The 2006 report of superconductivity at about 4 K in the layered iron oxy-pnictide LaOFeP posed a coupled agenda that founded the field of iron-based superconductors: determine the superconducting mechanism, including how the tetrahedral iron coordination and the Fe 3d3d orbital character at the Fermi level shape pairing; map how chemical substitution and aliovalent doping at the La, M(Fe), and P sites tune carrier concentration, superconducting volume fraction, and transition temperature TcT_c; and quantify band characters and pairing symmetry. Later literature has executed much of this agenda: fluorine-doped LaOFeAs raised TcT_c to 26 K and then to 43 K and beyond, angle-resolved photoemission confirmed itinerant multiband Fermi surfaces, and penetration-depth and thermal-conductivity measurements established that LaFePO has a nodal superconducting gap in contrast to the nodeless s±s_{\pm} state of most arsenides. The surviving open core is the microscopic pairing mechanism itself: which interaction produces superconductivity in the iron pnictides, and why the gap symmetry changes so drastically with small material-specific differences such as pnictogen type (P vs As) and Fermi-surface orbital content. This remains one of the central unresolved problems of unconventional superconductivity.

Background

Superconductivity is the lossless condensation of electron pairs below a transition temperature TcT_c. After the cuprate high-TcT_c superconductors, a long-standing goal was to find superconductivity in other layered transition-metal compounds, to learn which electronic structures support pairing. Kamihara, Hiramatsu, Hirano, Kawamura, Yanagi, Kamiya, and Hosono reported in 2006 that LaOFeP becomes superconducting at about 4 K. LaOFeP crystallizes in the tetragonal ZrCuSiAs-type structure, a stacking of insulating lanthanum-oxide (LaO) spacer layers and conductive iron-phosphide (FeP) layers; each Fe sits in a slightly distorted FeP4 tetrahedron, unlike the octahedrally coordinated Fe in previously known iron-phosphide superconductors. In this and related '1111' compounds (so named for the LaO-M-pnictogen layer stoichiometry, where M denotes the transition-metal site occupied by Fe in LaOFeP), aliovalent substitution — replacing an ion by one of a different formal charge — of F for O in the spacer layer dopes carriers into the Fe-P/Fe-As planes, analogous to doping in cuprates. The 2006 measurements showed metallic resistivity, a resistive transition suppressed by magnetic field, bulk diamagnetic shielding (a large Meissner fraction, raised from about 18% to 91% of perfect diamagnetism by F doping), a type-II superconducting response with lower critical field of about 17 Oe, and a negative Seebeck coefficient indicating electron-like carriers. The paper framed a coupled agenda: understand the pairing mechanism and the role of Fe coordination and Fe 3d3d orbitals at the Fermi level; explore substitutions at the La, M, and P sites to tune carriers, dimensionality, volume fraction, and TcT_c; and measure the electronic structure, pairing symmetry, and their relation to orbital occupation by spectroscopic and anisotropic probes beyond bulk transport and magnetization. The agenda was realized explosively in 2008, when the isostructural arsenide La[O1-xFx]FeAs showed Tc=26T_c = 26 K (Kamihara, Watanabe, Hirano, Hosono, 2008), soon raised to 43 K in LaO1-xFxFeAs under pressure (Takahashi et al., 2008) and to about 55 K in Sm-based analogs, spawning the whole iron-pnictide family. Key subsequent findings for the phosphide itself: angle-resolved photoemission spectroscopy (ARPES, which maps the band dispersions and Fermi surface) found quantitative agreement between LaOFeP and itinerant local-density-approximation (LDA, the standard density-functional band-structure scheme) calculations (Lu et al., 2008); stoichiometric LaFePO is actually metallic and non-superconducting above 0.35 K (McQueen et al., 2008), so superconductivity in 'undoped' LaOFeP samples depends on subtle off-stoichiometry, while Ca/F doping raises TcT_c to about 7 K (Kamihara et al., 2008); and low-temperature penetration depth (the depth to which a weak magnetic field is expelled, whose temperature dependence reflects low-energy quasiparticles) and thermal-conductivity measurements established a nodal gap — a superconducting gap that vanishes along lines on the Fermi surface — in LaFePO, in contrast to the fully gapped s±s_{\pm} state (a sign-changing superconducting order parameter between electron and hole Fermi-surface sheets) favored in most arsenide compounds. Theoretical work (Thomale, Platt, Hanke, Bernevig, 2011) attributes this difference to the orbital content of the Fermi surfaces, but no microscopic mechanism is settled.

Problem Statement

For the layered iron oxy-pnictide LaOFeP and the related layered iron pnictides it exemplifies, determine the superconducting pairing mechanism and its material dependence, and establish quantitatively how superconductivity is tuned across the family. Concretely: (1) identify the microscopic interaction responsible for superconductivity, including how the tetrahedral Fe coordination and the specific Fe 3d3d orbital characters at the Fermi level determine the pairing symmetry (why LaFePO realizes a nodal gap, i.e. a superconducting energy gap that vanishes along lines on the Fermi surface, while isostructural LaOFeAs-based superconductors are nodeless s±s_{\pm}, a fully gapped sign-changing order parameter between electron and hole Fermi-surface sheets); (2) determine, at the La, M, and P sites (M denoting the transition-metal, i.e. Fe, site), how carrier concentration, superconducting volume fraction, and TcT_c vary across the LaOMP family and which compositions optimize them; and (3) determine the band characters and superconducting gap symmetry. These three results jointly support one objective — an experimentally grounded, predictive account of superconductivity in layered iron pnictides. Items (2) and (3) are substantially carried out in the literature, so the open core is item (1): the microscopic mechanism and its explanation of the observed gap-structure variation across the family.

References

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