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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Terahertz Landau level spectroscopy of Dirac fermions in millimeter-scale twisted bilayer graphene

Exotic electronic physics including correlated insulating states and fractional Chern insulators have been observed in twisted bilayer graphene in a magnetic field when the Fermi velocity vanishes; however, a question remains as to the stability of these states, which is controlled by the gap to the first excited state. Free-space terahertz magneto-optics can directly probe the gap to charge excitations, which bounds the stability of electronic states, but this measurement has thus far been inaccessible due to the micron size of twisted bilayer graphene samples, while the wavelength of terahertz light is up to 1 mm. Here we leverage advances in fabrication to create twisted bilayer graphene samples over 5×5 mm in size with a uniform twist angle and study the magnetic field dependence of the cyclotron resonance by a complex Faraday rotation experiment in 𝑝-doped large angle twisted bilayer graphene. These measurements directly probe charge excitations in inter-Landau level transitions and determine the Fermi velocity as a function of twist angle.

Mead, Benjamin [University of Pennsylvania]↗
DOE OSTI2021

Theory of ultrafast photoinduced low-to-high spin crossover in divalent iron systems

Here a theory is developed for the ultrafast low-to-high spin transition in divalent iron. Following the photoinduced metal-to-ligand charge transfer (MLCT), the 1 A 1 MLCT state decays on a sub-100-fs timescale into the 5 T 2 MLCT using a 3 T 1 MLCT state as intermediary. Damping of nuclear motion is crucial in obtaining a full singlet-to-quintet transition mediated by the spin-orbit interaction. Destructive interference suppresses transitions to metal-centered (MC) states. The relaxation to the MC 5 T 2 is a result of Coulomb scattering with the surroundings.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anisotropic antiferromagnetic order in the spin-orbit coupled trigonal-lattice Ca 2 Sr 2 IrO 6

Here, we used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca 2 Sr 2 IrO 6 . The crystal structure is determined to be R ¯3 with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below T N = 13.1 K. The magnetic wave vector is identified as (0,0.5,1) with ferromagnetic coupling along the a axis and antiferromagnetic correlation along the b axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34° toward the c axis. The ordered moment is 0.66(3) μ B /Ir, larger than other iridates where iridium ions form corner- or edge-sharing IrO 6 octahedral networks. The tilting angle is reduced to ≈19° when a magnetic field of 4.9 T is applied along the c axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap ~0.5 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Cluster spin glass correlations and dynamics in Zn 0.5⁢ Mn 0.5⁢ Te

Here, we present a combined magnetometry, muon spin-relaxation (𝜇⁢SR), and neutron-scattering study of the insulating spin glass Zn 0.5 ⁢Mn 0.5 ⁢Te, for which magnetic Mn 2+ and nonmagnetic Zn 2+ ions are randomly distributed on a face-centered cubic lattice. The magnetometry and 𝜇⁢SR results confirm a spin freezing transition around 𝑇 𝑓 ≈ 23 K, with the spin-fluctuation rate decreasing gradually and somewhat inhomogeneously through the sample volume as the temperature decreases toward 𝑇 𝑓 . Characteristic spin-correlation times well above 𝑇 𝑓 are on the order of 10 −10 s, much slower than typically observed in canonical spin glasses but in line with expectations for a cluster spin glass. Using magnetic pair distribution function (mPDF) analysis and reverse Monte Carlo (RMC) modeling of the magnetic diffuse neutron-scattering data, we show that the spin-glass ground state consists of clusters of spins exhibiting short-range-ordered type-III antiferromagnetic correlations with a locally ordered moment of 3.1⁢(1)⁢𝜇 B between nearest-neighbor spins. The type-III correlations decay exponentially as a function of spin separation distance with a correlation length of approximately 5 Å. The diffuse magnetic scattering and corresponding mPDF show no significant changes across 𝑇 𝑓 , indicating that the dynamically fluctuating short-range spin correlations in the paramagnetic state retain the same basic type-III configuration that characterizes the spin-glass state; the only change apparent from the neutron-scattering data is a gradual reduction of the correlation length and locally ordered moment with increasing temperature. Taken together, these results paint a unique and detailed picture of the local magnetic structure and dynamics in Zn 0.5 ⁢Mn 0.5⁢ Te and provide strong evidence that this material is best described as a cluster spin glass. In addition, this work showcases a statistical method for extracting diffuse scattering signals from neutron powder diffraction data, which we developed to facilitate the mPDF and RMC analysis of the neutron data. This method has the potential to be broadly useful for neutron powder diffraction experiments on a variety of materials with short-range atomic or magnetic order.

magnetism↗
DOE OSTI2017

Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure

Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Detection of impurity diamagnetic susceptibility and its behavior in n-Ge:As in the region of the insulator–metal phase transition

