An ALMA survey of DCN/H$^{13}$CN and DCO$^+$/H$^{13}$CO$^+$ in
  protoplanetary disks
  The deuterium enrichment of molecules is sensitive to their formation
environment. Constraining patterns of deuterium chemistry in protoplanetary
disks is therefore useful for probing how material is inherited or reprocessed
throughout the stages of star and planet formation. We present ALMA
observations at $\sim0.6"$ resolution of DCO$^+$, H$^{13}$CO$^+$, DCN, and
H$^{13}$CN in the full disks around T Tauri stars AS 209 and IM Lup, the
transition disks around T Tauri stars V4046 Sgr and LkCa 15, and the full disks
around Herbig Ae stars MWC 480 and HD 163296. We also present ALMA observations
of HCN in the IM Lup disk. DCN, DCO$^+$, and H$^{13}$CO$^+$ are detected in all
disks, and H$^{13}$CN in all but the IM Lup disk. We find efficient deuterium
fractionation for the sample, with estimates of disk-averaged DCO$^+$/HCO$^+$
and DCN/HCN abundance ratios ranging from $\sim0.02-0.06$ and $\sim0.005-0.08$,
respectively, which is comparable to values reported for other ISM
environments. The relative distributions of DCN and DCO$^+$ vary between disks,
suggesting that multiple formation pathways may be needed to explain the
diverse emission morphologies. In addition, gaps and rings observed in both
H$^{13}$CO$^+$ and DCO$^+$ emission provide new evidence that DCO$^+$ bears a
complex relationship with the location of the midplane CO snowline.
