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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Color center in β -Ga 2 O 3 emitting at the telecom range

Transition metal (TM) ions incorporated into a host from a wide bandgap semiconductor are recognized as a promising system for quantum technologies with enormous potential. In this work, we report on a TM color center in β-Ga 2 O 3 with physical properties attractive for quantum information applications. The center is found to emit at 1.316 μm and exhibits weak coupling to phonons, with optically addressable higher-lying excited states, beneficial for single-photon emission within the telecom range (O-band). Using magneto-photoluminescence (PL) complemented by time-resolved PL measurements, we identify the monitored emission to be internal 1 E→ 3 A 2 spin-forbidden transitions of a 3d 8 TM ion with a spin-triplet ground state—a possible candidate for a spin qubit. We tentatively attribute this color center to a complex involving a sixfold coordinated Cu 3+ ion.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Noise-induced transition in optimal solutions of variational quantum algorithms

Variational quantum algorithms are promising candidates for delivering practical quantum advantage on noisy intermediate-scale quantum (NISQ) hardware. However, optimizing the noisy cost functions associated with these algorithms is challenging for system sizes relevant to quantum advantage. In this work, we investigate the effect of noise on optimization by studying a variational quantum eigensolver (VQE) algorithm calculating the ground state of a spin chain model, and we observe an abrupt transition induced by noise to the optimal solutions. We will present numerical simulations, a demonstration using an IBM quantum processor unit (QPU), and a theoretical analysis indicating the origin of this transition. Our findings suggest that careful analysis is crucial to avoid misinterpreting the noise-induced features as genuine algorithm results.

Li, Andy C.Y.↗
DOE OSTI2024

Spin-Orbit-Lattice Entangled State in A 2 ⁢MgReO 6 (A = Ca, Sr, Ba) Revealed by Resonant Inelastic X-Ray Scattering

The 5d 1 ordered double perovskites present an exotic playground for studying novel multipolar physics due to large spin-orbit coupling. Here, we present Re L 3 edge resonant inelastic Xray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A 2 MgReO 6 (A = Ca, Sr, Ba) family of 5d 1 double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric lineshape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbitlattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibroniccoupling for a complete description of the ground state physics of 5d 1 double perovskites.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Spin–Orbit Natural Transition Orbitals and Spin-Forbidden Transitions

Natural transition orbitals (NTOs) are in widespread use for visualizing and analyzing electronic transitions. Here, the present work introduces the analysis of formally spin-forbidden transitions with the help of complex-valued spin-orbit (SO) NTOs. The analysis specifically focuses on the components in such transitions that cause their intensity to be non-zero because of SO coupling. Transition properties such as transition dipole moments are partitioned into SO-NTO hole-particle pairs, such that contributions to the intensity from specific occupied and unoccupied orbitals are obtained. The method has been implemented within the restricted active space (RAS) self-consistent field wave function theory framework, with SO coupling treated by RAS state interaction. SO-NTOs have a broad range of potential applications, which is illustrated by the T 2 -S 1 state mixing in pyrazine, spin-forbidden vs. spin-allowed 4f–5d transitions in the Tb 3+ ion, and the phosphorescence of tris(2-phenylpyridine) iridium [Ir(ppy) 3 ].

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Orbital character of the spin-reorientation transition in TbMn 6 Sn 6

