Our results can pave the way to exploring the predicted superconductivity in this Co-based material.
We report temperature and thermal-cycling dependence of surface and bulk structures of double-layered perovskite Sr3Ru2O7 single crystals. The surface and bulk structures were investigated using low-energy electron diffraction (LEED) and single-crystal X-ray diffraction (XRD) techniques, respectively. Single-crystal XRD data is in good agreement with previous reports for the bulk structure with RuO6 octahedral rotation, which increases with decreasing temperature (~ 6.7(6)degrees at 300 K and ~ 8.1(2) degrees at 90 K). LEED results reveal that the octahedra at the surface are much more distorted with a higher rotation angle (~ 12 degrees between 300 and 80 K) and a slight tilt ((4.5pm2.5) degrees at 300 K and (2.5pm1.7) degrees at 80 K). While XRD data confirms temperature dependence of the unit cell height/width ratio (i.e. lattice parameter c divided by the average of parameters a and b) found in a prior neutron powder diffraction investigation, both bulk and surface structures display little change with thermal cycles between 300 and 80 K.
Single crystals of RbOs2O6 have been grown from Rb2O and Os in sealed quartz ampoules. The crystal structure has been identified at room temperature as cubic with the lattice constant a = 10.1242(12) A. The anisotropy of the tetrahedral and octahedral networks is lower and the displacement parameters of alkali metal atoms are smaller than for KOs2O6, so the rattling of the alkali atoms in RbOs2O6 is less pronounced. Superconducting properties of RbOs2O6 in the mixed state have been well described within the London approach and the Ginzburg-Landau parameter kappa(0) = 31 has been derived from the reversible magnetization. This parameter is field dependent and changes at low temperatures from kappa = 22 (low fields) to kappa = 31 at H_{c2}. The thermodynamic critical field H_{c}(0) = 1.3 kOe and the superconducting gap 2delta/k_{B}T_{c} = 3.2 have been estimated. These results together with slightly different H_{c2}(T) dependence obtained for crystals and polycrystalline RbOs2O6 proof evidently that this compound is a weak-coupling BCS-type superconductor close to the dirty limit.
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