Mechanism of hopping transport in disordered mott insulators

By using a combination of detailed experimental studies and simple theoretical arguments, we identify a novel mechanism characterizing the hopping transport in the Mott insulating phase of Ca2-xSrxRuO4 near the metal-insulator transition. The hopping exponent alpha shows a systematic evolution from a value of alpha=1/2 deeper in the insulator to the conventional Mott value alpha=1/3 closer to the transition. This behavior, which we argue to be a universal feature of disordered Mott systems close to the metal-insulator transition, is shown to reflect the gradual emergence of disorder-induced localized electronic states populating the Mott-Hubbard gap.     

Fermi surface, surface states, and surface reconstruction in Sr2RuO4

The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to sqrt[2]xsqrt[2] surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the alpha, beta, and gamma sheets.     

Observation of nonequilibrium radiation behind strong shock waves in low-density air

Nonequilibrium radiation phenomena behind strong shock waves in low-density air are observed by using a couple of CCD camera systems in a shock tube experiment. The simultaneous observation for total radiation and its spectral radiation is carried out in order to elucidate spaced-ependent contribution of an individual radiation spectrum to the total radiation intensity. The results are shown for the shock velocity range from 9.0 km/s to 12.1 km/s at the initial pressure 13.3 Pa. Wavelength range is selected from 300 nm to 445 nm to investigate mainly the contributions from UV radiation. It is found that the band spectra due to the molecular species N₂⁺ and CN mainly contribute to the first-peak, while the spectra due to the atomic species O⁺ and N mainly contribute to the formation of the second-peak. It is also found that the Balmer series in H spectra strongly contributes to the second-peak. The radiation along the tube wall surfaces is composed of the same constituents as those around the tube axis as well as the spectra coming from the impurities.     

Cyclotron resonance and effective mass renormalizations in Sr2RuO4

Using a microwave cavity perturbation technique, we have succeeded in observing cyclotron resonances in the layered Perovskite superconductor Sr₂RuO₄. Three cyclotron masses are deduced, corresponding to m_cγ=9.7m_e, m_cβ=5.8m_e and m_c=4.3 m_e. A comparison between these values and the effective masses deduced from other techniques provides us with a unique means of gauging the role of Coulomb correlations in the title compound.     

Tactile impression and friction of water on human skin

Water has a unique touch as well as characteristic physical properties. However, nobody knows the real identity of its touch. Here, we show that water creates a stick-slip feel when a small amount is rubbed using fingertip on an artificial skin that mimics the structure of human skin. The results of frictional analyses predict that this stick-slip feel is caused by a drastic change in frictional resistance. The present result is valuable for biologists and robot engineers as well as cognitive scientists and tribologists, because it is a new example of stick-slip phenomena on biological surfaces. The tactile texture of this most familiar material could also be applied to consumer products or virtual reality systems.     

Noncommutative algebras related with Schubert calculus on Coxeter groups

For any finite Coxeter system (W,S) we construct a certain noncommutative algebra, the so-called bracket algebra, together with a family of commuting elements, the so-called Dunkl elements. The Dunkl elements conjecturally generate an algebra which is canonically isomorphic to the coinvariant algebra of the Coxeter group W. We prove this conjecture for classical Coxeter groups and I₂(m). We define a “quantization” and a multiparameter deformation of our construction and show that for Lie groups of classical type and G₂, the algebra generated by Dunkl’s elements in the quantized bracket algebra is canonically isomorphic to the small quantum cohomology ring of the corresponding flag variety, as described by B. Kim. For crystallographic Coxeter systems we define the so-called quantum Bruhat representation of the corresponding bracket algebra. We study in more detail the structure of the relations in B_n-, D_n- and G₂-bracket algebras, and as an application, discover a Pieri-type formula in the B_n-bracket algebra. As a corollary, we obtain a Pieri-type formula for multiplication of an arbitrary B_n-Schubert class by some special ones. Our Pieri-type formula is a generalization of Pieri’s formulas obtained by A. Lascoux and M.-P. Schützenberger for flag varieties of type A. We also introduce a super-version of the bracket algebra together with a family of pairwise anticommutative elements, the so-called flat connections with constant coefficients, which describes “a noncommutative differential geometry on a finite Coxeter group” in the sense of S. Majid.     

Ion-size effects on the structure and superconductivity of La-214 compounds at 1/8 doping

A structural transition to a low-temperature tetragonal (TLT) phase occurs below T_d2≈60 K in La_2−xBa_xCuO₄ only around x=0.125, accompanied by sharp suppression of superconductivity. In this study we investigated the effects of partial substitution of smaller divalent cations for Ba²⁺ ions (Ba²⁺ > Sr²⁺ > Ca²⁺) on these anomalous low-temperature properties. The suppression of superconductivity is strongly affected by the appearance of the TLT phase for this carrier concentration. We also show that the variations of T_d2 and T_c are quantitatively scaled by the averaged size of cations at the La site. This dependence can be interpreted as chemical effects acting effectively as applied pressure. We discuss how our observation is related to recent reports which claim that the TLT structure itself does not necessarily destroy the superconductivity.