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

超快时间分辨光谱学为研究凝聚态物质中元激发的非平衡动力学提供了独特的可能性。这种方法在研究相变时弛豫过程的临界减慢，追踪复杂序参量的演变以及晶格和电子凝聚体之间的相互作用方面特别成功。然而，到目前为止，大多数研究仅通过改变温度来进行，其中电子有序被热涨落抑制。 这种方法不允许研究系统的基态量子相位（接近绝对零度）之间的相变。这些量子相变对于理解高Tc超导体是极其重要的，其中超导性与磁序的共存及其在超导转变温度的强烈增强中的作用被广泛争论。然而，在高温超导体的量子相变的详细的实验图片还没有得到。在这里，我们建议利用时间分辨太赫兹光谱来解决这个问题的新型类的铁基超导体。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。它使我们能够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的竞争类型的电子秩序。实验将在两类超导材料上进行：122型铁基磷属元素化物（例如BaFe 2As 2）和碲化铁硒化物（FeTe 1-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微观机制导致形成的库珀对，特别是，澄清在这个过程中的作用的磁性和电荷的顺序波动。

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.