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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Origins of electronic bands in the antiferromagnetic topological insulator MnBi 2 Te 4

Despite the rapid progress in understanding the first intrinsic magnetic topological insulator MnBi 2 Te 4 , its electronic structure remains a topic under debates. In this work, we perform a thorough spectroscopic investigation into the electronic structure of MnBi 2 Te 4 via laser-based angle-resolved photoemission spectroscopy. Through quantitative analysis, we estimate an upper bound of 3 meV for the gap size of the topological surface state. Furthermore, our circular dichroism measurements reveal band chiralities for both the topological surface state and quasi-2D bands, which can be well reproduced in a band hybridization model. A numerical simulation of energy-momentum dispersions based on a four-band model with an additional step potential near the surface provides a promising explanation for the origin of the quasi-2D bands. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi 2 Te 4 , and provides an important framework to understand the electronic structures of other relevant topological materials MnBi 2n Te 3n+1 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Intrinsic origin and enhancement of topological responses in ferrimagnetic antiperovskite Mn 4 N

Using first-principles calculations here we investigate the intrinsic origins of the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) in antiperovskite ferrimagnet Mn 4 N. We predict that the AHE is significantly enhanced under both compressive and tensile strain; however, the ANE generally decreases under epitaxial strain, except for 1% compressive strain. We connect this behavior to the evolution of the Berry curvature with strain, suggesting similar strategies for achieving large AHE and ANE changes with modest amounts of strain. Finally, we find that the nonmonotonic characteristics of the AHE and ANE stem from the formation and movement of new Weyl points at the periphery of the Brillouin zone under compressive and tensile strains.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Electronic structure and lattice dynamics of 1⁢𝑇−VSe 2 : Origin of the three-dimensional charge density wave

To characterize in detail the charge density wave (CDW) transition of 1⁢𝑇−VSe 2 , its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, muon spectroscopy, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering, and state-of-the-art first-principles density functional theory calculations. Resonant elastic x-ray scattering does not show any resonant enhancement at either V or Se, indicating that the CDW peak at the 𝐾 edges describes a purely structural modulation of the electronic ordering. ARPES experiments identify (i) a pseudogap at 𝑇 > 𝑇⁢ CDW , which leads to a depletion of the density of states in the ML-M'L' plane at 𝑇 < 𝑇⁢ CDW , and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon (𝜔 1 ) with propagation vector (0.25 0 −0.3) r.l.u. Here, we show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependent EPI with a small contribution from nesting. The pressure dependence of the 𝜔 1 soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic C2/m phase. The wide set of experimental data is well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals corrections in the exchange-correlation functional. The comprehensive description of the electronic and dynamical properties of VSe 2 reported here adds important pieces of information to the understanding of the electronic modulations in the family of transition-metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Origin of spin-driven ferroelectricity and effect of external pressure on the complex magnetism of the 6⁢ H perovskite Ba 3 ⁢Ho⁢Ru 2⁢ O 9

