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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Inducing Circularly Polarized Single-Photon Emission via Chiral-Induced Spin Selectivity

One of the central aims of the field of spintronics is the control of individual electron spins to effectively manage the transmission of quantized data. One well-known mechanism for controlling electronic spin transport is the chiral-induced spin-selectivity (CISS) effect in which a helical nanostructure imparts a preferential spin orientation on the electronic transport. One potential application of the CISS effect is as a transduction pathway between electronic spin and circularly polarized light within nonreciprocal photonic devices. Here in this work, we identify and quantify the degree of chiral-induced spin-selective electronic transport in helical polyaniline films using magnetoconductive atomic force microscopy (mcAFM). We then induce circularly polarized quantum light emission from CdSe/CdS core/shell quantum dots placed on these films, demonstrating a degree of circular polarization of up to ~21%. Utilizing time-resolved photoluminescence microscopy, we measure the radiative lifetime difference associated with left- and right-handed circular polarizations of single emitters. These lifetime differences, in combination with Kelvin probe mapping of the variation of surface potential with magnetization of the substrate, help establish an energy level diagram describing the spin-dependent transport pathways that enable the circularly polarized photoluminescence.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Writing and Detecting Topological Charges in Exfoliated Fe 5– x GeTe 2

Fe 5–x GeTe 2 is a promising two-dimensional (2D) van der Waals (vdW) magnet for practical applications, given its magnetic properties. These include Curie temperatures above room temperature, and topological spin textures–TST (both merons and skyrmions), responsible for a pronounced anomalous Hall effect (AHE) and its topological counterpart (THE), which can be harvested for spintronics. Here, we show that both the AHE and THE can be amplified considerably by just adjusting the thickness of exfoliated Fe 5–x GeTe 2 , with THE becoming observable even in zero magnetic field due to a field-induced unbalance in topological charges. Using a complementary suite of techniques, including electronic transport, Lorentz transmission electron microscopy, and micromagnetic simulations, we reveal the emergence of substantial coercive fields upon exfoliation, which are absent in the bulk, implying thickness-dependent magnetic interactions that affect the TST. We detected a “magic” thickness t ≈ 30 nm where the formation of TST is maximized, inducing large magnitudes for the topological charge density (~6.45 × 10 20 cm –2 ), and the concomitant anomalous (ρ xy A,max ≃22.6 μΩ cm) and topological (ρ xy u,T 1≃5 μΩ cm) Hall resistivities at T ≈ 120 K. These values for ρ xy A,max and ρ xy u,T are higher than those found in magnetic topological insulators and, so far, the largest reported for 2D magnets. Furthermore, the hitherto unobserved THE under zero magnetic field could provide a platform for the writing and electrical detection of TST aiming at energy-efficient devices based on vdW ferromagnets.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Interplay between Topological States and Rashba States as Manifested on Surface Steps at Room Temperature

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, in this study, we report room-temperature scanning tunneling microscopy/spectroscopy observation of interaction between Rashba states and topological surface states, which manifests local electronic structure along step edges controllable by the layer thickness of thin films. The first-principles theoretical calculation elucidates the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. Furthermore, the Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi 2 Se 3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling the quantum states.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Humidity Disrupts Structural and Chiroptical Properties of Chiral 2D Perovskites

Chiral two-dimensional (2D) hybrid organic–inorganic metal halide perovskite semiconductors have emerged as an exceptional material platform with many design opportunities for spintronic applications. However, a comprehensive understanding of changes to the crystal structure and chiroptical properties upon exposure to atmospheric humidity has not been established. We demonstrate phase degradation to the 1D (MBA)PbI 3 (MBA = methylbenzylammonium) and the hypothetical (MBA) 3 PbI 5 ·H 2 O hydrate phases, accompanied by a reduction and disappearance of the chiroptical response. Firstprinciple simulations show that water molecules preferentially locate at the interface between the organic cations and the inorganic framework, thereby disrupting the hydrogen bonding, impacting both the structural chirality and stability of the material. These findings provide critical insights into phase degradation mechanisms and their impact on chiroptical activity in chiral 2D perovskites.

