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Cheng, Hai-Ping

Publications and source records attributed to Cheng, Hai-Ping.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Magnetoelectric Coupling in a Mn 4 Na–Organic Complex under Pulsed Magnetic Fields up to 73 T

Research on the magnetoelectric (ME) effect (or spin-electric coupling) in molecule-based magnetic materials is a relatively nascent but promising topic. Molecule-based magnetic materials have diverse magnetic functionalities that can be coupled to electrical properties. Here, in this study, we investigate a realization of ME coupling that is fundamental but not heavily studied─the coupling of magnetic spin level crossings to changes in electric polarization. A mixed-valence Mn 4 Na complex with a total ground-state spin S = 5/2 under zero magnetic field and S = 17/2 under high magnetic field undergoes a cascade of ground-state level crossings of the S z states with increasing magnetic field. Magnetization and electrical polarization measurements under pulsed magnetic fields up to 73 T show that each spin level crossing is accompanied by a significant change in electric polarization that is an even function of the applied magnetic field. A molecular Hamiltonian describing antiferromagnetic exchange in a distorted tetrahedron of three Mn III and one Mn II ions matches the data well. We conclude that the ME coupling is caused by magnetostriction within the polar molecule as it distorts to lower its magnetic exchange energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

A computational approach to complex junctions and interfaces

Molecular- and nano- junctions, surfaces, interfaces, and interfacial processes have been the subjects of this DOE supported program for about 20 years. We have investigated electronic and magnetic structure, spin-dependent charge transport, and emerging phenomena at surfaces and interfaces in a number of physical systems ranging from nano-clusters, to nanowires, to two-dimensional (2D) crystals and their interactions, to bulk matter. Eighty-four refereed papers were published [1-84], with a majority effort from the group led by the PI, and over a hundred presentations were made at international conferences. We use first-principles methods in the framework of density functional theory in conjunction with nano-equilibrium Green function techniques, semi-empirical Boltzmann transport theory, tight-binding models, and the Hubbard model and many-body wavefunction approaches. In pursuing our scientific goals, we have also developed a number of computational algorithms and computational packages.

2D materials↗
DOE OSTI2023

Effects of elevated-temperature deposition on the atomic structure of amorphous Ta 2 O 5 films

Brownian thermal noise as a result of mechanical loss in optical coatings will become the dominant source of noise at the most sensitive frequencies of ground-based gravitational-wave detectors. Experiments found, however, that a candidate material, amorphous Ta 2 O 5 , is unable to form an ultrastable glass and, consequently, to yield a film with significantly reduced mechanical loss through elevated-temperature deposition alone. X-ray scattering PDF measurements are carried out on films deposited and subsequently annealed at various temperatures. Inverse atomic modeling is used to analyze the short and medium range features in the atomic structure of these films. Furthermore, in silico deposition simulations of Ta 2 O 5 are carried out at various substrate temperatures and an atomic level analysis of the growth at high temperatures is presented. It is observed that upon elevated-temperature deposition, short range features remain identical, whereas medium range order increases. After annealing, however, both the short and medium range orders of films deposited at different substrate temperatures are nearly identical. A discussion on the surface diffusion and glass transition temperatures indicates that future pursuits of ultrastable low-mechanical-loss films through elevated temperature deposition should focus on materials with a high surface mobility, and/or lower glass transition temperatures in the range of achievable deposition temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Symposium: Quantum Theory, Quantum Materials, Quantum Computing (Final Report, DE-SC0022551)

Quantum mechanics is fundamental to understanding, predicting, and controlling many physical processes in a vast range of scientific and technological areas. In molecules and materials, the quantum mechanics of Coulombic many-body systems determines far more than the ground state and bulk properties. Quantum mechanics governs magnetism, electron and spin transport, chemical reactions and catalysis, superconductivity, photosynthesis, etc. Each plays a vital role in modern society. As digital technologies push both temporal and length-scale limits, fundamental quantum principles become the current focus for a new class of technologies: quantum computing, quantum sensing, quantum encryption, etc. This oncoming quantum information sciences wave is manifest recently in frontier foci of all six Department of Energy Office of Science programs. The Sanibel Symposium offers a proven, influential platform for facilitating and advancing cross-fertilization among theoretical and computational chemists and physicists, computer scientists, and materials engineers. The Symposia have a long, successful record of driving progress on theory and computation of materials, nanostructures, and their molecular constituents. The Symposia thus are well-suited for advancing the quantum information science thrust in the Office of Basic Energy Sciences (BES). With that QIS thrust as the focus, we propose five sessions of the 61st Sanibel Symposium on the theme Quantum Theory, Quantum Materials, Quantum Computing. From a longer perspective, this thematic program for 2022 is the second year of a five year plan of program segments addressed at challenges in quantum information sciences and quantum materials. The rest of the Symposium program will be of considerable relevance, with, for example, sessions on quantum molecular and material dynamics, and new directions in coupled cluster theory. New ideas for methods and applications can be expected to emerge from the stimulus of the formal presentations and informal discussions. The proposed award is targeted at students, post-docs, and junior scientists. Conscious effort, characteristic of the Sanibel Symposia, to stimulate the participation of women and under-represented minorities will continue.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Scattering solution of an interacting Hamiltonian for the electronic control of molecular spin qubits

