Interplay of fluctuations, ordering phenomena, and emergent phases

具有较高转变温度的超导体终于有了应用价值。然而，它们仍然是主持公开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，这与超导电性本身同样有趣。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的关系。在许多情况下，相变之前有起伏。随之而来的磁化率增强被认为是高温超导转变背后的潜在驱动力之一。然而，甚至连波动的来源和性质都不清楚。这项提议的目的是研究和理解选定系统中的自旋、轨道、电荷和有序参数的涨落。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。基本的实验方法是可见光的非弹性(拉曼)散射，它是光子偏振和能量、磁场、流体静压和单轴压力以及温度的函数。为了确定涨落的物理来源，我们将定量分析它们的对称性、随激发能量、温度、压力和场的变化，以及响应的频谱形状。同位素交换将被用来分析涨落与晶格的耦合。其目的是观察可以通过调整控制参数(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、重费米子系统或铁多金属和硫族化合物。在所有情况下都观察到了竞争阶段，但波动的存在、作用和来源存在争议。除了临界涨落外，还存在与场论中的Higgs模等价的长程有序位相序参数的振幅涨落。这些幅度涨落可能对自旋和超导能隙激发的光谱性质有很大的影响。计划研究不同入射光子能量下自旋激发的线形。这使得人们能够解开振幅起伏和多磁振子激发的影响，并推导出一般的线形和耦合参数。同样，超导体中的振幅模式留下了库珀配对的指纹。预计可以解决和回答一些重要的开放问题，如临界波动的起源和与其他相的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.