Iron-based superconductors: Critical relationships

Antiferromagnetic spin fluctuations may not be the only phenomenon that determines the critical temperature in iron-based superconductors.

The discovery of high-temperature superconductivity in iron-based compounds in 2008 sparked renewed interest in superconductors after many years of uphill progress in the field. Various types of experimental probes have since been used in attempts to gain insight into the mechanism responsible for the zero-resistance superconducting state in these materials, with the hope that any such insight will also provide essential information on superconductivity in other superconducting compounds, like the cuprates.

The critical temperature (T_c), or the temperature below which a superconductor loses all electrical resistance, is one of the fundamental phenomenological properties of superconductors, and it has been widely theorized that this temperature is primarily related to a transition in magnetic properties. Hiroaki Kinouchi and colleagues from Osaka University and the National Institute of Advanced Industrial Science and Technology in Japan, however, have now shown that T_c must also be dependent on other factors.¹

Fig. 1: Atomic configuration of the iron-based superconductor (Ca₄Al₂O_6–y)(Fe₂As₂)

In their study, the researchers used nuclear quadrupole resonance — a form of nuclear magnetic resonance that does not require an external magnetic field — to investigate the superconducting properties of the iron-based compound (Ca₄Al₂O_6–y)(Fe₂As₂). This compound has a T_c of 27 K and belongs to a series of iron-based compounds in which the layers containing iron are separated by large perovskite-structured blocking layers (see image). These materials can have relatively high critical temperatures of up to 40 K, but unlike most in this class do not exhibit any known structural or magnetic transitions.

The researchers’ measurements of nuclear relaxation as a function of temperature provided information on the symmetry of the ‘superconducting gap’, or the energy needed by superconducting electrons to revert to their non-superconducting state. The results show that while no antiferromagnetic order is present in the material either above or below the transition temperature, the material still exhibits strong antiferromagnetic spin fluctuations. Interestingly, in another well-studied compound with similar T_c, LaFeAsO_1–y, such antiferromagnetic spin fluctuations are much weaker despite strong antiferromagnetic order. Kinouchi and his colleagues therefore conclude that factors other than antiferromagnetism must be involved in determining the critical temperature in iron-based superconductors, suggesting instead that optimization of T_c may be more successful if approached from the perspective of the lattice structure.