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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Revealing a 3D Fermi Surface Pocket and Electron-Hole Tunneling in UTe 2 with Quantum Oscillations

Spin triplet superconductor UTe 2 is widely believed to host a quasi-two-dimensional Fermi surface, revealed by first-principles calculations, photoemission and quantum oscillation measurements. An outstanding question still remains as to the existence of a three-dimensional Fermi surface pocket, which is crucial for our understanding of the exotic superconducting and topological properties of UTe 2 . This 3D Fermi surface pocket appears in various theoretical models with different physics origins, but has not been unambiguously detected in experiment. Here, in this study, for the first time we provide concrete evidence for a relatively isotropic, small Fermi surface pocket of UTe 2 via quantum oscillation measurements. In addition, we observed high frequency quantum oscillations corresponding to electron-hole tunneling between adjacent electron and hole pockets. The coexistence of 2D and 3D Fermi surface pockets, as well as the breakdown orbits, provides a test bed for theoretical models and aid the realization of a unified understanding of superconducting state of UTe 2 from the first-principles approach.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Li interaction-induced phase transition from black to blue phosphorene

A comprehensive first-principle calculation has been carried out and revealed that at sufficiently high Li concentration and certain well-defined configurations a phase transition from black to blue phosphorene can take place. Blue phosphorene, a newly predicted allotrope of phosphorus, possesses unique crystalline and electronic structure and is a promising candidate, not only for fundamental research but also for electronic and optoelectronic applications. Methods to growth high quality blue phosphorene layers are highly desirable but challenging. Here, a novel kinetic pathway to grow blue phosphorene layers from black phosphorene layers via Li intercalation is proposed based on first principle study. This study pointed out that Li atoms intercalated in black phosphorene could act as ‘catalyst’ in the ‘reactive region’ of the lone pair of P atoms, leading to a P-P bond breaking and subsequently, a local structural transformation from orthorhombic lattice to an assembly of parallel narrow nanoribbons with rhombohedra-like symmetry. During Li deintercalation, these nanoribbons are self-mended and form blue phosphorene layers. The interlayer distance was found 4.60 Å for double layer with AA stacking and 4.13 Å for multilayer with ABC stacking, respectively, indicating a monolayer blue phosphorene can be mechanically exfoliated. Furthermore, this study also points out the possibility of new phases in other systems, where intercalation can lead to an unexpected structural phase transition and even a discovery of novel materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Primitive to conventional geometry projection for efficient phonon transport calculations

Abstract The primitive Wigner-Seitz cell and corresponding first Brillouin zone (FBZ) are typically used in calculations of lattice vibrational and transport properties as they contain the smallest number of degrees of freedom and thus have the cheapest computational cost. However, in complex materials, the FBZ can take on irregular shapes where lattice symmetries are not apparent. Thus, conventional cells (with more atoms and regular shapes) are often used to describe materials, though dynamical and transport calculations are more expensive. Here we discuss an efficient anharmonic lattice dynamic method that maps conventional cell dynamics to primitive cell dynamics based on translational symmetries. Such symmetries have not been utilized in typical lattice dynamical calculations. This leads to phase interference conditions that act like conserved quantum numbers and a conservation rule for phonon scattering that is hidden in conventional dynamics which significantly reduces the computational cost. We demonstrate this method for phonon transport in a variety of materials with inputs from first-principles calculations and attribute its efficiency to reduced scattering phase space and fewer summations in scattering matrix element calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Cs X Si 15 P 21 ( X = Sn or Pb): Polar Noncentrosymmetric Si–P Frameworks Stabilized by Covalent X –P Bonding

Metal silicon phosphides composed of earth-abundant Si and P tend to exhibit semiconducting properties and adopt diverse crystal structures with relatively small additions of structure-directing elements. The potential of silicon phosphide materials in nonlinear optical applications has been hindered by the inability to systematically produce noncentrosymmetric structures with such a flexible framework. Here, in this work, two isostructural compounds with a novel noncentrosymmetric structure were made possible by the inclusion of elements with stereochemically active lone pairs (Sn 2+ and Pb 2+ ). The structures were determined through single-crystal and synchrotron powder X-ray diffraction. Analysis of chemical bonding in real space through the electron localization function revealed stereochemically active Pb 2+ and Sn 2+ species in a trigonal pyramidal coordination with {Pb/Sn}–P bonds. Such covalent bonding between Pb and P is quite uncommon in extended solids and has been reported in a few rare instances. Band structure calculations and linear optical measurements confirm the semiconducting nature of Cs X Si 15 P 21 ( X = Sn or Pb). The synthesis was optimized to yield high-purity polycrystalline samples. The nonlinear optical properties show promising second-harmonic generation (SHG) coefficients from the Kurtz–Perry method. First-principles calculations of the nonlinear optical properties support the experimentally determined SHG values and provide moderate values of birefringence, suggesting Cs X Si 15 P 21 could be phase-matchable and practical nonlinear optical materials in the mid-IR region.

