Interplay of fluctuations, ordering phenomena, and emergent phases

具有高转变温度的超导体最终成为相关的应用。然而，它们仍然是主持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，这与超导性本身一样有趣。主要的悬而未决的问题涉及这些相之间以及与超导性的相互关系。在许多情况下，相变之前有波动。同时增强的磁化率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，甚至波动的起源和性质都不清楚。这是这个建议的目的，研究和理解自旋，轨道，电荷，和序参数波动在选定的系统。重点将放在确定波动的类型及其相互之间和与新阶段的相互关系上。基本的实验方法将是可见光的非弹性（拉曼）散射作为光子偏振和能量、磁场、流体静力学和单轴压力和温度的函数。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光谱形状。同位素的交换将用于分析涨落与晶格的耦合。它的目的是看看系统中的波动可以通过调整控制参数，如掺杂和压力的影响。然后，一个相，例如磁性，通常可以被抑制并屈服于另一个相。如果被抑制的相位的波动继续存在，则它们可能与在改变控制参数时出现的新相位交织在一起。这种相互关系的原型材料是铜酸盐、有机金属、重费米子系统或铁磷族元素化物和硫族元素化物。在所有情况下，竞争阶段观察，但波动的存在，作用和起源是有争议的。除了临界涨落外，长程有序相的序参量还存在振幅涨落，这与场论中的Higgs模是等价的。这些振幅起伏可能对自旋和超导能隙激发的光谱性质有很大的影响。计划研究各种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发的影响，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。预计重要的开放性问题，如起源和相互关系的临界波动与其他阶段和微观参数的推导可以得到解决和回答。

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.