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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Nodal superconductivity in miassite Rh 17 S 15

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh 17 S 15 (T c = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δλ(T) and the disorder evolution of the critical superconducting temperature T c and the upper critical field H c2 (T) in single crystalline Rh 17 S 15 . We found a T - linear temperature variation of Δλ(T) below 0.3T c , which is consistent with the presence of nodal lines in the superconducting gap of Rh 17 S 15 . The nodal character of the superconducting state is supported by the observed suppression of T c and H c2 (T) in samples with a controlled level of non-magnetic disorder introduced by 2.5 MeV electron irradiation. We propose a nodal sign-changing superconducting gap in the A 1g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the derived superfluid density. To the best of our knowledge, this establishes miassite as the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form, though it is unlikely that it is present in natural crystals because of unavoidable impurities that quickly destroy nodal superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Tunable high Néel temperature and large anomalous Hall response in antiferromagnetic Weyl semimetal Mn 3 Sn 1− x Ga x thin films

Antiferromagnetic Weyl semimetals based on Mn 3 X(X = Ge, Sn, Ga) kagome compounds exhibit the same ferromagnetic-like responses, including anomalous Hall, Nernst, and magneto-optical effects, as recently discussed for altermagnets. Driven by the Berry curvature due to Weyl fermions, these materials show a disproportionately large magnitude of electromagnetic effects even in the absence of large magnetization. For applications it is crucial to realize these responses in a wide range of temperatures both below and above 300 K. While stoichiometric Mn 3 X materials do not offer optimal performance, we show that Mn 3 Sn 1−x Ga x sputtered films with a variable composition offers a tunable Néel temperature, T N ≈ 425 ± 6–500 ± 15 K, which is crucial for device applications, together with a large tunable anomalous Hall effect. Our thin film growth method enables continuous and precise control over the film composition between x = 0 and x = 1. Through a detailed magnetization and Hall transport, we establish the magnetic phase diagram for the hexagonal Mn 3 Sn 1−x Ga x . Our results reveal an enhanced T N and antichiral magnetic phase in Ga-doped Mn 3 Sn and an enhanced anomalous Hall magnitude in Sn-doped Mn 3 Ga compared to their stoichiometric undoped forms. Our work demonstrates a route to optimize the technologically relevant antiferromagnets for various applications.

Magnetic properties and materials↗
DOE OSTI2020

Quantum computation of silicon electronic band structure

Development of quantum architectures during the last decade has inspired hybrid classical–quantum algorithms in physics and quantum chemistry that promise simulations of fermionic systems beyond the capability of modern classical computers, even before the era of quantum computing fully arrives. Strong research efforts have been recently made to obtain minimal depth quantum circuits which could accurately represent chemical systems. Here, we show that unprecedented methods used in quantum chemistry, designed to simulate molecules on quantum processors, can be extended to calculate properties of periodic solids. In particular, we present minimal depth circuits implementing the variational quantum eigensolver algorithm and successfully use it to compute the band structure of silicon on a quantum machine for the first time. We are convinced that the presented quantum experiments performed on cloud-based platforms will stimulate more intense studies towards scalable electronic structure computation of advanced quantum materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Semiconductor to Topological Insulator Transition Induced by Stress Propagation in Metal Dichalcogenides Core-Shell Lateral Heterostructures

Polymorphic phase transition is an important route for engineering the properties of two-dimensional materials. Heterostructure construction, on the other hand, not only allows the integration of different functionalities for device applications, but also enables the exploration of new physics arising from proximity coupling. Yet, implementing a design that incorporates the advantages of both remains underexplored. Here in this paper, based on comprehensive experimental and theoretical studies of WSe 2 /SnSe 2 core-shell lateral heterostructure, we demonstrate an unexpected H to T' phase transition in transition metal dichalcogenides (TMDs), correlating to a change of materials properties from semiconductor to topological insulator (TI), and propose a novel shell-to-core stress propagation mechanism. This finding offers new insights into TMD phase transition empowered by the rational design of heterostructures. Owing to the superconducting properties of SnSe 2 at low temperatures, the unique TI/superconductor core-shell template is expected to add more arsenals to the ongoing search of Majorana fermions in condensed matter systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hexagonal layered group IV–VI semiconductors and derivatives: fresh blood of the 2D family

