The ground state of the stacked triangular antiferromagnet CsFeBr3 is a spin singlet due to the large single ion anisotropy D(S z ) 2 . The field-induced magnetic ordering in this compound was investigated by the magnetic susceptibility, the magnetization process and specific heat measurements for an external field parallel to the c-axis. Unexpectedly, two phase transitions were observed in the magnetic field H higher than 3 T. The phase diagram for temperature versus magnetic field was obtained. The mechanism leading to the successive phase transitions is discussed.KEYWORDS: CsFeBr 3 , triangular antiferromagnet, singlet ground state, field-induced magnetic ordering, successive phase transitions, single ion anisotropy, magnetic susceptibility, magnetization process, specific heat §1. IntroductionThe spin frustration effect often plays an important role in the magnetic ordering process and magnetic excitations. In the hexagonal antiferromagnets of ABX 3 -type with the CsNiCl 3 structure, magnetic B 2+ ions form infinite chains along the c-axis and triangular lattices in the basal c-plane. Since the exchange interaction in the c-plane is antiferromagnetic, they behave as triangular antiferromagnets (TAF) at low temperatures. Because of the spin frustration effect being characteristic of TAF, together with the quantum effect, a rich variety of phase transitions have been observed in the hexagonal ABX 3 antiferromagnets. 1)Low-temperature magnetic properties of AFeX 3 systems are described by the pseudo spin S = 1 anisotropic XXZ model with the large easy-plane anisotropy of the form D(S z ) 2 due to the crystalline field.2) In RbFeCl 3 , exchange interactions overcome the anisotropy, so that RbFeCl 3 undergoes magnetic phase transition in the absence of magnetic field. On the other hand, CsFeCl 3 has a singlet ground state at zero field, because exchange interactions are not sufficiently strong to produce the magnetic ordering. The ordering process in RbFeCl 3 is not simple. The phase transition occurs from the paramagnetic state to the commensurate (C) ground state with the 120• structure through two different incommensurate (IC) states.3, 4) The IC-C phase transition occurs due to the competition between the antiferromagnetic exchange interaction in the c-plane and the dipole-dipole (D-D) interaction, the latter of which is enhanced by the ferromagnetic exchange interaction along the c-axis.5, 6) A similar IC-C phase transition was observed in CsFeCl 3 Table I. Interaction parameters for CsFeBr 3 in the unit of K.

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The interface modes of CdSe/ZnSe superlattices were observed by Raman scattering experiments. These modes and their overtone spectra were found to be enhanced by the incident Ar + laser with an excitation energy near the confined exciton energy band in CdSe quantum wells. The observed frequencies of these modes agree with the theoretical dispersion relation of the interface mode based on an elastic continuum model. By using two kinds of laser excitation, the resonant Raman effect of CdSe/ZnSe was confirmed to reflect the miniband structure of the present systems.KEYWORDS: Raman scattering, semiconductor superlattice, interface mode, resonant effect, miniband structure, CdSe, ZnSe §1. Introduction II-VI wide-band-gap semiconductor superlattices and multiple quantum wells associated with them arouse great interest because of the potential applicability of their unique optical and electronic properties. In particular, ZnSe-based have attracted increasing attention as important materials for blue-green laseremitters and detectors. A binary multilayer CdSe/ZnSe superlattice is a typical system with a quantum-well band gap of 1.765 eV and is therefore regarded as a suitable candidate for the active medium of laser diodes emitting blue-green light. To date, however, most works have examined ZnSe/ZnCdSe alloy superlattices 1, 2) or a short-period-superlattice CdSe/ZnSe, 3, 4) because the large lattice mismatch between ZnSe and CdSe makes it very difficult to grow multilayer CdSe/ZnSe superlattices with high crystalline quality. Recently, a group including two of the present authors (Matsumoto and Nabetani) successfully achieved the growth of CdSe/ZnSe superlattices with a long period. 5) Thus, a Raman scattering study has been performed to investigate the electronic structure and several phonons of these systems. At the same time, an evaluation of the obtained superlattice structure was carried out.Raman scattering is one of the most widely used techniques for the observation of the electronic structure and phonon modes in semiconductor superlattices. Probable resonance phenomena have been exploited in a variety of ways to gain information on electron-phonon interactions in superlattices and excitonic excitations in multiple quantum wells.Three main features of the vibrational mode for the Raman scattering in superlattices are well known. 6,11,12) The first is the folded acoustic mode which is caused microstructures * Present address: 2nd Examination Dpt. Japanese Patent Office. by the expansion of lattice periodicity along the stacking direction of the multilayers. This phonon propagates through the entire structure exhibiting the folding effect of the phonon dispersion curves of the bulk material. Properties of acoustic phonon modes depend mainly on a superlattice period d. The is the confined mode of an optical restricted within the respective layers not propagating into another layer because of the difference in the dielectric constants of successive layers. Properties of the confined optical phonon modes are governe...