The Annealing Effects in the Iron-Based Superconductor FeTe0.8Se0.2 Prepared by the Self-Flux Method (original) (raw)
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Microstructural magnetic phases in superconducting FeTe 0.65 Se 0.35
Superconductor Science and Technology, 2012
In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogenities in the iron chalcogenide system FeTe 1-x Se x and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe 0.65 Se 0.35 studied with transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate a presence of nanometre scale hexagonal regions coexisting with tetragonal host lattice, a chemical disorder demonstrating non homogeneous distribution of host atoms in the crystal lattice, as well as hundreds-of-nanometres-long iron-deficient bands. From magnetic data and ferromagnetic resonance temperature dependence, we attribute magnetic phases in Fe-Te-Se to Fe 3 O 4 inclusions and to hexagonal symmetry nanometre scale regions with structure of Fe 7 Se 8 type. Our results suggest that nonhomogeneous distribution of host atoms might be an intrinsic feature of superconducting Fe-Te-Se chalcogenides and we find a surprising correlation indicating that faster grown crystal of inferior crystallographic properties is a better superconductor.
Synthesis and superconducting properties of FeTe1-xSe Single Crystals under high magnetic fields
2011
We compare the superconducting phase-diagram under high magnetic fields (up to H = 45 T) of Fe1+ySe0.4Te0.6 single crystals originally grown by the Bridgman-Stockbarger (BRST) technique, which were annealed to display narrow superconducting transitions and the optimal transition temperature Tc 14 K, with the diagram for samples of similar stoichiometry grown by the travelingsolvent floating-zone technique as well as with the phase-diagram reported for crystals grown by a self-flux method. We find that the so-annealed samples tend to display higher ratios Hc2/Tc, particularly for fields applied along the inter-planar direction, where the upper critical field Hc2(T ) exhibits a pronounced downward curvature followed by saturation at lower temperatures T . This last observation is consistent with previous studies indicating that this system is Pauli limited. An analysis of our Hc2(T ) data using a multiband theory suggests the emergence of the Farrel-Fulde-Larkin-Ovchnikov state at low temperatures. A detailed structural x-ray analysis, reveals no impurity phases but an appreciable degree of mosaicity in as-grown BRST single-crystals which remains unaffected by the annealing process. Energy-dispersive x-ray analysis showed that the annealed samples have a more homogeneous stoichiometric distribution of both Fe and Se with virtually the same content of interstitial Fe as the non-annealed ones. Thus, we conclude that stoichiometric disorder, in contrast to structural disorder, is detrimental to the superconducting phase diagram of this series under high magnetic fields. Finally, a scaling analysis of the fluctuation conductivity in the superconducting critical regime, suggests that the superconducting fluctuations have a two-dimensional character in this system.
Detailed physical property characterization of FeTe1-xSex x = 0.0 to 0.50 single crystals
arXiv (Cornell University), 2019
Here, we report self flux single crystal growth of FeTe 1-x Se x (0.00≤ x ≤ 0.50) series via solid state reaction route; the resulted crystals as seen are shiny. X-Ray diffraction (XRD) performed on the surface of crystals elucidated the growth in (00l) plane, i.e. orientation in cdirection only. Scanning electron microscopy (SEM) images showed slab like morphology and EDX (Energy dispersive X-ray analyzer) confirmed that the crystals are closed to their designed compositions. Rietveld analysis of the XRD patterns of crushed crystal powders showed that the cell parameters decrease with Se content increase. Coupled magnetic/structural phase transition temperature, seen as a step in resistivity for the lower Se concentration i.e. 0.00 ≤ x ≤ 0.07, decreases from around 65K for x=0.0 to 50K for x=0.07 and it is not detected for higher x values. Superconductivity is observed by resistivity measurement for higher Se concentration i.e. 0.07 ≤ x ≤ 0.50, up to a maximum temperature of 14K at x=0.50. Thermally Activated Flux Flow (TAFF) analysis based on high field transport measurements in superconducting region done for x=0.20 crystal exhibited activated flux energy to be decreasing from 12meV (0.5Tesla) to 4.6meV (14Tesla). Raman spectroscopy at room temperature of synthesized samples exhibits all the allowed phonon modes with slight shift to higher frequency with Se content. Mossbauer spectra of FeTe 1- x Se x crystals series were recorded at 300 and 5K. At 5K, the average hyperfine field decreases systematically with Se content increase from 10.6 to 6.1Tesla for x=0.0 to x=0.20 samples. This indicates a possibility of co-existing magnetism and superconductivity in 0.07 ≤ x ≤ 0.20 crystals. For x=0.50 sample, no hyperfine field related to magnetic ordering is seen. Based on above results, detailed phase diagram of the FeTe 1-x Se x (0.00 ≤ x ≤ 0.50) compounds is defined in the present study.
