We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh2Si2. The data provide evidence for an energy scale in the equilibrium excitation spectrum that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.

A continuous phase transition driven to zero temperature by a non-thermal parameter, such as pressure, terminates in a quantum critical point (QCP). At present, two main theoretical approaches are available for antiferromagnetic QCPs in heavyfermion systems. The conventional one is the quantum generalization of finite-temperature phase transitions, which reproduces the physical properties in many cases 1-5 . More recent unconventional models incorporate a breakdown of the Kondo effect, giving rise to a Fermi-surface reconstruction 6-8 -YbRh 2 Si 2 is a prototype of this category 5,9-11 . In YbRh 2 Si 2 , the antiferromagnetic transition temperature merges with the Kondo breakdown at the QCP. Here, we study the evolution of the quantum criticality in YbRh 2 Si 2 under chemical pressure. Surprisingly, for positive pressure we find the signature of the Kondo breakdown within the magnetically ordered phase, whereas negative pressure induces their separation, leaving an intermediate spin-liquid-type ground state over an extended range. This behaviour suggests a new quantum phase arising from the interplay of the Kondo breakdown and the antiferromagnetic QCP.In heavy-fermion systems, the Kondo effect leads to the formation of composite quasiparticles of the f and conductionelectron states with largely renormalized masses forming a Landau Fermi-liquid ground state in the paramagnetic regime well below the Kondo temperature T K . These quasiparticles are assumed to stay intact at the quantum critical point (QCP) in the conventional models in which magnetic order arises through a spin-densitywave (SDW) instability. However, the observation of magnetic correlations in CeCu 5.9 Au 0.1 being of local character 11 prompted a series of theoretical descriptions that discard this basic assumption. Rather, they focus on the breakdown of the Kondo effect, which causes the f states to become localized and decoupled from the conduction-band states at the QCP where one expects the Fermi surface to be reconstructed 7 . Consequently, a new energy scale T is predicted reflecting the finite-temperature T crossover of the Fermi-surface volume. This picture has been scrutinized in tetragonal YbRh 2 Si 2 (T K ≈ 25 K; ref. 12), a stoichiometric and very clean heavy-fermion metal that seems to be ideally suited for this kind of study 9,12 : antiferromagnetic order sets in at a very low temperature T N = 0.07 K and can easily be suppressed by a small magnetic field of µ 0 H N = 60 mT (H ⊥ c, with c being the magnetically hard axis). Hall-effect experiments 13 have detected a rapid change of the Hall coefficient along a line T (H ) that converges with H N , the width of the Hall crossover extrapolating LETTERS NATURE PHYSICS

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