arXiv subjects

Y. Mizuguchi

Publications and source records attributed to Y. Mizuguchi.

At least 37 records · Page 2Linked to original sources

arXiv2013

Observation of isosceles triangular electronic structure around excess iron atoms in Fe$_{1+δ}$Te

We present scanning tunneling microscopy and spectroscopy studies around an individual excess Fe atom, working as a local perturbation, in the parent material of the iron-chalcogenide superconductor Fe$_{1+δ}$Te. Spectroscopic imaging reveals a novel isosceles triangular electronic structure around the excess Fe atoms. Its spatial symmetry reects the underlying bicollinear antiferromagnetic spin state and the structural monoclinic symmetry. These findings provide important clues to understand the role of the excess Fe atoms, which complicate the understanding of the phenomena occurring in iron-chalcogenide materials.

cond-mat.str-el↗

arXiv2012

Crystal Structure, Lattice Vibrations, and Superconductivity of LaO$_{1-x}$F$_{x}$BiS$_{2}$

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, and its nonsuperconducting parent compound LaOBiS$_{2}$. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts significantly along $c$ and expands slightly along $a$, the buckling of the BiS$_2$ plane remains almost the same. In the inelastic measurements, a large difference in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almost unchanged between non-superconducting and superconducting states either by F-doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes, and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.

cond-mat.supr-con↗

arXiv2012

Evolution of superconductivity in LaO1-xFxBiS2 prepared by high pressure technique

Novel BiS2-based superconductors LaO1-xFxBiS2 prepared by the high pressure synthesis technique were systematically studied. It was found that the high pressure annealing strongly the lattice as compared to the LaO1-xFxBiS2 samples prepared by conventional solid state reaction at ambient pressure. Bulk superconductivity was observed within a wide F-concentration range of x = 0.2 ~ 0.7. On the basis of those results, we have established a phase diagram of LaO1-xFxBiS2.

cond-mat.supr-con↗

arXiv2012

Fermiological Interpretation of Superconductivity/Non-superconductivity of FeTe_{1-x}Se_{x} Thin Crystal Determined by Quantum Oscillation Measurement

We have successfully observed quantum oscillation (QO) for FeTe_{1-x}Se_{x}. QO measurements were performed using non-superconducting and superconducting thin crystals of FeTe_{0.65}Se_{0.35} fabricated by the scotch-tape method. We show that the Fermi surfaces (FS) of the non-superconducting crystal are in good agreement with the rigid band shift model based on electron doping by excess Fe while that of the superconducting crystal is in good agreement with the calculated FS with no shift. From the FS comparison of both crystals, we demonstrate the change of the cross-sectional area of the FS, suggesting that the suppression of the FS nesting with the vector Q_{s} = (π, π) due to excess Fe results in the disappearance of the superconductivity.

cond-mat.supr-con↗

arXiv2012

Coexistence of different electronic phases in the K0.8Fe1.6Se2 superconductor: a bulk-sensitive hard x-rays spectroscopy study

We have studied electronic and magnetic properties of the K0.8Fe1.6Se2 superconductor by x-ray absorption and emission spectroscopy. Detailed temperature dependent measurements alongwith a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with high spin 3+Fe state and a minority phase with intermediate spin 2+Fe state. The effect of high temperature annealing suggests that the compressed phase with lower spin 2+Fe state is directly related with the high Tc superconductivity in the title system. The results clearly underline glassy nature of superconductivity in the electronically inhomogeneous K0.8Fe1.6Se2, similar to the superconductivity in granular phases.

cond-mat.supr-con↗

arXiv2012

Evidence of Inhomogeneous Superconductivity in FeTe$_{1-x}$Se$_x$ Thin Film Using Scotch-Tape Method

We have fabricated thin films of FeTe$_{1-x}$Se$_x$ using a scotch-tape method. The superconductivities of the thin films are different from each other although these films were fabricated from the same bulk sample. The result clearly presents the inhomogeneous superconductivity in FeTe$_{1-x}$Se$_x$. The difference comes from inhomogeneity due to the excess Fe concentration. The resistivity of a thin film with low excess Fe shows good superconductivity with the sharp superconducting-transition width and more isotropic superconductivity.

