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Tohru Watanabe

Publications and source records attributed to Tohru Watanabe.

13 recordsLinked to original sources
arXiv2014

Electrical transport properties of small diameter single-walled carbon nanotubes aligned on ST-cut quartz substrates

A method is introduced to isolate and measure the electrical transport properties of individual single-walled carbon nanotubes (SWNTs) aligned on an ST-cut quartz, from room temperature down to 2 K. The diameter and chirality of the measured SWNTs are accurately defined from Raman spectroscopy and atomic force microscopy (AFM). A significant up-shift in the G-band of the resonance Raman spectra of the SWNTs is observed, which increases with increasing SWNTs diameter, and indicates a strong interaction with the quartz substrate. A semiconducting SWNT, with diameter 0.84 nm, shows Tomonaga-Luttinger liquid and Coulomb blockade behaviors at low temperatures. Another semiconducting SWNT, with a thinner diameter of 0.68 nm, exhibits a transition from the semiconducting state to an insulating state at low temperatures. These results elucidate some of the electrical properties of SWNTs in this unique configuration and help pave the way towards prospective device applications.

cond-mat.mes-hall↗
arXiv2013

Coexistence of bulk superconductivity and ferromagnetism in CeO1-xFxBiS2

We show the observation of the coexistence of bulk superconductivity and ferromagnetism in CeO1-xFxBiS2(x = 0 - 1.0) prepared by annealing under high-pressure. In CeO1-xFxBiS2 system, both superconductivity and two types of ferromagnetism with respective magnetic transition temperatures of 4.5 K and 7.5 K are induced upon systematic F substitution. This fact suggests that carriers generated by the substitution of O by F are supplied to not only the BiS2 superconducting layers but also the CeO blocking layers. Furthermore, the highest superconducting transition temperature is observed when the ferromagnetism is also enhanced, which implies that superconductivity and ferromagnetism are linked to each other in the CeO1-xFxBiS2 system.

cond-mat.supr-con↗
arXiv2013

α-FeAs-free SmFeAsO1-xFx by low temperature sintering with slow cooling

We obtained amorphous-FeAs-free SmFeAsO1-xFx using a low temperature sintering with slow cooling. SmFeAsO1-xFx is sintered at 980 °C for 40 hours and cooled slowly down to 600 °C. The low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces much fluorine into SmFeAsO1-xFx. The superconductivity of this sample appears at 57.8 K and the superconducting volume fraction reaches 96 %. To study the change of fluorine concentration during the cooling process, samples are quenched by water at 950 °C, 900 °C, 850 °C, 800 °C, 750 °C and 700 °C. It is found that fluorine is substituted not only at the maximum heating temperature but also during the cooling process. The low temperature sintering with slow cooling is very effective to obtain a homogeneous SmFeAsO1-xFx with high fluorine concentration.

cond-mat.supr-con↗
arXiv2013

Phase diagram and superconductivity at 58.1 K in α-FeAs free SmFeAsO1-xFx

The Phase diagram of SmFeAsO1-xFx in terms of x is exhibited in this study. SmFeAsO1-xFx from x = 0 to x = 0.3 were prepared by low temperature sintering with slow cooling. The low temperature sintering suppresses the formation of the amorphous FeAs, which is inevitably produced as an impurity by using high temperature sintering. Moreover, slow cooling is effective to obtain the high fluorine concentration. The compositional change from feedstock composition is quite small after this synthesis. We can reproducibly observe a record superconducting transition for an iron based superconductor at 58.1 K. This achievement of a high superconducting transition is due to the success in a large amount of fluorine substitution. A shrinking of the a lattice parameter caused by fluorine substitution is observed and the substitutional rate of fluorine changes at x =0.16.

