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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Conditions of coherent-phonon excitation in SrMnSb2 films and crystals

Excitation of coherent phonons has the potential to dramatically alter the electronic structure of Dirac and Weyl semimetals, enabling sub-picosecond control of their optical and electronic properties. The Dirac semimetal SrMnSb2 is a candidate for such control, with a coherent-phonon mode that is predicted to close and reopen a gap at the Dirac node. Here, through a series of ultrafast pump-probe experiments, we establish suitable samples and conditions for driving the coherent phonon to high amplitude and attempting to observe the gap’s closure. Films of SrMnSb2 grown by molecular-beam epitaxy are shown to have phononic properties matching those of bulk crystals. We find that the phonon can be strongly excited by pump pulses with wavelength near 1500 nm, which will excite a 30-nm film almost uniformly and will penetrate the arsenic capping layers that protect the films. We find that samples withstand pump pulses of fluence up to 20 mJ/cm2, and we demonstrate the potential for sequences of pulses to amplify the oscillation while suppressing other phonon modes. Armed with our new knowledge of the conditions for exciting the desired coherent phonon, future experiments will be well prepared to measure its motion and to observe phononic control of the Dirac-point gap.

Rai, Manita↗
DOE OSTI2020

Topological band structure transitions and goniopolar transport in honeycomb antimonene as a function of buckling

The electronic band topology of monolayer β-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low-energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the pz orbitals, and one with the {p x , p y } orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slight buckling opens a gap along the nodal line except at six remaining Dirac points, protected by symmetry. Under increasing buckling, pairs of Dirac points of opposite winding number annihilate at a critical buckling angle. At a second critical angle, the remaining Dirac points disappear when the band structure opens a gap. Spin-orbit coupling and edge states are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Twisted symmetric trilayer graphene. II. Projected Hartree-Fock study

The Hamiltonian of the magic-angle twisted symmetric trilayer graphene (TSTG) can be decomposed into a twisted-bilayer-graphene- (TBG-) like flat band Hamiltonian and a high-velocity Dirac fermion Hamiltonian. We use Hartree-Fock mean field approach to study the projected Coulomb interacting Hamiltonian of TSTG developed in Calugaru et al. [Phys. Rev. B 103, 195411 (2021)] at integer fillings ν = -3, -2, -1, and 0 measured from charge neutrality. We study the phase diagram with w 0 /w 1 , the ratio of AA and AB interlayer hoppings, and the displacement field, which introduces an interlayer potential U and hybridizes the TBG-like bands with the Dirac bands. At small U, we find the ground states at all fillings ν are in the same phases as the tensor products of a Dirac semimetal with the filling ν TBG insulator ground states, which are spin-valley polarized at ν = -3, and fully (partially) intervalley coherent at ν = -2, 0(ν = -1) in the flat bands. An exception is ν = -3 with w 0 /w 1 ≳ 0.7 , which possibly becomes a metal with competing orders at small U due to charge transfers between the Dirac and flat bands. At strong U where the bandwidths exceed interactions, all the fillings ν enter a metal phase with small or zero valley polarization and intervalley coherence. Lastly, at intermediate U, semimetal or insulator phases with zero intervalley coherence may arise for ν = -2, -1, 0. Finally, our results provide a simple picture for the electron interactions in TSTG systems, and reveal the connection between the TSTG and TBG ground states.

2-dimensional systems↗
DOE OSTI2024

Theory for Cd 3 As 2 thin films in the presence of magnetic fields

Here we present a theory for thin films of the Dirac semimetal Cd 3 ⁢As 2 in the presence of magnetic fields. We show that, above a critical thickness, specific subbands n of thin film Cd 3 ⁢As 2 are in a quantum spin Hall insulator regime and study their response to in- and out-of-plane magnetic fields. We find that sufficiently large in-plane Zeeman fields drive the system toward a 2D Dirac semimetal regime, provided the field is directed perpendicular to a high-symmetry mirror plane. For other directions, we find the Dirac points to be weakly gapped. We further investigate how the system responds to finite out-of-plane field components, both starting from the quantum spin Hall regime at small in-plane fields and from the 2D Dirac semimetal regimes at larger in-plane fields, addressing recent experimental observations in A. C. Lygo et al. [Phys. Rev. Lett. 130, 046201 (2023)] and B. Guo et al. [Phys. Rev. Lett. 131, 046601 (2023)].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2012

