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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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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Tensor train continuous time solver for quantum impurity models

The simulation of strongly correlated quantum impurity models is a significant challenge in modern condensed matter physics that has multiple important applications. Thus far, the most successful methods for approaching this challenge involve Monte Carlo techniques that accurately and reliably sample perturbative expansions to any order. However, the cost of obtaining high precision through these methods is high. Recently, tensor train decomposition techniques have been developed as an alternative to Monte Carlo integration. In this study, we apply these techniques to the single-impurity Anderson model at equilibrium by calculating the systematic expansion in power of the hybridization of the impurity with the bath. Furthermore, we demonstrate the performance of the method in a paradigmatic application, examining the first-order phase transition on the infinite-dimensional Bethe lattice, which can be mapped to an impurity model through dynamical mean field theory. Our results indicate that using tensor train decomposition schemes allows the calculation of finite-temperature Green's functions and thermodynamic observables with unprecedented accuracy. The methodology holds promise for future applications to frustrated multiorbital systems, using a combination of partially summed series with other techniques pioneered in diagrammatic and continuous time quantum Monte Carlo.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2015

Determination of the Contact Angle Based on the Casimir Effect

In several crystal growth processed based on capillarity, a melt comes into contact with a crucible wall at an angle defined as the contact angle. For molten metals and semiconductors, this contact angle is dependent upon both the crucible and melt material and typical values fall in the range 80-170deg. However, on a microscopic scale, there does not exist a precise and sharp contact angle but rather the melt and solid surfaces merge smoothly and continuously over a distance of up to several micrometers. Accurate modeling requires a more advanced treatment of this interaction. The interaction between the melt and solid surfaces can be calculated by considering two forces: a short-range repulsive force and a longer range (up to a few micrometers) Casimir force. The Casimir force between the two bodies of complex geometry is calculated using a retarded temperature Green's function (Matsubara type) for the photon in the medium. The governing equations are cast in the form of a set of boundary integral equations which are then solved numerically for the case of molten Ge on SiO2. The shape of the molten surface approaching the flat solid body is determined, and the contact angle is defined as the angle between the two surfaces at the microscopically asymptotic distance of 1-2 micrometers. The formulation of this model and the results of the numerical calculations will be presented and discussed.

Mazuruk, K.↗
DOE OSTI2021

Evaluation of two-particle properties within finite-temperature self-consistent one-particle Green’s function methods: Theory and application to GW and GF2

One-particle Green’s function methods can model molecular and solid spectra at zero or non-zero temperatures. One-particle Green’s functions directly provide electronic energies and one-particle properties, such as dipole moment. However, the evaluation of two-particle properties, such as $\langle$S 2 $\rangle$ and $\langle$N 2 $\rangle$, can be challenging because they require a solution of the computationally expensive Bethe–Salpeter equation to find two-particle Green’s functions. We demonstrate that the solution of the Bethe–Salpeter equation can be completely avoided. Applying the thermodynamic Hellmann–Feynman theorem to self-consistent one-particle Green’s function methods, we derive expressions for two-particle density matrices in a general case and provide explicit expressions for GF2 and GW methods. Such density matrices can be decomposed into an antisymmetrized product of correlated one-electron density matrices and the two-particle electronic cumulant of the density matrix. Cumulant expressions reveal a deviation from ensemble representability for GW, explaining its known deficiencies. We analyze the temperature dependence of $\langle$S 2 $\rangle$ and $\langle$N 2 $\rangle$ for a set of small closed-shell systems. Interestingly, both GF2 and GW show a non-zero spin contamination and a non-zero fluctuation of the number of particles for closed-shell systems at the zero-temperature limit.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Sensitivity of Surface Temperature to Oceanic Forcing via q -Flux Green’s Function Experiments. Part I: Linear Response Function

