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At least 1,675 records · Page 93
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DOE OSTI2024

Stoichiometry–Induced Ferromagnetism in Altermagnetic Candidate MnTe

The field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (T N ≈ 310 K) and semiconducting properties. The results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films are presented. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle-resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first-principles calculations for altermagnetic spin-splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn-richness that is intrinsic to the MBE-grown samples. Further, when combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Knight Shift and Leading Superconducting Instability from Spin Fluctuations in Sr 2 RuO 4

We report that recent nuclear magnetic resonance studies [A. Pustogow et al. , Nature 574, 72 (2019)] have challenged the prevalent chiral triplet pairing scenario proposed for Sr 2 RuO 4 . To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the γ band to the van Hove points. A surprising near degeneracy of the nodal s ' and d x 2 - y 2 wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken s ' + i d x 2 - y 2 pair state. Lastly, predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Ab Initio Nuclear Structure Calculations of Light Nuclei

We perform no-core configuration interaction calculations for nuclei in the p-shell. We show that for typical light nuclei, a truncation on the total number of quanta in the many-body system converges much more rapidly than a full configuration interaction (FCI) truncation, which is a truncation on the single-particle basis space. We present new results for the ground state energies of the Be isotopes with the nonlocal two-body potential JISP16, and discuss emerging phenomena such as clustering and rotational band structures in 9 Be. We also show that the anomalously suppressed beta decay of 14 C to the ground state of 14 N can be reproduced using two- and three-nucleon forces from chiral effective field theory. In particular the structure of the ground state of 14 N is sensitive to the three-nucleon force.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

High-temperature ferromagnetism in Cr 1+x Pt 5-x P

In this work, we present the growth and basic magnetic and transport properties of Cr 1+x Pt 5-x P. We show that single crystals can readily be grown from a high-temperature solution created by adding dilute quantities of Cr to Pt-P based melts. Like other 1-5-1 compounds, Cr 1+x Pt 5-x P adopts a tetragonal P4/mmm, structure composed facesharing CrPts like slabs that are broken up along the c-axis by sheets of P atoms. EDS and X-ray diffraction measurements both suggest Cr 1+x Pt 5-x P has mixed occupancy between Cr and Pt atoms, similar to what is found in the closely related compound CrPtg, giving real compositions of Cr 1.5 Pt 4.5 P (x = 0.5). We report that Cr 1.5 Pt 4.5 P orders ferromagnetically at T c = 464.5 K with a saturated moment of ≈ 2.1 μs/Cr at 1.8 K. Likely owing to the strong spin-orbit coupling associated with the large quantity of high Z, Pt atoms, Cr 1.5 Pt 4.5 P has exceptionally strong planar anisotropy with estimated anisotropy fields of 345 kOe and 220 kOe at 1.8 K and 300 K respectively. The resistance of Cr 1.5 Pt 4.5 P has a metallic temperature dependence with relatively weak magnetoresistance. Electronic band structure calculations show that CrPt 5 P has a large peak in the density of states near the Fermi level which is split into spin majority and minority bands in the ferromagnetic state. Furthermore, the calculations suggest substantial hybridization between Cr-3d and Pt-5d states near the Fermi level, in agreement with the experimentally measured anisotropy.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Polaritonic Chern Insulators in Monolayer Semiconductors

Systems with strong light-matter interaction open up new avenues for studying topological phases of matter. Examples include exciton polaritons, mixed light-matter quasiparticles, where the topology of the polaritonic band structure arises from the collective coupling between matter wave and optical fields strongly confined in periodic dielectric structures. Distinct from light-matter interaction in a uniform environment, the spatially varying nature of the optical fields leads to a fundamental modification of the well-known optical selection rules, which were derived under the plane wave approximation. Here, in this study, we identify polaritonic Chern insulators by coupling valley excitons in transition metal dichalcogenides to photonic Bloch modes in a dielectric photonic crystal slab. We show that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and photonic Dirac cones in the presence of both time-reversal and inversion symmetries. Lifting exciton valley degeneracy by breaking time-reversal symmetry leads to gapped polaritonic bands with nonzero Chern numbers. Through numerical simulations, we predict polaritonic chiral edge states residing inside the topological gaps. Our Letter paves the way for the further study of strong exciton-photon interaction in nanophotonic structures and for exploring polaritonic topological phases and their practical applications in polaritonic devices.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Dirac cone and double zero materials

Materials with zero permittivity and zero permeability (double zero) possess very interesting wave manipulation characteristics. Systems with Dirac cones in the band structure also possess amazing wave transport properties. These two classes of material are actually related to each other. We show that dielectric photonic crystals can be designed and fabricated which exhibit Dirac cones at k = 0 at a finite frequency. A subset of such materials behave as if they have zero permittivity and zero permeability at the Dirac point, as well as exhibiting properties intrinsic to a Dirac cone.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Physical Principles of Self-Consistent Simulation of the Generation of Interface States and the Transport of Hot Charge Carriers in Field-Effect Transistors Based on Metal–Oxide–Semiconductor Structures

