Observation of a gapped phase in the one-dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>S</mml:mi> <mml:mo>=</mml:mo> <mml:mfrac> <mml:mn>1</mml:mn> <mml:mn>2</mml:mn> </mml:mfrac> </mml:math> Heisenberg antiferromagnet

Spin-$\frac{1}{2}$ 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-$\frac{1}{2}$ compound, $\mathrm{Cu}\text{(Ampy)ClBr}\phantom{\rule{4.pt}{0ex}}(\text{Ampy}={\mathrm{C}}_{6}{\mathrm{H}}_{8}{\mathrm{N}}_{2}=2$-(aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, $^{1}\mathrm{H}$ nuclear magnetic resonance (NMR), electron spin resonance, and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic ${\mathrm{Cu}}^{2+}$ 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 ${\mathrm{Cu}}^{2+}$ spins as reflected by a Curie-Weiss temperature of about $\ensuremath{-}9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ 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 $^{1}\mathrm{H}$ 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 $\ensuremath{\mu}\mathrm{SR}$ evidences lack of any static magnetism.