Impact of Fe site Co substitution on superconductivity of Fe1-xCoxSe0.5Te0.5 (x = 0.0 to 0.10): A flux free single crystal study

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.


Introduction
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 T c (HTSc) cuprates [7,8] in condensed matter physics. The ground state of parent non superconducting Fe based compounds is known to be magnetically ordered one. Fe based superconducting compounds i.e., pnictides and chalcogenides are also known to be outside the well known conventional superconducting BCS theory [9], similar to high T c cuprates. For the condensed matter theorist's community, theoretical explanation for high T c superconductor is biggest challenge yet.
As far as iron chalcogenides [Fe(Se 1-x Te x )] are concerned, they possess simplest crystal structure. Though FeTe does not show any superconducting transition [10], the FeSe exhibits superconducting transition at around 8K [11]. In FeSe 1-x Te x series, the highest T c of around 15K is seen for the FeSe 0.5 Te 0.5 at ambient pressure. [12][13][14]. 15K superconductivity of FeSe 0.5 Te 0.5 is reported to decrease with 3d metal (Ni/Co/Cr/Zn) doping at Fe site [15][16][17][18][19]. For in depth understanding of the effect of 3d metal doping at Fe site in FeSe 0.5 Te 0.5 , one would prefer doped single crystals rather than poly-crystal ones. Theoreticians would like to model single crystals data rather than the poly-crystals, as the latter ones do have extrinsic (grain boundary etc.) contributions along with the intrinsic material characteristics. 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]. However, the flux free growth of large single crystals of FeSe 1-x Te x (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 Fe 1-x Ni x Se 0.5 Te 0.5 single crystals was reported by us and fast suppression of superconducting transition temperature was seen with Ni content [30].
Keeping in view, the need of flux free 3d metal doped FeSe 0.5 Te 0.5 large single crystals and in continuation to our recent work on Fe 1-x Ni x Se 0.5 Te 0.5 single crystals [30], here we report flux free large single crystal (cm size) growth and superconductivity properties of Fe site Co doped Fe 1- x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) series, with the similar protocol reported very recently for Fe 1- x Ni x Se 0.5 Te 0.5 [30]. Large single crystals of Fe 1-x Co x Se 0.5 Te 0.5 are obtained till 10% Co doping at Fe site. It is seen that Co substitution at Fe site in Fe 1-x Co x Se 0.5 Te 0.5 suppresses superconductivity, although the rate of T c suppression is much less in comparison to Fe 1-x Ni x Se 0.5 Te 0.5 [30]. In our knowledge, this is first detailed study on Fe 1-x Co x Se 0.5 Te 0.5 (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].

Experimental Details
All the Fe 1-x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) single crystals were grown in a normal automated furnace using flux free method. The essential elements i.e., Fe, Co, Se and Te powder of high purity better than 4N are taken in stoichiometric ratio i.e., Fe 1-x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) and grinded in Ar gas filled glove box. This grinded powder is pelletized by applying hydrostatic pressure of 100kg/cm 2 , and sealed in a quartz tube with the vacuum of less than 10 -3 torr. These sealed quartz tubes are kept in a normal programmable automated furnace, and heated up to 1000 0 C for 24 hours with an intermediate step of 450 0 C with the rate of 2 0 C/minute. Finally the furnace is cool down very slowly up to room temperature with the rate of 10 0 C/hour. Thus obtained crystals were very shiny and big in size and their photographs are shown in Figure 1. X-ray diffraction (XRD) of as obtained single crystals and their crushed powder was done using Rigaku X-ray diffractometer with CuKα radiation of 1.54184Å at room temperature. Scanning electron microscopy (SEM) pictures has been taken on ZEISS-EVO-10 electron microscope to understand the morphology of Fe 1- x Co x Se 0.5 Te 0.5 single crystals. Electrical and magnetic measurements were performed on Quantum Design Physical Property Measurement System (PPMS-14Tesla) down to 2K and up to magnetic field of 14Tesla. Figure 1 shows the photograph of a piece of as synthesized Fe 1-x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) single crystals after breaking the quartz tube. All the synthesized samples are looking very shiny and in single crystalline form. As synthesized all the crystals are of cm sizes. As all the samples are synthesized via self flux method so there is no requirement to remove foreign flux. The large size flux free method grown crystals are certainly more attractive for the physical property measurements. All the measurements were done on a small piece, taken from obtained crystals.

