Abstract
We report synthesis of Co substitution at Fe site in Fe1-xCoxSe0.5Te0.5 (x=0.0 to 0.10) single crystals via vacuum shield solid state reaction route using flux free method. Single crystal XRD results showed that these crystals grow in (00l) plane i.e., orientation in c-direction. All the crystals possess tetragonal structure having P4/nmm space group. Detailed scanning electron microscopy (SEM) images show that the crystals are grown in slab-like morphology. The EDAX results revealed the final elemental composition to be near stoichiometric. Powder X-Ray diffraction (PXRD) Rietveld analysis results show that (00l) peaks are shifted towards higher angle with increasing Co concentration. Both a and c lattice parameters decrease with increasing Co concentration in Fe1-xCoxSe0.5Te0.5 (x=0.0 to 0.10) single crystals. Low temperature transport and magnetic measurements show that the superconducting transition temperature (Tc), decreases from around 12K to 10K and 4K for x=0.03 and x=0.05 respectively. For x=0.10 crystal superconductivity is not observed down to 2K. Electrical resistivity measurement of Fe0.97Co0.03Se0.5Te0.5 single crystal under magnetic field up to 14Tesla for H//ab and H//c clearly showed the anisotropy nature of superconductivity in these crystals. The upper critical field Hc2(0), being calculated using conventional one band Werthamer–Helfand–Hohenberg (WHH) equation, for x=0.03 crystal comes around 70Tesla, 45Tesla and 35Tesla for normal state resistivity criterion ρn=90%, 50% and 10% criterion respectively for H//c and around 100Tesla, 75Tesla and 60Tesla respectively for H//ab. The activation energy of Fe0.97Co0.03Se0.5Te0.5 single crystal is calculated with the help of TAFF model for both H//c and H//ab direction. In conclusion, Co substitution at Fe site in Fe1-xCoxSe0.5Te0.5 suppresses superconductivity.
Highlights
Discovery of superconductivity in Fe based superconductor had been one of the most surprising discoveries for both experimental and theoretical condensed matter physicist.[1,2,3,4,5,6] Both iron pnictides[1,2,3,4] and chalcogenides[5,6] based superconductors had become one of the biggest surprises after the discovery of high Tc (HTSc) cuprates[7,8] in condensed matter physics
The flux free growth of Fe site Ni doped Fe1-xNixSe0.5Te0.5 single crystals was reported by us and fast suppression of superconducting transition temperature was seen with Ni content.[30]
Co substitution at Fe site in Fe1-xCoxSe0.5Te0.5 suppresses superconductivity, the rate of T c suppression is much less in comparison to Fe1-xNixSe0.5Te0.5.30 In our knowledge, this is first detailed study on Fe1-xCoxSe0.5Te0.5 (x=0.0 to 0.10) series of large flux free single crystals, earlier studies were mostly either on tiny and flux assisted single crystals[20,21,22,23] or on polycrystalline samples.[15,16,17,18]
Summary
Discovery of superconductivity in Fe based superconductor had been one of the most surprising discoveries for both experimental and theoretical condensed matter physicist.[1,2,3,4,5,6] Both iron pnictides[1,2,3,4] and chalcogenides[5,6] based superconductors had become one of the biggest surprises after the discovery of high Tc (HTSc) cuprates[7,8] in condensed matter physics. In case of Fe chalcogenide superconductors, the most of the literature reported pertaining to Fe site 3d metal doping is on polycrystalline samples.[15,16,17,18] The single crystal growth of Fe chalcogenide superconductors is quite complicated and is mostly possible with added flux (KCl/NaCl) only, further the obtained single crystal were of tiny (mm) size.[20,21,22,23] The flux free growth of Fe chalcogenides single crystal is possible by using Bridgman method with complicated heating schedules.[24,25,26] the flux free growth of large single crystals of FeSe1-xTex (x=0.00 and x=0.50) was reported recently[27,28,29] with simple heat treatment schedule and on normal automated furnace.[28,29] Very recently, the flux free growth of Fe site Ni doped Fe1-xNixSe0.5Te0.5 single crystals was reported by us and fast suppression of superconducting transition temperature was seen with Ni content.[30]. Co substitution at Fe site in Fe1-xCoxSe0.5Te0.5 suppresses superconductivity, the rate of T c suppression is much less in comparison to Fe1-xNixSe0.5Te0.5.30 In our knowledge, this is first detailed study on Fe1-xCoxSe0.5Te0.5 (x=0.0 to 0.10) series of large flux free single crystals, earlier studies were mostly either on tiny and flux assisted single crystals[20,21,22,23] or on polycrystalline samples.[15,16,17,18]
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