Ferromagnetic (FM) order in a two-dimensional kagome layer is predicted to generate a topological Chern insulator without an applied magnetic field. The Chern gap is largest when spin moments point perpendicular to the kagome layer, enabling the capability to switch topological transport properties, such as the quantum anomalous Hall effect, by controlling the spin orientation. In TbMn 6 Sn 6 , the uniaxial magnetic anisotropy of the Tb 3+ ion is effective at generating the Chern state within the FM Mn kagome layers while a spin-reorientation (SR) transition to easy-plane order above T SR = 310 K provides a mechanism for switching. Here, we use inelastic neutron scattering to provide key insights into the fundamental nature of the SR transition. The observation of two Tb excitations, which are split by the magnetic anisotropy energy, indicates an effective two-state orbital character for the Tb ion, with a uniaxial ground state and an isotropic excited state. The simultaneous observation of both modes below T SR confirms that orbital fluctuations are slow on magnetic and electronic time scales < ps and act as a spatially-random orbital alloy. A thermally-driven critical concentration of isotropic Tb ions triggers the SR transition.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Decay spectroscopy of 160 Eu: Quasiparticle configurations of excited states and structure of K π = 4 + bandheads in 160 Gd

Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the r-process path and following freeze-out region impact the resulting r-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei. Purpose: We aim to improve the understanding of the nuclear structure of 160 Gd, specifically the K π = 4 + bands, as well as study the β decay of 160 Eu into 160 Gd. Methods: High-statistics decay spectroscopy of 160 Gd resulting from the β-decay of 160 Eu was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. Results: Two new excited states and ten new transitions were observed in 160 Gd. The β-decaying half-lives of the low- and high-spin isomers in 160 Eu were determined, and the low-spin state's half-life was measured to be t 1/2 = 26.0 (8) s, ≈ 16% shorter than previous measurements. Lifetimes of the two K π = 4 + bandheads in 160 Gd were measured for the first time, as well as γ – γ angular correlations and mixing ratios of intense transitions out of those bandheads. Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the K π = 4 + bandheads in 160 Gd is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in 160 Gd heavily populated in β decay is shown to have positive parity, which raises questions regarding the structure of the high-spin β-decaying state in 160 Eu.

150 ≤ A ≤ 189↗
DOE OSTI2022

Anisotropic magnetic and transport properties of orthorhombic o-Pr 2 Co 3 Ge 5

Abstract The crystal structure, electron energy-loss spectroscopy (EELS), heat capacity, and anisotropic magnetic and resistivity measurements are reported for Sn flux grown single crystals of orthorhombic Pr 2 Co 3 Ge 5 (U 2 Co 3 Si 5 -type, Ibam ). Our findings show that o -Pr 2 Co 3 Ge 5 hosts nearly trivalent Pr ions, as evidenced by EELS and fits to temperature dependent magnetic susceptibility measurements. Complex magnetic ordering with a partially spin-polarized state emerges near T sp = 32 K, with a spin reconfiguration transition near T M = 15 K. Heat capacity measurements show that the phase transitions appear as broad peaks in the vicinity of T sp and T M . The magnetic entropy further reveals that crystal electric field splitting lifts the Hund’s rule degeneracy at low temperatures. Taken together, these measurements show that Pr 2 Co 3 Ge 5 is an environment for complex f state magnetism with potential strongly correlated electron states.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electrical and Magnetic Properties of FeGa 3 with Antisite Disorder

We investigated the electronic and magnetic ground states of the cage compound FeGa 3 under the presence of Fe antisite disorder (namely as Fe 0.258 Ga 0.742 ) by x-ray diffraction (XRD), Mössbauer spectroscopy (MS), magnetization and electrical resistivity. Our results show that approximately 4.5 (5)% Fe antisite occupying one of the two non-equivalent Ga sites changes significantly the electronic and magnetic ground states of the pristine compound. We uncover that Fe 0.258 Ga 0.742 undergoes two magnetic transitions, T 1 ≈ 60 K and T 2 ≈ 15 K, the latter associated with the suppression of semiconducting behavior. We focus our discussion on the role that in-gap impurity states and spin disorder play across semiconducting-metal and paramagnetic-magnetic transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Topological and magnetic phase transitions in the bilayer Kitaev-Ising model

