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Results for “Temperature Green function”

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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

A Green's Function Wind Turbine Induction Model That Incorporates Complex Inflow Conditions

ABSTRACT In this work, we develop a new analytical turbine induction model that can incorporate complex inflow conditions including cases where the wind velocity and temperature profiles can vary as functions of height. This induction model is derived from the linearized Navier–Stokes and leads to a second‐order ODE that can be solved using a Green's function formulation. The corresponding Green's function for several configurations are found including the infinite domain, semi‐infinite domain with ground plane, and a power law velocity inflow profile. The results of this approach are then compared with simulations of the turbine induction field using the AMR‐Wind CFD solver with a uniformly loaded actuator disk model. These comparisons show that the Green's function approach captures the centerline blockage, three‐dimensional blockage flow field, and streamwise velocity slow down, with very good agreement for lower thrust conditions and at larger distances away from rotor disk. The effects of shear on the turbine blockage were also compared using a power law inflow profile, and we show that this approach matches the CFD predictions for the cases considered.

17 WIND ENERGY↗
DOE OSTI2021

The dominant contribution of Southern Ocean heat uptake to time - evolving radiative feedback in CESM

Radiative feedbacks are found to vary with time in both historical records and future warming projections. Previous studies proposed two factors that determine the variation of radiative feedbacks: (i) the evolution of tropical sea surface warming patterns and (ii) the tropical-extratropical contrast of ocean heat uptake. Our results bridge the two factors by evaluating the remote impact from the extratropical ocean on tropical temperature patterns, accounting for the changes in radiative feedbacks. Based on the Green’s Function approach that quantifies the non-local contributions of regional ocean heat uptake, we show that the net radiative feedback evolution in CESM can be mostly attributed to the heat uptake variations in the Southern Ocean. The enhanced surface warming associated with the weakened heat uptake decades after quadrupling CO2 is not confined over the Southern Ocean, but extends to tropical Southeastern Pacific, which leads to decreasing tropospheric stability and more positive cloud feedbacks.

Lin, Yuan-Jen↗
DOE OSTI2015

Infrared conductivity of cuprates using Yang-Rice-Zhang ansatz: Review of our recent investigations

A review of our recent investigations related to the ac transport properties in the psedogapped state of cuprate high temperature superconductors is presented. For our theoretical calculations we use a phenomenological Green’s function proposed by Yang, Rice and Zhang (YRZ). This is based upon the renormalized mean-field theory of the Hubbard model and takes into account the strong electron-electron interaction present in Cuprates. The pseudogap is also taken into account through a proposed self energy. We have tested the form of the Green’s function by computing ac conductivity of cuprates and then compared with experimental results. We found agreement between theory and experiment in reproducing the doping evolution of ac conductivity but there is a problem with absolute magnitudes and their frequency dependence. This shows a partial success of the YRZ ansatz. The ways to rectify it are suggested and worked out.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Coherent Superconductivity with a Large Gap Ratio from Incoherent Metals

Here, a mysterious incoherent metallic (IM) normal state with T-linear resistivity is ubiquitous among strongly correlated superconductors. Recent progress with microscopic models exhibiting IM transport has presented the opportunity for us to study new models that exhibit direct transitions into a superconducting state out of IM states within the framework of connected Sachdev-Ye-Kitaev “quantum dots.” Here, local Sachdev-Ye-Kitaev interactions within a dot produce IM transport in the normal state, while local attractive interactions drive superconductivity. Through explicit calculations, we find two features of superconductivity arising from an IM normal state. First, despite the absence of quasiparticles in the normal state, the superconducting state still exhibits coherent superfluid transport. Second, the nonquasiparticle nature of the IM Green’s functions produces a large enhancement in the ratio of the zero-temperature superconducting gap Δ and transition temperature T SC , 2Δ/T SC , with respect to its BCS value of 3.53.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Wide ranging equation of state with Tartarus: A hybrid Green’s function/orbital based average atom code

Average atom models are widely used to make equation of state tables and for calculating other properties of materials over a wide range of conditions, from zero temperature isolated atom to fully ionized free electron gases. The numerical challenge of making these density functional theory based models work for any temperature, density or nuclear species is formidable. Here we present in detail a hybrid Green’s function/orbital based approach that has proved to be stable and accurate for wide ranging conditions. Algorithmic strategies are discussed. In particular the decomposition of the electron density into numerically advantageous parts is presented and a robust and rapid self consistent field method based on a quasi-Newton algorithm is given. Example application to the equation of state of lutetium (Z = 71) is explored in detail, including the effect of relativity, finite temperature exchange and correlation, and a comparison to a less approximate method. Furthermore, the hybrid scheme is found to be numerically stable and accurate for lutetium over at least 6 orders of magnitude in density and 5 orders of magnitude in temperature.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Exploring the effects of molecular beam epitaxy growth characteristics on the temperature performance of state-of-the-art terahertz quantum cascade lasers

