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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Synthesis, Crystal and Electronic Structure of the Quaternary Magnetic EuTAl4Si2 (T = Rh and Ir) Compounds

Single crystals of the quaternary europium compounds EuRhAl4Si2 and EuIrAl4Si2 were synthesized by using the Al–Si binary eutectic as a flux. The structure of the two quaternary compounds has been refined by single crystal X-ray diffraction. Both compounds are stoichiometric and adopt an ordered derivative of the ternary KCu4S3 structure type (tetragonal tP8, P4/mmm). The two compounds reported here represent the first example of a quaternary and truly stoichiometric 1:1:4:2 phase crystallizing with this structure type. In light of our present results, the structure of the BaMg4Si3 compound given in literature as representing a new prototype is actually isotypic with the KCu4S3 structure. Local spin density approximation including the Hubbard U parameter (LSDA + U) calculations show that Eu ions are in the divalent state, with a significant hybridization between the Eu 5d, Rh (Ir) 4d (5d), Si 3p and Al 3p states. Magnetic susceptibility measured along the [001] direction confirms the divalent nature of the Eu ions in EuRhAl4Si2 and EuIrAl4Si2, which order magnetically near 11 and 15 K, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Imaging local discharge cascades for correlated electrons in WS 2 /WSe 2 moiré superlattices

Transition metal dichalcogenide moiré heterostructures provide an ideal platform to explore the extended Hubbard model, where long-range Coulomb interactions play a critical role in determining strongly correlated electron states. This has led to experimental observations of Mott insulator states at half filling as well as a variety of extended Wigner crystal states at different fractional fillings. However, a microscopic understanding of these emerging quantum phases is still lacking. Here we describe a scanning tunnelling microscopy (STM) technique for the local sensing and manipulation of correlated electrons in a gated WS 2 /WSe 2 moiré superlattice, which enables the experimental extraction of fundamental extended Hubbard model parameters. We demonstrate that the charge state of the local moiré sites can be imaged by their influence on the STM tunnelling current. In addition to imaging, we are also able to manipulate the charge state of correlated electrons. When we ramp the bias on the STM tip, there is a local discharge cascade of correlated electrons in the moiré superlattice, which allows us to estimate the nearest-neighbour Coulomb interaction. Two-dimensional mapping of the moiré electron charge states also enables us to determine the on-site energy fluctuations at different moiré sites. Furthermore, our technique should be broadly applicable to many semiconductor moiré systems, offering a powerful tool for the microscopic characterization and control of strongly correlated states in moiré superlattices.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Photo-driven oxidation of water on α-Fe{sub 2}O{sub 3} surfaces: An ab initio study

Adopting the theoretical scheme developed by the Nørskov group [see, for example, Nørskov et al., J. Phys. Chem. B 108, 17886 (2004)], we conducted a density functional theory study of photo-driven oxidation processes of water on various terminations of the clean hematite (α-Fe{sub 2}O{sub 3}) (0001) surface, explicitly taking into account the strong correlation among the 3d states of iron through the Hubbard U parameter. Six best-known terminations, namely, Fe − Fe −O{sub 3}− (we call S{sub 1}), O− Fe − Fe − (S{sub 2}), O{sub 2}− Fe − Fe −(S{sub 3}), O{sub 3}− Fe − Fe − (S{sub 4}), Fe −O{sub 3}− Fe − (S{sub 5}), and O− Fe −O{sub 3}−(S{sub 6}), are first exposed to water, the stability of resulting surfaces is investigated under photoelectrochemical conditions by considering different chemical reactions (and their reaction free energies) that lead to surfaces covered by O atoms or/and OH groups. Assuming that the water splitting reaction is driven by the redox potential for photogenerated holes with respect to the normal hydrogen electrode, U{sub VB}, at voltage larger than U{sub VB}, most 3-oxygen terminated substrates are stable. These results thus suggest that the surface, hydroxylated in the dark, should release protons under illumination. Considering the surface free energy of all the possible terminations shows that O{sub 3}–S{sub 5} and O{sub 3}–S{sub 1} are the most thermodynamically stable. While water oxidation process on the former requires an overpotential of 1.22 V, only 0.84 V is needed on the latter.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Projected coupled cluster theory

