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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Evidence of a gate-tunable Mott insulator in a trilayer graphene moiré superlattice

The Mott insulator is a central concept in strongly correlated physics and manifests when the repulsive Coulomb interaction between electrons dominates over their kinetic energy. Doping additional carriers into a Mott insulator can give rise to other correlated phenomena such as unusual magnetism and even high-temperature superconductivity. A tunable Mott insulator, where the competition between the Coulomb interaction and the kinetic energy can be varied in situ, can provide an invaluable model system for the study of Mott physics. Here we report the possible realization of such a tunable Mott insulator in a trilayer graphene heterostructure with a moiré superlattice. The combination of the cubic energy dispersion in ABC-stacked trilayer graphene and the narrow electronic minibands induced by the moiré potential leads to the observation of insulating states at the predicted band fillings for the Mott insulator. Moreover, the insulating states in the heterostructure can be tuned: the bandgap can be modulated by a vertical electrical field, and at the same time the electron doping can be modified by a gate to fill the band from one insulating state to another. Here, this opens up exciting opportunities to explore strongly correlated phenomena in two-dimensional moiré superlattice heterostructures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Rigorous numerical study of strong microwave photon-magnon coupling in all-dielectric magnetic multilayers

We demonstrate theoretically a ∼350-fold local enhancement of the intensity of the in-plane microwave magnetic field in multilayered structures made from a magneto-insulating yttrium iron garnet (YIG) layer sandwiched between two non-magnetic layers with a high dielectric constant matching that of YIG. The enhancement is predicted for the excitation regime when the microwave magnetic field is induced inside the multilayer by the transducer of a stripline Broadband Ferromagnetic Resonance (BFMR) setup. By means of a rigorous numerical solution of the Landau-Lifshitz-Gilbert equation consistently with the Maxwell's equations, we investigate the magnetisation dynamics in the multilayer. We reveal a strong photon-magnon coupling, which manifests itself as anti-crossing of the ferromagnetic resonance magnon mode supported by the YIG layer and the electromagnetic resonance mode supported by the whole multilayered structure. The frequency of the magnon mode depends on the external static magnetic field, which in our case is applied tangentially to the multilayer in the direction perpendicular to the microwave magnetic field induced by the stripline of the BFMR setup. The frequency of the electromagnetic mode is independent of the static magnetic field. Consequently, the predicted photon-magnon coupling is sensitive to the applied magnetic field and thus can be used in magnetically tuneable metamaterials based on simultaneously negative permittivity and permeability achievable thanks to the YIG layer. We also suggest that the predicted photon-magnon coupling may find applications in microwave quantum information systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Physical properties of Ce3-xTe4 below room temperature

The physical properties of polycrystalline Ce{sub 3-x}Te{sub 4} were investigated by measurements of the thermoelectric properties, Hall coefficient, heat capacity, and magnetization. The fully filled, metallic x = 0 compound displays a soft ferromagnetic transition near 4 K, and analysis of the corresponding heat capacity anomaly suggests a doublet ground state for Ce{sup 3+}. The transition is suppressed to below 2 K in the insulating x = 0.33 composition, revealing that magnetic order in Ce{sub 3-x}Te{sub 4} is driven by a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction. The thermoelectric properties trend with composition as expected from simple electron counting, and the transport properties in Ce{sub 3}Te{sub 4} are observed to be similar to those in La{sub 3}Te{sub 4}. Trends in the low-temperature thermal conductivity data reveal that the phonons are efficiently scattered by electrons, while all compositions examined have a lattice thermal conductivity near 1.2 W/m K{sup -1} at 200 K.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The effect of magnetic and non-magnetic ion damage on the surface state in SmB 6

SmB 6 is a Kondo insulator with a band structure that is topologically distinct from the vacuum. We theoretically predict this in order to produce metallic topological surface states that are robust to perturbations that do not break time reversal symmetry, such as non-magnetic defects. But, the surface state may be destroyed by an impurity with a sufficiently large magnetic moment. In order to test this prediction we show measurements of the resistance of the surface state of single crystals of SmB 6 with varying levels of damage induced by magnetic and non-magnetic ion irradiation. Finally, we find that at a sufficiently high concentration of damage the surface state reconstructs below an amorphous damaged layer, whether the damage was caused by a magnetic or non-magnetic ion.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Quantum and classical contributions to linear magnetoresistance in topological insulator thin films

