EXAFS与热力学揭示B位无序自旋-1/2钙钛矿BaCu₁/₃Nb₂/₃O₃中随机单重态行为的微观起源
Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu_1/3Nb_2/3O_3 Revealed by EXAFS and Thermodynamics
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中文总结 AI 辅助
研究B位无序钙钛矿BCNO的随机单重态行为，结合XRD、XAFS和热力学，揭示其局域化学有序抑制Cu-Cu连接，解释无长程磁有序的原因，观测到低温比热的场致行为转变。
中文摘要 AI 辅助
我们报道了对ABO₃型无序钙钛矿BaCu₁/₃Nb₂/₃O₃（BCNO）的结构与热力学结合研究，其B位由Cu与Nb以1:2比例共同占据。利用同步辐射粉末X射线衍射（XRD）和X射线吸收精细结构（XAFS）谱，我们研究了准立方B亚晶格上Cu²⁺/Nb⁵⁺无序的微观本质及其与低温下观测到的涌现随机单重态（RS）行为的关联。XRD显示无长程Cu/Nb有序，平均位点占据符合化学计量比；XAFS则揭示了一种特殊的局域化学有序，其特征为优先的异原子Cu:Nb关联。尽管Cu浓度接近立方晶格的渗流阈值，这种局域排列仍强烈抑制了直接的Cu:Cu连接。由此产生的交换网络解释了在存在显著反铁磁相互作用（居里-外斯温度Θ_CW≈-50K）时，自旋玻璃冻结或长程磁有序的缺失。相反，磁化率χ(T)和比热c_p(T)在宽温度和磁场范围内呈现幂律行为及特征性单参数T/H标度，与随机单重态现象学一致。值得注意的是，在极低温下，零场下的比热行为从T^(1-γ)（由T/H标度得到γ≈0.6）转变为高场下的T线性依赖，表明存在向不同低能区的 crossover，其微观起源仍待确立。
英文摘要
We report a combined structural and thermodynamic study of the ABO$_3$-type disordered perovskite BaCu$_{1/3}$Nb$_{2/3}$O$_3$ (BCNO), whose B site is jointly occupied by Cu and Nb in the $1:2$ ratio. Using synchrotron powder x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) spectroscopy, we investigate the microscopic nature of Cu$^{2+}$/Nb$^{5+}$ disorder on the pseudo-cubic B-sublattice and its relation to the emergent random-singlet (RS) behavior evidenced at low temperatures. While XRD reveals no long-range Cu/Nb ordering and average site occupancy consistent with stoichiometry, XAFS reveals a peculiar local chemical order characterized by preferential heteroatomic Cu$:$Nb correlations. This local arrangement strongly suppresses direct Cu$:$Cu linkages, despite the Cu concentration being close to the percolation threshold of a cubic lattice. The resulting exchange network explains the absence of spin-glass freezing or long-range magnetic order in the presence of substantial antiferromagnetic interactions, as indicated by a Curie-Weiss temperature $Θ_{CW}\approx -50$ K. Instead, the magnetic susceptibility $χ(T)$ and specific heat $c_p(T)$ exhibit power-law behavior and characteristic single-parameter $T/H$ scaling over broad temperature and magnetic-field ranges, consistent with random-singlet phenomenology. Notably, at very low temperatures, the specific heat behavior transitions from $T^{1-γ}$ ($γ\approx 0.6$ from the $T/H$ scaling) in zero-field to a T-linear dependence under high field, indicating a crossover to a distinct low-energy regime whose microscopic origin remains to be established.
