Band narrowing and Mott localization in iron oxychalcogenides La2O2Fe2O(Se,S)2

Superconductivity in a Layered Cobalt Oxychalcogenide Na2CoSe2O with a Triangular Lattice

Unconventional superconductivity in bulk materials under ambient pressure is extremely rare among the 3d transition metal compounds outside the layered cuprates and iron-based family. It is predominantly linked to highly anisotropic electronic properties and quasi-two-dimensional (2D) Fermi surfaces. To date, the only known example of a Co-based exotic superconductor is the hydrated layered cobaltate, NaxCoO2·yH2O, and its superconductivity is realized in the vicinity of a spin-1/2 Mott state. However, the nature of the superconductivity in these materials is still a subject of intense debate, and therefore, finding a new class of superconductors will help unravel the mysteries of their unconventional superconductivity. Here, we report the discovery of superconductivity at ∼6.3 K in our newly synthesized layered compound Na2CoSe2O, in which the edge-shared CoSe6 octahedra form [CoSe2] layers with a perfect triangular lattice of Co ions. It is the first 3d transition metal oxychalcogenide superconductor with distinct structural and chemical characteristics. Despite its relatively low TC, this material exhibits very high superconducting upper critical fields, μ0HC2(0), which far exceeds the Pauli paramagnetic limit by a factor of 3-4. First-principles calculations show that Na2CoSe2O is a rare example of a negative charge transfer superconductor. This cobalt oxychalcogenide with a geometrical frustration among Co spins shows great potential as a highly appealing candidate for the realization of unconventional and/or high-TC superconductivity beyond the well-established Cu- and Fe-based superconductor families and opens a new field in the physics and chemistry of low-dimensional superconductors.

Iron Oxychalcogenides and Their Photocurrent Responses

We report here the results of an experimental investigation of the electronic properties and photocurrent responses of the CaFeOQ and La2O2Fe2OQ2 phases and a computational study of the electronic structure of polar CaFeOSe. We find that both CaFeOQ (Q = S and Se) have band gaps and conduction band edge positions compatible with light-driven photocatalytic water splitting, although the oxysulfide suffers from degradation due to the oxidation of Fe2+ sites. The higher O/Q ratio in the Fe2+ coordination environment in CaFeOSe increases its stability without increasing the band gap beyond the visible range. The photocurrent CaFeOSe shows fast electron-hole separation, consistent with calculated carrier effective masses. These results suggest that these iron oxychalcogenides warrant further study to optimize their stability and morphology for photocatalytic and other photoactive applications.

Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La₂O₃Fe₂Se₂ occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La₂O₃Fe₂Se₂ at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

Structures and Magnetic Ordering in Layered Cr Oxide Arsenides Sr2CrO2Cr2OAs2 and Sr2CrO3CrAs

Two novel chromium oxide arsenide materials have been synthesized, Sr₂CrO₂Cr₂OAs₂ (i.e., Sr₂Cr₃As₂O₃) and Sr₂CrO₃CrAs (i.e., Sr₂Cr₂AsO₃), both of which contain chromium ions in two distinct layers. Sr₂CrO₂Cr₂OAs₂ was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO₄As₂ octahedra containing Cr²⁺ and distorted CrO₂As₄ octahedra containing Cr³⁺. In contrast, Sr₂CrO₃CrAs incorporates Cr³⁺ in CrO₅ square-pyramidal coordination in [Sr₂CrO₃]⁺ layers and Cr²⁺ ions in CrAs₄ tetrahedra in [CrAs]⁻ layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr₂CrO₃CrAs the Cr²⁺ moments in the [CrAs]⁻ layers exhibit long-range ordering, while the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering. However, in Sr₂CrO₂Cr₂OAs₂, both the Cr²⁺ moments in the CrO₄As₂ environments and the Cr³⁺ moments in the CrO₂As₄ polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr³⁺ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr₂CrO₂Cr₂OAs₂ below the magnetic ordering transitions.

