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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Root-N Krylov-space correction vectors for spectral functions with the density matrix renormalization group

In this work, we propose a method to compute spectral functions of generic Hamiltonians using the density matrix renormalization group (DMRG) algorithm directly in the frequency domain, based on a modified Krylov-space decomposition to compute the correction vectors. Our approach entails the calculation of the root-N (N=2 is the standard square root) of the Hamiltonian propagator using Krylov-space decomposition and repeating this procedure N times to obtain the actual correction vector. We show that our method greatly alleviates the burden of keeping a large bond dimension at large target frequencies, a problem found with conventional correction-vector DMRG, whereas achieving better computational performance at large N. We apply our method to spin and charge spectral functions of t-J and Hubbard models in the challenging two-leg ladder geometry and provide evidence that the root-N approach reaches a much improved spectral resolution compared to the conventional correction vector.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Scaling within the spectral function approach

Scaling features of the nuclear electromagnetic response functions unveil aspects of nuclear dynamics that are crucial for interpreting neutrino-and electron-scattering data. In the large momentum-transfer regime, the nucleon-density response function defines a universal scaling function, which is independent of the nature of the probe. In this work, we analyze the nucleon-density response function of 12 C, neglecting collective excitations. We employ particle and hole spectral functions obtained within two distinct many-body methods, both widely used to describe electroweak reactions in nuclei. We show that the two approaches provide compatible nucleon-density scaling functions that for large momentum transfers satisfy first-kind scaling. Both methods yield scaling functions characterized by an asymmetric shape, although less pronounced than that of experimental scaling functions. Finally, this asymmetry, only mildly affected by final state interactions, is mostly due to nucleon-nucleon correlations, encoded in the continuum component of the hole spectral function.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Distorted spin dependent spectral function of {sup 3}He and semi-inclusive deep inelastic scattering processes

The spin dependent spectral function, relevant to describe polarized electron scattering off polarized {sup 3}He, is studied, within the Plane Wave Impulse Approximation and taking into account final state interaction effects (FSI). In particular, the case of semi-inclusive deep inelastic scattering (SiDIS) is considered, evaluating the FSI of the hadronizing quark with the nuclear remnants. It is shown that particular kinematical regions can be selected to minimize the latter effects, so that parton distributions in the neutron can be accessed. On the other side, in the regions where FSI dominates, the considered reactions can elucidate the mechanism of hadronization of quarks during the propagation in the nuclear medium. It is shown that the obtained spin dependent spectral function can be directly applied to investigate the SiDIS reaction e-vector + {sup 3}He-vector to h+X, where the hadron h originates from the current fragmentation. Experiments of this type are being performed at JLab to extract neutron transverse momentum dependent parton distributions. As a case study, a different SiDIS process, with detection of slow (A-1) systems in the final state, is considered in more details, in order to establish when nuclear structure effects and FSI can be distinguished from elementary reactions on quasi-free nucleons. It is argued that, by a proper choice of kinematics, the origin of nuclear effects in polarized DIS phenomena and the details of the interaction between the hadronizing quark and the nuclear medium can be investigated at a level which is not reachable in inclusive deep inelastic scattering.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Accessing the Spectral Function in a Current-Carrying Device

The presence of an electrical transport current in a material is one of the simplest and most important realizations of nonequilibrium physics. The current density breaks the crystalline symmetry and can give rise to dramatic phenomena, such as sliding charge density waves, insulator-to-metal transitions, or gap openings in topologically protected states. Almost nothing is known about how a current influences the electron spectral function, which characterizes most of the solid’s electronic, optical, and chemical properties. In this work, we show that angle-resolved photoemission spectroscopy with a nanoscale light spot provides not only a wealth of information on local equilibrium properties, but also opens the possibility to access the local nonequilibrium spectral function in the presence of a transport current. Unifying spectroscopic and transport measurements in this way allows simultaneous noninvasive local measurements of the composition, structure, many-body effects, and carrier mobility in the presence of high current densities. In the particular case of our graphene-based device, we are able to correlate the presence of structural defects with locally reduced carrier lifetimes in the spectral function and a locally reduced mobility with a spatial resolution of 500 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Stochastic reconstructions of spectral functions: Application to lattice QCD

Here, we present a detailed study of the applications of two stochastic approaches, stochastic optimization method (SOM) and stochastic analytical inference (SAI), to extract spectral functions from Euclidean correlation functions. SOM has the advantage that it does not require prior information. On the other hand, SAI is a more generalized method based on Bayesian inference. Under mean field approximation SAI reduces to the often-used maximum entropy method (MEM), and for a specific choice of the prior SAI becomes equivalent to SOM. To test the applicability of these two stochastic methods to lattice QCD, Firstly, we apply these methods to various reasonably chosen model correlation functions, and present detailed comparisons of the reconstructed spectral functions obtained from SOM, SAI and MEM. Next, we present similar studies for charmonia correlation functions obtained from lattice QCD computations using clover-improved Wilson fermions on large, fine, isotropic lattices at 0:75 and 1:5 T c , T c being the deconfinement transition temperature of a pure gluon plasma. We find that SAI and SOM give consistent results to MEM at these two temperatures.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Spectral functions with the density matrix renormalization group: Krylov-space approach for correction vectors

