High-resolution alternating-field technique to determine the magnetocaloric effect of metals down to very low temperatures

The magnetocaloric effect or "magnetic Gr\"uneisen ratio" $\Gamma_H=T^{-1}(dT/dH)_S$ quantifies the cooling or heating of a material when an applied magnetic field is changed under adiabatic conditions. Recently this property has attracted considerable interest in the field of quantum criticality. Here we report the development of a low-frequency alternating field technique which allows to perform continuous temperature scans of $\Gamma_H(T)$ on small single crystals with very high precision and down to very low temperatures. Measurements on doped YbRh$_2$Si$_2$ show that $\Gamma_H(T)$ can be determined with this technique in a faster and much more accurate way than by calculation from magnetization and specific heat measurements

Dispersion forces in macroscopic quantum electrodynamics

The description of dispersion forces within the framework of macroscopic quantum electrodynamics in linear, dispersing, and absorbing media combines the benefits of approaches based on normal-mode techniques of standard quantum electrodynamics and methods based on linear response theory in a natural way. It renders generally valid expressions for both the forces between bodies and the forces on atoms in the presence of bodies, while showing very clearly the intimate relation between the different types of dispersion forces. By considering examples, the influence of various factors like form, size, electric and magnetic properties, or intervening media on the forces is addressed. Since the approach based on macroscopic quantum electrodynamics does not only apply to equilibrium systems, it can be used to investigate dynamical effects such as the temporal evolution of forces on arbitrarily excited atoms.Comment: 112 pages, 7 figures, 4 tables, extended versio

Shareholding Networks in Japan

The Japanese shareholding network existing at the end of March 2002 is studied empirically. The network is constructed from 2,303 listed companies and 53 non-listed financial institutions. We consider this network as a directed graph by drawing edges from shareholders to stock corporations. The lengths of the shareholder lists vary with the companies, and the most comprehensive lists contain the top 30 shareholders. Consequently, the distribution of incoming edges has an upper bound, while that of outgoing edges has no bound. The distribution of outgoing degrees is well explained by the power law function with an exponential tail. The exponent in the power law range is gamma=1.7. To understand these features from the viewpoint of a company's growth, we consider the correlations between the outgoing degree and the company's age, profit, and total assets.Comment: 10 pages, 4 figures, International Conference Science of Complex Networks: from Biology to the Internet and WWW (CNET2004

Casimir effect for perfect electromagnetic conductors (PEMCs): A sum rule for attractive/repulsive forces

We discuss the Casimir effect for boundary conditions involving perfect electromagnetic conductors (PEMCs). Based on the corresponding reciprocal Green's tensor we construct the Green's tensor for two perfectly reflecting plates with magnetoelectric coupling (non-reciprocal media) within the framework of macroscopic quantum electrodynamics. We calculate the Casimir force between two PEMC plates in terms of the PEMC parameter M and the duality transformation angle ${\theta}$ resulting in a universal analytic expression that connects the attractive Casimir force with the repulsive Boyer force. We relate the results to the duality symmetry of electromagnetism