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The search for quantum criticality near the convergence of hidden order and ferromagnetism


The properties of the heavy fermion superconductor URu\₂Si\₂ have been investigated as a function of Re substitution. Single crystals of URu\₂-xRexSi\₂, for 0 <\̲ x <\̲ 0.6, were synthesized via the Czochralski technique and studied through measurements of x-ray diffraction, electrical resistivity, specific heat, and dc magnetization. In accord with previous studies, it has been found that superconductivity is quickly suppressed, while the hidden order state persists up to about x = 0.1. Ferromagnetic behavior is observed at intermediate concentrations, for x > 0.15, and its properties are consistent with itinerant ferromagnetism. The magnetic ordering transitions, interpreted via a scaled Arrott analysis, are characterized by unconventional exponents, strongly suggesting proximity to a quantum critical point. It is argued that in this system, low-temperature divergence in the specific heat divided by temperature and power-law temperature-dependence of the electrical resistivity, both archetypal characteristics of non-Fermi liquid behavior, may not be related to the ferromagnetic quantum criticality. The x-dependence of these properties and the evolution of the heavy fermion state is discussed in relation to the ordered phases in the URu\₂-xRexSi\₂ phase diagram

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