Electronic, magnetic or structural inhomogeneities ranging in size from nanoscopic to mesoscopic scales seem endemic, and are possibly generic, to colossal magnetoresistance manganites and other transition metal oxides. We show here that an extension, to include long range Coulomb interactions, of a quantum two-fluid $\ell-b$ model proposed recently for manganites [Phys. Rev. Lett., {\bf 92}, 157203 (2004)] leads to an excellent description of such inhomogeneities. In the $\ell-b$ model two very different kinds of electronic states, one localized and polaronic ($\ell$), and the other extended or broad band ($b$) co-exist. For model parameters appropriate to manganites, and even within a simple dynamical mean-filed theory (DMFT) framework, it describes many of the unusual phenomena seen in manganites, including colossal magnetoresistance (CMR), qualitatively and quantitatively. However, in the absence of long ranged Coulomb interaction, a system described by such a model would actually phase separate, into macroscopic regions of $l$ and $b$ electrons respectively. As we show in this paper, in the presence of Coulomb interactions, the {\em macroscopic} phase separation gets suppressed, and instead nanometer scale regions of polarons interspersed with band electron puddles appear, constituting a new kind of quantum Coulomb glass. Our work points to an interplay of strong correlations, long range Coulomb interaction and dopant ion disorder as the origin of nanoscale inhomogeneities, rather than disorder frustrated phase competition as is generally believed. Based on this, we argue that the observed micrometer(meso)-scale inhomogeneities owe their existence to extrinsic causes, eg. strain due to cracks and defects. We suggest possible experiments to validate our speculation.
Nanoscale Sr$_2$IrO$_4$ Freestanding Thin-Films for Flexible Electronics
Department of Physics and Astronomy, University of Kentucky（肯塔基大学物理与天文学系）；Department of Materials Science and Engineering, The Ohio State University（俄亥俄州立大学材料科学与工程系）；Department of Physics, Pusan National University（釜山国立大学物理系）；Advanced Photon Source, Argonne National Laboratory（阿贡国家实验室先进光子源）
Virtual Synthesis of Nanoscale Systems with Pre-Designed Properties: Fundamentals and Applications. Chapter 7. Nickel Oxide Quantum Dots and Polymer Nanowires
Evidence for nano-scale inhomogeneities in bilayer manganites in the Mn\sp{4+} rich region: $\bm{0.54 \leq x \leq 0.80}$
采用原子对分布函数（PDF）技术研究了掺杂范围为$0.54 \leq x \leq 0.80$的天然双层锰氧化物\lsmo中原子的局域结构响应。我们的低温中子粉末衍射测量表明，当其有序磁结构从A型（$0.54 \leq x \leq 0.66$）向C/C型$^*$（$0.74 \leq x \leq 0.90$）有序过渡时，两种不同的Jahn-Teller（JT）畸变MnO $_6$八面体共存。在低温下，在所有掺杂水平下，掺杂空穴主要存在于双层的平面中。在A型磁有序区中，$_g$电子在平面上明显离域，形成未畸变的八面体，而在C/C型$^*$区中，JT畸变八面体明显拉长.这与非均匀共存的离域和局域电子态的存在是一致的。没有观察到宏观相分离的证据。这种纳米级的不均匀性可以解释在自旋无序的“间隙”区域（$0.66 \leq x \leq 0.74$）中观察到的磁阻挫行为。
