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

TL;DR

This study investigates the magnetic properties of Cu2Te2O5Br2 using high-field magnetization and NQR/NMR techniques, revealing persistent singlet fluctuations and complex interactions within the tetrahedral spin system.

Contribution

It provides new insights into the interplay of intra- and inter-tetrahedral interactions and the persistence of singlet fluctuations in a frustrated spin tetrahedral system.

Findings

Observation of field-induced level crossing effects.

Detection of a large effective singlet-triplet spin gap.

Evidence of competing and cooperating interactions in the system.

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.