We report a comparative study of the series Fe1.1Te1-xSex and the stoichiometric FeTe1-xSex to bring out the difference in their magnetic, superconducting and electronic properties. The Fe1.1Te1-xSex series is found to be magnetic and its microscopic properties are elucidated through Moessbauer spectroscopy. The magnetic phase diagram of Fe1.1Te1-xSex is traced out and it shows the emergence of spin-glass state when the antiferromagnetic state is destabilized by the Se substitution. The isomer shift and quadrupolar splitting obtained from the Moessbauer spectroscopy clearly brings out the electronic differences in these two series.
We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide superconductor FeTe1-xSex to elucidate the electronic states relevant to the superconductivity. While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x = 0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a well-defined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realized. The present result suggests the evolution from incoherent to coherent electronic states and its close relationship to the emergence of superconductivity.
We investigated the effects of Se substitution on the lattice constants and superconducting properties of CeO0.5F0.5Bi(S1-xSex)2. With increasing Se concentration, the a lattice constant increased, while the c lattice constant did not show any significant increase between x = 0.1 and x = 0.5. Bulk superconductivity was observed in samples with x = 0.2-0.4, and the superconducting transition temperature was the highest at x = 0.3. The obtained superconductivity phase diagram was compared to those of LaO0.5F0.5Bi(S1-xSex)2 and NdO0.5F0.5Bi(S1-xSex)2.
We have systematically investigated the crystal structure, magnetic susceptibility, and electrical resistivity of the BiS2-based superconductor LaO0.5F0.5Bi(S1-xSex)2 (x = 0 - 0.7). With expanding lattice volume by Se substitution, bulk superconductivity was induced for x > 0.2, and the highest Tc of 3.8 K was observed in x = 0.5 (LaO0.5F0.5BiSSe). Metallic conductivity was observed for x > 0.3 in the resistivity measurement, whereas semiconducting-like behavior was observed for x < 0.2. The induction of bulk superconductivity by the partial substitution of S by Se in the LaO0.5F0.5BiS2 superconductor should be positively linked to the enhancement of metallic conductivity.
We synthesized Nax(H3O)zCoO2yH2O samples with various Na/H3O ratios but with the constant Co valence of s = +3.40, and measured their magnetic properties to draw phase diagrams of the system. The superconductivity is very sensitive to the Na/H3O ratio. With varying x under fixed s of +3.40, magnetically ordered phase appears in the intermediate range of x sandwiched by two separated superconducting phases, suggesting that the superconductivity is induced by moderately strong magnetic interactions. In the vicinity of the magnetic phase, transition from the superconducting state to the magnetically ordered state was induced by applying high magnetic field. This transition is of the second order, at least, above 1.8 K. The upper-critical field is expected to be much higher than the Pauli limit for a phase located far away from the magnetic phase regarding the Na/H3O ratio.
The recently discovered (Rb,Cs)EuFe4As4 compounds exhibit an unusual combination of superconductivity (Tc = 35 K) and ferromagnetism (Tm = 15 K). We have performed a series of x-ray diffraction, ac magnetic susceptibility, dc magnetization, and electrical resistivity measurements on both RbEuFe4As4 and CsEuFe4As4 to pressures as high as 30 GPa. We find that the superconductivity onset is suppressed monotonically by pressure while the magnetic transition is enhanced at initial rates of dTm/dP = 1.7 K/GPa and 1.5 K/GPa for RbEuFe4As4 and CsEuFe4As4, respectively. Near 7 GPa, Tc onset and Tm become comparable. At higher pressures, signatures of bulk superconductivity gradually disappear. Room temperature x-ray diffraction measurements suggest the onset of a transition from tetragonal (T) to a half collapsed-tetragonal (hcT) phase at 10 GPa (RbEuFe4As4) and 12 GPa (CsEuFe4As4). The ability to tune Tc and Tm into coincidence with relatively modest pressures highlights (Rb,Cs)EuFe4As4 compounds as ideal systems to study the interplay of superconductivity and ferromagnetism.
P. L. Paulose
,C. S. Yadav
,K. M. Subhedar
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(2009)
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"Magnetic phase diagram of Fe1.1Te1-xSex: A comparative study with the stoichiometric superconducting FeTe1-xSex system"
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P.L Paulose
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