New phases of group IV–VI semiconductors in 2D hexagonal structures are predicted and their unusual physical properties are revealed. The structures of monolayer group IV–VI semiconductors are similar to those of blue phosphorene and each unit has the same ten valence electrons. The band gap of 2D hexagonal group IV–VI semiconductors depends on both the thickness and stacking order. Atomic functionalization can induce ferromagnetism, and the Curie temperature can be tuned. Gapped Dirac fermions with zero mass are developed and this makes it exceed that of graphene. Finally, the Fermi velocity can be compared to or even above that of graphene.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Low-energy pion-nucleon scattering and the Δ resonance in lattice QCD

We report on our investigation of low-energy pion-nucleon scattering from lattice QCD with Wilson type fermions. Our focus is on the isospin I = 3/2 channel, which couples to the Delta resonance. We discuss our calculations aiming at the extraction of the Delta resonance mass, width and effective coupling at pion mass 250 MeV, using the Lüscher finite-volume method to extract the phase shift δ J=3/2,l=1 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

QED Mesons, the QED Neutron, and the Dark Matter

Schwinger’s boson solution for massless fermions in QED in 1+1D has been applied and generalized to quarks interacting in QED and QCD interactions, leading to stable and confined open-string QED and QCD boson excitations of the quark-QCD-QED system in 1+1D. Just as the open-string QCD excitations in 1+1D can be the idealization of QCD mesons with a flux tube in 3+1D, so the open-string QED excitations in 1+1D may likewise be the idealization of QED mesons with masses in the tens of MeV region, corresponding possibly to the anomalous X17 and E38 particles observed recently. A further search for bound states of quarks interacting in the QED interaction alone leads to the examination on the stability of the QED neutron, consisting of two d quarks and one u quark. Theoretically, the QED neutron has been found to be stable and estimated to have a mass of 44.5 MeV, whereas the analogous QED proton is unstable, leading to a long-lived QED neutron that may be a good candidate for the dark matter.

Wong, Cheuk-Yin↗
DOE OSTI2020

Sub-system quantum dynamics using coupled cluster downfolding techniques

In this paper, we discuss extending the sub-system embedding sub-algebra coupled cluster (SES-CC) formalism and the double unitary coupled cluster (DUCC) ansatz to the time domain. As we demonstrated in earlier studies, it is possible, using these formalisms, to calculate the energy of the entire system as an eigenvalue of downfolded/effective Hamiltonian in the active space, that is identifiable with the sub-system of the composite system. In these studies, we demonstrated that downfolded Hamiltonians integrate out Fermionic degrees of freedom that do not correspond to the physics encapsulated by the active space. We extend these results to the time-dependent Schrödinger equation, showing that a similar construct is possible to partition a system into a sub-system that varies slowly in time and a remaining subsystem that corresponds to fast oscillations. This time dependent formalism allows coupled cluster quantum dynamics to be extended to larger systems and for the formulation of novel quantum algorithms based on the quantum Lanczos approach, which have recently been considered in the literature.

coupled cluster, Electron correlation, quantum dyn↗
DOE OSTI2020

High quality epitaxial thin films and exchange bias of antiferromagnetic Dirac semimetal FeSn

FeSn is a topological semimetal (TSM) and kagome antiferromagnet (AFM) composed of alternating Fe 3 Sn kagome planes and honeycomb Sn planes. This unique structure gives rise to exotic features in the band structures such as the coexistence of Dirac cones and flatbands near the Fermi level, fully spin-polarized 2D surface Dirac fermions, and the ability to open a large gap in the Dirac cone by reorienting the Néel vector. In this paper, we report the synthesis of high-quality epitaxial (0001) FeSn films by magnetron sputtering. Using FeSn/Py heterostructures, we show a large exchange bias effect that reaches an exchange field of 220 Oe at 5 K, providing unambiguous evidence of antiferromagnetism and strong interlayer exchange coupling in our films. Field cycling studies show steep initial training effects, highlighting the complex magnetic interactions and anisotropy. Importantly, our work provides a simple, alternative means to fabricate FeSn films and heterostructures, making it easier to explore the topological physics of AFM TSMs and develop FeSn-based spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

