Charge fluctuations in the intermediate-valence ground state of SmCoIn₅

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn₅ and other Ce-based heavy fermion materials, depends strongly on the efficiency with which f electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce³⁺ (4f 1, J = 5/2) with Sm³⁺ (4f 5, J = 5/2), we show that a combination of crystal field and on-site Coulomb repulsion causes SmCoIn₅ to exhibit a Γ7 ground state similar to CeCoIn5 with multiple f electrons. Remarkably, we also find that with this ground state, SmCoIn₅ exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of f holes below a temperature scale set by the crystal field, Tv ≈ 60 K. Our result provides evidence that in the case of many f electrons, the crystal field remains the most important tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations are present and may play a generally important role in the ground state of "115" materials.

针对CeCoIn₅及其他铈基重费米子材料中的非传统超导现象，重能带形成的微观机制强烈依赖于f电子从稀土位点离域并参与近藤晶格(Kondo lattice)的效率。我们将铈离子Ce³⁺(4f¹，总角动量J=5/2)替换为钐离子Sm³⁺(4f⁵，J=5/2)后发现，晶体场与在位库仑排斥的共同作用使得SmCoIn₅呈现出与CeCoIn₅类似的Γ7基态，且其体系内f电子数目更多。尤为值得关注的是，我们还发现具备该基态的SmCoIn₅会出现温度诱导的价态交叉现象，这与近藤物理图景相符；当温度低于由晶体场决定的特征温度尺度Tᵥ≈60 K时，f空穴的离域程度会显著提升。我们的研究结果提供了证据表明，在f电子数目较多的情形下，晶体场仍是调控重费米子量子临界点附近离域效率的最关键调控手段；此外，本研究还阐明，“115”型材料的基态中存在电荷涨落，且该涨落或许普遍发挥着重要作用。