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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Evidence for I 2 loss from the perovskite–gas interface upon light-induced halide segregation

Sunlight-induced halide segregation in (CH 3 NH 3 )Pb(Br x I 1−x ) 3 (1 > x > 0.2), which limits obtainable voltages from solar cells with these perovskite absorbers, reverses upon resting in the dark. However, sustained illumination at ca. 1 sun opens a new decomposition pathway, leading to irreversible I 2 loss in an open system. We conclusively show I 2 off-gassing from halide-segregated (CH 3 NH 3 )Pb(Br 0.75 I 0.25 ) 3 by trapping gaseous I 2 and tracking the electronic conductivity of the perovskite, which increases from electron-doping as iodides are oxidized to iodine. Importantly, we show that this reaction occurs across the perovskite-air solid–gas interface, without confounding effects from solvent or reactive solid interfaces. This characterization was conducted under a nitrogen atmosphere, avoiding vacuum- and oxygen-driven I 2 loss pathways. Consistent observations of I 2 loss upon light-soaking CsPb(Br 0.75 I 0.25 ) 3 films show that this reaction is intrinsic to the inorganic framework. We propose that the disruption of iodide-rich domains in the halide-segregated films through I 2 loss can masquerade as a light-induced healing or apparent remixing of the segregated film, when in fact it leads to irreversible decomposition. Although I 2 off-gassing is less likely in bromide-rich solid solutions, light-induced halide segregation brings the iodides into proximity and forms electronic states that are energetically poised to trap and accumulate holes, providing a driving force for I 2 loss. Thus, even bromide-rich mixed-halide perovskite absorbers will benefit from I 2 -impermeable encapsulation for long-term stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Magnetic interactions and excitations in SrMnSb 2

The magnetic interactions in the antiferromagnetic (AFM) Dirac semimetal candidate SrMnSb 2 are investigated using ab initio linear response theory and inelastic neutron scattering (INS). Our calculations reveal that the first two nearest in-plane couplings (J 1 and J 2 ) are both AFM in nature, indicating a significant degree of spin frustration, which aligns with experimental observations. The orbital resolution of exchange interactions shows that J 1 and J 2 are dominated by direct and superexchange, respectively. In a broader context, a rigid-band model suggests that electron doping fills the minority spin channel and results in a decrease in the AFM coupling strength for both J 1 and J 2 . To better compare with INS measurements, we calculate the spin-wave spectra within a linear spin-wave theory, utilizing the computed exchange parameters. Although the calculated spin-wave spectra somewhat overestimate the magnon bandwidth, they exhibit overall good agreement with measurements from INS experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Unlikelihood of a phonon mechanism for the high-temperature superconductivity in La 3 Ni 2 O 7

The discovery of ~80 K superconductivity in nickelate La 3 Ni 2 O 7 under pressure has ignited intense interest. Here, we present a comprehensive first-principles study of the electron-phonon (e-ph) coupling in La 3 Ni 2 O 7 and its implications on the observed superconductivity. Our results conclude that the e-ph coupling is too weak (with a coupling constant λ ≲ 0.5) to account for the high T c , albeit interesting many-electron correlation effects exist. While Coulomb interactions (via GW self-energy and Hubbard U) enhance the e-ph coupling strength, electron doping (oxygen vacancies) introduces no major changes. Additionally, different structural phases display varying characteristics near the Fermi level, but do not alter the conclusion. The e-ph coupling landscape of La 3 Ni 2 O 7 is intrinsically different from that of infinite-layer nickelates. These findings suggest that a phonon-mediated mechanism is unlikely to be responsible for the observed superconductivity in La 3 Ni 2 O 7 , pointing instead to an unconventional nature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ca(Co 1-x Ni x ) 2-y As 2 single crystals