A comprehensive ARPES study on the type-II Dirac semimetal candidate Ir 1-x Pt x Te 2

The transition metal dichalcogenide Ir 1-x Pt x Te 2 displays both superconductivity and a topological band structure. Using angle-resolved photoemission spectroscopy, we obtain a comprehensive understanding of the three-dimensional electronic structure in the normal state of Ir 1-x Pt x Te 2 for doping levels from x = 0.1 to 0.4, which spans the composition range of a superconducting state to a non-superconducting state. Many features of the electronic structure can be attributed to strong Te–Te interactions between the layers of the layered crystal structure and can be resolved by photon energy dependent measurements. We demonstrate that the type-II Dirac fermions can be successfully tuned via Pt doping, where the Dirac point lies close to the Fermi level for x = 0.1. The band evolution vs doping provides a clearer understanding of the relationship between the superconductivity and electronic structure. In addition, the β band in the superconducting samples locates the system close to a type-II van Hove singularity, where spin triplet paring symmetry has been predicted. Our results provide a comprehensive understanding of the band structure of Ir 1-x Pt x Te 2 , and we discuss the possibilities of the existence of topological superconductivity in this system.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electronic structure of non-centrosymmetric PtBi2 studied by angle-resolved photoemission spectroscopy

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the electronic structure of non-centrosymmetric PtBi2 with a layered structure. Through photon energy dependent measurements, the surface state and the bulk state were identified. Consistent with the non-centrosymmetric structure, different surface states were observed on opposite PtBi2(001) surfaces. Experimental bulk bands agree nicely with first-principle calculations. By combining ARPES and calculations, energy bands that contribute to triply degenerate point fermion were all detected. We also suggest that there are “canted” tube-like Fermi surfaces that might cause strong anisotropy in electronic transportation in non-centrosymmetric PtBi2.

Jiang, Wenxiang (ORCID:000000020524259X)↗
DOE OSTI2020

Toward a systematic improvement of the fixed-node approximation in diffusion Monte Carlo for solids—A case study in diamond

While Diffusion Monte Carlo (DMC) is in principle an exact stochastic method for ab initio electronic structure calculations, in practice, the fermionic sign problem necessitates the use of the fixed-node approximation and trial wavefunctions with approximate nodes (or zeros). This approximation introduces a variational error in the energy that potentially can be tested and systematically improved. Here, we present a computational method that produces trial wavefunctions with systematically improvable nodes for DMC calculations of periodic solids. These trial wavefunctions are efficiently generated with the configuration interaction using a perturbative selection made iteratively (CIPSI) method. A simple protocol in which both exact and approximate results for finite supercells are used to extrapolate to the thermodynamic limit is introduced. This approach is illustrated in the case of the carbon diamond using Slater–Jastrow trial wavefunctions including up to one million Slater determinants. Fixed-node DMC energies obtained with such large expansions are much improved, and the fixed-node error is found to decrease monotonically and smoothly as a function of the number of determinants in the trial wavefunction, a property opening the way to a better control of this error. The cohesive energy extrapolated to the thermodynamic limit is in close agreement with the estimated experimental value. Interestingly, this is also the case at the single-determinant level, thus, indicating a very good error cancellation in carbon diamond between the bulk and atomic total fixed-node energies when using single-determinant nodes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Out of time order correlation of the Hubbard model with random local disorder

The out-of-time-order correlator (OTOC) serves as a powerful tool for investigating quantum information spreading and chaos in complex systems. We present a method employing non-equilibrium dynamical mean-field theory and coherent potential approximation combined with diagrammatic perturbation on the Schwinger–Keldysh contour to calculate the OTOC for correlated fermionic systems subjected to both random disorder and electron interaction. Furthermore, our key finding is that random disorder enhances the OTOC decay in the Hubbard model for the metallic phase in the weakly interacting limit. However, the current limitation of our perturbative solver restricts the applicability to weak interaction regimes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Toward first principles-based simulations of dense hydrogen

