Satoshi Watauchi

Charge Ordering, Commensurability, and Metallicity in the Phase Diagram of the Layered NaxCoO2

The phase diagram of nonhydrated NaxCoO2 has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2), then to a Curie-Weiss metal (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.

Large enhancement of the thermopower in NaxCoO2 at high Na doping

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate NaxCoO2 exhibits several interesting electronic phases as the Na content x is varied1,2,3,4, including water-induced superconductivity4 and an insulating state3 that is destroyed by field5. Initial measurements1 showed that, in the as-grown composition, NaxCoO2 has moderately large thermopower S and conductivity σ. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.60.75, S undergoes an even steeper enhancement. At the critical doping xp∼0.85, Z (at 80 K) reaches values ∼40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.

Crystal growth of Ca12Al14O33 by the floating zone method

We have grown single crystals of Ca 12 Al 14 O 33 (C12A7), a congruently melting compound, by the floating zone method. With the conventional method, the shape of the solid liquid interface was concave, and many bubbles and cracks remained in the grown crystal. To reduce them, the crystal growth at a lower growth rate and the control of the shape of the solid liquid interface were examined. For the growth rate < 0.3 mm/h, the amount of bubbles was reduced considerably. However, in such a condition, a significantly large bubble was often formed in the molten zone, which disturbed the stability of the molten zone. Introducing an alumina tube as a heat reservoir at a suitable position in the heated area, the shape of the solid-liquid interface could be changed from concave to a convex one. This may suggest that the effect of the concave interface could be eliminated and crystals with high quality may be grown at a higher growth rate.

Subgap structures in the current–voltage properties of La2−xSrxCuO4 intrinsic Josephson junctions

Abstract We investigated current–voltage ( I – V ) properties of intrinsic Josephson junctions in c -axis micro-bridges (∼20 μm 2 area for ab plane and ∼1 μm length along the c -axis) of La 2− x Sr x CuO 4 single crystals. The micro-bridges were fabricated by the focused-ion-beam etching technique. We found periodic conductance peaks below the gap voltage in the RCSJ-like I – V curves of the intrinsic Josephson junctions. We discuss possible explanations and show that the subgap structures can be explained by coupling between ac Josephson oscillation and Josephson plasma oscillation in La 2− x Sr x CuO 4 .

Unconventional Superconductivity in the BiS2 -Based Layered Superconductor NdO0.71F0.29BiS2

We investigate the superconducting-gap anisotropy in one of the recently discovered BiS_{2}-based superconductors, NdO_{0.71}F_{0.29}BiS_{2} (T_{c}∼5u2009u2009K), using laser-based angle-resolved photoemission spectroscopy. Whereas the previously discovered high-T_{c} superconductors such as copper oxides and iron-based superconductors, which are believed to have unconventional superconducting mechanisms, have 3d electrons in their conduction bands, the conduction band of BiS_{2}-based superconductors mainly consists of Bi 6p electrons, and, hence, the conventional superconducting mechanism might be expected. Contrary to this expectation, we observe a strongly anisotropic superconducting gap. This result strongly suggests that the pairing mechanism for NdO_{0.71}F_{0.29}BiS_{2} is an unconventional one and we attribute the observed anisotropy to competitive or cooperative multiple paring interactions.

Growth of La2−xSrxCuO4 single-crystalline films by liquid phase epitaxial technique

Abstract Single-crystalline films of La 2− x Sr x CuO 4 were prepared on nonsuperconducting substrates of Zn-doped La 2 CuO 4 single crystals by the liquid phase epitaxial (LPE) technique. The La 2− x Sr x CuO 4 single-crystalline films, less than 50 μm thick, were subgrain- and crack-free. The Sr content of the La 2− x Sr x CuO 4 films was determined to be 0.07–0.11, but the films were contaminated with less than 3 at.% aluminum as a result of the use of alumina crucibles. Some of the La 2− x Sr x CuO 4 films exhibited superconductivity with T c onset ≈30 K and Δ T c ≈20 K.

Preparation of La2−xSrxCuO4 single-crystalline films by infrared-heated liquid phase epitaxial technique

Abstract We have designed a new liquid phase epitaxial (LPE) technique, infrared heated LPE (IR-LPE) technique, using no crucible to avoid a contamination from it. La 2− x Sr x CuO 4 single-crystalline films were grown on the a tetra -axis-orientated bulk single crystals of Zn- or Ni-doped La 2 CuO 4 by the IR-LPE technique. La 2− x Sr x CuO 4 single-crystalline films of the thickness 35–250 μm and of Sr content 0.08⩽ x ⩽0.15 were obtained. The superconducting properties in the La 2− x Sr x CuO 4 films were improved to T c,onset ≈36 K for x =0.11, which is 10 K higher than that in the La 2− x Sr x CuO 4 bulk single crystals.

Characterization of intrinsic Josephson junctions for La2-xSrxCuO4 single crystals

Abstract We have fabricated c-axis micro-bridges of La2−xSrxCuO4 (LSCO) single crystals in order to characterize the LSCO intrinsic Josephson junctions (IJJs). The current–voltage characteristics of the micro-bridges exhibited a large hysteresis with a voltage jump of the order 0.5–3 V and no multiple branching structures. A superconducting energy gap was clearly observed on the quasi-particle branch and showed BCS-like temperature dependence. In addition, the temperature dependence of the critical current of the IJJ was in good agreement with the theoretical curves for superconductor–insulator–superconductor (SIS) Josephson junctions. These results demonstrate that the IJJs of LSCO are characterized as stacked series SIS junctions.

Coexistence of superconductivity and charge-density wave in the quasi-one-dimensional material HfTe 3

We present the first experimental evidence for metallicity, superconductivity (SC) and the co-existence of charge density waves (CDW) in the quasi-one-dimensional material HfTe3. The existence of such phenomena is a typical characteristic of the transition metal chalcogenides however, without the application of hydrostatic pressure/chemical doping, it is rare for a material to exhibit the co-existence of both states. Materials such as HfTe3 can therefore provide us with a unique insight into the relationship between these multiple ordered states. By improving on the original synthesis conditions, we have successfully synthesised single phase HfTe3 and confirmed the resultant structure by performing Rietveld refinement. Using low temperature resistivity measurements, we provide the first experimental evidence of SC at ~1.4u2009K as well as a resistive anomaly indicative of a CDW formation at ~82u2009K. By the application of hydrostatic-pressure, the resistivity anomaly shifts to higher temperature. The results show that HfTe3 is a promising new material to help study the relationship between SC and CDW.

Depression of the superconducting transition temperature in neutron irradiated La2CuO4+δ

Abstract The influence of neutron irradiation to the superconductivity of the oxygen loaded La 2 CuO 4+ δ , which were prepared from one single crystal of La 2 CuO 4 , was investigated. The neutron irradiation were performed at 10 K. The superconducting transition temperature ( T c ) dropped monotonously with increasing the neutron fluence. The depression rate of T c was −1.3 K/10 17 n/cm 2 . A rise of T c could not be observed by the neutron irradiation.