The compound Ba 3 HoRu 2 O 9 magnetically orders at 50 K (T N1 ), followed by another complex magnetic ordering at 10.2 K (T N2 ). The second magnetic phase transition was characterized by the coexistence of two competing magnetic ground states associated with two different magnetic wave vectors (K 1 = 0.5 0 0 and K 2 = 0.25 0.25 0). The multiferroicity and magnetoelectric coupling were predicted below T N2 in these 4d-based materials. Here, in this work, we have discussed the origin of spin-driven ferroelectricity, which is not known yet, and the nature of magnetoelectric domains. We have investigated the compound through time-of-flight neutron diffraction, synchrotron x-ray diffraction (XRD), ac susceptibility, frequency-dependent complex dielectric spectroscopy, and dc magnetization under external pressure. We have demonstrated that the noncollinear structure involving two different magnetic ions, Ru (4d) and Ho (4f), breaks the spatial inversion symmetry via inverse Dzyaloshinskii-Moriya (DM) interaction through strong 4d-4f magnetic correlation, which shifts the oxygen atoms and results in nonzero polarization. Such an observation of inverse DM interaction from two different magnetic ions which cause ferroelectricity is rarely observed. The stronger spin-orbit coupling of 4d orbital might play a major role in creating DM interaction of noncollinear spins. We have systematically studied the spin and dipolar dynamics, which exhibit intriguing behavior with shorter coherence lengths of second magnetic phase associated with the k 2 wave vector. The results manifest the development of finite-size magnetoelectric domains instead of true long-range ordering, which justifies the experimentally obtained low value of ferroelectric polarization. The lattice parameters and volume show a sharp anomaly at T N2 obtained by analyzing the temperature-dependence XRD, which is consistent with the ferroelectric transition, predicting a noncentrosymmetric space group, $P\bar{6}2c$, for this compound. Furthermore, we have investigated the effect of external pressure on this complex magnetism. The result reveals an enhancement of ordering temperature by the application of external pressure (~1.6 K/GPa). The external pressure might favor stabilizing the magnetic ground state associated with second magnetic phase. Our study shows an unconventional mechanism of spin-driven ferroelectricity involving inverse DM interaction between Ru (4d) and Ho (4f) magnetic ions due to strong 4d-4f cross coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electronic structure and the origin of the Dzyaloshinskii-Moriya interaction in MnSi

Here, the metallic helimagnet MnSi has been found to exhibit skyrmionic spin textures when subjected to magnetic fields at low temperatures. The Dzyaloshinskii-Moriya (DM) interaction plays a key role in stabilizing the skyrmion state. With the help of first-principles calculations, crystal field theory and a tight-binding model we study the electronic structure and the origin of the DM interaction in the B20 phase of MnSi. The strength of $\vec{D}$ parameter is determined by the magnitude of the spin-orbit interaction and the degree of orbital mixing, induced by the symmetry-breaking distortions in the B20 phase. We find that, strong coupling between Mn-$d$ and Si-$p$ states lead to a mixed valence ground state $|d^{7-x}p^{2+x}\rangle$ configuration. The experimental magnetic moment of $0.4~\mu_B$ is consistent with the Coulomb-corrected DFT+$U$ calculations, which redistributes electrons between the majority and minority spin channels. We derive the magnetic interaction parameters $J$ and $\vec{D}$ for Mn-Si-Mn superexchange paths using Moriya's theory assuming the interaction to be mediated by $e_g$ electrons near the Fermi level. Finally, using parameters from our calculations, we get reasonable agreement with the observations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Origin of localization in Ti-doped Si

Intermediate band semiconductors hold the promise to significantly improve the efficiency of solar cells but only if the intermediate impurity band is metallic. In this work, we apply a recently developed first principles method to investigate the origin of electron localization in Ti doped Si, a promising candidate for intermediate band solar cells. We compute the critical Ti concentration and compare it against the available experimental data. Although Anderson localization is often overlooked in the context of intermediate band solar cells, our results show that in Ti doped Si it plays a more important role in the metal insulator transition than Mott localization. To this end we have devised a way to gauge the relative strengths of these two localization mechanisms that can be applied to study localization in doped semiconductors in general. Our findings have important implications for the theory of intermediate band solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing the origin of extreme magnetoresistance in Pr/Sm mono-antimonides/bismuthides