Coupled cluster theory is the method of choice for weakly correlated systems. But in the strongly correlated regime, it faces a symmetry dilemma, where it either completely fails to describe the system or has to artificially break certain symmetries. On the other hand, projected Hartree-Fock theory captures the essential physics of many kinds of strong correlations via symmetry breaking and restoration. Here, we combine and try to retain the merits of these two methods by applying symmetry projection to broken symmetry coupled cluster wave functions. The non-orthogonal nature of states resulting from the application of symmetry projection operators furnishes particle-hole excitations to all orders, thus creating an obstacle for the exact evaluation of overlaps. Here we provide a solution via a disentanglement framework theory that can be approximated rigorously and systematically. Results of projected coupled cluster theory are presented for molecules and the Hubbard model, showing that spin projection significantly improves unrestricted coupled cluster theory while restoring good quantum numbers. The energy of projected coupled cluster theory reduces to the unprojected one in the thermodynamic limit, albeit at a much slower rate than projected Hartree-Fock.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Ab initio calculation of atomic solid hydrogen phases based on Gutzwiller many-body wave functions

We apply two ab initio many-body methods based on Gutzwiller wave functions, i.e., correlation matrix renormalization theory (CMRT) and Gutzwiller conjugate gradient minimization (GCGM), to the study of crystalline phases of atomic hydrogen. Both methods avoid empirical Hubbard U parameters and are free from double-counting issues. CMRT employs a Gutzwiller-type approximation that enables efficient calculations, while GCGM goes beyond this approximation to achieve higher accuracy at higher computational cost. By benchmarking against available quantum Monte Carlo (QMC) results, we demonstrate that while both methods are more accurate than the widely used density-functional theory, GCGM systematically captures additional correlation energy missing in CMRT, leading to significantly improved total energy predictions. We also show that by including the correlation energy Ec from local density approximation in the CMRT calculation, CMRT + E c produces energy in better agreement with the QMC results in these hydrogen lattice systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Prediction of topological phases in metastable ferromagnetic $MPX_3$ monolayers

Density functional theory calculations are carried out to study the electronic and topological properties of MPX 3 (M = Mn, Fe, Co,Ni; X = S, Se) monolayers in the ferromagnetic (FM) metastable magnetic state. Here we find that FM MnPSe 3 monolayers host topological semimetal signatures that are gapped out when spin-orbit coupling is included. These findings are supported by explicit calculations of the Berry curvature and the Chern number. The choice of the Hubbard-U parameter to describe the d-electrons is thoroughly discussed, as well as the influence of using a hybrid-functional approach. The presence of band inversions and the associated topological features are found to be formalism-dependent. Nevertheless, routes to achieve the topological phase via the application of external biaxial strain are demonstrated. Within the hybrid-functional picture, topological band structures are recovered under a pressure of 15% (17 GPa). The present work provides a potential avenue for uncovering new topological phases in metastable ferromagnetic phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Visible-light absorption and large band-gap bowing of GaN 1-x Sb x from first principles

Applicability of the Ga(Sb x )N 1-x alloys for practical realization of photoelectrochemical water splitting is investigated using first-principles density functional theory incorporating the local density approximation and generalized gradient approximation plus the Hubbard U parameter formalism. Our calculations reveal that a relatively small concentration of Sb impurities is sufficient to achieve a significant narrowing of the band gap, enabling absorption of visible light. Theoretical results predict that Ga(Sb x )N 1-x alloys with 2-eV band gaps straddle the potential window at moderate to low pH values, thus indicating that dilute Ga(Sb x )N 1-x alloys could be potential candidates for splitting water under visible light irradiation.