The method of superconducting quantum interference device (SQUID) magnetometry is used to measure and study low-temperature (T ≤ 100K) susceptibility in a series of samples of heavily doped Ge:As samples on the insulator side of the insulator–metal phase transition. Subtracting the known values of the magnetic susceptibility of the lattice from the measurement results, the values of the impurity magnetic susceptibility of the system are obtained. Using the method of electron spin resonance, the paramagnetic component of the impurity susceptibility is determined. Subtraction of the paramagnetic component from the total impurity susceptibility is used to obtain, for the first time, the values of the impurity diamagnetic susceptibility (∼5 × 10{sup –8} cm{sup 3}/g). The obtained result is consistent with estimates obtained for the localization radius of an electron at an As donor. Lowering the temperature to T ≤ 4 K leads to an increase in the diamagnetic susceptibility, which is consistent with the observed increase in the paramagnetic susceptibility. The observed effect is accounted for by the transition of impurity electrons from the singlet state to the triplet one.

36 MATERIALS SCIENCE↗
DOE OSTI2023

High pO2 Flux Growth and Characterization of NdNiO3 Crystals

Single crystals of the perovskite nickelate NdNiO3 with dimensions of up to 50 μm on edge have been successfully grown using the flux method at a temperature of 400 °C and oxygen pressure of 200 bar. The crystals were investigated by a combination of techniques, including high-resolution synchrotron X-ray single-crystal and powder diffraction and physical property measurements such as magnetic susceptibility and resistivity. Resistivity measurements revealed a metal-insulator transition (MIT) at TMIT~180 K with apparent thermal hysteresis; however, no superlattice peaks or peak splitting below TMIT, which corresponds to a structural transition from Pbnm to P21/n, was observed. The successful growth of NdNiO3 crystals at relatively low temperatures and oxygen pressure provides an alternative approach for preparing single crystals of interesting perovskites such as RNiO3 (R = Sm-Lu) and parent phases of superconducting square planar nickelates.

Wang, Xiaoli↗
DOE OSTI2025

Terahertz Landau level spectroscopy of Dirac fermions in millimeter-scale twisted bilayer graphene

Exotic electronic physics including correlated insulating states and fractional Chern insulators have been observed in twisted bilayer graphene in a magnetic field when the Fermi velocity vanishes, however a question remains as to the stability of these states which is controlled by the gap to the first excited state. Free-space terahertz magneto-optics can directly probe the gap to charge excitations which bounds the stability of electronic states, but this measurement has thus-far been inaccessible due to the micron size of twisted bilayer graphene samples, while the wavelength of terahertz light is up to a millimeter. Here we leverage advances in fabrication to create twisted bilayer graphene samples over 5 mm x 5 mm in size with a uniform twist angle and study the magnetic field dependence of the cyclotron resonance by a complex Faraday rotation experiment in p-doped large angle twisted bilayer graphene. These measurements directly probe charge excitations in inter-Landau level transitions and determine the Fermi velocity as a function of twist angle.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Theory of the spin-Seebeck effect at a topological-insulator/ferromagnetic-insulator interface

The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI surface states scattering off the nonequilibrium magnon population at the surface of the semi-infinite thermally driven FI. Our theory is readily generalized to spin-Seebeck voltages in any two-dimensional conductor that is exchange-coupled to the surface of a FI. Surface-state carrier-density-dependent signal strengths calculated using Bi 2 Te 3 and yttrium iron garnet material parameters are consistent with recent experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Magnetic, transport and magnetoresistance behavior of Ni doped La{sub 0.67}Sr{sub 0.33}Mn{sub 1−x}Ni{sub x}O{sub 3} (0.00≤x≤0.09) system

The magnetic, electrical and magnetoresistance behavior of Ni doped La{sub 2/3}Sr{sub 1/3}Mn{sub 1−x}Ni{sub x}O{sub 3} (0.00≤x≤0.09) samples have been studied systematically. Rietveld analysis of XRD shows the formation of pure crystalline phase with rhombohedral symmetry. It has been found that with increasing Ni concentration, magnetization, resistivity, magnetoresistance (MR), Curie temperature (T{sub c}) and metal–insulator transition temperature of the samples changes drastically. Doping of Ni at Mn site shows an inverse behavior of %MR at 300 K and 77 K. Further study revealed that all samples show a low field magnetoresistance at low temperature (77 K) which found to increase with the increase of Ni doping. Further, X-ray photoelectron spectrum (XPS) confirms the ionic state of Ni to be Ni{sup 2+}. In the light of this study, we can say that Ni doped La{sub 0.67}Sr{sub 0.33}MnO{sub 3} can be synthesized with fine tuning of the dopant ion to bring down T{sub c} further for its application in Magnetic Hyperthermia. - Graphical abstract: The magnetic, electrical and magnetoresistance behavior of Nickel doped La{sub 2/3}Sr{sub 1/3}Mn{sub 1−x}Ni{sub x}O{sub 3} (0.00≤x≤0.09) samples have been studied systematically. Highlights: • Ni doped La{sub 2/3}Sr{sub 1/3}Mn{sub 1−x}Ni{sub x}O{sub 3} samples synthesized by solid state reaction route. • Studied the magnetic, electrical and magnetoresistance behavior of the samples. • T{sub c} of the samples decreases whereas resistivity increases with Ni doping. • Samples show excellent properties for magnetic Hyperthermia application.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The first-order structural transition in NiO at high pressure

