The Project for the New High Field Facility

Since High Field Laboratory for Superconducting Materials attached to Institute for Materials Research of Tohoku University started, 15 years has passed. We have had plenty of fruitful scientific and technological results not only in a field of superconductivity but also in areas of magnetism, semiconductors, organic conductors, chemical and biological materials and crystallography. The equipments in the Laboratory, however, are now getting aged and the available high field of 31 T obtained by our hybrid magnet, which used to be the highest in the world, must be graded up. The committee of the Laboratory has been discussing on the development of equipments and expansion of research fields for the new facility

Theory of phonon-assisted "forbidden" optical transitions in spin-gapped systems

We consider the absorption of light with emission of one S(tot)=1 magnetic excitation in systems with a spin gap induced by quantum fluctuations. We argue that an electric dipole transition is allowed on the condition that a virtual phonon instantaneously breaks the inversion symmetry. We derive an effective operator for the transition and argue that the proposed theory explains the polarized experiments in CuGeO(3) and SrCu(2)[BO(3)](2).Comment: 9 pages, 4 figure

Coherent radiation by molecular magnets

The possibility of coherent radiation by molecular magnets is investigated. It is shown that to realize the coherent radiation, it is necessary to couple the considered sample to a resonant electric circuit. A theory for describing this phenomenon is developed, based on a realistic microscopic Hamiltonian, including the Zeeman terms, single-site anisotropy, and dipole interactions. The role of hyperfine interactions between molecular and nuclear spins is studied. Numerical solutions of the spin evolution equations are presented.Comment: Latex file, 11 pages, 3 figure

ESR investigation on the Breather mode and the Spinon-Breather dynamical crossover in Cu Benzoate

A new elementary-excitation, the so called "breather excitation", is observed directly by millimeter-submillimeter wave electron spin resonance (ESR) in the Heisenberg quantum spin-chain Cu benzoate, in which a field-induced gap is found recently by specific heat and neutron scattering measurements. Distinct anomalies were found in line width and in resonance field around the "dynamical crossover" regime between the gap-less spinon-regime and the gapped breather-regime. When the temperature becomes sufficiently lower than the energy gap, a new ESR-line with very narrow line-width is found, which is the manifestation of the breather excitation. The non-linear field dependence of the resonance field agrees well with the theoretical formula of the first breather-excitation proposed by Oshikawa and Affleck. The present work establishes experimentally for the first time that a sine-Gordon model is applicable to explain spin dynamics in a S=1/2 Heisenberg spin chain subjected to staggered field even in high fields.Comment: Revtex, 4 pages, 4 figures, submitted to Phys. Rev. Let

Magnetic structures of RbCuCl_3 in a transverse field

A recent high-field magnetization experiment found a phase transition of unknown character in the layered, frustrated antiferromagnet RbCuCl_3, in a transverse field (in the layers). Motivated by these results, we have examined the magnetic structures predicted by a model of RbCuCl_3, using the classical approximation. At small fields, we obtain the structure already known to be optimal, an incommensurate (IC) spiral with wave vector q in the layers. At higher fields, we find a staircase of long-period commensurate (C) phases (separated initially by the low-field IC phase), then two narrow IC phases, then a fourth IC phase (also with intermediate C phases), and finally the ferromagnetically aligned phase at the saturation field H_S. The three-sublattice C states familiar from the theory of the triangular antiferromagnet are never optimal. The C phases and the two intermediate IC phases were previously unknown in this context. The magnetization is discontinuous at a field \approx 0.4H_S, in qualitative agreement with experiment, though we find much fine structure not reported.Comment: 9 pages, 8 figure

Highly Strengthened Superconducting Magnet for a 40 T Compact Hybrid Magnet(Magnet Technology)

A 16 T outer superconducting magnet for a 40 T compact hybrid magnet is investigated. A highly strengthened superconducting magnet with a 360 mm room temperature bore can be made using newly developed (Nb, Ti)_3Sn wires with Cu-Al_2O_3 reinforcing stabilizer. The coil weight is outstandingly reduced by as much as 70 %