强磁场下强自旋轨道电子材料的多量子态与调控研究

In 4d/5d strongly correlated electron materials, many exotic physical properties were closely related to the strong spin-orbital interactions, including Mott insulator, chiral spin liquids, topological semimetals and topological insulators and so on. High magnetic field experimental condition has played an important role in wide fields, especially in 5d electronic system with strong spin-orbital interactions. For instance, a 30T high magnetic field amounts to an energy level of~ meV is no longer a small perturbative influence in materials based upon heavy transition elements, but comparable to those critical to the band topology in the 4d/5d materials, and this energy is expected to readily tip the balance between the competing energies, inducing novel phenomena never seen in other materials. In the past a few decades, most of the work gave rise to the pervasive emphasis on the 3d-elements in both basic and applied research, the phenomenology of the spin-orbital interaction and its fundamental consequences on material properties has been neglected until recently. In his proposal, we will turn our attention on three aspects: (1) Modulation of quantum phase and the exotic properties in 5d irridates; (2) New phase and quantum transport behavior in novel topological insulators under high magnetic fields; (3) Competition of multiple quantum ordering and interfacial effect in transition metal multilayers and artificial two dimensional heterogeneous interface. Our target is to investigate the possible novel phenomena in these systems induced by high magnetic field through effectively tuning the relative strengths of the spin orbital interaction, on-site Coulomb interactions and the crystal fields.

在4d/5d关联电子材料中，许多奇特物性是由强自旋轨道相互作用导致的，包括莫特绝缘体、手征自旋液体、拓扑半金属以及宽带隙拓扑绝缘体等。强磁场作为一种极端实验条件在很多领域显露出它的重要作用，比方说～30T的磁场在能量上为几个毫电子伏，已经不再是一个可忽略的微扰，能够很有效地调控各种竞争能量之间的平衡关系，诱导出新的量子序，从而导致新现象。在过去几十年的强关联研究中，人们主要强调了3d元素的作用而对自旋轨道作用以及该作用所诱导的基本相关效应大多被忽略了。本项目拟开展：（1）5d铱化物中的奇异量子现象与量子调控机制；（2）新型宽带拓扑绝缘体及由关结构中的新量子态与量子输运研究；（3）过渡金属氧化物薄膜和人工结构中序竞争及界面效应研究。目标就是通过强磁场等方法实现对电荷、自旋、轨道等量子有序态的调控，使之出现全新的物质状态，从而呈现出新现象、新物理，并推动最前沿的基础材料研究的发展。

本项目主要围绕过渡金属氧化物薄膜和人工超晶格、4d/5 d氧化物中的奇异量子现象与量子调控机制及新型拓扑量子材料在强磁场下中的量子输运与新量子态开展了系统研究，并取得如下创新性成果：（1）首次研制出具有层分辨磁化翻转特性的全氧化物人工反铁磁体超晶格，该磁化结构能在几百高斯的低磁场下实现有效调制转换，并且具有几近于室温的居里温度，使其在自旋电子学领域的应用成为可能；（2）理论预言和实验证明：三角PtBi2是新一类三重简并拓扑半金属，而立方PtBi2是一个载流子补偿的具有超大磁阻的新拓扑半金属；（3）系统研究了ZrTe5单晶的高磁场量子输运特性及磁场、压力和尺寸调控效应等,观察到手性磁效应、磁场诱导的三维Dirac半金属到二维外尔半金属的拓扑相变、压力诱导的三维Dirac半金属到平庸半金属的转变以及厚度调控拓扑半金属到平庸金属的转变等；（4）详细研究了狄拉克半金属Cd3As2纳米片在局域和非局域探测下的量子输运特性以及平面霍尔效应，观察到局域的二维表面态量子振荡信号以及角度满足余弦关系的平面霍尔信号，清晰给出手性磁效应的证据；（5）首次利用平面霍尔测量手段，研究了4d材料Sr4Ru3O10 低温奇异磁性的尺寸调控现象，首次发现随单晶厚度减小能够诱导低温50 K变磁转变温度降低并伴随磁矩从c轴取向反转到ab-面，这一发现澄清了人们对低温下50K附近奇异变磁转变机理的长期争论；（6）系统研究了拓扑磁性材料MnSi 和FeGe在受限体系下的新颖结构，成功实现在纳米条带和纳米盘中观察到单个磁斯格明子或团簇以及尺寸对其形状的调控，首次实现对单个磁斯格明子的电学探测及形状调控等。 在研究中，共发表论文52 篇，一作和通讯作者论文47篇，其中在国际顶级期刊发表论文16篇，如Science 1篇，Nat. Mater. 1 篇，Nat. Commun. 5篇，Phys. Rev. Lett. 2 篇， PNAS 2 篇等。