This study conducts a comparative analysis, using non-equilibrium Green’s functions (NEGF), of two state-of-the-art two-well (TW) Terahertz Quantum Cascade Lasers (THz QCLs) supporting clean 3-level systems. The devices have nearly identical parameters and the NEGF calculations with an abrupt-interface roughness height of 0.12 nm predict a maximum operating temperature (T max ) of ~ 250 K for both devices. However, experimentally, one device reaches a T max of ~ 250 K and the other a T max of only ~ 134 K. Both devices were fabricated and measured under identical conditions in the same laboratory, with high quality processes as verified by reference devices. The main difference between the two devices is that they were grown in different MBE reactors. Our NEGF-based analysis considered all parameters related to MBE growth, including the maximum estimated variation in aluminum content, growth rate, doping density, background doping, and abrupt-interface roughness height. From our NEGF calculations it is evident that the sole parameter to which a drastic drop in T max could be attributed is the abrupt-interface roughness height. We can also learn from the simulations that both devices exhibit high-quality interfaces, with one having an abrupt-interface roughness height of approximately an atomic layer and the other approximately a monolayer. However, these small differences in interface sharpness are the cause of the large performance discrepancy. This underscores the sensitivity of device performance to interface roughness and emphasizes its strategic role in achieving higher operating temperatures for THz QCLs. We suggest Atom Probe Tomography (APT) as a path to analyze and measure the (graded)-interfaces roughness (IFR) parameters for THz QCLs, and subsequently as a design tool for higher performance THz QCLs, as was done for mid-IR QCLs. Our study not only addresses challenges faced by other groups in reproducing the record T max of ~ 250 K and ~ 261 K but also proposes a systematic pathway for further improving the temperature performance of THz QCLs beyond the state-of-the-art.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Polarity dependent heating at the phase interface in metal-insulator transitions

We show that in a model of a metal photoexcited by a transient pump pulse resonant with a phonon mode, the absolute dc conductivity may become negative, depending on the interplay between the electronic structure, the phonon frequency, and the pump intensity. The analysis includes the effects of inelastic scattering and thermal relaxation. Results for the time evolution of the negative conductivity state are presented; the associated nonequilibrium physics may persist for long times after the pulse. Furthermore, our findings provide a theoretical justification for previously proposed phenomenology and indicate new routes to the generation and exploration of intrinsically nonequilibrium states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Analytical solution and parameter estimation for heat of wetting and vapor adsorption during spontaneous imbibition in tuff

An analytical expression is derived for the thermal response observed during spontaneous imbibition of water into a dry core of zeolitic tuff. Sample tortuosity, thermal conductivity, and thermal source strength are estimated from fitting an analytical solution to temperature observations during a single laboratory test. Here, the closed-form analytical solution is derived using Green's functions for heat conduction in the limit of “slow” water movement; that is, when advection of thermal energy with the wetting front is negligible. The solution has four free fitting parameters and is efficient for parameter estimation. Laboratory imbibition data used to constrain the model include a time series of the mass of water imbibed, visual location of the wetting front through time, and temperature time series at six locations. The thermal front reached the end of the core hours before the visible wetting front. Thus, the predominant form of heating during imbibition in this zeolitic tuff is due to vapor adsorption in dry zeolitic rock ahead of the wetting front. The separation of the wetting front and thermal front in this zeolitic tuff is significant, compared to wetting front behavior of most materials reported in the literature. This work is the first interpretation of a thermal imbibition response to estimate transport (tortuosity) and thermal properties (including thermal conductivity) from a single laboratory test.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Nature of the zero-bias conductance peak associated with Majorana bound states in topological phases of semiconductor-superconductor hybrid structures

Rashba spin-orbit coupled semiconductor-superconductor hybrid structures in the presence of Zeeman splitting have emerged as the first experimentally realizable topological superconductor supporting zero-energy Majorana bound states. However, recent experimental studies in these hybrid structures are not in complete agreement with the theoretical predictions, for example, the observed height of the zero-bias conductance peak (ZBCP) associated with the Majorana bound states is less than 10% of the predicted quantized value 2e 2 /h. We try to understand the sources of various discrepancies between the recent experiments and the earlier theories by starting from a microscopic theory and studying non-equilibrium transport in these systems at arbitrary temperatures and applied bias voltages. Our approach involves quantum Langevin equations and non-equilibrium Green’s functions. Here we are able to model the tunnel coupling between the one-dimensional semiconductor-superconductor hybrid structure and the metallic leads realistically; study the role of tunnel coupling on the height of the ZBCP and the subgap conductance; predict the nature of the splitting of the ZBCP with an increasing magnetic field beyond the critical field; show the behavior of the ZBCP with an increasing gate-controlled onsite potential; and study the evolution of the full differential conductance across the topological quantum phase transition. When the applied magnetic field is quite large compared to the Rashba splitting and the bulk energy gap is much reduced, we find the ZBCP even for an onsite potential much larger than the applied magnetic field. The height of the corresponding ZBCP depends on the tunnel coupling even at zero temperature and can be much smaller than 2e 2 /h.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Modeling graphene sheet growth and dynamical matrix calculations using molecular dynamics