Three dimensional topological insulators possess backscattering immune relativistic Dirac fermions on their surface due to nontrivial topology of the bulk band structure. Both metallic and bulk insulating topological insulators exhibit weak-antilocalization in the low magnetic field and linear like magnetoresistance in higher fields. We explore the linear magnetoresistance in bulk insulating topological insulator Bi{sub 2-x}Sb{sub x}Te{sub 3-y}Se{sub y} thin films grown by pulsed laser deposition technique. Thin films of Bi{sub 2-x}Sb{sub x}Te{sub 3-y}Se{sub y} were found to be insulating in nature, which conclusively establishes the origin of linear magnetoresistance from surface Dirac states. The films were thoroughly characterized for their crystallinity and composition and then subjected to transport measurements. We present a careful analysis taking into considerations all the existing models of linear magnetoresistance. We comprehend that the competition between classical and quantum contributions to magnetoresistance results in linear magnetoresistance in high fields. We observe that the cross-over field decreases with increasing temperature and the physical argument for this behavior is explained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetic field driven emergent phenomena: Insights from magneto-optics and nanoscopy

This review explores magnetic field-driven emergent phenomena across various material systems, emphasizing the pivotal roles of magneto-optical and nanoscopy techniques. We examine fundamental aspects of Landau electrodynamics in both 2D and 3D systems, including quantum Hall and topological magnetoelectric effects in graphene and topological insulators. Particularly attention is given to magnetic excitations and magnetopolaritons, such as surface magnon polaritons, magnetoplasmons, and magnetoexcitons in novel quantum materials, including quantum magnets and hybrid heterostructures. Advanced imaging techniques, such as scattering-type scanning near-field optical microscopy (SNOM) and microwave impedance microscopy, are showcased for their capability to resolve these phenomena with microscopic and nanoscopic resolution. These insights are complemented by discussions of advanced experimental approaches, including cryogenic environments, ultrafast pump-probe techniques, and the integration of magnetic fields into near-field optical methodologies. We further investigate the potential of these imaging techniques for unraveling complex magnetic orders, quantum phases, and correlated electronic behaviors. Finally, we offer perspectives on future research directions and highlight emerging opportunities in the evolving field of optical magneto-nanoscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Reentrant Insulating State in Ultrathin Manganite Films

The transport and magnetic properties of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} thin-films grown by pulsed laser deposition on (LaAlO{sub 3}){sub 0.3}(SrAl{sub 0.5}Ta{sub 0.5}O{sub 3}){sub 0.7} single crystal substrates have been investigated. A systematic series with various thicknesses of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} was used to establish a phase diagram - which showed a clear difference compared to films grown on SrTiO{sub 3} substrates, highlighting the importance of film thickness and substrate strain. At 8 unit cells, the boundary between the metallic and insulating ground states, a second abrupt metal-insulator transition was observed at low temperatures, which could be tuned with by magnetic field, and is interpreted as a signature of electronic phase separation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Kondo screening and magnetism at interfaces

The nature of magnetic order and transport properties near surfaces is a topic of great current interest. Here we model metal-insulator interfaces with a multilayer system governed by a tight-binding Hamiltonian in which the interaction is nonzero on one set of adjacent planes and zero on another. As the interface hybridization is tuned, magnetic and metallic properties undergo an evolution that reflects the competition between antiferromagnetism and (Kondo) singlet formation in a scenario similar to that occurring in heavy-fermion materials. For a few-layer system at intermediate hybridization, a Kondo insulating phase results, where magnetic order and conductivity are suppressed in all layers. As more insulating layers are added, magnetic order is restored in all correlated layers except that at the interface. Residual signs of Kondo physics are however evident in the bulk as a substantial reduction of the order parameter in the 2 to 3 layers immediately adjacent to the interfacial one. In conclusion, we find no signature of long-range magnetic order in the metallic layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

151 Eu Mössbauer study of magnetic ordering in flux-grown ferromagnetic and antiferromagnetic forms of EuCd 2 As 2

EuCd 2 As 2 is a remarkably complex magnetic semimetal that may behave as a topological insulator or host two pairs of Weyl points, depending on the growth conditions and the final magnetic state. Both antiferromagnetic (AFM) and ferromagnetic (FM) forms have been grown, and we show here, using 151 Eu Mössbauer spectroscopy, that the differences between the AFM and FM forms extend well beyond their ground state magnetic structures. Whereas the AFM form undergoes a conventional AFM → paramagnetic transition on warming, the FM form passes through a complex incommensurate modulated state before becoming paramagnetic.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Magnetoresistance anomaly during the electrical triggering of a metal-insulator transition

