10 research outputs found
Synthesis, crystal structure and chemical stability of the superconductor FeSe_{1-x}
We report on a comparative study of the crystal structure and the magnetic
properties of FeSe1-x (x= 0.00 - 0.15) superconducting samples by neutron
powder diffraction and magnetization measurements. The samples were synthesized
by two different methods: a 'low-temperature' one using powders as a starting
material at T =700 C and a "high-temperature' method using solid pieces of Fe
and Se at T= 1070 C. The effect of a starting (nominal) stoichiometry on the
phase purity of the obtained samples, the superconducting transition
temperature Tc, as well as the chemical instability of FeSe1-x at ambient
conditions were investigated. It was found that in the Fe-Se system a stable
phase exhibiting superconductivity at Tc~8K exists in a narrow range of
selenium concentration (FeSe0.974(2)).Comment: 7 pages, 7 figures, 1 tabl
Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe_1-x
The Fe isotope effect (Fe-IE) on the transition temperature T_c and the
crystal structure was studied in the Fe chalcogenide superconductor FeSe_1-x by
means of magnetization and neutron powder diffraction (NPD). The substitution
of natural Fe (containing \simeq 92% of ^{56}Fe) by its lighter ^{54}Fe isotope
leads to a shift of T_c of 0.22(5)K corresponding to an Fe-IE exponent of
\alpha_Fe=0.81(15). Simultaneously, a small structural change with isotope
substitution is observed by NDP which may contribute to the total Fe isotope
shift of T_c.Comment: 4 pages, 3 figure
Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5
Iron-chalcogenide single crystals with the nominal composition
FeSeTe and a transition temperature of 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, SQUID and torque
magnetometry, and muon-spin rotation. Room temperature neutron powder
diffraction reveals that 95% of the crystal volume is of the same tetragonal
structure as PbO. The structure refinement yields a stoichiometry of
Fe_1.045Se_0.406Te_0.594. Additionally, a minor hexagonal Fe_7Se_8 impurity
phase was identified. The magnetic penetration depth \lambda at zero
temperature was found to be 491(8) nm in the ab-plane and 1320(14) nm along the
c-axis. The zero-temperature value of the superfluid density \rho_s(0)
\lambda^-2(0) obeys the empirical Uemura relation observed for various
unconventional superconductors, including cuprates and iron-pnictides. The
temperature dependences of both \lambda_ab and \lambda_c are well described by
a two-gap s+s-wave model with the zero-temperature gap values of
\Delta_S(0)=0.51(3) meV and \Delta_L(0)=2.61(9) meV for the small and the large
gap, respectively. The magnetic penetration depth anisotropy parameter
\gamma_\lambda(T)=\lambda_c(T)/\lambda_{ab}(T) increases with decreasing
temperature, in agreement with \gamma_\lambda(T) observed in the iron-pnictide
superconductors
Coexistence of magnetism and superconductivity in the iron-based compound Cs0.8(FeSe0.98)2
We report on muon-spin rotation and relaxation (μSR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs(0.8)(FeSe(0.98))(2). Whereas our transport and magnetization data confirm the bulk character of the superconducting state below T(c)=29.6(2) K, the μSR data indicate that the system is magnetic below T(N)=478.5(3) K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs(0.8)(FeSe(0.98))(2) a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed T(N) is the highest reported to date for a magnetic superconductor
Coexistence of incommensurate magnetism and superconductivity in Fe_{1+y}Se_xTe_{1-x}
We report an investigation into the superconducting and magnetic properties
of Fe_{1+y}Se_{x}Te_{1-x} single crystals by magnetic susceptibility, muon spin
rotation, and neutron diffraction. We find three regimes of behavior in the
phase diagram for 0\leq x\leq 0.5: (i) commensurate magnetic order for x< 0.1,