cond-mat.supr-con↗

arXiv2011

Transport properties of iron-based FeTe0.5Se0.5 superconducting wire

FeTe0.5Se0.5 superconducting wires have been fabricated using ex-situ PIT method with an Fe sheath. Among the other FeAs-based superconductor, FeTe0.5Se0.5 has great advantage for application due to binary composition and less toxic. Surprisingly, superconducting current was observed in the as-fabricated wire without any heat treatments. Zero resistivity and transport critical current density (Jc) were 3.2 K and 2.8 A/cm2, respectively. In addition, by annealing at 200 degC for 2 h, critical temperature Tczero and Jc were enhanced up to 9.1 K, and 64.1 A/cm2, respectively. This suggests that the grain connectivity was improved by heat treatment, and superconducting property of FeTe0.5Se0.5 wire was enhanced.

cond-mat.supr-con↗

arXiv2011

Microstructure and transport properties of FeTe0.5Se0.5 superconducting wires fabricated by ex-situ Powder-in-tube process

We fabricated FeTe0.5Se0.5 superconducting wires using ex-situ Powder-in-tube method with an Fe sheath. It was amazing that superconducting current was observed in the as-drawn wire without any heat treatments. By heat treatment at 200C for 2 hours, Tczero and Jc at 4.2 K were enhanced up to 9.1 K and 64.1 A/cm2 (Ic = 182.6 mA), respectively. Furthermore, the Jc of FeTe0.5Se0.5 wire heat treated at 200C for 2 hours was not much sensitive to the applied magnetic fields. Therefore, FeTe0.5Se0.5 wires have a great potential for applications.

cond-mat.supr-con↗

arXiv2011

Nanoscale phase separation in the iron chalcogenide superconductor K0.8Fe1.6Se2 as seen via scanning nanofocused x-ray diffraction

Advanced synchrotron radiation focusing down to a size of 300 nm has been used to visualize nanoscale phase separation in the K0.8Fe1.6Se2 superconducting system using scanning nanofocus single-crystal X-ray diffraction. The results show an intrinsic phase separation in K0.8Fe1.6Se2 single crystals at T< 520 K, revealing coexistence of i) a magnetic phase characterized by an expanded lattice with superstructures due to Fe vacancy ordering and ii) a non-magnetic phase with an in-plane compressed lattice. The spatial distribution of the two phases at 300 K shows a frustrated or arrested nature of the phase separation. The space-resolved imaging of the phase separation permitted us to provide a direct evidence of nanophase domains smaller than 300 nm and different micrometer-sized regions with percolating magnetic or nonmagnetic domains forming a multiscale complex network of the two phases.

cond-mat.supr-con↗

arXiv2011

Pressure study of the new iron-based superconductor K0.8Fe2Se2

We investigated pressure effects on transition temperature (Tc) of the new iron-based superconductor K0.8Fe2Se2 using a BeCu/NiCrAl hybrid-type clamped piston-cylinder cell. The Tc(onset) was 33K at 0.85 GPa. With increasing pressure, Tc(onset) gradually increased and reached 36.6 K at 2.03 GPa.

cond-mat.supr-con↗

arXiv2010

Intrinsic pinning property of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe0.5Te0.5, which is the superconductor with Tc of about 14 K, were studied by the analysis of magnetization curves by the extended critical state model. In the magnetization measurements by SQUID magnetometer, the external magnetic fields were applied parallel and perpendicular to c-axis of the sample. The critical current density Jc's under the perpendicular field of 1 T were estimated by using the Kimishima model as about 1.6 x 10^4, 8.8 x 10^3, 4.1 x 10^3, and 1.5 x 10^3 A/cm2 at 5, 7, 9, and 11 K, respectively, and the temperature dependence of Jc could be fitted with the exponential law of Jc(0)xexp(-αT /Tc) up to 9 K and power law of Jc(0)x(1-T / Tc)n near Tc.

cond-mat.supr-con↗

arXiv2010

A study of electronic structure of FeSe$_{1-x}$Te$_{x}$ chalcogenides by Fe and Se K-edge x-ray absorption near edge structure measurements

