P. K. Maheshwari

Flux free growth of large FeSe1/2Te1/2 superconducting single crystals by an easy high temperature melt and slow cooling method

We report successful growth of flux free large single crystals of superconducting FeSe1/2Te1/2 with typical dimensions of up to few cm. The AC and DC magnetic measurements revealed the superconducting transition temperature (Tc) value of around 11.5K and the iso-thermal MH showed typical type-II superconducting behavior. The lower critical field being estimated by measuring the low field iso-thermal magnetization in superconducting regime is found to be above 200 Oe at 0K.

Structural, and Magnetic properties of flux free large FeTe Single crystal

We report synthesis of non-superconducting parent compound of iron chalcogenide, i.e., FeTe single crystal by self-flux method. The FeTe single crystal is crystallized in a tetragonal structure with the P4/nmm space group. The detailed scanning electron microscopy (SEM) results showed that the crystals are formed in slab-like morphology and are near (slight deficiency of Te) stoichiometric with homogenous distribution of Fe and Te. The coupled structural and magnetic phase transition is seen at around 70 K in both electrical resistivity and magnetization measurements, which is hysteric (ΔT = 5 K) in nature with cooling and warming cycles. Magnetic susceptibility (χ−T) measurements showed the magnetic transition to be of antiferromagnetic in nature, which is substantiated by isothermal magnetization (M-H) plots as well. The temperature-dependent electrical resistivity measured in 10-kOe field in both in-plane and out-of-plane field directions showed that the hysteric width nearly becomes double to ΔT = 10 K and is maximum for the out-of-plane field direction for the studied FeTe single crystal. We also obtained a sharp spike-like peak in heat capacity Cp(T) measurement due to the coupled structural and magnetic order phase transitions.

Flux free growth of superconducting FeSe single crystals

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.

Heat Capacity and Mössbauer Study of Self-Flux Grown FeTe Single Crystal

We report mainly the heat capacity and Mössbauer study of self-flux grown FeTe single crystal, which is a ground state compound of the Fe chalcogenides superconducting series i.e. FeTe1−x(Se/S)x The as grown FeTe single crystal is large enough to the tune of a few centimetres and the same crystallizes in tetragonal structure having space group of P4/nmm. FeTe shows the structural/magnetic phase transition at 70 K in both magnetic and resistivity measurements. Heat capacity measurement also confirms the coupled structural/magnetic transition at the same temperature. The Debye model fitting of low temperature (below 70 K) heat capacity exhibited Debye temperature (𝜃D) to be 324 K. Mössbauer spectra are performed at 300 and 5 K. The 300-K spectra showed two paramagnetic doublets and the 5-K spectra exhibited hyperfine magnetic sextet with an average hyperfine field of 10.6 Tesla matching with the results of Yoshikazu Mizuguchi et al.

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. 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.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=0.1...

Structural and Transport Studies of Under-Doped FeTe 1 − x Se x (x =0.0, 0.01, 0.03, 0.05) Single Crystals

We report the synthesis of under-doped non-superconducting parent compound of iron chalcogenide, i.e, FeTe 1−xSe x (x =0.0, 0.01, 0.03, 0.05) single crystals by flux-free method. These FeTe 1−xSe x (x =0.0, 0.01, 0.03, 0.05) single crystals are crystallized in the tetragonal structure with space group P4/nmm at room temperature. Detailed scanning electron microscopy (SEM) results showed that the crystals are formed in slab-like morphology structure. Powder X-ray diffraction (XRD) results show that (0 0 l) peaks are shifted towards higher angel with increasing Se concentration. Coupled magnetic and structural phase transition temperature, being seen as a step in resistivity, decreases from around 67K for x =0.0 to 55K for x =0.05 crystal. Further, the step in resistivity is seen to be hysteric (ΔT) in nature, i.e., different step temperatures in cooling and warming cycles. Interestingly, the hysteric (ΔT) becomes narrower from around 5.8 K for x =0.0 to 0.95 K for x = 0.05 crystal.