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

The Noncollinear Path to Two-Dimensional Topological Superconductivity

Two-dimensional magnet-superconductor hybrids (2D-MSH) are promising candidates to realize devices for topology-based quantum technologies and superconducting spintronics. So far, studies have focused on 2D-MSH systems with collinear ferro- or antiferromagnetic layers. Here, we present the discovery of topological superconductivity in a noncollinear MSH system where a magnetic spiral is realized in an Fe monolayer proximity coupled to a superconducting Ta(110) substrate. By combining low-temperature spin-polarized scanning tunneling spectroscopy with an in-depth theoretical study, we can conclude that the system is in a topological nodal-point superconducting phase with low-energy edge modes. Furthermore, we reveal that for this noncollinear spin texture, these edge modes exhibit a magnetization direction-dependent dispersion. This means that a spatial shift of the magnetic spiral could be used to reverse the chirality of an edge mode in future MSH-based devices.

chiral edge modes↗
DOE OSTI2025

Local Inversion Symmetry Breaking and Thermodynamic Evidence for Ferrimagnetism in Fe 3 GaTe 2

The layered compound Fe 3 GaTe 2 is attracting attention due to its high Curie temperature, low dimensionality, and the presence of topological spin textures above room temperature, making Fe 3 GaTe 2 a good candidate for applications in spintronics. Here, in this study, we show, through transmission electron microscopy (TEM) techniques, that Fe 3 GaTe 2 single crystals break local inversion symmetry while maintaining global inversion symmetry according to X-ray diffraction. Coupled to the observation of Néel skyrmions via Lorentz-TEM, our structural analysis provides a convincing explanation for their presence in centrosymmetric materials. Magnetization measurements as a function of the temperature displays a sharp first-order thermodynamic phase-transition leading to a reduction in the magnetic moment. This implies that the ground state of Fe 3 GaTe 2 is globally ferrimagnetic and not a glassy magnetic state composed of ferrimagnetic, and ferromagnetic domains as previously claimed. Neutron diffraction studies indicate that the ferromagnetic to ferrimagnetic transition upon reducing the external magnetic field is associated with a change in the magnetic configuration/coupling between Fe1 and Fe2 moments. We observe a clear correlation between the hysteresis observed in both the skyrmion density and the magnetization of Fe 3 GaTe 2 . This indicates that its topological spin textures are affected by the development of ferrimagnetism upon cooling. Observation, via magnetic force microscopy, of magnetic bubbles at the magnetic phase boundary suggests skyrmions stabilized by the competition among magnetic phases and distinct exchange interactions. Our study provides an explanation for the observation of Néel skyrmions in centrosymmetric systems, while exposing a correlation between the distinct magnetic phases of Fe 3 GaTe 2 and topological spin textures.

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

Curvature Induced Modifications of Chirality and Magnetic Configuration in Perpendicular Films

Designing curvature in three-dimensional (3D) magnetic nanostructures enables controlled manipulation of local energy landscapes, allowing for the modification of noncollinear spin textures relevant for next-generation spintronic devices. In this study, we experimentally investigate 3D magnetization textures in a Co/Pd multilayer film, exhibiting strong perpendicular magnetic anisotropy (PMA), deposited onto curved Cu nanowire meshes with diameters as small as 50 nm and lengths of several microns. Utilizing magnetic soft X-ray nanotomography, we achieve reconstructions of 3D magnetic domain patterns at approximately 30 nm spatial resolution. This approach provides detailed information on both the orientation and magnitude of magnetization within the film. Our results reveal that interfacial anisotropy in the Co/Pd multilayers drives the magnetization toward the local surface normal. In contrast to typical labyrinth domains observed in planar films, the presence of curved nanowires significantly alters the domain structure, with domains preferentially aligning along the nanowire axis in close proximity, while adopting random orientations farther away. We report direct experimental observation of a curvature-induced Dzyaloshinskii-Moriya interaction (DMI), which is quantified to be approximately one-third of the intrinsic DMI in Co/Pd stacks. The curvature induced DMI enhances stability of Néel-type domain walls. These experimental observations are further supported by micromagnetic simulations. Altogether, our findings demonstrate that introducing curvature into magnetic nanostructures provides a powerful strategy for tailoring complex magnetic behaviors, paving the way for the design of advanced 3D racetrack memory and neuromorphic computing devices.

Raftrey, David↗
DOE OSTI2025

Emergence of Moiré Dirac Fermions at the Interface of Topological and 2D Magnetic Insulators

