| LaFeAsO | |
|---|---|
| Name | LaFeAsO |
| Formula | LaFeAsO |
| System | Tetragonal (ZrCuSiAs-type) |
| Othernames | Lanthanum iron arsenide oxide |
LaFeAsO
LaFeAsO is a layered iron pnictide compound consisting of alternating La–O and Fe–As planes in a ZrCuSiAs-type structure. It gained prominence as the parent compound of the 1111 family of high-temperature superconductivity after carrier doping induced superconducting transitions, making it a central material for studies in condensed matter and Quantum Physics due to intertwined electronic, magnetic, and structural quantum phenomena.
LaFeAsO crystallizes in a tetragonal ZrCuSiAs-type lattice at room temperature, space group P4/nmm, with layered stacks of [LaO] and [FeAs] sheets. The Fe atoms form a square planar lattice coordinated tetrahedrally by As, producing quasi-two-dimensional electronic anisotropy important for nesting-driven instabilities. Substitutions on the La site (e.g., F doping: LaFeAsO1−xFx), oxygen vacancies, or isovalent replacement on As (e.g., P) modify carrier concentration and lattice parameters, tuning electronic correlations. Typical unit cell parameters are a ≈ 4.0 Å and c ≈ 8.7 Å; internal atomic positions and Fe–As bond angles strongly affect the electronic bandwidth and superconducting transition temperature, linking crystallography to quantum many-body behavior.
First-principles calculations using Density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES) reveal multiple Fe 3d-derived bands crossing the Fermi level, generating hole pockets near the Brillouin zone center (Γ) and electron pockets near the M point. The multi-band nature produces nested Fermi surface sections, promoting interband scattering channels that are central to pairing theories. Correlation effects beyond DFT—treated by Dynamical mean field theory (DMFT) or GW approaches—modify quasiparticle renormalization and effective masses. The proximity of van Hove singularities, orbital-selective renormalization of dxy versus dxz/dyz bands, and spin-orbit coupling combine to create a complex low-energy electronic structure that underpins anomalous transport, thermodynamics, and superconducting gap symmetries.
Undoped LaFeAsO exhibits a tetragonal-to-orthorhombic structural transition followed closely by antiferromagnetic order characterized as a stripe-like spin-density wave (SDW) with ordering vector (π,0) in the Fe square lattice notation. The SDW arises from Fermi surface nesting between hole and electron pockets and is stabilized by intra- and inter-orbital interactions described by Hubbard and Hund's coupling terms. Neutron scattering experiments at facilities such as the Institut Laue–Langevin and Oak Ridge National Laboratory quantify ordered moments (~0.3–0.9 μB per Fe) and spin-wave dispersions that inform itinerant versus localized magnetic descriptions. The competition and coexistence of SDW order with superconductivity upon doping or pressure reflect fundamental quantum many-body interplay between magnetism and pairing.
Superconductivity in doped LaFeAsO (e.g., LaFeAsO1−xFx or LaFe1−yCoyAsO) emerges with transition temperatures (Tc) up to ~26 K in early reports and higher in related rare-earth variants (e.g., SmFeAsO). Proposed pairing mechanisms center on spin-fluctuation mediated interband pairing producing sign-changing order parameters, notably s± symmetry, where the superconducting gap changes sign between hole and electron pockets. Competing proposals include orbital-fluctuation mediated s++ pairing and electron-phonon coupling contributions, but consensus favors unconventional pairing driven by electronic interactions. Measurements of the superconducting gap structure via tunneling spectroscopy, ARPES, and nuclear magnetic resonance (NMR) probe coherence factors and low-energy quasiparticle excitations consistent with multiband superconductivity and anisotropic gaps.
The phase diagram of LaFeAsO-derived materials shows suppression of SDW order and structural distortion with increasing carrier doping or applied pressure, giving way to superconductivity — behavior indicative of a nearby quantum critical point (QCP). Signatures of quantum criticality include non-Fermi-liquid resistivity, enhanced specific heat coefficients, and divergent magnetic fluctuations measured by inelastic neutron scattering and NMR. The interplay of magnetic, nematic (rotational symmetry breaking), and superconducting orders defines a multi-dimensional phase diagram studied through experiments at institutions such as Stanford University and Max Planck Society research groups. The presence of nematic order and its coupling to superconductivity makes LaFeAsO a prototype for investigating quantum phase transitions in correlated electron systems.
LaFeAsO and its doped derivatives have been characterized using a broad suite of experimental techniques central to quantum materials research: X-ray and neutron diffraction for structure and magnetic order; ARPES for momentum-resolved band structure; Scanning tunneling microscopy (STM) and spectroscopy for real-space gap mapping; NMR and muon spin rotation (μSR) for local magnetic and superconducting properties; transport and thermal measurements for carrier dynamics; and inelastic neutron scattering for spin excitations. High-pressure cells, molecular-beam epitaxy and solid-state synthesis enable tuning of parameters; synchrotron facilities and cryogenic measurement platforms permit access to low-temperature quantum regimes. Collaboration between experimental groups and theoretical efforts employing DFT, DMFT, and many-body models continues to refine understanding of LaFeAsO in the context of unconventional superconductivity and quantum critical phenomena.
Category:Iron-based superconductors Category:Lanthanum compounds Category:Arsenides