Accurate knowledge of the properties of hydrogen at high compression is crucial for astrophysics (e.g., planetary and stellar interiors, brown dwarfs, atmosphere of compact stars) and laboratory experiments, including inertial confinement fusion. There exists experimental data for the equation of state, conductivity, and Thomson scattering spectra. However, the analysis of the measurements at extreme pressures and temperatures typically involves additional model assumptions, which makes it difficult to assess the accuracy of the experimental data rigorously. On the other hand, theory and modeling have produced extensive collections of data. They originate from a very large variety of models and simulations including path integral Monte Carlo (PIMC) simulations, density functional theory (DFT), chemical models, machine-learned models, and combinations thereof. At the same time, each of these methods has fundamental limitations (fermion sign problem in PIMC, approximate exchange–correlation functionals of DFT, inconsistent interaction energy contributions in chemical models, etc.), so for some parameter ranges accurate predictions are difficult. Recently, a number of breakthroughs in first principles PIMC as well as in DFT simulations were achieved which are discussed in this review. Here we use these results to benchmark different simulation methods. We present an update of the hydrogen phase diagram at high pressures, the expected phase transitions, and thermodynamic properties including the equation of state and momentum distribution. Furthermore, we discuss available dynamic results for warm dense hydrogen, including the conductivity, dynamic structure factor, plasmon dispersion, imaginary-time structure, and density response functions. We conclude by outlining strategies to combine different simulations to achieve accurate theoretical predictions that are based on first principles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Thermal mean-field theories

Several closely related ab initio thermal mean-field theories for fermions, both well-established and new ones, are compared with one another at the formalism level and numerically. The theories considered are Fermi–Dirac theory; thermal Hartree–Fock (HF) theory; two modifications of the thermal single-determinant and the first-order finite-temperature many-body perturbation theory based on a zero-temperature or thermal HF reference. Furthermore, thermal full-configuration-interaction theory is used as the benchmark.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2026

Terahertz magneto-nanoscopy of encapsulated monolayer graphene

This study investigates the nanoscale conductivity of encapsulated monolayer graphene at temperatures down to 5 K and magnetic fields of up to 1 T. We use the scattering-type scanning near-field optical microscopy technique to probe magnetic-field-dependent responses from graphene close to charge neutrality in the terahertz spectral region. We observe the near-perfect high-q reflector behavior of graphene but with subtle changes by the presence of magnetic fields. Measurements align with calculations of the magneto-optical conductivity and the near-field spectroscopic contrast that describes the field-tunable cyclotron resonance of Dirac fermions. Our result provides an initial step toward understanding temperature and magnetic field effects on nanoscale terahertz transport in two-dimensional quantum materials.

Kim, Richard H. J. [Ames Laboratory (AMES), Ames, ↗
DOE OSTI2026

Seniority eigenstate configuration interaction

Zero-seniority methods have shown great promise for the description of strongly correlated electronic systems. Other seniority sectors have been much less explored, and in particular, the maximal seniority sector and zero seniority have the same underlying algebraic structure. We introduce a seniority eigenstate configuration interaction in which the wave function is constrained to have good fixed local seniority for each paired orbital, by which we mean we partition orbitals into a pairing set with seniority zero, and a spin set with seniority one. Here, we show how to build the effective Hamiltonian for this ansatz, and demonstrate that high-seniority wave functions have unexpectedly excellent accuracy for strongly correlated fermionic systems, with accuracy competitive with or better than seniority zero for the Hubbard model and for the dissociation of the nitrogen molecule.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Theory of the strange metal Sr 3 Ru 2 O 7

The bilayer perovskite Sr 3 Ru 2 O 7 has been widely studied as a canonical strange metal. It exhibits T -linear resistivity and a T log(1/ T ) electronic specific heat in a field-tuned quantum cr itical fan. Criticality is known to occur in “hot” Fermi pockets with a high density of states close to the Fermi energy. We show that while these hot pockets occupy a small fraction of the Brillouin zone, they are responsible for the anomalous transport and thermodynamics of the material. Specifically, a scattering process in which two electrons from the large, “cold” Fermi surfaces scatter into one hot and one cold electron renders the ostensibly noncritical cold fermions a marginal Fermi liquid. From this fact the transport and thermodynamic phase diagram is reproduced in detail. To conclude, we show that the same scattering mechanism into hot electrons that are instead localized near a 2D van Hove singularity explains theanomalous transport observed in strained Sr 2 RuO 4 .

36 MATERIALS SCIENCE↗