Dirac Fermions on the surface of the topological insulator are spin-momentum locked and topologically protected, making them interesting for spintronics and quantum computing applications. When in proximity to magnetism and superconductivity, these electronic states could result in quantum anomalous Hall effect and Majorana Fermions, respectively. An even more dramatic enrichment of the topological insulators’ physics is expected for moiré superlattices, where, analogously to the twisted graphene layers, electronic correlations could be strongly enhanced, a task previously notoriously difficult to achieve in topological matter. Until now, the experimental confirmation of such moiré properties has remained elusive. Here, we grow the two-dimensional van der Waals magnetic insulators FeX 2 (where X = Cl or Br) on top of the topological insulator Bi 2 Se 3 and establish a moiré superlattice formation at the interface. By means of scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we investigate the electronic properties of the formed moiré superlattice and demonstrate its tunability via the film choice. We reveal replicated Dirac cones and focus on their intersections, which, in the case of FeBr 2 /Bi 2 Se 3 , occur below the Fermi level. We identify the signatures of small gaps at the intersections around the M̅ i points that we attribute to the moiré interaction. These findings point to the specific type of magnetic moiré potential that breaks the time-reversal symmetry at these points but not at the $\barΓ$ point. Our observations provide an intriguing scenario of correlated topological phases induced by moiré superlattice that may result in topological superconductivity, high Chern number phases, and exotic noncollinear magnetic textures.

2D magnets↗
DOE OSTI2026

A Visual Understanding of Circular Dichroism Spectroscopy

Mapping chemical and structural properties to electronic and magnetic responses is critical to many applications such as quantum information science, where the precise storage and transmission of unique information is paramount. Specifically, constructing molecules and materials that provide strong polarized responses at tunable frequencies and with large anisotropies is key to optical processing of quantum information. Chiral molecules provide chiroptical response to circularly polarized light, making them attractive for quantum information science and other applications related to sensing, polarized photodetectors, and spintronics. Predicting a molecular design, a priori, with large anisotropies to circularly polarized light is challenging due to the complex interplay between electric and magnetic components of the optical response. In this work, we explore a visual representation of the electronic chiroptical response by decomposing the rotary strength into its constituent components. Here, we make use of the intuitive electronic oscillator framework to develop classical intuition regarding the rotary strength and its constituents. We explore three model chemical systems that exhibit local and global chirality. Our analysis reveals that local chirality necessarily exhibits competition between the local chiral center and chirality induced in other fragments of the molecule, resulting in both unexpected nonmonotonic trends and sign flips in chemically adjacent geometries. Furthermore, we can visually distinguish between local and global chirality via examination of the transition chiral tensor. Interestingly, we make strong connections to ferromagnetic and antiferromagnetic spin systems in that chiroptically inactive transitions exhibit antiferromagnetic-like alternating orbital patterns while active transitions show domain formation in an ferromagnetic-like alignment that produces a net chiroptical response.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Current-Driven Symmetry Breaking and Spin–Orbit Polarization in Chiral Wires

The spin dynamics of electrons in chiral molecular systems remains a topic of intense interest, particularly regarding whether geometric chirality inherently induces spin polarization in current-carrying electrons. In this work, we employ ab initio real-time time-dependent density functional theory (rt-TDDFT) to directly simulate the interplay among charge current, spin, and orbital. This realtime tracking extends beyond perturbative treatments, and we analyze how nonequilibrium currents effectively lift the symmetry constraints of screw rotation and time-reversal symmetry. We find that the emergence of spin and orbital angular momenta is dynamically correlated with a concomitant loss of translational (linear) momentum, which we interpret as an intrinsic consequence of current-driven symmetry lowering. The implications of this mechanism for chirality-induced spin selectivity and spintronics device design are discussed.

chiral wire↗
DOE OSTI2025

First-Principles Studies on Sc 2 RuZ (Z = Si, Ge, Sn) Inverse Heusler Alloys: Structural, Electronic, and Transport Properties

The continuous demand for efficient, nontoxic, and thermally stable materials for room-temperature energy conversion motivates the exploration of novel thermoelectric systems beyond the traditional magnetic Heusler alloys. While full and half-Heusler compounds, especially Co-, Ni-, and Mn-based systems, have demonstrated promising thermoelectric properties, their typically high operating temperatures and magnetic complexities limit their applicability in ambient thermal management. In this context, we investigate whether Sc-based inverse Heusler alloys can offer a viable nonmagnetic alternative with competitive thermoelectric performance. In this work, we perform a systematic first-principles study of the inverse Heusler compounds Sc 2 RuZ (Z = Si, Ge, Sn), focusing on their structural, electronic, mechanical, and thermodynamic-thermoelectric properties. Density Functional Theory (DFT) was employed to compute optimized lattice structures and band dispersion, while dynamical stability was assessed via phonon calculations. Thermoelectric transport coefficients, including Seebeck coefficient, electrical conductivity, and thermal conductivity, were estimated using the semiclassical Boltzmann transport theory within the constant relaxation time approximation. Our results show that all Sc 2 RuZ compounds are thermodynamically stable semiconductors with indirect band gaps of 0.12–0.16 eV and exhibit high elastic moduli, especially Sc 2 RuSn, which demonstrates superior stiffness and incompressibility. Importantly, all compounds display promising room-temperature thermoelectric characteristics, including high Seebeck coefficients and power factors. These findings reveal that Sc 2 RuZ alloys represent a rare class of stable, nonmagnetic inverse Heusler semiconductors with intrinsic thermoelectric potential at room temperature, unlike many existing Heusler systems optimized for spintronics or high-temperature operation. This work expands the known design space for Heusler-based thermoelectrics and offers a theoretical basis for experimental realization of efficient, low-temperature, nonmagnetic thermoelectric materials.