Positron Interactions with Atoms and Ions

Dirac, in 1928, combining the ideas of quantum mechanics and the ideas of relativity invented the well-known relativistic wave equation. In his formulation, he predicted an antiparticle of the electron of spin n-bar/2. He thought that this particle must be a proton. Dirac published his interpretation in a paper 'A theory of electrons and protons.' It was shown later by the mathematician Hermann Weyl that the Dirac theory was completely symmetric between negative and positive particles and the positive particle must have the same mass as that of the electron. In his J. Robert Oppenheimer Memorial Prize Acceptance Speech, Dirac notes that 'Blackett was really the first person to obtain hard evidence for the existence of a positron but he was afraid to publish it. He wanted confirmation, he was really over cautious.' Positron, produced by the collision of cosmic rays in a cloud chamber, was detected experimentally by Anderson in 1932. His paper was published in Physical Review in 1933. The concept of the positron and its detection were the important discoveries of the 20th century. I have tried to discuss various processes involving interactions of positrons with atoms and ions. This includes scattering, bound states and resonances. It has not been possible to include the enormous work which has been carried out during the last 40 or 50 years in theory and measurements.

Bhatia, Anand K.↗
DOE OSTI2023

High-order finite element method for atomic structure calculations

We introduce featom, an open source code that implements a high-order finite element solver for the radial Schrödinger, Dirac, and Kohn-Sham equations. The formulation accommodates various mesh types, such as uniform or exponential, and the convergence can be systematically controlled by increasing the number and/or polynomial order of the finite element basis functions. The Dirac equation is solved using a squared Hamiltonian approach to eliminate spurious states. Here, to address the slow convergence of the $κ=±1$ states due to divergent derivatives at the origin, we incorporate known asymptotic forms into the solutions. We achieve a high level of accuracy (10 -8 Hartree) for total energies and eigenvalues of heavy atoms such as uranium in both Schrödinger and Dirac Kohn-Sham solutions. We provide detailed convergence studies and computational parameters required to attain commonly required accuracies. Finally, we compare our results with known analytic results as well as the results of other methods. In particular, we calculate benchmark results for atomic numbers (Z) from 1 to 92, verifying current benchmarks. We demonstrate significant speedup compared to the state-of-the-art shooting solver dftatom. An efficient, modular Fortran 2008 implementation, is provided under an open source, permissive license, including examples and tests, wherein particular emphasis is placed on the independence (no global variables), reusability, and generality of the individual routines.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Electronic and magnetic properties of the topological semimetal SmMg 2 Bi 2

Dirac semimetals show nontrivial physical properties and can host exotic quantum states like Weyl semimetals and topological insulators under suitable external conditions. Here, by combining angle-resolved photoemission spectroscopy measurements (ARPES) and first-principle calculations, we demonstrate that the Zintl-phase compound SmMg 2 Bi 2 is in close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone center near the Fermi level (E F ), which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg 2 Bi 2 belongs to a Z 2 topological class and hosts spin-polarized states around the E F . Zintl's theory predicts that the valence state of Sm in this material should be Sm 2+ , however, we detect many Sm-4f multiplet states (flat-bands) whose energy positions and relative intensities suggest the presence of both dominant Sm 2+ and minor Sm 3+ . The small concentration (2.5%) of Sm 3+ in the bulk of a crystal is inferred to arise from Sm vacancies in the crystal. It is also evident that these flat bands and other dispersive states are strongly hybridized when they cross each other. Due to the presence of Sm 3+ ions, the temperature dependence of the magnetic susceptibility χ (T) shows a Curie-Weiss-like contribution in the low-temperature region, in addition to the Van Vleck-like behavior expected for the Sm 2+ ions. The present study will help to better understand the electronic structure, magnetism, and transport properties of related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Twisted bilayers of thin film magnetic topological insulators

