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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Quantum anomalous Hall insulator phases in Fe-doped GaBi honeycomb

We discuss electronic and magnetic properties of the Fe-doped GaBi honeycomb using first principles calculations. Our analysis shows that the pristine GaBi honeycomb transitions from being a two-dimensional quantum spin Hall (QSH) insulator to a quantum anomalous Hall (QAH) insulator when it is doped with one Fe atom in a 4 × 4 GaBi honeycomb. Additionally, the QAH phase in Fe-doped GaBi is found to be robust in that it maintains its Chern number (C = 1) under fairly large strains (~ 4%) and supports a gap as large as 112 meV at 2.21% strain. The QAH phase is also retained when the Fe-doped GaBi is placed on a CdTe substrate, suggesting that Fe-doped GaBi films could be useful for spintronics applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Extrinsic and intrinsic properties in metal–insulator transition of hydrothermally prepared vanadium dioxide crystals

The clear insulator (monoclinic-VO{sub 2}) to metal (rutile-VO{sub 2}) transition (IMT) was observed in electrical conductivity and differential scanning calorimeter (DSC) measurements at around 340 K, which is IMT temperature (T{sub H}), in the hydrothermally prepared VO{sub 2} crystals. The occurrence of metal to insulator transition (MIT) temperature (T{sub C}) was observed below 333 K during the first resistance measurement cycle in the most of cases. The sudden jump of the electrical resistance at IMT and MIT points was amplified several times than that of the first cycle during the repeated successive thermal cycles (heating and cooling across the IMT and MIT temperatures). T{sub C} and T{sub H} shifted to higher temperature by the repeated successive thermal cycles. This shift and the amplified jump might be related to the mechanical stress between the VO{sub 2} crystals, i.e. extrinsic properties. However, the starting point of MIT, T{sub CS} = ∼ 336 K, and the starting point of IMT, T{sub HS} = ∼ 338 K, kept almost constant during the repeated thermal cycles (< 10 times). These two temperatures may be related to the intrinsic properties of the VO{sub 2}: the phase transitions initiated at these temperatures regardless of the number of the repeated thermal cycles. The neat surface of the VO{sub 2} crystals was severely damaged and the average size of particles reduced from 110 nm to 70–90 nm after extensively repeated thermal cycles (> 70 times). The damaged surface and the smaller particles, which would be originated from the mechanical stress caused by crystal volume change during the first order transition of the VO{sub 2}, would weaken the electrical conduction path (loosen grain boundaries) between the VO{sub 2} single crystals and would result in the amplified jump at the following MIT. This report may boost the study for the improved stability and lifetime of the VO{sub 2} based electronic devices. - Highlights: • The sharp phase transition in cluster of VO{sub 2} crystals depends on repeated thermal cycles. • Two intrinsic and two extrinsic temperatures are observed during the phase transition. • The mechanical stress change and surface damage may cause the extrinsic properties in transport measurement.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Highly silanized cellulose biocomposites for sustainable insulation materials

Microfibrillated lignocellulose networks, derived from agricultural byproducts, represent an environmentally friendly biogenic material production due to their abundant availability to circular bioeconomy and inherent carbon sink in life cycle analysis. Yet, its vulnerability to moisture and flammability, coupled with challenges in creating highly reinforced insulation materials, poses challenges for the carbon-zero green building sector. Here we address these challenges with a new concept of in-situ grafting polymerization of nanoporous silica in pre-formed lignocellulosic fiber networks. The seamlessly integrating nanoporous silica with cellulose through hydrogen bonding networks enabled us to prepare highly reinforced biogenic composites for green building insulations. A high reinforcement biocomposite with hierarchal arrangements of nanoporous silica within the cellulose network exhibits remarkable attributes. It boasts a thermal conductivity of 24.2 mW·m –1 ·K –1 , a flexural modulus of 942 MPa, and soundproofing with a 20.8 % noise reduction, as well as the fire resistance characterized by an extended time to ignition and a reduced peak heat release rate of 144 kW·m –2 at 35 kW·m –2 of incident radiant heat flux. Furthermore, it demonstrates a reduced water absorption capacity, dropping from 5.12 g·g –1 to 0.75 g·g –1 . Altogether, this study opens the new pathways towards sustainable carbon-zero building materials in the context of circular bioeconomy.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Effect of insulator end cap thickness on time-dependent Hartmann flow in a rotating mirror

