Geometrical Doping at the Atomic Scale in Oxide Quantum Materials.

Chemical dopants enabling a plethora of emergent physical properties have been treated as randomly and uniformly distributed in the frame of a three-dimensional doped system. However, in nanostructured architectures, the location of dopants relative to the interface or boundary can greatly influence device performance. This observation suggests that chemical dopants need to be considered as discrete defects, meaning that geometric control of chemical dopants becomes a critical aspect as the physical size of materials scales down into the nanotechnology regime.

The 2021 room-temperature superconductivity roadmap.

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization.

Topological and thermoelectric properties of double antiperovskite pnictides.

Doubling the perovskite cell (double perovskite) has been found to open new possibilities for engineering functional materials, magnetic materials in particular. This route should be applicable to the antiperovskite (aPV) class. In the pnictide based double aPV (2aPV) class introduced here magnetism is very rare, and we address them as new topological materials, possibly with thermoelectric interest.

Pressure-tuned Frustration of Magnetic Coupling in Elemental Europium.

Applying linear response and the magnetic force theorem in correlated density functional theory, the intersublattice exchange constants of antiferromagnetic Eu are calculated and found to vanish near the pressure of P_{c}=82  GPa, just where magnetic order is observed experimentally to be lost. The Eu 4f^{7} moment remains unchanged at high pressure, again in agreement with spectroscopic measurements, leaving the picture of perfect frustration of interatomic Ruderman-Kittel-Kasuya-Yoshida couplings in a broad metallic background, leaving a state of electrons strongly exchange coupled to arbitrarily oriented, possibly quasistatic local moments. This strongly frustrated state gives way to superconductivity at T_{c}=1.

Probing hole-doping of the weak antiferromagnet TiAu with first principles methods.

We investigate hole-doping by Sc substitution for Ti in the weak antiferromagnet (wAFM) system Ti[Formula: see text]Sc[Formula: see text]Au. Behavior reported so far fits a weak itinerant AFM picture, and experimentally leads to a quantum critical point at [Formula: see text]. Here we study supercells with several rational fractions [Formula: see text] of Ti replaced by Sc.

Superconducting Phases in Lithium Decorated Graphene LiC.

A study of possible superconducting phases of graphene has been constructed in detail. A realistic tight binding model, fit to ab initio calculations, accounts for the Li-decoration of graphene with broken lattice symmetry, and includes s and d symmetry Bloch character that influences the gap symmetries that can arise. The resulting seven hybridized Li-C orbitals that support nine possible bond pairing amplitudes.

Author Correction: Fermiology and electron dynamics of trilayer nickelate LaNiO.

The original version of this Article contained errors in Fig. 2, Fig. 3a-c and Supplementary Fig.

Local Moment Instability of Os in Honeycomb LiOsO.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, LiOsO (C2/c, a = 5.09 Å, b = 8.

A maximally particle-hole asymmetric spectrum emanating from a semi-Dirac point.

Tight binding models have proven an effective means of revealing Dirac (massless) dispersion, flat bands (infinite mass), and intermediate cases such as the semi-Dirac (sD) dispersion. This approach is extended to a three band model that yields, with chosen parameters in a two-band limit, a closed line with maximally asymmetric particle-hole dispersion: infinite mass holes, zero mass particles. The model retains the sD points for a general set of parameters.

Single Nodal Loop of Accidental Degeneracies in Minimal Symmetry: Triclinic CaAs_{3}.

The existence of closed loops of degeneracies in crystals has been intimately connected with associated crystal symmetries, raising the following question: What is the minimum symmetry required for topological character, and can one find an example? Triclinic CaAs_{3}, in the space group P1[over ¯] with only a center of inversion, has been found to display, without need for tuning, a nodal loop of accidental degeneracies with topological character, centered on one face of the Brillouin zone that is otherwise fully gapped. The small loop is very flat in energy, yet is cut four times by the Fermi energy, a condition that results in an intricate repeated touching of inversion related pairs of Fermi surfaces at Weyl points. Spin-orbit coupling lifts the fourfold degeneracy along the loop, leaving trivial Kramers pairs.

Magnetic correlations and pairing in the 1/5-depleted square lattice Hubbard model.

We study the single-orbital Hubbard model on the 1/5-depleted square-lattice geometry, which arises in such diverse systems as the spin-gap magnetic insulator CaV4O9 and ordered-vacancy iron selenides, presenting new issues regarding the origin of both magnetic ordering and superconductivity in these materials. We find a rich phase diagram that includes a plaquette singlet phase, a dimer singlet phase, a Néel and a block-spin antiferromagnetic phase, and stripe phases. Quantum Monte Carlo simulations show that the dominant pairing correlations at half filling change character from d wave in the plaquette phase to extended s wave upon transition to the Néel phase.

Charge states of ions, and mechanisms of charge ordering transitions.

To gain insight into the mechanism of charge ordering transitions, which conventionally are pictured as a disproportionation of an ion M as 2M(n+)→M((n+1)+) + M((n-1)+), we (1) review and reconsider the charge state (or oxidation number) picture itself, (2) introduce new results for the putative charge ordering compound AgNiO2 and the dual charge state insulator AgO, and (3) analyze the cationic occupations of the actual (not formal) charge, and work to reconcile the conundrums that arise. We establish that several of the clearest cases of charge ordering transitions involve no disproportion (no charge transfer between the cations, and hence no charge ordering), and that the experimental data used to support charge ordering can be accounted for within density functional-based calculations that contain no charge transfer between cations. We propose that the charge state picture retains meaning and importance, at least in many cases, if one focuses on Wannier functions rather than atomic orbitals.