Sheetz, R. Michael↗
DOE OSTI2011

Computational study of the energetics of charge and cation mixing in U 1-x Ce x O₂

The formalism of electronic density-functional theory (DFT), with Hubbard-U corrections (DFT+U), is employed in a computational study of the energetics of fluorite-structured U 1-x Ce x O₂ mixtures. The computational approach makes use of a procedure which facilitates convergence of the calculations to multiple self-consistent DFT+U solutions for a given cation arrangement, corresponding to different charge states for the U and Ce ions in several prototypical cation arrangements. Results indicate a significant dependence of the structural and energetic properties on the nature of both charge and cation ordering. With the effective Hubbard-U parameters that reproduce well the measured oxidation-reduction energies for urania and ceria, we find that charge transfer between U⁴⁺ and Ce⁴⁺ ions, leading to the formation of U⁵⁺ and Ce³⁺, gives rise to an increase in the mixing energy in the range of 4–14 kJ/mol of the formula unit, depending on the nature of the cation ordering. The results suggest that although charge transfer between uranium and cerium ions is disfavored energetically, it is likely to be entropically stabilized at the high temperatures relevant to the processing and service of urania-based solid solutions.

Hanken, B. E.↗
DOE OSTI2023

Distinguishing erbium dopants in Y 2 O 3 by site symmetry: Ab initio theory of two spin-photon interfaces

Here, we present a first-principles study of the defect formation and electronic structure of erbium (Er)–doped yttria (Y 2 O 3 ). This is an emerging material for spin-photon interfaces in quantum information science due to the narrow-linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories and transducers. We calculate formation energies of neutral and negatively and positively charged Er dopants and find the charge-neutral configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the C 2 (more relevant for quantum memories) and C 3i (more relevant for quantum transduction), we identify the former as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof functional along with the Hubbard U parameter and spin-orbit coupling, which yields a ~ 6 μ B orbital and a ~ 3 μ B spin magnetic moment, and 11 electrons in the Er 4 f shell, confirming the formation of charge-neutral Er 3+ . This standard density functional theory approach underestimates the band gap of the host and lacks a first-principles justification for U . To overcome these issues we performed screened hybrid functional calculations, including a negative U for the 4 f orbitals, with mixing (α) and screening (w) parameters. These produced robust electronic features with slight modifications in the band gap and the 4 f splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Assessing Relativistic Effects and Electron Correlation in the Actinide Metals Th to Pu

Density functional theory (DFT) calculations are employed to explore and assess the effects of the relativistic spin–orbit interaction and electron correlations in the actinide elements. Specifically, we address electron correlations in terms of an intra-atomic Coulomb interaction with a Hubbard U parameter (DFT + U). Contrary to recent beliefs, we show that for the ground-state properties of the light actinide elements Th to Pu, the DFT + U makes its best predictions for U = 0. Actually, our modeling suggests that the most popular DFT + U formulation leads to the wrong ground-state phase for plutonium. Instead, extending DFT and the generalized gradient approximation (GGA) with orbital–orbital interaction (orbital polarization; OP) is the most accurate approach. We believe the confusion in the literature on the subject mostly originates from incorrectly accounting for the spin–orbit (SO) interaction for the p 1/2 state, which is not treated in any of the widely used pseudopotential plane-wave codes. Here, we show that for the actinides it suffices to simply discard the SO coupling for the p states for excellent accuracy. We thus describe a formalism within the projector-augmented-wave (PAW) scheme that allows for spin–orbit coupling, orbital polarization, and non-collinear magnetism, while retaining an efficient calculation of Hellmann–Feynman forces. We present results of the ground-state phases of all the light actinide metals (Th to Pu). Furthermore, we conclude that the contribution from OP is generally small, but substantial in plutonium.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