This paper explores the use of the linear response function (LRF) to relate the mean sea surface temperature (SST) response to prescribed ocean heat convergence ( q flux) forcings. Two methods for constructing the LRF based on the fluctuation–dissipation theorem (FDT) and Green’s function (GRF) are examined. A 900-yr preindustrial simulation by the Community Earth System Model coupled with a slab ocean model (CESM–SOM) is used to estimate the LRF using FDT. For GRF, 106 pairs of CESM–SOM simulations with warm and cold q-flux patches are performed. FDT is found to have some skill in estimating the SST response to a q-flux forcing when the local SST response is strong, but it fails in inverse estimation of the q-flux forcing for a given SST pattern. In contrast, GRF is shown to be reasonably accurate in estimating both SST response and q-flux forcing. Possible degradation in FDT may be attributed to insufficient data sampling, significant departure of the SST distribution from Gaussianity, and the nonnormality of the constructed operator. The GRF-based LRF is then used to (i) generate a global surface temperature sensitivity map that shows the q-flux forcing in higher latitudes to be 3–4 times more effective than low latitudes in producing global surface warming, and (ii) identify the most excitable SST mode (neutral vector) that shows marked resemblance to the interdecadal Pacific oscillation (IPO). The latter discovery suggests that the IPO-like fluctuation exists in the absence of the coupling to the ocean dynamics. Coupling to the ocean dynamics in CESM, on the other hand, only enhances the spectral power of the IPO at interannual time scales.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Theoretical Study of Superconducting Gap Parameters, Density of States, and Condensation Energy of Two-Band Iron-Based Superconductor BaFe{sub 2}(As{sub 1−x}P{sub x}){sub 2}

This research work focuses on the theoretical study of superconducting gap parameters, density of states, and condensation energy of two-band iron-based superconductor BaFe{sub 2}(As{sub 1−x}P{sub x}){sub 2}. By developing a model Hamiltonian for the given system and by using the double time temperature-dependent Green’s function formalism, we obtained mathematical expressions for superconducting order parameters for the electron intra-band (Δ{sub e}), hole intra-band (Δ{sub h}), inter-band between the two bands (Δ{sub eh}) and superconducting transition temperature (T{sub C}). Furthermore, we obtained the density of states for each intra-band, N(ε) and condensation energy (E{sub c}) for BaFe{sub 2}(As{sub 1−x}P{sub x}){sub 2}. By using the experimental values and by considering some plausible approximations of the parameters in the obtained expressions, phase diagrams of superconducting order parameters for Δ{sub e}, Δ{sub h}, and Δ{sub eh} versus temperature and superconducting transition temperature (T{sub C}) versus the inter-band pairing potential (V{sub eh}) are plotted. We also plotted the density of states for the electron intra-band, N{sub e}(ε) and hole intra-band, N{sub h}(ε) versus excitation energy (ε) at T = 0K, and the variation of density of states with temperature. Similarly, phase diagrams of condensation energy (E{sub c}) versus temperature, inter-band pairing potential, and superconducting transition temperature are plotted for the material. Our results are in agreement with previous findings.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Sensitivity of Surface Temperature to Oceanic Forcing via q -Flux Green’s Function Experiments. Part II: Feedback Decomposition and Polar Amplification

A large set of Green’s function-type experiments is performed with q-flux forcings mimicking the effects of the ocean heat uptake (OHU) to examine the global surface air temperature (SAT) sensitivities to the location of the forcing. The result of the experiments confirms the earlier notion derived from experiments with different model complexities that the global mean SAT is far more sensitive to the oceanic forcing from high latitudes than the tropics. Remarkably, no matter in which latitude the q-flux forcings are placed, the SAT response is always characterized by a feature of polar amplification, implicating that it is intrinsic to our climate system. Considerable zonal asymmetry is also present in the efficacy of the tropical OHU, with the tropical eastern Pacific being much more efficient than the Indian Ocean and tropical Atlantic in driving global SAT warming by exciting the leading neutral mode of the SAT that projects strongly onto global mean warming. Using a radiative kernel, feedback analysis is also conducted to unravel the underlying processes responsible for the spatial heterogeneity in the global OHU efficacy, the polar amplification structures, and the tropical altruism of sharing the warmth with remote latitudes. Warming “altruism” for a q flux at a given latitude is also investigated in terms of the ratio of the induced remote latitudes versus the directly forced local warming. It is found that the tropics are much more altruistic than higher latitudes because of the high-energy transport efficiency of the Hadley circulation.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Sensitivity of surface temperature to oceanic forcing via q-Flux Green’s function experiments. Part III: asymmetric response to warming and cooling

