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Magnetic ordering and ergodicity in the Cu2Te2O5X2 family of frustrated quantum magnets

We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu2Te2O5X2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coupled Cu2+ spins and (ii) coupled tetrahedra within onedimensional chains, in both cases involving mean-field coupling to other chains. The results show that Cu2Te2O5X2 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 singlet-like excitations that contribute to finite specific heat at temperatures far below the singlet-triplet gap, but do not contribute to 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 Cu2+ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length ξ extends beyond the single tetrahedral cluster dimension, Cu2Te2O5X2 compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagomé and pyrochlore antiferromagnets.

spin glasses and other random magnets; intrinsic properties of magnetically ordered materials; critical-point effects; specific heats; short range order

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