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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Single crystal growth, chemical defects, magnetic and transport properties of antiferromagnetic topological insulators ( Ge 1 – δ – x Mn x ) 2 Bi 2 Te 5 ( x ≤ 0.47 , 0.11 ≤ δ ≤ 0.20 )

Magnetic topological insulators provide a platform for emergent phenomena arising from the interplay between magnetism and band topology. Here we report the single crystal growth, crystal structure, magnetic and transport properties, as well as the neutron scattering studies of topological insulator series (Ge 1–δ–x Mn x ) 2 ⁢Bi 2 ⁢Te 5 (x ≤ 0.47, 0.11 ≤ δ ≤ 0.20). Upon doping up to x = 0.47, the lattice parameter c decreases by 0.8%, while the lattice parameter a remains nearly unchanged. Significant Ge vacancies and Ge/Bi site mixing are revealed via elemental analysis as well as refinements of the neutron and x-ray diffraction data, resulting in holes dominating the charge transport. At x = 0.47, below 10.8 K, a bilayer A-type antiferromagnetic ordered state emerges, featuring an ordered moment of 3.0(3) μ B /Mn at 5 K, with the c axis as the easy axis. Magnetization data unveils a much stronger effective interlayer antiferromagnetic exchange interaction and a much smaller uniaxial anisotropy compared to MnBi 2 ⁢Te 4 . We attribute the former to the shorter nearest-neighbor Mn-Mn interlayer superexchange path and the latter to the smaller ligand-field splitting in (Ge 1–δ–x ⁢Mn x ) 2 ⁢Bi 2 ⁢Te 5 . Finally, our study demonstrates that this series of materials holds promise for the investigation of the layer Hall effect and quantum metric nonlinear Hall effect.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Multi-mode THz quantum-cascade VECSELs based on disordered metasurfaces

Quantum-cascade vertical-external-cavity surface-emitting-lasers (VECSELs) based on disordered amplifying metasurfaces are demonstrated and explored as potential broadband, multi-mode THz sources. The disorder is introduced along one spatial axis of the metasurface by pseudo-randomly varying the width of its resonant ridge antennas. Compared to a quantum-cascade (QC) VECSEL based on a uniform metasurface, the disordered structure supports much more localized transverse modes with reduced spatial overlap within the QC gain material. This localization is hypothesized to facilitate the spatial hole burning of the gain material and, therefore, enable multi-mode lasing, particularly for short cavities on the order of a few wavelengths. Several devices have been fabricated and shown to differ from uniform QC-VECSELs in a few key ways, possessing highly nonlinear light-current characteristics, angle-dependent emission spectra and broadband multi-mode lasing. At most, 17 modes are simultaneously observed, spanning 680 GHz. The number of VECSEL modes is shown to have an inverse relationship with the cavity length, which is attributed to increased diffractive losses in the open plano-plano cavity. As the cavity length is tuned, the device emits over a quasi-continuous band from 3.15 to 3.97 THz or up to 4.24 THz with a longitudinal mode hop.

Morag, Eilam [Univ. of California, Los Angeles, CA↗
DOE OSTI2017

Pink-beam serial crystallography

Serial X-ray crystallography allows macromolecular structure determination at both X-ray free electron lasers (XFELs) and, more recently, synchrotron sources. The time resolution for serial synchrotron crystallography experiments has been limited to millisecond timescales with monochromatic beams. The polychromatic, “pink”, beam provides a more than two orders of magnitude increased photon flux and hence allows accessing much shorter timescales in diffraction experiments at synchrotron sources. Here we report the structure determination of two different protein samples by merging pink-beam diffraction patterns from many crystals, each collected with a single 100 ps X-ray pulse exposure per crystal using a setup optimized for very low scattering background. In contrast to experiments with monochromatic radiation, data from only 50 crystals were required to obtain complete datasets. The high quality of the diffraction data highlights the potential of this method for studying irreversible reactions at sub-microsecond timescales using high-brightness X-ray facilities.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

B a 2 NiOs O 6 : A Dirac-Mott insulator with ferromagnetism near 100 K

In this study, the ferromagnetic semiconductor Ba 2 NiOsO 6 ( T mag ~ 100 K ) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [Fm - 3m ; a = 8.0428 ( 1 ) Å], where the Ni 2+ and Os 6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os 6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te ( T mag < 180 K ), the spin-gapless semiconductor Mn 2 CoAl ( T mag ~ 720 K ), and the ferromagnetic insulators EuO ( T mag ~ 70 K ) and Bi 3 Cr 3 O 11 ( T mag ~ 220 K ). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba 2 NiOsO 6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Characterizing extreme laser intensities by ponderomotive acceleration of protons from rarified gas

