Interplay of fluctuations, ordering phenomena, and emergent phases

具有高转变温度的超导体终于有了应用价值。然而，它们仍然是包含开放科学问题的顶级学科之一。它们表现出各种额外的有序态和不稳定性，这与超导本身同样有趣。主要悬而未决的问题是这些相之间的相互关系以及它们与超导性的关系。在许多情况下，相变之前有波动。伴随而来的磁化率增强被认为是高转变温度到超导性背后的潜在驱动力之一。然而，甚至连波动的起源和性质都不清楚。本提案的目的是研究和理解所选系统的自旋、轨道、电荷和序参量波动。重点将放在确定波动的类型及其相互之间和与新阶段之间的相互关系。实验的基本方法将可见光的非弹性（拉曼）散射作为光子偏振和能量、磁场、静流体和单轴压力以及温度的函数。为了确定波动的物理起源，将定量分析其对称性，随激发能，温度，压力和场的变化以及响应的频谱形状。同位素交换将用于分析波动与晶格的耦合。它旨在研究通过调整控制参数（如掺杂和压力）来影响波动的系统。然后，一个相，例如磁性，通常可以被抑制并生成另一个相。如果被抑制相位的波动幸存下来，它们可能与改变控制参数后出现的新相位交织在一起。这种相互关系的典型材料是铜酸盐、有机金属、重费米子系统或铁-羟基和-硫族化合物。在所有情况下都观察到相互竞争的阶段，但波动的存在、作用和起源是有争议的。除了临界涨落外，长阶相的阶数参数也存在幅值涨落，相当于场论中的希格斯模。这些振幅波动可能对自旋和超导间隙激发的光谱特性有很大的影响。计划研究不同入射光子能量下自旋激发的线形。这使人们能够解开振幅波动和多磁振子激励的影响，并推导出一般的线形和耦合参数。类似地，超导体中的振幅模式留下了库珀配对的指纹。预期一些重要的悬而未决的问题，如临界波动与其他相的起源和相互关系以及微观参数的推导，将得到处理和解答。

Superconductors with high transition temperature became finally relevant for applications. Yet, they are still among the top subjects hosting open scientific problems. They exhibit a variety of additional ordered states and instabilities which are similarly interesting as superconductivity itself. The major open question concerns the interrelation of these phases among each other and with superconductivity. In many cases the phase transitions are preceded by fluctuations. The concomitantly enhanced susceptibility is considered one of the potential driving forces behind the high transition temperatures to superconductivity. Yet, not even the origin and nature of the fluctuations are clear. It is the purpose of this proposal to study and understand spin, orbital, charge, and order parameter fluctuations in selected systems. The focus will be placed on the identification of the type of fluctuations and their interrelation among each other and with new phases. The basic experimental method will be inelastic (Raman) scattering of visible light as a function of photon polarization and energy, magnetic field, hydrostatic and uniaxial pressure and temperature. For identifying the physical origin of the fluctuations their symmetry properties, variation with excitation energy, temperature, pressure and field, and the spectral shape of the response will be analyzed quantitatively. The exchange of isotopes will be used to analyze the coupling of fluctuations with the lattice. It is intended to look at systems in which the fluctuations can be influenced by tuning a control parameter such as doping and pressure. Then one phase, for example magnetism, can usually be suppressed and yields to another phase. If the fluctuations of the suppressed phase survive they may be intertwined with the new phase emerging upon changing the control parameter. Materials prototypical for this interrelation are the cuprates, organic metals, heavy fermion systems or the ferro-pnictides and -chalcogenides. In all cases competing phases were observed but the existence, role and origin of fluctuations are controversial. In addition to critical fluctuations, there exist also amplitude fluctuations of the order parameter in phases with long-ranged order which are equivalent to the Higgs modes in field theory. These amplitude fluctuations may have a strong influence on the spectral properties of spin and superconducting gap excitations. It is planned to study the line shapes of spin excitations for various incident photon energies. This enables one to disentangle the influence of amplitude fluctuations and multi-magnon excitations and derive the generic line shape and the coupling parameters. Similarly, the amplitude mode in a superconductor leaves a fingerprint of Cooper pairing. It is expected that important open questions such as the origin and interrelation of critical fluctuations with other phases and the derivation of the microscopic parameters can be addressed and answered.