We theoretically study how a scattered electron can entangle molecular spin qubits (MSQs). This requires solving the inelastic transport of a single electron through a scattering region described by a tight-binding interacting Hamiltonian. We accomplish this using a Green's-function solution. We can model realistic physical implementations of MSQs by parametrizing the tight-binding Hamiltonian with first-principles descriptions of magnetic anisotropy and exchange interactions. As a result, we find that, for two-MSQ systems with inversion symmetry, projective measurement of the spin degree of freedom of the scattered electron offers probabilistic control of the degree of entanglement between the MSQs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Electron-nuclear decoupling at a spin clock transition

The ability to design quantum systems that decouple from environmental noise sources is highly desirable for development of quantum technologies with optimal coherence. The chemical tunability of electronic states in magnetic molecules combined with advanced electron spin resonance techniques provides excellent opportunities to address this problem. Indeed, so-called clock transitions have been shown to protect molecular spin qubits from magnetic noise, giving rise to significantly enhanced coherence. Here we conduct a spectroscopic and computational investigation of this physics, focusing on the role of the nuclear bath. Away from the clock transition, linear coupling to the nuclear degrees of freedom causes a modulation and decay of electronic coherence, as quantified via electron spin echo signals generated experimentally and in silico. Meanwhile, the effective hyperfine interaction vanishes at the clock transition, resulting in electron-nuclear decoupling and an absence of quantum information leakage to the nuclear bath, providing opportunities to characterize other decoherence sources.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Ampere field fluctuation from acoustic phonons as a possible source of spin decoherence

Here, we present a simple theory for estimating spin decoherence due to spin–phonon coupling in a lattice, and apply the theory to the Ampere field fluctuation from acoustic phonons. The Ampere field is generated by charge current loops around each lattice site due to acoustic phonon motion of the ions. The spin decoherence time due to the Ampere field fluctuation is estimated for NV centers and Mo impurities in SiC, and quantum dot qubits in Ge, GaAs, InAs, and InSb for temperature range 2 to 300 K. For most materials the estimated decoherence time is orders of magnitude longer than reported experimental values at the same temperature. The only exception is Mo impurity in SiC, for which the experimental decoherence time at 2 K is close to that due to Ampere field fluctuation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Dzyaloshinskii-Moriya interaction induced magnetoelectric coupling in a tetrahedral molecular spin-frustrated system

We have investigated magnetoelectric (ME) coupling in the single-molecule magnet Mn 4 Te 4 (PEt 3 ) 4 with tetrahedral spin frustration. Our density functional studies found that an electric dipole moment can emerge with various noncollinear spin orderings. The forms of spin-dependent dipole are determined and consistent with that in noncentrosymmetric magnets driven by the Dzyaloshinskii-Moriya interaction. Writing a parameterized spin Hamiltonian, after solving for eigenvalues and eigenstates, we quantified the ME coupling by calculating the thermal average of the electric and magnetic susceptibilities, which can be influenced by external magnetic and electric fields, respectively. The quadratic relations are expected to be observable in experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2022

First-principles calculation of gate-tunable ferromagnetism in magic-angle twisted bilayer graphene under pressure

Magic-angle twisted bilayer graphene (MATBG) is notable as a highly tunable platform for investigating strongly correlated phenomena such as unconventional superconductivity and quantum spin liquids, due to easy control of doping level through gating and sensitive dependence of the magic angle on hydrostatic pressure. Experimental observations of correlated insulating states, unconventional superconductivity and ferromagnetism in MATBG indicate that this system exhibits rich exotic phases. In this work, using density functional theory calculations in conjunction with the effective screening medium method, we find the MATBG under pressure at a twisting angle of 2.88 °and simulate how its electronic states evolve when doping level and electric field perpendicular to plane are tuned by gating. Our calculations show that, at doping levels between two electrons and four holes per moiré unit cell, a ferromagnetic (FM) solution with spin density localized at AA stacking sites is lower in energy than the nonmagnetic solution. The magnetic moment of this FM state decreases with both electron and hole doping and vanishes at four electrons/holes doped per moiré unit cell. Hybridization between the flat bands at the Fermi level and the surrounding dispersive bands can take place at finite doping. On increasing the out-of-plane electric field at zero doping, a transition from the FM state to the nonmagnetic one is seen. An investigation of impurity effects shows that both absorption of O 2 molecules and occurrence of Stone–Wales impurities suppress the FM state, and the mechanisms are understood from our calculations. We also analyze the interlayer bonding character due to flat bands via Wannier functions. Finally, we report trivial band topology of the flat bands in the FM state at a certain doping level.