crystal structure↗
DOE OSTI2023

Dynamics of Aqueous Electrolyte Solutions: Challenges for Simulations

This Perspective article focuses on recent simulation work on the dynamics of aqueous electrolytes. It is well-established that full-charge, nonpolarizable models for water and ions generally predict solution dynamics that are too slow in comparison to experiments. Models with reduced (scaled) charges do better for solution diffusivities and viscosities but encounter issues describing other dynamic phenomena such as nucleation rates of crystals from solution. Polarizable models show promise, especially when appropriately parametrized, but may still miss important physical effects such as charge transfer. First-principles calculations are starting to emerge for these properties that are in principle able to capture polarization, charge transfer, and chemical transformations in solution. Finally, while direct ab initio simulations are still too slow for simulations of large systems over long time scales, machine-learning models trained on appropriate first-principles data show significant promise for accurate and transferable modeling of electrolyte solution dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Magnetocaloric effect in Mn-rich Heusler-derived alloys for room temperature-based applications

Magnetic refrigeration-based technologies rely on the magnetocaloric properties of materials and are crucial for improving energy efficiency and for supporting clean environments. However, materials possessing significant magnetocaloric properties that can be used for room temperature-based applications are still lacking. Heusler alloys (such as Mn–Ni–Ga) show giant magnetocaloric effects, which can be further improved by tuning their composition. Herein, we optimized the ratio of Ni to Ga to achieve an alloy with unique magnetocaloric properties. Here, the alloy demonstrated a substantial magnetocaloric effect, attributed to its narrow temperature range for structural and magnetic phase transitions (near room temperature with minimal hysteresis). Structural phase changes were also observed through high-resolution transmission electron microscopy. Optimum entropy changes, calculated from isothermal magnetization curves (4 Tesla magnetic field), were found to be 24.50 J kg −1 K −1 , which is higher than that of state-of-the-art alloys. Experimental demonstration of the magnetocaloric effect (refrigeration cycle) was also carried out using a thermal imaging camera. Furthermore, first-principles calculations were conducted to validate the experimental findings, specifically focusing on the compositional effects on the structural transitions and magnetic properties of the investigated MNG alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Cobalt-based magnetic Weyl semimetals with high-thermodynamic stabilities

Abstract Recent experiments identified Co 3 Sn 2 S 2 as the first magnetic Weyl semimetal (MWSM). Using first-principles calculation with a global optimization approach, we explore the structural stabilities and topological electronic properties of cobalt (Co)-based shandite and alloys, Co 3 MM’X 2 (M/M’ = Ge, Sn, Pb, X = S, Se, Te), and identify stable structures with different Weyl phases. Using a tight-binding model, for the first time, we reveal that the physical origin of the nodal lines of a Co-based shandite structure is the interlayer coupling between Co atoms in different Kagome layers, while the number of Weyl points and their types are mainly governed by the interaction between Co and the metal atoms, Sn, Ge, and Pb. The Co 3 SnPbS 2 alloy exhibits two distinguished topological phases, depending on the relative positions of the Sn and Pb atoms: a three-dimensional quantum anomalous Hall metal, and a MWSM phase with anomalous Hall conductivity (~1290 Ω −1 cm −1 ) that is larger than that of Co 2 Sn 2 S 2 . Our work reveals the physical mechanism of the origination of Weyl fermions in Co-based shandite structures and proposes topological quantum states with high thermal stability.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Infrared spectroscopy data- and physics-driven machine learning for characterizing surface microstructure of complex materials

There is a need to characterize complex materials and their dynamics under reaction conditions to accelerate materials design. Adsorbate vibrational excitations are selective to adsorbate/surface interactions and infrared (IR) spectra associated with activating adsorbate vibrational modes are accurate, capture details of most modes, and can be obtained operando. Current interpretation depends on heuristic peak assignments for simple spectra, precluding the possibility of obtaining detailed structural information. Here, we combine data-based approaches with chemistry-dependent problem formulation to develop physics-driven surrogate models that generate synthetic IR spectra from first-principles calculations. Using synthetic IR spectra of carbon monoxide on platinum, we implement multinomial regression via neural network ensembles to learn probability distributions functions (pdfs) that describe adsorption sites and quantify uncertainty. We use these pdfs to infer detailed surface microstructure from experimental spectra and extend this methodology to other systems as a first step towards characterizing complex interfaces and closing the materials gap.

