Phase Structure of a Compact U(1) Gauge Theory from the Viewpoint of a Sine-Gordon Model

We discuss the phase structure of the four-dimensional compact U(1) gauge theory at finite temperature using a deformation of the topological model. Its phase structure can be determined by the behavior of the Coulomb gas (CG) system on the cylinder. We utilize the relation between the CG system and the sine-Gordon (SG) model, and investigate the phase structure of the gauge theory in terms of the SG model. Especially, the critical-line equation of the gauge theory in the strong-coupling and high-temperature region is obtained by calculating the one-loop effective potential of the SG model.Comment: 9 pages, 6 figures, REVTeX4, typos corrected, reference added; to appear in Phys.Rev.

Controlling Decoherence of Transported Quantum Spin Information in Semiconductor Spintronics

We investigate quantum coherence of electron spin transported through a semiconductor spintronic device, where spins are envisaged to be controlled by electrical means via spin-orbit interactions. To quantify the degree of spin coherence, which can be diminished by an intrinsic mechanism where spin and orbital degrees of freedom become entangled in the course of transport involving spin-orbit interaction and scattering, we study the decay of the off-diagonal elements of the spin density matrix extracted directly from the Landauer transmission matrix of quantum transport. This technique is applied to understand how to preserve quantum interference effects of fragile superpositions of spin states in ballistic and non-ballistic multichannel semiconductor spintronic devices.Comment: 7 pages, 3 color EPS figures, prepared for Proceedings of International Symposium on Mesoscopic Superconductivity and Spintronics 2004 (Atsugi, Japan, March 1-4, 2004

Non-Trivial Ultraviolet Fixed Point in Quantum Gravity

The non-trivial ultraviolet fixed point in quantum gravity is calculated by means of the exact renormalization group equation in d-dimensions $(2\simeq d\leq4)$. It is shown that the ultraviolet non-Gaussian fixed point which is expected from the perturbatively $\epsilon$-expanded calculations in $2+\epsilon$ gravity theory remains in d=4. Hence it is possible that quantum gravity is an asymptotically safe theory and renormalizable in 2<d.Comment: 17 pages with 5 eps figures, to be published in Prog. Theor. Phy

Spatial Characteristics of Joint Application Networks in Japanese Patents

Technological innovation has extensively been studied to make firms sustainable and more competitive. Within this context, the most important recent issue has been the dynamics of collaborative innovation among firms. We therefore investigated a patent network, especially focusing on its spatial characteristics. The results can be summarized as follows. (1) The degree distribution in a patent network follows a power law. A firm can then be connected to many firms via hubs connected to the firm. (2) The neighbors' average degree has a null correlation, but the clustering coefficient has a negative correlation. The latter means that there is a hierarchical structure and bridging different modules may shorten the paths between the nodes in them. (3) The distance of links not only indicates the regional accumulations of firms, but the importance of time it takes to travel, which plays a key role in creating links. (4) The ratio of internal links in cities indicates that we have to consider the existing links firms have to facilitate the creation of new links

Recent Competition in the Japanese Life Insurance Industry

This paper examines a change in the level of competition in the Japanese life insurance industry over the last 17 years. We estimate the first order condition for profit-maximizing insurance oligopolies to obtain the degree of non-competition and collusion. Estimation results suggest that: 1) not only stock companies, but also mutual companies maximize their own profits rather than pay out dividends to policyholders; 2) competition has become stronger since 1995; 3) revision of Insurance Industry Law and failures of insurance companies promoted the competition; and 4) the competition in the recent years is still more lax than the pre-war period.