A Quantum Critical Point Lying Beneath the Superconducting Dome in Iron Pnictides

Abstract

Whether a quantum critical point (QCP) lies beneath the superconducting dome has been a long-standing issue that remains unresolved in many classes of unconventional superconductors, notably cuprates, heavy fermion compounds and most recently iron-pnictides. The existence of a QCP may offer a route to understand: the origin of their anomalous non-Fermi liquid properties, the microscopic coexistence between unconventional superconductivity and magnetic or some exotic order, and ultimately the mechanism of superconductivity itself. The isovalent substituted iron-pnictide BaFe2(As1−xPx)2 offers a new platform for the study of quantum criticality, providing a unique opportunity to study the evolution of the electronic properties in a wide range of the phase diagram. Recent experiments in BaFe2(As1−xPx)2 have provided the first clear and unambiguous evidence of a second order quantum phase transition lying beneath the superconducting dome.

Disorder-induced topological change of the superconducting gap structure in iron pnictides

“…17). This system is particularly suitable for the study of the impurity effect on gap structure, because several experiments have indicated that the pristine crystals are very clean and exhibit nodes in the superconducting gap 18 . To detect changes in the low-energy quasiparticle excitations, we measure the magnetic penetration depth l at low temperatures, a fundamental property of superconductors whose T-dependence is directly related to the excited quasiparticles.…”

“…Single crystals of BaFe 2 (As 1 À x P x ) 2 were grown by the self-flux method 17 and characterized by several techniques as reported previously 18 . The observation of the quantum oscillations in this series of crystals and the sharp superconducting transition indicate the very high quality of our pristine crystals.…”

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…6(a), 6(b) and 6(c)] (Inosov et al, 2013;Kim et al, 2010;Marty et al, 2011), isoelectronic substitution by replacing As with P [ Fig. 6(d)] (Jiang et al, 2009;Shibauchi et al, 2014) or Fe with Ru [ Fig. 6(e)] in BaFe 2 As 2 can induce superconductivity.…”

Impact of Disorder on the Superconducting Phase Diagram in BaFe2(As1−xPx)2

“…6) Among the iron pnictides, Phosphorus(P)-substituted BaFe 2 As 2 is a particularly clean system, and moreover, is unique in the fact that there is growing evidence for the existence of a QCP inside the superconducting dome near the optimal composition. [8][9][10][11][12][13] Although a QCP located at the maximum T c naturally leads to the consideration that the quantum-critical fluctuations help to enhance superconductivity, there has been no direct evidence against a scenario that it is just a coincidence. A direct test to address this issue is to investigate how the superconducting dome traces when the AFM phase is shifted.…”

“…In the phase diagram of the pristine sample, the optimal P concentration coincides with the extrapolated end point of the AFM phase, where the AFM QCP is considered to be located. 8) Here we make the phase diagram for the 2.0 C/cm 2 case by interpolating the data points linearly in the dose dependence of T N and T c . The monotonic decrease of T N for each composition leads to a shift of the AFM phase.…”