We investigate the phase diagram of a bilayer Kitaev honeycomb model with Ising interlayer interactions, deriving effective models via perturbation theory and performing Majorana mean-field theory calculations. We show that a diverse array of magnetic and topological phase transitions occur, depending on the direction of the interlayer Ising interaction and the relative sign of Kitaev interactions. When two layers have the same sign of the Kitaev interaction, a first-order transition from a Kitaev spin liquid to a magnetically ordered state takes place. The magnetic order points along the Ising axis and it is (anti)ferromagnetic for (anti)ferromagnetic Kitaev interactions. However, when two layers have opposite signs of the Kitaev interaction, we observe a notable weakening of magnetic ordering tendencies and the Kitaev spin liquid survives up to a remarkably larger interlayer exchange. Our mean-field analysis suggests the emergence of an intermediate gapped ℤ 2 spin-liquid state, which eventually becomes unstable upon vison condensation. The confined phase is described by a highly frustrated 120° compass model. We furthermore use perturbation theory to study the model with the Ising axis pointing along the $\hat{z}$ axis or lying in the 𝑥⁢𝑦 plane. In both cases, our analysis reveals the formation of one-dimensional Ising chains, which remain decoupled in perturbation theory, resulting in a subextensive ground-state degeneracy. Our results highlight the interplay between topological order and magnetic ordering tendencies in bilayer quantum spin liquids.

Kitaev model↗
DOE OSTI2026

Three-Qubit Encoding in Ytterbium-171 Atoms for Simulating 1+1D Quantum Chromodynamics

Simulating nuclear matter described by quantum chromodynamics using quantum computers is notoriously inefficient because of the assortment of quark degrees of freedom such as matter/antimatter, flavor, color, and spin. Here, we propose to address this resource efficiency challenge by encoding three qubits within individual ytterbium-171 atoms of a neutral atom quantum processor. The three qubits are encoded in three distinct sectors: an electronic “clock” transition, the spin-1/2 nucleus, and the lowest two motional states in one radial direction of the harmonic trapping potential. We develop a family of composite sideband pulses and demonstrate a universal gate set and readout protocol for this three-qubit system. We then apply it to single-flavor quantum chromodynamics in 1+1D axial gauge for which the three qubits directly represent the occupancy of quarks in the three colors. We show that two atoms are sufficient to simulate both vacuum persistence oscillations and a screened hadron-number transition. We consider resource requirements and connections to error detection/correction. Our work is a step toward resource-efficient digital simulation of nuclear matter and opens new opportunities for versatile qubit encoding in neutral atom quantum processors.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Reconfigurable Magnetotransport in MnBi 2 Te 4 via Gate and Magnetic Field Tuning

The intrinsic magnetic topological insulator MnBi 2 Te 4 is a promising platform for exploring quantum phases with nontrivial band topology and for enabling electrical control over coupled magnetic and electronic phase transitions. In-plane magnetic fields, in particular, offer a distinct means of tuning these properties by strengthening quantized Hall effects, enhancing surface energy gaps, and driving spin reorientation transitions. However, a systematic understanding of how such fields affect magnetotransport is limited. Here, the magnetotransport behavior of few-layer MnBi 2 Te 4 as a function of gate voltage, temperature, and magnetic field angle, with a primary focus on in-plane field effects, are investigated. A gate-tunable crossover in magnetoresistance is observed from positive to negative values under in-plane magnetic fields as the gate voltage is swept below the charge neutrality point at temperatures below the Néel temperature. The in-plane field drives a transition from the antiferromagnetic ground state to a ferromagnetic configuration with spins aligned in-plane, while simultaneously altering the electronic structure, as revealed by gate-dependent transport features. The angle-dependent measurements reveal strongly gate-tunable magnetotransport anisotropy. These results establish in-plane magnetic fields as an effective tuning parameter for modulating spin and charge transport in MnBi 2 Te 4 , advancing prospects for reconfigurable spintronic and topological devices.