Sr₂CrO₂Cr₂OAs₂ and Sr₂CrO₃CrAs are both mixed-anion materials containing chromium ions in two unique layers. In Sr₂CrO₂Cr₂OAs₂, Cr³⁺ ions in CrO₂As₄ environments order antiferromagnetically at around 600 K and Cr²⁺ ions in CrO₄As₂ environments also order antiferromagnetically at a lower temperature of 530(10) K. In contrast, only the Cr²⁺ moments in the [CrAs]⁻ layers exhibit long-range ordering in Sr₂CrO₃CrAs as the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering.

Structural Diversity of Rare-Earth Oxychalcogenides

Mixed-anion systems have garnered much attention in the past decade with attractive properties for diverse applications such as energy conversion, electronics, and catalysis. The discovery of new materials through mixed-cation and single-anion systems proved highly successful in the previous century, but solid-state chemists are now embracing an exciting design opportunity by incorporating multiple anions in compounds such as oxychalcogenides. Materials containing rare-earth ions are arguably a cornerstone of modern technology, and herein, we review recent advances in rare-earth oxychalcogenides. We discuss ternary rare-earth oxychalcogenides whose layered structures illustrate the characters and bonding preferences of oxide and chalcogenide anions. We then review quaternary compounds which combine anionic and cationic design strategies toward materials discovery and describe their structural diversity. Finally, we emphasize the progression from layered two-dimensional compounds to three-dimensional networks and the unique synthetic approaches which enable this advancement.

Local magnetism, magnetic order and spin freezing in the 'nonmetallic metal' FeCrAs

We present the results of x-ray scattering and muon-spin relaxation ([Formula: see text]SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120° periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. [Formula: see text]SR measurements indicate a magnetically ordered phase throughout the bulk of the material below [Formula: see text] K. There are signs of fluctuating magnetism in a narrow range of temperatures above [Formula: see text] involving low-energy excitations, while at temperatures well below [Formula: see text] behaviour characteristic of freezing of dynamics is observed, likely reflecting the effect of disorder in our polycrystalline sample. Using density functional theory we propose a distinct muon stopping site in this compound and assess the degree of distortion induced by the implanted muon.

Hund's metal regimes and orbital selective Mott transitions in three band systems

We analyze the electronic properties of interacting crystal field split three band systems. Using a rotationally invariant slave boson approach we analyze the behavior of the electronic mass renormalization as a function of the intralevel repulsion U, the Hund's coupling J, the crystal field splitting, and the number of electrons per site n. We first focus on the case in which two of the bands are identical and the levels of the third one are shifted by [Formula: see text] with respect to the former. We find an increasing quasiparticle mass differentiation between the bands, for system away from half-filling (n  =  3), as the Hubbard interaction U is increased. This leads to orbital selective Mott transitions where either the higher energy band (for 4  >  n  >  3) or the lower energy degenerate bands (2  <  n  <  3) become insulating for U larger than a critical interaction [Formula: see text]. Away from the half-filled case [Formula: see text] there is a wide range of parameters for [Formula: see text] where the system presents a Hund's metal phase with the physics dominated by the local high spin multiplets. Finally, we study the fate of the n  =  2 Hund's metal as the energy splitting between orbitals is increased for different possible crystal distortions. We find a strong sensitivity of the Hund's metal regime to crystal fields due to the opposing effects of J and the crystal field splittings on the charge distribution between the bands.

A possible family of Ni-based high temperature superconductors

We suggest that a family of Ni-based compounds, which contain [Ni₂M₂O]^2- (M = chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, NiM₄O₂, can be potential high temperature superconductors (high-T_c) upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La₂B₂Se₂O₃ (B = Mn, Fe, Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in iron-based high temperature superconductors. The electronic physics near Fermi energy is controlled by two e_gd-orbitals with completely independent in-plane kinematics. We predict that the superconductivity in this family is characterized by strong competition between extended s-wave and d-wave pairing symmetries.

