Unconventional Quantum Criticality in β-YbAlB4 Detached from Its Magnetically Ordered Phase

We studied the effect of pressure on the Yb-based heavy-fermion superconductor β-YbAlB4 (Tc=80 mK) near its quantum critical point (QCP) at ambient pressure. Here, the electrical resistivity ρ(T) of the high-quality single crystal (RRR = 300) is measured under hydrostatic pressure P ≃ 2.7 GPa at low temperatures down to 70 mK. The superconducting transition Tc gradually decreases with the application of pressure and disappears near 0.9 GPa. However, the non-Fermi liquid (NFL) behavior is found to be robust over a broad range of pressure 0 ≤ P ≤ 0.5 GPa. The NFL state above Tc gradually changes to the Fermi liquid state with the application of pressure above P ≃ 0.5 GPa. The magnetic order transition is first observed at 50 mK and Pc = 2.7 GPa. The P –T phase diagram indicates that the QCP lying beneath the superconductivity dome is detached from the magnetically ordered phase.