Anomalous frequency and intensity scaling of collective and local modes in a coupled-spin tetrahedral system ${\text{Cu}}_{2}{\text{Te}}_{2}{\text{O}}_{5}{\text{Cl}}_{2}$

Anomalous frequency and intensity scaling of collective and local modes in a coupled-spin tetrahedral system ${Cu}_{2} {Te}_{2} O_{5} {Cl}_{2}$

Kwang-Yong Choi, Hiroyuki Nojiri, Naresh S. Dalal, Helmuth Berger, Wolfram Brenig, and Peter Lemmens

Phys. Rev. B 79, 024416 – Published 14 January 2009

Abstract

We report on the magnetic excitation spectrum of the coupled-spin tetrahedral system ${Cu}_{2} {Te}_{2} O_{5} {Cl}_{2}$ using Raman scattering on single crystals. The transition to an ordered state at $T_{N}^{Cl} = 18.2 K$ evidenced from thermodynamic data leads to the evolution of distinct low-energy magnetic excitations superimposed by a broad maximum. These modes are ascribed to magnons with different degree of localization and a two-magnon continuum. Two of the modes develop a substantial energy shift with decreasing temperature similar to the order parameter of other Néel ordered systems. The other two modes show only a negligible temperature dependence and dissolve above the ordering temperature in a continuum of excitations at finite energies. These observations point to a delicate interplay of magnetic intertetrahedra and intratetrahedra degrees of freedoms as expected for a system in the proximity to quantum criticality.