Akio Yamanaka

Detailed analysis of transmission spectra and Bragg-reflection spectra of a two-dimensional photonic crystal with a lattice constant of 1.15 µm

We investigated the transmission spectra and the Bragg-reflection spectra of a two-dimensional photonic crystal composed of a triangular array of circular air rods formed in PbO glass, for which the laser oscillation peculiar to the two-dimensional photonic band structure was observed recently. The sample parameters, i.e., the lattice constant, the radius of the air rods, and the dielectric constant of the host PbO glass, were evaluated from the observation angle of the Bragg reflection and by comparison of observed with calculated band gaps. The transmission spectra and the Bragg-reflection spectra were calculated with the plane-wave expansion method, and a good agreement with both the dispersion relation and the observed Bragg-reflection spectra was shown.

Scattering Angle Dependence of Brillouin Shift near the Normal-Incommensurate Phase Transition Point of Rb2ZnCl4

Brillouin shifts of the C 11 mode were measured near the normal-incommensurate phase transition temperature with two scattering angles: 90 and 45 degrees. The scattering angle dependence of the Brillouin shifts is found and is inferred to be caused by the fluctuation of an order parameter.

Uniaxial Stress Effect on the Transition Temperature of Squaric Acid (H2C4O4)

Dielectric measurements were made under various uniaxial stresses up to 800 bar applied perpendicularly to the layer plane of squaric acid crystal. The transition temperature T tr increases almost linearly with the stress and the rate of T tr shift is +3.9 K/kbar. This result seems to show that the uniaxial stress increases the interlayer interaction. The experimental results obtained under uniaxial stresses were compared with those of hydrostatic pressure studies; the effect of two-dimensional pressure parallel to the layer plane on T tr is estimated to be -17 K/kbar. The deuteration effect is also discussed from these data.

Electronic Raman scattering in Bi2Sr2CaCu2O8

Abstract Raman scattering has been investigated in Bi 2 Sr 2 CaCu 2 O 8 . It is found that the electronic Raman scattering is observed as a continuum spectrum and this continuum is redistributed itself due to the gap formation below T c . These indicate that the origin of the electronic continuum may be attributed to the single particle excitations of holes on the Fermi surface. Further, the intensity of the phonon Raman scattering has been measured as a function of the excitation photon energy in the visible region. A resonant enhancement of the intensity is found in the vibrational modes related to the CuO 2 SrO layers, but not observed in that of the BiO layer. On the basis of these results, the electronic (hole) states near E F are discussed.

Observation of the coupling between TA and TO modes in SrTiO3 by Brillouin scattering

Brillouin scattering showing an unusual low temperature softening of some transverse modes in SrTiO3 had been reported previously. The earlier data, complemented by newer Brillouin results, and supplemented by high quality hyper Raman data on the soft mode, demonstrate that this anomalous softening is to a large extent caused by a bilinear coupling of the strain with the gradient of the electrical polarization fluctuations. This coupling had been identified long ago in KTaO3, but at the large momentum exchanges that are achieved in neutron spectroscopy. To our knowledge, it is the first time that the signature of this coupling is observed in any material at the small wave vectors of optical Brillouin scattering

On the isotope effect of Tc in squaric acid (H2C4O4)

Abstract The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on Tc is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H2SQ) is stretched to 2R(D2SQ) the phase transition temperature 7H c of H2SQ coincides with TD c of D2SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.

Observation of a polar soft-phonon in hexagonal barium titanate

Abstract By a hyper-Raman spectroscopy, it is found that a polar soft optic phonon with A2u symmetry exists in the hexagonal BaTiO3 polymorphic to perovskite BaTiO3, independently of another non-polar E2u optic phonon responsible for a phase transition at Ta = 222 K. The frequency is monotonously soft from 670 K to Ta, and thereby a virtual characteristic temperature of ferroelectric instability is deduced as 40K. Further, the magnitude of the TO-LO splitting of the relevant mode is found to be very large as is the case for perovskite BaTiO3 and SrTiO3. The remarkable temperature dependence of the dielectric constant ec can be well accounted for from the lattice-dynamical point of view, including an anomalous bump at Ta. Implication of the present findings is discussed.

A new spectroscopic system for the measurement of hyper‐Raman and Raman signals

A new and highly sensitive spectroscopic system which allows one to observe Raman and hyper‐Raman scattering signals in a successive or simultaneous way has been developed for the first time. The present apparatus consists of a repetitively Q‐switched Nd‐yttrium aluminum garnet (YAG) laser, a single‐grating monochromator supplemented with a premonochromator which is a double‐grating monochromator of zero‐dispersive type, and a gatable optical multichannel detector cooled below −40 °C. With this, a greatly improved sensitivity for hyper‐Raman signals has been accomplished. For exciting hyper‐Raman signals the original beam of the Nd‐YAG laser was used, whereas the second‐harmonic light of the laser was used for Raman signals. The second‐harmonic light beam, collimated to the original one, was obtained from a phase‐matched LiNbO3 crystal mounted on a kinematic mount. Thereby, either the original beam of the Nd‐YAG laser or the second‐harmonic one could excite the same portion of the sample, so that both hyp...

Raman Scattering Studies in Squaric Acid under the Uniaxial Stress

Uniaxial stress effect on optic modes in squaric acid was studies by Raman scattering. The out-of-plane librational modes (Bg species lines near 150 cm -1 ) and the translational mode (Bg species line at 158 cm -1 ) are found to be sensitive to a uniaxial stress applied along the b axis, but the in-plane librational mode (Ag species line at 238 cm -1 ) and several internal modes are not. These results are discussed in relation to the two dimensional character of the layered structure of the compound.

Temperature and Uniaxial Stress Dependence of Rigid Layer Mode in Squaric Acid (H2C4O4)

The rigid layer mode of a shear type in squaric acid has been studied at several temperatures by Brillouin and Raman scatterings under a uniaxial stress applied perpendicularly to layer planes. The temperature and uniaxial stress dependences of a transverse acoustic mode ( c 44 ) in the low temperature phase are essentially the same as those of the low frequency A g -optic mode below T c , which corresponds to the zone-boundary acoustic mode in the high temperature phase. This rigid layer mode is analyzed by a one dimensional model with a single force constant f 1 between the nearest neighbor layers and the value of f 1 is calculated to be 1.18 ×10 4 dyn/cm at room temperature. The variation of f 1 with interlayer distance is also estimated.