  Quantum spin liquids (QSLs) are an exotic state of matter that is subject to
extensive research. However, the relationship between the ubiquitous disorder
and the QSL behaviors is still unclear. Here, by performing comparative
experimental studies on two kagom\'{e}-lattice QSL candidates,
Tm$_3$Sb$_3$Zn$_2$O$_{14}$ and Tm$_3$Sb$_3$Mg$_2$O$_{14}$, which are
isostructural to each other but with strong and weak structural disorder,
respectively, we show unambiguously that the disorder can induce
spin-liquid-like features. In particular, both compounds show dominant
antiferromagnetic interactions with a Curie-Weiss temperature of -17.4 and
-28.7 K for Tm$_3$Sb$_3$Zn$_2$O$_{14}$ and Tm$_3$Sb$_3$Mg$_2$O$_{14}$,
respectively, but remain disordered down to about 0.05 K. Specific heat results
suggest the presence of gapless magnetic excitations characterized by a
residual linear term. Magnetic excitation spectra obtained by inelastic neutron
scattering (INS) at low temperatures display broad continua. All these
observations are consistent with those of a QSL. However, we find in
Tm$_3$Sb$_3$Zn$_2$O$_{14}$ which has strong disorder resulting from the random
mixing of the magnetic Tm$^{3+}$ and nonmagnetic Zn$^{2+}$, that the low-energy
magnetic excitations observed in the specific heat and INS measurements are
substantially enhanced, compared to those of Tm$_3$Sb$_3$Mg$_2$O$_{14}$ which
has much less disorder. We believe that the effective spins of the Tm$^{3+}$
ions in the Zn$^{2+}$/Mg$^{2+}$ sites give rise to the low-energy magnetic
excitations, and the amount of the random occupancy determines the excitation
strength. These results provide direct evidence of the mimicry of a QSL caused
by disorder.
