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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Study of spin symmetry in the doped t-J model using infinite projected entangled pair states

We study the two-dimensional t-J model on a square lattice using infinite projected entangled pair states (iPEPS). At small doping, multiple orders, such as antiferromagnetic order, stripe order and superconducting order, are intertwined or compete with each other. We demonstrate the role of spin symmetry at small doping by either imposing SU(2) spin symmetry or its U(1) subgroup in the iPEPS ansatz, thereby excluding or allowing spontaneous spin-symmetry breaking, respectively, in the thermodynamic limit. From a detailed comparison of our simulations, we provide evidence that stripe order is pinned by long-range antiferromagnetic order. We also find SU(2) iPEPS, enforcing a spin-singlet state, yields a uniform charge distribution and favors d-wave singlet pairing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Electronic structure of chromium trihalides beyond density functional theory

In this work, we explore the electronic band structure of freestanding monolayers of chromium trihalides Cr X 3 , X = Cl, Br, I, within an advanced ab initio theoretical approach based on the use of Green's function functionals. We compare the local density approximation with the quasiparticle self-consistent GW (QS GW ) approximation and its self-consistent extension ( QS G W ^ ) by solving the particle-hole ladder Bethe-Salpeter equations to improve the effective interaction W . We show that, at all levels of theory, the valence band consistently changes shape in the sequence Cl → Br → I , and the valence band maximum shifts from the M point to the Γ point. By analyzing the dynamic and momentum-dependent self-energy, we show that QS G W ^ adds to the localization of the systems in comparison with QS GW , thereby leading to a narrower band and reduced amount of halogens in the valence band manifold. Further analysis shows that X = Cl is most strongly correlated, and X = I is least correlated (most bandlike) as the hybridization between Cr d and X p enhances in the direction Cl → Br → I . For CrBr 3 and CrI 3 , we observe remarkable differences between the QS GW and QS G W ^ valence band structures, while their eigenfunctions are very similar. We show that weak perturbations, like moderate strain, weak changes to the d - p hybridization, and adding small U , can flip the valence band structures between these two solutions in these materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Charge ordering in Ir dimers in the ground state of Ba 5 AlIr 2 O 11

It has been well established experimentally that the interplay of electronic correlations and spin-orbit interactions in Ir 4+ and Ir 5+ oxides results in insulating J eff = 1/2 and J eff = 0 ground states, respectively. However, in compounds where the structural dimerization of iridium ions is favorable, the direct Ir d-d hybridization can be significant and takes a key role. Here, in this study, we investigate the effects of direct Ir d-d hybridization in comparison with electronic correlations and spin-orbit coupling in Ba 5 AlIr 2 O 11 , a compound with Ir dimers. Using a combination of ab initio many-body wave-function quantum chemistry calculations and resonant inelastic x-ray scattering experiments, we elucidate the electronic structure of Ba 5 AlIr 2 O 11 . We find excellent agreement between the calculated and the measured spin-orbit excitations. Contrary to expectations, the analysis of the many-body wave function shows that the two Ir (Ir 4+ and Ir 5+ ) ions in the Ir 2 O 9 dimer unit in this compound preserve their local J eff character close to 1/2 and 0, respectively. The local point group symmetry at each of the Ir ions plays an important role, significantly limiting the direct d-d hybridization. Our results emphasize that minute details in the local crystal field environment can lead to dramatic differences in the electronic states in iridates and 5d oxides in general.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Unconventional skyrmions in an interfacial frustrated triangular lattice

Magnetic skyrmions are spin topological textures of potential interest in spintronics-related data storage and processing devices. Here, in this work, we show the emergence of unconventional skyrmions in a geometrically frustrated triangular lattice on an inversion-symmetry-breaking two-dimensional electron gas substrate. Starting with a classical double-exchange mechanism, this generic interface induces exotic skyrmionic and unique noncoplanar magnetic states not observed in the equivalent square lattice interface. We study the model by deriving an effective spin Hamiltonian. Large scale classical Monte Carlo simulations provide a quantitative evidence for the emergence of these exotic magnetic states. We found that these chiral magnetic states exhibit a substantial and nonzero topological Hall conductivity. As potential material candidates, we propose Cr/MoS 2 , Fe/MoS 2 , and Fe/WSe 2 interfaces because they have the requisite underlying triangular lattice structures and large spin-orbit coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Charge order driven multiferroic behavior in Sr 4 Fe 6 O 12 : An ab-initio study

