arXiv subjects

Yoshikazu Mizuguchi

Publications and source records attributed to Yoshikazu Mizuguchi.

At least 163 records · Page 9Linked to original sources

arXiv2012

Unidirectional Electronic Structure in the Parent State of Iron-Chalcogenide Superconductor Fe$_{1+δ}$Te

We use scanning tunnelling microscopy and spectroscopy to explore the electronic structure of Fe$_{1.07}$Te which is the parent compound of the iron-chalcogenide superconductors. A unidirectional electronic structure with a period of $a_{0}$ (where $a_{0}$ is the lattice constant) along the a-axis is observed. The unidirectional pattern is visible within an energy range from approximately -200 to +130 meV. Since the direction of the unidirectional electronic structure coincides with those of the underlying antiferromagnetic and the predicted orbital orders, it is presumable that the observed unidirectional structure is closely tied to these orders and is peculiar to the parent state in iron-based superconductors.

cond-mat.str-el↗

arXiv2012

Inducement of superconductivity in Fe(Te,S) by sulfuric acid treatment

Superconductivity of Fe0.95Te0.85S0.15 was induced and enhanced by immersing the as-grown (non-superconducting) sample into sulfuric acid. Dissolved Fe was detected in the sulfuric acid treatment, indicating that excess Fe was removed from the Fe0.95Te0.85S0.15 sample.

cond-mat.supr-con↗

arXiv2012

Large local disorder in the superconducting K_{0.8}Fe_{1.6}Se_{2} studied by extended x-ray absorption fine structure

We have measured local structure of superconducting K_{0.8}Fe_{1.6}Se_{2} chalcogenide (T_{c}=31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe-Se and Fe-Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for Fe-Se bondlength (k ~ 5.8 eV/Å^{2}) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to get flexible (k ~ 2.1 eV/Å^{2}) in comparison to the binary FeSe (k ~ 3.5 eV/Å^{2}), an indication of partly relaxed Fe-Fe networks in K_{0.8}Fe_{1.6}Se_{2}. The results suggest glassy nature of the title system, with the superconductivity being similar to the one in the granular materials.

cond-mat.supr-con↗

arXiv2012

Field-Induced Magnetostructural Transitions in Antiferromagnetic Fe1+yTe1-xSx

The transport and structural properties of Fe1+yTe1-xSx (x=0, 0.05, and 0.10) crystals were studied in pulsed magnetic fields up to 65 T. The application of high magnetic fields results in positive magnetoresistance effect with prominent hystereses in the antiferromagnetic state. Polarizing microscope images obtained at high magnetic fields showed simultaneous occurrence of structural transitions. These results indicate that magnetoelastic coupling is the origin of the bicollinear magnetic order in iron chalcogenides.

cond-mat.supr-con↗

arXiv2012

Evolution of two-step structural phase transition in Fe1+dTe detected by low-temperature x-ray diffraction

The low-temperature crystal structure of Fe1.13Te, which exhibits an anomalous two-step magnetic transition, was clarified by the systematic x-ray diffraction measurements. It was found that two-step structural phase transition, tetragonal-orthorhombic-monoclinic, occurred correspondingly to the two-step magnetic transition. The detailed analysis of the profile at 5 K indicated the coexistence of the minor orthorhombic area inside the major monoclinic lattice, which could explain the lower-shift (suppression) of the antiferromagnetic transition temperature in Fe1.13Te and suggest a possibility of superconductivity at the domain boundary.

cond-mat.supr-con↗

arXiv2012

Structural and magnetic properties of Fe1+dTe single crystals

We have grown single crystals of Fe1+dTe by a conventional self-flux method. We obtained plate-like single crystals with dnom \geq 0.1. The value of the magnetization increased with increasing excess Fe concentration, and a broadening of the antiferromagnetic transition was observed for dnom >1.15. Further, we noted that the antiferromagnetic transition of Fe1.134Te (dnom = 0.15) was clearly suppressed to a lower temperature, which would indicate a possibility of controllability of magnetism by excess Fe concentration.

cond-mat.supr-con↗

arXiv2012

Enhancement of superconducting properties in FeSe wires using a quenching technique