Combining angle-resolved photoemission spectroscopy and magnetotransport measurements, we systematically investigated the possible origin of the extreme magnetoresistance in Pr/Sm mono-antimonides/bismuthides (PrSb, SmSb, PrBi, SmBi). Our photoemission measurements reveal that the bulk band inversion and surface states are absent (present) in Pr/Sm antimonides (bismuthides), implying that topological surface states are unlikely to play an important role for the observed extreme magnetoresistance. We found that the electron- hole compensation is well satisfied in all these compounds and the bulk band structure exhibits no obvious temperature dependence from 10 up to 150 K. Simultaneous fittings of the magnetoresistance and Hall coefficient reveal that the carrier mobility is dramatically enhanced at low temperature, which naturally explains the suppression of extreme magnetoresistance at high temperatures. Our results therefore show that the extreme magnetoresistance in these compounds can be well accounted for by the two-band model with good electron-hole compensation. Lastly, we found that both PrSb and SmSb exhibit highly linear bulk bands near the X point and lie close to the transition point between a topologically trivial and nontrivial phase, which might be relevant for the observed anomalous quantum oscillations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Exploring a cosmic-ray origin of the multiwavelength emission in M31

A recent detection of spatially extended gamma-ray emission in the central region of the Andromeda galaxy (M31) has led to several possible explanations being put forth, including dark matter annihilation and millisecond pulsars. Another possibility is that the emission in M31 can be accounted for with a purely astrophysical cosmic-ray (CR) scenario. This scenario would lead to a rich multiwavelength emission that can, in turn, be used to test it. Relativistic cosmic-ray electrons (CRe) in magnetic fields produce radio emission through synchrotron radiation, while x-rays and gamma rays are produced through inverse Compton scattering. Additionally, collisions of primary cosmic-ray protons (CRp) in the interstellar medium produce charged and neutral pions that then decay into secondary CRe (detectable through radiative processes) and gamma rays. Here, we explore the viability of a CR origin for multiwavelength emission in M31, taking into consideration three scenarios: a CR scenario dominated by primary CRe, one dominated by CRp and the resulting secondary CRe and gamma rays from neutral pion decay, and a final case in which both of these components exist simultaneously. Here, we find that the multicomponent model is the most promising, and is able to fit the multiwavelength spectrum for a variety of astrophysical parameters consistent with previous studies of M31 and of cosmic-ray physics. However, the CR power injection implied by our models exceeds the estimated CR power injection from typical astrophysical cosmic-ray sources such as supernovae.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Origin of single transverse-spin asymmetries in high-energy collisions

In this paper, we perform the first simultaneous QCD global analysis of data from semi-inclusive deep inelastic scattering, Drell-Yan, e + e - annihilation into hadron pairs, and proton-proton collisions. Consequently, we are able to extract a universal set of nonperturbative functions that describes the observed asymmetries in these reactions. The outcome of our analysis indicates single transverse-spin asymmetries in high-energy collisions have a common origin. Furthermore, we achieve the first phenomenological agreement with lattice QCD on the up and down quark tensor charges.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Unified origin of inflation, baryon asymmetry, and neutrino mass

In this work, we present a unified theoretical framework that simultaneously addresses some of the most intriguing puzzles in particle physics and cosmology, namely the origins of neutrino mass, baryon asymmetry, and cosmic inflation. In our model, inflation is driven by a combination of the Standard Model Higgs, the type II seesaw Higgs responsible for neutrino mass generation, and the unified symmetry-breaking Higgs field. During inflation, nonzero values of the latter field ensure the absence of the monopole problem. The baryon asymmetry is generated through the Affleck-Dine mechanism, facilitated by the nonzero angular motion in the phase of a complex scalar field, which is part of the inflaton. We find that the successful parameter region for generating baryon asymmetry through a renormalizable term in the scalar potential requires a rather heavy type II seesaw triplet, with a mass well beyond the TeV scale. Inflationary observables, in particular, the spectral index is in excellent agree with experimental observation, whereas tensor-to scalar ratio is expected to be probed by the future LiteBIRD and CMB-S4 missions. Published by the American Physical Society 2024

