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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Direct Imaging of Antiferromagnet-Ferromagnet Phase Transition in van der Waals Antiferromagnet CrSBr

The advent of van der Waals (vdW) ferromagnetic (FM) and antiferromagnetic (AFM) materials offers unprecedented opportunities for spintronics and magneto-optic devices. Combining magnetic Kerr microscopy and density functional theory calculations, the AFM-FM transition is investigated and a surprising abnormal magneto-optic anisotropy in vdW CrSBr associated with different magnetic phases (FM, AFM, or paramagnetic state) is discovered. This unique magneto-optic property leads to different anisotropic optical reflectivity from different magnetic states, permitting direct imaging of the AFM Néel vector orientation and the dynamic process of the AFM-FM transition within a magnetic field. Using Kerr microscopy, not only the domain nucleation and propagation process is imaged but also the intermediate spin-flop state in the AFM-FM transition is identified. In conclusion, the unique magneto-optic property and clear identification of the dynamics process of the AFM-FM phase transition in CrSBr demonstrate the promise of vdW magnetic materials for future spintronic technology.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Preparation of Ultrathin Gold Films with Subatomic Surface Roughness

Nanoscale device fabrication requires control over film growth at the atomic scale. Growth conditions must be tuned in consideration of interface parameters like chemical bonding, surface free energy, and lattice matching. In metals, electronic properties may also be utilized for control of physical parameters. Quantum size effects can induce metals to spontaneously form specific shapes and sizes according to their electronic structure. Unfortunately, such electronic growth is generally known only for a few systems and is typically only stable under cryogenic conditions. In this work, we explore a recently discovered class of electronic growth systems in which metal films are grown upon the relatively inert surfaces of van der Waals crystals. In this class of materials, the electronic growth is highly stable at room temperature and actually requires higher temperature annealing to achieve proper equilibrium. We work with the Au/MoS 2 system, which shows excellent stability and can readily form discrete and atomically flat nanostructures. Here, we show how the electronic growth modes facilitate the formation of atomically flat films with nanometer scale thickness. Here, the surface roughness of these films was found to be less than a single atom over several square microns, creating nearly perfect surfaces for studies of self-assembled monolayers or other applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Nanoscale Periodic Trapping Sites for Interlayer Excitons Built by Deformable Molecular Crystal on 2D Crystal

The nanoscale moiré pattern formed at 2D transition-metal dichalcogenide crystal (TMDC) heterostructures provides periodic trapping sites for excitons, which is essential for realizing various exotic phases such as artificial exciton lattices, Bose–Einstein condensates, and exciton insulators. At organic molecule/TMDC heterostructures, similar periodic potentials can be formed via other degrees of freedom. Here, we utilize the structure deformability of a 2D molecular crystal as a degree of freedom to create a periodic nanoscale potential that can trap interlayer excitons (IXs). Specifically, two semiconducting molecules, PTCDI and PTCDA, which possess similar band gaps and ionization potentials but form different lattice structures on MoS 2 , are investigated. The PTCDI lattice on MoS 2 is distorted geometrically, which lifts the degeneracy of the two molecules within the crystal’s unit cell. The degeneracy lifting results in a spatial variation of the molecular orbital energy, with an amplitude and periodicity of ~0.2 eV and ~2 nm, respectively. On the other hand, no such energy variation is observed in PTCDA/MoS 2 , where the PTCDA lattice is much less distorted. Furthermore, the periodic variation in molecular orbital energies provides effective trapping sites for IXs. For IXs formed at PTCDI/MoS 2 , rapid spatial localization of the electron in the organic layer toward the interface is observed, which demonstrates the effectiveness of these interfacial IX traps.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Magnetic Imaging with Spin Defects in Hexagonal Boron Nitride

Optically active spin defects hosted in hexagonal boron nitride (ℎ-BN) are promising candidates for the development of a two-dimensional (2D) quantum sensing unit. Here, we demonstrate quantitative magnetic imaging with ℎ-BN flakes doped with negatively charged boron-vacancy ($V^{-}_{B}$) centers through neutron irradiation. As a proof-of-concept, we image the magnetic field produced by Cr⁢Te 2 , a van der Waals ferromagnet with a Curie temperature slightly above 300 K. Compared to other quantum sensors embedded in 3D materials, the advantages of the ℎ-BN-based magnetic sensor described in this work are its ease of use, high flexibility, and, more importantly, its ability to be placed in close proximity to a target sample. Further, such a sensing unit will likely find numerous applications in 2D materials research by offering a simple way to probe the physics of van der Waals heterostructures.

