  We present the first lattice-QCD calculation of the unpolarized strange and
charm parton distribution functions using large-momentum effective theory
(LaMET). We use a lattice ensemble with 2+1+1 flavors of highly improved
staggered quarks (HISQ) generated by MILC collaboration, with lattice spacing
$a\approx 0.12$ fm and $M_\pi \approx 310$ MeV, and clover valence fermions
with two valence pion masses: 310 and 690 MeV. We use momentum-smeared sources
to improve the signal up to nucleon boost momentum $P_z=2.18$ GeV, and
determine nonperturbative renormalization factors in RI/MOM scheme. We compare
our lattice results with the matrix elements obtained from matching the PDFs
from CT18NNLO and NNPDF3.1NNLO global fits. Our data support the assumptions of
strange-antistrange and charm-anticharm symmetry that are commonly used in
global PDF fits, and we find smaller than expected parton distribution at mid
to small $x$.
