We discuss non-Fermi liquid and quantum critical behavior in heavy fermion materials, focussing on the mechanism by which the electron mass appears to diverge at the quantum critical point. We ask whether the basic mechanism for the transformation involves electron diffraction off a quantum critical spin density wave, or whether a break-down in the composite nature of the heavy electron takes place at the quantum critical point. We show that the Hall constant changes continously in the first scenario, but may "jump" discontinuously at a quantum critical point where the composite character of the electron quasiparticles changes.

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder 1-5 that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study 6 on theheavy-electron material Y bRh 2 Si 2 found that near a field-induced quantum critical point electrons move ever more slowly and scatter off one-another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and cross-section. These studies could not shed light on whether these properties were an artifact of the applied field 7,8 , or a more general feature of field-free QCPs. Here we report that when Germanium-doped Y bRh 2 Si 2 is tuned away from a chemically induced quantum critical point by magnetic fields there is a universal behavior in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual 1 electrons decay into collective current carrying motions of the electron fluid.Recent work 6 on the heavy electron material YbRh 2 Si 2 9 has demonstrated that a magnetic field can be used to probe the heavy electron quantum critical point. This material exhibits a small antiferromagnetic (AFM) ordering temperature T N = 70 mK (Fig. 1a) that is driven to zero by a critical magnetic field B c = 0.66 T (if the field is applied parallel to the crystallographic c-axis, perpendicular to the easy magnetic plane) 6 . For 2 Past experience 7,8 suggested that a finite field quantum critical point has properties which are qualitatively different to a zero field transition, shedding doubt on the reliability of these measurements as an indicator of the physics of a quantum phase transition at zero field. However, the zero-field properties of YbRh 2 (Si 1−x Ge x ) 2 above T ≈ 70 mK for the undoped (x = 0) and doped (x = 0.05) crystals are essentially identical (Fig. 2a), suggesting that by suppressing the critical field we are still probing the same quantum critical point.In both compounds, the ac-susceptibility follows a temperature dependence χ −1 ∝ T α from 0.3 K to ≤ T ≤ 1.5 K, with α = 0.75 14 , and the coefficient of the electronic specific heat, C el (T )/T , exhibits 9 a logarithmic divergence between 0.3 K and 10 K. However, in the low-T paramagnetic regime, i. e. , T N < T < ∼ 0.3 K, the ac-susceptibility follows a CurieWeiss law (inset of Fig. 2a) with a Weiss temperature Θ W ≈ 0.3 K, and a surprisingly large effective moment µ eff ≈ 1.4µ B /Yb 3+ , indicating the emergence of coupled, unquenched spins at the quantum critical point. The electronic specific heat coefficient, C e...

No abstract