Pressure dependence of ferroelectric quantum critical fluctuations
Pressure dependence of ferroelectric quantum critical fluctuations
复制标题
DOI：
10.1103/physrevb.100.214111
复制
发表时间：
2019-12-13
影响因子：
3.7
通讯作者：
Saxena, S. S.
中科院分区：
文献类型：
--
作者：
Coak, M. J.;Haines, C. R. S.;Saxena, S. S.
在凝聚态物理中，由软化的声子模驱动的到长程有序态的相变是普遍存在的，但直到现在，这种模式的演化还从未被明确地测量过。我们报道了压力对原型量子临界顺电SrTiO_3中铁电性软模的影响。对于探索这些效应，这是一个理想的、干净的模型系统，压力只直接处理声子模式。我们测量和报道了当系统调谐远离临界点时，量子临界涨落对铁电软声子模压强和温度依赖性的影响。我们证明了平均场近似在实验上得到了证实。此外，使用量子临界激发的自洽模型，包括与体积应变的耦合，并且没有可调参数，我们确定了只有在非常接近量子临界点时才能观察到的对数修正。因此，压力依赖的平均场特征比温度依赖的平均场特征对涨落的影响要强得多。我们预测，较低维度的修正会更强烈。同样的计算证实了Lydanne-Sachs-Teller关系在所考虑的整个压力和温度范围内都是有效的。因此，测量的介电常数可以用来提取软模的频率，最低可达1.5K，最高可达20kbar。观察到软模进一步变硬，提高了低温能隙，并返回到预期的光学模式的浅温度依赖关系。这一行为与钛酸锶压力-温度相图上铁电量子临界点的存在是一致的，施加压力使系统远离。这项工作代表了一项实验测量，当系统调谐远离临界点时，区域中心软声子模的僵化，这是凝聚态物理中各种相变和系统中潜在的普遍现象。
Phase transitions to a long-range-ordered state driven by a softened phonon mode are ubiquitous across condensed matter physics, but the evolution of such a mode as the system is tuned to or from the transition has never been explicitly measured until now. We report the effect of pressure on the soft mode associated with ferroelectricity in the archetypal quantum critical paraelectric SrTiO3. This is an ideal, clean, model system for exploring these effects, with pressure directly addressing the phonon modes only. We measure and report the effect of quantum critical fluctuations on the pressure and temperature dependence of the ferroelectric soft phonon mode as the system is tuned away from criticality. We show that the mean-field approximation is confirmed experimentally. Furthermore, using a self-consistent model of the quantum critical excitations including coupling to the volume strain and without adjustable parameters, we determine logarithmic corrections that would be observable only very close to the quantum critical point. Thus, the mean-field character of the pressure dependence is much more robust to the fluctuations than is the temperature dependence. We predict stronger corrections for lower dimensionalities. The same calculation confirms that the Lydanne-Sachs-Teller relation is valid over the whole pressure and temperature range considered. Therefore, the measured dielectric constant can be used to extract the frequency of the soft mode down to 1.5 K and up to 20 kbar of applied pressure. The soft mode is observed to stiffen further, raising the low-temperature energy gap and returning towards the expected shallow temperature dependence of an optical mode. This behavior is consistent with the existence of a ferroelectric quantum critical point on the pressure-temperature phase diagram of SrTiO3, which applied pressure tunes the system away from. This work represents an experimental measurement of the stiffening of a zone center soft phonon mode as a system is tuned away from criticality, a potentially universal phenomenon across a variety of phase transitions and systems in condensed matter physics.