Quantum bicriticality in the heavy-fermion metamagnet YbAgGe

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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 TBCP 0.3 K at a magnetic field of mu₀ HBCP 4.5 T. In its vicinity, YbAgGe exhibits anomalous behavior that we attribute to the influence of a quantum bicritical point (QBCP), that is close in parameter space yet can be reached by tuning TBCP further to zero. Using high-resolution measurements of the magnetocaloric effect, we demonstrate that the magnetic Grueneisen parameter Gammaₕ indeed both changes sign and diverges as required for quantum criticality. Moreover, Gammaₕ displays a characteristic scaling behavior but only on the low-field side, H < HBCP, indicating a pronounced asymmetry with respect to the critical field. We speculate that the small value of TBCP is related to the geometric frustration of the Kondo-lattice of YbAgGe.