A Search for Gravitationally Lensed Quasars and Quasar Pairs in
  Pan-STARRS1: Spectroscopy and Sources of Shear in the Diamond 2M1134$-$2103
  We present results of a systematic search for gravitationally lensed quasars
in Pan-STARRS1. Our final sample of candidates comprises of 91 systems, not
including 25 rediscovered lensed quasars and quasar pairs. In the absence of
spectroscopy to verify the lensing nature of the candidates, the main sources
of contaminants are likely to be quasar pairs, which we consider to be a
byproduct of our work, and a smaller number of quasar$+$star associations.
Amongst the independently discovered quads is 2M1134$-$2103, for which we
obtained spectroscopy for the first time, finding a redshift of 2.77 for the
quasar. There is evidence for microlensing in at least one image. We perform
detailed mass modeling of this system using archival imaging data, and find
that the unusually large shear responsible for the diamond-like configuration
can be attributed mainly to a faint companion $\sim 4''$ away, and to a galaxy
group/cluster $\sim 30''$ away. We also set limits of $z\sim0.5-1.5$ on the
redshift of the lensing galaxy, based on its brightness, the image separation
of the lensed images, and an analysis of the observed photometric flux ratios.
