  Generally, studies of the critical current Ic are necessary if
superconductors are to be of practical use because Ic sets the current limit
below which there is a zero-resistance state. Here, we report a peak in the
pressure dependence of the zero-field Ic, Ic(0), at a hidden quantum critical
point (QCP), where a continuous antiferromagnetic transition temperature is
suppressed by pressure toward 0 K in CeRhIn5 and 4.4% Sn-doped CeRhIn5. The
Ic(0)s of these Ce-based compounds under pressure exhibit a universal
temperature dependence, underlining that the peak in zero-field Ic(P) is
determined predominantly by critical fluctuations associated with the hidden
QCP. The dc conductivity is a minimum at the QCP, showing anti-correlation with
Ic(0). These discoveries demonstrate that a quantum critical point hidden
inside the superconducting phase in strongly correlated materials can be
exposed by the zero-field Ic, therefore providing a direct link between a QCP
and unconventional superconductivity.