Elucidating the Impact of Chalcogen Content on the Photovoltaic Properties of Oxychalcogenide Perovkskites: NaMO3-x Qx (M=Nb, Ta; Q=S, Se, Te)

In the quest for nontoxic and stable perovskites for solar cells, we have conducted a systematic study of the effect of chalcogen content in oxychalcogenide perovskite by using DFT and quasi-particle perturbation theory. We explored the changes in the electronic structure due to the substitution of O atoms in NaNbO₃ and NaTaO₃ perovskite structures with various chalcogens (S, Se, Te) at different concentrations. Interestingly, the introduction of the chalcogen atoms resulted in a drastic reduction in the electronic band gap, which made some of the compounds fall within the visible range of the solar spectrum. In addition, our analysis of the electronic structure shows that the optical transition becomes direct as a result of the strong hybridization between the orbitals of the transition metal and those of the chalcogen ion, in contrast to the indirect band feature of NaNbO₃ and NaTaO₃ . We identified candidates with a high theoretical solar conversion efficiency that approached the Shockley-Queisser limit, which makes them suitable for thin-film solar cell applications. The present work serves as a guideline for experimental efforts by identifying the chalcogen content that should be targeted during the synthetic route of thermodynamically stable and strongly photoactive absorbers for oxychalcogenide perovskites in thin-film solar cells.

Structure Evolution and Spin-Glass Transition of Layered Compounds ALiFeSe2 (A = Na, K, Rb)

Three new layered compounds, namely NaLiFeSe₂, KLiFeSe₂, and RbLiFeSe₂, have been discovered. NaLiFeSe₂ adopts a trigonal CaAl₂Si₂-type structure with space group P3̅m1, while the other two possess a tetragonal ThCr₂Si₂-type structure with space group I4/mmm. Structural refinements reveal that Li and Fe atoms randomly occupy the same sites in all these compounds without ordering. It is found that the radius of the alkali metals plays a vital role in determining the symmetry of this series of compounds. The substitution of Li at the Fe site shortens the layer spacing and elongates the A-Se bond length in the ThCr₂Si₂-type structure. The elongated Na-Se bond length would destabilize the ThCr₂Si₂-type structure in NaLiFeSe₂, suggesting that Na_xFe_2-ySe₂ lies at the border of ThCr₂Si₂-type and CaAl₂Si₂-type structures. Magnetic and resistivity measurements demonstrate that these compounds exhibit anisotropic spin-glass and narrow-band-gap semiconducting characteristics. First-principles calculations indicate that the introduction of Li enhances strong localization and weakens the correlation of the 3d electrons of Fe, which are responsible for the observed spin-glass transition and semiconducting conductions.

A Mott insulator continuously connected to iron pnictide superconductors

Iron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe_1−xCu_xAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe_1−xCu_xAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-T_c superconductivity.

Whether an actual Mott insulator phase exists in iron pnictides remains elusive. Here, Song et al. demonstrate an antiferromagnetic insulator phase persisting above the Néel temperature in NaFe_1−xCu_xAs, indicative of a Mott insulator, highlighting the role of electron correlations in high-T_c superconductivity.

The magnetic and electronic properties of oxyselenides-influence of transition metal ions and lanthanides

Magnetic oxyselenides have been a topic of research for several decades, firstly in the context of photoconductivity and thermoelectricity owing to their intrinsic semiconducting properties and ability to tune the energy gap through metal ion substitution. More recently, interest in the oxyselenides has experienced a resurgence owing to the possible relation to strongly correlated phenomena given the fact that many oxyselenides share a similar structure to unconventional superconducting pnictides and chalcogenides. The two dimensional nature of many oxyselenide systems also draws an analogy to cuprate physics where a strong interplay between unconventional electronic phases and localised magnetism has been studied for several decades. It is therefore timely to review the physics of the oxyselenides in the context of the broader field of strongly correlated magnetism and electronic phenomena. Here we review the current status and progress in this area of research with the focus on the influence of lanthanides and transition metal ions on the intertwined magnetic and electronic properties of oxyselenides. The emphasis of the review is on the magnetic properties and comparisons are made with iron based pnictide and chalcogenide systems.