A detailed simulation of degradation (caused by hot charge carriers) based on self-consistent consideration of the transport of charge carriers and the generation of defects at the SiO{sub 2}/Si interface is carried out for the first time. The model is tested using degradation data obtained with decananometer n-type-channel field-effect transistors. It is shown that the mutual influence of the above aspects is significant and their independent simulation gives rise to considerable quantitative errors. In calculations of the energy distribution for charge carriers, the actual band structure of silicon and such mechanisms as impact ionization, scattering at an ionized impurity, and also electron–phonon and electron–electron interactions are taken into account. At the microscopic level, the generation of defects is considered as the superposition of single-particle and multiparticle mechanisms of breakage of the Si–H bond. A very important applied aspect of this study is the fact that our model makes it possible to reliably assess the operating lifetime of a transistor subjected to the effects of “hot” charge carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Polarity control at interfaces: Quantifying pseudo-solvent effects in nano-confined systems

Surface functionalization controls local environments and induces solvent-like effects at liquid–solid interfaces. We explored structure–property relationships between organic groups bound to pore surfaces of mesoporous silica nanoparticles and Stokes shifts of the adsorbed solvatochromic dye Prodan. Correlating shifts of the dye on the surfaces with its shifts in solvents resulted in a local polarity scale for functionalized pores. The scale was validated by studying the effects of pore polarity on quenching of Nile Red fluorescence and on the vibronic band structure of pyrene. Measurements were done in aqueous suspensions of porous particles, proving that the dielectric properties in the pores are different from the bulk solvent. The precise control of pore polarity was used to enhance the catalytic activity of TEMPO in the aerobic oxidation of furfuryl alcohol in water. Furthermore, an inverse relationship was found between pore polarity and activity of TEMPO in the pores, demonstrating that controlling the local polarity around an active site allows modulating the activity of nanoconfined catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2015

(CaO)(FeSe): A layered wide-gap oxychalcogenide semiconductor

A new iron-oxychalcogenide (CaO)(FeSe) was obtained which crystallizes in the orthorhombic space group Pnma (No. 62) with a = 5.9175(12) Å, b = 3.8797(8) Å, c = 13.170(3) Å. The unique structure of (CaO)(FeSe) is built up of a quasi-two-dimensional network of corrugated infinite layers of corner-shared FeSe 2 O 2 tetrahedra that extend in the ab-plane. The FeSe 2 O 2 layers stack along the c-axis with Ca 2+ cations sandwiched between the layers. Optical spectroscopy and resistivity measurements reveal semiconducting behavior with an indirect optical band gap of around 1.8 eV and an activation energy of 0.19(1) eV. Furthermore, electronic band structure calculations at the density function level predict a magnetic configuration as ground state and confirm the presence of an indirect wide gap in (CaO)(FeSe).

36 MATERIALS SCIENCE↗
DOE OSTI2026

Cs X Si 15 P 21 ( X = Sn or Pb): Polar Noncentrosymmetric Si–P Frameworks Stabilized by Covalent X –P Bonding

Metal silicon phosphides composed of earth-abundant Si and P tend to exhibit semiconducting properties and adopt diverse crystal structures with relatively small additions of structure-directing elements. The potential of silicon phosphide materials in nonlinear optical applications has been hindered by the inability to systematically produce noncentrosymmetric structures with such a flexible framework. Here, in this work, two isostructural compounds with a novel noncentrosymmetric structure were made possible by the inclusion of elements with stereochemically active lone pairs (Sn 2+ and Pb 2+ ). The structures were determined through single-crystal and synchrotron powder X-ray diffraction. Analysis of chemical bonding in real space through the electron localization function revealed stereochemically active Pb 2+ and Sn 2+ species in a trigonal pyramidal coordination with {Pb/Sn}–P bonds. Such covalent bonding between Pb and P is quite uncommon in extended solids and has been reported in a few rare instances. Band structure calculations and linear optical measurements confirm the semiconducting nature of Cs X Si 15 P 21 ( X = Sn or Pb). The synthesis was optimized to yield high-purity polycrystalline samples. The nonlinear optical properties show promising second-harmonic generation (SHG) coefficients from the Kurtz–Perry method. First-principles calculations of the nonlinear optical properties support the experimentally determined SHG values and provide moderate values of birefringence, suggesting Cs X Si 15 P 21 could be phase-matchable and practical nonlinear optical materials in the mid-IR region.