2010
Syntheses of superconducting iron chalcogenides FeSe1-x (x = 0-0.15) and FeTe1-ySey (y = 0.3-0.55) were performed. Superconducting phase of iron selenide was obtained by the solid-state reaction and from liquid phase. The highest values of critical temperature (T-c = 8.2-8.7 K) exhibit FeSe1-x obtained by the crystallization from a melt with excess of iron less than 1 mol\%. The samples from a melt contain up to 78\% of tetragonal phase, as estimated by the X-ray diffraction. Lattice parameters and unit cell volume for the samples exhibiting highest T-c and sharpest transition to superconducting state are limited to narrow range, with c/a ratio close to 1.469. The samples with excess of selenium contain higher amount of hexagonal phase than stoichiometric one. Superconducting single-crystalline samples of FeTe1-ySey (up to 100\% of tetragonal phase) were obtained using Bridgman's method. When y value increases, the volume of unit cell decreases. The critical temperature T-c cha...
Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5
2010
Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of T-c similar or equal to 14.6 K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal x-ray and neutron powder diffraction, superconducting quantum interference device and torque magnetometry, and muon-spin rotation (mu SR). Room
Physical properties, crystal and magnetic structure of layered Fe1.11Te1- x Se x superconductors
The European Physical Journal B, 2011
The physical and structural properties of Fe1.11Te and Fe1.11Te0.5Se0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe1.11Te0.5Se0.5 compound, the superconducting transition with Tc = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H c2 is obtained by measuring the resistivity under different magnetic fields. The Kim's critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe1.11Te0.5Se0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.
Composition and annealing effects on superconductivity in sintered and arc-melted Fe1+εTe0.5Se0.5
Journal of Physics: Conference Series, 2018
We present the results of x-ray diffraction, electrical resistivity, and ac magnetic susceptibility measurements on specimens of the "11"-structure superconductor Fe 1+ε Te 0.50 Se 0.50 (0 ≤ ε ≤ 0.15). Samples were initially either sintered in sealed quartz tubes or melted in a zirconium-gettered arc furnace. Sintered samples were fired two to three times at temperatures of 425°C, 600°C, or 675°C, while arc-melted samples were studied both asmelted and after annealing at 650°C. X-ray diffraction data show a predominant PbO-type tetragonal phase, with a secondary hexagonal NiAs-type phase; for sintered specimens annealed at 600°C, the secondary phase decreases as ε increases over the range 0 ≤ ε ≤ 0.10, with the composition Fe 1.10 Te 0.5 Se 0.5 exhibiting x-ray phase purity. A higher annealing temperature of 675°C provided such tetragonal phase purity at the composition Fe 1.05 Te 0.5 Se 0.5. The resistive superconducting transition temperature T c was nearly independent of the iron concentration 1+ε, suggesting a single superconducting phase, while the magnetic screening fraction varied greatly with concentration and conditions, peaking at ε = 0.07, indicating that the amount of superconducting phase is strongly dependent on conditions. We propose that the behaviour can also be viewed in terms of an electron-doped, chalcogen-deficient stoichiometry.
Role of interstitial “caged” Fe in the superconductivity of FeTe1/2Se1/2
Solid State Communications, 2011
All samples are synthesized through standard solid state reaction route and are quenched to room temperature systematically at 700 0 C, 500 0 C, 300 0 C and room temperature (RT); named as 700Q, 500Q, 300Q and RTQ respectively. The structural and magnetic properties are studied. Careful Reitveld analysis of XRD patterns revealed that though all samples except 700Q are crystallized in single phase with space group P4/nmm, the presence of interstitial Fe (Fe int) at 2c site is increased from 5% for RTQ to 8% for 500Q. The 700Q sample is crystallized in Fe 7 Se 8 phase. The transport and magnetization results revealed that though RTQ and 300Q are superconducting at 10 K and 13 K respectively, while the 500Q and 700Q are not. Magnetic ordering (T mag) is observed at around 125 K for all the samples. The prominence of T mag in terms of effective moment is sufficiently higher for 500Q and 700Q than RTQ and 300Q. Summarily it is found that quenching induced disorder affects the occupancy of interstitial Fe in FeTe 1/2 Se 1/2 and thus both its superconducting and magnetic properties. Further it clear that limited presence of interstitial Fe at 2c site is not fully against observation of superconductivity, because 300Q sample possesses higher T c (13 K) for higher Fe int (6%) than RTQ sample with relatively lower T c (10 K) having lower Fe int (5%). Further the 500Q sample with much higher Fe int (8%) is though non-superconducting.
Magnetism and superconductivity of S-substituted FeTe
Journal of Alloys and Compounds, 2015
The influence of a partial substitution with sulphur into Te sites on the crystal, electronic and magnetic structures of FeTe is investigated by DFT calculations. The results reveal a phase transition from the antiferromagnetic double-stripe order for pure FeTe to the single-stripe order for S-substituted samples, which coincides with the previously observed appearance of the superconducting state. The magnetic transition is caused by the variations of the average chalcogen position in the unit cell. The analyzed normalstate properties of Fe(Te,S) and Fe(Se;S) compounds allow a detection of the well resolved nesting-driven magnetic fluctuations only for superconducting samples, consistent with their antiferromagnetic ground state. Thus, the role of an S-substitution is a suppression of the double-stripe antiferromagnetic order to give rise to the single-stripe correlations, which are associated with an occurrence of superconductivity in Fe(Te,S) solid solutions.