Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5Br2 revealed by high-field magnetization and 79Br NQR - 125Te NMR

Abstract

We present high-field magnetization and ${}^{79}$Br nuclear quadrupole resonance (NQR) and ${}^{125}$Te nuclear magnetic resonance (NMR) studies in the weakly coupled Cu${}^{2+}$ ($S=1/2$) tetrahedral system Cu${}_2$Te${}_2$O${}_5$Br${}_2$. The field-induced level crossing effects were observed by the magnetization measurements in a long-ranged magnetically ordered state which was confirmed by a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much larger than that observed by static bulk measurements. Our results imply that the inter- and the intra-tetrahedral interactions compete, but at the same time they cooperate strengthening effectively the local intratetrahedral exchange couplings. We discuss that the unusual feature originates from the frustrated intertetrahedral interactions.

arXiv comment5 pages, 4 figures, accepted in Phys. Rev. B as a Rapid Communications

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