Interplay of fluctuations, ordering phenomena, and emergent phases

具有高转变温度的超导体最终变得有意义。然而，它们仍然是解决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状态和不稳定性，这些与超导本身同样有趣。主要的悬而未决的问题涉及这些相之间的相互关系以及与超导性的相互关系。在许多情况下，相变之前都会发生波动。随之而来的敏感性增强被认为是超导高转变温度背后的潜在驱动力之一。然而，甚至波动的起源和性质也不清楚。本提案的目的是研究和理解选定系统中的自旋、轨道、电荷和有序参数波动。重点将放在确定波动类型及其相互之间以及与新阶段的相互关系上。基本实验方法是可见光的非弹性（拉曼）散射作为光子偏振和能量、磁场、静水压力和单轴压力和温度的函数。为了确定波动的物理起源，将对它们的对称性、激发能量、温度、压力和场的变化以及响应的光谱形状进行定量分析。同位素交换将用于分析涨落与晶格的耦合。其目的是研究可以通过调整掺杂和压力等控制参数来影响波动的系统。然后一个相，例如磁力，通常可以被抑制并产生另一相。如果被抑制相位的波动继续存在，它们可能会与改变控制参数时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、重费米子系统或亚铁磷族化物和硫族化物。在所有情况下都观察到了竞争相，但波动的存在、作用和起源是有争议的。除了临界涨落之外，长程有序相位中还存在有序参数的振幅涨落，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产生强烈影响。计划研究各种入射光子能量的自旋激发线形状。这使得人们能够解开振幅波动和多磁振子激励的影响，并导出通用线形状和耦合参数。类似地，超导体中的振幅模式留下了库珀配对的指纹。期望能够解决和回答重要的开放性问题，例如临界涨落与其他相的起源和相互关系以及微观参数的推导。

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.