Kensuke Tani

Anomalous behaviour of sound near the Curie points in displacive-type ferroelectrics

Abstract The attenuation of sound is studied theoretically using the Silverman model, which predicts an anomalously large attenuation constant near the Curie point.

Life time of soft modes in displacive - type ferroelectrics

Abstract With the use of the Silverman model the temperature and wave number dependence of the lifetime of the soft modes in displacive ferroelectrics is calculated explicitly, whereby Cowleys experiments SrTiO 3 by neutron scattering are explained.

Coupled mode of sound with soft mode in displacive-type ferroelectrics

Abstract It is pointed out that the sound mode in displacive-type ferro-electrics changes into a new type collective mode near the Curie point through strong coupling with the soft mode. The temperature and wave number dependence of this coupled mode are calculated explicitly.

Anomalous attenuation of sound near the spin-flop transition point in uniaxial antiferromagnets

Abstract Attenuation of sound near the spin-flop transition point is studied theoretically, and an explicit dependence of the attenuation constant on magnetic field, temperature and frequency is obtained.

Theory of Neutron Scattering from Lattice Vibrations. II. Application of a Phonon Theory

A phonon theory developed previously which is a new approach besides the self consistent phonon theory and which is generalization of the Born phonon theory giving the rigorous phonon frequencies and normal coordinates of a crystal at an arbitrary temperature, is applied to neutron scattering from lattice vibrations in an anharmonic crystal. This application is done using a moment expansion of the canonical correlation function in the form of a continued fraction. A displacive ferroelectric and a perovskite crystal which exhibits a structural phase transition are discussed in some detail. A magnetic system having the spin-phonon interaction due to modulation of the exchange interaction caused by lattice vibrations is also discussed. A scattering cross section which depends explicitly upon the interaction acting on a phonon and has an anomalous temperature dependence near the transition point is found in some cases.

Photon pumped antiferromagnetic magnon instability associated with two magnon excitation

Abstract A possibility for light absorption different from those observed previously is pointed out.

Longitudinal spin pair correlation in antiferromagnets

Abstract Longitudinal spin pair correlation in antiferromagnets is studied by the thermal Green function method. The dependence of the correlation on the wave length and the temperature is determined explicitly for temperatures below the Neel point T N .

Dynamical Behaviour of Displacive-Type Ferroelectrics near the Transition Point

Dynamical behaviour of both optical and acoustic modes near the transition point of displacive-type ferroelectrics such as BaTiO 3 and SrTiO 3 is investigated theoretically with the use of the correlation function method. Particular attention is paid to the behaviour concerning the memory effect due to the critical fluctuation of polarizations and the possibility of existence of a new-type oscillation peculiar to the critical regime is shown adopting the Silverman Hamiltonian. The dynamic scaling law in the original form is shown not to be applicable to these substances and its physical origin is clarified. An extended form of this law is given.

Frequency shift and damping constant of ferrimagnetic spin waves

Abstract The temperature and wave length dependence of the frequency shift and damping constant of the ferrimagnetic spin waves are obtained explicitly. Use is made of the equation of motion and the correlation function methods.

Anomalous behavior of sound near the critical point in heisenberg ferro- and antiferromagnets with uniaxial anisotropy

Abstract Explicit dependence of the attenuation constant on temperature, wave number and anisotropy is studied both below and above the critical point Tλ.