Ca2O3Fe2.6S2: an antiferromagnetic Mott insulator at proximity to bad metal

We report here the first layered iron oxychalcogenide Ca2O3Fe2.6S2 that contains both planar [Ca2FeO2](2+) and [Fe2OS2](2-) layers with the shortest Fe-Fe bond length. This compound is a narrow band gap (~0.073 eV) Mott insulator. The observed antiferromagnetic (AFM) transition at 77 K is due to the ordered Fe vacancies, which can be suppressed by partial substitution of Se for S. We show that the vacancy-free phase Ca2O3Fe3S2 may become a metal with moderate electron correlation comparable to the parent compound LaOFeAs of corresponding superconductors. Our results imply that iron oxychalcogenide can be converted from an AFM Mott insulator into a bad metal like iron pnictides through Fe-Fe bond length shrinking.

Bandwidth and Electron Correlation-Tuned Superconductivity in Rb_{0.8}Fe_{2}(Se_{1-z}S_{z})_{2}

We present a systematic angle-resolved photoemission spectroscopy study of the substitution dependence of the electronic structure of Rb_{0.8}Fe_{2}(Se_{1-z}S_{z})_{2} (z=0, 0.5, 1), where superconductivity is continuously suppressed into a metallic phase. Going from the nonsuperconducting Rb_{0.8}Fe_{2}S_{2} to superconducting Rb_{0.8}Fe_{2}Se_{2}, we observe little change of the Fermi surface topology, but a reduction of the overall bandwidth by a factor of 2. Hence, for these heavily electron-doped iron chalcogenides, we have identified electron correlation as explicitly manifested in the quasiparticle bandwidth to be the important tuning parameter for superconductivity, and that moderate correlation is essential to achieving high T_{C}.

Induced magnetization in La0.7Sr0.3MnO3/BiFeO3 superlattices

Using polarized neutron reflectometry, we observe an induced magnetization of 75 ± 25 kA/m at 10 K in a La(0.7)Sr(0.3)MnO(3) (LSMO)/BiFeO(3) superlattice extending from the interface through several atomic layers of the BiFeO(3) (BFO). The induced magnetization in BFO is explained by density functional theory, where the size of band gap of BFO plays an important role. Considering a classical exchange field between the LSMO and BFO layers, we further show that magnetization is expected to extend throughout the BFO, which provides a theoretical explanation for the results of the neutron scattering experiment.

Induced ferromagnetism at BiFeO3/YBa2Cu3O7 interfaces

Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO₃/YBa₂Cu₃O₇ interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO₃, we are able to show that a metallic ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interaction strengths, from the first-principles calculations, into a resultant generic phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the presence of a hysteresis loop in field-dependent magnetization data. The emergence of interfacial ferromagnetism should have implications to electronic and transport properties.

Orbital-selective Mottness in layered iron oxychalcogenides: the case of Na2Fe2OSe2

Using a combination of local density approximation and dynamical mean-field theory calculations, we explore the correlated electronic structure of a member of the layered iron oxychalcogenides group, Na2Fe2OSe2. We find that the parent compound is a multi-orbital Mott insulator. Surprisingly, and somewhat reminiscently of the underdoped high-Tc cuprate scenario, carrier localization is found to persist upon hole doping because the chemical potential lies in a gap structure with almost vanishing density of states. On the other hand, in remarkable contrast, electron doping drives an orbital-selective metallic phase with coexisting pseudogapped (Mott-localized) and itinerant carriers. These remarkably contrasting behaviors in a single system thus stem from drastic electronic reconstruction caused by large-scale transfer of dynamical spectral weight involving states with distinct orbital character at low energies, fitting the oxychalcogenides neatly into the increasingly visible pattern for Fe-based systems of having orbital-selective Mott phases. We detail the implications that follow from our analysis, and discuss the nature and symmetries of the superconductive states that may arise upon appropriately doping or pressurizing Na2Fe2OSe2.