We present a framework for analysing plasma flow in a rotating mirror. By making a series of physical assumptions, we reduce the magnetohydrodynamic (MHD) equations in a three-dimensional cylindrical system to a one-dimensional system in a shallow, cuboidal channel within a transverse magnetic field, similar to the Hartmann flow in ducts. We then solve the system both numerically and analytically for a range of values of the Hartmann number and calculate the dependence of the plasma flow speed on the thickness of the insulating end cap. We observe that the mean flow overshoots and decelerates before achieving a steady-state value, a phenomenon that the analytical model cannot capture. This overshoot is directly proportional to the thickness of the insulating end cap and the external electric field, with a weak dependence on the external magnetic field. Our simplified model can act as a benchmark for future simulations of the supersonic mirror device CMFX (centrifugal magnetic fusion experiment), which will employ more sophisticated physics and realistic magnetic field geometries.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Molecule-Adsorbed Topological Insulator and Metal Surfaces: A Comparative First-Principles Study

We compare electronic structure characteristics of three different kinds of benzene-adsorbed (111) surfaces: that of Bi 2 Te 3 , a prototypical topological insulator, that of Au, a prototypical inert metal, and that of Pt, a prototypical catalytic metal. Using first-principles calculations based on dispersion-corrected density functional theory, we show that benzene is chemisorbed on Pt, but physisorbed on Au and Bi 2 Te 3 . Here, the adsorption on Bi 2 Te 3 is particularly weak, consistent with a minimal perturbation of the electronic structure at the surface of the topological insulator, revealed by a detailed analysis of the interaction of the molecular orbitals with the topological surface states.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Study of Polycrystalline Bulk Sr 3 OsO 6 Double-Perovskite Insulator: Comparison with 1000 K Ferromagnetic Epitaxial Films

Polycrystalline Sr 3 OsO 6 , which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 ° C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr3OsO6 toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr 3 OsO 6 films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Thus, robust ferromagnetic order appears only when Sr 3 OsO 6 is under the influence of interfaces. A specific heat capacity of 39.6(9) X 10 -3 J mol -1 K -2 is found at low temperatures (<17 K). This value is remarkably high, suggesting the presence of possible Fermionic-like excitations at the magnetic ground state. While the bulk and film forms of Sr 3 OsO 6 share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

The Role of Structural and Compositional Heterogeneities in the Insulator-to-Metal Transition in Hole-Doped APd 3 O 4 (A = Ca, Sr)

The cubic semiconducting compounds APd 3 O 4 (A = Ca, Sr) can be hole-doped by Na substitution on the A site and driven toward more conducting states. This process has been followed here by a number of experimental techniques to understand the evolution of electronic properties. While an insulator-to-metal transition is observed in Ca 1–x Na x Pd 3 O 4 for x ≥ 0.15, bulk metallic behavior is not observed for Sr 1–x Na x Pd 3 O 4 up to x = 0.20. Given the very similar crystal and (calculated) electronic structures of the two materials, the distinct behavior is a matter of interest. We present evidence of local disorder in the A = Sr materials through the analysis of the neutron pair distribution function, which is potentially at the heart of the distinct behavior. Solid-state 23 Na nuclear magnetic resonance studies additionally suggest a percolative insulator-to-metal transition mechanism, wherein presumably small regions with a signal resembling metallic NaPd 3 O 4 form almost immediately upon Na substitution, and this signal grows monotonically with substitution. Here, some signatures of increased local disorder and a propensity for Na clustering are seen in the A = Sr compounds.

