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At least 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

First-Principles Investigation of Equiatomic Quaternary Heusler Alloys NbVMnAl and NbFeCrAl and a Discussion of the Generalized Electron-Filling Rule

Plane-wave pseudo-potential methods based on density functional theory are used to study the electronic structures, magnetic, and half-metallic properties of the equiatomic quaternary Heusler alloys NbVMnAl and NbFeCrAl. Calculated results reveal that NbVMnAl and NbFeCrAl alloys are newly designed half-metallic ferrimagnets with a total magnetic moment (M{sub t}) of 2 μ{sub B}. We found that the Slater-Pauling rule, i.e., the relationship between the M{sub t} and the total number of valence electrons (Z{sub t}) of these two alloys was different. For NbVMnAl alloy, it obeys the M{sub t} = |Z{sub t} − 18| rule. For NbFeCrAl, it satisfies the M{sub t} = |Z{sub t} − 24| rule. Also, we observed that the half-metallic band gap (HM-BG) of these two alloys was different. Namely, for one material (NbFeCrAl), it is located in the spin-up channel, and in the other (NbVMnAl), it is located in the spin-down channel. To explain these phenomena, in this work, the generalized electron-filling rule was investigated, and the origin of the HM-BG of NbVMnAl and NbFeCrAl was studied. We hope that our work may provide theoretical guidance in searching for Nb-based equiatomic quaternary Heusler half-metals in the future.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Evolution of magnetism in the magnetic topological semimetal NdSb x Te 2–x+δ

Magnetic topological semimetals LnSbTe (Ln = Lanthanide) have attracted intensive attention because of the presence of interplay between magnetism, topological, and electron correlations depending on the choices of magnetic Ln elements. Recently, varying Sb-Te composition has been found to effectively control the electronic and magnetic states in LnSb x Te 2-x . Here, with this motivation, we report the evolution of magnetic properties with Sb-Te substitution in NdSb x Te 2-x+δ , (0 ≤ x ≤ 1). Our work reveals the interesting non-monotonic change in magnetic ordering temperature with varying composition stoichiometry. In addition, reducing the Sb content x drives the reorientation of moments from in-plane (ab-plane) to out-of-plane (c-axis) direction that results in the distinct magnetic structures for two end compounds NdTe 2 (x = 0) and NdSbTe (x = 1). Furthermore, the moment orientation in NdSb x Te 2-x+δ is also found to be strongly tunable upon application of weak magnetic field, leading to rich magnetic phases depending on the composition stoichiometry, temperature, and magnetic field. Such strong tuning of magnetism in this material establishes it as a promising platform for investigating tunable topological states and correlated topological physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Magneto-acoustic study near the quantum critical point of the frustrated quantum antiferromagnet Cs{sub 2}CuCl{sub 4}

Magneto-acoustic investigations of the frustrated triangular-lattice antiferromagnet Cs{sub 2}CuCl{sub 4} were performed for the longitudinal modes c{sub 11} and c{sub 33} in magnetic fields along the a-axis. The temperature dependence of the sound velocity at zero field shows a mild softening at low temperature and displays a small kink-like anomaly at T{sub N}. Isothermal measurements at T < T{sub N} of the sound attenuation α reveal two closely spaced features of different characters on approaching the material's quantum-critical point (QCP) at B{sub s} ≈ 8.5 T for B || a. The peak at slightly lower fields remains sharp down to the lowest temperature and can be attributed to the ordering temperature T{sub N}(B). The second anomaly, which is rounded and which becomes reduced in size upon cooling, is assigned to the material's spin-liquid properties preceding the long-range antiferromagnetic ordering with decreasing temperature. These two features merge upon cooling suggesting a coincidence at the QCP. The elastic constant at lowest temperatures of our experiment at 32 mK can be well described by a Landau free energy model with a very small magnetoelastic coupling constant G/k{sub B} ≈ 2.8 K. The applicability of this classical model indicates the existence of a small gap in the magnetic excitation spectrum which drives the system away from quantum criticality.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Intervalence charge transfer luminescence: The anomalous luminescence of cerium-doped Cs{sub 2}LiLuCl{sub 6} elpasolite