Layered pnictide-oxide Na2Ti2Pn2O (Pn=As, Sb): a candidate for spin density waves

From first-principles calculations, we have studied the electronic and magnetic structures of compound Na2Ti2Pn2O (Pn=As or Sb), whose crystal structure is a bridge between or a combination of those of high-Tc superconducting cuprates and iron pnictides. We find that in the ground state Na2Ti2As2O is a novel blocked checkerboard antiferromagnetic semiconductor with a small band gap of about 0.15 eV. In contrast, Na2Ti2Sb2O is a bi-collinear antiferromagnetic semimetal, with a small moment of about 0.5 μ(B) around each Ti atom. We show that there is a strong Fermi surface nesting in Na2Ti2Pn2O, and we verify that the blocked checkerboard and bi-collinear antiferromagnetic states both are the spin density waves induced by the Fermi surface nesting. A tetramer structural distortion is found in company with the formation of a blocked checkerboard antiferromagnetic order, in good agreement with the experimentally observed commensurate structural distortion but with space group symmetry retained after the anomaly happens.

Orbital-selective Mott phase in multiorbital models for alkaline iron selenides K1-xFe2-ySe2

We study a multiorbital model for the alkaline iron selenides K(1-x)Fe(2-y)Se(2) using a slave-spin method. With or without ordered vacancies, we identify a metal-to-Mott-insulator transition at the commensurate filling of six 3d electrons per iron ion. For Hund's couplings beyond a threshold value, this occurs via an intermediate orbital-selective Mott phase, in which the 3d xy orbital is Mott localized while the other 3d orbitals remain itinerant. This phase is still stabilized over a range of carrier dopings. Our results lead to an overall phase diagram for the alkaline iron selenides, which provides a unified framework to understand the interplay between the strength of the vacancy order and carrier doping. In this phase diagram, the orbital-selective Mott phase provides a natural link between the superconducting K(1-x)Fe(2-y)Se(2) and its Mott-insulating parent compound.

Magnetic structure of La2O3FeMnSe2: neutron diffraction and physical property measurements

We report on the characterization of the mixed layered lanthanum iron manganese oxyselenide La(2)O(3)FeMnSe(2), where Fe and Mn share the same crystallographic position. The susceptibility data show a magnetic transition temperature of 76 K and a strong difference between field cooled and zero field cooled (ZFC) data at low fields. While the ZFC magnetization curve exhibits negative values below about 45 K, hysteresis measurement reveals, after an initial negative magnetic moment, a hysteresis loop typical for ferromagnetic material, pointing to competing ferromagnetic and antiferromagnetic interactions. Resistivity and dielectric permittivity measurements indicate that La(2)O(3)FeMnSe(2) is a semiconductor. We performed x-ray diffraction at 295 K and neutron diffraction at 90 and 1.7 K. The nuclear and magnetic structure was refined in the space group I4/mmm with a = 4.11031 (3) Å and c = 18.7613 (2) Å at 295 K. We did not detect a structural distortion and the Fe and Mn atoms were randomly distributed. The magnetic order was found to be antiferromagnetic, with a propagation vector q = (0,0,0) and magnetic moments of 3.44 (5) μ(B) per Fe/Mn atom aligned within the a-b plane. This magnetic order is different with respect to the pure Fe or Mn compositions reported in other studies.

Strongly correlated materials

Strongly correlated materials are profoundly affected by the repulsive electron-electron interaction. This stands in contrast to many commonly used materials such as silicon and aluminum, whose properties are comparatively unaffected by the Coulomb repulsion. Correlated materials often have remarkable properties and transitions between distinct, competing phases with dramatically different electronic and magnetic orders. These rich phenomena are fascinating from the basic science perspective and offer possibilities for technological applications. This article looks at these materials through the lens of research performed at Rice University. Topics examined include: Quantum phase transitions and quantum criticality in "heavy fermion" materials and the iron pnictide high temperature superconductors; computational ab initio methods to examine strongly correlated materials and their interface with analytical theory techniques; layered dichalcogenides as example correlated materials with rich phases (charge density waves, superconductivity, hard ferromagnetism) that may be tuned by composition, pressure, and magnetic field; and nanostructure methods applied to the correlated oxides VO₂ and Fe₃O₄, where metal-insulator transitions can be manipulated by doping at the nanoscale or driving the system out of equilibrium. We conclude with a discussion of the exciting prospects for this class of materials.