Frequency-dependent correlations, such as the spectral function and the dynamical structure factor, help illustrate condensed matter experiments. Within the density matrix renormalization group (DMRG) framework, an accurate method for calculating spectral functions directly in frequency is the correction-vector method. The correction vector can be computed by solving a linear equation or by minimizing a functional. Our paper proposes an alternative to calculate the correction vector: to use the Krylov-space approach. Further, this paper also studies the accuracy and performance of the Krylov-space approach, when applied to the Heisenberg, the t-J, and the Hubbard models. The cases we studied indicate that the Krylov-space approach can be more accurate and efficient than the conjugate gradient, and that the error of the former integrates best when a Krylov-space decomposition is also used for ground state DMRG.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Light-front spin-dependent spectral function and nucleon momentum distributions for a three-body system

Poincare covariant definitions for the spin-dependent spectral function and for the momentum distributions within the light-front Hamiltonian dynamics are proposed for a three-fermion bound system, starting from the light-front wave function of the system. The adopted approach is based on the Bakamjian–Thomas construction of the Poincaré generators, which allows one to easily import the familiar and wide knowledge on the nuclear interaction into a light-front framework. The proposed formalism can find useful applications in refined nuclear calculations, such as those needed for evaluating the European Muon Collaboration effect or the semi-inclusive deep inelastic cross sections with polarized nuclear targets, since remarkably the light-front unpolarized momentum distribution by definition fulfills both normalization and momentum sum rules. As a result, also shown is a straightforward generalization of the definition of the light-front spectral function to an A -nucleon system.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Determination of the argon spectral function from ( e , e ' p ) data

The E12-14-012 experiment, performed in Jefferson Lab Hall A, has measured the $(e, e'p)$ cross section in parallel kinematics using a natural argon target. Here, we report the full results of the analysis of the data set corresponding to beam energy 2.222 GeV, and spanning the missing momentum and missing energy range $15 \lesssim p_m \lesssim 300$ MeV/c and $12 \lesssim E_m \lesssim 80$ MeV. The reduced cross section, determined as a function of $p_m$ and $E_m$ with $\approx$4\% accuracy, has been fitted using the results of Monte Carlo simulations involving a model spectral function and including the effects of final state interactions. The overall agreement between data and simulations turns out to be quite satisfactory ($\chi^2$/n.d.o.f.=1.9). Furthermore, the resulting spectral function will provide valuable new information, needed for the interpretation of neutrino interactions in liquid argon detectors.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1999

A Critical Examination of Wind-Wave Spectral Functional Form

Traditionally, data from random ocean waves are presented in spectral functions. The spectra are the result of Fourier analysis. Fourier spectral analysis has dominated data analysis for, at least, the last hundred years. It has been the standard method for is examining the global amplitude-frequency distributions. Although Fourier transform valid under extremely general conditions, there are some crucial restrictions for the Fourier spectral analysis. The system must be linear, and the data must be stationary- otherwise, the resulting spectrum will make little physical sense. The stationarity requirement is also a common required criterion for most of other available data analysis methods. Nevertheless, few, if any, natural phenomena are linear and stationary. To compound these complications is the imperfection of our probes or numerical schemes the interactions of the imperfect probes even with a perfect linear system can make the final data nonlinear. Furthermore, all the available data are usually of finite duration. Under these conditions, Fourier analysis is of limited use, For lack of alternatives, however, Fourier analysis is still used to process such data. The loose application of Fourier analysis and the insouciant adoption of the stationary and linear assumptions may lead to misleading conclusions. Ocean waves are know to be nonlinear, and the wind system generating the wave field are seldom stationary- As a result, the traditional examination of the spectral form hardly made physical sense. A new method for analyzing nonlinear and nonstationary data has been developed. The key part is the Empirical Mode Decomposition (EMD) method with which any complicated data set can be decomposed into a finite and often small number of Intrinsic Mode Functions (IMF) that serve as the basis of the representation of the data, This decomposition method is adaptive, and, therefore, highly efficient. The IMFs admit well-behaved Hilbert transforms, and yield instantaneous energy and frequency as functions of time that give sharp identifications of imbedded structures. The final presentation of the results is an energy-frequency-time distribution, designated as the Hilbert Spectrum. Among the main conceptual innovations is the introduction of the instantaneous frequencies for complicated data sets, which eliminate the need of spurious harmonics to represent nonlinear and nonstationary signals. The spectral forms of the ocean waves are very different. This new method offers the first physical meaningful view of the wave spectrum. Data from laboratory and field will be presented to illustrate the differences.

