与量子自旋液体相关的烧绿石稀土氧化物的磁性、磁激发与热传导性质

Rare-earth titanates R2Ti2O7 (R = rare earth) with pyrochlore structure are a typical class of three-dimensional spin frustrated materials, which exhibit various exotic magnetic properties. This project plans to study the magnetic ground states, phase diagrams, magnetic excitations and heat transport of the quantum-spin-liquid related materials, such as Tb2Ti2O7 and Yb2Ti2O7. The main research contents include: studying the thermal Hall effect of Tb2Ti2O7 and Yb2Ti2O7 under ultra-low temperatures and high magnetic fields; growing chemically doped or element substituted R2+xTi2−xO7 (R = Tb, Yb) and R2Ti2−xMxO7 (M = Sn, Zr, Ge, etc.) single crystals and studying their ground-state magnetisms, magnetic transitions and magnetic excitations by measuring anisotropic magnetic property, specific heat, thermal conductivity and thermal Hall effect; growing and characterizing the Ti-free rare-earth pyrochlore oxides like Er2AlSbO7, Tb2AlSbO7, Yb2AlSbO7, etc. Based on these experimental works, we try to explore the main parameters determining the quantum disordered and ordered magnetic states, to figure out the temperature-(magnetic)field-(chemical)composition phase diagrams, and to reveal the natures of magnetic excitations and their heat transport properties.

具有烧绿石结构的稀土钛氧化物R2Ti2O7（R为稀土元素）是典型的三维自旋阻挫材料，展现出多种奇特的磁性。本项目以Tb2Ti2O7、Yb2Ti2O7等与量子自旋液体相关的体系为研究对象，以这些体系的基态磁性、磁相图、磁激发特征和热传导行为为主要问题，拟开展以下工作：极低温强磁场下Tb2Ti2O7、Yb2Ti2O7的热霍尔效应探索和对比；通过化学掺杂和元素替代，制备R2+xTi2−xO7（R = Tb, Yb）和R2Ti2−xMxO7（M = Sn、Zr、Ge等）单晶，利用各向异性磁学、比热、热导率、热霍尔效应的测量，表征基态磁性、磁转变和磁激发性质；Er2AlSbO7、Tb2AlSbO7、Yb2AlSbO7等不含Ti的稀土烧绿石结构单晶材料的制备和低温物性表征。通过以上实验研究，探讨这些材料中决定量子无序和磁有序状态的主要参数，描绘温度－磁场－组份相图，揭示磁激发特性和其热传导机制。

具有烧绿石结构的稀土钛氧化物R2Ti2O7（R为稀土元素）是典型的三维自旋阻挫材料，展现出多种奇特的磁性，包括最受瞩目的量子自旋液体。这类材料的磁结构特征一方面形成共顶点的四面体阻挫结构，另一方面沿其立方晶轴[111]方向构成二维三角层和kagomé层的依次堆垛。因此，本项目以烧绿石结构、三角和kagomé结构等与量子自旋液体相关的体系为研究对象，通过极低温强磁场下磁性、比热、热导率、热霍尔等测量，研究基态磁性、磁相图、磁激发特征和热传导行为。研究工作按照计划顺利进行，取得的主要进展包括：（1）合成了一种新型的XY型烧绿石氧化物材料Er2AlSbO7，发现其至几十mK的温度都没有形成长程磁有序，而是具有自旋冻结行为和较强的自旋涨落；（2）表征了高压诱导的Dy2Ti2O7中晶体结构与能带结构转变规律，发现TiO6八面体的畸变、Ti的3d轨道和O48f的2p轨道杂化决定了带隙的变化；（3）发现Tb2Ti2-xZrxO7单晶在零场中具有自旋―声子散射所导致的极低声子热导率，外加磁场可以显著增大热导率，霍尔热导率呈非单调的磁场依赖关系，表明Tb2Ti2O7中巨大的热霍尔效应起因于磁场中自旋子的横向传导；（4）针对二维三角结构的量子自旋液体候选材料Na2BaCo(PO4)2和YbMgGaO4，从实验上揭示其具有巡游自旋子磁激发与磁场诱导的量子自旋态转变；（5）利用极低温和强磁场下的磁化率、电输运和热输运等测量，揭示了天然kagomé冰材料HoAgGe中自旋阻挫所引起的量子自旋态转变对电子和声子的输运行为所产生的显著影响。这些研究结果为深入理解相关材料的物理机制提供了必要的实验基础。共发表SCI论文17篇，包括2篇Nat. Commun.和9篇Phys. Rev.；培养6名博士生和1名硕士生，其中3名博士生已经完成学位论文答辩并获得博士学位；在国内大型学术会议做邀请报告2次。