  We present the observational results from the 43-GHz Very Long Baseline Array
(VLBA) observations of 124 compact radio-loud active galactic nuclei (AGNs)
that were conducted between 2014 November and 2016 May. The typical dimensions
of the restoring beam in each image are about 0.5 mas $\times$ 0.2 mas. The
highest resolution of 0.2 mas corresponds to a physical size of 0.02 pc for the
lowest redshift source in the sample. The 43-GHz very long baseline
interferometry (VLBI) images of 97 AGNs are presented for the first time. We
study the source compactness on milli-arcsec (mas) and sub-mas scales, and
suggest that 95 sources in our sample are suitable for future space VLBI
observations. By analyzing our data supplemented with other VLBA AGN surveys
from literature, we find that the core brightness temperature increases with
increasing frequency below a break frequency ~ 7 GHz, and decreases between
~7--240~GHz but increases again above~240 GHz in the rest frame of the sources.
This indicates that the synchrotron opacity changes from optically thick to
thin. We also find a strong statistical correlation between radio and
$\gamma$-ray flux densities. Our correlation is tighter than those in
literature derived from lower-frequency VLBI data, suggesting that the
$\gamma$-ray emission is produced more co-spatially with the 43-GHz VLBA core
emission. This correlation can also be extrapolated to the un-beamed AGN
population, implying that a universal $\gamma$-ray production mechanism might
be at work for all types of AGNs.
