Nanometer scale dipole moments in the polarization clusters in BaTiO3 are believed to be thermally excited and thermally relaxed within a picosecond time scale. However, so far, there have been no reports on the direct observation of the dynamics of these dipole moments in such a very short time scale. The limitation here is mainly due to the low spatial coherence of the X-ray beam, in particular when synchrotron radiation is used as a light source. Professor K. Namikawa (Tokyo Gakugei Univ, Japan) and his colleagues have recently obtained some interesting results. To measure the time correlation of speckle intensities, they employed a soft X-ray pulse laser (7 ps in pulse width, 3.5×1010 photons/sec/pulse, 13.9 nm in wavelength, band width 10-4, angular spread 0.5 mrad) at Japan Atomic Energy Agency, Kizugawa, Japan, and a Michelson-type delay pulse generator as well as an X-ray streak camera. Spatial coherence in their system was estimated at more than 90 %. The evolution of the relaxation time of the dipole moment near the Curie temperature (TC) was studied. It was found that the maximum relaxation time (~90 ps) appears at a temperature of 4.5 K above the TC, being coincident with the one where the maximum polarization takes place. For more information, see the paper, "Direct observation of the critical relaxation of polarization clusters in BaTiO3 using a pulsed X-ray laser technique", K. Namikawa et al., Phys. Rev. Lett., 103, 197401 (2009).