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

Tetragonal BaCoO 3 : A Co 4+ Ferromagnetic Mott Insulator

We report the stabilization of the metastable body-centered tetragonal (BCT) phase of BaCoO 3 (BCT-BaCoO 3 ) under high-pressure (15 GPa) and hightemperature (1200 °C) conditions using a mixture precursor. This double perovskite adopts the EuTiO 3 -type structure (space group I4/mcm), as confirmed by powder Xray diffraction and high-resolution STEM. X-ray photoelectron spectroscopy indicates a predominant Co 4+ oxidation state without detectable oxygen vacancies. Magnetization and heat capacity measurements reveal ferromagnetic ordering at T C ∼ 107 K, attributable to the BCT-BaCoO 3 phase. Above this temperature, the mixed-phase sample exhibits Curie−Weiss paramagnetism, a low-spin to high-spin crossover upon cooling, and a possible intermediate-spin state at elevated temperatures. Resistivity data indicates insulating behavior with weak magnetoresistance. DFT and DFT + DMFT calculations suggest that the insulating state originates from an orbitally selective transition sensitive to the nominal valence of the Co-d shell. The metastable BCT-BaCoO 3 phase cannot be retained in pure form at ambient pressure but can be stabilized by embedding it in a disordered mixture, offering a potential route to discover and preserve other high-pressure phases under ambient conditions.

Magnetic properties↗
DOE OSTI2026

Direct Simulations of H–He Mixtures at Planetary Interior Conditions: Demixing, Insulator–Metal Transition and Miscibility Boundaries

Accurate knowledge of the electrical and thermal conductivities and structural properties of hydrogen–helium mixtures under thermodynamic conditions within and beyond the immiscibility range is very important to predict the thermal evolution and internal structure of gas giant planets like Jupiter and Saturn. Here, we propose a novel method to determine the immiscibility boundary accurately without the need for free energy calculations, while providing consistent insights into structural and transport properties of mixtures. We show with direct large-scale ab initio simulations that the insulator–metal transition (IMT) of the hydrogen subsystem is strongly affected by an admixture with a small fraction of helium and occurs at temperatures significantly higher than those of pure hydrogen. At pressures below 150 GPa, the IMT boundary is not related anymore to the H 2 subsystem dissociation, the system remains insulating even after the full dissociation of H 2 molecules and its transition to an H–He mixture. The offset of the IMT in the H–He mixture relative to the dissociation region in the hydrogen subsystem and the significant reduction of static electrical and thermal conductivity by a factor between two and a few thousand relative to pure hydrogen found in mixtures have consequences for Jupiter and Saturn’s thermal evolution, internal structure, and dynamo action, affecting a large fraction of the interior of both planets.

Helium↗
DOE OSTI2024

Picosecond Ultrasonics in Magnetic Topological Insulator MnBi 2 Te 4

MnBi 2 Te 4 is a magnetic topological insulator with layered A-type antiferromagnetic order. It exhibits a rich layer- and magnetic-state dependent topological phase diagram; however, much about the coupling between spin, charge, and lattice remains to be explored. Here, in this work, we report that MnBi 2 Te 4 is an excellent acoustic phonon cavity by realizing phonon frequency combs using picosecond ultrasonics. With the generated acoustic phonon wavepackets, we demonstrate that the timing and phase of acoustic echoes can be used to detect the presence of stacking faults between van der Waals layers buried deep within the crystal. Furthermore, by implementing this nondestructive ultrafast optical measurement in conjunction with time-resolved magneto-optical Kerr effect experiments, we uncover that out-of-plane vibrations in MnBi 2 Te 4 do not couple to the magnetic order, i.e. there is no appreciable magnetostriction. Our work points out how a well-developed technique can probe the structural defects and phonon pulse engineering in layered topological insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Insulating nanomagnets driven by spin torque

