三角阻挫磁性材料的分数磁化平台及相关新效应研究

Frustrated magnet is now an important topic in condensed matter physics owing to their degenerate ground state, elementary excitations and behavior of quantum phase transition. With the application of magnetic fields, this kind of materials usually undergoes quantum phase transition from disorder to order, leading to a variety of new quantum states and physical phenomena. Particular interest is given by the novel quantum state called fractional magnetization plateau. This phenomenon is only observed in sufficient magnetic fields due to relatively large exchange interactions in these materials. With respect to this reason, pulsed high magnetic field becomes one of the most powerful tools to study this macroscopic quantum effect. In the present project, we will focus our interests on two-dimensional triangular magnets. Various pulsed field techniques will be employed to investigate origin of magnetization plateau, the relationship of multiferroic, and the effect of itinerant electrons. Through these works, we expect to solve some basic problem in frustrated magnets, establish the correlation between various interactions and quantum states in complex many-body system, and also explore some new physics and novel states in high magnetic fields.

阻挫磁性材料通常具有能量简并的量子基态、元激发和量子相变行为，是目前凝聚态物理的研究热点。在外加磁场下，这类材料往往会发生无序到有序的量子相变，导致各种新奇量子态的产生，并呈现出许多内涵极为丰富的物理特性。其中一种物理现象就是分数磁化平台。由于材料内部交换作用力往往比较大，这种宏观量子效应一般需要在强磁场下才能观测和研究。脉冲强磁场作为一种极端实验条件，成为研究这类量子现象的有力手段之一。本项目计划利用脉冲强磁场实验条件，研究具有二维特征的三角基阻挫磁性材料，具体包括研究磁化平台的形成机制、多铁性与平台之间的关联、巡游电子对平台的影响等科学问题。通过本项目，希望解决阻挫磁性材料研究中的某些问题，建立复杂多体物理中各种相互作用与量子态之间的关联，并探索极端实验条件下的新现象、新物态。

由于同时具有低维和阻挫两种特性，低维阻挫磁性材料在低温和磁场下呈现出丰富的物理现象，如简并量子基态、分数磁化平台、多铁性等，成为研究量子效应和新奇量子物态的理想载体。本项目重点研究了四类不同的低维阻挫磁性材料：(1)具有准一维螺旋自旋链结构的R2V2O7(R=Ni,Co)；(2)准一维Ising自旋链结构的alpha-CoV2O6和Zigzag链FeNbO4；(3)准二维六角锰氧化物RMnO3(R=Ho-Yb)；(4)准二维Kagome结构Ni3V2O8。我们成功合成了以上磁性材料的单晶样品，并开展了系统的低温、强磁场下的物性研究。通过本项目，我们获得了以下研究进展：首先，发现了具有量子磁化平台的新材料，揭示R2V2O7系统在强磁场下不仅具有很宽的1/2磁化平台，还具有磁场诱导的多铁性；第二，发现了一些新物理现象，如Ni3V2O8反常磁电记忆效应、Co2V2O7磁场驱动的铁电极化反转和翻转，并给出了物理解释；第三，探索了新的量子物态，如Ni2V2O7自旋向列态、Co2V2O7磁子波色-爱因斯坦凝聚；第四，澄清了一些物理争议，如RMnO3磁对称性和磁相图确定、alpha-CoV2O6多步磁化台阶和自旋动力行为的成因；第五，加深对磁化平台物理本质的理解，结合理论计算合理解释了R2V2O7和Ni3V2O8的量子磁化平台物理起源。