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Superconductivity and Quantum Phase Transitions in Weak Itinerant Ferromagnets

T. R. Kirkpatrick·Thomas Vojta·D. Belitz·R. Narayanan
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Abstract

It is argued that the phase transition in low-T_c clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical endpoint. A semi-quantitative discussion is given in terms of recent experiments on MnSi and UGe_2. It is then shown that the critical temperature for spin-triplet, p-wave superconductivity mediated by spin fluctuations is generically much higher in a Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling of magnons to the longitudinal magnetic susceptibility. This qualitatively explains the phase diagram recently observed in UGe_2 and ZrZn_2.

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T. R. Kirkpatrick, Thomas Vojta, D. Belitz, R. Narayanan. 2001-08-27. Superconductivity and Quantum Phase Transitions in Weak Itinerant Ferromagnets. https://doi.org/10.1142/9789812777843_0017

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