Abstract A new method to diagnose extreme laser intensities through measurement of angular and spectral distributions of protons directly accelerated by the laser focused into a rarefied gas is proposed. We simulated a laser pulse focused by an off-axis parabolic mirror by Stratton–Chu integrals, that enables description of laser pulse with different spatial-temporal profiles focusing in a focal spot down to the diffraction limit, that makes our theoretical predictions be a basis for experimental realization. The relationship between characteristics of the proton distributions and parameters of the laser pulse have been analyzed. The analytical and numerical results obtained justify the new method of laser diagnostics. The proposed scheme should be valuable for the commissioning of new extreme intensity laser facilities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Revisiting spin ice physics in the ferromagnetic Ising pyrochlore Pr 2 Sn 2 O 7

Pyrochlore materials are characterized by their hallmark network of corner-sharing rare-earth tetrahedra, which can produce a wide array of complex magnetic ground states. Ferromagnetic Ising pyrochlores often obey the “two-in-two-out” spin ice rules, which can lead to a highly degenerate spin structure. Large moment systems, such as Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , tend to host a classical spin ice state with low-temperature spin freezing and emergent magnetic monopoles. Systems with smaller effective moments, such as Pr 3+ -based pyrochlores, have been proposed as excellent candidates for hosting a “quantum spin ice” characterized by entanglement and a slew of exotic quasiparticle excitations. However, experimental evidence for a quantum spin ice state has remained elusive. Here, in this work, we show that the low-temperature magnetic properties of Pr 2 Sn 2 O 7 satisfy several important criteria for continued consideration as a quantum spin ice. We find that Pr 2 Sn 2 O 7 exhibits two distinct spin-correlation time scales of τ ≥ 10 –4 and ~10 –10 s in the spin ice regime. Our comprehensive bulk characterization and neutron scattering measurements enable us to map out the magnetic field-temperature phase diagram, producing results consistent with expectations for a ferromagnetic Ising pyrochlore. We identify key hallmarks of spin ice physics and show that the application of small magnetic fields (μ 0 H c ~0.5 T) suppresses the spin ice state and induces a field-polarized, ordered spin-ice phase. Together, our work clarifies the current state of Pr 2 Sn 2 O 7 and encourages future studies aimed at exploring the potential for a quantum spin ice ground state in this system.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ternary molecular switching in a single-crystal optical actuator with correlated crystal strain

A growing portfolio of single-crystal optical actuators is forging a new class of photonic materials that hold prospects for quantum technologies. Ruthenium-based complexes that exhibit this phenomenon via SO 2 -linkage photoisomerisation are of particular interest since they display multiple metastable states, once induced by green light; yet, complete photoconversion into each SO 2 -isomeric state is rarely achieved. We discover a new complex, trans-[Ru(SO 2 )(NH 3 ) 4 (4-bromopyridine)]tosylate 2 , that produces 100% photoconverted η 1 -OSO isomeric crystal structures at 90 K, which fully transition into η 2 -(OS)O photoisomers upon warming to 100 K, while the dark-state η 1 -SO 2 structure is wholly recovered by heating the crystal to room temperature. Crystal structures and optical-absorption profiles of each state are captured via in-situ light-induced single-crystal X-ray diffraction and optical-absorption spectroscopy. Results show that both photoisomeric species behave as optical switches, but with distinct optical properties. The photoisomerisation process causes thermally-reversible micro- and nanoscopic crystal strain, as characterised by optical microscopy and in-situ light-induced atomic-force microscopy.

actuators↗
DOE OSTI2024

Improving equations of state calibrations in the toroidal DAC—The case study of molybdenum

