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We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the high temperature superconductors (HTS) YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), La1.85Sr0.15CuO4 (LSCO) and Nd1.85Ce0.15CuO4−y (NCCO). In all experiments the tunneling direction was along the CuO2 planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak (ZBCP) was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.To appear in Physical Review B There is strong evidence that the superconducting order parameter (OP) in the HTS has a dominating d-wave symmetry [1,2]. For this pairing symmetry there is a π-phase shift of the OP in orthogonal k-space directions resulting in a positive and negative sign of the pair potential in those directions. This also means that there are directions with nodes of the pair potential, e. g. for a pure d x 2 −y 2 -symmetry, the nodes are along the [110] direction in the CuO 2 plane. For the tunneling spectra of junctions employing HTS electrode materials with a d-wave symmetry of the OP, a pronounced ZBCP has been predicted originating from mid-gap surface (interface) states or zero energy bound states (ZES) at the Fermi level [3][4][5][6][7][8]. The physical reason for these states originates from the fact that quasiparticles incident and reflecting from the surface propagate through different order parameter fields which leads to Andreev reflection. The constructive interference between incident and Andreev reflected quasiparticles results in bound states. Stable ZES are formed if the scattering induces a change in sign of the OP. For a d x 2 −y 2 -wave symmetry such sign change and, hence, the presence of ZES is possible for all surfaces parallel to the c-axis except for those with the lobe directions perpendicular to the surface, whereas for a s-wave symmetry no ZES are possible. The spectral weight of the ZES for a d x 2 −y 2 -wave symmetry depends on the orientation of the surface with respect to the crystal axis. The maximum spectral weigth is expected for a (110) surface and, hence, a maximum ZBCP is expected for tunneling in the direction of the nodal lines, i. e., the [110] direction. This has been observed recently using low temperature scanning tunneling spectroscopy (LTSTS) [9] and planar type junctions [10]. We note that the ZBCP is sensitive to surface roughness making it difficult to distinguish between the directions in the plane [11][12][13].Initially, the ZBCP in the tunneling spectra...

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We have measured the temperature dependence of the in-plane London penetration depth λ ab (T ) and the maximum Josephson current Ic(T ) using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor Nd1.85Ce0.15CuO4−y. Both quantities reveal an anomalous temperature dependence below about 4 K. In contrast to the usual monotonous decrease (increase) of λ ab (T ) (Ic(T )) with decreasing temperature, λ ab (T ) and Ic(T ) are found to increase and decrease, respectively, with decreasing temperature below 4 K resulting in a non-monotonous overall temperature dependence. This anomalous behavior was found to be absent in analogous measurements performed on Pr1.85Ce0.15CuO4−y. From this we conclude that the anomalous behavior of Nd1.85Ce0.15CuO4−y is caused by the presence of the Nd 3+ paramagnetic moments. Correcting the measured λ ab (T ) dependence of Nd1.85Ce0.15CuO4−y for the temperature dependent susceptibility due to the Nd moments, an exponential dependence is obtained indicating isotropic s-wave pairing. This result is fully consistent with the λ ab (T ) dependence measured for Pr1.85Ce0.15CuO4−y. The vast majority of experiments on the cuprate superconductors are performed on hole-doped materials. Much less attention has been paid to the system Ln 2−x Ce x CuO 4−y (with Ln = Pr, Nd, Sm, Eu) [1] which represents an electron-doped material. Both hole-and electron-doped cuprates have in common the copper oxygen planes as the central building blocks of the high-temperature superconductors (HTS) suggesting similar superconducting properties. However, as can already be seen from the differences of the generic phase diagram on the electron-and hole-doped side, the physics of electron-and hole-doped HTS is different. In particular, the order parameter (OP) symmetry of the electron-doped cuprates is most likely of s-wave type [2][3][4][5], in contrast to the d-wave OP symmetry in the hole-doped HTS. To clarify the specific differences and similarities between the electron-and hole-doped HTS a more detailed experimental study of the electron-doped HTS is required.

One of the fabrication methods for functionally graded materials (FGMs) is a centrifugal solid-particle method, which is an application of the centrifugal casting technique. However, it is the difficult to fabricate FGMs containing nano-particles by the centrifugal solid-particle method. Recently, we proposed a novel fabrication method, which we have named the centrifugal mixed-powder method, by which we can obtain FGMs containing nano-particles. Using this processing method, Cu-based FGMs containing SiC particles and Al-based FGMs containing TiO2 nano-particles on their surfaces have been fabricated. In this article, the microstructure and mechanical property of Cu/SiC and Al/TiO2 FGMs, fabricated by the centrifugal mixed-powder method are reviewed.

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Using nitrogen-dioxide (NO2) adsorption treatment and Al2O3 passivation technique, we improved drain current (I DS) of hydrogen-terminated (H-terminated) diamond field-effect transistors (FETs). The Al2O3 passivation layer also serves as a gate-insulator in a gate region. Maximum I DS (I DSmax) of -1.35 A/mm was obtained for the diamond FETs with NO2 adsorption and the Al2O3 passivation layer. This I DSmax is the highest ever reported for diamond FETs and indicates that the Al2O3 passivation layer can stabilize adsorbed NO2, which increases the hole carrier concentration on the H-terminated diamond surface. In RF small-signal characteristics, the diamond FETs with NO2 adsorption and the Al2O3 passivation layer showed high cutoff-frequency (f T) and maximum frequency of oscillation (f max) in a wide gate–source voltage (V GS) range (>10 V). This is because the Al2O3 gate insulator with a high potential barrier against hole carriers can confine and control the high concentration of hole carriers and then high forward-bias voltage can be applied without noticeable gate leakage current.