Enhancements of superconducting properties were observed in FeSe wires using a quenching technique. Zero resistivity was achieved at about 10 K in quenched wires, which is about 2 K higher than that of polycrystalline FeSe bulk. Furthermore, transport Jc of quenched wires showed three times higher than that of furnace-cooled wires. In contrast, the quenched polycrystalline FeSe bulks did not show the enhancement of Tc. The quenching technique is a greatly promising for fabricating FeSe wires with high Tc and high Jc, and quenched FeSe wires have high potential for superconducting wire applications.

cond-mat.supr-con↗

arXiv2011

Transport properties and microstructure of mono- and seven-core wires of FeSe1-xTex superconductor by Fe-diffusion powder-in-tube method

We report the successful fabrication of mono- and seven-core superconducting wires of FeSe1-xTex using an in-situ Fe-diffusion process based on the powder-in-tube (Fe-diffusion PIT) method. The reacted layer in these wires were found to have composite structure with composition nearly FeSe and FeTe for the inner and outer layers, although a single layer of composition FeSe0.5Te0.5 was supposed to be formed. The self-field transport Jc values at 4.2 K were found to be 226.2 A/cm2 and 100.3 A/cm2 respectively for mono- and seven-core wires. The Jc's of mono- and seven-core wires dropped rapidly at low fields and then showed a gradual decrease with increasing magnetic fields. In addition, the seven-core wire showed higher Jc than the mono-core wire under higher magnetic fields, indicating that the seven-core wire of FeSe1-xTex superconductor using Fe-diffusion PIT method is advantageous for the superconducting-wire application under high magnetic fields.

cond-mat.supr-con↗

arXiv2011

Electrochemical synthesis of iron-based superconductor FeSe films

The superconducting FeSe films were successfully fabricated using the electrochemical synthesis. The composition ratio of Fe and Se can be controlled by the electric potential and pH value. We found that the FeSe films deposited at the electric potential -1.75 V and pH 2.3 show the superconducting transition at 3.5 K. The establishment of this electrochemical synthesis technique will provide many advantages for application.

cond-mat.supr-con↗

arXiv2011

Intrinsic phase separation in superconducting K0.8Fe1.6Se2 (Tc= 31.8 K) single crystals

Temperature dependent single-crystal x-ray diffraction (XRD) in transmission mode probing the bulk of the newly discovered K0.8Fe1.6Se2 superconductor (Tc = 31.8 K) using synchrotron radiation is reported. A clear evidence of intrinsic phase separation at 520 K between two competing phases, (i) a first majority magnetic phase with a ThCr2Si2-type tetragonal lattice modulated by the iron vacancy ordering and (ii) a minority non-magnetic phase having an in-plane compressed lattice volume and a weak superstructure, is reported. The XRD peaks due to the Fe vacancy ordering in the majority phase disappear by increasing the temperature at 580 K, well above phase separation temperature confirming the order-disorder phase transition. The intrinsic phase separation at 520K between a competing first magnetic phase and a second non-magnetic phase in the normal phase both having lattice superstructures (that imply different Fermi surface topology reconstruction and charge density) is assigned to a lattice-electronic instability of the K0.8Fe1.6Se2 system typical of a system tuned at a Lifshitz critical point of an electronic topological transition that gives a multi-gaps superconductor tuned a shape resonance.

cond-mat.supr-con↗

arXiv2011

Evolution of tetragonal phase in the FeSe wire fabricated by a novel chemical-transformation PIT process

We fabricated superconducting FeSe wires by the chemical-transformation PIT process. The obvious correlation between annealing temperature and phase transformation was observed. Annealing above 500^{\circ}C produced wire-core transformation from hexagonal to tetragonal phase. Furthermore the hexagonal phase completely transformed into the tetragonal phase by annealing at 1000^{\circ}C. With increasing annealing temperature, the superconducting property was dramatically improved, associated with the evolution of the tetragonal phase.

cond-mat.supr-con↗

arXiv2011

Transport properties of the single- and 3-core Fe-Se wires fabricated by a novel chemical-transformation PIT process

We fabricated single- and 3-core superconducting Fe-Se wires using a novel process based on a chemical transformation from hexagonal FeSe1+d (non-superconducting) to tetragonal FeSe (superconducting) via an optimal supply of Fe from the Fe sheath by annealing. This process enhanced a packing density of superconducting core inside the sheath, owing to an expansion of the lattice volume via a chemical transformation from high-density hexagonal FeSe1+d to low-density tetragonal FeSe. The obtained superconducting wire showed superconductivity below ~10 K and the upper critical field was estimated to be 19.3 T at 0 K. The obtained transport critical current densities at 0 T were 588 A/cm2 for the 3-core wire and 218 A/cm2 for the single-core wire.

