  Identifying the elusive spinon excitations in quantum spin liquid (QSL)
materials is what scientists have long sought for. Recently, thermal
conductivity ($\kappa$) has emerged to be a decisive probe because the
fermionic nature of spinons leads to a characteristic nonzero linear
$\kappa_0/T$ term while approaching zero Kelvin. So far, only a few systems
have been reported to exhibit such term. Here, we report a $\kappa_0/T \approx$
0.01 WK$^{-2}$m$^{-1}$, the largest $\kappa_0/T$ value ever observed in
magnetic oxide QSL candidates, in a new quantum magnet Pr$_2$Ga$_2$BeO$_7$ with
a Shastry-Sutherland lattice (SSL). Its QSL nature is further supported by the
power-law temperature dependence of the specific heat, a plateau of muon spin
relaxation rate, and gapless inelastic neutron spectra. Our theoretical
analysis reveals that the introduction of XY spin anisotropy is the key for
Pr$_2$Ga$_2$BeO$_7$ to be the first QSL realized on the SSL, after more than
four decades of extensive studies on this celebrated magnetically frustrated
lattice.
