Magnetic ordering and ergodicity of the spin system in theCu2Te2O5X2family of quantum magnets

Abstract

We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu 2 Te 2 O 5 X 2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of ͑i͒ isolated tetrahedra of four antiferromagnetically coupled Cu 2+ spins and ͑ii͒ coupled tetrahedra within one-dimensional chains, in both cases involving mean-field coupling to other chains. The results show that Cu 2 Te 2 O 5 X 2 compounds are true three-dimensional systems of coupled spins. Susceptibility results are consistent with the existence of a singlet-triplet gap, whereas specific heat analysis shows that the singlet-triplet gap is filled with dense singletlike excitations that contribute to finite specific heat at temperatures far below the singlet-triplet gap, but do not contribute to a magnetic response of the system. Furthermore, measured specific heat data show excessive entropy when compared to the numerical results based on a pure spin system, which we attribute to the presence of phonons. Though Cu 2+ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length extends beyond the single tetrahedral cluster dimension, Cu 2 Te 2 O 5 X 2 compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagomé and pyrochlore antiferromagnets.

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

“…The extremely narrow transition width (≪ T 0 ) above T 0 corroborates the 3D character of the magnetic order suggested in previous studies. 17,20,21 Surprisingly, T 0 identified in our study is considerably higher than 11.4 K in Ref. 9 and 10.5 K in Ref.…”

NMR and NQR study of the tetrahedral frustrated quantum spin systemCu2Te2O5Br2in its paramagnetic

“…The temperature dependence of the electron spin susceptibility of Cu 2 Te 2 O 5 Br 2 has been extensively studied by means of DC and AC susceptometry 1,2,13,14,16 . NMR measurements provide a way to probe the local electron spin susceptibility through hyperfine interactions with the advantage of being essentially insensitive to paramagnetic impurities.…”

“…Nevertheless, the results of Jensen et al and Jaglicić et al appear to favor a 3D over 1D nature of the magnetic transition [13][14][15] . It is likely that both the intratetrahedral (and thus the frustration), leading to a creation of spingaps, and the intertetrahedral interactions, inducing a magnetic long range order, are present and compete together.…”

Triplons, magnons, and spinons in a single quantum spin system:SeCuO3

“…Candidate spinon excitations also coexist with square-lattice antiferromagnetism [19], and many low-dimensional metal-organic systems offer the possibility of controlling this coexistence [20]. One structural route to the same phenomena is provided by the spin-tetrahedron material Cu 2 Te 2 O 5 X 2 (X = Cl, Br) [21], where the magnetic response is dominated by the nonordered spins [22], a situation anticipated in theory [23], and incommensurate magnetism [24] coexists with anomalous coupled-cluster excitations [25]; similar coupled-cluster physics has been pursued in a number of other materials [26][27][28]. Another is the recent discovery [29] of both spinons and magnons in very weakly coupled chains of two different types.…”