Phase separation naturally occurs in a variety of magnetic materials and it often has a major impact on both electric and magnetotransport properties. In resistive switching systems, phase separation can be created on demand by inducing local switching, which provides an opportunity to tune the electronic and magnetic state of the device by applying voltage. Here we explore the magnetotransport properties in the ferromagnetic oxide La 0.7 ⁢Sr 0.3 ⁢MnO 3 (LSMO) during the electrical triggering of an intrinsic metal-insulator transition (MIT), which produces volatile resistive switching. This switching occurs in a characteristic spatial pattern, i.e., the formation of a high-resistance barrier perpendicular to the current flow, enabling an electrically actuated ferromagnetic-paramagnetic-ferromagnetic phase separation. At the threshold voltage of the MIT triggering, both anisotropic and colossal magnetoresistances exhibit anomalies including a large increase in magnitude and a sign flip. Computational analysis revealed that these anomalies originate from the coupling between the switching-induced phase separation state and the intrinsic magnetoresistance of LSMO. Furthermore, this work demonstrates that driving the MIT material into an out-of-equilibrium resistive switching state provides the means for electrical control of the magnetotransport phenomena.

Complex oxides↗
DOE OSTI2014

Different response of transport and magnetic properties of BaIrO 3 to chemical and physical pressure

A combination of x-ray absorption, x-ray-diffraction, and transport measurements at high pressure is used to investigate the interplay between the electronic properties of Ir 5d states and lattice degrees of freedom in the weakly ferromagnetic insulator BaIrO3. Although the Ir 5d local magnetic moment is highly stable against lattice compression, remaining nearly unperturbed to at least 30 GPa, the weak ferromagnetism (net ordered moment) is quickly quenched by 4.5 GPa (3% volume reduction). Under chemical pressure, where Sr is substituted for the larger Ba in BaIrO3, the local magnetic moment on Ir remains stable, but the weak ferromagnetism is quenched after only 1.7% volume reduction. The magnetic ordering temperature Tm is also more strongly suppressed by chemical pressure compared to physical pressure. In addition, under ~23-at.% Sr doping, BaIrO3 undergoes a transition to a paramagnetic metallic state. Resistivity measurements indicate that BaIrO3 remains an electrical insulator to at least 9 GPa, a much higher pressure than required to quench the weak ferromagnetism (~4.5 GPa). Such a disparate response of transport and magnetic properties to chemical and physical pressure is likely rooted in the different compression rates of the (a,c) lattice parameters with Sr doping and applied pressure and the effect of related lattice distortions on electronic bandwidth and exchange interactions in this strongly spin-orbit-coupled system.

Laguna-Marco, M. A.↗
DOE OSTI2015

Crystallographic and magnetic properties of the spinel-type ferrites Zn{sub x}Co{sub 1-x}Fe{sub 2}O{sub 4} (0.0 ≤ x ≤ 0.75)

Ultrahigh frequencies (UHF) have applications in signal and power electronics to minimize product sizes, increase production quantity and lower manufacturing cost. In the UHF range of 300 MHz to 3 GHz, ferrimagnetic iron oxides (ferrites) are especially useful because they combine the properties of a magnetic material with that of an electrical insulator. Ferrites have much higher electrical resistivity than metallic ferromagnetic materials, resulting in minimization of the eddy current losses, and total penetration of the electromagnetic (EM) field. Hence ferrites are frequently applied as circuit elements, magnetic storage media like read/write heads, phase shifters and Faraday rotators. The electromagnetic properties of ferrites are affected by operating conditions such as field strength, temperature and frequency. The spinel system Zn{sub x}Co{sub 1-x}Fe{sub 2}O{sub 4} (x=0.0, 0.25, 0.50 and 0.75) has been prepared by the standard solid state sintering method. X-ray and neutron powder diffraction measurements were performed at room temperature. Neutron diffraction data analysis confirms the cubic symmetry corresponding to the space group Fd3m. The distribution of three cations Zn{sup 2+}, Co{sup 2+} and Fe{sup 3+} over the spinel lattice and other crystallographic parameters like lattice constant, oxygen position parameter, overall temperature factor and occupancies of different ions in different lattice sites for the samples have been determined from the analysis of neutron diffraction data. The lattice constant increases with increasing Zn content in the system. The magnetic structure was found to be ferrimagnetic for the samples with x≤0.50. Magnetization measurements show that with the increase of Zn content in the system the value of saturation magnetization first increases and then decreases. The variation of the magnetic moment with Zn substitution has been discussed in terms of the distribution of magnetic and non-magnetic ions over the A and B sub-lattices and their exchange coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Local optical control of ferromagnetism and chemical potential in a topological insulator

