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Register a new userElectronic and Superconducting Properties of Some FeSe-based Single Crystals and Films Grown Hydrothermally
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Our recent years studies of the prototypal FeSe and molecule-intercalated (Li,Fe)OHFeSe superconductor systems are briefly reviewed here, with emphasis on experimental observations of the link between the superconductivity and normal-state electronic property in the single crystals and films. These samples were successfully synthesized by our recently developed soft-chemical hydrothermal methods, which are also briefly described. Particularly in the Mn-doped high-Tc (Li,Fe)OHFeSe film, a strong enhancement of the superconducting critical current density was achieved, which is promising for practical application of the superconductivity.
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We report flux free growth of superconducting FeSe single crystals by an easy and versatile high temperature melt and slow cooling method for first time. The room temperature XRD on the surface of the piece of such obtained crystals showed single 101 plane of Beta-FeSe tetragonal phase. The bulk powder XRD, being obtained by crushing the part of crystal chunk showed majority tetragonal and minority FeSe hexagonal crystalline phases. Detailed HRTEM images along with SAED (selected area electron diffraction) showed the abundance of both majority and minority FeSe phases. Both transport (RT) and magnetization (MT) exhibited superconductivity at below around 10K. Interestingly, the magnetization signal of these crystals is dominated by the magnetism of minority magnetic phase, and hence the isothermal magnetization (MH) at 4K was seen to be ferromagnetic (FM) like. Transport (R-T) measurements under magnetic field showed superconductivity onset at below 12K, and R = 0 (Tc) at 9K. Superconducting transition temperature (Tc) decreases with applied field to around 6K at 7Tesla, with dTc/dH of 0.4K/Tesla, giving rise to an Hc2 value of around 50 Tesla, 30 Tesla and 20 Tesla for Rn = 90, 50 and 10 percent respectively. FeSe single crystal activation energy is calculated from Thermally Activated Flux Flow (TAFF) model which is found to decreases with field.
We report on the synthesis of superconducting single crystals of FeSe, and their characterization by X-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that TC increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure X-ray diffraction study up to 12 GPa on a powder sample, and we find that between 8.5 GPa and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16%, and is accompanied by the appearance of an intermediate, probably orthorhombic phase.
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.
We report scanning tunnelling microscopy and spectroscopy (STM/STS) studies on one and two unit cell (UC) high FeSe thin films grown on Bi$_2$Se$_3$(0001). In our thin films, we find the tetragonal phase of FeSe and dumb-bell shaped defects oriented along Se-Se bond directions. In addition, we observe striped moire patterns with a periodicity of ($7.3pm 0.1$) nm generated by the mismatch between the FeSe lattice and the Bi$_2$Se$_3$ lattice. We could not find any signature of a superconducting gap in the tunneling spectra measured on the surface of one and two UC thick islands of FeSe down to 6.5 K. The spectra rather show an asymmetric behavior across and a finite density of states at the Fermi level ($E_F$) resembling those taken in the normal state of bulk FeSe.
The results of the comprehensive ultrasonic research of high quality single crystals of FeSe are presented. Absolute values of sound velocities and their temperature dependences were measured; elastic constants and Debye temperature were calculated. The elastic C11-C12 and C11 constants undergo significant softening under the structural tetra-ortho transformation. The significant influence of the superconducting transition on the velocity and attenuation of sound was revealed and the value of the superconducting energy gap was estimated.
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