First-principles study on the electronic structure and magnetism of layered oxyselenide La2Mn2Se2O3

The electronic structure and magnetism of layered oxyselenide La(2)Mn(2)Se(2)O(3) have been studied by using first-principles calculations within the generalized gradient approximation (GGA) and GGA + U methods. The G-type antiferromagnetic (AF) state is calculated to be the most stable phase among the various magnetic configurations of interest, irrespective of the choice of the functional used, which is in good agreement with the experiments. In contrast to La(2)Fe(2)Se(2)O(3) and La(2)Co(2)Se(2)O(3), in which the AF states show metallic behavior under the GGA method, we predict the ground state of La(2)Mn(2)Se(2)O(3) is a semiconductor with an indirect band gap of ∼0.52 eV via the GGA calculations. This is closely related to a closed shell configuration and large exchange splitting (∼3.5 eV) in the Mn 3d states. Moreover, the magnetic properties are also discussed in terms of the calculated Heisenberg spin exchange constants, suggesting that La(2)Mn(2)Se(2)O(3) is a strong two-dimensional magnetically frustrated system.

From antiferromagnetic insulator to correlated metal in pressurized and doped LaMnPO

Widespread adoption of superconducting technologies awaits the discovery of new materials with enhanced properties, especially higher superconducting transition temperatures T(c). The unexpected discovery of high T(c) superconductivity in cuprates suggests that the highest T(c)s occur when pressure or doping transform the localized and moment-bearing electrons in antiferromagnetic insulators into itinerant carriers in a metal, where magnetism is preserved in the form of strong correlations. The absence of this transition in Fe-based superconductors may limit their T(c)s, but even larger T(c)s may be possible in their isostructural Mn analogs, which are antiferromagnetic insulators like the cuprates. It is generally believed that prohibitively large pressures would be required to suppress the effects of the strong Hund's rule coupling in these Mn-based compounds, collapsing the insulating gap and enabling superconductivity. Indeed, no Mn-based compounds are known to be superconductors. The electronic structure calculations and X-ray diffraction measurements presented here challenge these long held beliefs, finding that only modest pressures are required to transform LaMnPO, isostructural to superconducting host LaFeAsO, from an antiferromagnetic insulator to a metallic antiferromagnet, where the Mn moment vanishes in a second pressure-driven transition. Proximity to these charge and moment delocalization transitions in LaMnPO results in a highly correlated metallic state, the familiar breeding ground of superconductivity.

Quantum criticality in the iron pnictides and chalcogenides

Superconductivity in the iron pnictides and chalcogenides arises at the border of antiferromagnetism, which raises the question of the role of quantum criticality. In this topical review, we describe the theoretical work that led to the prediction of a magnetic quantum critical point arising out of a competition between electronic localization and itinerancy, and the proposal for accessing it by using isoelectronic P substitution for As in the undoped iron pnictides. We go on to compile the emerging experimental evidence in support of the existence of such a quantum critical point in isoelectronically tuned iron pnictides. We close by discussing the implications of these results for the physics of the iron pnictides and chalcogenides.

Mott transition in modulated lattices and parent insulator of (K,Tl)(y)Fe(x)Se(2) superconductors

The degree of electron correlations remains a central issue in the iron-based superconductors. The parent iron pnictides are antiferromagnetic, and their bad-metal behavior has been interpreted in terms of proximity to a Mott transition. We study such a transition in multiorbital models on modulated lattices containing an ordered pattern of iron vacancies, using a slave-rotor method. We show that the ordered vacancies lead to a band narrowing, which pushes the system to the Mott insulator side. This effect is proposed to underlie the insulating behavior observed in the parent compounds of the newly discovered (K,Tl)(y)Fe(x)Se(2) superconductors.