Climate response is often assumed to be linear in climate sensitivity studies. However, by examining the surface temperature (TS) response to pairs of oceanic forcings of equal amplitude but opposite sign in a large set of local q-flux perturbation experiments with CAM5 coupled to a slab, we find strong asymmetry in TS responses to the heating and cooling forcings, indicating a strong nonlinearity intrinsic to the climate system examined. Regardless of where the symmetric forcing is placed, the cooling response to the negative forcing always exceeds the warming to the positive forcing, implying an intrinsic inclination towards cooling of our current climate. Thus, the ongoing global warming induced by increasing greenhouse gases may have already been alleviated by the asymmetric component of the response. The common asymmetry in TS response peaks in high latitudes, especially along sea ice edges, with notable seasonal dependence. Decomposition into different radiative feedbacks through a radiative kernel indicates that the asymmetry in the TS response is realized largely through lapse rate and albedo feedbacks. Further process interference experiments disabling the seasonal cycle and/or sea ice reveal that the asymmetry originates ultimately from the presence of the sea ice component and is further amplified by the seasonal cycle. The fact that a pair of opposite tropical q-flux forcings can excite very similar asymmetric response as a pair placed at 55°S strongly suggests the asymmetric response is a manifestation of an internal mode of the climate model system.

Liu, Fukai↗
DOE OSTI2020

Pairing instability on a Luttinger surface: A non-Fermi liquid to superconductor transition and its Sachdev-Ye-Kitaev dual

Superconductivity results from an instability of the Fermi surface—contour of poles of the single-particle propagator—to an infinitesimally small attraction between electrons. Here, we instead discuss the analogous problem on a model Luttinger surface, or contour of zeros of the Green function. At zero temperature ($β → \infty$) and a critical interaction strength ($u_{c∞}$) characterized by the residue of self-energy pole, we find that the pair susceptibility diverges leading to a superconducting instability. We evaluate the pair fluctuation partition function and find that the spectral density in the normal state has an interaction-driven, power-law $\frac{1}{\sqrt{ω}}$ type, van-Hove singularity (vHS), indicating non-Fermi liquid (NFL) physics. Crucially, in the strong coupling limit ($βu \gg 1$), the leading-order fluctuation free-energy terms in the normal state of this NFL-SC transition resembles the equivalent [$\textit{O}(1)$] terms of the Sachdev-Ye-Kitaev (SYK) model. This free-energy contribution takes a simple form $–βF = βu_{c∞} – γ \text{ln}(βu_{c∞}$), where $\gamma$ is a constant equal to $\frac{1}{2}$. Weak impurity scattering ($τ \gg β^{–1}$) leaves the low-energy spectral density unaffected but leads to an interaction-driven enhancement of superconductivity. Our results in this work shed light on the role played by order-parameter fluctuations in providing the key missing link between Mott physics and strongly coupled toy models exhibiting gravity duals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Real-space Green's function approach for x-ray spectra at high temperature

There has been considerable interest in properties of condensed matter at high temperature, including nonequilibrium behavior and extreme conditions up to the warm dense matter regime. Such behavior is encountered, e.g., in experimental time-resolved x-ray absorption spectroscopy in the presence of intense laser fields. In an effort to simulate such behavior, we present an approach for calculations of high-temperature x-ray absorption spectra in arbitrary materials, using a generalization of the real-space Green's function formalism. Here, the method is incorporated as an option in the core-level x-ray spectroscopy code FEFF10. To illustrate the approach, we present calculations for several materials together with comparisons to experiment and with other methods.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Metastable antiphase boundary ordering in CaFe 2 O 4

