We investigate the electronic and magnetic properties of the frustrated triangular-lattice antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ in the framework of density functional theory. Analysis of the exchange couplings J and J' using the available X-ray structural data corroborates the values obtained from experimental results for Cs$_2$CuBr$_4$ but not for Cs$_2$CuCl$_4$. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs$_2$CuCl$_4$ employing different exchange-correlation functionals. We find that the exchange couplings depend on rather subtle details of the structural optimization and that only when the insulating state (mediated through spin polarization) is present in the structural optimization, we do have good agreement between the calculated and the experimentally determined exchange couplings. Finally, we discuss the effect of interlayer couplings as well as longer-ranged couplings in both systems.
Atomic and magnetic structures of (CuCl)LaNb$_2$O$_7$ and (CuBr)LaNb$_2$O$_7$: Density functional calculations
用密度泛函方法研究了（Cu $X$）LaNb $_2$ O $_7$（$X$ =Cl和Br）的原子结构和磁性.在几十个被检查的结构中，正交畸变的$2\times 2$结构，其中$X$卤素的位移模式类似于先前基于经验信息所描述的模型，被确定为最稳定的一个。$X$卤素的置换与Cu离子的置换一起导致在两种材料中形成$X$ -Cu-$X$-Cu-$X$之字形链。Z字形链中的最近邻相互作用对（CuCl）LaNb $_2$ O $_7$是反铁磁的（AFM），而对（CuBr）LaNb $_2$ O $_7$是铁磁的（FM）.另一方面，前两个相邻的铜阳离子之间的相互作用从相邻的链被发现是AFM和FM分别为这两个化合物。在（CuBr）LaNb $_2$ O $_7$中所有这些面内交换耦合的大小被估计为在（CuCl）LaNb $_2$ O $_7$中的那些的大约三倍。此外，一个相当大的AFM面间相互作用被发现由两个NbO $_6$八面体分离的Cu离子之间。本研究强烈地表明，为了正确地解释（Cu $X)$ LaNb $_2$ O $_7$）的磁性，有必要超越正方形$J_1-J_2$模型。
We investigate the electronic structure of Cs$_2$CuCl$_4$, a material discussed in the framework of a frustrated quantum antiferromagnet, by means of resonant inelastic x-ray scattering (RIXS) and Density Functional Theory (DFT). From the non-dispersive highly localized dd excitations, we resolve the crystal field splitting of the Cu$^{2+}$ ions in a strongly distorted tetrahedral coordination. This allows us to model the RIXS spectrum within the Crystal Field Theory (CFT), assign the dd orbital excitations and retrieve experimentally the values of the crystal field splitting parameters D$_q$,…
Antiferromagnetic spin-frustrated layers of corner-sharing Cu4 tetrahedra on the kagome lattice in volcanic minerals Cu5O2(VO4)2(CuCl), NaCu5O2(SeO3)2Cl3, and K2Cu5Cl8(OH)4 2H2O
$μ$SR studies of the frustrated quasi-2d square-lattice spin system Cu(Cl,Br)La(Nb,Ta)$_{2}$O$_{7}$: evolution from spin-gap to antiferromagnetic state
我们报道了μ子自旋弛豫（$μ$ SR）和对Cu（Cl，Br）La（Nb，Ta）$_{2}$ O $_{7}$的磁化率测量，其证明：（a）在低至15 mK的（CuCl）LaNb $_{2}$ O $_{7}$中不存在静态磁性，证实了自旋能隙基态;（b）在（CuCl）La（Nb，Ta）$_{2}$ O $_{7}$中具有自旋能隙和静态磁性的部分体积之间的相分离;（c）（Nb，Ta）和（Cl，Br）替代系统中的历史相关磁化;（d）（CuBr）LaNb $_{2}$ O $_{7}$中的均匀长程共线反铁磁态;以及（e）Cu（Cl $_{1-x}$ Br $_{x}$）LaNb $_{2}$ O $_{7}$中的Néel温度随Br浓度的降低而降低，而$0.33 \leq x \leq 1$的有序Cu矩大小没有变化。与其他几个$μ$ SR研究的量子相变的几何挫折自旋系统，目前的结果表明，从一个自旋能隙到磁有序状态的演变往往与相分离和/或一级相变。