The thermal neutron scattering law for hydrogen bound in plutonium dihydride and predicted critical mass for several configurations

The thermal neutron scattering law (TSL) for hydrogen bound in plutonium dihydride (H-PuH{sub 2}) is developed using first-principles density functional theory and lattice dynamics calculations. The GGA+U exchange correlation with spin-polarized magnetism is used to simulate the electronic structure of the material with a Hubbard U parameter that is selected to reproduce experimental lattice constant measurements. The partial phonon density of states from these calculations is used to develop a thermal neutron scattering law evaluation for H-PuH{sub 2} in the incoherent approximation. This thermal scattering law is used to predict the critical mass of {sup 239}PuH{sub 2} in various bare, reflected, and moderated spherical configurations with MC21 using ENDF/B-VII.1 cross sections. The H-PuH{sub 2} TSL is designed to be suitable for the analysis of bulk PuH{sub x} compositions with 0.0 < x ≤ 2.0. Water volume fractions from 0.0 to 0.95 are analyzed for the homogenized slurry. A small positive difference in k{sub eff} is observed for the water reflected configuration when comparing calculations using the {sup 1}H free-gas treatment instead of the H-PuH{sub 2} thermal scattering law.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Hubbard Model Physics in Transition Metal Dichalcogenide Moiré Bands

Flexible long period moire superlattices form in two-dimensional van der Waals crystals containing layers that differ slightly in lattice constant or orientation. In this Letter we show theoretically that isolated flat moire bands described by generalized triangular lattice Hubbard models are present in twisted transition metal dichalcogenide heterobilayers. The hopping and interaction strength parameters of the Hubbard model can be tuned by varying the twist angle and the three-dimensional dielectric environment. In conclusion, when the flat moire bands are partially filled, candidate many-body ground states at some special filling factors include spin-liquid states, quantum anomalous Hall insulators, and chiral d-wave superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2014

ARPES-parameterized Hubbard approach to d-wave cuprate superconductors

In the last decade, the Angle Resolved Photoemission Spectroscopy (ARPES) has achieved important advances in both energy and angular resolutions, providing a direct measurement of the single-particle dispersion relation and superconducting gap. These dispersion relation data allow a full determination of the self-energy, first and second neighbor parameters in the Hubbard model. This model and its generalizations offer a simple and general way to describe the electronic correlation in solids. In particular, the parameters of correlated hopping interactions, responsible of the d-wave superconductivity in the generalized Hubbard model, are determined from ARPES data and the critical temperature within the mean-field approximation. In this work, we determine the model parameters for Bi{sub 2}Sr{sub 2−x}La{sub x}CuO{sub 6+δ} and study its d-wave superconducting gap as a function of temperature by solving numerically two coupled integral equations. Finally, the calculated electronic specific heat is compared with experimental results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of Short-Range Correlations on Spectral Properties of Doped Mott Insulators

In the framework of the cluster perturbation theory for the 2D Hubbard and Hubbard-Holstein models at low hole doping, we have studied the effect of local and short-range correlations in strongly correlated systems on the anomalous features in the electronic spectrum by investigating the fine structure of quasiparticle bands. Different anomalous features of spectrum are obtained as the result of intrinsic properties of strongly correlated electron and polaron bands in the presence of short-range correlations. Particularly, features similar to the electron-like Fermi-pockets of cuprates at hole doping p ∼ 0.1 are obtained without ad hoc introducing a charge density wave order parameter within the Hubbard model in a unified manner with other known peculiarities of the pseudogap phase like Fermi-arcs, pockets, waterfalls, and kink-like features. The Fermi surface is mainly formed by dispersive quasiparticle bands with large spectral weight, formed by coherent low-energy exications. Within the Hubbard-Holstein model at moderate phonon frequencies, we show that modest values of local electron-phonon interaction are capable of introducing low-energy kink-like features and affecting the Fermi surface by hybridization of the fermionic quasiparticle bands with the Franck-Condon resonances.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Fermionic Symmetry-Protected Topological Phase in a Two-Dimensional Hubbard Model