Here, in this paper, we report the structural, electronic, and ferroelectric properties of the layered mixed-valent transition-metal compound Sr 4 ⁢ Fe 6 ⁢O 12 (SFO). We demonstrate how SFO undergoes a phase transition from a high-temperature (𝑇) centrosymmetric tetragonal phase (𝑃⁢4 2 /𝑚⁢𝑛⁢𝑚) to a low-𝑇 polar orthorhombic phase (𝑃⁢𝑚⁢𝑛⁢2 1 ). The transition is primarily driven by charge ordering at tetrahedral Fe-layer creating Fe 3+ and Fe 2+ cations between two edge-sharing tetrahedra. This charge ordering induces electronic polarization, which is remarkably larger (3.5 times) in magnitude than ionic polarization and oppositely directed, giving a net polarization of 0.05 C/m 2 which is comparable to the state-of-the-art rare-earth nickelets and manganite perovskites. The direction of structural distortion, governed by the polar mode irrep $Γ^−_5$, depends sensitively on the type of magnetic ordering in the Fe-octahedral layer. Consequently, both the ionic and electronic polarization directions are influenced by magnetic ordering, suggesting the potential for multiferroic behavior with strong magnetoelectric coupling in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Differential conductance and defect states in the heavy-fermion superconductor CeCoIn 5

We demonstrate that the electronic band structure extracted from quasiparticle interference spectroscopy [Nat. Phys. 9, 468 (2013)] and the theoretically computed form of the superconducting gaps [Proc. Natl. Acad. Sci. USA 111, 11663 (2014)] can be used to understand the dI/dV line shape measured in the normal and superconducting state of CeCoIn 5 [Nat. Phys. 9, 474 (2013)]. In particular, the dI/dV line shape, and the spatial structure of defect-induced impurity states, reflects the existence of multiple superconducting gaps of d x 2 –y 2 symmetry. As a result, these results strongly support a recently proposed microscopic origin of the unconventional superconducting state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

High-precision mass measurements of the isomeric and ground states of 44 V: Improving constraints on the isobaric multiplet mass equation parameters of the A=44, 0 + quintet

Background: The quadratic isobaric multiplet mass equation (IMME) has been very successful at predicting the masses of isobaric analog states in the same multiplet, while its coefficients are known to follow specific trends as functions of mass number. The Atomic Mass Evaluation 2016 [Chin. Phys. C 41, 030003 (2017)] 44 V mass value results in an anomalous negative c coefficient for the IMME quadratic term; a consequence of large uncertainty and an unresolved isomeric state. The b and c coefficients can provide useful constraints for construction of the isospin-nonconserving Hamiltonians for the pf shell. In addition, the excitation energy of the 0 + , T = 2 level in 44 V is currently unknown. This state can be used to constrain the mass of the more exotic 44 Cr. Purpose: The aim of the experimental campaign was to perform high-precision mass measurements to resolve the difference between 44 V isomeric and ground states, to test the IMME using the new ground state mass value and to provide necessary ingredients for the future identification of the 0 + , T = 2 state in 44 V. Method: High-precision Penning trap mass spectrometry was performed at LEBIT, located at the National Superconducting Cyclotron Laboratory, to measure the cyclotron frequency ratios of [ 44g,m VO] + versus [ 32 SCO] + , a well-known reference mass, to extract both the isomeric and ground state masses of 44 V. Results: The mass excess of the ground and isomeric states in 44 V were measured to be -23 804.9(80) keV/c 2 and -23 537.0(55) keV/c 2 , respectively. This yielded a new proton separation energy of S p = 1 773(10) keV. Conclusion: The new values of the ground state and isomeric state masses of 44 V have been used to deduce the IMME b and c coefficients of the lowest 2 + and 6 + triplets in A = 44. The 2 + c coefficient is now verified with the IMME trend for lowest multiplets and is in good agreement with the shell-model predictions using charge-dependent Hamiltonians. The mirror energy differences were determined between 44 V and 44 Sc, in line with isospin-symmetry for this multiplet. Finally, the new value of the proton separation energy determined, to an uncertainty of 10 keV, will be important for the determination of the 0 + , T = 2 state in 44 V and, consequently, for prediction of the mass excess of 44 Cr.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Mass measurement of 51 Fe for the determination of the 51 Fe($p, γ$) 52 Co reaction rate

