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  Singular Effects of Impurities near the Ferromagnetic Quantum-Critical  Point



A quantum-critical phenomenon has been one of the central issues in condensed matter physics for the last few decades. The typical case is a ferromagnetic (FM) transition at absolute zero (T = 0) which, for example, has been observed in a metallic ferromagnet MnSi under pressure. Figure A presents a schematic phase diagram around a ferromagnetic quantum-critical point (QCP) at r = 0 where r is a "disordering" parameter, the pressure for MnSi. In the paper, we obtained systematic theoretical results for the effect of a dilute concentration of magnetic impurities, i.e., Kondo effect near the QCP to which much attention had not been paid so far. From figure B, the Born scattering amplitude Γ(σ) of the conduction electron is seen to be singular due to the divergence of the magnetic susceptibility x at the QCP, so that an individual magnetic impurity necessarily scatters in many angular momentum channels. Figure C shows temperature-dependences of the quasiparticle's damping rate 1/τimp and the electrical resistivity ρimp on the basis of the two-loop perturbative renormalization group (RG) theory. In the regime I in figure A, the result of the coupling between the quantum fluctuation of the localized spin of the impurity and the dynamical spin fluctuations of the conduction electrons is to enhance the Kondo temperature. In the regime II, this enhancement decreases rapidly and an anomalous behavior of the resistivity results in due to the multichannel Kondo effect within a certain range of temperatures.


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