Magnetic Raman scattering of the ordered tetrahedral spin-½ clusters in Cu ₂ Te ₂ O ₅ (Br _{1 − x} Cl _x ) ₂

Abstract

Abstract. -Raman light-scattering experiments in the antiferromagnetic phase of the Cu2Te2O5(Br1−xClx)2 compounds are analyzed in terms of a dimerized spin model for the tetrahedral Cu-clusters. It is shown that the longitudinal magnetic excitation in the pure Br system hybridizes with a localized singlet excitation due to the presence of a DzyaloshinskiiMoriya anisotropy term. The drastic change of the magnetic scattering intensities observed when a proportion of Br is replaced by Cl ions, is proposed to be caused by a change of the magnetic order parameter. Instead of being parallel/antiparallel with each other, the spins in the two pairs of spin-1 2 order perpendicular to each other, when the composition x is larger than about 0.25.The compounds Cu 2 Te 2 O 5 Br 2 and Cu 2 Te 2 O 5 Cl 2 are spin-1 2 Cu-systems with coupled spin tetrahedra. The tetragonal P 4 crystal structure of the two compounds and the susceptibility measurements are presented in ref. [1]. While the high-temperature susceptibility approaches that of uncoupled spins, at low temperatures the susceptibility is reduced and goes through a maximum at, respectively, 30 and 23 K. The maximum indicates that the 2 4 spin-1 2 states of the Cu-ions in the (distorted) tetrahedral clusters are dimerized so to create a singlet ground state separated from the excited states by a gap of about 40 K. Both the susceptibility and the heat capacity measurements clearly indicate a transition to an ordered phase at a temperature, 11.4 K in the Br and 18.2 K in the Cl system [2,3], well below that of the maximum. Some of the bulk properties and the Raman-scattering results of the Br compound have been analyzed successfully by Gros et al. [4] in terms of a dimerized model for the four Cu spins of one tetrahedron, as determined by the following Hamiltonian:J 1 and J 2 are both positive. Defining the ratio r = J 2 /J 1 then the energy differences between the singlet ground state |s 1 and the excited states are: 2(1 − r)J 1 to a singlet |s 2 , J 1 to a within the mean-field (MF) approximation, i.e., the order parameter is one where the spins on each of the pairs 1-2 and 3-4 are parallel, but antiparallel with respect to the other pair of spins. This ordering takes full advantage of the J 1 -interaction on the expense of the J 2 -interaction. In the model of Gros et al. [4] for the Br compound r = 0.66 is smaller than 1, and the system is close to quantum criticality with a coupling parameter J c = 0.85J 1 , only 13% larger than the critical value J (qc) c = 0.75J 1 . The model was investigated analytically neglecting the modifications due to higher lying levels. The high-temperature susceptibility is accounted for using J 1 = 47.7 K, the Néel temperature increases with an applied field, and the frequency of the longitudinal magnetic excitation in the ordered phase is estimated to be close to that observed by Raman spectroscopy. Here we shall present a complete MF analysis, which includes the calculation of the susceptibility and the bulk magnetization in the ordered ...

Incommensurate magnetic ordering inCu2Te2O5X2Zaharko1, Rønnow2, Mesot3 et al. 2006Phys. Rev. B321170View full textAdd to dashboardCite

“…2 The group is non-centrosymmetric and racemic twins were found to be present with the volume ratio 37(4):63 (4). No features which could explain the coexistence of wave vectors k and k ′ below T N were detected.…”

“…In these compounds the magnetic susceptibility reaches a maxima at T ∼ 25 K before dropping sharply at low temperatures, which was attributed to the presence of a singlet-triplet spin-gap. 2,3 Further evidence of spingapped behavior in the bromide is observed in Raman scattering, 3,4,5 which also reveals evidence of what is suggested to be a low energy longitudinal magnon. Fitting the susceptibility data to an isolated tetrahedral model with four nearest neighbour (J 1 ) and two next nearest neighbour (J 2 ) exchange interactions gives the coupling strengths J 1 =J 2 ∼ 43 K and 38.5 K for X = Br and Cl respectively.…”

“…4,5 The consequences of antisymmetric Dzyaloshinsky-Moriya (DM) interactions have been also analysed.…”

Neutron inelastic scattering investigation of the magnetic excitations inCu2Te2O5X2Crowe1, Majumdar2, Lees3 et al. 2005Phys. Rev. B

“…One can model the magnetic structure of Cu 2 Te 2 O 5 X 2 as isolated tetrahedra by neglecting the inter-tetrahedral coupling and considering only nearest neighbor (J 1 ) and next nearest neighbor(J 2 ) interactions, see figure 1. The ground state of this model consists either of two singlet dimers or a quadrumer singlet in-dral model for this system has since been investigated, with further theoretical treatments 2,5,6,7,8,9,10 looking at various inter-tetrahedral coupling configurations. Both Valenti et al 9 and Whangbo et al 10 argue that the intercluster super-superexchange paths Cu-X··X-Cu along the (110) direction and the (100) and (010) directions (denoted J a and J b respectively in figure 1) are significant.…”

Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5BrBaek1, Choi2, Berger3 et al. 2012Phys. Rev. B

“…8 The anomalous field dependence of the transition temperature 9,11 T 0 and the presence of a longitudinal magnon 3,12 coexisting with a gapped singlet-like mode manifests the dominance of quantum fluctuations. Chemical and hydrostatic pressure measurements suggest the closeness of the Br system to a QCP.…”