Direct experimental observation of the molecular Jeff=3/2 ground state in the lacunar spinel GaTa4Se8
Min Yong Jeong, Seo Hyoung Chang, Beom Hyun Kim, Jae-Hoon Sim, Ayman Said, Diego Casa, Thomas Gog, Etienne Janod, Laurent Cario, Seiji Yunoki, Myung Joon Han, Jungho Kim
Strong spin-orbit coupling lifts the degeneracy of t2g orbitals in 5d transition-metal systems, leaving a Kramers doublet and quartet with effective angular momentum of Jeff = 1/2 and 3/2, respectively. These spin-orbit entangled states can host exotic quantum phases such as topological Mott state, unconventional superconductivity, and quantum spin liquid. The lacunar spinel GaTa4Se8 was theoretically predicted to form the molecular Jeff = 3/2 ground state. Experimental verification of its existence is an important first step to exploring the consequences of the Jeff = 3/2 state. Here, we report direct experimental evidence of the Jeff = 3/2 state in GaTa4Se8 by means of excitation spectra of resonant inelastic x-rays scattering at the Ta L3 and L2 edges. We found that the excitations involving the Jeff = 1/2 molecular orbital were suppressed only at the Ta L2 edge, manifesting the realization of the molecular Jeff = 3/2 ground state in GaTa4Se8.
Skyrmionic order and magnetically induced polarization change in lacunar spinel compounds GaV$_{4}$S$_{8}$ and GaMo$_{4}$S$_{8}$: comparative theoretical study
Microscopic theory of electric polarization induced by skyrmionic order in GaV$_{4}$S$_{8}$
空隙尖晶石GaV $_{4}$ S $_{8}$最近被认为是具有与磁序耦合的相当大的极化$\boldsymbol{P}$的Skyrmion晶格态的原型多铁性材料。我们在微观层面上解释了这一现象。在密度泛函理论的基础上，我们构造了一个描述（V $_{4}$ S $_{4}$）$^{5+}$团簇分子轨道形成的弱耦合晶格中磁活性电子行为的有效模型.应用超交换理论和Berry相理论，导出了磁键中能量和极化变化与自旋方向的关系模型.我们认为，虽然每个skyrmion层主要是由超交换相互作用在同一平面上形成的，自旋依赖的$\boldsymbol{P}$产生的堆叠错位，这些平面在垂直方向上，这是固有的空隙尖晶石结构。我们预测各向同性，PDMATHAMR，反对称，$\sim \boldsymbol{e}_{i} \times \boldsymbol{e}_{j}$，贡献$\boldsymbol{P}$，解释实验观察到的效果的强烈竞争。
Possible emergence of a skyrmion phase in ferroelectric GaMo$_4$S$_8$
提出了在磁场下，极性空隙尖晶石，例如GaV $_4$ S $_8$和GaV $_4$ Se $_8$，作为Skyrmion相的宿主。在本工作中，我们提出了一种等构的GaMo $_4$ S $_8$作为Néel型skyrmion晶格的候选晶格，并通过第一性原理计算和模型哈密顿量的蒙特卡罗模拟对其进行了系统的研究.由Jahn-Teller失真驱动的电极化的强度预计将出现在GaMo $_4$ S $_8$中，显示出与基于V的对应物中评估的尺寸相当但相反的符号，并根据不同的电子计数参数和产生的失真进行解释。有趣的是，在钒尖晶石中，4d轨道相对于3d轨道的更大的自旋-轨道耦合导致更强的Dzyaloshinskiii-Moriya相互作用，这有利于稳定摆线自旋织构，以及更小尺寸的skyrmions（半径<10 nm）。此外，GaMo_4S_8的交换各向异性可能导致铁电-铁磁基态，作为铁电-skyrmionic基态的替代，需要进一步的实验验证。
