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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Temperature-Induced Topological Phase Transitions: Promoted versus Suppressed Nontrivial Topology

Contrary to previous two-band model studies which find increasing temperature would induce a topological phase transition, we show here through first-principles calculations that the opposite is also realizable, depending on the material’s full band structure and symmetry of the electron-phonon coupling potential. This finding explains recent experimental results by Wojek et al . We show that the topological phase diagram of BiTl$(S_{1-δ}Se_δ)_2$ as a function of doping and temperature contains two distinct regions with nontrivial topology. In BiTlS 2 , the phonons promote the topological phase at high temperature, while in BiTlSe 2 , the system is driven back into the trivial phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anisotropic angular magnetoresistance and Fermi surface topology of the candidate novel topological metal Pd 3 Pb

In this paper, the recent realization of topological electronic states such as Dirac and Weyl fermions in real materials and their potential for future energy and electronics applications has motivated interest in the study of new forms of topological behavior embodied through new materials. Pd 3 Pb is one such candidate predicted recently to host unique topological features, including a dispersionless band near the Fermi level and triple nodal points hosting Dirac fermions and open Fermi arcs. Here, we report the crystal growth and electric transport properties of Pd 3 Pb. Our low field magnetoresistance measurements indicate an anisotropic Fermi surface. We found that Pd 3 Pb manifests a large transverse magnetoresistance, which reaches 650% at 1.8 K and 14 T, and pronounced Shubnikov-de Haas (SdH) oscillations. Preliminary analysis of the field dependence of the SdH oscillations points to the likelihood of nontrivial Berry phase in Pd 3 Pb. Further studies in high field limit are desirable to extend the realization of the topological properties of the predicted novel fermions in this material.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Twisted bulk-boundary correspondence of fragile topology

A topological insulator reveals its nontrivial bulk through the presence of gapless edge states: This is called the bulk-boundary correspondence. However, the recent discovery of “fragile” topological states with no gapless edges casts doubt on this concept. We propose a generalization of the bulk-boundary correspondence: a transformation under which the gap between the fragile phase and other bands must close. We derive specific twisted boundary conditions (TBCs) that can detect all the two-dimensional eigenvalue fragile phases. We develop the concept of real-space invariants, local good quantum numbers in real space, which fully characterize these phases and determine the number of gap closings under the TBCs. Realizations of the TBCs in metamaterials are proposed, thereby providing a route to their experimental verification.

Science & Technology - Other Topics↗
DOE OSTI2017

Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1

The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Lastly, topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Quantum Imaging of Magnetic Phase Transitions and Spin Fluctuations in Intrinsic Magnetic Topological Nanoflakes

Topological materials featuring exotic band structures, unconventional current flow patterns, and emergent organizing principles offer attractive platforms for the development of next-generation transformative quantum electronic technologies. The family of MnBi 2 T e4 (Bi 2 Te 3 )n materials is naturally relevant in this context due to their nontrivial band topology, tunable magnetism, and recently discovered extraordinary quantum transport behaviors. Despite numerous pioneering studies, to date, the local magnetic properties of MnBi 2 Te 4 (Bi 2 Te 3 )n remain an open question, hindering a comprehensive understanding of their fundamental material properties. Exploiting nitrogen-vacancy (NV) centers in diamond, we report nanoscale quantum imaging of magnetic phase transitions and spin fluctuations in exfoliated MnBi 2 Te 4 (Bi 2 Te 3 )n flakes, revealing the underlying spin transport physics and magnetic domains at the nanoscale. Furthermore, our results highlight the unique advantage of NV centers in exploring the magnetic properties of emergent quantum materials, opening new opportunities for investigating the interplay between topology and magnetism.

Antiferromagnetic materials↗
DOE OSTI2022

A topological kagome magnet in high entropy form

AbstractTopological kagome magnets RMn 6 Sn 6 (R = rare earth element) attract numerous interests due to their non-trivial band topology and room-temperature magnetism. Here, we report a high entropy version of kagome magnet, (Gd 0.38 Tb 0.27 Dy 0.20 Ho 0.15 )Mn 6 Sn 6 . Such a high entropy material exhibits multiple spin reorientation transitions, which is not seen in all the related parent compounds and can be understood in terms of competing magnetic interactions enabled by high entropy. Furthermore, we also observed an intrinsic anomalous Hall effect, indicating that the high entropy phase preserves the non-trivial band topology. These results suggest that high entropy may provide a route to engineer the magnetic structure and expand the horizon of topological materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

SU(N) fractional quantum Hall effect in topological flat bands

We study N-component interacting particles (hardcore bosons and fermions) loaded in topological lattice models with SU(N)-invariant interactions based on exact diagonalization and density matrix renormalization group method. By tuning the interplay of interspecies and intraspecies interactions, we demonstrate that a class of SU(N) fractional quantum Hall states can emerge at fractional filling factors nu=N/(N+1) for bosons [nu=N/(2N+1) for fermions] in the lowest Chern band, characterized by the nontrivial fractional Hall responses and the fractional charge pumping. Moreover, we establish a topological characterization based on the K matrix and discuss the close relationship to the fractional quantum Hall physics in topological flat bands with Chern number N.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Two-dimensional ferroelectric topological insulators in functionalized atomically thin bismuth layers