CaFe 2 O 4 is an $\textit{S}$ = 5/2 antiferromagnet exhibiting two magnetic orders that shows regions of coexistence at some temperatures. Using a Green’s function formalism, we model neutron scattering data of the spin wave excitations in this material, elucidating the microscopic spin Hamiltonian. In doing so, we suggest that the low-temperature A phase order (↑↑↓↓) finds its origins in the freezing of antiphase boundaries created by thermal fluctuations in a parent B phase order (↑↓↑↓). In this work, the low-temperature magnetic order observed in CaFe 2 O 4 is thus the result of a competition between the exchange coupling along $\textit{c}$, which favors the B phase, and the single-ion anisotropy, which stabilizes thermally generated antiphase boundaries, leading to static metastable A phase order at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Study of ultrasonic attenuation in f-electron systems in the paramagnetic limit of Coulomb interaction

We report here a microscopic model study of ultrasonic attenuation in f-electron systems based on Periodic Anderson Model in which Coulomb interaction is considered within a mean-field approximation for a weak interaction. The Phonon is coupled to the conduction band and f-electrons. The phonon Green's function is calculated by Zubarev's technique of the Green's function method. The temperature dependent ultrasonic attenuation co-efficient is calculated from the imaginary part of the phonon self-energy in the dynamic and long wave length limit. The f-electron occupation number is calculated self-consistently in paramagnetic limit of Coulomb interaction. The effect of the Coulomb interaction on ultrasonic attenuation is studied by varying the phonon coupling parameters to the conduction and f-electrons, hybridization strength, the position of f-level and the Coulomb interaction Strength. Results are discussed on the basis of experimental results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

The electronic heat capacity of YBa{sub 2}Cu{sub 3}O{sub 7−δ} superconductor

The contributions due to the point defects or disorder and anharmonicities which play deterministic role in the understanding of electronic heat capacity (EHC) of high temperature superconductors (HTS) have been investigated via electron density of states (EDOS) approach on the basis of quantum dynamical many body theory. The evaluation of EDOS has been carried out with the help of most versatile method of double time temperature dependent electron Green’s functions (GF) via a Hamiltonian (non BCS type) which includes the effects of electrons, phonons, defects, anharmonicity, and electron-phonon interactions which enables to account the effects of cubic anharmonicity besides with both the force constant changes and mass difference caused by the impurities in developing the results for EDOS and EHC. The new results reveal some striking features of EHC of HTS.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Chebyshev polynomial representation of imaginary-time response functions

Problems of finite-temperature quantum statistical mechanics can be formulated in terms of imaginary (Euclidean) -time Green's functions and self-energies. In the context of realistic Hamiltonians, the large energy scale of the Hamiltonian (as compared to temperature) necessitates a very precise representation of these functions. In this paper, we explore the representation of Green's functions and self-energies in terms of a series of Chebyshev polynomials. Here we show that many operations, including convolutions, Fourier transforms, and the solution of the Dyson equation, can straightforwardly be expressed in terms of the series expansion coefficients. We then compare the accuracy of the Chebyshev representation for realistic systems with the uniform-power grid representation, which is most commonly used in this context.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Self-consistent second-order Green’s function perturbation theory for periodic systems

Despite recent advances, systematic quantitative treatment of the electron correlation problem in extended systems remains a formidable task. Systematically improvable Green’s function methods capable of quantitatively describing weak and at least qualitatively strong correlations appear as promising candidates for computational treatment of periodic systems. We present a periodic implementation of temperature-dependent self-consistent 2nd-order Green’s function (GF2) method, where the self-energy is evaluated in the basis of atomic orbitals. Evaluating the real-space self-energy in atomic orbitals and solving the Dyson equation in k-space are the key components of a computationally feasible algorithm. We apply this technique to the one-dimensional hydrogen lattice — a prototypical crystalline system with a realistic Hamiltonian. By analyzing the behavior of the spectral functions, natural occupations, and self-energies, we claim that GF2 is able to recover metallic, band insulating, and at least qualitatively Mott regimes. We observe that the iterative nature of GF2 is essential to the emergence of the metallic and Mott phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