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When a magnetic phase transition in a metal is driven to absolute zero temperature by a non-thermal control parameter, such as pressure or composition, the quantum fluctuations of the order parameter induce new universality classes and, possibly, new ground states of matter. Recent experiments suggest that near such a quantum phase transition in a heavy-fermion compound, notably CeCuAux, the heavy quasiparticles, formed by the Kondo effect below the Kondo temperature, disintegrate. The conditions for this breakdown have, however, remained obscure. This talk presents the direct investigation of quasiparticle formation and breakdown in CeCuAux by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the Kondo temperature exhibits an abrupt step near the quantum critical Au concentration of 10%. |
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