层状Co基氧化物薄膜的输运各向异性研究

Layered cobaltite NaxCoO2 with CoO2 plane has peculiar thermoelectric properties and rich physical pictures, which has also been reckoned as the atomic layer thermopile structure due to its obvious anisotropy. As the easy cleaving along interlayers in NaxCoO2 crystal, less attention has been paid to the anisotropies of transport properties. In this project, epitaxial NaxCoO2 thin films with c-axis tilting will be grown. Resistivity measurements along [100]L and [010]T directions of tilted NaxCoO2 thin films, and the process of separating variables will be employed to obtain the resistivity anisotropy ρc/ρab; while laser induced transverse voltage (LITV)effect will be used to get the Seebeck coefficient anisotropy ΔS=(Sab-Sc). The doping concentration x and partial ion substitution to the influence of transport anisotropies will be studied systematically. Combined with the thermal conductivity and electronic structure of NaxCoO2, we will discuss the electrical and heat conducting mechanisms of carriers and phonons in ab-plane and along c-axis, and the interplays of spin, orbital and charge degrees of freedom. Compared with the behaviors of perovskite structure of La1-xSrxCoO3 thin films, the intrinsic physical parameters to affect the LITV signal responsivity and rise speed will be studied for the applications of transverse thermoelectric effect.

含CoO2面的层状NaxCO2具有奇特的热电输运性质和丰富物理图像，且是具有显著各向异性的原子层热电堆结构；由于晶体易于层间解理，故其c轴输运性质和各向异性的研究薄弱。项目拟生长c轴倾斜的外延NaxCO2薄膜，通过测量薄膜面内 [100] L和[010] T两个方向的电阻率并分离变量以获得NaxCO2的电阻率各向异性ρc/ρab，以及测量薄膜的激光感生横向电压来研究其Seebeck系数的各向异性ΔS=(Sab-Sc)。将系统研究在不同掺杂浓度x及部分离子替代情况下，NaxCO2的输运各向异性的行为规律；并结合热导率和电子结构，探讨载流子、声子如何在面内面间进行热、电的传导，以及与电子自旋、轨道简并、电荷的序等的耦合。同时对照钙钛矿La1-xSrxCoO3薄膜的行为，探讨影响材料体系的激光感生横向电压效应的信号响应率和响应速度的本征因素，以推动横向热电的应用。

在四个材料体系开展了输运各向异性的研究：1. 在层状NaxCoO2体系外延薄膜(x=0.45~0.75，跨越顺磁金属相，电荷有序绝缘体相和居里外斯金属相)的制备、表征和热电输运各向异性等方面开展了系统的工作。在系列薄膜中均测量到激光感生横向电压(LITV)信号，其信号响应率为0.017~1.24 Vcm2/mJ；由一维热传导模型推导了Seebeck张量在ab面和c 轴的各向异性ΔS=Sab-Sc=为0.83~60.8 μV/K；组分x=0.6的居里外斯金属相具有最大的响应率1.24Vc m2/mJ和最大ΔS=60.8 μV/K。通过测量倾斜外延薄膜不同方向电阻率获得了电荷有序绝缘体Na0.5CoO2和居里外斯金属Na0.6CoO2室温电阻率各向异性ρab/ρc分别为750和270. 2. 对错层Ca3Co4O9多晶和薄膜的热电性能做了较系统探索。首次在c轴择优Ca3Co4O9多晶块材中获得了LITV信号，峰值电压为50 mV，分析了多晶块材偏离<001>取向的晶粒分布抵消了部分本征各向异性，导致较小的 宏观热电各向异性和电压信号。在不同单晶衬底上生长了外延Ca3Co4O9薄膜，结合薄膜形貌和生长动力学分析得出平衬底上 薄膜生长模式主要为为“层+岛，倾斜衬底上为“台阶流动”模式；获得了不同厚度平直/ 倾斜薄膜的电输运、热电势性质； 由c轴倾斜Ca3Co4O9薄膜的LITV信号响应率推算出热电各向异性ΔS=20 μV/K。3. 在高电导率钙钛矿La0.5Sr0.5CoO3薄膜（电阻率为0.66 mΩcm）中测量到上升延为7 ns的超 快横向热电信号，并提出归一化响应速度的概念来评价不同材料的响应速度，通过建立热传导模型分析得出超快响应主要是高电导率导致穿透深度较小，而响应速度与穿透深度正相关，这为基于横向热电效应探测器的设计提供了指导思路。4. 对复杂钙钛矿Sr3ReCo4O10.5+δ (Re为稀土元素Y, Er等)材料体系的热电磁性能进行了积极的探索。通过组分调制实现了顺磁态立方ABO3型钙钛矿结构到室温铁磁的四方314相超结构的转变，初步分析室温铁磁性与A位和氧陷有序密切关联, Tc=320K~340K，并首次测量到LITV信号，这为进一步探索横向热电信号与磁性的关联提供可能。5. 透明导电CuCrO2外延薄膜的LITV热电输运各向异性的研究，ρab/ρc~35.