Insulator-metal transition in NiO and concomitant collapse of spin and magnetism in the material are important subjects in studies of strong electronic correlations and as such they generate multiple theoretical and experimental efforts aimed at understanding the complicated physics of strongly correlated electronic systems. We report here the iso-structural transition in NiO accompanying the previously observed by us transition to the metallic state. These findings resolve some experimental controversies regarding the transition. We also discuss the latest theoretical results in detail and show that our data confirm some recent theoretical models. Given the simplicity of the material and its fundamental importance to the understanding of strongly correlated electronic systems, our findings should stimulate further experimental and theoretical efforts to understand this prototype Mott insulator.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Spinons and damped phonons in the spin- 1 2 quantum liquid Ba 4 Ir 3 O 10 observed by Raman scattering

In spin-1/2 Mott insulators, nonmagnetic quantum liquid phases are often argued to arise when the system shows no magnetic ordering, but identifying positive signatures of these phases or related spinon quasiparticles can be elusive. Here, we use Raman scattering to provide three signatures for spinons in a possible spin-orbit quantum liquid material Ba 4 Ir 3 O 10 : (1) a broad hump, which we show can arise from Luttinger liquid spinons in Raman with parallel photon polarizations normal to one-dimensional chains; (2) strong phonon damping from phonon-spin coupling via the spin-orbit interaction; and (3) the absence of (1) and (2) in the magnetically ordered phase that is produced when 2% of Ba is substituted by Sr [(Ba 0.98 Sr 0.02 ) 4 Ir 3 O 10 ]. Furthermore, the phonon damping via itinerant spinons seen in this quantum liquid insulator suggests a mechanism for enhancing thermoelectricity in strongly correlated conductors, through a neutral quantum liquid that need not affect electronic transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

High pO(2) Floating Zone Crystal Growth of the Perovskite Nickelate PrNiO3

Single crystals of PrNiO3 were grown under an oxygen pressure of 295 bar using a unique high-pressure optical-image floating zone furnace. The crystals, with volume in excess of 1 mm(3), were characterized structurally using single crystal and powder X-ray di ff raction. Resistivity, specific heat, and magnetic susceptibility were measured, all of which evidenced an abrupt, first order metal-insulator transition (MIT) at similar to 130 K, in agreement with previous literature reports on polycrystalline specimens. Temperature-dependent single crystal di ff raction was performed to investigate changes through the MIT. Our study demonstrates the opportunity space for high fugacity, reactive environments for single crystal growth specifically of perovskite nickelates but more generally to correlated electron oxides.

floating zone method↗
DOE OSTI2020

Transition-metal phthalocyanine monolayers as new Chern insulators

To explore new materials for the realization of the quantum anomalous Hall effect (QAHE), we studied the electronic, magnetic and topological properties of transition-metal phthalocyanine (TMPc) monolayers in a square lattice. Many of them have large topologically nontrivial band gaps and integer Chern numbers. In particular, the Fermi level of MoPc lies right in the band gap (E g = 8.1 meV) and has large magnetic anisotropy energy (MAE = 2.0 meV per Mo atom) and a Curie temperature of 16 K, showing its usefulness for applications. The presence of the topologically protected edge state in a MoPc nanoribbon further confirms it as a new two-dimensional topological insulator. This much widens the search for QAHE materials for the design of quantum devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Ambipolar and non-ambipolar diffusion in an rf plasma source containing a magnetic filter

By placing a magnetic filter across a rectangular plasma source (closed at one end with a ceramic plate and an rf antenna, and terminated at the opposite end by a grounded grid), we experimentally investigate the effect of conducting and insulating source walls on the nature of the plasma diffusion phenomena. The use of a magnetic filter creates a unique plasma, characterized by a high upstream electron temperature (T{sub e{sub u}}∼5 eV) near the rf antenna and a low downstream electron temperature (T{sub e{sub d}}∼1 eV) near the grid, which more clearly demonstrates the role of the source wall materials. For conducting walls a net ion current to ground is measured on the grid, and the plasma potential is determined by a mean electron temperature within the source. For insulating walls the plasma potential is determined by the downstream electron temperature (i.e., V{sub p}∼5.2T{sub e{sub d}} in argon), and the net current to the grid is exactly zero. Furthermore, by inserting a small additional upstream conductor (that can be made floating or grounded through an external circuit switch), we demonstrate that the plasma potential can be controlled and set to a low (V{sub p}∼5.2T{sub e{sub d}}), or high (V{sub p}∼5.2T{sub e{sub u}}) value.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Crystals, magnetic and electronic properties of a new ThCr{sub 2}Si{sub 2}-type BaMn{sub 2}Bi{sub 2} and K-doped compositions

