Spin frustration and orbital physics in vanadates

自旋受挫和轨道物理的研究是当代凝聚态物理研究中最令人兴奋的前沿之一。钒酸盐化合物中模型自旋-轨道系统的实验实现的发现，加上飞行时间和极化中子的最新发展，以及中央设施的共振X射线散射技术，为在这一领域做出重大贡献提供了前所未有的机会。然而，为了最大限度地发挥优势，提供单晶样品是强制性的。钒酸盐单晶的生长还处于初级阶段，但我们建议使用一位已经使用浮动区域技术生产了这些化合物的一些最大单晶的PDRA。我们还建议与采用互补通量生长技术的欧洲领先的材料发现实验室合作，并与在这一领域工作的凝聚态理论家合作。首先，我们在Pb2VO(PO4)2及其相关材料中发现了具有显著交键交换的正方晶格中的受挫铁磁性。通过化学取代，我们将能够实现量子无序相。在自旋液体相中预测了全新的激发，我们建议在非弹性中子散射实验中非常直接地成像这种行为。其次，钙钛矿型正钒酸盐RVO_3表现出新的磁化现象，包括LaVO_3的磁化反转、GdVO_3的磁记忆效应和PrVO_3的阶梯状磁化。我们的测量将决定轨道有序和轨道激发，并将告诉我们准一维轨道特征的量子涨落相对于Jahn-Teller扭曲的相对重要性。最后，对于V离子，AV2O4尖晶石具有高度受挫的焦绿石几何构型，对于二价A=锌、镁、镉和锰，轨道自由度开始发挥作用。我们将比较用共振x射线散射确定的轨道基态与理论预测，并使用大型单晶的非弹性中子散射来测量复杂的交换相互作用。该项目的主要目标之一是培训博士生不仅在一个令人兴奋和重要的科学领域中互补使用中子和x射线，而且还掌握合成技术，以生产能够充分利用世界领先的中子和同步加速器设施的材料。

The study of spin frustration and orbital physics is one of the most exciting frontiers of contemporary research in condensed matter physics. The discovery of experimental realisations of model spin-orbital systems in vanadate compounds, coupled with recent developments of time-of-flight and polarised neutron, and resonant x-ray scattering techniques at central facilities, gives unprecedented opportunities to make major contributions in this field. However, in order to take maximum advantage the provision of single-crystal samples is mandatory. The growth of single-crystal vanadates is in its infancy, but we propose to employ a PDRA who has already produced some of the largest single crystals of these compounds using the floating-zone technique. We also propose to collaborate with a leading European materials discovery laboratory employing the complementary flux growth technique, and with condensed matter theorists working in this area. First, we have discovered frustrated ferromagnetism in square-lattices with significant cross-bond exchange, in Pb2VO(PO4)2 and related materials. By chemical substitution we will be able to realise a quantum disordered phase. Entirely new excitations are predicted in the spin liquid phase, and we propose to image this behaviour very directly in inelastic neutron scattering experiments. Secondly, the perovskite orthovanadates RVO3 exhibit novel magnetic phenomena including magnetisation reversal for LaVO3, a magnetic memory effect for GdVO3, and a staircase-like magnetisation for PrVO3. Our measurements will determine the orbital ordering and orbital excitations, and will tell us about the relative importance of the quantum fluctuations of quasi-one-dimensional orbital character, versus Jahn-Teller distortions. Finally, the AV2O4 spinels have a highly frustrated pyrochlore geometry for the V ions, and for divalent A = Zn, Mg, Cd and Mn, the orbital degree of freedom comes into play. We shall compare the orbital ground states determined using resonant x-ray scattering with theoretical predictions, and measure the complex exchange interactions using inelastic neutron scattering from large single crystals.One of the primary aims of the project is to train PhD students not only in the complementary use of neutrons and x-rays in an exciting and important area of science, but also with the synthesis know-how to produce materials allowing the fullest use to be made of world-leading neutron and synchrotron facilities.