Kaladharan, Ajay (ORCID:0000000165096427)↗
DOE OSTI2025

Possible origin of α -vacua as the initial state of the Universe

We investigate the cosmological observables using the Euclidean path integral approach. Specifically, we study both the no-boundary compact instantons scenario and the Euclidean wormholes scenario that can induce the creation of two universes from nothing. It is known that perturbations associated with the no-boundary scenario can only be consistent with the Bunch-Davies vacuum. Here, in this study, we demonstrate that the Euclidean wormholes can allow for a de Sitter invariant vacuum, the so-called α -vacua state, where the Bunch-Davies vacuum is a special case. This therefore provides the α -vacua a geometrical origin. As an aside, we discuss a subtle phase issue when considering the power spectrum related to α -vacua in the closed universe framework.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Origin of the tentative AMS antihelium events

We demonstrate that the tentative detection of a few antihelium events with the Alpha Magnetic Spectrometer (AMS) on board the International Space Station can, in principle, be ascribed to the annihilation or decay of Galactic dark matter, when accounting for uncertainties in the coalescence process leading to the formation of antinuclei. We show that the predicted antiproton rate, assuming the antihelium events came from dark matter, is marginally consistent with AMS data, as is the antideuteron rate with current available constraints. We argue that a dark matter origin can be tested with better constraints on the coalescence process, better control of misidentified events, and with future antideuteron data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Origin of liquid fragility

Liquid fragility characterizes how steeply the viscosity of a glass-forming liquid decreases with increasing temperature above the glass transition. It is one of the most fundamental properties of a liquid, with high importance for science and application. Yet, its origin is unclear. Here we show that it is directly related to the structural coherence of the medium-range order (MRO) in liquid defined by the decay of the pair-distribution function with distance. The MRO can also be evaluated from the first peak of the structure function determined by x-ray or neutron diffraction, and it is a measure of the cooperativity of atomic motion in a diffusive event in supercooled liquids. These findings shed light on the mechanism of atomic transport in supercooled liquids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Origin of Magic Angles in Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) was recently shown to host superconductivity when tuned to special “magic angles” at which isolated and relatively flat bands appear. However, until now the origin of the magic angles and their irregular pattern have remained a mystery. Here we report on a fundamental continuum model for TBG which features not just the vanishing of the Fermi velocity, but also the perfect flattening of the entire lowest band. When parametrized in terms of α ~ 1/θ, the magic angles recur with a remarkable periodicity of Δα ≃ 3/2. We show analytically that the exactly flat band wave functions can be constructed from the doubly periodic functions composed of ratios of theta functions—reminiscent of quantum Hall wave functions on the torus. We further report on the unusual robustness of the experimentally relevant first magic angle, address its properties analytically, and discuss how lattice relaxation effects help justify our model parameters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Charge-Lattice Coupling in Hole-Doped LuFe 2 O 4 + δ : The Origin of Second-Order Modulation

Understanding singularities in ordered structures, such as dislocations in lattice modulation and solitons in charge ordering, offers great opportunities to disentangle the interactions between the electronic degrees of freedom and the lattice. Specifically, a modulated structure has traditionally been expressed in the form of a discrete Fourier series with a constant phase and amplitude for each component. We report atomic scale observation and analysis of a new modulation wave in hole-doped ${\mathrm{LuFe}}_{2}{\mathrm{O}}_{4+{\delta}}$ that requires significant modifications to the conventional modeling of ordered structures. This new modulation with an unusual quasiperiodic singularity can be accurately described only by introducing a well-defined secondary modulation vector in both the phase and amplitude parameter spaces. Correlated with density-functional-theory (DFT) calculations, our results reveal that those singularities originate from the discontinuity of lattice displacement induced by interstitial oxygen in the system. The approach of our work is applicable to a wide range of ordered systems, advancing our understanding of the nature of singularity and modulation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Higher-Order Topology, Monopole Nodal Lines, and the Origin of Large Fermi Arcs in Transition Metal Dichalcogenides XTe 2 ( X =Mo,W )

