Observation of the dimer-singlet phase in the one-dimensional S = 1/2 Heisenberg antiferromagnet Cu(Ampy)ClBr (Ampy= C6H8N2 = 2-(Aminomethyl)pyridine)

Abstract: Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C6H8N2 = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, 1H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation (muSR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu2+ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu2+ spins as reflected by a Curie-Weiss temperature of about -9 K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the 1H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field muSR evidences lack of any static magnetism. We attribute these experimental results to the stabilization of a dimer-singlet phase in the presence of a next-near neighbor interaction and of a randomness in the exchange coupling driven by Cl/Br mixing.