Fe K-edge and Se K-edge x-ray absorption near edge structure (XANES) measurements are used to study FeSe$_{1-x}$Te$_{x}$ electronic structure of chalcogenides. An intense Fe K-edge pre-edge peak due to Fe 1s$\to$3d (and admixed Se/Te $p$ states) is observed, showing substantial change with the Te substitution and X-ray polarization. The main white line peak in the Se K-edge XANES due to Se 1s $\to$ 4p transition appear similar to the one expected for Se$^{2-}$ systems and changes with the Te substitution. Polarization dependence reveals that unoccupied Se orbitals near the Fermi level have predominant $p_{x,y}$ character. The results provide key information on the hybridization of Fe $3d$ and chalcogen $p$ states in the Fe-based chalcogenide superconductors.

cond-mat.supr-con↗

arXiv2010

Evidence of local structural inhomogeneity in FeSe(1-x)Te(x) from extended x-ray absorption fine structure

Local structure of FeSe(1-x)Te(x) has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and Fe K edge EXAFS has permitted to quantify the local interatomic distances and their mean-square relative displacements. The Fe-Se and Fe-Te bond lengths in the ternary system are found to be very different from the average crystallographic Fe-Se/Te distance, and almost identical to the Fe-Se and Fe-Te distances for the binary FeSe and FeTe systems, indicating distinct site occupation by the Se and Te atoms. The results provide a clear evidence of local inhomogeneities and coexisting electronic components in the FeSe1-xTex, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors.

cond-mat.supr-con↗

arXiv2010

Weak superconducting fluctuations and small anisotropy of the upper critical fields in an Fe1.05Te0.85Se0.15 single crystal

We have investigated the temperature dependence of the resistive upper critical fields ($μ_{0}H_{\rm c2}(T)$) for an Fe$_{1.05}$Te$_{0.85}$Se$_{0.15}$ single crystal, which exhibit superconductivity at $T_{\rm c} \sim 14 K, in magnetic fields of up to 55 T. Two-dimensional feature and superconducting fluctuations of the samples are found to be weak, because the resistive broadening effect on applied magnetic fields of up to 14 T is small. The Pauli paramagnetic effect is obviously evidenced by the strong suppression of the $μ_{0}H_{\rm c2}^{ab}(T)$ ($H \parallel ab$) curve and nearly isotropic $μ_{0}H_{\rm c2}(0) \approx 47 T is seen for both $H \parallel ab$ and $H \parallel c$. This fact is almost identical to the results of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystals reported previously. We have discussed that the small anisotropy of the upper critical field at low temperatures in Fe$_{1+y}$(Te,Se) systems against the variation of the Te/Se ratio.

cond-mat.supr-con↗

arXiv2009

Substitution Effects on FeSe Superconductor

We have investigated the effect of atomic substitutions in the FeSe system, which exhibits the simplest crystal structure among the iron-based superconductors. An enhancement of the superconducting transition temperature Tc was observed with the substitution of S or Te for Se; the Tc increased with S substitution by up to 20 %, and also increased with Te substitution up to 75 %. In contrast, Co or Ni substitutions for the Fe site significantly suppressed superconductivity. In this work we present a detailed description of the substitution technique employed to determine Tc in the FeSe system.

cond-mat.supr-con↗

arXiv2009

Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (T_c) increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa. The maximum T_c, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using 77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T_1T just above T_c shows a plateau as well as the pressure dependence of T_c, which gives clear evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2009

Pressure evolution of low-temperature crystal structure and bonding of 37 K $T_c$ FeSe superconductor

FeSe with the PbO structure is a key member of the family of new high-$T_c$ iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe$_4$ tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases $T_c$ which attains a broad maximum of 37 K at $\sim$7 GPa (this is one of the highest $T_c$ ever reported for a binary solid) before decreasing to 6 K upon further compression to $\sim$14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $α$-FeSe, revealing an extremely soft solid with a bulk modulus, $K_0$ = 30.7(1.1) GPa and strong bonding anisotropy between inter- and intra-layer directions that transforms to the more densely packed $β$-polymorph above $\sim$9 GPa. The non-monotonic $T_c$($P$) behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties.

cond-mat.supr-con↗

arXiv2008

Superconductivity in S-substituted FeTe

We have successfully synthesized a new superconducting phase of FeTe1-xSx with a PbO-type structure. It has the simplest crystal structure in iron-based superconductors. Superconducting transition temperature is about 10 K at x = 0.2. The upper critical field Hc2 was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic elements, this material is a candidate for applications and will activate more and more research on iron-based superconductor.

cond-mat.supr-con↗