Twisted bilayer graphene (TBG) near "magic angles" has emerged as a rich platform for strongly correlated states of two-dimensional (2D) Dirac semimetals. Here we show that twisted bilayers of thin film magnetic topological insulators (MTIs) with large in-plane magnetization can realize flat bands near 2D Dirac nodes. Using a simple model for thin films of MTIs, we derive a continuum model for two such MTIs, twisted by a small angle with respect to each other. When the magnetization is in plane, we show that interlayer tunneling terms act as effective SU (2) vector potentials, which are known to lead to flat bands in TBG. We show that by changing the in-plane magnetization, it is possible to tune the twisted bilayer MTI band dispersion to quadratic band touching or to flat bands, similar to the TBG. If realized, this system can be a highly tunable platform for strongly correlated phases of two-dimensional Dirac semimetals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

UOTe: Kondo‐Interacting Topological Antiferromagnet in a Van der Waals Lattice

Since the initial discovery of 2D van der Waals (vdW) materials, significant effort has been made to incorporate the three properties of magnetism, band structure topology, and strong electron correlations—to leverage emergent quantum phenomena and expand their potential applications. However, the discovery of a single vdW material that intrinsically hosts all three ingredients has remained an outstanding challenge. Here, in this work, the discovery of a Kondo-interacting topological antiferromagnet is reported in the vdW 5f electron system UOTe. It has a high antiferromagnetic (AFM) transition temperature of 150 K, with a unique AFM configuration that breaks the combined parity and time reversal (PT) symmetry in an even number of layers while maintaining zero net magnetic moment. This angle-resolved photoemission spectroscopy (ARPES) measurements reveal Dirac bands near the Fermi level, which combined with the theoretical calculations demonstrate UOTe as an AFM Dirac semimetal. Within the AFM order, the presence of the Kondo interaction is observed, as evidenced by the emergence of a 5ƒ flat band near the Fermi level below 100 K and hybridization between the Kondo band and the Dirac band. The density functional theory calculations in its bilayer form predict UOTe as a rare example of a fully-compensated AFM Chern insulator.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Metal-to-insulator transition in oxide semimetals by anion doping

Dirac semimetals exhibiting nontrivial topological characteristics stand as exemplary parent compounds, offering great promise for the realization of novel electronic states. These materials also serve as a versatile platform for systematically investigating topological quantum phase transitions. In this study, we present compelling evidence of profound structural and transport phase shifts in a recently uncovered oxide Dirac semimetal, SrNbO3, achieved through effective in situ anion doping. Notably, a remarkable increase in resistivity of more than three orders of magnitude at room temperature is observed upon nitrogen-doping. The extent of electronic modulation in SrNbO3 is strongly correlated with the misfit strain, underscoring its phase instability to both chemical doping and crystallographic symmetry variations. Using first-principles calculations, we discern that elevating the level of nitrogen doping induces an upward shift in the conductive bands of SrNbO3-?N?. Consequently, a transition from a metallic state to an insulating state becomes apparent as the nitrogen concentration reaches a threshold of 1/3. This investigation sheds light on the potential of anion engineering in oxide Dirac semimetals, offering pathways for manipulating their physical properties. These insights hold promise for future applications that harness these materials for tailored functionalities.