MnBi 2 Te 4↗
DOE OSTI2025

Phase Diagram of the Easy-Axis Triangular-Lattice 𝐽 1 −𝐽 2 Model

Our work studies the largely unexplored phase diagram of the S = 1/2 easy-axis triangular-lattice J 1 -J 2 model, motivated by recent interest in rare-earth and transition-metal compounds that exhibit strong quantum fluctuations and broad spin excitation continua—hallmarks of spin-liquid behavior. Combining density-matrix renormalization group simulations with analytical insights, we present a comprehensive study of the model that describes these materials. Our results provide compelling evidence for a robust spin-liquid phase stabilized between the exotic supersolid Y phase and the collinear stripe phase, remarkably persistent even in the presence of symmetry-breaking anisotropy. Additionally, we analyze the supersolid Y phase, offering a quantitative characterization of its order parameters and clarifying the surprising absence of a ferromagnetic moment; both features relevant to recent experiments on Ising-like triangular-lattice magnets. Altogether, our work provides the necessary framework and important theoretical guidance to the ongoing searches of the spin liquid and other exotic states in the rare-earth and transition-metal triangular-lattice compounds, as well as for advancing the understanding of anisotropic-exchange magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Monolayer 1T-LaN 2 : Dirac spin-gapless semiconductor of p -state and Chern insulator with a high Chern number

Two-dimensional transition-metal dinitrides have attracted considerable attention in recent years due to their rich magnetic properties. Here, we focus on rare-earth-metal elements and propose a monolayer of lanthanum dinitride with a 1T structural phase, 1T-LaN 2 . Using first-principles calculations, we systematically investigated the structure, stability, magnetism, and band structure of this material. It is a flexible and stable monolayer exhibiting a low lattice thermal conductivity, which is promising for future thermoelectric devices. Here, the monolayer shows the ferromagnetic ground state with a spin-polarized band structure. Two linear spin-polarized bands cross at the Fermi level forming a Dirac point, which is formed by the p atomic orbitals of the N atoms, indicating that monolayer 1T-LaN 2 is a Dirac spin-gapless semiconductor of p-state. When the spin–orbit coupling is taken into account, a large nontrivial indirect bandgap (86/354 meV) can be opened at the Dirac point, and three chiral edge states are obtained, corresponding to a high Chern number of C = 3, implying that monolayer 1T-LaN 2 is a Chern insulator. Importantly, this kind of band structure is expected to occur in more monolayers of rare-earth-metal dinitride with a 1T structural phase.

2D materials↗
DOE OSTI2025

Field-tunable BKT and quantum phase transitions in spin-$\frac{1}{2}$ triangular lattice antiferromagnet

Quantum magnetism is one of the most active fields for exploring exotic phases and phase transitions. The recently synthesized Na 2 BaCo(PO 4 ) 2 (NBCP) is an ideal material incarnation of the spin-$\frac{1}{2}$ easy-axis triangular lattice antiferromagnet (TLAF). Experimental evidence shows that NBCP hosts the spin supersolid state with a giant magnetocaloric effect. Theory further predicts that magnetic fields can drive NBCP through Berezinskii-Kosterlitz-Thouless (BKT) and other richer quantum phase transitions. However, detecting these transitions is challenging, as they onset at ultralow temperatures near 60 mK and require high magnetization sensitivity. Using a newly developed gradient force magnetometer in a dilution refrigerator, we mapped the magnetic susceptibility phase diagram down to 30 mK. Our results provide a more comprehensive and accurate understanding of BKT melting of spin supersolidity and several field-tunable quantum phase transitions, which establish NBCP as a model platform for frustrated magnetism and highlight potential applications of its giant magnetocaloric effects.

