FeSi: a new building block for iron-based superconductivity

28-05-2018

Traditional iron-based superconductors are limited by the invariable presence of toxic arsenic or scarce selenium elements in their superconducting building blocks. This limit has been surpassed by the discovery of LaFeSiH, which is the first silicide displaying unconventional superconductivity with onset at 11 K.

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Superconducting materials provide electric-current transport without dissipation and the reduction of mechanical friction by means of magnetic levitation. Thus, they hold great promise for energy savings through more efficient energy transportation and storage, and to reduce our transportation carbon footprint. At present, superconductors are commonly used in medical applications, particle accelerators and detectors (crucial for the discovery of the Higgs boson at LHC-CERN) and bolometers (for example, used in the detection of gravitational waves). However, before superconducting technology can be widely deployed, a major challenge lies in the discovery of appropriate materials with high superconducting transition temperatures (Tc) that can be manufactured from abundant and non-toxic elements.

The so-called iron-based superconductors have revolutionised the field of superconductivity since ten years ago [1]. These systems provide the latest platform for high-temperature unconventional superconductivity with record high Tc 's of 56 K in bulk doped materials [2] and 100 K in special single-layer setups. However, the remarkable superconducting properties of this class of materials are invariably obtained when the Fe atom is associated with either the toxic pnictide As or the scarce chalcogenide Se. This raises the important fundamental question about the link between iron-based superconductivity and the apparent need of these special elements. At the same time, alternative materials containing less harmful and/or more abundant elements are clearly desirable.

These limitations have been surpassed by the discovery of LaFeSiH. This novel material is the first silicide in the family of the iron-based superconductors displaying superconductivity, with onset at 11 K (Figure 1). In fact, LaFeSiH proves to be one of the very few parent compounds that superconducts without the need of doping nor pressure. The origin of its superconductivity is unconventional, in the sense that it cannot be explained via the standard electron-phonon mechanism [3], and the corresponding Tc sets the second highest Tc among the 1111 parent compounds. In addition, high-resolution X-ray diffraction experiments under high pressure and low temperature carried out at beamline ID27 reveal that superconductivity in LaFeSiH coexists with an orthorhombic distortion of the lattice (Figure 2). This type of distortion is known to be related to an underlying magnetic order with a strong interplay with electron nematicity and Cooper pairing [4]. This makes LaFeSiH a rather singular material since all the intriguing — but generally disjointed — features that characterise the family of iron-based superconductors as a whole emerge spontaneously together in this novel parent compound.

Ball-and-stick model of the crystal structure of LaFeSiH and resistivity as a function of temperature in LaFeSiH measured for different values of the magnetic field

Figure 1. a) Ball-and-stick model of the crystal structure of LaFeSiH, with the unprecedented superconducting FeSi building-block highlighted in colour. The structure of this novel silicide has the P4/nmm space-group symmetry, and hence represents an original 1111 parent compound of the iron-based superconductors. b) Resistivity as a function of temperature in LaFeSiH measured for different values of the magnetic field. The novel silicide displays unconventional superconductivity with onset at 11 K.

The discovery of superconductivity in LaFeSiH thus defines a new playground to understand the rich physics of iron-based superconductors. This novel material demonstrates, for the first time, that this physics can emerge from a building-block such as FeSi containing non-toxic, extremely abundant and comparatively inexpensive elements. In addition, the parent compound LaFeSiH displays an intriguing interplay between charge, spin and lattice degrees of freedom that should make it a reference system in the field of high-temperature superconductivity.

Orthorhombic distortion in LaFeSiH at 15 K as a function of the applied pressure determined from X-ray diffraction experiments

Figure 2. Orthorhombic distortion in LaFeSiH at 15 K as a function of the applied pressure determined from X-ray diffraction experiments carried out at beamline ID27. The distortion displays an unusual re-entrant behaviour that matches the evolution of the magnetic order predicted from density-functional-theory calculations.

 

Principal publication and authors
Iron-based superconductivity extended to the novel silicide LaFeSiH, F. Bernardini (a), G. Garbarino (b), A. Sulpice (c), M. Núñez-Regueiro (c), E. Gaudin (d), B. Chevalier (d), M.-A. Méasson (c), A. Cano (d), and S. Tencé (d), Phys. Rev. B 97, 100504(R) (2018); doi: 10.1103/PhysRevB.97.100504.
(a) CNR-IOM-Cagliari and Dipartimento di Fisica, Universitá di Cagliari, Monserrato (Italy)
(b) ESRF
(c) CNRS, Université Grenoble Alpes, Institut Néel, Grenoble (France)
(d) CNRS, Université Bordeaux, ICMCB, UPR 9048, Pessac (France)

 

References
[1] See e.g. H. Alloul and A. Cano, Special issue on Iron-based superconductors, C.R. Phys. 17, 1 (2016) and the references therein.
[2] G. Garbarino et al., Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsO1-xFx, Phys. Rev. B 84, 024510 (2011).
[3]  L. Hung and T. Yildirim, First-principles study of magnetism, lattice dynamics, and superconductivity in LaFeSiHx, arXiv:1711.01764.
[4] A. Cano, M. Civelli, I. Eremin, and I. Paul, Interplay of magnetic and structural transitions in iron-based pnictide superconductors, Phys. Rev. B 82, 020408(R) (2010).