Science & Technology - Other Topics↗
DOE OSTI2026

Insulating ground state and 2−𝑘 magnetic structure of candidate Weyl hydrogen-atom K2⁢Mn3⁢(AsO4)3

The ideal Weyl "hydrogen-atom" semimetal exhibits only a single pair of Weyl nodes and no other trivial states at the Fermi energy. Such a material would be a panacea in the study of Weyl quasi-particles, allowing direct unambiguous observation of their topological properties. The alluaudite-like K2⁢Mn3⁢(AsO4)3 compound was recently proposed as such a material. Here, we use comprehensive experimental work and first-principle calculations to assess this prediction. We find K2⁢Mn3⁢(AsO4)3 crystallizes in the 𝐶⁢2/𝑐 symmetry with a quasi-one-dimensional Mn sublattice, growing as small needle-like crystals. Bulk property measurements reveal magnetic transitions at ≈8 and ≈4 K, which neutron scattering experiments show correspond to two distinct magnetic orders, first a partially ordered ferrimagnetic 𝐤𝟏=(0,0,0) structure at 8 K and a second transition of 𝐤𝟐=(1,0,0) at 4 K to a fully ordered state. Below the second transition, both ordering vectors are necessary to describe the complex magnetic structure with modulated spin magnitudes. Both of the best-fit magnetic structures in this work are found to break the symmetry necessary for the generation of Weyl nodes, though one of the magnetic structures allowed by 𝐤𝟏 does preserve this symmetry. However, the crystals are optically transparent and ellipsometry measurements reveal a large band gap, undermining expectations of semimetallic behavior. Density functional theory calculations predict an insulating antiferromagnetic ground state, in contrast to previous reports, and suggest potential frustration on the magnetic sublattice. Given the wide tunability of the alluaudite structure, we consider ways to push the system closer to a semimetallic state.

Taddei, Keith M [Argonne National Laboratory]↗
DOE OSTI2021

The role of substrate on stabilizing new phases of two-dimensional tin

In this work, we investigate the role of substrates in determining the morphologies and electronic structures of two-dimensional (2D) Sn. Here, we use Ir(111) and 2D hexagonal boron nitride (h-BN) predeposited on Ir(111) as substrates. The latter contains Moiré patterns with varying binding strength between h-BN and metal. Using scanning tunneling microscopy and first-principles calculations, we reveal the distinct morphologies of 2D Sn on different substrates. Of particular interest, we discover a new √7 × √7 Sn phase, originating from the local charge transfer with metal beneath the “decoupling” h-BN monolayer at the strongly interacting Moiré region. To the best of our knowledge, this work demonstrates, for the first time via combined experimental and theoretical approach, that new 2D Sn phases can be achieved using Moiré patterns on h-BN/metal. This finding should be instrumental in the development of 2D-based topological materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Active and Transfer Learning of High-Dimensional Neural Network Potentials for Transition Metals

Classical molecular dynamics (MD) simulations represent a very popular and powerful tool for materials modeling and design. The predictive power of MD hinges on the ability of the interatomic potential to capture the underlying physics and chemistry. There have been decades of seminal work on developing interatomic potentials, albeit with a focus predominantly on capturing the properties of bulk materials. Such physics-based models, while extensively deployed for predicting the dynamics and properties of nanoscale systems over the past two decades, tend to perform poorly in predicting nanoscale potential energy surfaces (PESs) when compared to high-fidelity first-principles calculations. These limitations stem from the lack of flexibility in such models, which rely on a predefined functional form. Machine learning (ML) models and approaches have emerged as a viable alternative to capture the diverse size-dependent cluster geometries, nanoscale dynamics, and the complex nanoscale PESs, without sacrificing the bulk properties. Here, in this study, we introduce an ML workflow that combines transfer and active learning strategies to develop high-dimensional neural networks (NNs) for capturing the cluster and bulk properties for several different transition metals with applications in catalysis, microelectronics, and energy storage, to name a few. Our NN first learns the bulk PES from the high-quality physics-based models in literature and subsequently augments this learning via retraining with a higher-fidelity first-principles training data set to concurrently capture both the nanoscale and bulk PES. Our workflow departs from status-quo in its ability to learn from a sparsely sampled data set that nonetheless covers a diverse range of cluster configurations from near-equilibrium to highly nonequilibrium as well as learning strategies that iteratively improve the fingerprinting depending on model fidelity. All the developed models are rigorously tested against an extensive first-principles data set of energies and forces of cluster configurations as well as several properties of bulk configurations for 10 different transition metals. Our approach is material agnostic and provides a methodology to transfer and build upon the learnings from decades of seminal work in molecular simulations on to a new generation of ML-trained potentials to accelerate materials discovery and design.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Twin Boundaries Contribute to The First Cycle Irreversibility of LiNiO 2