Background: The 51 Fe(p, γ) 52 Co reaction lies along the main rp-process path leading up to the 56 Ni waiting point. The uncertainty in the reaction Q value, which determines the equilibrium between the forward protoncapture and reverse photodisintegration 52 Co(γ,p) 51 Fe reaction, contributes to considerable uncertainty in the reaction rate in the temperature range of interest for Type I x-ray bursts and thus to an ≈10% uncertainty in burst ashes lighter than A = 56. Purpose: With a recent Penning trap mass measurement of 52 Co reducing the uncertainty on its mass to 6.6 keV [Nesterenko et al., J. Phys. G 44, 065103 (2017)], the dominant source of uncertainty in the reaction Q value is now the mass of 51 Fe, reported in the 2016 atomic mass evaluation to a precision of 9 keV [Wang et al., Chin.Phys. C 41, 030003 (2017)]. A new, high-precision Penning trap mass measurement of 51 Fe was performed to allow the determination of an improved precision Q value and thus new reaction rates. Method: 51 Fe was produced using projectile fragmentation at the Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory, and separated using the A1900 fragment separator. The resulting secondary beam was then thermalized in the beam stopping area before a mass measurement was performed using the LEBIT 9.4T Penning trap mass spectrometer. Results: The new mass excess, ME = -40189.2(1.6) keV, is sixfold more precise than the current AME value, and 1.6σ less negative. This value was used to calculate a new proton separation energy for 52 Co of 1431(7) keV. New excitation levels were then calculated for 52 Co using the NUSHELLX code with the GXPF1A interaction, and a new reaction rate and burst ash composition was calculated. Conclusions:With a new measured Q value, the uncertainty on the 51 Fe(p, γ ) reaction rate is dominated by the poorly measured 52 Co level structure. Finally, reducing this uncertainty would allow a more precise rate calculation and a better determination of the mass abundances in the burst ashes.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Arrested Phase Separation in Double-Exchange Models: Large-Scale Simulation Enabled by Machine Learning

In this work, we present large-scale dynamical simulations of electronic phase separation in the single-band double-exchange model based on deep-learning neural-network potentials trained from small-size exact diagonalization solutions. We uncover an intriguing correlation-induced freezing behavior as doped holes are segregated from half filled insulating background during equilibration. While the aggregation of holes is stabilized by the formation of ferromagnetic clusters through Hund’s coupling between charge carriers and local magnetic moments, this stabilization also creates confining potentials for holes when antiferromagnetic spin-spin correlation is well developed in the background. The dramatically reduced mobility of the self-trapped holes prematurely disrupts further growth of the ferromagnetic clusters, leading to an arrested phase separation. Implications of our findings for phase separation dynamics in materials that exhibit colossal magnetoresistance effect are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Breaking Rayleigh’s Law with Spatially Correlated Disorder to Control Phonon Transport

Controlling thermal transport in insulators and semiconductors is crucial for many technological fields such as thermoelectrics and thermal insulation, for which a low thermal conductivity (κ) is desirable. A major obstacle for realizing low κ materials is Rayleigh’s law, which implies that acoustic phonons, which carry most of the heat, are insensitive to scattering by point defects at low energy. We demonstrate, with large scale simulations on tens of millions of atoms, that isotropic long-range spatial correlations in the defect distribution can dramatically reduce phonon lifetimes of important low-frequency heat-carrying modes, leading to a large reduction of κ—potentially an order of magnitude at room temperature. We propose a general and quantitative framework for controlling thermal transport in complex functional materials through structural spatial correlations, and we establish the optimal functional form of spatial correlations that minimize κ. Finally, we end by briefly discussing experimental realizations of various correlated structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Calculation of the Physicochemical Characteristics of a New Orthorhombic Form of Diamond

A geometrically optimized structure and some properties of a new diamond polymorph have been assessed using density-functional theory calculations in the local-density approximation. The new phase has a primitive orthorhombic lattice (Pmma). The calculation results indicate that the density, sublimation energy, bulk modulus, Vickers hardness, and band gap of this phase are lower than those of cubic diamond by 5–18%. The orthorhombic diamond polymorph can be obtained by compressing graphite in the [001] direction at a pressure of 59.4 GPa. For this phase transition, we calculated theoretical X-ray diffraction patterns of the new phase and intermediate states, which can be used for their identification in experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Band and Impurity States in Dimond with the (MV){sup –} (M = Si, Ge, Sn) Centers Based on ab Initio Calculations

Ab initio calculations were used to determine the structure, electronic zone dispersion, partial (atomic and orbital) densities of states, and levels of impurity–vacancy cluster (MV){sup –} (M = Si, Ge, Sn) in diamond. Wavefunctions were determined in the density functional theory approximation using hybrid exchange-correlation functional, localized basis set of Gaussians, and a supercell consisting of 192 atoms. Results are compared with data on undoped diamond and with experimental results. The dependence of the direct and indirect energy gap on the cluster type is discussed. Calculations are carried out taking into account the spin–orbit interaction to identify impurity levels in the valence band. The effect of electron–electron interaction on the position of impurity levels is discussed. The dominant contribution of the d orbitals to the density of states at the impurity level in all clusters is detected.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structure and Dynamic and Nonlinear Optical Properties of Hg{sub 2}F{sub 2} Crystals

