双层钙钛矿钌氧化物(Sr,Ca)3Ru2O7中多重量子序及其调控研究

Double layered perovskite ruthenate is a typical 4d transition metal oxides (TMOs), in which spin, charge, orbital and lattice degrees of freedom are strongly coupled. Compared to their 3d cousins, orbitals of 4d transition metal oxides (TMOs) are more extended in the space, leading to comparable electron bandwidth U and Coulomb repulsion U and pushing the system close to the Mott-Hubbard limit. This results in unstable ground states and makes the system sensitive to external stimulations such as the magnetic field, pressure or chemical composition, resulting in exceptional rich quantum phenomena. The study in this area has been a focus in comtemporary condensed matter physics. Nevertheless, there are still many questions remain open. The coexistence and competition among various flucutaitons, the coupling between multiple quantum order parameters, and the underlying physics behind the manipulation of quantum orders are still not fully understood. In this project, we choose to investigate the double layered perovskite ruthenates (Sr,Ca)3Ru2O7, on the basis of our previous studies. We plan to tune the single-electron bandwidth, carrier density, and lattice distortion through the combination of material design and the application of external pressure. On this basis, and with the help of the high magneitic field, we plan to systematically invesigated the complex transitions among multiple quantum orders and resulted unusual quantum phenomena. We want to investigate the spin flucutations and collective excitations in the system, clarify the correlation among multiple order parameters, and try to unravel their underlying physics. This project is expected to provide critical experimental information towards the in depth understanding of the cooperation and competition among charge, spin, orbital and lattice degrees of freedom.

双层钙钛矿钌氧化物是典型的4d过渡金属氧化物，其电荷、自旋、轨道、晶格等自由度间存在强烈耦合。由于轨道相对于3d电子更为扩展，电子带宽W与库仑势U较为接近，体系位于Mott-Hubbard极限，对外加微扰更为敏感。针对该领域的研究已成为凝聚态物理研究的重点之一，但多重量子序中各种涨落的共存与竞争、多种序参量间的关联和耦合，以及量子序调控背后的物理机制等还有待澄清。本项目将以双层钙钛矿钌氧化物(Sr,Ca)3Ru2O7为研究对象，在现有工作基础上，通过材料设计与压力相结合，对单电子带宽、载流子浓度、晶格畸变等参数进行调制，结合强磁场开展调控，系统研究由此引发的多重量子序间的复杂相变，发掘由此引发的奇异量子物性，探究其自旋涨落和集体激发，阐明各种序参量之间的竞争与制约关系，明晰其背后的物理机制，为深入理解4d过渡金属氧化物中电荷、自旋、轨道、晶格等多种自由度之间相互竞争与合作提供关键实验信息。

双层钙钛矿钌氧化物是典型的4d过渡金属氧化物，其电荷、自旋、轨道、晶格等自由度间存在强烈耦合。由于轨道相对于3d电子更为扩展，电子带宽W与库仑势U较为接近，体系位于Mott-Hubbard极限，对外加微扰更为敏感。本项目以4d钌氧化物为研究对象，通过材料设计与强磁场相结合，研究该体系中多重量子序间的复杂相变，发掘由此引发的奇异量子物性并明晰其背后的物理机制，取得如下进展：.1.通过强磁场下光谱测量，澄清双层钙钛矿钌氧化物中温度和强磁场调控金属绝缘体相变机理，指出双层钙钛矿钌氧化物中金属绝缘体转变与反铁磁/铁性轨道有序之间的内在联系。.2.采用机械剥离方法将Sr3Ru2O7解理成为纳米片，通过对其尺寸的调控和采用超高灵敏度的悬臂梁静态测磁法对其磁性进行测量，澄清了双层钙钛矿钌氧化物中变磁性相变的机理，指出变磁相变与磁阻峰可能都来源于费米面在磁场下的Lifshitz相变。.3.通过磁性测量对这一掺杂体系中无序对磁性的影响进了深入探究，发现在Ca含量8%-15%附近存在经典的类Griffiths行为，表明(Sr,Ca)3Ru2O7可能是研究海森堡自旋体系中Griffiths物理的理想体系。.4.使用机械剥离方法将Sr4Ru3O10解理成为纳米片，研究尺寸的调制对物理性质的调控，证明Tm处的相变是由于自旋重取向导致的，其根源是形状各向异性和磁晶各向异性之间的竞争。.5.利用Fe元素进行元素替代，抑制了Sr4Ru3O10在第二个磁转变温度Tm处发生自旋重取向的行为，并在TC以下的整个测量温度区间内发现平面霍尔电阻Rpxy均有明显的类似于“自旋阀”现象的电阻开关行为。.6.此外，为更好的与属于4d电子体系的钌氧化物进行对比分析，项目组还针对3d电子体系开展了一些研究。.这些结果为研究该体系中多重量子序积累了实验数据，为深入理解这一体系中电荷、自旋、轨道、晶格等多种自由度之间相互竞争与合作提供了有用信息。.在在本项目资助下，共培养四名研究生，包括一名博士、三名硕士，发表SCI论文18篇，另外有1篇论文正在投稿中。