Physics↗
DOE OSTI2022

First-principles study of bilayer polymeric manganese phthalocyanine

Here we study bilayer manganese phthalocyanine (MnPc) molecules and MnPc polymeric sheets using first-principles simulations with a focus on the magnetic interactions between Mn atoms. We find that the most stable position of the upper layer with respect to the lower layer is shifted about 1/8 of a lattice vector from the center of the bottom layer along the direction toward a nearest-neighbor N atom. The magnetic ground state is the Néel antiferromagnetic (AFM) configuration within a layer and ferromagnetic (FM) between Mn atoms in adjacent layers. In this state, the system becomes a semiconductor with an indirect band gap of 11meV. The strongest interaction is the interlayer coupling between the closest Mn atoms. A maximally localized Wannier analysis suggests that the dominant coupling pathway is Mn-(N,C)-Mn rather than a direct Mn-Mn coupling. The maximum calculated magnetic anisotropy energy is found to be 1.0meV per Mn atom. We also find that the bilayer molecule shows a significant stacking angle change from FM to AFM configurations accompanied by a change of orbital filling ordering.

2-dimensional systems↗
DOE OSTI2022

Clar’s Goblet on Graphene: Field-Modulated Charge Transfer in a Hydrocarbon Heterostructure

In certain configurations, the aromatic properties of benzene ring structured molecules allow for unpaired, reactive valence electrons (known as radicals). Clar’s goblets are such molecules. With an even number of unpaired radicals, these nanographenes are topologically frustrated hydrocarbons in which π-bonding network and topology of edges give rise to the magnetism. Clar’s goblets are therefore valued as prospective qubits provided they can be modulated between magnetic states. Here, using first-principles DFT, we demonstrate the effects of adsorption on both molecule and substrate in a graphene–Clar’s goblet heterostructure. We look at the energy difference bewteen FM and AFM states of the system and discuss underlying physical and chemical mechanisms in reference to the highest occupied molecular orbital (HOMO) and second HOMO (HOMO–1). We find that the HOMO of the molecule in the FM state is right at the Fermi surface, which leads to the hybridization between molecular state and the graphene state near the Dirac point. Furthermore, we investigate qualitative changes in charge realignment and magnetic state under a variable electric field. Transitions from FM to AFM and back to FM states are observed.

Bruce, Adam V.↗
DOE OSTI2022

Performance Enhancement of APW+lo Calculations by Simplest Separation of Concerns

Full-potential linearized augmented plane wave (LAPW) and APW plus local orbital (APW+lo) codes differ widely in both their user interfaces and in capabilities for calculations and analysis beyond their common central task of all-electron solution of the Kohn–Sham equations. However, that common central task opens a possible route to performance enhancement, namely to offload the basic LAPW/APW+lo algorithms to a library optimized purely for that purpose. To explore that opportunity, we have interfaced the Exciting-Plus (“EP”) LAPW/APW+lo DFT code with the highly optimized SIRIUS multi-functional DFT package. This simplest realization of the separation of concerns approach yields substantial performance over the base EP code via additional task parallelism without significant change in the EP source code or user interface. We provide benchmarks of the interfaced code against the original EP using small bulk systems, and demonstrate performance on a spin-crossover molecule and magnetic molecule that are of size and complexity at the margins of the capability of the EP code itself.

Zhang, Long↗
DOE OSTI2022

Universal quantum operation of spin-3/2 Blume-Capel chains

We propose a logical qubit based on the Blume-Capel model: A higher-spin generalization of the Ising chain which allows for an on-site anisotropy-preserving rotational invariance around the Ising axis. We show that such a spin-3/2 Blume-Capel model can also support localized Majorana zero modes at the ends of the chain. Inspired by known braiding protocols of these Majorana zero modes, upon appropriate manipulation of the system parameters, we demonstrate a set of universal gate operations which act on qubits encoded in the doubly degenerate ground states of the chain.

1-dimensional spin chains↗