We study the two-dimensional (2D) Hubbard model using exact diagonalization for spin-1/2 fermions on the triangular and honeycomb lattices decorated with a single hexagon per site. In certain parameter ranges, the Hubbard model maps to a quantum compass model on those lattices. On the triangular lattice, the compass model exhibits collinear stripe antiferromagnetism, implying d-density wave charge order in the original Hubbard model. On the honeycomb lattice, the compass model has a unique, quantum disordered ground state that transforms nontrivially under lattice reflection. The ground state of the Hubbard model on the decorated honeycomb lattice is thus a 2D fermionic symmetry-protected topological phase. This state—protected by time-reversal and reflection symmetries—cannot be connected adiabatically to a free-fermion topological phase.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Engineering 2D Square Lattice Hubbard Models in 90° Twisted GeX/SnX (X =S, Se) Moiré Superlattices

Because of the large-period superlattices emerging in moiré two-dimensional (2D) materials, electronic states in such systems exhibit low energy flat bands that can be used to simulate strongly correlated physics in a highly tunable setup. While many investigations have thus far focused on moiré flat bands and emergent correlated electron physics in triangular, honeycomb, and quasi-one-dimensional lattices, tunable moiré realizations of square lattices subject to strong correlations remain elusive. Here, in this work, we propose a feasible scheme to construct moiré square lattice systems by twisting two or more layers of 2D materials in a rectangular lattice by 90°. We demonstrate the concept with twisted GeX/SnX (X =S, Se) moiré superlattices and calculate their electronic structures from first principles. We show that the lowest conduction flat band in these systems can be described by a square lattice Hubbard model with parameters which can be controlled by varying the choice of host materials, number of layers, and external electric fields. In particular, twisted double bilayer GeSe realizes a square lattice Hubbard model with strong frustration due to the next-nearest-neighbor hopping that could host unconventional superconductivity, in close analogy to the Hubbard model for copper-oxygen planes of cuprate high-temperature superconductors. The presented scheme uses 90° twisted 2D materials with rectangular unit cells as a promising platform for realizing the physical phenomena of square lattice Hubbard models, establishing a new route for studying its rich phase diagram of magnetism, charge order, and unconventional superconductivity in a highly tunable setting.

2-dimensional systems↗
DOE OSTI2019

First-Principle Studies of Ferrimagnetic Double Perovskite Ca{sub 2}FeMoO{sub 6} Compound

Using first-principle calculations, the structural, electronic, and magnetic properties of the Ca{sub 2}FeMoO{sub 6} double perovskite compound is investigated. Different spin-ordering: ferrimagnetic (FiM), ferromagnetic (FM), and anti-ferromagnetic (AFM1 and AFM2) using the generalized gradient approximation (GGA) and GGA + U (Hubbard Coulomb onsite correction) are evaluated to determine the theoretical ground state. The value of the Hubbard Coulomb U parameter is varied from 1 to 4 eV. The ground state is found to be a FM spin-ordering within the GGA approach and FiM spin-ordering within the GGA + U approach (where U ≥ 3 eV) which is the experimental preferred configuration. We obtain the FiM spin-ordered half semiconducting state within the GGA + U approach for the Ca{sub 2}FeMoO{sub 6} compound. Within the GGA + U (where U ≥ 3 eV) approach, the FM phase maintains a half-metallic character with a net magnetic moment of 4 0 μ{sub B}, whereas the FiM phase have a spin gapless semiconducting (SGS) behavior at U = 3 eV, and an insulating character at U = 4 eV, with a net magnetic moment of 4 0 μ{sub B}. The main features found in the density of states profile show that the hybridization of the Fe and Mo d orbitals play an important role in determining the electronic and magnetic character of this compound.

36 MATERIALS SCIENCE↗