Large positive correlation between the effective electron mass and the multipolar fluctuation in the heavy-fermion metal Ce1 − x La x B6

For the last few decades, researchers have been intrigued by multipolar ordering phenomena and related quantum phase transitions in heavy-fermion Kondo systems. However, a criticality induced by substitution level ( x ), temperature ( T ), or magnetic field ( B ) is poorly understood even in the prototypical material, Ce 1− x La x B 6 , despite a large collection of experimental results is available. In this work, we present T – B , x – T , and x – B phase diagrams of Ce 1− x La x B 6 ( B || [110]). These are completed by investigating heat capacity, magnetocaloric effect (MCE), and elastic neutron scattering. A drastic increase of the Sommerfeld coefficient γ 0 , which is estimated from the heat capacity down to 0.05 K, is observed with increasing x . The precise T – B phase diagram including a high-entropy region is derived from the MCE analysis in which a knowledge beyond the equilibrium thermodynamics is involved. Finally, the x – B phase diagram at T = 0, which supports the existence of a quantum critical point at x > 0.75, is obtained by the same analysis. A detailed interpretation of phase diagrams strongly indicates positive correlation between the fluctuating multipoles and the effective electron mass.