Separation of Energy Scales in Undoped YbRh₂Si₂Under Hydrostatic Pressure

Abstract

The temperature-magnetic field (T-H) phase diagram of YbRh 2 Si 2 in the vicinity of its quantum critical point is investigated by low-T magnetization measurements. Our analysis reveals that the energy scale T ? ðHÞ, previously related to the Kondo breakdown and terminating at 0.06 T for T ! 0, remains unchanged under pressure, whereas the antiferromagnetic critical field increases from 0.06 T (P ¼ 0) to 0.29 T (P ¼ 1:28 GPa), resulting in a crossing of T N ðHÞ and T ? ðHÞ. Our results are very similar to those on Yb(Rh 1Àx Co x ) 2 Si 2 , proving that the Co-induced disorder can not be the reason for the detachment of both scales under chemical pressure.KEYWORDS: heavy fermion, quantum critical point DOI: 10.1143/JPSJ.78.123708 Heavy fermion (HF) metals, i.e., periodic lattices of certain f-elements, are ideally suited to study the interplay of competing interactions (Kondo-vs RKKY-interaction), which can lead to a continuous phase transition at zerotemperature, driven by pressure, doping or magnetic field. 1)In the approach of the quantum critical point (QCP) the magnetic order parameter fluctuations grow continuously in spatial and temporal dimensions, causing strong deviations from Landau's Fermi liquid (LFL) theory. Such non-Fermi liquid (NFL) states are characterized by a single-excitation energy scale, which vanishes at the QCP.2-4) This results, e.g., in a divergence of the Grüneisen ratio, À / =C, or magnetic Grüneisen parameter À mag / ðÀdM=dTÞ=C, (: volume thermal expansion, C: electronic specific heat, M: magnetization) for QCPs tuned by pressure and magnetic field, respectively. 5) Experiments on the HF metals CeCu) and YbRh 2 Si 2 8-10)are incompatible with the standard picture of an antiferromagnetic (AF) QCP, which describes a spin-densitywave transition. Unconventional quantum criticality, [11][12][13][14] which qualitatively differs from the predictions of the standard theory, may arise due to a destruction of Kondo screening, leading to a decomposition of the heavy quasiparticles into conduction electrons and local magnetic moments.In this letter, we focus on the clean stoichiometric HF metal YbRh 2 Si 2 , located very close to a QCP.15) Because of the very weak AF order below T N ¼ 0:07 K, a tiny variation of an external control parameter is sufficient to tune the system through the QCP. The AF order may be suppressed either by small amounts of Ge-, La-, or Ir-doping 8,16,17) or magnetic fields of H N % 0:06 T and % 0:7 T, applied perpendicular and parallel to the tetragonal c-axis, respectively. 18) At H > H N , heavy LFL behavior is found in the low-T electronic specific heat, magnetic susceptibility and electrical resistivity, with diverging coefficients in the approach of the critical field.19) Correspondingly, a stronger than logarithmic divergence of CðTÞ=T and linear Tdependence of the electrical resistivity has been found in the quantum critical regime.8) These NFL effects, together with the Grüneisen ratio divergences 20,21) could not be described within the itinerant theory for a...

Break Up of Heavy Fermions at an Antiferromagnetic Instability

“…This finding is confirmed by a study on pure YbRh 2 Si 2 under physical pressure. 35) Also no relation between the second transition at T L and the T (B) line is evident as only for Yb(Rh 0.93 Co 0.07 ) 2 Si 2 T L vanishes in proximity to B line rendering this an accidental coincidence. With B assigned to the Kondo breakdown, we infer from B N > B that the magnetism extends into the regime where the Kondo effect is operating.…”

Microscopic investigation of electronic inhomogeneity induced by substitutions in a quantum critical metalCeCoIn5

“…To understand non-universal NFL behaviors [2] in a unified way, dynamics of Kondo holes in a quantum critical system should be further investigated, since heterogeneous electronic states appear to be realized in a variety of correlated electron systems. For instance, it is interesting to ask what is the degree and influence of electronic heterogeneity in YbRh 2 Si 2 , particularly, given the nearly field-independent local to itinerant crossover in pressure tuned and Co-and Ir-doped YbRh 2 Si 2 [45,46]. Moreover, a dynamical electronic heterogeneity at a QCP accessed by an applied magnetic field was also revealed by NMR studies even in pure YbRh 2 Si 2 [47].…”

Critical quasiparticle theory applied to heavy fermion metals near an antiferromagnetic quantum phase transition

“…The origin of the T Ã ðHÞ line is not clear at present. Recent experiments on YRS under pressure (12) or YRS doped with Co or Ir (13) have shown that while the critical field for the magnetic transition is found to shift, the T Ã line stays unchanged. This suggests that the T Ã feature is only weakly or not at all tied to the AFM transition.…”