The existence of intervalence charge transfer (IVCT) luminescence is reported. It is shown that the so called anomalous luminescence of Ce-doped elpasolite Cs{sub 2}LiLuCl{sub 6}, which is characterized mainly by a very large Stokes shift and a very large band width, corresponds to an IVCT emission that takes place in Ce{sup 3+}–Ce{sup 4+} pairs, from the 5de{sub g} orbital of Ce{sup 3+} to 4f orbitals of Ce{sup 4+}. Its Stokes shift is the sum of the large reorganization energies of the Ce{sup 4+} and Ce{sup 3+} centers formed after the fixed-nuclei electron transfer and it is equal to the energy of the IVCT absorption commonly found in mixed-valence compounds, which is predicted to exist in this material and to be slightly larger than 10 000 cm{sup −1}. The large band width is the consequence of the large offset between the minima of the Ce{sup 3+}–Ce{sup 4+} and Ce{sup 4+}–Ce{sup 3+} pairs along the electron transfer reaction coordinate. This offset is approximately 2√(3) times the difference of Ce–Cl equilibrium distances in the Ce{sup 3+} and Ce{sup 4+} centers. It is shown that the energies of the peaks and the widths of IVCT absorption and emission bands can be calculated ab initio with reasonable accuracy from diabatic energy surfaces of the ground and excited states and that these can be obtained, in turn, from independent calculations on the donor and acceptor active centers. We obtained the energies of the Ce{sup 3+} and Ce{sup 4+} active centers of Ce-doped Cs{sub 2}LiLuCl{sub 6} by means of state-of-the-art wave-function-theory spin-orbit coupling relativistic calculations on the donor cluster (CeCl{sub 6}Li{sub 6}Cs{sub 8}){sup 11+} and the acceptor cluster (CeCl{sub 6}Li{sub 6}Cs{sub 8}){sup 12+} embedded in a quantum mechanical embedding potential of the host. The calculations provide interpretations of unexplained experimental observations as due to higher energy IVCT absorptions, and allow to reinterpret others. The existence of another IVCT emission of lower energy, at around 14 000-16 000 cm{sup −1} less than the 5dt{sub 2g} emission, is also predicted.

36 MATERIALS SCIENCE↗
NASA NTRS2021

All-sky search in early O3 LIGO data for continuous gravitational-wave signals from unknown neutron stars in binary systems

Rapidly spinning neutron stars are promising sources of continuous gravitational waves. Detecting such a signal would allow probing of the physical properties of matter under extreme conditions. A significant fraction of the known pulsar population belongs to binary systems. Searching for unknown neutron stars in binary systems requires specialized algorithms to address unknown orbital frequency modulations. We present a search for continuous gravitational waves emitted by neutron stars in binary systems in early data from the third observing run of the Advanced LIGO and Advanced Virgo detectors using the semicoherent, GPU-accelerated, binaryskyhough pipeline. The search analyzes the most sensitive frequency band of the LIGO detectors, 50–300 Hz. Binary orbital parameters are split into four regions, comprising orbital periods of three to 45 days and projected semimajor axes of two to 40 light seconds. No detections are reported. We estimate the sensitivity of the search using simulated continuous wave signals, achieving the most sensitive results to date across the analyzed parameter space.

Gravitation↗
DOE OSTI2023

Quasilinear theory of Brillouin resonances in rotating magnetized plasmas

Both spin and orbital angular momentum can be exchanged between a rotating wave and a rotating magnetized plasma. Through resonances the spin and orbital angular momentum of the wave can be coupled to both the cyclotron rotation and the drift rotation of the particles. It is, however, shown that the Landau and cyclotron resonance conditions which classically describe resonant energy–momentum exchange between waves and particles are no longer valid in a rotating magnetized plasma column. In this case a new resonance condition which involves a resonant matching between the wave frequency, the cyclotron frequency modified by inertial effects and the harmonics of the guiding centre rotation is identified. A new quasilinear equation describing orbital and spin angular momentum exchanges through these new Brillouin resonances is then derived, and used to expose the wave-driven radial current responsible for angular momentum absorption.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Complete particle-pair annihilation as a dynamical signature of the spectral singularity