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

Atomic-Scale Insights into the Lateral and Vertical Epitaxial Growth in Two-Dimensional Pd 2 Se 3 –MoS 2 Heterostructures

Two-dimensional (2D) materials form heterostructures in both the lateral and vertical directions when two different materials are interfaced, but with totally different bonding mechanisms of covalent in-plane to van der Waal’s layered interactions. Understanding how the competition between lateral and vertical forces influences the epitaxial growth is important for future materials development of complex mixed layered heterostructures. In this study, we use atomic-resolution annular dark-field scanning transmission electron microscopy to study the detailed atomic arrangements at mixed 2D heterostructure interfaces composed of two semiconductors with distinctly different crystal symmetry and elemental composition, Pd 2 Se 3 :MoS 2 , in order to understand the role of different chemical bonds on the resultant epitaxy. Pd 2 Se 3 is grown off the step edge in bilayer MoS 2 , and the vertical and lateral epitaxial relationships of the Pd 2 Se 3 –MoS 2 heterostructures are investigated. We find that the similarity of geometry at the interface with one metal (Pd or Mo) atoms bonded with two chalcogens (S or Se) are the crucial factors to make the atomically stitched lateral junction of 2D heterostructures. In addition, the vertical van der Waal interactions that are normally dominant in layered materials can be overcome by in-plane forces if the interfacial atomic stitching is high in quality and low in defect density. This knowledge should help guide the approaches for improving the epitaxy in mixed 2D heterostructures and seamless stitching of in-plane 2D heterostructures with various complex monolayer structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Evidence for anisotropic spin-triplet Andreev reflection at the 2D van der Waals ferromagnet/superconductor interface

Fundamental symmetry breaking and relativistic spin–orbit coupling give rise to fascinating phenomena in quantum materials. Of particular interest are the interfaces between ferromagnets and common s-wave superconductors, where the emergent spin-orbit fields support elusive spin-triplet superconductivity, crucial for superconducting spintronics and topologically-protected Majorana bound states. Here, we report the observation of large magnetoresistances at the interface between a quasi-two-dimensional van der Waals ferromagnet Fe 0.29 TaS 2 and a conventional s-wave superconductor NbN, which provides the possible experimental evidence for the spin-triplet Andreev reflection and induced spin-triplet superconductivity at ferromagnet/superconductor interface arising from Rashba spin-orbit coupling. The temperature, voltage, and interfacial barrier dependences of the magnetoresistance further support the induced spin-triplet superconductivity and spin-triplet Andreev reflection. This discovery, together with the impressive advances in two-dimensional van der Waals ferromagnets, opens an important opportunity to design and probe superconducting interfaces with exotic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electrical Transport Interplay with Charge Density Waves, Magnetization, and Disorder Tuned by 2D van der Waals Interface Modification via Elemental Intercalation and Substitution in ZrTe 3 , 2H-TaS 2 , and Cr 2 Si 2 Te 6 Crystals

