Ultrafast Dynamics in High-Temperature Superconductors near a Quantum Critical Point

超快时间分辨光谱为研究凝聚态物质中基本激发的非平衡动力学提供了独特的可能性。该方法特别成功地研究了相变弛豫过程的临界慢化，追踪了复杂有序参数的演化以及晶格和电子凝聚物之间的相互作用。然而，到目前为止，大多数研究都是通过改变温度进行的，其中电子顺序受到热波动的抑制。这种方法不允许研究系统的基态量子相（接近绝对零度）之间的相变。这些量子相变对于理解高tc超导体非常重要，其中超导与磁序的共存及其在超导转变温度的强增强中的作用被广泛争论。然而，高tc超导体中量子相变的详细实验图像尚未得到。在这里，我们提出利用时间分辨太赫兹光谱来解决这类新型铁基超导体的问题。我们的方法依赖于超快非线性光谱和低温高压技术的协同作用。它允许我们通过比较它们的超快动力学和光谱响应来区分量子临界点附近竞争类型的电子顺序。实验将在两类超导材料上进行：122型铁基化合物（例如BaFe2As2）和碲化铁硒化物（FeTe1-xSex），它们在超导圆顶附近表现出量子临界性。我们的研究结果将揭示导致库珀对形成的微观机制，特别是澄清磁和电荷顺序波动在这一过程中的作用。

Ultrafast time-resolved spectroscopy offers unique possibilities to study nonequilibrium dynamics of elementary excitations in condensed matter. This method has been particularly successful in studying the critical slowing down of relaxation processes at phase transitions, and tracing the evolution of the complex order parameter and the interplay between the lattice and electronic condensates. However, up to now, most studies have been performed only by varying temperature, where an electronic order is suppressed by thermal fluctuations. This approach does not allow investigating phase transitions between ground-state quantum phases (close to absolute zero) of a system. These quantum phase transitions are extremely important for understanding of high-Tc superconductors, where the coexistence of superconductivity with a magnetic order and its role in the strong enhancement of the superconducting transition temperature is widely debated. Nevertheless a detailed experimental picture of quantum phase transitions in high-Tc superconductors has not been obtained yet.Here we propose to utilize time-resolved terahertz spectroscopy to resolve this problem for the novel class of iron-based superconductors. Our approach relies on the synergy of ultrafast nonlinear spectroscopy and low-temperature high-pressure technology. It allows us to distinguish competing types of electronic order near the quantum critical point by comparing their ultrafast dynamics and spectral response. The experiments will be performed on two classes of superconducting materials: iron-based pnictides of 122 type (e.g. BaFe2As2) and iron telluride-selenides (FeTe1-xSex) which demonstrate a quantum criticality in the vicinity of the superconducting dome. Results of our studies will shed light on microscopic mechanisms leading to the formation of Cooper pairs and, in particular, clarify the role of magnetic and charge order fluctuations in this process.