Evidence for an Unconventional Magnetic Instability in the Spin-Tetrahedra SystemCu2Te2O5

Abstract

Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu2Te2O5X2, X=Cl and Br. While the phase transition observed in the Cl system at To=18.2 K is consistent with 3D AF ordering, the phase transition at To=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets. 75.40.Gb, 75.40.Cx, 75.10.Jm, Reduced dimensionality of a quantum spin system in combination with frustration leads in many cases to unconventional and interesting ground states or magnetic phase diagrams. Prominent examples are the frustrated and dimerized spin-1/2 chain, represented by the lowtemperature phase of CuGeO 3 , or the two-dimensional Shastry-Sutherland lattice with orthogonally arranged spin dimers and a frustrating inter-dimer coupling, realized in SrCu 2 (BO 3 ) 2 . These systems show a spin liquid ground state with a singlet-triplet gap. Frustration is evident in the latter system as dispersionless elementary triplets and multi-particle bound states of triplet and singlet character [1][2][3].Spin triangles and tetrahedra that are strongly coupled into Kagomé or pyrochlore structures are at the origin of another important class of frustrated spin systems [4,5]. Although the consequences of the classical ground-state degeneracy for the quantum case have not been fully elucidated theoretically, there are good reasons to believe that such models do not possess magnetic long-range order (LRO) but low-lying singlets [6,7]. Scenarios leading to the development of LRO within this non-magnetic manifold have been put forward [8,9].The limit of weakly-coupled tetrahedra with spin S=1/2 has been studied in some detail in 3D and 1D, and the physics is expected to be very interesting [10][11][12][13]. However, they have not been investigated experimentally so far due to the lack of appropriate materials. The recently found spin system Cu 2 Te 2 O 5 X 2 , with X=Cl, Br, contains tetrahedral clusters of Cu 2+ with S = 1/2 in a distorted square planar CuO 3 X-coordination [14]. These tetrahedra align to tubes or chains along the [001] direction, as they are separated along [100] and [010] direction by different Te-O coordinations (see Fig. 1a). Substituting Br for Cl in this system widens up the unit cell and increases its volume from 367 to 391Å 3 , by 7%, while bond angles or anisotropies do not change essentially. Therefore, this system allows in a unique way to study the interplay of frustration and coupling in a tetrahedra quantum spin system. Preliminary measurements of the magnetic susceptibility χ(T) of both compounds showed a maximum at T χmax = 23 K and 30 K for X=Cl, Br and a strong reduction at low temperatures, typical for a spin gap system. Assuming that the compounds consist of weakly-coupled units of 4 spins with couplings J 1 and J 2 (see Fig. 1b), the best fit of the susceptibility -quite a good one actually -was obtained for J 1 = J 2 = 38.5 K ...

Magnetic Raman scattering of the ordered tetrahedral spin-½ clusters in Cu 2 Te 2 O 5 (Br 1 − x Cl x ) 2Jensen1, Lemmens2, Gros3 2003Europhys. Lett.291390View full textAdd to dashboardCite

“…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: (2) 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.…”

“…It is remarkable that T N increases with the field. The Raman spectra of the Cu 2 Te 2 O 5 (Br 1−x Cl x ) 2 compounds have been studied in detail [2,3,4]. The longitudinal magnetic excitation at zero wave vector is observed in the Br system at 2.1 K, when the incoming and scattered light are polarized along the c axis.…”

Structural and magnetic dimers in the spin-gapped systemCuTe2O5

“…2, 3 For example, it was proposed that Cu 2 Te 2 O 5 X 2 (X=Cl, Br), which consists of tetrahedral clusters of Cu 2+ linked by bridging Te-O units, is an example of quasi-zerodimensional systems in which the extreme limits of magnetic insulation via Te 4+ ions are reached. 4 The transition from a spin-gapped paramagnetic state to an antiferromagnetically ordered state at T N = 18.2 K (X=Cl) and T N = 11.4 K (X=Br) was attributed to the proximity of a quantum phase transition. 5 The magnetization, specific heat and Raman scattering data of Cu 2 Te 2 O 5 Br 2 were also interpreted by considering weakly coupled Cu 4 tetrahedra within a mean field approximation.…”

Anomalous spin dynamics in the coupled spin tetramer systemCuSeO3

“…The ground state of isolated antiferromagnetic spin clusters is given by either a spin singlet or a spin triplet state 1,2 . Applying an external field or pressure, on the one hand, can induce long-range magnetic order [3][4][5][6][7][8][9][10][11][12][13] . On the other hand, intercluster interactions can render the discrete energy level of an isolated cluster to form an energy band.…”