Dipartimento di Fisica, Università di Roma 'La Sapienza', P. Aldo Moro 2, Roma 00185, Italy.
Nat Commun. 2011 Jul 19;2:398. doi: 10.1038/ncomms1407.
In several materials, unconventional superconductivity appears nearby a quantum phase transition where long-range magnetic order vanishes as a function of a control parameter like charge doping, pressure or magnetic field. The nature of the quantum phase transition is of key relevance, because continuous transitions are expected to favour superconductivity, due to strong fluctuations. Discontinuous transitions, on the other hand, are not expected to have a similar role. Here we determine the nature of the magnetic quantum phase transition, which occurs as a function of doping, in the iron-based superconductor LaFeAsO(1-x)F(x). We use constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The outcome is intriguing, as this material turns out to be remarkably close to a quantum tricritical point, where the transition changes from continuous to discontinuous, and several susceptibilities diverge simultaneously. We discuss the consequences for superconductivity and the phase diagram.
在几种材料中，非常规超导性出现在量子相变附近，随着电荷掺杂、压力或磁场等控制参数的变化，长程磁有序消失。量子相变的性质至关重要，因为由于强涨落，连续相变有望促进超导性。另一方面，不连续相变预计不会有类似的作用。在这里，我们确定了铁基超导体 LaFeAsO(1-x)F(x) 中掺杂函数下的磁量子相变的性质。我们使用约束密度泛函计算，为朗道序参量分析提供了从头算系数。结果令人着迷，因为这种材料非常接近于量子三临界点，在这个临界点处，相变从连续变为不连续，并且几个磁化率同时发散。我们讨论了对超导性和相图的影响。