Motivated by the physical relevance of a spectral singularity of interacting many-particle system, we explore the dynamics of two bosons as well as fermions in one-dimensional system with imaginary delta interaction strength. Based on the exact solution, it shows that the two-particle collision leads to amplitude-reduction of the wave function. For fermion pair, the amplitude-reduction depends on the spin configuration of two particles. In both cases, the residual amplitude can vanish when the relative group velocity of two single-particle Gaussian wave packets with equal width reaches the magnitude of the interaction strength, exhibiting complete particle-pair annihilation at the spectral singularity. - Highlights: • We investigate the physical relevance of a spectral singularity. • The two-particle collision leads to amplitude-reduction of the wave function. • There is a singularity spectrum which leads to complete particle-pair annihilation. • Complete particle-pair annihilation can only occur for two distinguishable bosons and singlet fermions. • Pair annihilation provides a detection method of the spectral singularity in the experiment.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1994

Development of New Open-Shell Perturbation and Coupled-Cluster Theories Based on Symmetric Spin Orbitals

A new spin orbital basis is employed in the development of efficient open-shell coupled-cluster and perturbation theories that are based on a restricted Hartree-Fock (RHF) reference function. The spin orbital basis differs from the standard one in the spin functions that are associated with the singly occupied spatial orbital. The occupied orbital (in the spin orbital basis) is assigned the delta(+) = 1/square root of 2(alpha+Beta) spin function while the unoccupied orbital is assigned the delta(-) = 1/square root of 2(alpha-Beta) spin function. The doubly occupied and unoccupied orbitals (in the reference function) are assigned the standard alpha and Beta spin functions. The coupled-cluster and perturbation theory wave functions based on this set of "symmetric spin orbitals" exhibit much more symmetry than those based on the standard spin orbital basis. This, together with interacting space arguments, leads to a dramatic reduction in the computational cost for both coupled-cluster and perturbation theory. Additionally, perturbation theory based on "symmetric spin orbitals" obeys Brillouin's theorem provided that spin and spatial excitations are both considered. Other properties of the coupled-cluster and perturbation theory wave functions and models will be discussed.

Lee, Timothy J.↗
DOE OSTI2012

Self-induced acoustic transparency of three-component longitudinal-transverse pulses

The features of the nonlinear dynamics of three-component elastic pulses in a low-temperature crystal containing paramagnetic impurities of electron and nuclear spins have been analyzed in the slowly varying envelope approximation. The presence of the electron spin subsystem makes it possible to equate the velocities of longitudinal sound and transverse acoustic waves; as a result, all components of the strain field efficiently interact with each other through the nuclear spin subsystem. The system of equations for envelopes of harmonics of the components of the strain field and the spin variables has been derived. The relations between the amplitudes and phases of the components have been obtained, the spectral composition has been analyzed, and the regimes of acoustic transparency of three-component longitudinal-transverse pulses have been discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Chronus Quantum software package

The Chronus Quantum (ChronusQ) software package is an open source software infrastructure which targets the solution of challenging problems that arise in ab initio electronic structure theory. Special emphasis is placed on the consistent treatment of time dependence and spin in the electronic wave function, as well as the inclusion of relativistic effects in said treatments. In addition, ChronusQ provides support for the inclusion of uniform finite magnetic fields as external perturbations through the use of gauge-including atomic orbitals. ChronusQ is a parallel electronic structure code written in modern C++ which utilizes both message passing implementation and shared memory (OpenMP) parallelism. In addition to the examination of the current state of code base itself, a discussion regarding ongoing developments and developer contributions will also be provided.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Gluon fragmentation into quarkonium at next-to-leading order

Here, we present the first calculation at next-to-leading order (NLO) in α s of a fragmentation function into quarkonium whose form at leading order is a nontrivial function of z, namely the fragmentation function for a gluon into a spin-singlet S-wave state at leading order in the relative velocity. To calculate the real NLO corrections, we introduce a new subtraction scheme that allows the phase-space integrals to be evaluated in 4 dimensions. We extract all ultraviolet and infrared divergences in the real NLO corrections analytically by calculating the phase-space integrals of the subtraction terms in 4 – 2ϵ dimensions. We also extract the divergences in the virtual NLO corrections analytically, and detail the cancellation of all divergences after renormalization. The NLO corrections have a dramatic effect on the shape of the fragmentation function, and they significantly increase the fragmentation probability.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

