Masako Saito

Antiferromagnetic Long Range Order in Disordered Spin-Peierls Systems

A theoretical possibility of the existence of disorder-induced antiferromagnetic long range order in spin-Peierls system has been presented, which will be in accordance with the recent experimental indication of the coexistence of dimerization and antiferromagnetism in CuGe 1- y Si y O 3 .

Impurity Induced Antiferromagnetism in Spin-1/2 Heisenberg Two-Leg Ladder Systems

Based on the phase Hamiltonian representation of a spin-1/2 Heisenberg two-leg ladder system coupled antiferromagnetically, which has a spin gap if clean, it is shown that a non-magnetic impurity replacing a spin induces a staggered spin modulation. This results in the long range order of antiferromagnetism in the presence of three-dimensional coupling between the ladders. This will be in accordance with a recent experimental observation by Azuma et al. on Sr(Cu 1- x Zn x ) 2 O 3 .

Magnetic Excitations in Disordered Spin-Peierls Systems.

The magnetic excitations in disordered spin-Peierls systems are investigated theoretically. Reflecting the characteristic properties of the ground state, where the true long range orders of antiferromagnetism and spin-Peierls lattice dimerization coexist with spatially varying order parameters, two kinds of excitations with intrinsic mode broadenings are found. They are acoustic antiferromagnetic (AF) spin wave mode and gapped spin-Peierls (SP) lattice distortion mode. These results are qualitatively consistent with inelastic neutron scattering experiments by Martin et al. on Cu 1- y Zn y GeO 3 with y =0.032.

Effects of Magnetic Fields in Disordered Spin-Peierls Systems

Our former theory [H. Fukuyama et al. : J. Phys. Soc. Jpn. 65 (1996) 1182.] of the coexistence of two long range orderings of spin-Peierls (SP) lattice dimerization and antiferromagnetic staggered moments in disordered SP systems have been extended in the treatment of impurity effects. First it is shown that long range order of the lattice dimerization can be destroyed above a critical density of impurity in accordance with recent experimental results. Secondly effects of magnetic fields are investigated in terms of soliton formations. In weakly disordered systems magnetic discommensurations are found to be formed around each impurity even in relatively weak fields and the magnetization process is expected to show two steps.

Theoretical Possibility of First-Order Phase Transition in Disordered Spin-Peierls Systems

Impurity effects on spin-Peierls (SP) systems are investigated by using the phase Hamiltonian including mean field interchain interactions at T =0. It is shown that impurities induce antiferromagne...

A theoretical study on orbital magnetism of mesoscopic ring systems

Abstract Spatial distribution of current and resultant magnetization of mesoscopic ring systems with finite width are investigated in various regimes of temperature and magnetic field based on the exactly solvable model. The Aharonov–Bohm (AB) oscillation is observed in low temperatures when electrons occupy only one Landau sublevel. When several Landau sublevels are occupied, the coherence of the AB oscillation is destroyed and mesoscopic fluctuations appear instead. The finite Landau sublevels leads to also de Haas-van Alphen oscillations in relatively low temperatures. In high temperatures, the Landau diamagnetism is observed like in the disk systems.

Midgap States in Disordered Spin-Peierls Systems

We investigate effects of a single impurity on the spin-Peierls system using a simplified one-dimensional XY model coupled with lattice distortion in terms of fermions representing spin operators in the continuum limit. An impurity is assumed to reduce the spin-lattice coupling around it. It is shown that midgap bound states appear around the impurity and consequently the lattice distortion is reduced there. Magnetization can be induced there under adequate boundary conditions. The results are compared with the recent results by a phase Hamiltonian.

Impurity induced midgap state and magnetism in spin-Peierls systems

Abstract Effects of impurities on the spin-Peierls systems are investigated by using fermion representation of spin operators within the continuum limit approximation. We use XY model instead of the isotropic Heisenberg exchange interaction. The Bogoliubov–de Gennes-type equations are solved on the assumption that an impurity changes the spin–lattice coupling at the impurity site. It is shown that midgap bound states are formed. The lattice distortion is reduced around the impurities and its spatial dependence is expressed by a composition of parts of the soliton solution in polyacetylenes. There are a few possible impurity states depending on the total S z , which is determined by the filling of the midgap states. Uniform magnetic moments will be finite around impurities. These results are compared with the results of previous theoretical studies based on the bosonized phase Hamiltonian. Since the previous works are for the isotropic Heisenberg exchange interaction, the behavior of the magnetic moments around the impurity is different from the present case.

Effects of disorder on magnetic excitation in Spin-Peierls systems

Abstract It has been experimentally and theoretically established that disorder in a spin-Peierls systems can induce long-range-ordered staggered moments, whose amplitude vary spatially. We investigate magnetic excitations in such a system theoretically and obtain two modes, gapless spin wave mode due to disorder-induced antiferromagnetic ordering and gapped mode due to the lattice dimerization. Each mode has characteristic broadening caused by disorder. These results are compared with experiments.

MAGNETISM IN DISORDERED SPIN-PEIERLS SYSTEMS

Abstract The magnetic excitations in disordered spin-Peierls systems are investigated theoretically. Reflecting the characteristic properties of the ground state, where the true long-range orders of antiferromagnetism and spin-Peierls lattice dimerization coexist with spatially varying order parameters, two kinds of excitations with intrinsic mode broadenings are found. They are acoustic antiferromagnetic (AF) spin wave mode and gapped spin-Peierls (SP) lattice distortion mode. The effects of magnetic field is also discussed. These results are compared with experimental results.