Abstract LiNiO 2 remains a target for layered oxide Li‐ion cathode development as it can theoretically deliver the highest energy density of this materials class. In practice, LiNiO 2 suffers from poor capacity retention due to electrochemically‐induced structural changes. While the impact of Ni off‐stoichiometry on the electrochemical performance has been extensively studied, that of planar defects present in the as‐synthesized cathode is not well understood. Advanced ex situ and operando structure probes are used to identify and quantify point and planar defects present in as‐synthesized Li 1‐y Ni 1+y O 2 cathodes and monitor their evolution during the first cycle. Specifically, a 7 Li nuclear magnetic resonance (NMR) signature characteristic of Li environments near twin boundaries is identified; an assignment supported by first‐principles calculations and scanning transmission electron microscopy (STEM) images of twin boundary defects. The NMR results suggest that the concentration of twin boundaries depends on the amount of Ni excess. Moreover, operando magnetometry and ex situ synchrotron X‐ray diffraction and NMR demonstrate that these planar defects impede Li reinsertion into the bulk cathode at reasonable discharge rates and contribute to the first cycle irreversible capacity. These findings provide new design rules for Li 1‐y Ni 1+y O 2 cathodes, whereby a reduced concentration of twin boundaries in the pristine material leads to reduced kinetic limitations and improved cathode utilization.

Nguyen, H.↗
DOE OSTI2024

Ghost states and surface structures of the charge density wave kagome metal ScV 6 Sn 6

In this work, we investigate the high-temperature phase of the kagome metal ScV 6 Sn 6 using scanning tunneling microscopy/spectroscopy (STM/S) and density functional theory calculations. STM topographic images of the cleaved sample reveal two distinct surface terminations: flat islands with Sn termination and trenches terminated by kagome layers with Sn as the outermost atomic layer. STS measurements on the Sn-terminated and kagome-terminated surfaces show significant differences, in particular the presence of large density of states near the Fermi level in the former case. Our first-principles calculations reveal that the charge density on the kagome-terminated surface gives rise to “ghost states” which show intensity away from surface atoms, arising due to hybridization of orbitals above the surface. These states can obscure the intrinsic properties of the surface, potentially leading to misattribution of the surface termination. This underscores the need for careful interpretation in STM studies, especially when discerning surface states of localized states. Understanding the surface structure of this versatile quantum material provides essential information for interpreting surface-sensitive experiments, tailoring material properties, engineering interfaces, and controlling stability and reactivity. This knowledge paves the way for further exploration and potential applications of kagome lattice materials in various fields, including quantum computing, topological physics, and advanced electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic-field-induced nontrivial electronic state in the Kondo-lattice semimetal CeSb

Synergic effect of electronic correlation and spin-orbit coupling is an emerging topic in topological materials. Central to this rapidly developing area are the prototypes of strongly correlated heavy-fermion systems. Recently, some Ce-based compounds are proposed to host intriguing topological nature, among which the electronic properties of CeSb are still under debate. Herein, we report a comprehensive study combining magnetic and electronic transport measurements, and electronic band-structure calculations of this compound to identify its topological nature. Quantum oscillations are clearly observed in both magnetization and magnetoresistance at high fields, from which one pocket with a nontrivial Berry phase is recognized. Angular-dependent magnetoresistance shows that this pocket is elongated in nature and corresponds to the electron pocket as observed in LaBi. Nontrivial electronic structure of CeSb is further confirmed by first-principle calculations, which arises from spin splitting in the fully polarized ferromagnetic state. These features indicate that magnetic field can induce nontrivial topological electronic states in this prototypical Kondo semimetal.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Stable Cuprous Hydroxide Nanostructures by Organic Ligand Functionalization