Electrical transport in 2D materials exhibits unique behaviors due to reduced dimensionality, broken symmetries, and quantum confinement. It serves as both a sensitive probe for the emergence of coherent electronic phases and a tool to actively manipulate many-body correlated states. Exploring their interplay and interdependence is crucial but remains underexplored. This review integratively cross-examines the atomic and electronic structures and transport properties of van der Waals-layered crystals ZrTe 3 , 2H-TaS 2 , and Cr 2 Si 2 Te 6 , providing a comprehensive understanding and uncovering new discoveries and insights. A common observation from these crystals is that modifying the atomic and electronic interface structures of 2D van der Waals interfaces using heteroatoms significantly influences the emergence and stability of coherent phases, as well as phase-sensitive transport responses. In ZrTe 3 , substitution and intercalation with Se, Hf, Cu, or Ni at the 2D vdW interface alter phonon–electron coupling, valence states, and the quasi-1D interface Fermi band, affecting the onset of CDW and SC, manifested as resistance upturns and zero-resistance states. We conclude here that these phenomena originate from dopant-induced variations in the lattice spacing of the quasi-1D Te chains of the 2D vdW interface, and propose an unconventional superconducting mechanism driven by valence fluctuations at the van Hove singularity, arising from quasi-1D lattice vibrations. Short-range in-plane electronic heterostructures at the vdW interface of Cr 2 Si 2 Te 6 result in a narrowed band gap. The sharp increase in in-plane resistance is found to be linked to the emergence and development of out-of-plane ferromagnetism. The insertion of 2D magnetic layers such as Mn, Fe, and Co into the vdW gap of 2H-TaS 2 induces anisotropic magnetism and associated transport responses to magnetic transitions. Overall, 2D vdW interface modification offers control over collective electronic behavior, transport properties, and their interplays, advancing fundamental science and nanoelectronic devices.

2D van der Waals interfaces↗
DOE OSTI2025

Revealing the electronic structure of van der Waals antiferromagnetic NiPS 3 through synchrotron-based 𝜇-ARPES and alkali metal dosing

Antiferromagnetic NiPS 3 has recently emerged as a quantum material of considerable interest, thanks to the discovery of multiple new couplings involving electrons, spins, orbitals, phonons, and magnons. However, controversies and open questions persist concerning the fundamental origins of these couplings. A critical piece of information required to advance the understanding is the precise electronic band structure of NiPS 3 . Angle-resolved photoemission spectroscopy (ARPES), combined with alkali metal dosing (AMD), can enable us to directly observe the subtle electronic states that appear around the Fermi surface, offering valuable insights into the intriguing quantum properties and interplays of the examined material. Here, in this study, we present a comprehensive characterization and analysis of the band structure of van der Waals layered antiferromagnet NiPS 3 , leveraging state-of-the-art μ-ARPES measurements supported by density functional theory (DFT) calculations. Theoretical DFT results identify the orbital contributions to the observed bands, providing a precise understanding of the experimental ARPES data. Crucially, AMD enables the observation of conduction band and defect-related states above the valence band maximum in NiPS 3 . Furthermore, temperature dependent ARPES results across the Néel transition temperature of NiPS 3 reveal that the paramagnetic and antiferromagnetic phases have nearly identical band structures, underlining the highly localized character of Ni d states. These findings substantially deepen our understanding of the electronic properties of NiPS 3 and lay a vital foundation for exploring the intriguing quantum phenomena it exhibits.

Cao, Yifeng [Boston Univ., MA (United States); Law↗
DOE OSTI2026

A New Platform for Twistronics: Perovskite Moiré Superlattices Beyond van der Waals

Moiré superlattices have reshaped condensed‐matter physics by enabling twist‐angle engineering of electronic and excitonic states, traditionally in van der Waals (vdW) systems. This Perspective focuses on perovskites as a transformative non‐van der Waals (non‐vdW) platform for twistronics. These materials are distinguished by ionic‐covalent bonding and soft octahedral lattices that support deep moiré potentials (≈100–200 meV), which in turn yield bright, room‐temperature excitons and tunable quantum phenomena. We organize recent advances into two core classes: (i) moiré superlattices constructed from 2D Ruddlesden‐Popper (RP) perovskites via spontaneous or guided twisted stacking; and (ii) moiré architectures in 3D perovskite lamellae, achieved through topotactic 2D RP‐to‐3D conversion and ligand‐induced self‐assembly. Key experimental observations in these systems include twist‐tunable moiré periods, suppressed exciton–exciton annihilation, flat band‐like transport, and ferroelectric vortex lattices in their oxide perovskite counterparts. Here, we outline key bottlenecks, including defects arising during phase conversion, stringent lattice‐matching requirements, and the absence of in situ twist metrology, and highlight opportunities in lattice‐mismatched heterostructures and multifunctional quantum‐optoelectronic devices. Collectively, these advances establish perovskite moiré systems as a critical bridge between fundamental moiré physics and room‐temperature technological applications.