Magnetic insulators, such as yttrium iron garnet (Y 3 Fe 5 O 12 ), are ideal materials for ultra-low power spintronics applications due to their low energy dissipation and efficient spin current generation and transmission. Recently, it has been realized that spin dynamics can be driven very effectively in micrometer-sized Y 3 Fe 5 O 12 /Pt heterostructures by spin-Hall effects. We demonstrate here the excitation and detection of spin dynamics in Y 3 Fe 5 O 12 /Pt nanowires by spin-torque ferromagnetic resonance. The nanowires defined via electron-beam lithography are fabricated by conventional room temperature sputtering deposition on Gd 3 Ga 5 O 12 substrates and lift-off. We observe field-like and anti-damping-like torques acting on the magnetization precession, which are due to simultaneous excitation by Oersted fields and spin-Hall torques. The Y 3 Fe 5 O 12 /Pt nanowires are thoroughly examined over a wide frequency and power range. We observe a large change in the resonance field at high microwave powers, which is attributed to a decreasing effective magnetization due to microwave absorption. By comparing different nanowire widths, the importance of geometrical confinements for magnetization dynamics becomes evident. In conclusion, our results are the first stepping stones toward the realization of integrated magnonic logic devices based on insulators, where nanomagnets play an essential role.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Doped Twisted Bilayer Graphene near Magic Angles: Proximity to Wigner Crystallization, Not Mott Insulation

We devise a model to explain why twisted bilayer graphene exhibits insulating behavior when ν = 2 or 3 charges occupy a unit moiré cell, a feature attributed to Mottness per previous work but not for ν = 1, clearly inconsistent with Mott insulation. We compute $r_s$ = EU/EK, where EU and EK are the potential and kinetic energies, respectively, and show that (i) the Mott criterion lies at a density larger than experimental values by a factor of 104 and (ii) a transition to a series of Wigner crystalline states exists as a function of ν. We find that, for ν = 1, $r_s$ fails to cross the threshold ($r_s$ = 37) for the triangular lattice, and metallic transport ensues. However, for ν = 2 and ν = 3, the thresholds $r_s$ = 22 and $r_s$ = 17, respectively, are satisfied for a transition to Wigner crystals (WCs) with a honeycomb (ν = 2) and a kagome (ν = 3) structure. We posit that such crystalline states form the correct starting point for analyzing superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Manipulating Topological Domain Boundaries in the Single-Layer Quantum Spin Hall Insulator 1T'–WSe 2

We report the creation and manipulation of structural phase boundaries in the single-layer quantum spin Hall insulator 1T'–WSe 2 by means of scanning tunneling microscope tip pulses. We observe the formation of one-dimensional interfaces between topologically nontrivial 1T' domains having different rotational orientations, as well as induced interfaces between topologically nontrivial 1T' and topologically trivial 1H phases. Scanning tunneling spectroscopy measurements show that 1T'/1T' interface states are localized at domain boundaries, consistent with theoretically predicted unprotected interface modes that form dispersive bands in and around the energy gap of this quantum spin Hall insulator. We also observe a qualitative difference in the experimental spectral line shape between topologically “unprotected” states at 1T'/1T' domain boundaries and protected states at 1T'/1H and 1T'/vacuum boundaries in single-layer WSe 2 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Pressure-Induced Chemical Bonding Effects on Lattice and Magnetic Instabilities in Antiferromagnetic Insulating CaMn 2 Sb 2