Hong, Haitao↗
DOE OSTI2024

Uniform Diffusion of Cooper Pairing Mediated by Hole Carriers in Topological Sb 2 Te 3 /Nb

Spin-helical Dirac Fermions at a doped topological insulator’s boundaries can support Majorana quasiparticles when coupled with s-wave superconductors, but in n-doped systems, the requisite induced Cooper pairing in topological states is often buried at heterointerfaces or complicated by degenerate coupling with bulk conduction carriers. Rarely probed are p-doped topological structures with nondegenerate Dirac and bulk valence bands at the Fermi level, which may foster long-range superconductivity without sacrificing Majorana physics. Using ultrahigh-resolution photoemission, we report proximity pairing with a large decay length in p-doped topological Sb 2 Te 3 on superconducting Nb. Despite no momentum-space degeneracy, the topological and bulk states of Sb 2 Te 3 /Nb exhibit the same isotropic superconducting gaps at low temperatures. Furthermore, our results unify principles for realizing accessible pairing in Dirac Fermions relevant to topological superconductivity.

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

Origin of the quasi-quantized Hall effect in ZrTe 5

The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the Dirac semimetal ZrTe 5 . It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe 5 samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe 5 electronic structure and its Dirac-type semi-metallic character.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

A topological Hund nodal line antiferromagnet

The interplay of topology, magnetism, and correlations gives rise to intriguing phases of matter. In this study, through state-of-the-art angle-resolved photoemission spectroscopy, density functional theory, and dynamical mean-field theory calculations, we visualize a fourfold degenerate Dirac nodal line at the boundary of the bulk Brillouin zone in the antiferromagnet YMn 2 Ge 2 . We further demonstrate that this gapless, antiferromagnetic Dirac nodal line is enforced by the combination of magnetism, space-time inversion symmetry, and nonsymmorphic lattice symmetry. The corresponding drumhead surface states traverse the whole surface Brillouin zone. YMn 2 Ge 2 thus serves as a platform to exhibit the interplay of multiple degenerate nodal physics and antiferromagnetism. Interestingly, the magnetic nodal line displays a d-orbital dependent renormalization along its trajectory in momentum space, thereby manifesting Hund's coupling. Our findings offer insights into the effect of electronic correlations on magnetic Dirac nodal lines, leading to an antiferromagnetic Hund nodal line.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Light control of surface–bulk coupling by terahertz vibrational coherence in a topological insulator

The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipationless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such a goal. Yet, surface-bulk coupling can lead to strong Dirac electron scattering with bulk carriers and phonons as well as impurities, assisted by such dissipative channel, which results in “topological breakdown”. Here, we demonstrate that coherent lattice vibrations periodically driven by a single-cycle terahertz (THz) pulse can significantly suppress such dissipative channel in topological insulators. This is achieved by reducing the phase space in the bulk available for Dirac fermion scattering into during coherent lattice oscillations in Bi 2 Se 3 . This light-induced suppression manifests as a remarkable transition exclusively in surface transport, absent for bulk, above the THz electric fields for driving coherent phonons, which prolongs the surface transport lifetime. These results, together with simulations, identify the critical role of spin–orbit coupling for the “phase space contraction” mechanism that suppresses the surface-bulk coupling. Imposing vibrational quantum coherence into topological states of matter may become a universal light control principle for reinforcing the symmetry-protected helical transport.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Crystalline symmetry-protected non-trivial topology in prototype compound BaAl 4

The BaAl 4 prototype crystal structure is the most populous of all structure types, and is the building block for a diverse set of sub-structures including the famous ThCr 2 Si 2 family that hosts high-temperature superconductivity and numerous magnetic and strongly correlated electron systems. The MA 4 family of materials (M = Sr, Ba, Eu; A = Al, Ga, In) themselves present an intriguing set of ground states including charge and spin orders, but have largely been considered as uninteresting metals. We predict the exemplary compound BaAl 4 to harbor a three-dimensional Dirac spectrum with non-trivial topology and possible nodal lines crossing the Brillouin zone, wherein one pair of semi-Dirac points with linear dispersion along the k z direction and quadratic dispersion along the k x / k y direction resides on the rotational axis with C 4 v point group symmetry. An extremely large, unsaturating positive magnetoresistance in BaAl 4 despite an uncompensated band structure is revealed, and quantum oscillations and angle-resolved photoemission spectroscopy measurements confirm the predicted multiband semimetal structure with pockets of Dirac holes and a Van Hove singularity (VHS) remarkably consistent with the theoretical prediction. We thus present BaAl 4 as a topological semimetal, casting its prototype status into a role as a building block for a vast array of topological materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Spin Berry curvature-enhanced orbital Zeeman effect in a kagome metal