On unitarity of the Coon amplitude

The Coon amplitude is a one-parameter deformation of the Veneziano amplitude. We explore the unitarity of the Coon amplitude through its partial wave expansion using tools from q-calculus. Our analysis establishes manifest positivity on the leading and sub-leading Regge trajectories in arbitrary spacetime dimensions D, while revealing a violation of unitarity in a certain region of (q, D) parameter space starting at the sub-sub-leading Regge order. A combination of numerical studies and analytic arguments allows us to argue for the manifest positivity of the partial wave coefficients in fixed spin and Regge asymptotics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

On the correspondence between quantum and classical variational principles

Here, classical variational principles can be deduced from quantum variational principles via formal reparameterization of the latter. It is shown that such reparameterization is possible without invoking any assumptions other than classicality and without appealing to dynamical equations. As examples, first principle variational formulations of classical point-particle and cold-fluid motion are derived from their quantum counterparts for Schrodinger, Pauli, and Klein-Gordon particles.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Ab initio calculations of the carbon and oxygen isotopes: Energies, correlations, and superfluid pairing

We perform ab initio nuclear lattice calculations of the neutron-rich carbon and oxygen isotopes using high-fidelity chiral interactions. We find good agreement with the observed binding energies and compute correlations associated with each two-nucleon interaction channel. For the isospin T = 1 channels, we show that the dependence on T z provides a measure of the correlations among the extra neutrons in the neutron-rich nuclei. For the spin-singlet S-wave channel, we observe that any paired neutron interacts with the nuclear core as well as its neutron pair partner, while any unpaired neutron interacts primarily with only the nuclear core. For the other partial waves, the correlations among the extra neutrons grow more slowly and smoothly with the number of neutrons. These general patterns are observed in both the carbon and oxygen isotopes and may be universal features that appear in many neutron-rich nuclei.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (T c ) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of T c has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ~15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ~6 GPa the sudden enhancement of superconductivity (T c ≤ 38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-T c phase above 6 GPa. In conclusion, the obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-T c cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Strain tuning of vestigial three-state Potts nematicity in a correlated antiferromagnet

Electronic nematicity is a state of matter in which rotational symmetry is spontaneously broken and translational symmetry is preserved. In strongly correlated materials, nematicity often emerges from fluctuations of a multicomponent primary order, such as spin or charge density waves, and is termed vestigial nematicity. One widely studied example is Ising nematicity, which arises as a vestigial order of collinear antiferromagnetism in the tetragonal iron pnictide superconductors. Because nematic directors in crystals are restricted by the underlying crystal symmetry, recently identified quantum materials with three-fold rotational symmetry offer a new platform to investigate nematic order with three-state Potts character. Here, in this study, we demonstrate strain control of three-state Potts nematicity as a vestigial order of zigzag antiferromagnetism in FePSe 3 . Optical linear dichroism measurements reveal the nematic state and demonstrate the rotation of the nematic director by uniaxial strain. We show that the nature of the nematic phase transition can also be controlled by strain, inducing a smooth crossover transition between a Potts nematic transition and an Ising nematic flop transition. Elastocaloric measurements demonstrate the signatures of two coupled phase transitions, indicating that the vestigial nematic transition is separated from the antiferromagnetic transition. This establishes FePSe 3 as a system to explore three-state Potts vestigial nematicity.

Hwangbo, Kyle↗
DOE OSTI2012

Transition metal dimer on Au(111) surface: A first principle study

The adsorption behaviour of transition metal dimers M{sub 2} (M= Cu, Ag, Au) on the Au(111) surface have been studied using the density functional theory formalism. The projector augmented wave method under the spin polarized version of generalized gradient approximation scheme was employed to calculate the total energy. The results suggest that all dimers prefer to orient in parallel to the surface plane, where two M atoms are adsorbed on two nearby threefold fcc sites. We have investigated the chemical interaction between M atoms and Au surface through electronic density of state analysis. It is found that on deposition, the electronic density of states (EDOS) of M{sub 2} dimer becomes broader in comparison to their gas phase spectrum.

36 MATERIALS SCIENCE↗