  A search for Lorentz- and CPT-violating signals in the double beta decay
spectrum of $^{136}$Xe has been performed using an exposure of 100 kg$\cdot$yr
with the EXO-200 detector. No significant evidence of the spectral modification
due to isotropic Lorentz-violation was found, and a two-sided limit of $-2.65
\times 10^{-5 } \; \textrm{GeV} < \mathring{a}^{(3)}_{\text{of}} < 7.60 \times
10^{-6} \; \textrm{GeV}$ (90% C.L.) is placed on the relevant coefficient
within the Standard-Model Extension (SME). This is the first experimental study
of the effect of the SME-defined oscillation-free and momentum-independent
neutrino coupling operator on the double beta decay process.
