  The Kepler observations indicate that many exoplanets are super-Earths, which
brings about a puzzle for the core-accretion scenario. Since observed
super-Earths are in the range of critical mass, they would accrete gas
efficiently and become gas giants. Theoretically, super-Earths are predicted to
be rare in the core-accretion framework. To resolve this contradiction, we
propose that the tidally-forced turbulent diffusion may affect the heat
transport inside the planet. Thermal feedback induced by turbulent diffusion is
investigated. We find that the tidally-forced turbulence would generate
pseudo-adiabatic regions within radiative zones, which pushes the
radiative-convective boundaries (RCBs) inwards. This would decrease the cooling
luminosity and enhance the Kelvin-Helmholtz (KH) timescale. For a given
lifetime of protoplanetary disks (PPDs), there exists a critical threshold for
the turbulent diffusivity, $\nu_{\rm critical}$. If $\nu_{\rm turb}>\nu_{\rm
critical} $, the KH timescale is longer than the disk lifetime and the planet
would become a super-Earth rather than a gas giant. We find that even a small
value of turbulent diffusion has influential effects on evolutions of
super-Earths. $\nu_{\rm critical}$ increases with the core mass. We further
ascertain that, within the minimum mass extrasolar nebula (MMEN), $\nu_{\rm
critical}$ increases with the semi-major axis. This may explain the feature
that super-Earths are common in inner PPD regions, while gas giants are common
in the outer PPD regions. The predicted envelope mass fraction (EMF) is not
fully consistent with observations. We discuss physical processes, such as late
core assembly and mass loss mechanisms, that may be operating during
super-Earth formation.