Molecular dynamics (MD) has been an incredibly useful tool to model physical processes that were synthesized experimentally but not fully understood. MD, through the use of semi-empirical inter-atomic potentials, has allowed understanding of different physical processes in materials science. Yet as well as providing useful insights into materials science, molecular dynamics has a wider range of usability. In this report, I will be detailing how MD can be used to study graphene formation from a carbon liquid which requires high temperatures and pressures. Beyond this, I will describe the usefulness of MD for understanding the physics for phonon transport quantum sensors. To do this, MD was employed to determine the dynamical matrix by treating atoms as coupled oscillators. An accurate understanding of the dynamical matrix of a system is required to calculate the non-equilibrium Green’s function used to describe the phonon transport within phonon wave-guides. I found that, across multiple pressures and temperatures, randomly placed carbon atoms will show evidence of pent-first formation with semi-empirical models. Density functional theory (DFT), on the other hand, was too computationally expensive to use for full scale MD simulations, but we have the possibility of training a machine learned interatomic potential to approximate DFT for carbon in the environments being studied for pent-first graphene sheet formation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Exact calculation of the time convolutionless master equation generator: Application to the nonequilibrium resonant level model

The generalized quantum master equation provides a powerful tool to describe the dynamics in quantum impurity models driven away from equilibrium. Two complementary approaches, one based on Nakajima–Zwanzig–Mori time-convolution (TC) and the other on the Tokuyama–Mori time-convolutionless (TCL) formulations provide a starting point to describe the time-evolution of the reduced density matrix. A key in both approaches is to obtain the so called “memory kernel” or “generator,” going beyond second or fourth order perturbation techniques. While numerically converged techniques are available for the TC memory kernel, the canonical approach to obtain the TCL generator is based on inverting a super-operator in the full Hilbert space, which is difficult to perform and thus, nearly all applications of the TCL approach rely on a perturbative scheme of some sort. Here, the TCL generator is expressed using a reduced system propagator which can be obtained from system observables alone and requires the calculation of super-operators and their inverse in the reduced Hilbert space rather than the full one. This makes the formulation amenable to quantum impurity solvers or to diagrammatic techniques, such as the nonequilibrium Green’s function. We implement the TCL approach for the resonant level model driven away from equilibrium and compare the time scales for the decay of the generator with that of the memory kernel in the TC approach. Furthermore, the effects of temperature, source-drain bias, and gate potential on the TCL/TC generators are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Two-Dimensional Mixed Problem of Thermoelasticity for a Semistrip

We study the stress state of a semistrip with conditions of smooth contact imposed on one of its lateral surfaces and coupling conditions given on the other surface under the action of normal loading and temperature on the end face. The original problem is reduced to a one-dimensional vector boundary-value problem by the method of integral transformations. By using the tools of matrix differential calculus and Green matrix function, we reduce the obtained problem to a singular integral equation for the derivative of displacements on the end face of the semistrip and solve this equation by the method of orthogonal polynomials. We perform the numerical analysis of fields of displacements and stresses inside the semistrip and establish the zones of tensile stresses on the lateral face of the semistrip and the conditions of their formation.

42 ENGINEERING↗
DOE OSTI2014

Simulation-based design of a strained graphene field effect transistor incorporating the pseudo magnetic field effect

We present a numerical study on the performance of strained graphene-based field-effect transistors. A local strain less than 10% is applied over a central channel region of the graphene to induce the shift of the Dirac point in the channel region along the transverse momentum direction. The left and the right unstrained graphene regions are doped to be either n-type or p-type. By using the atomistic tight-binding model and a Green's function method, we predict that the gate voltage applied to the central strained graphene region can switch the drain current on and off with an on/off ratio of more than six orders of magnitude at room temperature. This is in spite of the absence of a bandgap in the strained channel region. Steeper subthreshold slopes below 60 mV/decade are also predicted at room temperature because of a mechanism similar to the band-to-band tunneling field-effect transistors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Finite-temperature calculations of the Compton profile of Be, Li, and Si

