Exploring Reionization-Era Quasars IV: Discovery of Six New $z \gtrsim
  6.5$ Quasars with DES, VHS and unWISE Photometry
  This is the fourth paper in a series of publications aiming at discovering
quasars at the epoch of reionization. In this paper, we expand our search for
$z\sim 7$ quasars to the footprint of the Dark Energy Survey (DES) Data Release
One (DR1), covering $\sim 5000$ deg$^2$ of new area. We select $z\sim 7$ quasar
candidates using deep optical, near-infrared (near-IR) and mid-IR photometric
data from the DES DR1, the VISTA Hemisphere Survey (VHS), the VISTA Kilo-degree
Infrared Galaxy (VIKING) survey, the UKIRT InfraRed Deep Sky Surveys -- Large
Area Survey (ULAS) and the unblurred coadds from the Wide-field Infrared Survey
Explore ($WISE$) images (unWISE). The inclusion of DES and unWISE photometry
allows the search to reach $\sim$ 1 magnitude fainter, comparing to our $z
\gtrsim 6.5$ quasar survey in the northern sky (Wang et al. 2018). We report
the initial discovery and spectroscopic confirmation of six new luminous
quasars at $z>6.4$, including an object at $z=7.02$, the fourth quasar yet
known at $z>7$, from a small fraction of candidates observed thus far. Based on
the recent measurement of $z \sim 6.7 $ quasar luminosity function using the
quasar sample from our survey in the northern sky, we estimate that there will
be $\gtrsim$ 55 quasars at $z > 6.5$ at $M_{1450} < -24.5$ in the full DES
footprint.