Exotic quantum phenomena often emerge near an electronic delocalization transition (EDT) from an antiferromagnetic insulating phase to a strongly correlated metallic state under pressure. We report the pressure-induced structural and magnetic evolution of the antiferromagnetic insulator CaMn 2 Sb 2 . Single-crystal X-ray diffraction reveals a first-order phase transition near 5.4 GPa from a trigonal P-3m1 structure to a monoclinic P2 1 /m phase accompanied by a ∼7% volume collapse. Residual electron density analysis at intermediate pressures reveals charge localization along Mn–Sb chains, signaling electronic instability preceding the structural transition. Bonding analysis indicates anisotropic Mn–Sb orbital reconfiguration under pressure, driving a distorted square-pyramidal geometry. Neutron scattering confirms the transition and identifies a pressure-induced incommensurate magnetic order, distinct from the ambient antiferromagnetic state. In the monoclinic phase, zigzag Mn chains exhibit antiferromagnetic coupling along the ac-plane, enabled by enhanced orbital overlap. These results establish CaMn 2 Sb 2 as a model system for studying the coupling of structural distortion, charge redistribution, and magnetic order in layered Mn pnictides under pressure.

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

Photoelectrochemical Imaging of Charge Separation between MoS 2 Triangles and Insulating SiO 2 Support

The role of the insulating support in photocatalysis is poorly understood. Using high-resolution photo-scanning electrochemical microscopy (photo-SECM), we observed significant spatial charge separation in few-layer-thick molybdenum disulfide (MoS 2 ) triangles attached to a SiO 2 substrate. Spatially resolved surface photovoltage (SPV) measurements revealed that photogenerated holes migrate from MoS 2 to the SiO 2 surface and travel laterally over distances exceeding 2 μm, driven by the builtin electric field of ~1.7 kV/cm. In thicker and less uniform flakes, the charge separation is dominated by internal driving forces within MoS 2 , without significant contribution from SiO 2 . These findings underscore the importance of insulator–semiconductor interactions for effective charge separation, suggesting a new strategy for optimizing photocatalytic systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Ferromagnetic Insulator to Metal Transition in Noncentrosymmetric Graphene Nanoribbons

Engineering sublattice imbalance within the unit cell of bottom-up synthesized graphene nanoribbons (GNRs) represents a versatile tool for realizing custom-tailored quantum nanomaterials. The interaction between low-energy zero-modes (ZMs) not only contributes to frontier bands but can form the basis for magnetically ordered phases. Here, we present the bottom-up synthesis of a noncentrosymmetric GNR that places all ZMs on the majority sublattice sites. Scanning tunneling microscopy and spectroscopy reveal that strong electron–electron correlations, leading to the Stoner magnetic instability, drive the system into a ferromagnetically ordered insulating ground state featuring a sizable band gap of E g ∼1.2 eV. At higher temperatures, a chemical transformation induces an insulator-to-metal transition that quenches the ferromagnetic order. Tight-binding (TB), density functional theory, and GW calculations corroborate our experimental observations. Furthermore, this work showcases how control over molecular symmetry, sublattice polarization, and ZM hybridization in bottom-up synthesized nanographenes can open a path to the exploration of many-body physics in rationally designed quantum materials.

Band structure↗
DOE OSTI2016

Hallmarks of the Mott-metal crossover in the hole-doped pseudospin-1/2 Mott insulator Sr 2 IrO 4

The physics of doped Mott insulators remains controversial after decades of active research, hindered by the interplay among competing orders and fluctuations. It is thus highly desired to distinguish the intrinsic characters of the Mott-metal crossover from those of other origins. Here we investigate the evolution of electronic structure and dynamics of the hole-doped pseudospin-1/2 Mott insulator Sr 2 IrO 4 . The effective hole doping is achieved by replacing Ir with Rh atoms, with the chemical potential immediately jumping to or near the top of the lower Hubbard band. The doped iridates exhibit multiple iconic low-energy features previously observed in doped cuprates - pseudogaps, Fermi arcs and marginal-Fermi-liquid-like electronic scattering rates. We suggest these signatures are most likely an integral part of the material's proximity to the Mott state, rather than from many of the most claimed mechanisms, including preformed electron pairing, quantum criticality or density-wave formation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet

When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi 2 Se 3 . Furthermore, our findings not only allow characterizing magnetism at the TI–FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces.

36 MATERIALS SCIENCE↗