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Y. Takano

Publications and source records attributed to Y. Takano.

At least 37 records · Page 2Linked to original sources
arXiv2014

On the bad metallicity and phase diagrams of Fe$_{1+δ}X$ ($X$ =Te, Se, S, solid solutions): an electrical resistivity study

Based on a systematic analysis of the thermal evolution of the resistivities of Fe-based chalcogenides Fe$_{1+δ}$Te$_{1-x}X_{x}$ ($X$= Se, S), it is inferred that their often observed nonmetallic resistivities are related to a presence of two resistive channels: one is a high-temperature thermally-activated process while the other is a low-temperature log-in-$T$ process. On lowering temperature, there are often two metal-to-nonmetall crossover events: one from the high-$T$ thermally-activated nonmetallic regime into a metal-like phase and the other from the log-in-$T$ regime into a second metal-like phase. Based on these events, together with the magnetic and superconducting transitions, a phase diagram is constructed for each series. We discuss the origin of both processes as well as the associated crossover events. We also discuss how these resistive processes are being influenced by pressure, intercalation, disorder, doping, or sample condition and, in turn, how these modifications are shaping the associated phase diagrams.

cond-mat.supr-con↗
arXiv2014

Temperature Dependence of the Electrical Transport Properties of Multilayer Graphene

Multilayer graphene (MLG) thin films are deposited on silicon oxide substrates by mechanical exfoliation (or 'scotch-tape method') from Kish graphite. The thickness and number of layers are determined from both Atomic Force Microscopy (AFM) and Raman Spectroscopy. Electrical terminals are deposited on MLGs in a four-probe configuration by electron-beam lithography, gold/titanium thermal evaporation, and lift-off. The electrical resistance is measured from room temperature down to 2 K. The electrical resistance of the MLGs shows an increase with decreasing temperature, and then decreases after reaching a maximum value. These results are compared with recent experimental and theoretical data from the literature.

cond-mat.mes-hall↗
arXiv2014

Spectromicroscopy of electronic phase separation in K$_x$Fe$_{2-y}$Se$_2$ superconductor

Structural phase separation in A$_x$Fe$_{2-y}$Se$_2$ system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexist, and what is their spatial distribution. Here, we have used novel scanning photoelectron microscopy (SPEM) to study the electronic phase separation in K$_x$Fe$_{2-y}$Se$_2$, providing a direct measurement of the topological spatial distribution of the different electronic phases. The SPEM results reveal a peculiar interconnected conducting filamentary phase that is embedded in the insulating texture. The filamentary structure with a particular topological geometry could be important for the high T$_c$ superconductivity in the presence of a phase with a large magnetic moment in A$_x$Fe$_{2-y}$Se$_2$ materials.

cond-mat.supr-con↗
arXiv2014

Effect of high pressure annealing on the normal state transport of LaO0.5F0.5BiS2

We study normal state electrical, thermoelectrical and thermal transport in polycrystalline BiS2-based compounds, which become superconducting by F doping on the O site. In particular we explore undoped LaOBiS2 and doped LaO0.5F0.5BiS2 samples, prepared either with or without high pressure annealing, in order to evidence the roles of doping and preparation conditions. The high pressure annealed sample exhibits room temperature values of resistivity ro around 5 mohmcm, Seebeck coefficient S around -20 microV/K and thermal conductivity k around 1.5 W/Km, while the Hall resistance RH is negative at all temperatures and its value is -10-8 m3/C at low temperature. The sample prepared at ambient pressure exhibits RH positive in sign and five times larger in magnitude, and S negative in sign and slightly smaller in magnitude. These results reveal a complex multiband evolution brought about by high pressure annealing. In particular, the sign inversion and magnitude suppression of RH, indicating increased electron-type carrier density in the high pressure sample, may be closely related to previous findings about change in lattice parameters and enhancement of superconducting Tc by high pressure annealing. As for the undoped sample, it exhibits the 10 times larger resistivity, 10 times larger |S| and 10 times larger |RH| than its doped counterpart, consistently with its insulating nature. Our results point out the dramatic effect of preparation conditions in affecting charge carrier density as well as structural, band and electronic parameters in these systems.

cond-mat.supr-con↗
arXiv2014

Magnetic ordering induced by interladder coupling in the spin-1/2 Heisenberg two-leg ladder antiferromagnet C$_9$H$_{18}$N$_2$CuBr$_4$