We introduce a class of two-dimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spin-orbit coupling (SOC). Here, we showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by CH 2 OH, which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth layer exhibits an additional quantum order driven by the valley splitting at the K and K' points in the Brillouin zone stemming from the symmetry breaking and strong SOC in the system, resulting in a remarkable state of matter with the simultaneous presence of the quantum spin Hall and quantum valley Hall effect. These phenomena are predicted to exist in other similarly constructed 2D FETIs, thereby offering a unique quantum material platform for discovering novel physics and exploring innovative applications.

2-dimensional systems↗
DOE OSTI2017

Indium Substitution Effect on the Topological Crystalline Insulator Family (Pb 1$-$x Sn x )1 $-$y InyTe: Topological and Superconducting Properties

Topological crystalline insulators (TCIs) have been of great interest in the area of condensed matter physics. We investigated the effect of indium substitution on the crystal structure and transport properties in the TCI system (Pb 1-x Sn x ) 1-y In y Te. For samples with a tin concentration x ≤ 50% , the low-temperature resisitivities show a dramatic variation as a function of indium concentration: with up to ~2% indium doping, the samples show weak-metallic behavior similar to their parent compounds; with `6% indium doping, samples have true bulk-insulating resistivity and present evidence for nontrivial topological surface states; with higher indium doping levels, superconductivity was observed, with a transition temperature, T c , positively correlated to the indium concentration and reaching as high as 4.7 K. We address this issue from the view of bulk electronic structure modified by the indium-induced impurity level that pins the Fermi level. The current work summarizes the indium substitution effect on (Pb,Sn)Te, and discusses the topological and superconducting aspects, which can be provide guidance for future studies on this and related systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Coupled Dirac Plasmons in Topological Insulators

Topological insulators are predicted to house spin-polarized two-dimensional Dirac plasmons. In topological insulator thin films, Dirac plasmons on the top and bottom surfaces will be coupled, giving rise to an unusual dispersion relationship. These plasmons are of interest both for fundamental science as well as applications in THz sensing and waveguiding. In this article, we present conclusive evidence for coupled Dirac plasmons excited in the surface states of topological insulator thin film stripe arrays by investigating how the plasmon frequency depends on both film thickness and stripe width. Bi 2 Se 3 films of thickness 50-200nm are patterned into periodic stripes of width 1-4μm to excite plasmons. The plasmon frequency dependence on both film thickness and stripe width is found to be in good agreement with theoretical models for couple 2D Dirac plasmons. Alternative explanations such as 2D massive plasmons are shown to be inconstant with data. Finally, effective mode indexes of up to 211 are reported, demonstrating incredible confinement of light in the THz frequency range.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Topologically guided tuning of Zr-MOF pore structures for highly selective separation of C6 alkane isomers

Effective separation of alkane isomers is a critical step in the petroleum refining process to produce gasoline with high octane rating. Current technology relies on energy intensive distillation method. Adsorptive separation using porous solids offers an alternative route that is more cost effective, lower energy and higher efficiency. Guided by topology-directed design and synthesis, we have developed a series of zirconium-based metal-organic frameworks with optimized pore structure for efficient separation of C6 alkane isomers, Zr-bptc adsorbs a large amount of n-hexane while excluding branched isomers. The n-hexane uptake is ~70% higher than that of a benchmark adsorbent, zeolite-5A. A derivative structure, Zr-abtc, is capable of discriminating all three C6 isomers with a high separation factor for 3-methylpentane over 2,3-dimethylbutane. This property is important for producing gasoline with further improved quality.

36 MATERIALS SCIENCE↗
DOE OSTI2021

The Zintl phases A In 2 As 2 (A = Ca, Sr, Ba): new topological insulators and thermoelectric material candidates

Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their surface. The surface states observed in such materials behave as a perfect conductor thereby making them more suited for several cutting-edge technological applications such as spintronic devices. Here, we report the synthesis and structural characterization of the Zintl phases AIn 2 As 2 (A = Ca, Sr, Ba), which could become a new class of TIs. Crystal structure elucidation by single-crystal X-ray diffraction reveals that CaIn 2 As 2 and SrIn 2 As 2 are isostructural and crystallize in the EuIn 2 P 2 structure type (space group P6 3 /mmc, no. 194, Z = 2) with unit cell parameters a = 4.1482(6) Å, c = 17.726(4) Å; and a = 4.2222(6) Å, c = 18.110(3) Å, respectively. Their hexagonal structure is made up of alternating [In 2 As 2 ] 2– layers separated by slabs of A 2+ cations. BaIn 2 As 2 on the other hand crystallizes in the monoclinic EuGa 2 P 2 structure type (space group P2/m, no. 10, Z = 4) with unit cell parameters a = 10.2746(11) Å, b = 4.3005(5) Å, c = 13.3317(14) Å and β = 95.569(2)°. This structure is also layered, and it is made up of different type of polyanionic [In 2 As 2 ] 2– units and Ba 2+ cations. The valence electron count for all three compounds adheres to the Zintl-Klemm formalism, and all elements achieve closed-shell electronic configurations. Bulk electronic structure calculations indicate the opening of a bandgap E g ~ 0.03 eV (CaIn 2 As 2 and Sr 2 In 2 As 2 ), and E g ~0.21 eV (BaIn 2 As 2 ) in the absence of strain and spin–orbit coupling (SOC). Furthermore, these results argue in favor of the realization of a nontrivial topological insulator state under the influence of tensile strain and SOC. Preliminary transport properties on BaIn 2 As 2 are suggestive of a degenerate p-type semiconductor—a behavior which is sought after in thermoelectric (TE) materials. Since both TIs and excellent TE materials are known to favor the same material properties such as narrow bandgap, heavy elements, and strong SOC, these three Zintl phases are also projected as candidates TE materials.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Topologically Nontrivial Magnon Bands in Artificial Square Spin Ices with Dzyaloshinskii-Moriya Interaction [Topologically Non-Trivial Magnon Bands in Artificial Square Spin Ices Subject to Dzyaloshinskii-Moriya Interaction]

Systems that exhibit topologically protected edge states are interesting both from a fundamental point of view as well as for potential applications, the latter because of the absence of backscattering and robustness to perturbations. It is desirable to be able to control and manipulate such edge states. Here, we demonstrate using a semi-analytical model that artificial square ices can incorporate both features: an interfacial Dzyaloshinksii-Moriya gives rise to topologically non-trivial magnon bands, and the equilibrium state of the spin ice is reconfigurable with different states having different magnon dispersions and topology. Micromagnetic simulations are used to determine the magnetization equilibrium states and to validate the semi-analytical model. Lastly, our results are amenable to experimental verification via, e.g., lithographic patterning and micro-focused Brillouin light scattering.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Topological crystalline magnets: Symmetry-protected topological phases of fermions

Here, we introduce a novel class of interaction-enabled topological crystalline insulators in two- and three-dimensional electronic systems, which we call “topological crystalline magnet.” It is protected by the product of the time-reversal symmetry T and a mirror symmetry or a rotation symmetry R . A topological crystalline magnet exhibits two intriguing features: (i) it cannot be adiabatically connected to any Slater insulator and (ii) the edge state is robust against coupling electrons to the edge. These features are protected by the anomalous symmetry transformation property ( RT ) 2 = -1 of the edge state. Finally, an anisotropic response to the external magnetic field can be an experimental signature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anomalous helicity-dependent photocurrent in the topological insulator ( Bi 0.5 Sb 0.5 ) 2 Te 3 on a GaAs substrate [Spin manipulation at the interface of a topological insulator and a non-magnetic semiconductor]

We present that the emerging material, topological insulator, has provided new opportunities for spintronic applications, owing to its strong spin-orbit character. Topological insulator based heterostructures that display spin-charge coupling driven by topology at surfaces have great potential for the realization of novel spintronic devices. Here, we report the observation of anomalous photogalvanic effect in (Bi 0.5 Sb 0.5 ) 2 Te 3 thin films grown on GaAs substrate. We demonstrate that the magnitude, direction, and temperature dependence of the helicity-dependent photocurrent (HDPC) can be modulated by the gate voltage. From spatially resolved photocurrent measurements, we show that the line profile of HDPC in (Bi 0.5 Sb 0.5 ) 2 Te 3 /GaAs is unaffected by the variation of beam size, in contrast to the photocurrent response measured in a (Bi 0.5 Sb 0.5 ) 2 Te 3 /mica structure.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Can Topology and Geometry be Measured by an Operator Measurement in Quantum Gravity?

In the context of LLM geometries, we show that superpositions of classical coherent states of trivial topology can give rise to new classical limits where the topology of spacetime has changed. We argue that this phenomenon implies that neither the topology nor the geometry of spacetime can be the result of an operator measurement. We address how to reconcile these statements with the usual semiclassical analysis of low energy effective field theory for gravity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Twisted bilayer graphene: topological pumping and switching of topology by current [Slides]

The presentation was prepared for a BLABS seminar and includes various graphical images. The summary given highlighted: Topological pumping can be triggered by interlayer sliding in TBG due to the nontrivial topology associated with the synthetic dimension; Time-reversal symmetry of TBG can be spontaneously broken by Coulomb interaction, that results in the valley-polarized QAH state at 3/4 filling; A large enough current can pump electrons from the valence band to conductance band, that redistributes the electron occupation; and, the redistribution of electron occupation combined with the Coulomb interaction can switch the valley polarization and topology.

36 MATERIALS SCIENCE↗