The superconducting electronic states of Nd 1.85 Ce 0.15 CuO 4Ϫ␦ ͑NCCO͒ are studied by applying the phasesensitive capability of tunneling spectroscopy. Unlike the YBa 2 Cu 3 O 7Ϫ␦ cases, the conductance spectra measured on ͑100͒-, ͑110͒and ͑001͒-oriented surfaces showed V-shaped gap structures, and no zero-bias conductance peaks were observed even around steps on the surfaces. The present results reject the possibility of d x 2 Ϫy 2-wave and d xy -wave symmetries for the pair potential in NCCO. The experimental spectra are well fitted to theoretical curves of anisotropic s-wave symmetry with a large smearing factor.

A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with seven-femtosecond laser pulses can detect the optically induced spin dynamics in Nd2CuO4, a typical cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at ~18 fs. This time constant is attributable to the spin-relaxation time during magnetic-polaron formation, which is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolution of the reflectivity changes, and their frequencies (1400–2700 cm−1) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations can be interpreted as the interference between charge excitations with two magnons originating from charge–spin coupling.

We calculate the 1-loop effect in super-Yang-Mills which preserves 1/4supersymmetries and is holographically dual to the null-like cosmology with a big-bang singularity. Though the bosonic and fermionic spectra do not agree precisely, we do obtain vanishing 1-loop vacuum energy for generic warped plane-wave type backgrounds with a big-bang singularity. Moreover, we find that the cosmological "constant" contributed either by bosons or fermions is time-dependent. The issues about the particle production of some background and about the UV structure are also commented. We argue that the effective higher derivative interactions are suppressed as long as the Fourier transform of the timedependent coupling is UV-finite. Our result holds for scalar configurations that are BPS but with arbitrary time-dependence. This suggests the existence of non-renormalization theorem for such a new class of time-dependent theories. Altogether, it implies that such a super-Yang-Mills is scale-invariant, and that its dual bulk quantum gravity might behave regularly near the big bang.Keywords: M(atrix) Theories, AdS-CFT Correspondence, Penrose limit and pp-wave background.1 Though the conformal invariance of N=4 SYM is broken, as shown in [10], there is still a residual scale-invariance that guarantees the power-law structure of the SYM correlators.2 We thank C. Bachas for pointing this out to us.

The zero bias anomaly dependent on the preferred surface grain orientation of YBa2Cu3O7-δ ceramics has been investigated in the conductance curve of the tunneling characteristics. A tunnel junction was fabricated by evaporating Al metal onto the ceramics sample and then annealed to remove the oxygen-deficient layer. The preferred orientation and the oxygen deficiency on the surface were examined based on X-ray diffraction patterns and X-ray photoelectron spectra. There exist oriented grains with their c-plane parallel to the ceramics sample surface. These surface layers are oxygen-deficient. Using the junction fabricated on the oxygen-deficient grains, the zero bias anomaly appears on the dynamic conductance curve of the tunneling characteristics. This anomaly disappears when the junction is fabricated after mechanical polishing of the surface and annealed. One of the sources of zero bias anomaly is the existence of oxygen-deficient grains on the YBa2Cu3O7-δ surface.

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