Neutron powder diffraction study of magnetic order in geometrically frustrated triangular lattice: CeAuAl4Ge2

Recently the compounds with geometrically frustrated crystal structure have attracted considerable attention due to frustration induced quantum spin-liquid ground state. CeAuAl4Ge2, which crystallizes in a triangular lattice (rhombohedral, space group R-3m) structure, is reported to exhibit a phase transition near 1.4 K in heat capacity and electrical resistivity. The entropy is found to be only 52% of Rln2 near the transition temperature, but Rln2 at 15 K, indicating a presence of short-range ordering above 1.4 K. Initial interpretation for this phase transition is a magnetic ordering of an antiferromagnetic character. The Kondo hybridization as inferred from the electrical resistivity is almost negligible. In the rhombohedral structure of CeAuAl4Ge2, the Ce-moments form the equilateral triangle arrangement and hence are subject to geometrical frustration and could stabilize complex magnetic structure. Further investigations using neutron diffraction measurements are highly desired to determine the magnetic structure of this compound and the presence of short-range magnetic correlations above TN=1.4 K. We propose to carry out neutron powder diffraction measurements on CeAuAl4Ge2 using the WISH diffractometer to determine the magnetic structure of this geometrically frustrated compound and short-range correlations present above the phase transition as evident from the magnetic entropy.

近年来，具有几何阻挫晶体结构（geometrically frustrated crystal structure）的化合物因阻挫诱导产生的量子自旋液体基态（quantum spin-liquid ground state）而受到广泛关注。CeAuAl4Ge2以三角晶格（菱面体晶系，空间群R-3m）结构结晶，据报道其在热容与电阻率测试中于1.4 K附近出现相变。在相变温度附近，其熵仅为Rln2的52%，而在15 K时则达到Rln2，这表明在1.4 K以上存在短程有序（short-range ordering）。该相变最初被认为是具有反铁磁特性的磁有序。从电阻率推断出的近藤杂化（Kondo hybridization）几乎可以忽略。在CeAuAl4Ge2的菱面体晶系结构中，铈磁矩（Ce-moments）形成等边三角形排布，因此受到几何阻挫作用，有望形成复杂的磁结构。亟需开展中子衍射（neutron diffraction）测量以明确该化合物的磁结构，以及在奈尔温度（TN, Néel temperature）1.4 K以上是否存在短程磁关联（short-range magnetic correlations）。我们提议利用WISH衍射仪（WISH diffractometer）对CeAuAl4Ge2开展中子粉末衍射（neutron powder diffraction）测量，以确定该几何阻挫化合物的磁结构，以及由磁熵所证实的相变温度以上存在的短程磁关联。