Disorder-induced broadening of the spin waves in the triangular-lattice quantum spin liquid candidate ${\mathrm{YbZnGaO}}_4$

Disorder is important in the study of quantum spin liquids, but its role on the spin dynamics remains elusive. Here, we explore the disorder effect by investigating the magnetic-field dependence of the low-energy magnetic excitations in a triangular-lattice frustrated magnet ${\mathrm{YbZnGaO}}_4$ with inelastic neutron scattering. With an intermediate field of 2.5 T applied along the $c$ axis, the broad continuum at zero field becomes more smeared both in energy and momentum. With a field up to 10 T, which fully polarizes the magnetic moments, we observe clear spin-wave excitations with a gap of $\sim 1.4\mathrm{meV}$ comparable to the bandwidth. However, the spectra are significantly broadened. The excitation spectra both at zero and high fields can be reproduced by performing classical Monte Carlo simulations which take into account the disorder effect arising from the random site mixing of nonmagnetic ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Ga}}^{3+}$ ions. These results elucidate the critical role of disorder in broadening the magnetic excitation spectra and mimicking the spin-liquid features in frustrated quantum magnets.

Figure 1

Magnetization data for a ${\mathrm{YbZnGaO}}_4$ single crystal. Magnetic-field dependence of the magnetization for a ${\mathrm{YbZnGaO}}_4$ single crystal at $T=2\mathrm{K}$ with a field applied parallel to the $c$ axis up to 14 T. The dashed line represents a linear fit to the data for ${\mu }_{0}H\ge 8\mathrm{T}$. The inset shows a photograph of coaligned nine pieces of single crystals glued on a copper plate.

Figure 2

Magnetic excitation spectra obtained by INS experiment under different magnetic field strengths at 50 mK. Magnetic dispersions along high-symmetry directions of ${\mathrm{M}}_1$-K-${\mathrm{\Gamma }}_1$ and ${\mathrm{\Gamma }}_1\text{−}{\mathrm{M}}_2\text{−}{\mathrm{\Gamma }}_2$ as illustrated by the arrows in (b) at 0-T (a), 2.5-T (d), and 10-T (g) fields. The dispersions are obtained by plotting together a series of constant-energy scans displayed in (c), (f), and (i), with an energy interval of 0.1 meV. Dashed lines in (a), (d), and (g) indicate constant-energy scans of 0.6, 0.6, and 2.4 meV, respectively. Solid lines in (c), (f), and (i) are guides to the eye. Contour plots of the INS spectra at $E=0.6\mathrm{meV}$ at 0 T (b), 2.5 T (e), and at $E=2.4\mathrm{meV}$ at 10 T (h). The contour maps are obtained by plotting together a series of constant-energy scans along the $[H,0,0]$ direction with a step of 0.1 r.l.u. and an interval of 0.1 r.l.u. along the $[0,K,0]$ direction. Black solid lines represent the Brillouin zone boundaries. Errors represent one standard deviation throughout the paper.

Figure 3

Results of Monte Carlo simulations. Calculated magnetic dispersions along two high-symmetry routes of ${\mathrm{M}}_1$-K-${\mathrm{\Gamma }}_1$ and ${\mathrm{\Gamma }}_1\text{−}{\mathrm{M}}_2\text{−}{\mathrm{\Gamma }}_2$ as illustrated by the arrows in (b) at 0-T (a) and 10-T (c) fields. Contour maps of the calculated spectra at $E$ = 0.6 meV at 0-T field (b) and $E=2.4\mathrm{meV}$ at 10-T field (d).