We report an updated isothermal equation of state (EoS) of molybdenum (Mo) obtained by compression in beveled and toroidal diamond-anvil cells (DACs). For an improved compression environment, we developed a copper (Cu) pressure-transmitting medium (PTM) for the toroidal diamond-anvil cell samples, as it is a soft metal compared to Mo with a well calibrated EoS. A Ne PTM was used for the conventional beveled DAC samples. The unit-cell volumes of Mo were measured to 336(1) GPa in the Cu PTM and 231.2(6) GPa in the Ne PTM at room temperature. We additionally calculated elastic stiffness and compliance constants and evaluated the uniaxial stress of Mo and Cu with pressure. A new EoS for Mo is presented from data collected in all sample environments and compared to our theoretical predictions as well as previous compression studies of Mo. The (200) lattice plane of Mo produced the lowest volumes across the pressure range of this study for all compression environments, suggesting that it is less affected by nonhydrostatic stresses in the DAC compared to the other observed diffraction planes. The presented Mo EoS is compatible with extrapolations of EoS fits of Mo in helium (He) within ~1% at 330 GPa. Results from this work demonstrate that compressing a sample in a softer metal in the toroidal DAC can improve the compression environment and result in measured sample volumes comparable to those collected in noble-gas media at multi-megabar conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Stochastic frequency fluctuation super-resolution imaging

The inherent non-linearity of intensity correlation functions can be used to spatially distinguish identical emitters beyond the diffraction limit, as achieved, for example, in super-resolution optical fluctuation imaging (SOFI). Here, we propose a complementary concept based on spectral correlation functions, termed spectral fluctuation super-resolution (SFSR) imaging. Through theoretical and computational analysis, we show that spatially resolving time-frequency correlation functions in the image plane can improve the imaging resolution by a factor of $\sqrt2$ in most cases and up to twofold for strictly two emitters. This improvement is achieved by quantifying the degree of correlation in spectral fluctuations across the spatial domain. Experimentally, SFSR can be implemented using a combination of interferometry and photon-correlation measurements. The method works for non-blinking emitters and stochastic spectral fluctuations with arbitrary temporal statistics. This suggests its utility in super-resolution microscopy of quantum emitters at low temperatures, where spectral diffusion is often more pronounced than emitter blinking.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

USc 2 C 2 and USc 2 NC Clusters with U–C Triple Bond Character Stabilized Inside Fullerene Cages

The chemistry of f-block metal–carbon multiple bonds is underdeveloped compared to well-established carbene complexes of the d-block transition metals. Herein, we report two new actinide-rare earth mixed metal carbides and nitrogen carbide cluster fullerenes, USc 2 C 2 @D 5h (6)-C 80 and USc 2 NC@D 5h (6)-C 80 , which contain U–C bonds with triple bond character and were successfully synthesized and characterized by mass spectrometry, UV–vis–NIR spectroscopy, Fourier transform infrared spectroscopy, single crystal X-ray diffraction, and DFT calculations. Crystallographic studies show that the two previously unreported clusters, USc 2 C 2 and USc 2 NC, are stabilized in the D 5h (6)-C 80 carbon cage and adopt unique trifoliate configurations, in which C 2 /NC units are almost vertically inserted into the plane defined by the U and two Sc atoms. Combined experimental and theoretical studies further reveal the bonding structure of USc 2 C 2 and USc 2 NC, which contain C=U(VI)=C and C=U(V)=N bonding motifs. The electronic structures of the two compounds are determined as U 6+ (Sc 2 ) 6+ (C 4– ) 2 @D 5h (6)-C 80 4– and U 5+ (Sc 2 ) 6+ (N) 3– (C) 4– @D 5h (6)-C 80 4– , respectively. Quantum-chemical studies confirm that the U–C bonds in both molecules show unprecedented multicenter triple-bond character. Furthermore, the discovery of this unique U–C multiple bond offers a deeper understanding of the fundamentals of uranium chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Scalable ultrafast epitaxy of large-grain and single-crystal II-VI semiconductors

A general problem for semiconductor applications is that very slow deposition on expensive single-crystal substrates yields high crystalline quality with excellent electro-optical properties, but at prohibitive costs and throughput for many applications. In contrast, rapid deposition on inexpensive substrates or nanocrystalline films yields low costs, but comparatively inferior crystallinity, carrier transport, and recombination. Here, we present methods to deposit single-crystal material at rates 2-3 orders of magnitude faster than state-of-the-art epitaxy with low-cost methods without compromising crystalline or electro-optical quality. For example, single-crystal CdTe and CdZnTe films that would take several days to grow by molecular-beam epitaxy are deposited in 8 minutes by close-spaced sublimation, yet retain the same crystalline quality measured by X-ray diffraction rocking curves. The fast deposition is coupled with effective n- and p-type in-situ doping by In, P, and As. The epitaxy can be extended to nanocrystalline substrates. For example, we recrystallize thin CdTe films on glass to deposit large grains with low defect density. The results provide new research paths for photovoltaics, detectors, infrared imaging, flexible electronics, and other applications.