(ii) bulk superconductivity for , and (iii) a range \sim
0.25\leq x\leq 0.45 in which superconductivity coexists with static
incommensurate magnetic order. The results are qualitatively consistent with a
two-band mean-field model in which itinerant magnetism and extended s-wave
superconductivity are competing order parameters.Comment: 4 pages, 4 figure
Coexistence of incommensurate magnetism and superconductivity in Fe_{1+y}Se_xTe_{1-x}
We report an investigation into the superconducting and magnetic properties
of Fe_{1+y}Se_{x}Te_{1-x} single crystals by magnetic susceptibility, muon spin
rotation, and neutron diffraction. We find three regimes of behavior in the
phase diagram for 0\leq x\leq 0.5: (i) commensurate magnetic order for x< 0.1,
(ii) bulk superconductivity for , and (iii) a range \sim
0.25\leq x\leq 0.45 in which superconductivity coexists with static
incommensurate magnetic order. The results are qualitatively consistent with a
two-band mean-field model in which itinerant magnetism and extended s-wave
superconductivity are competing order parameters.Comment: 4 pages, 4 figure
Effects of Disorder in FeSe : An Ab Initio Study
Using the coherent-potential approximation, we have studied the effects of
excess Fe, Se-deficiency, and substitutions of S, Te on Se sub-lattice and Co,
Ni and Cu on Fe sub-lattice in FeSe. Our results show that (i) a small amount
of excess Fe substantially disorders the Fe-derived bands while Se-deficiency
affects mainly the Se-derived bands, (ii) the substitution of S or Te enhances
the possibility of Fermi surface nesting, specially in FeSeTe,
in spite of disordering the Se-derived bands, (iii) the electron doping through
Co, Ni or Cu disorders the system and pushes down the Fe-derived bands, thereby
destroying the possibility of Fermi surface nesting. A comparison of these
results with the rigid-band, virtual-crystal and supercell approximations
reveals the importance of describing disorder with the coherent-potential
approximation.Comment: Redone VCA calculations, and some minor changes. (Accepted for
publication in Journal of Physics:Condensed Matter
Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc=27K
We report on the synthesis of large single crystals of a new FeSe-layer
superconductor Cs0.8(FeSe0.98)2. X-ray powder diffraction, neutron
powder-diffraction and magnetization measurements have been used to compare the
crystal structure and the magnetic properties of Cs0.8(FeSe0.98)2 with those of
the recently discovered potassium intercalated system KxFe2Se2. The new
compound Cs0.8(FeSe0.98)2 shows a slightly lower superconducting transition
temperature (Tc=27.4 K) in comparison to 29.5 in K0.8(FeSe0.98)2). The volume
of the crystal unit cell increases by replacing K by Cs - the c-parameter grows
from 14.1353(13) {\AA} to 15.2846(11) {\AA}. For the so far known alkali metal
intercalated layered compounds (K0.8Fe2Se2 and Cs0.8(FeSe0.98)2) the Tc
dependence on the anion height (distance between Fe-layers and Se-layers) was
found to be analogous to those reported for As-containing Fe-superconductors
and Fe(Se1-xChx), where Ch=Te, S.Comment: 8 pages, 4 figure
Low-temperature Synthesis of FeTe0.5Se0.5 Polycrystals with a High Transport Critical Current Density
We have prepared high-quality polycrystalline FeTe0.5Se0.5 at temperature as
low as 550{\deg}C. The transport critical current density evaluated by the
current-voltage characteristics is over 700 A/cm2 at 4.2 K under zero field,
which is several times larger than FeTe0.5Se0.5 superconducting wires. The
critical current density estimated from magneto-optical images of flux
penetration is also similar to this value. The upper critical field of the
polycrystalline FeTe0.5Se0.5 at T = 0 K estimated by
Werthamer-Helfand-Hohenberg theory is 585 kOe, which is comparable to that of
single crystals. This study gives some insight into how to improve the
performance of FeTe0.5Se0.5 superconducting wires.Comment: 12 pages, 6 figure