Abstract Copper compounds have been extensively investigated for diverse applications. However, studies of cuprous hydroxide (CuOH) have been scarce due to structural metastability. Herein, a facile, wet‐chemistry procedure is reported for the preparation of stable CuOH nanostructures via deliberate functionalization with select organic ligands, such as acetylene and mercapto derivatives. The resulting nanostructures are found to exhibit a nanoribbon morphology consisting of small nanocrystals embedded within a largely amorphous nanosheet‐like scaffold. The acetylene derivatives are found to anchor onto the CuOH forming CuC linkages, whereas CuS interfacial bonds are formed with the mercapto ligands. Effective electronic coupling occurs at the ligand‐core interface in the former, in contrast to mostly non‐conjugated interfacial bonds in the latter, as manifested in spectroscopic measurements and confirmed in theoretical studies based on first principles calculations. Notably, the acetylene‐capped CuOH nanostructures exhibit markedly enhanced photodynamic activity in the inhibition of bacteria growth, as compared to the mercapto‐capped counterparts due to a reduced material bandgap and effective photocatalytic generation of reactive oxygen species. Results from this study demonstrate that deliberate structural engineering with select organic ligands is an effective strategy in the stabilization and functionalization of CuOH nanostructures, a critical first step in exploring their diverse applications.

Liu, Qiming↗
DOE OSTI2020

Ca 2 MnO 4 structural path: Following the negative thermal expansion at the local scale

The oxygen octahedral rotations in Ca 2 MnO 4 , the first member of the CaO(CaMnO 3 )n Ruddlesden-Popper family, is probed through a set of complementary techniques, including temperature-dependent neutron and x-ray diffraction, combined with local probe studies and ab initio calculations. Here we demonstrate the enhancement of the uniaxial negative thermal expansion coefficient from -1.26 ± 0.25 to -21 ± 1.8 ppm/K at the second order I4 1 /acd to I4/mmm structural phase transition, providing direct evidence for the corkscrew atomic mechanism. We establish, also, that the predicted I4/mmm high symmetry is attained around 1050 K. At lower temperatures, within the 10–1000 K temperature range, our first-principles calculations and detailed analysis of the Ca local environment reveals that the reported Aba2 structural phase, coexisting with the I4 1 /acd one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical I4 1 /acd and Acam structural phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

𝜂 and 𝜂′ Production in 𝐽/𝜓 Radiative Decays from Quantum Chromodynamics

We present a first principles calculation within quantum chromodynamics (QCD) of the radiative decays of the 𝐽/𝜓 into the light pseudoscalar mesons 𝜂 and 𝜂′. Within a lattice computation we obtain the transition form factors as a function of photon virtuality from the timelike region, accessible experimentally via the “Dalitz” decay 𝐽/𝜓 →𝑒 + ⁢𝑒 − ⁢𝜂 (′) , through to the real photon point corresponding to 𝐽/𝜓 →𝛾⁢𝜂 (′) . This is the first calculation in lattice QCD with two (heavier than physical) degenerate flavors of light quark and a heavier strange quark, in which the 𝜂′ appears as the first excited state with pseudoscalar isoscalar quantum numbers. We access it reliably by using variationally optimized operators , the use of which also improves the purity of the 𝐽/𝜓 and 𝜂 signals, reducing systematic uncertainties. High quality results at a large number of kinematic points are obtained in a typically noisy disconnected process by using a novel correlator averaging procedure. Our results show the expected enhanced production of the 𝜂′ over the 𝜂 in this process, and suggest that the demonstrated lattice technology is suitable for future calculations considering processes in which light meson resonances are produced.

form factors↗
DOE OSTI2020

Piezoelectric Electron-Phonon Interaction from Ab Initio Dynamical Quadrupoles: Impact on Charge Transport in Wurtzite GaN

First-principles calculations of $\textit{e}$-ph interactions are becoming a pillar of electronic structure theory. However, the current approach is incomplete. The piezoelectric (PE) $\textit{e}$-ph interaction, a long-range scattering mechanism due to acoustic phonons in noncentrosymmetric polar materials, is not accurately described at present. Current calculations include short-range $\textit{e}$-ph interactions (obtained by interpolation) and the dipolelike Frölich long-range coupling in polar materials, but lack important quadrupole effects for acoustic modes and PE materials. Here we derive and compute the long-range $\textit{e}$-ph interaction due to dynamical quadrupoles, and apply this framework to investigate $\textit{e}$-ph interactions and the carrier mobility in the PE material wurtzite GaN. In this work, we show that the quadrupole contribution is essential to obtain accurate $\textit{e}$-ph matrix elements for acoustic modes and to compute PE scattering. Our work resolves the outstanding problem of correctly computing $\textit{e}$-ph interactions for acoustic modes from first principles, and enables studies of $\textit{e}$-ph coupling and charge transport in PE materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