应兰州大学物理科学与技术学院、萃英学院、理论物理交流平台邀请，中国人民大学物理系于伟强教授将来我校进行学术合作交流并做学术报告，欢迎广大师生参加！

主讲人：于伟强教授

题目：核磁共振探测自旋阻挫材料中磁场诱导的量子相变

时间：10月14日上午10：30

地点：格致楼3016

主持人：赵继泽教授

报告摘要：

Magnetic frustration usually leads to highly competing states, and therefore frustrated magnets can usually be tuned by external magnetic fields and give rise to rich quantum phase transitions, which opens the window for looking for exotic states and excitations. Condensed-matter NMR (nuclear magnetic resonance), taking advantage of the strong hyperfine couplings between electrons and the nuclei, is a sensitive probe of magnetic states and low-energy excitations. Here I report our recent NMR studies which reveal novel field-induced quantum phase transitions in several low-dimensional, frustrated magnetic materials.

For a honeycomb latticeα-RuCl₃which contains Kitaev interactions, the magnetic field suppresses the magnetic ordering and leads to a disordered phase. The disordered phase demonstrates characters of gapless, fractional excitations, and is consistent with a proximate Kitaev spin liquid [1]. For a quasi-1D XXZ chain compound with Ising anisotropy, SrCo₂V₂O₈, we establish a strong suppression of the magnetic ordering by an effective transverse field, and resolve two quantum critical points (QCPs). The low-field one is consistent with a 3D QCP, and the high-field one is consistent with a 1D QCP in the (1+1) Ising universality class [2]. For a triangular lattice antiferromagnet Ba₈CoNb₆O₂₄, we show that this material approaches a real 2D, spin-1/2, Heisenberg limit. A Quantum spin liquid is suggested at zero field. The magnetic field first induces magnetic ordering, and then leads to a fully polarized state at finite temperatures, with a distinctive (pressure, temperature) phase diagram compared to the classical Heisenberg model [3].

References:

[1] J. Zheng et al., Phys. Rev. Lett. 119, 227208 (2017).

[2] Y. Cui et al., Phys. Rev. Lett. 123, 067203 (2019).

[3] Y. Cui et al., Phys. Rev. M 2, 044403 (2018)

报告人简介：

于伟强，中国人民大学物理系教授。2004年获得UCLA博士学位，后在马里兰大学、马克马斯特大学做博士后，2008年起任现职。2011年获国家基金委首届优青资助。结合极低温、高压和高磁场技术，搭建了特色的低温固体核磁共振系统，专注于铁基高温超导材料和量子磁性材料的物性研究，目前共发表60多篇学术论文。近几年以通讯作者在《物理评论快报》发表6篇学术论文。在铁基高温超导材料的超导对称性、向列序、自旋涨落和超导的关联，以及低维量子磁性材料的自旋液体态、量子相变等方面做出前沿研究成果。