In recent years, transition metal dichalcogenides (TMDs) have garnered great interest as topological materials. In particular, monolayers of centrosymmetric β-phase TMDs have been identified as 2D topological insulators (TIs), and bulk crystals of noncentrosymmetric γ-phase MoTe 2 and WTe 2 have been identified as type-II Weyl semimetals. However, angle-resolved photoemission spectroscopy and STM probes of these semimetals have revealed huge, arclike surface states that overwhelm, and are sometimes mistaken for, the much smaller topological surface Fermi arcs of bulk type-II Weyl points. In this Letter, we calculate the bulk and surface electronic structure of both β- and γ-MoTe 2 . We find that β-MoTe 2 is, in fact, a $\mathbb Z_4$-nontrivial higher-order TI (HOTI) driven by double band inversion and exhibits the same surface features as γ-MoTe 2 and γ-WTe 2 . Further, we discover that these surface states are not topologically trivial, as previously characterized by the research that differentiated them from the Weyl Fermi arcs but, rather, are the characteristic split and gapped fourfold Dirac surface states of a HOTI. In β-MoTe 2 , this indicates that it would exhibit helical pairs of hinge states if it were bulk insulating, and in γ-MoTe 2 and γ-WTe 2 , these surface states represent vestiges of HOTI phases without inversion symmetry that are nearby in parameter space. Using nested Wilson loops and first-principles calculations, we explicitly demonstrate that, when the Weyl points in γ-MoTe 2 are annihilated, which may be accomplished by symmetry-preserving strain or lattice distortion, γ-MoTe 2 becomes a nonsymmetry-indicated, noncentrosymmetric HOTI. We also show that, when the effects of spin-orbit coupling are neglected, β-MoTe 2 is a nodal-line semimetal with $\mathbb Z_2$-nontrivial monopole nodal lines (MNLSM). This finding confirms that MNLSMs driven by double band inversion are the weak-spin-orbit coupling limit of HOTIs, implying that MNLSMs are higher-order topological semimetals with flat-band-like hinge states, which we find to originate from the corner modes of 2D “fragile” TIs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Anharmonic Origin of the Giant Thermal Expansion of NaBr

ll phonons in a single crystal of NaBr are measured by inelastic neutron scattering at temperatures of 10, 300, and 700 K. Even at 300 K, the phonons, especially the longitudinal-optical phonons, show large shifts in frequencies and show large broadenings in energy owing to anharmonicity. Ab initio computations are first performed with the quasiharmonic approximation (QHA) in which the phonon frequencies depend only on V and on T only insofar as it alters V by thermal expansion. This QHA is an unqualified failure for predicting the temperature dependence of phonon frequencies, even 300 K, and the thermal expansion is in error by a factor of 4. Ab initio computations that include both anharmonicity and quasiharmonicity successfully predict both the temperature dependence of phonons and the large thermal expansion of NaBr. The frequencies of longitudinal-optical phonon modes decrease significantly with temperature owing to the real part of the phonon self-energy from explicit anharmonicity originating from the cubic anharmonicity of nearest-neighbor NaBr bonds. Anharmonicity is not a correction to the QHA predictions of thermal expansion and thermal phonon shifts but dominates the behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Unmasking the Origin of Kinks in the Photoemission Spectra of Cuprate Superconductors

The origin of a ubiquitous bosonic coupling feature in the photoemission spectra of high- T c cuprates, an energy-momentum dispersion “kink” observed at ~70 meV binding energy, remains a two-decade-old mystery. Understanding this phenomenon requires an accurate description of the coupling between the electron and some collective modes. In this work, we report ab initio calculations based on G W perturbation theory and show that correlation-enhanced electron-phonon interaction in cuprates gives rise to the strong kinks, which not only explains quantitatively the observations but provides new understanding of experiments. Our results reveal it is the electron density of states being the predominant factor in determining the doping dependence of the kink size, manifesting the multiband nature of the cuprates, as opposed to the prevalent belief of it being a measure of the mode-coupling strength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