alloys↗
DOE OSTI2026

Charge Carrier and Spin Diffusion in a Polycrystalline and Single Crystal Lead Halide Perovskite Semiconductor

The directed movement of spin-bearing excitations in semiconductors can enable many potential schemes for spintronic applications. Understanding the mechanism of spin motion, as opposed to charge carrier or exciton motion, may require specialized techniques and sample conditions. Here we employ the noncontact and time-resolved technique of light-induced transient grating spectroscopy (LITG) to measure both carrier and spin motion in a perovskite semiconductor with varying crystallinity. The carrier motion aligns with expectations from past studies undertaken at low fluence, revealing an ambipolar diffusion coefficient on the order of 1 cm 2 /sec and diffusion length of roughly 1.5 μm for single crystals that is strongly attenuated as crystallite size decreases. The spin diffusion is measured with cross-polarized excitation beams and uncovers an intensity-dependent coefficient rising above 100 cm 2 /sec. The fast room-temperature spin relaxation limits the spin diffusion length, but the remarkable speedup of spin over carrier diffusion suggests a mechanism involving a combination of exchange-mediated and doping effects that enhance spin transport.

14 SOLAR ENERGY↗
DOE OSTI2025

Orientation-Driven Chirality Funnels in Chiral Low-Dimensional Lead-Halide Perovskite Heterostructures

Chiral hybrid metal-halide perovskites show low-symmetry crystal structures, large Rashba splitting, spin-filtering, and strong chiroptical activity. Circular dichroism and circularly polarized photoluminescence have been investigated in chiral perovskites with increasingly distorted chiral structures. Here, we report the fabrication of chiral (R/S)-EBAPbI3 (EBA = α-ethylbenzylamine) single crystals, which possess highly distorted octahedral structures with a high angle variance value of ∼68 degree2. Using control in the fabrication conditions, we transfer chiral single crystals to thin films and achieve different crystal orientation preferences that induce tunable chiroptical properties to their heterostructures with PbI2 nanodomains, which we characterize with in situ X-ray diffraction and grazing-incidence wide-angle X-ray scattering measurements. Using transient chiroptical spectroscopies, we resolve photoexcited charge carrier dynamics and chirality transfer processes in such heterostructures down to cryogenic temperatures. We observe rapid carrier transfer along the in-plane (002) facets in chiral perovskite phases to PbI2 nanostructures within the initial few picoseconds, while carrier transfer along the out-of-plane (002) facets occurs at a slower rate. This fast transfer process leads to high photoluminescence intensities and large degrees of circular polarization in the emission from PbI2 nanodomains at cryogenic temperatures. Our findings report a multidimensional chiral-achiral heterostructure which takes advantage of controllable chirality transfer and offers new routes for future spintronic and chiroptical applications.

Liu, Shangpu↗
DOE OSTI2025

Unconventional Superlattice Ordering in Intercalated Transition Metal Dichalcogenide V1/3NbS2

The interplay between symmetry and topology in magnetic materials makes it possible to engineer exotic phases and technologically useful properties. A key requirement for these pursuits is achieving control over local crystallographic and magnetic structure, usually through sample morphology (such as synthesis of bulk crystals versus thin films) and application of magnetic or electric fields. Here we show that V1/3NbS2 can be crystallized in two ordered superlattices, distinguished by the periodicity of out-of-plane magnetic intercalants. Whereas one of these structures is metallic and displays the hallmarks of altermagnetism, the other superlattice, which has not been isolated before in this family of intercalation compounds, is a semimetallic noncollinear antiferromagnet that may enable access to topologically nontrivial properties. This observation of an unconventional superlattice structure establishes a powerful route for tailoring the tremendous array of magnetic and electronic behaviors hosted in related materials and may expand their use in low-power spintronic or topological quantum devices.