Huang, Norden E.↗
DOE OSTI2022

Spectral Function of the Chiral One-Dimensional Fermi Liquid in the Regime of Strong Interactions

We study momentum-resolved tunneling into a system of spinless chiral one-dimensional fermions, such as electrons at the edge of an integer quantum Hall system. Interactions between particles give rise to broadening of the spectral function of the system. We develop an approach that enables one to obtain the shape of the peak in the spectral function in the regime of strong interaction. We apply our technique to the special cases of short-range and Coulomb interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Interfacing electron and neutrino quasielastic scattering cross sections with the spectral function in GENIE

Progress in neutrino-nucleus cross section models is being driven by the need for highly accurate predictions for the neutrino oscillation community. These sophisticated models are being developed within a microscopic description of the nucleus with the goal of encompassing all reaction modes relevant for the accelerator neutrino program. The disconnect between these microscopic models and the event generators that will be used in the next generation of experiments represents a critical obstacle that must be overcome in order to precisely measure the neutrino oscillation parameters. To this end we have developed a hadron tensor interface for lepton-nucleus quasielastic (QE) scattering within the GENIE event generator as a proof of principle, with the broader goal of creating an efficient pipeline for incorporating advanced theoretical models in event generators. As a demonstration of this interface we have implemented the spectral function model into GENIE by connecting theorist provided fortran code through the hadron tensor interface. The spectral function model offers a more complete description of the nuclear ground state, as well as the ability to provide quantifiable theoretical uncertainties. Finally, we validate this implementation and compare its predictions against data and against QE models already available in GENIE.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Heavy-impurity resonance, hybridization, and phonon spectral functions in Fe 1-x M x Si, M=Ir,Os

The vibrational behavior of heavy substitutional impurities (M=Ir,Os) in Fe 1-x M x Si (x = 0, 0.02, 0.04, 0.1) was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. In this paper, our INS measurements on single-crystals mapped the four-dimensional dynamical structure factor, S(Q;E), for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers and increases electron-phonon coupling, leads to softened interatomic force-constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon S(Q,E) from INS through a Green's function model incorporating the phonon self-energy based on first-principles density functional theory (DFT) simulations. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Finally, our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Spectral function of the $J_1$ – $J_2$ Heisenberg model on the triangular lattice

Spectral probes, such as neutron scattering, are crucial for characterizing excitations in quantum many-body systems and the properties of quantum materials. Among the most elusive phases of matter are quantum spin liquids, which have no long-range order even at zero temperature and host exotic fractionalized excitations with nontrivial statistics. These phases can occur in frustrated quantum magnets, such as the paradigmatic Heisenberg model with nearest- and next-nearest-neighbor exchange interactions on the triangular lattice, the so-called $J_1$ – $J_2$ model. In this work, we compute the spectral function using large-scale matrix product state simulations across the three different phases of this model's phase diagram, including a quantum spin-liquid phase at intermediate $J_2$/$J_1$. Despite a plethora of theoretical and experimental studies, the exact nature of this phase is still contested, with the dominant candidates being a gapped $\mathbb{Z}_2$, a gapless U(1) Dirac, and a spinon Fermi surface quantum spin-liquid state. We find a V-shaped spectrum near the center of the Brillouin zone ($Γ$ point), a key signature of a spinon Fermi surface, observed in prior neutron scattering experiments. However, we find a small gap near the $Γ$ point, ruling out such a phase. Furthermore, we find localized gapless excitations at the corner of the Brillouin zone boundary (K point) and the middle of the edge of the Brillouin zone boundary (M point), ruling out the gapped $\mathbb{Z}_2$ spin-liquid phase. Here, our results imply that the intermediate spin-liquid phase is a gapless $U(1)$ Dirac spin liquid, and provide clear signatures to detect this phase in future neutron scattering experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Spin-orbit-proximitized ferromagnetic metal by monolayer transition metal dichalcogenide: Atlas of spectral functions, spin textures, and spin-orbit torques in Co/MoSe 2 , Co/WSe 2 and Co/TaSe 2 heterostructures