Massive symmetry breaking in LaAlO3/SrTiO3(111) quantum wells: a three-orbital strongly correlated generalization of graphene.

Density functional theory calculations with an on-site Coulomb repulsion term reveal competing ground states in (111)-oriented (LaAlO(3))(M)/(SrTiO(3))(N) superlattices with n-type interfaces, ranging from spin, orbitally polarized (with selective e(g)('), a(1g), or d(xy) occupation), Dirac point Fermi surface, to charge-ordered flat band phases. These phases are steered by the interplay of (i) Hubbard U, (ii) SrTiO(3) quantum well thickness, and (iii) crystal field splitting tied to in-plane strain. In the honeycomb lattice bilayer N = 2 under tensile strain, inversion symmetry breaking drives the system from a ferromagnetic Dirac point (massless Weyl semimetal) to a charge-ordered multiferroic (ferromagnetic and ferroelectric) flat band massive (insulating) phase.

Formal valence, 3d-electron occupation, and charge-order transitions.

While the formal valence and charge state concepts have been tremendously important in materials physics and chemistry, their very loose connection to actual charge leads to uncertainties in modeling behavior and interpreting data. We point out, taking several transition metal oxides (La(2) VCuO(6), YNiO(3), CaFeO(3), AgNiO(2), V(4)O(7)) as examples, that while dividing the crystal charge into atomic contributions is an ill-posed activity, the 3d occupation of a cation (and more particularly, differences) is readily available in first principles calculations. We discuss these examples, which include distinct charge states and charge-order (or disproportionation) systems, where different "charge states" of cations have identical 3d orbital occupation.

Termination control of electronic phases in oxide thin films and interfaces: LaAlO3/SrTiO3(001).

A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO(3) and SrTiO(3) such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO(3) films on SrTiO(3)(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects.

Superconducting gap structure of the 115s revisited.

Density functional theory calculations of the electronic structure of Ce- and Pu-based heavy fermion superconductors in the so-called 115 family are performed. The gap equation is used to consider which superconducting order parameters are most favorable assuming a pairing interaction that is peaked at (π, π, qz)—the wavevector for the antiferromagnetic ordering found in close proximity. In addition to the commonly accepted dx2−y2 order parameter, there is evidence that an extended s-wave order parameter with nodes is also plausible.

Probing bulk electronic structure with hard X-ray angle-resolved photoemission.

Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.

Dirac point degenerate with massive bands at a topological quantum critical point.

The quasilinear bands in the topologically trivial skutterudite insulator CoSb(3) are studied under adiabatic, symmetry-conserving displacement of the Sb sublattice. In this cubic, time-reversal and inversion symmetric system, a transition from trivial insulator to topological point Fermi surface system occurs through a critical point in which massless (Dirac) bands appear, and moreover are degenerate with massive bands. Spin-orbit coupling, while small due to the type of band character, coupled with tetragonal strain opens the gap required to give the topological insulator.

Electronic phenomena at complex oxide interfaces: insights from first principles.

Oxide interfaces have attracted considerable attention in recent years due to the emerging novel behavior which does not exist in the corresponding bulk parent compounds. This opens possibilities for future applications in oxide-based electronics and spintronics devices. Among the different materials combinations, heterostructures containing the two simple band insulators LaAlO(3) and SrTiO(3) have advanced to a model system exhibiting unanticipated properties ranging from conductivity, to magnetism, even to superconductivity.

Quantum confinement induced molecular correlated insulating state in La4Ni3O8.

The recently synthesized layered nickelate La4Ni3O8, with its cupratelike NiO2 layers, seemingly requires a Ni1 (d(8))+2Ni2 (d(9)) charge order, together with strong correlation effects, to account for its insulating behavior. Using density functional methods including strong intra-atomic repulsion (Hubbard U), we obtain an insulating state via a new mechanism: without charge order, correlated (Mott) insulating behavior arises based on quantum-coupled, spin-aligned molecular Ni2-Ni1-Ni2 d(z)(2) trimer states across the trilayer (molecular rather than atomic states), with antiferromagnetic ordering within layers. The weak and frustrated magnetic coupling between cells may account for the small spin entropy that is removed at the Néel transition at 105 K and the lack of any diffraction peak at the Néel point.

Parallel electron-hole bilayer conductivity from electronic interface reconstruction.

The perovskite SrTiO3-LaAlO3 structure has advanced to a model system to investigate the rich electronic phenomena arising at polar oxide interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO3 capping layer prevents atomic reconstruction at the LaAlO3 surface and triggers the electronic reconstruction at a significantly lower LaAlO3 film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.

Tight-binding modeling and low-energy behavior of the semi-Dirac point.

We develop a tight-binding model description of semi-Dirac electronic spectra, with highly anisotropic dispersion around point Fermi surfaces, recently discovered in electronic structure calculations of VO2-TiO2 nanoheterostructures. We contrast their spectral properties with the well-known Dirac points on the honeycomb lattice relevant to graphene layers and the spectra of bands touching each other in zero-gap semiconductors. We also consider the lowest order dispersion around one of the semi-Dirac points and calculate the resulting electronic energy levels in an external magnetic field.