Tanaka Yu, Yoshida Makoto, Takigawa Masashi, Okamoto Yoshihiko, Hiroi Zenji
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
Phys Rev Lett. 2014 Nov 28;113(22):227204. doi: 10.1103/PhysRevLett.113.227204. Epub 2014 Nov 26.
We report ^{7}Li-NMR studies on LiInCr_{4}O_{8} and LiGaCr_{4}O_{8}, in which Cr^{3+} ions with spin 3/2 form a breathing pyrochlore lattice, a network of tetrahedra with alternating sizes. In LiInCr_{4}O_{8} with large alternation, the nuclear relaxation rate 1/T_{1} shows an activated temperature (T) dependence down to 18 K, indicating a singlet ground state with a spin gap. This behavior, however, is disrupted by an antiferromagnetic transition at 13 K, which is preceded by another, most likely structural, transition at 16 K. In contrast, LiGaCr_{4}O_{8} with a small alternation shows no spin gap but exhibits a first-order antiferromagnetic transition over a distributed T range 13-20 K. Nevertheless, 1/T_{1} of the paramagnetic phase diverges toward 13 K, indicating proximity to a second-order transition. The results indicate that LiGaCr_{4}O_{8} is located in the vicinity of a tricritical point in the phase diagram.
我们报道了对LiInCr₄O₈和LiGaCr₄O₈的⁷Li核磁共振研究，其中自旋为3/2的Cr³⁺离子形成了一个呼吸型焦绿石晶格，即一个由大小交替的四面体组成的网络。在具有较大交替度的LiInCr₄O₈中，核弛豫率1/T₁在低至18 K时呈现出活化温度（T）依赖性，表明其基态为具有自旋能隙的单重态。然而，这种行为在13 K时被反铁磁转变破坏，在该转变之前，在16 K时还有另一个很可能是结构上的转变。相比之下，具有较小交替度的LiGaCr₄O₈没有自旋能隙，但在13 - 20 K的分布温度范围内呈现出一级反铁磁转变。尽管如此，顺磁相的1/T₁在接近13 K时发散，表明接近二级转变。结果表明，LiGaCr₄O₈位于相图中的三临界点附近。