Fender, Shannon S↗
DOE OSTI2025

Highly Conjugated Porphyrin Arrays Enable Optical Resolution of Ferromagnetic and Antiferromagnetic Aligned States of the Triplet Exciton and an Incorporated Stable Radical

Well-defined photogenerated molecular spin systems have potential utility in spintronics and quantum information science (QIS). Because molecular magnetic, optical, and electronic properties can be controlled by design, diverse spin systems can be prepared at modest temperatures. Photogenerated molecular spin systems often involve states prepared from the interaction of excitons and charges. Resolving the nature of electron spin alignment in photogenerated spin states described by the coupling of a triplet exciton and a stable radical commonly relies on EPR spectroscopy. Here, we describe ethyne-bridged (porphinato)metal ( PM n ) oligomers that incorporate a macrocycle-bound Cu(II) radical center. Upon photoexcitation of such PM n arrays, a singdoublet ( 2 S 1 ) state is formed; ultrafast internal conversion (IC) then produces a tripdoublet ( 2 T 1 ) state, which undergoes intersystem crossing (ISC) to produce a tripquartet ( 4 T 1 ) state, before relaxation to the ground state ( 2 S 0 ). These highly conjugated Cu(II) radical-containing PMn arrays enable direct observation of copper porphyrin 2 T 1 → 4 T 1 ISC dynamics from the biexponential decay of the near-infrared (NIR) 2,4 T 1 → 2,4 T n transient absorption manifold. Multireference n-electron valence perturbation theory (NEVPT2) computations illuminate how PM n electronic structure controls the relaxation dynamics of these long-lived (>10 ns) electronically excited multiplet states. These studies show that highly conjugated and polarizable porphyrin arrays incorporating stable spin centers provide rare π-delocalized systems where the ferromagnetic and antiferromagnetic alignment between a triplet exciton and a stable radical are both spectrally resolved and addressable using transient optical spectroscopy at wavelengths exceeding 1 μm, providing new opportunities to QIS.

14 SOLAR ENERGY↗
DOE OSTI2025

Discovery and Synthesis of a Family of Boride Altermagnets

Borides are a rich material family. To push the boundaries of borides’ properties and applications into broader fields, we have conducted systematic theoretical and experimental searches for synthesizable phases in ternary borides TM 2 B 2 (T = 3d, M = 4d/5d transition metals). We find that TM 2 B 2 in the FeMo 2 B 2 -type and CoW 2 B 2 -type structures form a large family of stable/metastable materials of 120 members. Among them, we identify 40 materials with stable magnetic solutions. Further, we discover 11 altermagnets in the FeMo 2 B 2 -type structure. So far, boride altermagnets are rare. In these altermagnets, T = Fe or Mn atoms are arranged in parallel T-chains with strong ferromagnetic intrachain couplings and antiferromagnetic interchain couplings. They simultaneously exhibit electronic band spin splitting, typical of ferromagnetism, and zero net magnetization, typical of antiferromagnetism. They also exhibit magnonic band chiral splitting. Both effects originate from the unique altermagnetic symmetries crucially constrained by the nonmagnetic atoms in the structure. Transport properties of relevance to spintronic applications, including the strain-induced spin-splitter effect and anomalous Hall effect, are predicted. An iodine-assisted synthesis method for TM 2 B 2 is developed, using which 7 of the predicted low-energy phases are experimentally synthesized and characterized, including 4 altermagnets. This work expands the realm of borides by offering new opportunities for studying altermagnetism and altermagnons in borides. It also provides valuable insights into the discovery and design of altermagnets. Here, by demonstrating that altermagnets can exist as families sharing a common motif, this work paves a feasible route for discovering altermagnets by elemental substitutions and high-throughput computations.

Chemical structure↗
DOE OSTI2025

Skyrmion-like Spin Textures Emerging in the Material Derived from Structural Frustration

Magnetic materials with complex spin textures present both fundamental and practical appeal. The complex patterns of magnetic moments emerging on underlying crystal lattices hold potential for robust information storage and processing, including the promise of topological quantum computing. The scope of materials that host such patterns, however, remains rather limited. Here, in this study, we report a discovery of a complex spin texture in a noncentrosymmetric material that emerges from the structural frustration at the boundary between centrosymmetric parent structures MnCoGe (the hexagonal Ni 2 In or the orthorhombic TiNiSi structure type) and MnCoAs (the TiNiSi structure type). Our findings demonstrate that such structural frustration provides a powerful handle for identifying compositional spaces where complex magnetic behavior and associated nontrivial magnetic structures are likely to emerge. Thus, the new phase MnCoGe 1/3 As 2/3 exhibits a modulated cycloidal antiferromagnetic arrangement of electron spins on a noncentrosymmetric lattice (of the hexagonal ZrNiAl type) that materializes in the space between centrosymmetric collinear ferromagnets. This work provides a pathway for discovering novel materials with exotic spin textures for next-generation spintronics and quantum technologies.

Wang, YiXu [Florida State University, Tallahassee,↗