High-Pressure effects on a honeycomb iridate - Drs. Yang Ding and Yongjae Lee
AUGUST 17, 2018
New work from a team of scientists including Drs. Yang Ding and Yongjae Lee from HPSTAR confirm that Kitaev-quantum-spin candidate material, a-Li2IrO3 will lose its exotic quantum state under compression instead of becoming predicted Kitaev spin liquid from multiple in-situ characterizations combined with theoretical calculations. This work is published in recent NPJ Quantum Materials.
Generally, in a magnetic material, the electrons will order themselves with long range when the material is cooled to a specific low temperature.
But in a state, so-called quantum spin liquid, even if cooled to absolute zero temperature, the electrons in the material will not align orderly but disordered.
“Quantum spin liquid is new mysterious state of matter, in which electrons are free, said Dr. Yang Ding, one co-author of the work and a staff scientist in physics of HPSTAR. “This is similar to the liquid or amorphous structure of materials contains disordered atoms or molecular”.
Quantum spin liquids are currently the focus for physicists, because they can be used to explain high temperature superconductivity and have high potential for using in future quantum computation.
Kitaev spin liquid is one of quantum spin liquids, has been predicted theoretically to exist in a “honeycomb” lattice. This model proposed a hexagonal honeycomb structure which offered a promising route to quantum spin liquid.
Structurally, the 5d transition metal oxides have a two-dimensional honeycomb lattice structure, and magnetically, they have special quantum ground state showing strong spin-orbit coupling, so they are predicted to be ideal candidates for realizing Kitaev spin liquid.
While, unfortunately, these ideal materials will become electronically/ magnetically ordered at low temperature, rather than turn to quantum spin liquid.
Therefore, scientists try to tune the structure and related spin interactions of these materials for realizing the Kitaev state.
Pressure as a clear tool, has been successfully used in tuning the electronic structures of magnetic materials.
An international group co-authored by Drs. Yang Ding and Yongjae Lee, used in-situ high-pressure x-ray diffraction, x-ray spectroscopy techniques together with calculations to probe the crystal and electronic structural evaluations of honeycomb-lattice structured lithium iridate—a-Li2IrO3 — a 5d transition metal oxide.
“From high pressure studies on other similar materials, a-Li2IrO3 is promisingly to realize Kitaev state by pressure”, said Dr. Yongjae Lee.
Unexpectedly, the team observed that even minor pressure (0.1 GPa) will distort the honeycomb-lattice of a-Li2IrO3 , which even lead to the basic ambient quantum state disappear—not to mention the realizing Kitaev spin liquid.
“Our study shows that for 5d transition metal oxides, the lattice distortion will affect the magnetic related properties through electrons and orbitals which is not commonly observed in the 3d transition oxides”, said Dr. Yang Ding. “However, this was neglected before”.
“Such fragility of the ground state could have important implications for the understanding of pressure-driven magnetic transitions in other Kitaev magnets”, stated in the paper.
Caption: Kitaev model of spin 1/2 onthe honeycomb lattice with the interacting component of the spin.
具有强自旋-轨道耦合的蜂窝晶格量子磁体, 如 Li2IrO3 和 Na2IrO3, 是实现 Kitaev 量子三维自旋液体的理想候选者，也是目前凝聚态物理领域一项重要且热门的研究课题。北京高压科学研究中心丁阳研究员，Yongjae Lee研究员合作的研究小组通过原位 x 射线衍射，吸收光谱，共振非弹性散射，及第一性原理等技术，研究了 蜂窝晶格量子材料a-Li2IrO3 的晶体及电子结构在压力作用下的演变。研究者发现和其他蜂窝晶格量子材料不一样， a-Li2IrO3在极低的压力下， 晶体结构就变得不稳定而出现畸变，使得Kitaev自旋液体赖以存在的奇特量子态消失，所以a-Li2IrO3无法显现自旋液体。该研究指出，与以往的3d过渡金属氧化物不同，5d过渡金属氧化物中由于存在强自选-轨道耦合，所以其晶格的变化通过轨道对其量子态和种种性质对会起到极大的影响，但这一重要现象却一直被忽视。因而本研究所揭示的这一现象不仅仅在理论上要求新的理论认识，而且也为人们在蜂窝晶格量子磁体中寻找高压下的三维自旋液体提供一种新视野。