cond-mat.supr-con↗

arXiv2011

Preparation of Thin Crystals of FeTe1-xSx Using the Scotch-Tape Method

We have investigated an applicability of the scotch-tape method to fabricating thin crystals of Fe chalcogenide FeTe1-xSx. Thin crystals with a typical thickness of 40 nm were expectedly left on a Si substrate. Thin films prepared using this process will develop both detailed studies on intrinsic nature of Fe-based superconductivity and application to superconducting devices.

cond-mat.supr-con↗

arXiv2011

Superconductivity in PbO-type Fe chalcogenides

PbO-type Fe chalcogenide has been drawing much attention as the Fe-based superconductor with the most simple crystal structure. Whereas FeSe is an intrinsic superconductor, FeTe, while having a structure analogous to FeSe, exhibits antiferromagnetic ordering. The relationship between antiferromagnetism and superconductivity provides key information to understand better superconductivity in Fe chalcogenides. Furthermore the significant pressure effect on Tc in the Fe-chalcogenide superconductors, which is likely to be correlated with the crystal structure and/or magnetism, is important in elucidating the mechanism of Fe-based superconductivity. Due to the simple structure and composition, Fe-chalcogenide superconductor is one candidate for applications to such areas as superconducting wires and thin films.

cond-mat.supr-con↗

arXiv2011

Possible Superconducting Symmetry and Magnetic Correlations in K0.8Fe2Se2: A 77Se-NMR Study

We report ^77Se-NMR measurements in the recent discovered K_0.8Fe_2Se_2 with the superconducting transition temperature T_c=32 K. The characteristic point of this system is that the high density of state is located in the vicinity of the Fermi level. The comparison between Knight shift and nuclear spin lattice relaxation rate 1/T_1 shows that the antiferromagnetic spin correlation is not strong in this system, but it is reasonable to consider that its development toward low temperatures occurs. 1/T_1 in the superconducting state can be reproduced well by an s+- wave model, but a d wave model also roughly reproduces the data. Non exponential behavior in 1/T_1 at low temperatures disagrees with a single isotropic gap.

cond-mat.supr-con↗

arXiv2010

Transport properties of the new Fe-based superconductor KxFe2Se2 (Tc = 33 K)

We synthesized the new Fe-based superconductor K0.8Fe2Se2 single crystals. The obtained single crystal exhibited a sharp superconducting transition, and the onset and zero-resistivity temperature was estimated to be 33 and 31.8 K, respectively. A high upper critical field of 192 T was obtained. Anisotropy of superconductivity of K0.8Fe2Se2 was ~3.6. Both the high upper critical field and comparably low anisotropy are advantageous for the application under high magnetic field.

cond-mat.supr-con↗

arXiv2010

Single crystal growth and structural characterization of FeTe1-xSx

We have synthesized single crystals of FeTe1-xSx and investigated the structural and physical properties. The actual S concentration was found to be significantly smaller than the starting nominal value. The solubility limit of S for the Te site for the crystals grown using a self-flux method was estimated to be around 12 %. Although a suppression of antiferromagnetic transition was observed by S substitution, bulk superconductivity was not observed. A phase diagram of as-grown FeTe1-xSx single crystal was established on the basis of the structural and physical properties.

cond-mat.supr-con↗

arXiv2010

Superconductivity in oxygen-annealed FeTe1-xSx single crystal

We investigated the S-doping-driven phase transition from antiferromagnetic to superconducting in FeTe1-xSx single crystals. The partial substitution of Te by S suppresses antiferromagnetism in Fe-square lattice. Superconductivity is induced by oxygen annealing for only FeTe1-xSx in which the long-range magnetic ordering is suppressed. To realize superconductivity in FeTe1-xSx, both S concentration enough to suppress antiferromagnetism and oxygen annealing are required. Anisotropy of superconductivity in oxygen-annealed FeTe0.886S0.114 was estimated to be 1.17.

cond-mat.supr-con↗