Observation of a gapped phase in the one-dimensional $S = {\frac{1}{2}}$ Heisenberg antiferromagnetic chain Cu(Ampy)ClBr

TL;DR

This study characterizes Cu(Ampy)ClBr as a quasi-1D $S=1/2$ zigzag Heisenberg chain with moderate AFM interactions and no magnetic long-range order down to 0.06 K. A broad 1D-like susceptibility peak near 9 K and a hump in $C_{ ext{mag}}$ indicate short-range correlations, while evidence from $C_{ ext{mag}}$ and $^{1}$H NMR $1/T_1$ reveals a gapped excitation spectrum with gaps around a few kelvin. Muon spin relaxation shows persistent spin dynamics and two relaxation channels, consistent with diffusive spinon transport in a disordered chain; ESR and NMR support anisotropic Cu$^{2+}$ interactions and short-range correlations. The findings point to a disorder- or defect-induced gapped ground state, such as a dimer-singlet or pseudogap, rather than a plain gapless Luttinger liquid, and motivate further single-crystal and spectroscopic studies to unravel the microscopic Hamiltonian and spinon dynamics.

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= C$_6$H$_8$N$_2$ = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, $^1$H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation ($μ$SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic 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 Cu$^{2+}$ 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 $^1$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 $μ$SR evidences lack of any static magnetism.

Figures (12)

• Figure 1: Rietveld refinement of the powder x-ray diffraction pattern for Cu(Ampy)ClBr at 300 K. The red circles represent the observed (Y$_{obs}$) intensity, whereas the calculated (Y$_{cal}$) patterns and the difference (Y$_{obs}-$Y$_{cal}$) are shown in black and blue lines, respectively. The green tick marks denote the allowed Bragg positions.
• Figure 2: (a) Schematic diagram of Cu(Ampy)ClBr molecular unit with two positions of C(6) atom. Hydrogen atoms are omitted for clarity. Each unit is bridged to form an infinite chain-like structure. (b) Molecular structure of Cu(Ampy)ClBr. (c) View of the bridging network in Cu(Ampy)ClBr running along the crystallographic $b$ axis. (d) The possible exchange couplings between Cu ions forming zigzag spin-chain structure. The solid and dashed lines denote the nearest neighbor coupling and the next-nearest neighbor coupling respectively.
• Figure 3: (a) Temperature dependence of the static magnetic susceptibility $\chi(T)$ measured at $H=$ 100 Oe in field cooled (FC) (orange close symbols) and zero field cooled (ZFC) (brown open symbols) processes on a log-log scale. No bifurcation between ZFC and FC $\chi(T)$ in is seen down to 0.4 K. The solid blue line is the fit to Eq. (\ref{['eq1']}), as described in the text. (b) Left $y$-axis: Temperature dependence of the static magnetic susceptibility $\chi(T)$ measured at $H=$ 10 kOe. Right $y$-axis: The temperature dependence of inverse magnetic susceptibility data free from $\chi_{0}$. The Curie-Weiss fitting is shown by the solid blue line.
• Figure 4: (a) Field dependent isothermal magnetization ($M$vs$H$) curve for the first quadrant at various temperatures. (b) Isothermal magnetization($M$) vs$H$ curve at 0.4 K in the field range $-70$ kOe to 70 kOe. The absence of hysteresis indicates that there is no history dependence of magnetization in Cu(Ampy)ClBr. The solid line represents the fit obtained by combining the Brillouin function with a linear term.
• Figure 5: Temperature dependence of the real component of the ac-susceptibility $\chi^\prime$, measured at various frequencies. No frequency-dependent behavior can be observed in the whole temperature range.