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