Terahertz spectroscopy of collective excitations in ordered phases of overdoped manganites

在钙钛矿氧化物锰氧化物R1-xAxMnO3（R-稀土，A-碱性元素）中，几乎相等强度的自旋、电荷、晶格和轨道相互作用的精细相互作用导致了各种电子相关基态。它提供了通过适度改变系统参数（如温度、成分、颗粒尺寸、内部应变、磁场等）来调整它们的可能性，在过去的几十年里吸引了很多关注。然而，基态的起源和相应的相变机制尚未完全解决。目前，在电子相关的过渡金属氧化物的背景下，澄清系统中的电荷，轨道和磁序的物理模型具有更广泛的兴趣，特别是过掺杂区域，x> 0.5，与欠掺杂区域相比，相图的一个扇区较少探索，在那里观察到巨大的磁阻。通过研究纳米尺寸对磁性和电荷有序度的影响，以及薄膜、A位阳离子有序度和应变对电子跃迁、能隙和声子模的影响，利用光学光谱，重点研究了几个毫伏及以下的低能量，其中预期固体中的集体电子效应将出现。将采用一套光谱仪，包括光学椭圆偏振仪、红外傅里叶变换光谱仪以及用于低能的太赫兹和微波装置，并辅以磁测量。研究了一系列体相（多晶）和薄膜（外延生长）化合物R1-xAxMnO3（R = La，Pr，A = Ca）（0.5 <x <1）。

In perovskite oxide manganites R1-xAxMnO3 (R – rare earth, A – alkaline element) the fineinterplay of nearly equal-strength spin, charge, lattice, and orbital interactions leads to a varietyof electronically correlated ground states. It provides the possibility to tune them by moderatevariations of system parameters, such as temperature, composition, particle size, internal strain,magnetic field, etc., which has attracted a lot of attention during past decades. However, theorigin of the ground states and the mechanisms of the correspondent phase transitions are notcompletely resolved. Presently, clarification of physical models of charge, orbital, and magneticorders in the systems is of broader interest in the context of electronically correlated transitionmetal oxides, in particular of the overdoped regime, x > 0.5, a sector of the phase diagram that isless explored compared to the underdoped one, where colossal magnetoresistance is observed.We plan to study the microscopic origin and collective modes of various charge, magneticallyand orbitally ordered phases of the family R1-xAxMnO3 (0.5≤x≤1) by investigating the nano-sizeeffects on magnetic and charge order, and thin-film, A-site cation ordering and strain effects onthe electronic transitions, the energy gaps and the phonon modes using optical spectroscopy withthe focus on low energies, of few millielectronvolts and below, where collective electroniceffects in solids are expected to show up. A set of spectrometers will be employed, includingoptical ellipsometer, infrared Fourier-transform spectrometer and terahertz and microwavesetups for low energies, complemented with magnetic measurements. A series of bulk (polycrystalline)and thin-film (epitaxially grown) compounds R1-xAxMnO3 (R = La, Pr, A = Ca) willbe studied with 0.5 < x < 1, respectively.