crystal structure↗
DOE OSTI2015

A new form of Ca{sub 3}P{sub 2} with a ring of Dirac nodes

We report the synthesis and crystal structure of a new high-temperature form of Ca{sub 3}P{sub 2}. The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data. This form of Ca{sub 3}P{sub 2} has a crystal structure of the hexagonal Mn{sub 5}Si{sub 3} type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry. This yields a stable, charge-balanced compound of Ca{sup 2+} and P{sup 3−}. We also report the observation of a secondary hydride phase, Ca{sub 5}P{sub 3}H, which again is a charge-balanced compound. The calculated band structure of Ca{sub 3}P{sub 2} indicates that it is a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to what is seen for graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Exact two-component relativistic energy band theory and application

An exact two-component (X2C) relativistic density functional theory in terms of atom-centered basis functions is proposed for relativistic calculations of band structures and structural properties of periodic systems containing heavy elements. Due to finite radial extensions of the local basis functions, the periodic calculation is very much the same as a molecular calculation, except only for an Ewald summation for the Coulomb potential of fluctuating periodic monopoles. For comparison, the nonrelativistic and spin-free X2C counterparts are also implemented in parallel. As a first and pilot application, the band gaps, lattice constants, cohesive energies, and bulk moduli of AgX (X = Cl, Br, I) are calculated to compare with other theoretical results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural, electronic and elastic properties of RERu{sub 2} (RE=Pr and Nd) Laves phase intermetallic compounds

We have performed the first-principles calculations to study the structural, electronic and elastic properties of RERu{sub 2} (RE = Pr and Nd) Laves phase intermetallic compounds using full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT) within the generalized gradient approximation (GGA) for exchange and correlation potential. The optimized lattices constant are in reasonable agreement with available experimental data. The electronic properties are analyzed in terms of band structures, total and partial density of states, which confirm their metallic character. The calculated elastic constants infer that these compounds are mechanically stable in C15 (MgCu{sub 2} type) structure and found to be ductile in nature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Spin-reorientation transitions in the Cairo pentagonal magnet Bi 4 Fe 5 O 13 F

Here, we show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi 4 Fe 5 O 13 F, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mössbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi 4 Fe 5 O 13 F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe 3+ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Multi‐Species Energy‐Banded Ions in the Ionosphere During the 21 January 2005 Magnetic Storm: Low‐Altitude Edge of the Warm Plasma Cloak

During the 21–22 January 2005 magnetic storm, the FAST satellite observed warm (< few keV) ions in discrete energy bands on the dayside at ∼3,000 km altitude for more than 6.5 hr. We suggest that the ionospheric energy-banded ions represent the low-altitude edge of the warm plasma cloak observed simultaneously by magnetospheric satellites. This is a clear example of the multi-species ion energy bands (10 eV to several keV) observed during strong magnetic storms by the FAST satellite, stretching from the diffuse auroral region to the plasmapause with lifetimes up to 12 hr. The close association of these energy-banded ions with magnetic storms, their broad latitudinal extent, and the presence of multiple ion species in the same energy band, rather than at the same velocity, indicate that this is a distinct phenomenon from other types of energy-banded ions. During the 21–22 January 2005 magnetic storm, the dayside ion energy band structures, centered at 10 eV (H + ), 40 eV (H + and He + ), and 160 eV (H + , He + , and O + ), were consistent with a “time-of-flight and velocity filter” formation process acting on a near-cusp, impulsive outflow of a <200 eV multi-species ion-source population, poleward and in the same hemisphere as FAST. Understanding the sources and dynamics of warm energy-banded ions and their linkage to the warm plasma cloak is important because during superstorms these ions are transported to L values as low as L ∼ 1.2 in the dawn sector, significantly altering the energetics of the mid-latitude ionosphere.

58 GEOSCIENCES↗
DOE OSTI2024

Uncovering the spin ordering in magic-angle graphene via edge state equilibration

Abstract The flat bands in magic-angle twisted bilayer graphene (MATBG) provide an especially rich arena to investigate interaction-driven ground states. While progress has been made in identifying the correlated insulators and their excitations at commensurate moiré filling factors, the spin-valley polarizations of the topological states that emerge at high magnetic field remain unknown. Here we introduce a technique based on twist-decoupled van der Waals layers that enables measurement of their electronic band structure and–by studying the backscattering between counter-propagating edge states–the determination of the relative spin polarization of their edge modes. We find that the symmetry-broken quantum Hall states that extend from the charge neutrality point in MATBG are spin unpolarized at even integer filling factors. The measurements also indicate that the correlated Chern insulator emerging from half filling of the flat valence band is spin unpolarized and suggest that its conduction band counterpart may be spin polarized.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Theoretical investigation on structural and electronic properties of PdO{sub 2}

Theoretical studies on rutile type Palladium Dioxide were carried out with the aim of analyzing structural and electronic properties at ambient condition using the first principle calculation based on density functional theory. Within the framework of density functional theory, we used full potential linearized augmented plane wave method(FP-LAPW) in Wien 2k code. The exchange and correlation effect is treated with generalized gradient approximation (GGA) using the Perdew, Burke and Eruzeroff form. The charge density plots, density of states and band structure are plotted and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