Quantum Bicriticality in the Heavy-Fermion Metamagnet YbAgGe

Bicritical points, at which two distinct symmetry-broken phases become simultaneously unstable, are typical for spin-flop metamagnetism. Interestingly, the heavy-fermion compound YbAgGe also possesses such a bicritical point (BCP) with a low temperature $T_{\mathrm{BCP}}\approx 0.3\mathrm{K}$ at a magnetic field of ${\mu }_0{H}_{\mathrm{BCP}}\approx 4.5\mathrm{T}$. In its vicinity, YbAgGe exhibits anomalous behavior that we attribute to the influence of a quantum bicritical point that is close in parameter space yet can be reached by tuning $T_{\mathrm{BCP}}$ further to zero. Using high-resolution measurements of the magnetocaloric effect, we demonstrate that the magnetic Grüneisen parameter ${\Gamma }_H$ indeed both changes sign and diverges as required for quantum criticality. Moreover, ${\Gamma }_H$ displays a characteristic scaling behavior but only on the low-field side $H\lesssim H_{\mathrm{BCP}}$, indicating a pronounced asymmetry with respect to the critical field. We speculate that the small value of $T_{\mathrm{BCP}}$ is related to the geometric frustration of the Kondo lattice of YbAgGe.