  High-angular resolution imaging by ALMA has revealed the near-universality
and diversity of substructures in protoplanetary disks. However, disks around
M-type pre-main-sequence stars are still poorly sampled, despite the prevalence
of M-dwarfs in the galaxy. Here we present high-resolution (~50 mas, 8 au) ALMA
Band 6 observations of six disks around mid-M stars in Taurus. We detect dust
continuum emission in all six disks, 12CO in five disks, and 13CO line in two
disks. The size ratios between gas and dust disks range from 1.6 to 5.1. The
ratio of about 5 for 2M0436 and 2M0450 indicates efficient dust radial drift.
Four disks show rings and cavities and two disks are smooth. The cavity sizes
occupy a wide range: 60 au for 2M0412, and ~10 au for 2M0434, 2M0436 and
2M0508. Detailed visibility modeling indicates that small cavities of 1.7 and
5.7 au may hide in the two smooth disks 2M0450 and CIDA 12. We perform
radiative transfer fitting of the infrared SEDs to constrain the cavity sizes,
finding that micron-sized dust grains may have smaller cavities than millimeter
grains. Planet-disk interactions are the preferred explanation to produce the
large 60 au cavity, while other physics could be responsible for the three ~10
au cavities under current observations and theories. Currently, disks around
mid-to-late M stars in Taurus show a higher detection frequency of cavities
than earlier type stars, although a more complete sample is needed to evaluate
any dependence of substructure on stellar mass.