cond-mat.supr-con↗
arXiv2013

Evolution of superconductivity in isovalent Te-substituted KxFe2-ySe2 crystals

We report the evolution of superconductivity and the phase diagram of the KxFe2-ySe2-zTez (z=0-0.6) crystals grown by a simple one-step synthesis. No structural transition is observed in any crystals, while lattice parameters exhibit a systematic expansion with Te content. The Tc exhibits a gradual decrease with increasing Te content from Tconset = 32.9 K at z = 0 to Tconset = 27.9 K at z = 0.5, followed by a sudden suppression of superconductivity at z = 0.6. Upon approaching a Te concentration of 0.6, the shielding volume fraction decreases and eventually drops to zero. Simultaneously, hump positions in r-T curve shift to lower temperatures. These results suggest that isovalent substitution of Te for Se in KxFe2-ySe2 crystals suppresses the superconductivity in this system.

cond-mat.supr-con↗
arXiv2012

Effect of the indium addition on the superconducting property and the impurity phase in polycrystalline SmFeAsO1-xFx

We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induces the bringing together the superconducting grains. It means the total contact surfaces of grains are increased. We suggest that the enhancement of the magnetic critical current density is a direct effect of the indium addition.

cond-mat.supr-con↗
arXiv2012

BiS2 - based superconductivity in F-substituted NdOBiS2

We have successfully synthesized a new BiS2-based superconductor NdOBiS2 with F-doping. This compound is composed of superconducting BiS2 layers and blocking NdO layers, which indicates that the BiS2 layer is the one of the common superconducting layers like the CuO2 layer of cuprates or Fe-As layer of Fe-based superconductors. We can obtain NdO1-xFxBiS2 with bulk superconductivity by a solid-state reaction under ambient pressure. Therefore, NdO1-xFxBiS2 should be the suitable material to elucidate the mechanism of superconductivity in the BiS2-layer.

cond-mat.supr-con↗
arXiv2012

Electrodeposition as a new route to synthesize superconducting FeSe

We have successfully synthesized FeSe films by the electrochemical deposition in the electrolyte containing FeCl_{2}\cdot4H_{2}O, SeO_{2} and Na_{2}SO_{4}. The composition ratio of Fe and Se was controlled by the synthesis voltage and pH value. The FeSe film with the composition ratio of Fe : Se = 1 : 1 is fabricated at a voltage of -0.9 V and pH 2.1 in our electrochemical deposition. This sample has a highly crystalline tetragonal FeSe structure and exhibits a superconducting transition at 8.1 K, comparable to FeSe synthesized by other methods.

cond-mat.supr-con↗
arXiv2012

One-step synthesis of KxFe2-ySe2 single crystals for high critical current density

We have established a simple process that allows for the one-step synthesis of KxFe2-ySe2 single crystals, which exhibit high critical current density Jc. The post annealing and quenching technique has improved the homogeneity of as-grown crystals, resulting in full shielding of the external magnetic field. The quenched crystals show a superconducting transition at Tconset = 32.9 K and Tczero = 32.1 K. The upper critical fields μ_{0}Hc2(0) for H//ab and H//c are estimated to be ~206 and ~50 T, respectively. The critical current densities Jc for H//ab and H//c reach as high as 1.0\times10^{5} and 3.4\times10^{4} A/cm2 at 5 K. Furthermore, Jc exhibits a high field performance and a significantly weak temperature dependence up to 5 T, suggesting strong pinning. These results demonstrate that KxFe2-ySe2 would be a promising candidate material for practical applications.

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

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↗
arXiv2008

Resistivity reduction of boron-doped multi-walled carbon nanotubes synthesized from a methanol solution containing a boric acid

Boron-doped multi-walled carbon nanotubes (MWNTs) were synthesized using a methanol solution of boric acid as a source material. Accurate measurements of the electrical resistivity of an individual boron-doped MWNT was performed with a four-point contact, which was fabricated using an electron beam lithography technique. The doped boron provides conduction carriers, which reduces the resistivity of the MWNT.

cond-mat.mtrl-sci↗