14 SOLAR ENERGY↗
DOE OSTI2022

Atomic structure of an FeCrMoCBY metallic glass revealed by high energy x-ray diffraction

Amorphous bulk metallic glasses with the composition Fe 48 Cr 15 Mo 14 C 15 B 6 Y 2 have been of interest due to their special mechanical and electronic properties, including corrosion resistance, high yield-strength, large elasticity, catalytic performance, and soft ferromagnetism. Here, we apply a reverse Monte Carlo technique to unravel the atomic structure of these glasses. The pair-distribution functions for various atomic pairs are computed based on the high-energy x-ray diffraction data we have taken from an amorphous sample. Monte Carlo cycles are used to move the atomic positions until the model reproduces the experimental pair-distribution function. The resulting fitted model is consistent with our ab initio simulations of the metallic glass. Our study contributes to the understanding of functional properties of Fe-based bulk metallic glasses driven by disorder effects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Magnetism in EuAlSi and the Eu 1−𝑥 ⁢Sr 𝑥 ⁢ AlSi solid solution solid solution

The magnetic properties of EuAlSi, a compound comprising a honeycomb lattice of Al/Si atoms and a triangular lattice of Eu atoms, are presented. By means of single-crystal x-ray diffraction, we find that EuAlSi crystallizes in an AlB 2 -type structure with space group 𝑃⁢6/mmm and unit cell parameters 𝑎 = 4.2229⁢ (10) ⁢Å and 𝑐 = 4.5268 ⁢(12)⁢ Å. Our magnetic measurements indicate that EuAlSi is a soft ferromagnetic material with 𝑇 Curie = 25.8 K. The susceptibility follows the Curie-Weiss law at high temperatures, which allowed us to determine the paramagnetic Curie temperature 𝜃 𝑃 = 36.2 ⁢(1) ⁢K and an effective magnetic moment 𝜇 eff = 8.07 ⁢(1)⁢ µ 𝐵 /Eu. This value is in agreement with the theoretical value of 7.9 µ 𝐵 for Eu 2+ free ion. Moreover, we have prepared the Eu 1−𝑥 ⁢Sr 𝑥 ⁢ AlSi solid solution, where the atoms in the triangular lattice were systematically exchanged, in order to study the evolution of the collective quantum properties from the ferromagnetic EuAlSi toward the superconducting SrAlSi. Across the Eu 1−𝑥 ⁢Sr 𝑥 AlSi solid solution, the unit cell parameters change linearly, following Vegard’s law, and making the system reliable for studying composition dependence of the interplay between the crystal structure and physical properties. As the Sr content increases, i.e., 𝑥 increases, we note a consistent reduction of 𝜇 eff and 𝑇 Curie . Long-range magnetic order in Eu 1−𝑥 ⁢Sr 𝑥 ⁢ AlSi persists up to 𝑥 = 0.95, whereas superconductivity is only observed for samples with 𝑥 > 0.97.

Walicka, Dorota I. [University of Geneva (Switzerl↗
DOE OSTI2026

Temperature-dependent dielectric function of tantalum nitride formed by atomic layer deposition for tunnel barriers in Josephson junctions

Here, we report the dielectric functions of insulating tantalum nitride (TaN) films deposited by atomic layer deposition (ALD) on 300 mm Si/SiO 2 substrates to evaluate their suitability as tunnel barriers in tantalum-based Josephson junctions (JJs) for superconducting quantum circuits. Temperature-dependent ellipsometric angles were measured for ALD TaN films with nominal thicknesses of 13 and 25 nm at an incidence angle of 70°, over photon energy ranges of 0.03–0.7 eV (80–300 K) and 0.5–6.5 eV (80–600 K). These data were used to develop a dispersion model for insulating ALD TaN that incorporates a Tauc–Lorentz oscillator with an optical bandgap of 1.5–1.8 eV to describe interband transitions. The extracted dielectric function shows insulating behavior (mid-infrared transparency) at all temperatures and for both film thicknesses tested. In particular, ALD TaN does not exhibit infrared absorption due to free carriers, even at elevated temperatures, consistent with the insulating behavior required for a JJ tunnel barrier. Results from transmission electron microscopy (including selected area electron diffraction) and x-ray diffraction are also discussed. Sputter depth-profile x-ray photoelectron spectroscopy shows an N/Ta ratio of ∼1.2 throughout the film. The lower bandgap, low roughness, and thermal stability of ALD TaN compared to AlO x suggest the possibility of fabricating JJs with thicker barriers while achieving critical current densities required for qubits, enabling improved thickness and composition control, reduced topography, and resistance to aging.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Signature of magnetoelectric coupling driven finite momentum pairing in 3D ising superconductor

