Takahito Terashima

Single-Crystal YBa2Cu3O7-x Thin Films by Activated Reactive Evaporation

By means of the activated reactive evaporation, YBa2Cu3O7-x single-crystal thin films with the c axis perpendicular to the substrate plane have been directly and epitaxially grown on the (100) surface of SrTiO3. The substrate temperature was kept below 600°C and the oxidation treatment was performed at below 500°C. The ac-resistive measurement for a film with 2000 A thickness gave a sharp superconducting transition, where the endpoint was 90.2 K and ΔTc(10–90%)=1.7 K. From the complex susceptibility measurement we confirmed the Meissner effect as well as the structural uniformity of the specimen.

Preparation of ferroelectric BaTiO3 thin films by activated reactive evaporation

Ferroelectric BaTiO3 thin films were directly and epitaxially grown on SrTiO3 single crystal and epitaxial Pt film substrates by activated reactive evaporation. The substrate temperature was around 600 °C. For (100) oriented as‐grown films, a typical ferroelectric hysteresis loop and a maximum of dielectric constant at about 115 °C were observed. The resistivity was as high as 109 Ω cm and the breakdown voltage was 2.7 MV/cm for as‐grown BaTiO3 films.

Atomic layer growth of oxide thin films with perovskite‐type structure by reactive evaporation

Epitaxial growth of BaTiO3 and SrTiO3 films by the reactive evaporation method was investigated using reflection high‐energy electron diffraction (RHEED). The investigations were carried out using two growth methods: coevaporation and alternate evaporation of the metal elements in an oxygen atmosphere. Atomic layer growth was achieved by the alternate supply of Ba or Sr and Ti on the growing surface. In the case of coevaporation, epitaxial growth occurred in a two‐dimensional unit‐cell‐by‐unit‐cell mode. The surface of each unit cell is terminated by a (TiO2) layer. Artificial superlattices of BaTiO3/SrTiO3 were fabricated by monitoring the film thickness with the RHEED oscillations.

Epitaxial growth and dielectric properties of BaTiO3 films on Pt electrodes by reactive evaporation

Thin films of BaTiO3 have been epitaxially grown on Pt(001)/MgO(100) substrates by reactive evaporation. Structural and electrical properties were investigated as a function of film thickness. In situ reflection high‐energy electron diffraction and cross‐sectional transmission electron microscope observations have revealed that the BaTiO3 films are epitaxially grown on Pt/MgO substrates from the initial stage without any other phase formation. From the images of an atomic force microscope, it has been found that islands of BaTiO3 are present on the bare Pt surface at the initial stage of deposition; the island structure changes to a continuous layer above 1.2 nm in thickness and BaTiO3 grows in a two‐dimensional mode. The lattice parameters and the dielectric properties are dependent on the film thickness. Thermodynamic theory was introduced to explain the thickness dependence of the relative dielectric constant er. Good agreement between the experimental results and the theoretical calculations leads to ...

Electronic nematicity above the structural and superconducting transition in BaFe2(As1-xPx)2

Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice, has been observed in the iron pnictide and copper oxide high-temperature superconductors. Whether nematicity plays an equally important role in these two systems is highly controversial. In iron pnictides, the nematicity has usually been associated with the tetragonal-to-orthorhombic structural transition at temperature Ts. Although recent experiments have provided hints of nematicity, they were performed either in the low-temperature orthorhombic phase or in the tetragonal phase under uniaxial strain, both of which break the 90° rotational C4 symmetry. Therefore, the question remains open whether the nematicity can exist above Ts without an external driving force. Here we report magnetic torque measurements of the isovalent-doping system BaFe2(As1−xPx)2, showing that the nematicity develops well above Ts and, moreover, persists to the non-magnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides. By combining these results with synchrotron X-ray measurements, we identify two distinct temperatures—one at T*, signifying a true nematic transition, and the other at Ts (<T*), which we show not to be a true phase transition, but rather what we refer to as a ‘meta-nematic transition’, in analogy to the well-known meta-magnetic transition in the theory of magnetism.S. Kasahara, H. J. Shi, K. Hashimoto, S. Tonegawa, Y. Mizukami, T. Shibauchi, K. Sugimoto, T. Fukuda, T. Terashima, AndriyH. Nevidomskyy & Y. Matsuda Department of Physics, Kyoto University, Kyoto 606-8502, Japan Research Center for Low Temperature and Materials Sciences, Kyoto University, Kyoto 606-8501, Japan Research & Utilization Division, JASRI SPring-8, Sayo, Hyogo 679-5198, Japan Structural Materials Science Laboratory, RIKEN SPring-8, Sayo, Hyogo 679-5148, Japan Quantum Beam Science Directorate, JAEA SPring-8, Sayo, Hyogo 679-5148, Japan Materials Dynamics Laboratory, RIKEN SPring-8, Sayo, Hyogo 679-5148, Japan JST, Transformative Research-Project on Iron Pnictides (TRIP), Chiyoda, Tokyo 102-0075, Japan Department of Physics and Astronomy, Rice University, 6100 Main St., Houston, TX 77005, USA and ∗Present address: Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan (Dated: May 2, 2014)

Line nodes in the energy gap of superconducting BaFe2(As1-xPx)2 single crystals as seen via penetration depth and thermal conductivity

We report magnetic penetration depth and thermal conductiv ity data for high-quality single crystals of BaFe2(As1−xPx)2 (Tc = 30K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap foun d for (Ba,K)Fe2As2 which has similarTc and phase diagram. Our results indicate that repulsive electronic in teractions play an essential role for Fe-based highTc superconductivity but that uniquely there are distinctly d ifferent pairing states, with and without nodes, which have comparable Tc.

Epitaxial growth of YBa2Cu3O7−x thin films on (110) SrTiO3 single crystals by activated reactive evaporation

The orientation of YBa2Cu3O7−x thin films grown on (110) planes of SrTiO3 by activated reactive evaporation was investigated by means of reflection high‐energy electron diffraction. The orientation of the films depended on the substrate temperature. The films with (110) planes parallel to the substrate surface grew in a narrow range of substrate temperatures around 530 °C, while the films with (103) planes parallel to the surface grew at temperatures above 600 °C. The change of the epitaxial orientation with the substrate temperature is discussed in terms of the temperature dependence of the lattice mismatch between YBa2Cu3O7−x and SrTiO3.

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1–xPx)2

A Spike Inside the Dome The transition temperature Tc of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554; see the Perspective by Sachdev) measured the penetration depth of the pnictide series BaFe2(As1−xPx)2 as a function of x. A sharp peak right around the point where Tc has a maximum (x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30. A quantum critical point may be lurking inside the superconducting dome of a pnictide series. In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two-dimensional systems, this ratio n/m* (~1/λL2) determines the effective Fermi temperature, TF. We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1–xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.

Unconventional Superconductivity and Antiferromagnetic Quantum Critical Behavior in the Isovalent-Doped BaFe_{2}(As_{1-x}P_{x})_{2}

Spin dynamics evolution of BaFe2(As(1-x)Px){2} was probed as a function of P concentration via 31P NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility on approaching the AF phase from the superconducting dome. Moreover, the magnetically ordered temperature θ deduced from the relaxation rate vanishes at optimal doping. These results provide clear-cut evidence for a quantum-critical point, suggesting that the AF fluctuations associated with the quantum-critical point play a central role in the high-T(c) superconductivity.

Blue luminescence from electron-doped SrTiO3

Blue-light emissions from electron-doped SrTiO3 single crystals at room temperature were observed. Substituting La3+ for Sr2+ and Nb5+ for Ti4+ in SrTiO3 provides electron carriers in Ti 3d conduction bands; these carriers are responsible for the room-temperature blue-light emission. This blue-light emission is essentially the same as that observed in Ar+-irradiated oxygen-deficient SrTiO3. This blue luminescence is independent of the defect type. The chemical substitution of La3+ for Sr2+ changes the temperature of the structural phase transition from cubic to tetragonal symmetry. The relation between the photoluminescence properties and the structural phase transition is also discussed.