2D Ruddlesden–Popper perovskites↗
DOE OSTI2022

Effects of thin metal contacts on few-layer van der Waals ferrielectric CuInP 2 S 6

Out-of-plane polarized ferroelectric materials in a capacitive structure provide a key component for several technological applications. Furthermore, two-dimensional materials are expected to aid in the quest for both ultrathin and flexible electronics. Of the various two-dimensional ferroelectrics with out-of-plane polarization, CuInP 2 S 6 is special in that the Cu atoms are highly mobile and it has been shown to possess both low- and high-polarization states. Here, using density-functional-theory calculations, we explore the stabilization of the ferroelectric state for several prototypical metal contacts (Gr, Ni, Cu, Au, and Ag). In all cases, we find that the ferroelectric state can be stabilized at fewer layers than in the freestanding case. For all of the considered conventional metal contacts, we also find the existence of a quasi-ferroelectric state that stabilizes a polar phase for thicknesses greater than two layers of CIPS. In the cases of Au and Ag, interfacial alignment and strain can be used to stabilize ferroelectricity at the bilayer limit. Furthermore, we find that the strength of the interaction between the contact and CuInP 2 S 6 also leads to stabilization of the high-polarization state when ferroelectricity is stabilized. Lastly, energy-barrier calculations show that the system is still switchable in the presence of contact doping from the metal contacts.

2D materials↗
DOE OSTI2020

Quantum anomalous Hall effect in two-dimensional magnetic insulator heterojunctions

Abstract Recent years have witnessed tremendous success in the discovery of topological states of matter. Particularly, sophisticated theoretical methods in time-reversal-invariant topological phases have been developed, leading to the comprehensive search of crystal database and the prediction of thousands of topological materials. In contrast, the discovery of magnetic topological phases that break time reversal is still limited to several exemplary materials because the coexistence of magnetism and topological electronic band structure is rare in a single compound. To overcome this challenge, we propose an alternative approach to realize the quantum anomalous Hall (QAH) effect, a typical example of magnetic topological phase, via engineering two-dimensional (2D) magnetic van der Waals heterojunctions. Instead of a single magnetic topological material, we search for the combinations of two 2D (typically trivial) magnetic insulator compounds with specific band alignment so that they can together form a type-III broken-gap heterojunction with topologically non-trivial band structure. By combining the data-driven materials search, first-principles calculations, and the symmetry-based analytical models, we identify eight type-III broken-gap heterojunctions consisting of 2D ferromagnetic insulators in the MXY compound family as a set of candidates for the QAH effect. In particular, we directly calculate the topological invariant (Chern number) and chiral edge states in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work illustrates how data-driven material science can be combined with symmetry-based physical principles to guide the search for heterojunction-based quantum materials hosting the QAH effect and other exotic quantum states in general.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bilayer twisting as a mean to isolate connected flat bands in a kagome lattice throughWigner crystallization*

The physics of flat band is novel and rich but difficult to access. In this regard, recently twisting of bilayer van der Waals (vdW)-bounded two-dimensional (2D) materials has attracted much attention, because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy. Alternatively, one may start with a 2D kagome lattice, which already possesses flat bands at the Fermi level, but unfortunately these bands connect quadratically to other (dispersive) bands, leading to undesirable effects. Here, we propose, by first-principles calculation and tight-binding modeling, that the same bilayer twisting approach can be used to isolate the kagome flat bands. As the starting kinetic energy is already vanishingly small, the interlayer vdW potential is always sufficiently large irrespective of the twisting angle. As such the electronic states in the (connected) flat bands become unstable against a spontaneous Wigner crystallization, which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.

