  The microscopic mechanism for electron pairing in heavy-fermion
superconductors remains a major challenge in quantum materials. Some form of
magnetic mediation is widely accepted with spin fluctuations as a prime
candidate. A novel mechanism, 'composite pairing' based on the cooperative
two-channel Kondo effect directly involving the f-electron moments has also
been proposed for some heavy fermion compounds including CeCoIn$_5$. The origin
of the spin resonance peak observed in neutron scattering measurements on
CeCoIn$_5$ is still controversial and the corresponding hump-dip structure in
the tunneling conductance is missing. This is in contrast to the cuprate and
Fe-based high-temperature superconductors, where both characteristic signatures
are observed, indicating spin fluctuations are likely involved in the pairing
process. Here, we report results from planar tunneling spectroscopy along three
major crystallographic orientations of CeCoIn5 over wide ranges of temperature
and magnetic field. The pairing gap opens at T$_p$ ~ 5 K, well above the bulk
T$_c$ = 2.3 K, and its directional dependence is consistent with d$_{x^2-y^2}$
symmetry. With increasing magnetic field, this pairing gap is suppressed as
expected but, intriguingly, a gaplike structure emerges smoothly, increasing
linearly up to the highest field applied. This field-induced gaplike feature is
only observed below T$_p$. The concomitant appearance of the pairing gap and
the field-induced gaplike feature, along with its linear increase with field,
indicates that the f-electron local moments are directly involved in the
pairing process in CeCoIn$_5$.
