Searching for Isotropic Stochastic Gravitational-Wave Background in the
  International Pulsar Timing Array Second Data Release
  We search for isotropic stochastic gravitational-wave background (SGWB) in
the International Pulsar Timing Array second data release. By modeling the SGWB
as a power-law, we find very strong Bayesian evidence for a common-spectrum
process, and further this process has scalar transverse (ST) correlations
allowed in general metric theory of gravity as the Bayes factor in favor of the
ST-correlated process versus the spatially uncorrelated common-spectrum process
is $30\pm 2$. The median and the $90\%$ equal-tail amplitudes of ST mode are
$\mathcal{A}_{\mathrm{ST}}= 1.29^{+0.51}_{-0.44} \times 10^{-15}$, or
equivalently the energy density parameter per logarithm frequency is
$\Omega_{\mathrm{GW}}^{\mathrm{ST}} = 2.31^{+2.19}_{-1.30} \times 10^{-9}$, at
frequency of 1/year. However, we do not find any statistically significant
evidence for the tensor transverse (TT) mode and then place the $95\%$ upper
limits as $\mathcal{A}_{\mathrm{TT}}< 3.95 \times 10^{-15}$, or equivalently
$\Omega_{\mathrm{GW}}^{\mathrm{TT}}< 2.16 \times 10^{-9}$, at frequency of
1/year.