Physics↗
DOE OSTI2024

Investigation of topologically trivial and non-trivial spin textures and their relationships with the topological Hall effect

This proposal has two main goals: (1) to investigate ways to create a variety of spin textures, both topologically trivial and non-trivial, and (2) to establish the relationships between the created spin textures and the topological Hall effect (THE) signal. The approach is achieved by utilizing the van der Waals (vdW) two-dimensional (2D) magnetic materials, such as CrX 3 (X=I, Br, and Cl) and Fe 3 GeTe 2 , transferred onto various substrates with different interactions to create a wide range of spin textures; and utilize Spin helical system, such as Eu metal, and the impact of the thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Observation of van der Waals phonons in the single-layer cuprate ( Bi , Pb ) 2 ( Sr , La ) 2 CuO 6 + δ

Interlayer van der Waals (vdW) coupling is generic in two-dimensional materials such as graphene and transition-metal dichalcogenides, which can induce very low-energy phonon modes. Using high-resolution inelastic hard x-ray scattering, we uncover the ultralow energy phonon mode along the Cu-O bond direction in the high- T c cuprate (Bi,Pb) 2 (Sr,La) 2 ⁢CuO 6+$\delta$ (Bi2201). The energy and full-width half-maximum (FWHM) of these modes are independent of temperature, while their intensity decreases with doping in accordance with an increasing c -axis lattice parameter. Here, we compare the experimental results to first-principles density functional theory simulations and identify the observed mode as a van der Waals phonon, which arises from the shear motion of the adjacent Bi-O layers. This shows that Bi-based cuprate has vibrational properties similar to graphene and transition-metal dichalcogenides, which can be exploited to engineer novel heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Preserving the Josephson Coupling of Twisted Cuprate Junctions via Tailored Silicon Nitride Circuits Boards

Controlled fabrication of twisted van der Waals heterostructures is essential to unlock the full potential of moiré materials. However, achieving reproducibility remains a major challenge, particularly for air-sensitive materials such as Bi 2 Sr 2 CaCu 2 O 8 + δ (BSCCO), where it is crucial to preserve the intrinsic and delicate superconducting properties of the interface throughout the entire fabrication process. Here, a dry, inert and cryogenic assembly method is presented that combines silicon nitride nanomembranes (NMBs) with pre-patterned electrodes and the cryogenic stacking technique (CST) to fabricate high-quality twisted BSCCO Josephson junctions (JJs). This protocol prevents thermal and chemical degradation during both interface formation and electrical contact integration. It is also found that asymmetric membrane designs, such as a double cantilever, effectively suppress vibration-induced disorder due to wire bonding, resulting in sharp and hysteretic current–voltage characteristics. The junctions exhibit a twist-angle-dependent Josephson coupling with magnitudes comparable to the highest-performing devices reported to date, but achieved through a straightforward and versatile contact method, offering a scalable and adaptable platform for future applications. These findings highlight the importance of both interface and contact engineering in addressing reproducibility in superconducting van der Waals heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Three-dimensional ferromagnetism and magnetotransport in van der Waals Mn-intercalated tantalum disufide

Van der Waals (vdW) ferromagnets are an important class of materials for spintronics applications. The recent discovery of atomically vdW magnets CrI 3 and Cr 2 Ge 2 Te 6 has triggered a renaissance in the area of two-dimensional (2D) magnetism. Herein we systematically studied 2H-Mn 0.28 TaS 2 single crystal, a 2D vdW ferromagnet with T c ~ 82.3K, and a large in-plane magnetic anisotropy. Mn K-edge x-ray absorption spectroscopy was measured to provide information on its electronic state and local atomic environment. The detailed magnetic isotherms measured in the vicinity of T c indicates that the spin coupling inside 2H-Mn 0.28 TaS 2 is of a 3D Heisenberg type coupled with the attractive long-range interaction between spins that decay as J(r) ≈ r –4.85 . Both resistivity ρ(T) and thermopower S(T) exhibit anomalies near T c , confirming that the hole-type transport carriers strongly interact with local moments. Furthermore, an unusual angle-dependent magnetoresistance is further observed, suggesting a possible field-induced novel magnetic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