arXiv subjects
Publications and source records attributed to Yoshihiko Takano.
Superconductivity in FeTe0.8S0.2 is successfully induced by an electrochemical reaction using an ionic liquid solution. A clear correlation between the Fe concentration in the solution and the manifestation of superconductivity was confirmed, suggesting that superconductivity was induced by the deintercalation of excess iron.
We succeeded in growing single crystalline SmFeAsO1-xFx with exceptionally high fluorine concentration by using a CsCl flux method. Comparing to conventional flux methods, this method can introduce about double the amount of fluorine into the oxygen site. The obtained single crystal shows the highest superconducting transition temperature (Tc = 57.5 K) in single crystalline iron pnictides. In addition, the residual resistivity ratio (RRR) is almost three times as large as that of previously reported single crystals. This suggests that our single crystals are suitable for investigation of the intrinsic superconducting properties since they have few defects and impurities. Using both the Werthamer-Helfand-Hohenberg model and the effective mass model, we demonstrated that a higher fluorine concentration suppresses the anisotropic superconductivity of SmFeAsO1-xFx.
F-substituted LaOBiSe2 single crystals were grown using CsCl flux. The obtained single crystals showed a plate-like shape with a size of about 1.0 mm square. The c-axis lattice constant of the grown crystals was determined to be 14.114(3) Å. The superconducting critical temperature of the single crystal was approximately 3.5 K. The superconducting anisotropies were determined to be 49 and 24 using the upper critical field and the effective mass model, respectively.
Single crystal of La(O,F)BiSe2 was successfully grown for the first time by the CsCl flux method. Single crystal X-ray structural analysis clearly showed that La(O,F)BiSe2 crystalizes with space group P4/nmm (lattice parameters a = 4.1408(2) Å, c = 14.1096(6) Å). The structure is composed of alternating BiSe2 and LaO layers. Magnetic susceptibility and resistivity measurements showed the superconducting transition at 3.6 K, which is higher than that of as-grown La(O,F)BiS2. The structural analysis implies that La(O,F)BiSe2 is potential superconductor having higher Tc under high pressure.
We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO$_{1-x}$F$_x$BiS$_2$, in which the superconductivity of the BiS$_2$ layer and the ferromagnetism of the CeO$_{1-x}$F$_x$ layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS$_2$ exhibits coexistence of $4f^1$ (Ce$^{3+}$) and $4f^0$ (Ce$^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F-doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS$_2$ layer and the ferromagnetic CeO$_{1-x}$F$_x$ layer.
Superconductivity is successfully induced by utilizing a battery-like reaction found in a typical Li-ion battery. Excess Fe in FeTe0.8S0.2 is electrochemically de-intercalated by applying a voltage in a citric acid solution. The superconducting properties improve with an increase in the applied voltage up to 1.5 V. This result suggests that an electrochemical reaction can be used as a novel method to develop new superconducting materials.
We investigated the crystal structure and superconducting properties of As-grown and high-pressure-annealed PrO0.5F0.5BiS2. We found that the high-pressure annealing generates uniaxial lattice contraction along the c axis. Both As-grown and high-pressure-annealed PrO0.5F0.5BiS2 show bulk superconductivity. The Tc of PrO0.5F0.5BiS2 is clearly enhanced from Tczero = 3.6 K to Tczero = 5.5 K by high-pressure annealing. Unexpectedly, the semiconducting characteristics is relatively enhanced by high-pressure annealing. Namely, we assume that the enhancement of Tc can not be understood by an increase of electron carriers. Having considered these facts, we conclude that the enhancement of Tc correlates with uniaxial lattice contraction along the c axis in PrO0.5F0.5BiS2.
Synthesis of a series of layered iron arsenides Ca1-xRExFeAs2 (112) was attempted by heating at 1000 C under a high-pressure of 2 GPa. The 112 phase successfully forms with RE = La, Ce, Nd, Sm, Eu and Gd, while Tb, Dy and Ho substituted and RE free samples does not contain the 112 phase. The Ce, Nd, Sm, Eu and Gd doped Ca1-xRExFeAs2 are new compounds. All of them exhibit superconducting transition except for the Ce doped sample. The behaviour of the critical temperature, with the RE ionic radii have been investigated.
Correlation between crystal structure and superconducting properties of the BiS2-based superconductor LaO0.5F0.5BiS2 was investigated. We have prepared LaO0.5F0.5BiS2 polycrystalline samples with various lattice constants. It was found that the annealing the sample under high pressure generated uniaxial strain along the c axis. Further, the highly-strained sample showed higher superconducting properties. We concluded that the uniaxial strain along the c axis was positively linked with the enhancement of superconductivity in the LaO1-xFxBiS2 system.
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.
F-substituted ROBiS2 (R = La, Ce, Nd) superconducting single crystals with different F concentration were grown successfully using CsCl/KCl flux. All the obtained single crystals had a plate-like shape with a well-developed ab-plane of 1-2 mm in size. The flux components of Cs, K, and Cl were not detected in the obtained single crystals by electron probe microanalysis. The grown single crystals of F-substituted LaOBiS2 and CeOBiS2 showed superconducting at about 3 K while the Tc of the F-substituted NdOBiS2 exhibited approximately 5 K. The superconducting anisotropy of the single crystals of F-substituted LaOBiS2 and NdOBiS2 was estimated to be 30-45 according to the effective mass model whereas those values were 13-21 for the F-substituted CeOBiS2 single crystals. The F-substituted CeOBiS2 single crystals exhibited magnetic order at about 7 K that apparently coexisted with superconductivity below around 3 K.
F-substituted NdOBiS2 superconducting single crystals were grown using CsCl/KCl flux. This is the first example of the single-crystal growth of a BiS2-based superconductor. The obtained single crystals had a plate-like shape with a size of 1-2 mm and a well-developed ab-plane. The crystal structure of the grown crystals was determined by single-crystal X-ray diffraction analysis to be the tetragonal space group P4/nmm (#129) with a = 3.996(3) A and c = 13.464(6) A. The chemical formula of the grown crystals was approximately Nd0.98(0.06)O0.7(0.1)F0.3(0.1)Bi0.98(0.04)S2, and Cs, K, and Cl were not detected in the grown crystals by electron probe microanalysis. The grown crystals had a critical temperature of approximately 5 K. The superconducting anisotropy of the single crystals was estimated to be about 30 from the effective mass model and the upper critical field.
The newly discovered BiS$_2$-based LaO$_{1-x}$F$_{x}$BiS$_2$ ($x$=0.5) becomes superconductive at $T_c$=2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature $T_c$. We observe that $T_c$ abruptly increases from 2.5 K to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements, a structural phase transition from a tetragonal phase ($P$4/$nmm$) to a monoclinic phase ($P$2$_1/m$) also occurs at around $\sim$ 1 GPa. We consider that a pressure-induced enhancement of superconductivity is caused by the structural phase transition.
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.
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.
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.
Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.
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.