Results and Discussion
SEM is very useful tool to understand the morphology of the as synthesized material. Figure   2 (a-d) shows the SEM and EDAX results of the Fe 1-x Co x Se 0.5 Te 0.5 for x=0.03 and 0.10 crystals.
Un-doped x=0.00 i.e., FeSe 0.5 Te 0.5 single crystal SEM and EDAX results are already shown elsewhere [28]. Fig 2 (a) and 2(b) show the SEM images of Fe 0.97 Co 0.03 Se 0.5 Te 0.5 and Fe 0.90 Co 0.10 Se 0.5 Te 0.5 single crystal respectively. It is clearly seen that the morphology of the synthesized crystals is slab like and the growth is layer by layer. The morphology of these crystals is similar to FeSe 1-x Te x single crystals being reported earlier [28][29][30]. EDAX analysis is used for compositional analysis of the studied crystals.  Further they are near stoichiometric, i.e., the starting composition is intact. It is clear from the structural analysis that all the studied crystals of Fe 1-x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) are cm size (Fig.1), with slab like morphology (Fig 2) and their growth is in single oriented direction i.e., in (00l) plane (Fig 3). Powder XRD and Rietveld refinement results (Fig. 4) show shrinkage in volume, as both a and c lattice parameter decrease, with Co concentration in Fe 1- x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) crystals. Thus confirming that smaller ion Co is substituted successfully at Fe site in Fe 1-x Co x Se 0.5 Te 0.5 till x = 0.10.
The resistivity verses temperature (ρ-T) plots for all the Fe 1-x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10) single crystals in temperature range of 300K to 2K at zero magnetic field are shown in Though clearly the T c decreases with Co substitution at Fe site in Fe 1-x Co x Se 0.5 Te 0.5 (x=0.0 to 0.10), the rate of suppression is not as fast as in case of Ni doping at Fe site in Fe 1-x Ni x Se 0.5 Te 0.5 single crystals [30]. For example, though superconductivity is completely suppressed for x = 0.07 in Fe 1- x Ni x Se 0.5 Te 0.5 and semiconducting behavior was seen down to 2K [30], but the same happens only at x =0.10 for studied Fe 1-x Co x Se 0.5 Te 0.5 . for H//c, and is plotted in Fig 7(a). While for H//ab normal resistivity criterion of ρ n = 90%, 50% and 10%, the H c2 (0) is around 100Tesla, 70Tesla and 60Tesla respectively, which is plotted in Fig 7(b).
Clearly is lower than the pristine FeSe 0.5 Te 0.5 single crystal [28], the lower critical field [H c1 (0)] is higher.  [31,32]. clearly seen that in TAFF region, Lnρ vs 1/T graph would be linearly fitted and the fitted linear region with magnetic fields is shown in Fig 9(a) and Fig 9(b) for H//c and H//ab respectively. All the linearly fitted extrapolated lines are intercepted at the same temperature i.e., one being nearly equal to the superconducting transition temperature (T c ) which comes around 11.5K and 11.1K for H//c and H//ab respectively. Resistivity broadening with magnetic field is known to be due to thermally assisted flux motion [36]. Seemingly, the resistivity broadening in Fe 0.97 Co 0.03 Se 0.5 Te 0.5 single crystal is similar to FeSe 1-x Te x based superconductor with increasing magnetic field [37].
Thermally Activation energy of Fe 0.97 Co 0.03 Se 0.5 Te 0.5 single crystal is calculated for different magnetic fields from range 1Tesla to 14Tesla for both the direction . Fig 9(c) shows the magnetic fields dependency of thermally activation energy for both the direction. As the activation energy is different for both the directions, superconducting anisotropy effect is clearly evident in the studied crystal. Activation energy variation is in wide range with magnetic fields for both directions, this result shows that creep of thermally activated vortices gets affected with increasing magnetic fields.
Activation energy varies from 43meV to 12meV with the range of magnetic field from 1Tesla to 12Tesla for H//c direction and 36meV to 11meV with the range of magnetic field from 2Tesla to 14Tesla for H//ab direction. This activation energy is smaller than the activation energy of FeSe 0.5 Te 0.5 single crystal [28]. is similar to that as reported earlier by us for pristine FeSe 0.5 Te 0.5 single crystal [28].

Conclusion
We have successfully grown