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

Resistivity, specific heat and magnetic susceptibility measurements performed on SrFe2As2 samples evidence a behavior very similar to that observed in LaFeAsO and BaFe2As2 with the difference that the formation of the SDW and the lattice deformation occur in a pronounced first order transition at T0 = 205 K. Comparing further data evidences that the Fe-magnetism is stronger in SrFe2As2 and in EuFe2As2 than in the other layered FeAs systems investigated up to now. Full potential LDA band structure calculations confirm the large similarity between the compounds, especially for the relevant low energy Fe 3d states. The relation between structural details and magnetic order is analyzed.The discovery of superconductivity in doped LaFeAsO [1] and the subsequent raising of the superconducting (SC) transition temperature T c to 56 K [2, 3, 4] initiated a surge of interest in layered FeAs systems. Undoped RFeAsO compounds (R = La -Gd) present a structural transition at T 0 ∼ 150 K followed by the formation of a spin density wave (SDW) at a slightly lower temperature T N ∼ 140 K [5,6]. Electron or hole doping by substituting O by F [7], or R by a tetravalent or divalent cation [8,9], or by reducing the O-content [10] leads to the suppression of the SDW and to the onset of superconductivity. This connection between a vanishing magnetic transition and the simultaneous formation of a SC state is reminiscent of the behavior in the cuprates and in the heavy fermion systems, and therefore suggests the SC state in these doped RFeAsO systems to be of unconventional nature, too. While this has to be confirmed by further studies, there seems to be a general belief that the intriguing properties of these compounds are connected with very peculiar properties of the FeAs layers. The tetragonal ZrCuSiAs structure type in which these compounds crystallize [11] results in a square Fe lattice with As in the center of the square but being alternately shifted above and below the Fe-plane. It is well known that the ThCr 2 Si 2 structure type presents a very similar arrangement of the transition metal and p-element. Thus it was natural to look for appropriate candidates within the huge amount of compounds crystallizing in this structure type. In a very primitive approach, the RFeAsO compounds can be rationalized as a stack of alternating (Fe 2 As 2 ) 2− and (2+ layer by a layer with a single large atom A leads to the ThCr 2 Si 2 structure type. In order to keep the same electron counts as in the RFeAsO materials, A has to be a divalent atom. Therefore, appropriate candidates are A 2+ Fe 2 As 2 compounds. To the best of our knowledge, three of the possible candidates have already been investigated. M. Pfisterer and G. Nagorsen have re- * Electronic address: geibel@cpfs.mpg.de ported the synthesis, the structure as well as preliminary susceptibility data of SrFe 2 As 2 and BaFe 2 As 2 [12, 13]. From their susceptibility data they concluded the occurrence of a magnetic phase transition around 200 K and 130 K, respectively and suggested it to be ...