Abstract
An appropriate description of the state of matter that appears as a second order phase transition is tuned toward zero temperature, viz. quantum-critical point (QCP), poses fundamental and still not fully answered questions. Experiments are needed both to test basic conclusions and to guide further refinement of theoretical models. Here, charge and entropy transport properties as well as AC specific heat of the heavy-fermion compound CeRh0.58Ir0.42In5, measured as a function of pressure, reveal two qualitatively different QCPs in a single material driven by a single non-symmetry-breaking tuning parameter. A discontinuous sign-change jump in thermopower suggests an unconventional QCP at pc1 accompanied by an abrupt Fermi-surface reconstruction that is followed by a conventional spin-density-wave critical point at pc2 across which the Fermi surface evolves smoothly to a heavy Fermi-liquid state. These experiments are consistent with some theoretical predictions, including the sequence of critical points and the temperature dependence of the thermopower in their vicinity.
Highlights
Heavy-fermion metals have emerged as prototypes for discovering quantum-critical states[1,2] that are of broad interest as they are believed to be the origin of non-Fermi-liquid (NFL) and unconventional superconducting (SC) phases in classes of strongly correlated electron materials, ranging from organics to metallic oxides
A model of Kondo-breakdown and SDW quantum-critical point (QCP) anticipates the behaviors we find around pc[1] and pc2.39 This theory predicts a strong increase in S/T as T goes to zero following an a − bT0.5 law and that this increase is symmetric about a SDW QCP as it is approached from AFM and paramagnetic states, just as we find at pc[2] (Fig. 4a,g)
Agreement between experiment and theory at both pc[1] and pc[2] is appealing and evidence that the two critical points are likely different in nature, pc[1] being a Kondo-breakdown QCP and pc[2] a SDW QCP. These results provide an example where two qualitatively different QCPs appear to be realized in a single material driven by a single “clean" tuning parameter that does not break symmetry or induce spin-polarization
Summary
Heavy-fermion metals have emerged as prototypes for discovering quantum-critical states[1,2] that are of broad interest as they are believed to be the origin of non-Fermi-liquid (NFL) and unconventional superconducting (SC) phases in classes of strongly correlated electron materials, ranging from organics to metallic oxides. A QCP is an end point at absolute zero temperature of a continuous transition that separates ordered and disordered phases and is accessed by a non-thermal control parameter g, such as chemical doping (x), pressure (p) and magnetic field (B).[3,4] The conventional model of quantum criticality is based on a quantum extension of the Landau–Ginzburg–Wilson theory of classical, thermally-driven phase transitions and considers only fluctuations of a spindensity-wave (SDW) order parameter.[4] In this model, which does not treat electronic degrees of freedom as part of the critical excitations, the Fermi surface (FS) evolves smoothly as a function of g across the QCP.[5,6] Though this model provides a reasonable account of physical properties in some systems near a QCP,[4] it fails fundamentally to describe critical responses in other metallic systems in which there is accumulating evidence for unconventional quantum criticality, most notably in heavy-fermion compounds.[7,8,9,10,11,12,13] Alternatives to the conventional model, frequently called local, selective Mott or Kondo-breakdown theories, invoke criticality of electronic degrees of freedom that may be concurrent with magnetic criticality,[1,2,14,15] and the QCP is accompanied by a sharp reconstruction of the FS. It is important for experiments to both test their basic conclusions, such as the evolution of the FS across the QCP, and to guide their further development
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