Effect of pressure and magnetic field on the superconducting state of a heavy fermion superconductor NpPd₅Al₂

We have carried out the electrical resistivity measurements under high pressures and high magnetic fields in NpPd₅Al₂ in order to investigate a quantum critical behavior and the stability of the superconducting phase. A T-linear dependence of the electrical resistivity is changed into the Fermi liquid relation of ρ ∼ AT² under the magnetic fields, and A value indicates a divergent feature around the upper critical field H_c2, indicating the existence of a field-tuned quantum critical point (QCP) around H_c2. With increasing pressure, the superconducting transition temperature T_sc decreases and becomes zero above 5.7 GPa, together with a decrease of the upper critical field, indicating that the electronic state of NpPd₅Al₂ is located in the vicinity of QCP at ambient pressure and deviates from QCP with increasing pressure. Furthermore, we have constructed the superconducting phase diagram at 3.71 GPa.