High resolution inelastic x-ray scattering experiments are widely used to study the electronic and chemical properties of materials under a range of conditions, from ambient temperature to the warm dense matter regime. Here, we use the real-space multiple scattering (RSMS) Green's function formalism coupled with density functional theory molecular dynamics (DFT-MD) to study thermal effects on the Compton profile (CP) of disordered systems. The RSMS method is advantageous for calculations of highly disordered, aperiodic systems because it places no restriction on symmetry. As a test, we apply our approach to thermally disordered Be, Li, and Si in both liquid and solid phases. We find good agreement with experimental and other theoretical results, showing that the real-space multiple scattering approach coupled with DFT-MD is an efficient and reliable method for calculating the CP of disordered systems at finite temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Low-temperature thermal properties of Fe-doped Co 3 Sn 2 S 2

Recent studies on Co 3 Sn 2 S 2 usually focus on its electrical and magnetic behaviors as it is a Weyl semimetal, half metal, and anisotropic ferromagnet at the same time. Only a few works looked into its low-temperature thermal characteristics. Here, in this study, we report the low-temperature heat capacity, Seebeck coefficient, and thermal conductivity of Fe-doped Co 3 Sn 2 S 2 , as well as the formation energies, band structures, and spin-polarized density of states calculated by using the spin-polarized relativistic Korringa-Kohn-Rostoker – Green’s function method (sprKKR). The heat capacity data reveal that the sample with a heavier Fe-doping level has a lower Debye temperature and a larger density of states at the Fermi level, the latter confirmed by our ab initio computational results. The Fe-doping increases the absolute value of the Seebeck coefficient at temperatures above 120 K and dramatically suppresses the thermal conductivity. Furthermore, we discuss how the external applied magnetic field affects the Seebeck coefficient and thermal conductivity of pristine Co 3 Sn 2 S 2 . Our findings provide a detailed reference and may stimulate future investigations into the low-temperature thermal transport properties of Co 3 Sn 2 S 2 and other compounds in the shandite family.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Nitrogen doped carbon quantum dots as a green luminescent sensitizer to functionalize ZnO nanoparticles for enhanced photovoltaic conversion devices

Highlights: • Hydrothermal synthesis for highly green luminescent nitrogen doped carbon quantum dots (NCQDs) was proposed. • ZnO NPs were prepared by CTAB assisted low temperature reaction method. • High green luminescence was observed from NCQDs. • NCQDs are applied as light harvesting material in ZnO photoanodes. • Enhanced photoconversion efficiency was observed with NCQDs. - Abstract: A simple and facile hydrothermal approach for the synthesis of highly green luminescent nitrogen doped carbon quantum dots (NCQDs) is reported. The prepared NCQDs show narrow size distribution of 2–3 nm, abundant surface functional groups and good water solubility. To showcase the potential of these NCQDs as a solar light harvesting material, we designed NCQDs sensitized ZnO nanoparticles based photoanodes for photovoltaic solar cells. As a result, NCQDs sensitized ZnO photoanodes exhibited 75% higher photoconversion efficiency when compared to that of bare ZnO photoanodes. The characterization and optical properties of NCQDs sensitized ZnO photoanodes is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Using the Chebychev expansion in quantum transport calculations

Irradiation by laser pulses and a fluctuating surrounding liquid environment can, for example, lead to time-dependent effects in the transport through molecular junctions. From the theoretical point of view, time-dependent theories of quantum transport are still challenging. In one of these existing transport theories, the energy-dependent coupling between molecule and leads is decomposed into Lorentzian functions. This trick has successfully been combined with quantum master approaches, hierarchical formalisms, and non-equilibrium Green’s functions. The drawback of this approach is, however, its serious limitation to certain forms of the molecule-lead coupling and to higher temperatures. Tian and Chen [J. Chem. Phys. 137, 204114 (2012)] recently employed a Chebychev expansion to circumvent some of these latter problems. Here, we report on a similar approach also based on the Chebychev expansion but leading to a different set of coupled differential equations using the fact that a derivative of a zeroth-order Bessel function can again be given in terms of Bessel functions. Test calculations show the excellent numerical accuracy and stability of the presented formalism. The time span for which this Chebychev expansion scheme is valid without any restrictions on the form of the spectral density or temperature can be determined a priori.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Spin-Dependent Thermoelectric Effects in Double-Barrier Magnetic Tunnel Junctions with a Non-magnetic Metal Spacer

Spin-dependent thermoelectric (TE) effects are studied in MgO-based double-barrier magnetic tunnel junctions with a non-magnetic metal spacer (DBMTJs-NM) in the linear response regime. Using non-equilibrium Green’s function (NEGF) formalism, the results show resonant peaks at specific thicknesses of the NM spacer because of the resonant tunneling effect through the DBMTJs-NM. Effects of average temperature and magnetization alignment are also described. Therefore, thermally charge and spin voltage lead to large spin-dependent TE effects by the DBMTJs-NM structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