Discovery of a bi-critical point between antiferromagnetic and superconducting phases in pressurized single crystal Ca0.73La0.27FeAs2

TL;DR

This study discovers a bi-critical point in pressurized Ca0.73La0.27FeAs2 where antiferromagnetism abruptly vanishes and superconductivity appears simultaneously, challenging the conventional quantum critical point scenario.

Contribution

It reports the first observation of a bi-critical point in a high-temperature superconductor under pressure, revealing a sudden transition between antiferromagnetism and superconductivity.

Findings

Antiferromagnetism disappears abruptly at 2.88 GPa.

Superconductivity emerges simultaneously with the disappearance of antiferromagnetism.

Magnetic field suppresses superconductivity but not antiferromagnetism.

Abstract

One of the most strikingly universal features of the high temperature superconductors is that the superconducting phase emerges in the close proximity of the antiferromagnetic phase, and the interplay between these two phases poses a long standing challenge. It is commonly believed that,as the antiferromagnetic transition temperature is continuously suppressed to zero, there appears a quantum critical point, around which the existence of antiferromagnetic fluctuation is responsible for the development of the superconductivity. In contrast to this scenario, we report the discovery of a bi-critical point identified at 2.88 GPa and 26.02 K in the pressurized high quality single crystal Ca0.73La0.27FeAs2 by complementary in situ high pressure measurements. At the critical pressure, we find that the antiferromagnetism suddenly disappears and superconductivity simultaneously emerges at almost the same temperature, and that the external magnetic field suppresses the superconducting transition temperature but hardly affects the antiferromagnetic transition temperature.