Dynamical perturbations modify the states of classical systems in surprising ways and give rise to important applications in science and technology. For example, Floquet engineering exploits the possibility of band formation in the frequency domain when a strong, periodic variation is imposed on parameters such as spring constants. We describe here Kapitza engineering, where a drive field oscillating at a frequency much higher than the characteristic frequencies for the linear response of a system changes the potential energy surface so much that maxima found at equilibrium become local minima, in precise analogy to the celebrated Kapitza pendulum where the unstable inverted configuration, with the mass above rather than below the fulcrum, actually becomes stable. Our starting point is a quantum field theory of the Ginzburg-Devonshire type, suitable for many condensed matter systems, including particularly ferroelectrics and quantum paralectrics. We show that an off-resonance oscillatory electric field generated by a laser-driven terahertz source can induce ferroelectric order in the quantum-critical limit. Heating effects are estimated to be manageable using pulsed radiation; “hidden” radiation-induced order can persist to low temperatures without further pumping due to stabilization by strain. We estimate the Ginzburg-Devonshire free-energy coefficients in ${\text{SrTiO}}_3$ using density-functional theory and the stochastic self-consistent harmonic approximation accelerated by a machine-learned force field. Although we find that ${\text{SrTiO}}_3$ is not an optimal choice for Kapitza stabilization, we show that scanning for further candidate materials can be performed at the computationally convenient density-functional theory level. We suggest second harmonic generation, soft-mode spectroscopy, and x-ray diffraction experiments to characterize the induced order.

中文翻译：

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量子临界序的卡皮查稳定性

动态扰动以令人惊讶的方式改变经典系统的状态，并在科学和技术中产生重要的应用。例如，Floquet 工程利用了当对弹簧常数等参数施加强烈的周期性变化时，在频域中形成能带的可能性。我们在这里描述卡皮察工程，其中驱动场以远高于系统线性响应的特征频率的频率振荡，改变势能表面如此之大，以至于平衡时发现的最大值变成局部最小值，这与著名的卡皮察精确类比在钟摆中，不稳定的倒置结构（质量位于支点上方而不是下方）实际上变得稳定。我们的出发点是金茨堡-德文郡类型的量子场论，适用于许多凝聚态物质系统，特别是铁电体和量子顺电体。我们证明，激光驱动太赫兹源产生的偏共振振荡电场可以在量子临界极限内诱导铁电有序。据估计，使用脉冲辐射可以控制热效应；由于应变的稳定作用，“隐藏的”辐射诱导有序可以持续到低温，而无需进一步泵浦。我们估计金茨堡-德文郡自由能系数${\text{钛酸锶}}_3$使用密度泛函理论和机器学习力场加速的随机自洽谐波近似。虽然我们发现${\text{钛酸锶}}_3$不是卡皮查稳定的最佳选择，我们表明可以在计算方便的密度泛函理论水平上扫描更多候选材料。我们建议通过二次谐波产生、软模式光谱和 X 射线衍射实验来表征诱导阶。