The finite momentum superconducting paring states (FMPs) represent a forefront of condensed matter physics. Here we report experimental evidence of FMP in a locally noncentrosymmetric bulk superconductor 4H b -TaS 2 . Using hard X-ray diffraction and angle-resolved photoemission spectroscopy, we reveal unusual 2D ferro-rotational charge density wave (CDW) and weak interlayer hopping in 4H b -TaS 2 . The superconducting upper critical field, H c2 , linearly increases via decreasing temperature, and well exceeds the Pauli limit, suggesting the dominant orbital pair-breaking mechanism. Remarkably, we observed evidence of field-induced superconductivity-to-superconductivity transition that breaks continuous rotational symmetry of the s-wave uniform pairing in the Bardeen-Cooper-Schrieffer theory down to the six-fold rotation symmetry. Ginzburg-Landau free energy analysis shows that magnetoelectric coupling, induced by 2D ferro-rotational CDW, stabilizes FMP that provides an explanation of the lowering rotation symmetry. Our results provide a new understanding of unconventional superconducting behaviors of the bulk quantum heterostructure 4H b -TaS 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Coherent addition of high power broad-area laser diodes with a compact VBG V-shaped external Talbot cavity

In this paper, we introduced a compact V-shaped external Talbot cavity for phase locking of high power broad-area laser diodes. The length of compact cavity is ~25 mm. Near diffraction-limit coherent addition of 10 broad-area laser diodes indicated that high quality phase locking was achieved. We measured the near-field emission mode of each individual broad-area laser diode with different feedback, such as a volume Bragg grating and a high reflection mirror. Finally, we found out that the best result of phase locking broad-area laser diodes was achieved by the compact V-shaped external Talbot cavity with volume Bragg grating feedback.

42 ENGINEERING↗
DOE OSTI2021

Potential Skyrmion Host Fe(IO3)3: Connecting Stereoactive Lone-Pair Electron Effects to the Dzyaloshinskii-Moriya Interaction

Magnetic skyrmions, which are topologically distinct magnetic spin textures, are gaining increased attention for their unique physical properties and potential applications in spintronic devices. Here we present a design strategy for skyrmion host candidates based on combinations of magnetic spin, asymmetric building units having stereoactive lone-pair electrons, and polar lattice symmetry. To demonstrate the viability of the proposed rational design principles, we successfully synthesized a Fe(IO 3 ) 3 polycrystalline sample and single crystals by using a new simplified low-temperature pathway, which is experimentally feasible for extending materials growth of transition metal iodates. Single crystal X-ray and powder synchrotron X-ray diffraction measurements demonstrated that Fe(IO 3 ) 3 crystallizes in the polar chiral hexagonal lattice with space group P63. The combined structural features of the macroscopic electric polarization along the c-axis stemming from the coalignment of the stereoactive lone-pairs of the IO 3 – trigonal pyramid and the magnetic Fe 3+ cation residing on the 3-fold rotation axis were selected to promote asymmetric exchange coupling. We find evidence of a predicted skyrmion phase at 14 K ≤ T ≤ 16 K and 2.5 T ≤ μ 0 H ≤ 3.2 T driven by a Dzyaloshinskii–Moriya (DM) interaction, a conclusion supported by the appreciable DM exchange and the zero-field spiral antiferromagnetic ground state of Fe(IO 3 ) 3 deduced from neutron diffraction experiments. The associated magnetic modulation wavelength of the putative skyrmions is expected to be short ~18 nm, comparable to the period of the DM-driven incommensurate order. This work links stereoactive lone-pair electron effects to enhanced DM interaction, demonstrating a new approach for chemical guidelines in the search for skyrmionic states of matter.

36 MATERIALS SCIENCE↗