Markovian property of cooperative fluctuations investigated by light scattering spectroscopy

本文研究了顺电性磷酸二氢钾（KDP）晶体低频光散射光谱的斯托克斯与反斯托克斯强度比。由玻尔兹曼因子确定的通常关系式在293 K下成立。但在相变温度附近，当ω<30 cm-1时，软模的强度与玻尔兹曼因子之比约为0.94。引入时间不可逆性的软模式的马尔可夫性质对于理解像经典波动一样的模式的行为可能是必不可少的。随着温度的升高，软模从微观量子运动到经典和宏观动力学的演化规律还有待进一步研究。我们将软模的结果与液体中的合作转动动力学的结果进行了比较。在液体二硫化碳CS2中，Im（ω）的强度随着温度低于50 cm-1而变化。另一方面，在292 K下，仅在ω<5cm-1时观察到强度比偏离玻尔兹曼因子。该频率区域比Im（ω）随温度变化的频率区域窄得多。结果与KDP的情况相反，在KDP的情况下，几乎在Im（ω）随温度明显变化的相同区域中观察到偏差。这意味着，虽然这两个动力学是耗散的，导致软模式的马尔可夫性质的耗散程度高于合作的旋转动力学。这可能与软模在Tc附近作为临界涨落的宏观特性有关。

We have Ivestigated the Stokes to anti-Stokes intensity ratio for the low-frequency light scattering spectra of a paraelectric potassium dihydrogen phosphate (KDP) crystal. The usual relation determined by the Boltzmann factor holds at 293K. However the relation is not satisfied for the soft mode occurring for ω<30 cm-1 near the phase transition temperature, and the intensity ratio relative to the Boltzmann factor is about 0.94 at ω=15cm-1. The Markovian nature of the soft mode that introduces time-irreversibility may be essential for understanding the behavior of the mode like a classical fluctuation. Further investigation on the temperature dependence is needed for clarifying the evolution of the soft mode from microscopic quantum motions to the classical and macroscopic dynamics as temperature approaches Tc.We compare the present result of the soft mode with that of the cooperative rotational dynamics in liquid. In liquid carbon disulfide CS2, the intensity of Im(ω) changes with temperature below 50 cm-1. On the other hand, the deviation of the intensity ratio from the Boltzmann factor is observed only for ω<5cm-1 at 292K. This frequency region is much narrower than that where Im(ω) varies with temperature. The result is in contrast to the case of KDP where the deviation is observed almost in the same region where Im(ω) appreciably changes with temperature. This implies that although both the dynamics are dissipative, the degree of dissipation causing the Markovian nature of the soft mode is higher than that for the cooperative rotational dynamics. This may be related to the macroscopic character of the soft mode near Tc as a critical fluctuation.