This is a report on the new 122 ternary transition-metal pnictide of BaMn{sub 2}Bi{sub 2}, which is crystallized from bismuth flux. BaMn{sub 2}Bi{sub 2} adopts ThCr{sub 2}Si{sub 2}-type structure (I4/mmm) with a=4.4902(3) Å and c=14.687(1) Å; it is antiferromagnetic with anisotropic magnetic susceptibility and semiconducting with a band gap of E{sub g}=6 meV. Heat capacity result confirms the insulating ground state in BaMn{sub 2}Bi{sub 2} with the electronic residual Sommerfeld coefficient of γ=0. The high temperature magnetization results show that magnetic ordering temperature is T{sub N} ∼400 K. Hole-doping in BaMn{sub 2}Bi{sub 2} via potassium in Ba{sub 1−x}K{sub x}Mn{sub 2}Bi{sub 2} results in metallic behavior for x=0.10(1), 0.32(1) and 0.36(1). With K-doping, more magnetically anisotropic behavior is observed. Although there is a downturn in electrical resistivity and low-field magnetization data below 4 K in>30%-doped crystals, there is no sign of zero resistance or diamagnetism. - Graphical abstract: Local moment antiferromagnet BaMn{sub 2}Bi{sub 2}, the first bismuthide with ThCr{sub 2}Si{sub 2} structure, turns metallic upon K-doping. Highlights: • BaMn{sub 2}Bi{sub 2} and Ba{sub 1−x}K{sub x}Mn{sub 2}Bi{sub 2} (0≤x<0.4) measuring up to 1 cm in length have been synthesized from Bi flux reactions. • BaMn{sub 2}Bi{sub 2} is the first bismuthide with ThCr{sub 2}Si{sub 2}-type structure. • BaMn{sub 2}Bi{sub 2} is a small gap semiconductor but, upon hole-doping, turns metallic with a downturn in resistivity below 4 K. • BaMn{sub 2}Bi{sub 2} is a local moment antiferromagnet with T{sub N} ∼400 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Thin films of topological Kondo insulator candidate SmB 6 : Strong spin-orbit torque without exclusive surface conduction

The advent of topological insulators (TIs), a novel class of materials that harbor a metallic spin-chiral surface state coexisting with band-insulating bulk, opens up new possibilities for spintronics. One promising route is current-induced switching of an adjacent magnetic layer via spin-orbit torque (SOT), arising from the large spin-orbit coupling intrinsically possessed by TIs. The Kondo insulator SmB 6 has been recently proposed to be a strongly correlated TI, supported by the observation of a metallic surface state in bulk SmB 6 , as evidenced by the thickness independence of the low-temperature resistance plateau. We report the synthesis of epitaxial (001) SmB 6 /Si thin films and a systematic thickness-dependent electrical transport study. Although the low-temperature resistance plateau is observed for all films from 50 to 500 nm in thickness, the resistance is distinctively thickness-dependent and does not support the notion of surface conduction and interior insulation. On the other hand, we demonstrate that SmB 6 can generate a large SOT to switch an adjacent ferromagnetic layer, even at room temperature. The effective SOT generated from SmB 6 is comparable to that from β-W, one of the strongest SOT materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetism in Boron Nitride Monolayer Induced by Cobalt or Nickel Doping

We have investigated the electronic structure and magnetic properties of cobalt (Co)- or nickel (Ni)-doped hexagonal boron nitride (h-BN) monolayer using density functional theory calculations. The h-BN monolayer without any doping is a nonmagnetic insulator. Our studies show that isolated Co or Ni atom can both induce the local magnetic state in h-BN monolayer. And the impurity energy levels will be formed in the band gap. Results of our first-principle calculations reveal that isolated Co atom can result in a magnetic moment of 3.57 μ{sub B}, while the Ni atom is 0.87 μ{sub B}. And the magnetic moments mainly come from d orbitals of the doped atoms. The studies of magnetic coupling show that the two Co atoms at different distances in h-BN monolayer do not always couple ferromagnetically. When the distances are 2.504 and 6.625 Å, the spins induced by the two Co atoms will form antiferromagnetic ground states. While the two Ni atoms at different distances in the monolayer always couple ferromagnetically. So our results imply that the Ni-doped h-BN is more suitable for spintronic material than Co doping. These results are useful for spintronic application.

74 ATOMIC AND MOLECULAR PHYSICS↗