In this work, the heterostructures composed of an ultrathin ferromagnetic metal (FM) and a material hosting strong spin-orbit (SO) coupling are the principal resource for SO torque and spin-to-charge conversion nonequilibrium effects in spintronics. The key quantity in theoretical description of these effects is nonequilibrium spin density, which can appear on any monolayer of the heterostructure through which the current is flowing as long as the monolayer bands are affected by the native or proximity induced SO coupling. Here we demonstrate how hybridization of wave functions of Co layer and a monolayer of transition metal dichalcogenides (TMDs)—such as semiconducting MoSe 2 and WSe 2 or metallic TaSe 2 —can lead to dramatic transmutation of electronic and spin structure of Co within some distance away from its interface with TMD, when compared to the bulk of Co or its surface in contact with a vacuum. This is due to proximity induced SO splitting of Co bands encoded in the spectral functions and spin textures on its monolayers, which we obtain using noncollinear density functional theory (ncDFT) combined with equilibrium Green function (GF) calculations. In fact, SO splitting is present due to structural inversion asymmetry of the bilayer—i.e., just the presence of the interface—even if SO coupling within TMD monolayer is artificially switched off in ncDFT calculations. However, switching it on makes the effects associated with proximity SO coupling within Co layer about five times larger. Injecting spin-unpolarized charge current through SO-proximitized monolayers of Co generates nonequilibrium spin density over them, so that its cross product with the magnetization of Co determines SO torque. The SO torque computed via first-principles quantum transport methodology, which combines ncDFT with nonequilibrium GF calculations, can be used as the screening parameter to identify optimal combination of materials and their interfaces for applications in spintronics. In particular, we identify heterostructure two-monolayer-Co/monolayer-WSe 2 as the most optimal one, at least in the clean limit.

2-dimensional systems↗
DOE OSTI2015

Spectral function in electro-weak interactions and its impact on neutrino oscillation experiments

Neutrino oscillation experiments have entered the high-precision era in the last few years. The oscillation parameters, as a measure of the neutrino properties, are extracted from the energy-dependent oscillation probability function. Different types of nuclear dynamics deeply influence the determination of neutrino energies in neutrino oscillation experiments. As a consequence, a comprehensive understanding of various nuclear dynamics interprets the scenario behind the neutrino interaction with nucleus and nuclei. The initial ground-state structure of the target nucleus is categorized in one typical nuclear dynamics, and its realistic description is generally referred as the spectral function (SF). Implementing the SF for each target nucleus into the GENIE neutrino event generator is the preliminary step necessary to obtain a reliable determination of the kinematics of all detectable final-products from neutrino interactions. At the intermedium-range of neutrino energies (∼ 1 GeV), the kinematic energy reconstruction is the vastly used approach and consists in identifying final-products as coming from the charged-current quasi-elastic-like (CCQE-like) neutrino interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Determination of the titanium spectral function from ($e,e'p$) data

The E12-14-012 experiment, performed in Jefferson Lab Hall A, has measured the (e, e'p) cross section in parallel kinematics using a natural titanium target. In this paper, we report the analysis of the dataset obtained in different kinematics for our solid natural titanium target. Data were obtained in a range of missing momentum and missing energy between 15 ≲ p m ≲ 250 MeV / c and 12 ≲ E m ≲ 80 MeV, respectively, and using an electron beam energy of 2.2 GeV. We measured the reduced cross section with ~7% accuracy as a function of both missing momentum and missing energy. Furthermore, our Monte Carlo simulation, including both a model spectral function and the effects of final-state interactions, satisfactorily reproduces the data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Mixed quantum-classical methods for polaron spectral functions

In this work, using two distinct semiclassical approaches—namely, the mean-field Ehrenfest method and the mapping approach to surface hopping—we investigate the spectral function of a single charge interacting with phonons on a lattice. This quantity is relevant for the description of angle-resolved photoemission experiments. Focusing on the one-dimensional Holstein model, we compare the performance of these approaches across a range of coupling strengths and lattice sizes, exposing the relative strengths and weaknesses of each. We demonstrate that these approaches can be efficiently applied with reasonable accuracy to ab initio polaron models. Furthermore, our work provides a route to the calculation of spectral properties in realistic electron–phonon-coupled systems in a computationally inexpensive manner with encouraging accuracy.

Atomic and molecular spectra↗
DOE OSTI2018

Accuracy of the microcanonical Lanczos method to compute real-frequency dynamical spectral functions of quantum models at finite temperatures

We examine the accuracy of the microcanonical Lanczos method (MCLM) developed by Long et al. [Phys. Rev. B 68, 235106 (2003)] to compute dynamical spectral functions of interacting quantum models at finite temperatures. The MCLM is based on the microcanonical ensemble, which becomes exact in the thermodynamic limit. To apply the microcanonical ensemble at a fixed temperature, one has to find energy eigenstates with the energy eigenvalue corresponding to the internal energy in the canonical ensemble. Here in this paper, we propose to use thermal pure quantum state methods by Sugiura and Shimizu [Phys. Rev. Lett. 111, 010401 (2013)] to obtain the internal energy. After obtaining the energy eigenstates using the Lanczos diagonalization method, dynamical quantities are computed via a continued fraction expansion, a standard procedure for Lanczos-based numerical methods. Using one-dimensional antiferromagnetic Heisenberg chains with S = 1/2, we demonstrate that the proposed procedure is reasonably accurate, even for relatively small systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