Exploration of topological defects in ferroelectrics

铁电体（FEs）是在一定温度下表现出自发极化的材料，并且具有两种或两种以上稳定的极化状态，可以通过施加外场可逆地获得。以前的应用涉及域-具有一致极化感的FE区域-作为材料的功能部分。最近的焦点已经转移到结构域壁（DWs）上，这是一种分离结构域的纳米级膜状结构。DWs已被证明具有不同于其周围结构域的特性。该项目旨在研究这一不断发展的领域的两个领域：畴壁传导和FE天空。在绝缘体(1)中，dw的导电性增强已经在许多材料中得到证实。此外，由于极化随外场的应用是可逆的，dw可以移动和控制，甚至可以创建和移除，从而产生了一种新的可重构电路的想法，其中畴壁是一个移动的电连接(2)。传导是不同的，但表征传导是实现畴壁纳米电子学的关键，也是该项目的主要目标之一。通过测量几何磁电阻（在施加磁场时电阻率的变化）来测量铌酸锂（LN）中导电DWs载流子迁移率的工作正在进行中。这种效应的产生是由于可以在LN薄膜中写入的dw的独特的锥形几何形状(1)，使得一种新的测量畴壁迁移率的方法成为可能。此外，还计划对导电dw的电子能带结构进行首次直接测量。利用大型设施x射线源在DWs局部激发载流子，可以测量载流子的能量和动量。这些参数包括电子能带结构，从而深入了解电导率增强背后的机制。该项目的另一个方面是研究一个与磁性粒子相似的有限元。磁skyrmion是一种受限的自旋模式，被描述为“拓扑保护”，这意味着它不能连续变形和移除，并且在一定程度上是稳定的。它们的前景来自于它们的纳米尺寸，使它们成为高密度数据存储应用的理想选择，比如最近设想的赛道存储器(3)。在复杂结构中，极化取代自旋的电对应体已经在理论上得到了预测(4)和观察(5)，但一般情况还有待观察(4)。如果被发现，FE skyrmions可以以类似于上面提到的dw的方式由场控制，因此有望成为功能元素(4)。该项目寻求使用聚焦离子束切割FE钛酸铅薄片，编写一个预计是skyrmionic的域模式，并使用原子分辨率透射电子显微镜（与利默里克大学合作）来验证这一点。如果观察到，我们可以继续使用新的原子力显微镜方法来写入，移动和控制这些偏振模式与应用领域，研究它们的响应和输运性质，探索它们的物理起源，并最终表征它们的潜在应用。Schröder， Mathias等。“铌酸锂单晶的导电畴壁。”先进功能材料（2012）：3936-3944.2。麦奎德，雷蒙德·GP等。“不适当铁电性Cu-Cl -硼砂中导电畴壁的注入和控制运动。”自然通讯8(2017):15105.3。Tomasello， Riccardo等人。“ skyrmion赛道存储器的设计策略”科学报告4(2014):6784.4。gonalalves， MA Pereira等。“创造和调节电子泡泡的理论指南”科学进展5.2 (2019):eaau7023.5。Das， Sujit等。“观察室温下的极地天空。”Nature（自然）：568.7752(2019):368。

Ferroelectrics (FEs) are materials which exhibit spontaneous polarisation below a certain temperature, and have two or more stable polarisation states, reversibly accessible by application of external fields. Previous applications have involved domains -regions of the FE with a consistent sense of polarisation- as the functional part of the material. Recent focus has switched to the domain walls (DWs), the nanoscale membrane-like structures that separate domains. DWs have been shown to exhibit properties distinct from the domains surrounding them. This project aims to investigate two areas of this evolving field: domain wall conduction and FE skyrmions.Enhanced electrical conductivity at DWs in an otherwise insulating bulk(1) has now been demonstrated in many materials. Also, since the polarisation is reversible with the application of external fields, DWs can be moved and controlled, even created and removed, giving rise to the idea of a new kind of reconfigurable circuitry, where the domain wall is a mobile electrical connection(2). Conduction varies, but characterising the conduction is key to realising domain wall nanoelectronics, and is one of the main aims of the project. Work is under way to obtain a measurement for the carrier mobility of conducting DWs in lithium niobate (LN) by measuring the geometric magnetoresistance- a change in resistivity upon application of a magnetic field. This effect arises due to the unique conical geometry of the DWs that can be written in LN thin films(1), making a novel measurement of mobility in domain walls possible. Furthermore, plans are in place to make some of the first direct measurements of the electronic band structure specifically at conducting DWs. Using a large facility X-Ray source to excite carriers locally at the DWs, energy and momentum of carriers can be measured. These parameters comprise the electronic band structure, giving insight into the mechanism behind the enhanced conductivity.Another aspect of the project is to investigate a FE analogy to the magnetic skyrmion. A magnetic skyrmion is a confined spin pattern, described as being 'topologically protected', meaning it cannot be continuously deformed and removed, and is somewhat stable. Their promise comes from their nanometer size, making them ideal for high density data storage applications, such as the recently envisioned racetrack memory(3). An electric counterpart, where polarisation replaces spin, has been theoretically predicted(4) and observed(5) in complex structures, but the general case is yet to be seen(4). If found, FE skyrmions could be controlled by fields in a similar way to the DWs mentioned above, and thus hold promise as functional elements(4). This project seeks to use focussed ion beams to cut thin slices of FE lead titanate, write a domain pattern which is expected to be skyrmionic, and use atomic resolution transmission electron microscopy (in collaboration with University of Limerick), to verify this. If observed, we could move forward into using novel atomic force microscopy methods to write, move and control these polarisation patterns with applied fields, investigating their response and transport properties, exploring their physical origin and ultimately characterising them for potential application.1. Schröder, Mathias, et al. "Conducting domain walls in lithium niobate single crystals." Advanced Functional Materials 22.18 (2012): 3936-3944.2. McQuaid, Raymond GP, et al. "Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite." Nature communications 8 (2017): 15105.3. Tomasello, Riccardo, et al. "A strategy for the design of skyrmion racetrack memories." Scientific reports 4 (2014): 6784.4. Gonçalves, MA Pereira, et al. "Theoretical guidelines to create and tune electric skyrmion bubbles." Science advances 5.2 (2019):eaau7023.5. Das, Sujit, et al. "Observation of room-temperature polar skyrmions." Nature 568.7752 (2019): 368.