The understanding of the mechanism underlying the so-called unconventional high temperature superconductivity is one of the most difficult scientific challenges. Today, the most consensual approach suggests that the pairing of Cooper pairs originates from magnetic fluctuations of neighboring phases. However, the exact mechanism remains to be elucidated. In order to meet this challenge, the study of materials of reduced dimension and simple structure is an essential first step because it allows a simpler theoretical modeling. Researchers from the Laboratoire de Physique des Solides, in collaboration with colleagues from the Léon Brillouin Laboratory, the Institut Laue Langevin, the Institut Néel and the Condensed State Physics Department have taken a step forward in this direction by studying the magnetic order at the boundary of the superconducting phase of the spin scales of BaFe2X3 (X=Se, S) pnictures.
The physics of superconductors has been revolutionized by the discovery of superconducting cuprates in the mid-80s. Indeed, the record critical temperatures obtained on these compounds required the development of a new theoretical model. Cooper pairs, the elementary building block of superconductivity, remain conserved but the pairing mechanism of these electrons can no longer be based on the electron-phonon coupling of the standard BCS theory. In this new model, magnetism seems to play a central role. Indeed, all these two-dimensional compounds develop superconductivity in the vicinity of a magnetic phase. A second revolution took place in 2006, with the discovery of superconductivity in iron-based compounds: the copper age gave way to the iron age. The latter compounds show very strong similarities with their cuprate cousins: a two-dimensional structure, a magnetic phase in the vicinity of superconductivity and access to the superconducting phase by charge doping. It is confirmed: magnetism is essential to obtain superconductivity at high temperature.
A new twist occurs in 2015, superconductivity is discovered under pressure in two families of quasi-one-dimensional iron-based compounds: BaFe2S3 and BaFe2Se3. While the superconducting phase appears to have similar behavior in these two parent compounds, the magnetic phase, and in particular the symmetries of the magnetic order, differs strongly at least at room pressure. In both compounds, the iron atoms form ladders. In the sulfur compound, one observes a stripe-like magnetic structure with ferromagnetically aligned spins on each rungs of the ladder, and which change direction from one rung to another (center figure). The magnetic propagation vector is k=(1/2,0,1/2). In the selenium compound, the magnetic moments form blocks of 4 ferromagnetically coupled spins with alternation of the spin direction from one block to another along the scale (k=(1/2,1/2,1/2)). This means that the magnetic fluctuations potentially responsible for the pairing of Cooper pairs are different from one compound to another with a very similar superconducting phase. Our pressure neutron diffraction measurements at low temperature (3 K) on a BaFe2Se3 powder have resolved this dilemma. Indeed, we show that in BaFe2se3, the bloc-like magnetic phase disappears around 3 GPa to become the same stripe order as in BaFe2S3. This universal phase remains at least until 8 GPa, in the vicinity of the superconducting phase. The magnetic fluctuations associated with the stripe structure thus seem to be a prerequisite for the stabilization of the superconducting phase.
Figure. (from left to right) Neutron diffraction result on BaFe2Se3 powder at low temperature for different pressures; magnetic structure at low (bottom) and high (top) pressures; (P,T) phase diagram of BaFe2Se3 compound.
Reference
Universal stripe order as a precursor of the superconducting phase in pressurized BaFe2Se3 Spin Ladder
WG. Zheng, V. Balédent, C.V. Colin, F. Damay, JP. Rueff, A. Forget, D. Colson, P. Foury-Leylekian
Communications Physics 5, 1-6 (2022)
DOI: 10.1038/s42005-022-00955-7