Michio Naito

Reflection high-energy electron diffraction study on the SrTiO3 surface structure

Abstract This paper reports a reflection high-energy electron diffraction investigation of the surface structures of the (001) and (110) crystal faces of SrTiO 3 given various surface treatments. The as-polished surface has a (1×1) unreconstructed structure on both the (001) and (110) crystal faces. The vacuum annealed surface has a two-domain (2×1) superstructure on the (001) face and a (4×4) structure on the (110) face. The oxygen annealed surface has two superstructures on the (001) face: a two-domain (√13×√13-R33.7°) structure and a c(6×2) structure, and a (4×2) or a (4×4) structure on the (110) face. Of these surfaces, the oxygen annealed surface has the highest quality.

MBE growth of (La,Sr)2CuO4 and (Nd,Ce)2CuO4 thin films

Abstract The growth of (La,Sr) 2 CuO 4 and (Nd,Ce) 2 CuO 4 thin films by reactive coevaporation is reviewed. By (1) careful adjustment of the cation stoichiometry, (2) optimum oxidation, and (3) appropriate choice of substrates, we have successfully grown c -axis and non- c -axis oriented high-quality thin films of (La,Sr) 2 CuO 4 and (Nd,Ce) 2 CuO 4 . The key points for thin film growth and the characterization of grown films are discussed.

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(4-y).

We present measurements of the magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-y) and La(2-x)Ce(x)CuO(4-y) films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

All-MgB2 Josephson tunnel junctions

Sandwich-type all-MgB2 Josephson tunnel junctions (MgB2∕AlOx∕MgB2) have been fabricated with as-grown MgB2 films formed by molecular-beam epitaxy. The junctions exhibit substantial superconducting current (IcRN product ∼0.8mV at 4.2 K), a well-defined superconducting gap (Δ=2.2–2.3mV), and clear Fraunhofer patterns. The superconducting gap voltage of Δ agrees well with the smaller gap in the multigap scenario. The results demonstrate that MgB2 has great promise for superconducting electronics that can be operated at T∼20K.

MgB2 thin films for superconducting electronics

This review article describes the developments over the last 30 months in the thin film growth and junction fabrication of superconducting MgB2, including a brief summary of the chemistry and physics of MgB2. The most serious problem is the high Mg vapour pressure required for the phase stability. This problem makes in situ film growth difficult. So, most of the initial efforts were performed by two-step growth, namely ex situ post-annealing. Later, in situ physical vapour deposition and recently in situ chemical vapour deposition were reported. Each growth method is described in detail and compared. The past efforts at fabricating MgB2 junctions are summarized, and the future prospects for MgB2 superconducting electronics are briefly described.

Broadening Mechanism of Resistive Transition under Magnetic Field in Single Crystalline (La1-xSrx)2CuO4

The resistive transitions near the superconducting critical temperature were measured under magnetic field up to 5 T on a FZ-grown high-quality single crystal (La0.93Sr0.07)2CuO4 (Tc=37.5 K) The crystal was sufficiently large along the c-axis that the dc four-probe method could be applied along this axis as well as along the usual a-b plane. It was found that the width of the temperature region over which the resistive transition is broadened depends only on the relative orientation between the magnetic field and the crystalline c-axis, but not on that between the field and the current. It indicates that the mechanisms based on the fluxoid motion, such as the flux creep model, the Kosterlitz-Thouless transition model and the superconducting glassy state model, are unlikely to explain the broadening of the resistive transition under magnetic fields in (La1-xSrx)2CuO4.

High-Tc and high-Jc SmFeAs(O,F) films on fluoride substrates grown by molecular beam epitaxy

Superconducting thin films of SmFeAs(O,F) were prepared by molecular beam epitaxy on fluoride substrates. In our process, F-free SmFeAsO films were grown first, and F was subsequently introduced to the films by diffusion from an overlayer of SmF3. By this simple process, record high Tc, namely, Tcon (Tcend) = 57.8 K (56.4 K) was obtained in a film on CaF2. Furthermore, the films on CaF2 showed high critical current density over 1 MA/cm2 in the self-field at 5 K. The correlation between superconductivity and epitaxial strain in SmFeAs(O,F) films is discussed.

Stoichiometry control of atomic beam fluxes by precipitated impurity phase detection in growth of (Pr,Ce)2CuO4 and (La,Sr)2CuO4 films

This letter describes a method to sense slight off‐stoichiometry (less than 2%–3%) in the cation ratio [Cu/(Pr+Ce) or Cu/(La+Sr)] of atomic beam fluxes by detecting precipitated impurity phases on the surface using reflection high‐energy electron diffraction during growth of T′‐(Pr,Ce)2CuO4 and T‐(La,Sr)2CuO4 films. The method is based on the facts that off‐stoichiometry of fluxes inevitably produces precipitates of certain impurity phases on the surface in film growth of these oxides, and that the species of the impurity phases are solely determined by the type of off‐stoichiometry; copper rich or lanthanoid rich. Furthermore, it has been found that it is possible to recover a precipitate‐free surface, from a surface with precipitates produced by lanthanoid‐rich deposition, by later flux readjustment, but it is difficult from a surface with precipitates produced by copper‐rich deposition.

Reflection high-energy electron diffraction and atomic force microscopy studies on homoepitaxial growth of SrTiO3(001)

Abstract This paper reports a systematic study on the homoepitaxial growth of SrTiO 3 (001) conducted using reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) in order to improve our understanding of the basic processes in the epitaxial growth of perovskite thin films. Under certain growth conditions, the homoepitaxial growth of SrTiO 3 essentially proceeds in the layer-by-layer (2D nucleation) growth mode with the basic unit of a molecular layer. This cyclic process due to unit-cell-by-unit-cell (uc-by-uc) growth is confirmed by undamped RHEED oscillations and the corresponding temporal evolution of AFM images of the growth front. Furthermore, the surface mobility is controlled by growth temperature and oxidation condition. By changing growth temperature or oxidation condition, crossovers from layer-by-layer growth to step-flow like growth and from layer-by-layer growth to 3D island growth through Stranski–Krastanov growth are observed. On the basis of these results, a universal phase diagram is proposed for the growth mode of the epitaxial film growth of complex oxides.

Superconducting thin films of La2CuO4+δ by oxygen doping using ozone

Abstract Superconducting (001) thin films of La 2 CuO 4+δ were synthesized by cooling in ozone gas, demonstrating that this simple process produces a strong oxidizing environment. The films cooled in ozone gas until 150°C exhibit the superconducting onset temperature of 52 K and the zero-resistance temperature of 48 K on LaSrAlO 4 substrates. The room-temperature resistivity is as low as 500 μΩ cm for these films, indicating that they were homogeneously oxidized. The superconducting transition temperatures for the films on LaSrAlO 4 substrates are higher than those for the films on SrTiO 3 substrates, suggesting a strain effect caused by lattice mismatch with the substrates, which was investigated by X-ray diffraction. Surface analysis by reflection high energy electron diffraction and photo-emission spectroscopy indicate that degraded layers were formed on the surfaces of the films during the oxidation process. The thickness of these degraded layers seems to be much less than 50 A because they do not seriously affect the resistivity of the films with thickness of 50 A.