Muon spin relaxation studies of the frustrated quasi-two-dimensional square-lattice spin system Cu ( Cl , Br ) La ( Nb , Ta ) 2 O 7 : Evolution from spin-gap to antiferromagnetic state

摘要: We report muon spin relaxation $(\ensuremath{\mu}\text{SR})$ and magnetic-susceptibility measurements on $\text{Cu}(\text{Cl},\text{Br})\text{La}{(\text{Nb},\text{Ta})}_{2}{\text{O}}_{7}$, which demonstrate: (a) the absence of static magnetism in $(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ down to 15 mK confirming a spin-gapped ground state; (b) phase separation between partial volumes with spin-gap $(\text{CuCl})\text{La}{(\text{Nb},\text{Ta})}_{2}{\text{O}}_{7}$; (c) history-dependent magnetization (Nb,Ta) (Cl,Br) substitution systems; (d) uniform long-range collinear antiferromagnetic state $(\text{CuBr}){\text{LaNb}}_{2}{\text{O}}_{7}$; (e) decrease N\'eel temperature decreasing Br concentration $x$ $\text{Cu}({\text{Cl}}_{1\ensuremath{-}x}{\text{Br}}_{x}){\text{LaNb}}_{2}{\text{O}}_{7}$ no change ordered Cu moment size for $0.33\ensuremath{\le}x\ensuremath{\le}1$. Together several other $\ensuremath{\mu}\text{SR}$ studies quantum transitions geometrically frustrated systems, present results reveal that evolution from magnetically is often associated and/or first-order transition.