We present specific-heat and neutron-scattering results for the \emph{S}=1/2 quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr$_4$. The material orders magnetically at \emph{T}$_N$=1.99(2)\,K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange $J_{\rm leg}$=0.60(2)~meV, rung exchange $J_{\rm rung}$=0.64(9)~meV, interladder exchange $J_{\rm int}$=0.19(2)~meV, and an interaction-anisotropy parameter $λ$=0.93(2), according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Néel-ordered phase without an assistance of a magnetic field.

cond-mat.mtrl-sci↗
arXiv2013

s-wave pairing in the optimally-doped LaO0.5F0.5BiS2 superconductor

We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy measurements (uSR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-uSR data give access to the superfluid density, which shows a marked 2D character with a dominant s-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.

cond-mat.supr-con↗
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↗
arXiv2013

Macroscopic quantum tunneling and phase diffusion in a La$_{2-x}$Sr$_x$CuO$_4$ intrinsic Josephson junction stack

We performed measurements of switching current distribution in a submicron La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) intrinsic Josephson junction (IJJ) stack in a wide temperature range. The escape rate saturates below approximately 2\,K, indicating that the escape event is dominated by a macroscopic quantum tunneling (MQT) process with a crossover temperature $T^{*}\approx2\,$K. We applied the theory of MQT for IJJ stacks, taking into account dissipation and the phase re-trapping effect in the LSCO IJJ stack. The theory is in good agreement with the experiment both in the MQT and in the thermal activation regimes.

cond-mat.supr-con↗
arXiv2013

Low-Temperature Low-Field Phases of the Pyrochlore Quantum Magnet Tb$_2$Ti$_2$O$_7$

By means of ac magnetic-susceptibility measurements, we find evidence for a new magnetic phase of Tb$_2$Ti$_2$O$_7$ below about 140 mK in zero magnetic field. In magnetic fields parallel to [111], this phase---exhibiting frequency- and amplitude-dependent susceptibility and an extremely slow spin dynamics---extends to about 70 mT, at which it gives way to another phase. The field dependence of the susceptibility of this second phase, which extends to about 0.6 T, indicates the presence of a weak magnetization plateau below 50 mK, as has been predicted by a single-tetrahedron four-spin model, giving support to the underlying proposal that the disordered low-field ground state of Tb$_2$Ti$_2$O$_7$ is a quantum spin ice.

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

Superconductivity in Fe1+dTe0.9Se0.1 induced by deintercalation of excess Fe using alcoholic beverage treatment

Superconductivity in polycrystalline Fe1+dTe0.9Se0.1 was enhanced by heating in alcoholic beverages. We found that part of the excess Fe was deintercalated from the interlayer sites by the treatment and the shielding volume fractions were increased with the reduction of excess Fe content in sample. This behavior is similar to the case of Fe1+dTe1-xSx. Thus the technique of alcoholic beverage treatment can be applied generally to the Fe-chalcogenide superconductors.

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

Phase diagram and oxygen annealing effect of FeTe1-xSex iron-based superconductor

Phase diagrams of as-grown and O2-annealed FeTe1-xSex determined from magnetic susceptibility measurement were obtained. For as-grown samples, the antiferromagnetic order was fully suppressed in the range region x \geq 0.15, and weak superconductivity appeared when x \geq 0.1. Beginning at x = 0.5, weak superconductivity was found to evolve into bulk superconductivity. Interestingly, for O2-annealed samples, complete suppression of magnetic order and the occurrence of bulk superconductivity were observed when x \geq 0.1. We found that O2-annealing induces bulk superconductivity for FeTe1-xSex. Oxygen probably plays a key role in the suppression of the magnetic order and the appearance of bulk superconductivity.

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↗
arXiv2012

Clarification as to why alcoholic beverages have the ability to induce superconductivity in Fe_{1+d}Te_{1-x}S_x

To elucidate the mechanism as to why alcoholic beverages can induce superconductivity in Fe_{1+d}Te_{1-x}S_x samples, we performed component analysis and found that weak acid such as organic acid has the ability to induce superconductivity. Inductively-coupled plasma spectroscopy was performed on weak acid solutions post annealing. We found that the mechanism of inducement of superconductivity in Fe_{1+d}Te_{1-x}S_x is the deintercalation of excess Fe from the interlayer sites.

cond-mat.supr-con↗