Many proposed experiments involving topological insulators (TIs) require spatial control over time-reversal symmetry and chemical potential. We demonstrate reconfigurable micron-scale optical control of both magnetization (which breaks time-reversal symmetry) and chemical potential in ferromagnetic thin films of Cr-(Bi,Sb) 2 Te 3 grown on SrTiO 3 . By optically modulating the coercivity of the films, we write and erase arbitrary patterns in their remanent magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO 3 substrates, we control the local chemical potential of the films. This optical gating effect allows us to write and erase p-n junctions in the films, which we study with photocurrent microscopy. Both effects are persistent and may be patterned and imaged independently on a few-micron scale. As a result, dynamic optical control over both magnetization and chemical potential of a TI may be useful in efforts to understand and control the edge states predicted at magnetic domain walls in quantum anomalous Hall insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Magnetic Field-Stabilized Wigner Crystal States in a Graphene Moiré Superlattice

ABC-stacked trilayer graphene on boron nitride (ABC-TLG/hBN) moiré superlattices provides a tunable platform for exploring Wigner crystal states in which the electron correlation can be controlled by electric and magnetic fields. Here, in this work, we report the observation of magnetic field-stabilized Wigner crystal states in a ABC-TLG/hBN. We show that correlated insulating states emerge at multiple fractional and integer fillings corresponding to ν = 1/3, 2/3, 1, 4/3, 5/3, and 2 electrons per moiré lattice site under a magnetic field. These correlated insulating states can be attributed to generalized Mott states for the integer fillings and generalized Wigner crystal states for the fractional fillings. The generalized Wigner crystal states are stabilized by a vertical magnetic field and are strongest at one magnetic flux quantum per three moiré superlattices. The ν = 2 insulating state persists up to 30 T, which can be described by a Mott–Hofstadter transition at a high magnetic field.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Spin Crossover and Magnetic Properties of Ba-Substituted Cobaltites

The dependence of the properties of the La{sub 0.45}Ba{sub 0.55}CoO{sub 3 – δ} and La{sub 0.4}Ba{sub 0.6}CoO{sub 3 – δ} cobaltites on the oxygen content has been studied. A decrease in the oxygen content leads to the stabilization of cobalt ions in the 3+ oxidation state and to a transition from a conducting state with positive exchange interaction to an antiferromagnetic insulating state with a long range magnetic order with decreasing temperature. Application of a magnetic field gradually transforms the antiferromagnetic state to the ferromagnetic one. The cubic lattice parameter is independent of temperature and an anomalous dependence of the magnetization, electrical conductivity, and magnetoresistance is treated within a spin crossover scenario. It is assumed that the high-temperature conducting phase and ferromagnetic clusters correspond to a collective intermediate spin state of cobalt ions, whereas the low-temperature antiferromagnetic phase corresponds to a mixed high-spin/low-spin state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Topological descendants of a multicritical Dirac semimetal with magnetism and strain

The past decades have witnessed a transformation in characterizing condensed matter systems with topology. Aided by a refined understanding of topological band structures with crystalline symmetries that has emerged recently, many electronic phases have been identified, and a plethora of materials have been predicted to host properties and functionalities. A key underlying question, also with respect to future application, is to what extent the related physical features can be manipulated, especially in the context of magnetic order. Here, we describe a paradigmatic semimetal that simultaneously incorporates multiple and sometimes conflicting topology which guarantees gaplessness and leads to an exceptionally rich family of descendent phases on lowering symmetry. We predict that this multicritical phase is realized in EuTl 2 . Starting from the parent semimetallic state, which already separates two topological insulating regimes, the interplay of inherent magnetism and strain allows for an exceptionally rich phase diagram of topological descendant states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