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Register a new userSingular Effects of Impurities near the Ferromagnetic Quantum-Critical Point
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Systematic theoretical results for the effects of a dilute concentration of magnetic impurities on the thermodynamic and transport properties in the region around the quantum critical point of a ferromagnetic transition are obtained. In the quasi-classical regime, the dynamical spin fluctuations enhance the Kondo temperature. This energy scale decreases rapidly in the quantum fluctuation regime, where the properties are those of a line of critical points of the multichannel Kondo problem with the number of channels increasing as the critical point is approached, except at unattainably low temperatures where a single channel wins out.
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Heavy fermion systems, and other strongly correlated electron materials, often exhibit a competition between antiferromagnetic (AF) and singlet ground states. Using exact Quantum Monte Carlo (QMC) simulations, we examine the effect of impurities in the vicinity of such AF- singlet quantum critical points, through an appropriately defined impurity susceptibility, $chi_{imp}$. Our key finding is a connection, within a single calculational framework, between AF domains induced on the singlet side of the transition, and the behavior of the nuclear magnetic resonance (NMR) relaxation rate $1/T_1$. We show that local NMR measurements provide a diagnostic for the location of the QCP which agrees remarkably well with the vanishing of the AF order parameter and large values of $chi_{imp}$. We connect our results with experiments on Cd-doped CeCoIn$_5$.
Using high frequency (up to 450 GHz) ESR and low temperature specific heat measurements we find that insertion of 1% Fe and 2% Co damps spin-Peierls and Neel transitions and for T<30K gives rise to onset of a quantum critical behaviour characteristic for a random dimer Griffiths phase.
We address the quantum-critical behavior of a two-dimensional itinerant ferromagnetic systems described by a spin-fermion model in which fermions interact with close to critical bosonic modes. We consider Heisenberg ferromagnets, Ising ferromagnets, and the Ising nematic transition. Mean-field theory close to the quantum critical point predicts a superconducting gap with spin-triplet symmetry for the ferromagnetic systems and a singlet gap for the nematic scenario. Studying fluctuations in this ordered phase using a nonlinear sigma model, we find that these fluctuations are not suppressed by any small parameter. As a result, we find that a superconducting quasi-long-range order is still possible in the Ising-like models but long-range order is destroyed in Heisenberg ferromagnets.
In metals near a quantum critical point, the electrical resistance is thought to be determined by the lifetime of the carriers of current, rather than the scattering from defects. The observation of $T$-linear resistivity suggests that the lifetime only depends on temperature, implying the vanishing of an intrinsic energy scale and the presence of a quantum critical point. Our data suggest that this concept extends to the magnetic field dependence of the resistivity in the unconventional superconductor BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ near its quantum critical point. We find that the lifetime depends on magnetic field in the same way as it depends on temperature, scaled by the ratio of two fundamental constants $mu_B/k_B$. These measurements imply that high magnetic fields probe the same quantum dynamics that give rise to the $T$-linear resistivity, revealing a novel kind of magnetoresistance that does not depend on details of the Fermi surface, but rather on the balance of thermal and magnetic energy scales. This opens new opportunities for the investigation of transport near a quantum critical point by using magnetic fields to couple selectively to charge, spin and spatial anisotropies.
Strange metal behavior is ubiquitous to correlated materials ranging from cuprate superconductors to bilayer graphene. There is increasing recognition that it arises from physics beyond the quantum fluctuations of a Landau order parameter which, in quantum critical heavy fermion antiferromagnets, may be realized as critical Kondo entanglement of spin and charge. The dynamics of the associated electronic delocalization transition could be ideally probed by optical conductivity, but experiments in the corresponding frequency and temperature ranges have remained elusive. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh$_2$Si$_2$, a model strange metal compound. We observe frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery implicates critical charge fluctuations as playing a central role in the strange metal behavior, thereby elucidating one of the longstanding mysteries of correlated quantum matter.
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