BKT transition↗
DOE OSTI2024

Spin Coupling Effect on Geometry-Dependent X-Ray Absorption of Diradicals

Here, we theoretically investigate the influence of diradical electron spin coupling on the time-resolved X-ray absorption spectra of the photochemical ring opening of furanone. We predict geometry-dependent carbon K-edge signals involving transitions from core orbitals to both singly and unoccupied molecular orbitals. The most obvious features of the ring opening come from the carbon atom directly involved in the bond breaking through its transition to both the newly formed singly occupied and the available lowest unoccupied molecular orbitals (SOMO and LUMO, respectively). In addition to this primary feature, the singlet spin coupling of four unpaired electrons that arises in the core-to-LUMO states creates additional geometry dependence in some spectral features with both oscillator strengths and relative excitation energies varying observably as a function of the ring opening. We attribute this behavior to a spin-occupancy-induced selection rule, which occurs when singlet spin coupling is enforced in the diradical state. Notably, one of these geometry-sensitive core-to-LUMO transitions excites core electrons from a backbone carbon not involved in the bond breaking, providing a novel nonlocal X-ray probe of chemical dynamics arising from electron spin coupling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Quantum Spin Hall Edge States and Interlayer Coupling in Twisted Bilayer WTe 2

The quantum spin Hall (QSH) effect, characterized by topologically protected spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide (TMD) WTe 2 . However, the robustness of this topological protection remains largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. Here, we use scanning tunneling microscopy and spectroscopy (STM/STS) to explore the topological nature of twisted bilayer (tBL) WTe 2 . Furthermore, at the tBL edges, we observe the characteristic spectroscopic signatures of the QSH edge states. For small twist angles, a rectangular moiré pattern develops, which results in local modifications of the band structure. Using first-principles calculations, we quantify the interactions in tBL WTe 2 and its topological edge states as a function of interlayer distance and conclude that it is possible to engineer the topology of WTe 2 bilayers via the twist angle as well as interlayer interactions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Ground and excited state properties of ThBe and AcBe

In this work, the ground and excited states of ThBe and AcBe were investigated by performing high-level multireference and single-reference coupled-cluster quantum chemical calculations with large correlation consistent basis sets. Full potential energy curves (PECs), chemical bonding patterns, energetics, spectroscopic parameters (T e , r e , ω e , and ω e x e ), and spin–orbit effects of 13 and 8 electronic states of ThBe and AcBe, respectively, are reported. The ground electronic states of ThBe and AcBe are single-reference 1 3 Σ − (1σ 2 2σ 2 1π 2 ) and 1 2 Π (1σ 2 2σ 2 1π 1 ), respectively, and originate from their corresponding ground state fragments. The chemical bonding of ThBe (1 3 Σ − ) and AcBe (1 2 Π) are π-dative in character and are formed by d-electron transfers from Th/Ac to the empty 2p x and 2p y of the Be atom. The electron populations of the f-orbitals of both ThBe (1 3 Σ − ) and AcBe (1 2 Π) are minor which exhibit their “transition-metal-like” nature. The estimated bond energies of the spin–orbit ground states of ThBe (1 3 Σ − 0+ ) and AcBe (1 2 Π 1/2 ) are 12.79 and 11.02 kcal mol −1 , respectively. Finally, the bond energy of ThBe was used to estimate its heat of formation ΔH 0 f (298 K) of 869.61 ± 6 kJ mol −1 .

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Efficient charge to spin conversion in iridium oxide thin films

Many 5d transition metal oxides have a unique electronic structure, where the density of states near the Fermi level is dominated by only 5d electrons with strong spin–orbit coupling. IrO 2 , a Dirac nodal line semi-metal, is the simplest of these oxides. The presence of 5d electrons and gap opening of Dirac nodal lines via strong spin–orbit coupling allows for the hybridization of the 5d electrons of the oxide with the itinerant d electrons of a ferromagnet, while simultaneously increasing the intrinsic spin Hall effect. We report large charge-to-spin conversion in thin films of this material using spin-torque ferromagnetic resonance experiments. Finally, by independently performing line shape analysis and linewidth modulation experiments, we conclusively determine the spin Hall angle of optimized IrO 2 films to be ∼8 times larger than that of Pt.

Ferromagnetic materials↗