Berry phases and the related concept of Berry curvature can give rise to many unconventional phenomena in solids. Here, we discover a colossal orbital Zeeman effect of topological origin in a bilayer kagome metal, TbV 6 Sn 6 . Using spectroscopic imaging scanning tunnelling microscopy, we reveal that the magnetic field leads to a splitting of the gapped Dirac dispersion into two branches with enhanced momentum-dependent g factors, resulting in a substantial renormalization of the Dirac band. These measurements provide a direct observation of a magnetic field-controlled orbital Zeeman coupling to the orbital magnetic moments of up to 200 Bohr magnetons near the gapped Dirac points. Our work provides direct insight into the momentum-dependent nature of topological orbital moments and their tunability via the magnetic field, concomitant with the evolution of the spin Berry curvature. Furthermore, these results can be extended to explore large orbital magnetic moments driven by the Berry curvature governed by other quantum numbers beyond spin, such as the valley in certain graphene-based structures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Observation of hydrodynamic plasmons and energy waves in graphene

Thermally excited electrons and holes form a quantum-critical Dirac fluid in ultraclean graphene and their electrodynamic responses are described by a universal hydrodynamic theory. The hydrodynamic Dirac fluid can host intriguing collective excitations distinctively different from those in a Fermi liquid. Here we report the observation of the hydrodynamic plasmon and energy wave in ultraclean graphene. In this study, we use the on-chip terahertz (THz) spectroscopy technique to measure the THz absorption spectra of a graphene microribbon as well as the propagation of the energy wave in graphene close to charge neutrality. We observe a prominent high-frequency hydrodynamic bipolar-plasmon resonance and a weaker low-frequency energy-wave resonance of the Dirac fluid in ultraclean graphene. The hydrodynamic bipolar plasmon is characterized by the antiphase oscillation of massless electrons and holes in graphene. The hydrodynamic energy wave is an electron-hole sound mode with both charge carriers oscillating in phase and moving together. The spatial–temporal imaging technique shows that the energy wave propagates at a characteristic speed of $V_f$ / $ \sqrt2$ near the charge neutrality. Our observations open new opportunities to explore collective hydrodynamic excitations in graphene systems.

36 MATERIALS SCIENCE↗
DOE OSTI2025

First Look at Quartic-in-Spin Binary Dynamics at Third Post-Minkowskian Order

We compute the conservative and radiation-reaction contributions to classical observables in the gravitational scattering between a spinning and a spinless black hole to the fourth order in spin and third order in the gravitational constant. The conservative results are obtained from two-loop amplitudes for the scattering process of a massive scalar with a massive spin-𝑠 field (𝑠=0, 1, 2) minimally coupled to gravity, employing the recently introduced spin interpolation method to resolve all spin-Casimir terms. The two-loop amplitude exhibits a spin-shift symmetry in both probe limits, which we conjecture to be a sign of yet unknown integrability of Kerr orbits through the quartic order in spin and to all orders in the gravitational constant. We obtain the radial action from the finite part of the amplitude and use it to compute classical observables, including the impulse and spin kick. This is done using the recently introduced covariant Dirac brackets, which allow for the computation of classical scattering observables for general (nonaligned) spin configurations. Finally, employing the radiation-reaction amplitude proposed by Alessio and Di Vecchia, together with the Dirac brackets, we obtain radiation-reaction contributions to observables at all orders in spin and beyond the aligned-spin limit. We find agreement with known results up to the quadratic order in spin for both conservative and radiation-reaction contributions. Our results advance the state of the art in the understanding of spinning binary dynamics in general relativity and demonstrate the power and simplicity of the Dirac bracket formalism for relating scattering amplitudes to classical observables.

general relativity↗