The structure of the Hg{sub 2}F{sub 2} compound was studied experimentally and theoretically within the density functional theory. X-ray diffraction studies enabled us to determine the space group of the crystal (I4/mmm), and the comparison of the experimentally obtained Raman spectra of these crystals with the theoretical ones enabled us to find and uniquely identify all bands in the spectra. The stability of the I4/mmm phase was analyzed in a wide range of uniaxial and hydrostatic pressures. It is shown that the uniaxial compression of a crystal along a tetrad axis results in a phase transition to the Cmcm structure at P{sub c} = 8 GPa, whereas the hydrostatic compression results in a previously unknown phase transition to the Cmca phase at P{sub c} = 9 GPa. The calculations of the cubic nonlinear susceptibility of Hg{sub 2}F{sub 2} predict a giant value of this quantity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Band Gap Modulation by Two-Dimensional h-BN Nanostructure

Two-dimensional hexagonal boron nitride (h-BN) as a graphene-like material was investigated due to its impending applications in electronics. The h-BN band gap E{sub g} as an important factor and its variation between bilayer ZrSe{sub 2} sheets were explored under an external electric field. The initially indirect band gap is found to convert to direct band gap by means of density functional theory. Additionally, the band gap is modulated by van der Waals corrections from 0.21220 to 0.01770 eV. Based on the results, the proposed heterostructure is converted to the direct band gap, and band gap smoothly decreased from 0.25440 to 0.0436 eV following the application of external electric field from 0.2 to 0.6 eV. Moreover, ZrSe{sub 2}|h-BN|ZrSe{sub 2} is investigated under the applied biaxial compressive strain from 1 to 4%. The findings demonstrated that the gap was decreased by any compressive strain amplification, while the semiconducting behavior in the heterostructure attained to the semi-metallic performance under the increasing strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Average Atom models

This report discusses average atom models, relatively cheap EOS models based on DFT that can access any temperature or density point for any element.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1990

A technique to measure the thermal diffusivity of high T(sub c) superconductors

High T(sub c) superconducting electrical current leads and ground straps will be used in cryogenic coolers in future NASA Goddard Space Flight Center missions. These superconducting samples will be long, thin leads with a typical diameter of two millimeters. A longitudinal method is being developed to measure the thermal diffusivity of candidate materials for this application. This technique will use a peltier junction to supply an oscillatory heat wave into one end of a sample and will use low mass thermocouples to follow the heat wave along the sample. The thermal diffusivity will be calculated using both the exponential decay of the heat wave and the phase shift of the wave. Measurements will be done in a cryostat between 10 K and room temperature.

Powers, Charles E.↗
NASA NTRS1994

Method for forming single phase, single crystalline 2122 BCSCO superconductor thin films by liquid phase epitaxy

A substantially single phase, single crystalline, highly epitaxial film of Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8 superconductor which has a T.sub.c (zero resistance) of 83 K is provided on a lattice-matched substrate with no intergrowth. This film is produced by a Liquid Phase Epitaxy method which includes the steps of forming a dilute supercooled molten solution of a single phase superconducting mixture of oxides of Bi, Ca, Sr, and Cu having an atomic ratio of about 2:1:2:2 in a nonreactive flux such as KCl, introducing the substrate, e.g., NdGaO.sub.3, into the molten solution at 850.degree. C., cooling the solution from 850.degree. C. to 830.degree. C. to grow the film and rapidly cooling the substrate to room temperature to maintain the desired single phase, single crystalline film structure.

Pandey, Raghvendra K.↗
NASA NTRS1995

Epitaxial layers of 2122 BCSCO superconductor thin films having single crystalline structure

A substantially single phase, single crystalline, highly epitaxial film of Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8 superconductor which has a T.sub.c (zero resistance) of 83K is provided on a lattice-matched substrate with no intergrowth. This film is produced by a Liquid Phase Epitaxy method which includes the steps of forming a dilute supercooled molten solution of a single phase superconducting mixture of oxides of Bi, Ca, Sr, and Cu having an atomic ratio of about 2:1:2:2 in a nonreactive flux such as KCl, introducing the substrate, e.g., NdGaO.sub.3, into the molten solution at 850.degree. C., cooling the solution from 850.degree. C. to 830.degree. C. to grow the film and rapidly cooling the substrate to room temperature to maintain the desired single phase, single crystalline film structure.

Pandey, Raghvendra K.↗