CaCo 2–y As 2 is a unique itinerant system having strong magnetic frustration. Here, we report the effect of electron doping on the physical properties resulting from Ni substitutions for Co. The single crystals of Ca(Co 1–x Ni x ) 2–y As 2 were characterized by single-crystal x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetization M versus temperature T, magnetic field H, time t, and heat capacity C p (H, T) measurements. The A-type antiferromagnetic (AFM) transition temperature T N = 52 K for x = 0 decreases to 22 K with only 3% Ni substitution and is completely suppressed for x > 0.16. For 0.11 ≤ x ≤ 0.52 strong ferromagnetic (FM) fluctuations develop as revealed by magnetic susceptibility χ(T) = M(T) / H measurements. For x = 0.11 and 0.16 competing AFM and FM interactions result in a reentrant spin-glass behavior below T N , as evidenced by the observations of thermomagnetic hysteresis and magnetic relaxation. Enhanced FM fluctuations are also found for the x = 0.21 and 0.31 crystals, where χ c increases significantly at low T. A large χ anisotropy in these compositions where χ c is up to a factor of two larger than χ ab suggests that the FM spin fluctuations are quasi-1D in nature. Weak ferromagnetic contributions to the magnetization are found at T = 2 K for x = 0.11 –0.31. Heat-capacity Cp(T) measurements reveal the presence of FM quantum spin fluctuations for 0.11 ≤ x ≤ 0.52, where a logarithmic T dependence of Cp(T) / T is observed at low T. Here, the suppression of AFM order by the development of strong FM fluctuations in Ca(Co 1–x Ni x ) 2–y As 2 crystals suggests the presence of a FM quantum-critical point at x ≈ 0.20. Our density-functional theory (DFT) calculations confirm that FM fluctuations are enhanced by Ni substitutions for Co in CaCo 2–y As 2 . The Sommerfeld electronic heat-capacity coefficient is enhanced for x = 0, 0.21, and 0.42 by about a factor of two compared to DFT calculations of the density of states (DOS) at the Fermi energy, suggesting an enhancement of the DOS from electron-phonon and/or electron-electron interactions. The crystals with x > 0.52 do not exhibit FM spin fluctuations or magnetic order at T ≥ 1.8 K, which was found from the DFT calculations to arise from a Stoner transition. Superconductivity is not observed above 1.8 K for any of the compositions. Neutron-diffraction studies of crystals with x = 0.11 and 0.16 in the crossover regime (0.1 ≲ x ≲ 0.2) show no evidence of A-type ordering as observed in the parent compound with x = 0. Furthermore, no other common magnetic structures, such as ferromagnetic (FM), helical stacking of in-plane FM layers, or in-plane AFM structure, are found with an ordered moment greater than the uncertainty of 0.05 μ B per transition-metal atom.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Engineering collinear magnetization in hexagonal LuFeO 3 and magnetoelectric control of skyrmions in hexagonal 2D epilayers

Cubic, perovskite-structure ABO 3- and A 1–x A’ x BO 3 -type oxides have been investigated extensively while their hexagonal structure versions have not, even though they are multiferroic and can form heterostructures with hexagonal 2D materials. In particular, multiferroic 2D epilayers may lead to strong magnetoelectric coupling. Hexagonal RFeO 3 ferrites, where R is a rare-earth element (Lu, Yb, etc.), are excellent candidates, but their ferromagnetism isweak. In this paper, we employ density-functional-theory (DFT) calculations and first show that heavy electron doping of hexagonal LuFeO 3 (h-LFO), namely Lu 1–x Hf x FeO 3 (h-LHFO), leads to spin-disproportionation of the Fe sublattices and, especially for x = 1/2 and 2/3, to robust, room-temperature, out-of-plane, collinear ferrimagnetism that is stabilized by a Jahn-Teller metal-to-insulator transition. We then show that h-LHFO/h-2D heterostructures, where h-2D is the FE/ FM monolayer MnSTe, stabilize skyrmions without an external magnetic field and control their chirality by an external electric field through the h-LHFO polarization, opening up a new realm for magnetoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Charge Density Wave and Ferromagnetism in Intercalated CrSBr

In materials with 1D electronic bands, electron–electron interactions can produce intriguing quantum phenomena, including spin-charge separation and charge density waves (CDW). Most of these systems, however, are non-magnetic, motivating a search for anisotropic materials where the coupling of charge and spin may affect emergent quantum states. Here, in this study, chemical intercalation of the van der Waals magnetic semiconductor CrSBr yields Li 0.17(2) (tetrahydrofuran) 0.26(3) CrSBr, which possesses an electronically driven quasi-1D CDW with an onset temperature above room temperature. Concurrently, electron doping increases the magnetic ordering temperature from 132 to 200 K and switches its interlayer magnetic coupling from antiferromagnetic to ferromagnetic. The spin-polarized nature of the anisotropic bands that give rise to this CDW enforces an intrinsic coupling of charge and spin. The coexistence and interplay of ferromagnetism and charge modulation in this exfoliatable material provide a promising platform for studying tunable quantum phenomena across a range of temperatures and thicknesses.

2D magnets↗
DOE OSTI2025

Unusual Ferromagnetic Band Evolution and High Curie Temperature in Monolayer 1T‐CrTe2 on Bilayer Graphene

2D van der Waals ferromagnets hold immense promise for spintronic applications due to their controllability and versatility. Despite their significance, the realization and in-depth characterization of ferromagnetic materials in atomically thin single layers, close to the true 2D limit, has been scarce. Here, a successful synthesis of monolayer (ML) 1T-CrTe2 is reported on a bilayer graphene (BLG) substrate via molecular beam epitaxy. Using angle-resolved photoemission spectroscopy and magneto-optical Kerr effect measurements, that the ferromagnetic transition is observed at the Curie temperature (TC) of 150 K in ML 1T-CrTe2 on BLG, accompanied by unconventional temperature-dependent band evolutions. The spectroscopic analysis and first-principle calculations reveal that the ferromagnetism may arise from Goodenough-Kanamori super-exchange and double-exchange interactions, enhanced by the lattice distortion and the electron doping from the BLG substrate. These findings provide pivotal insight into the fundamental understanding of mechanisms governing 2D ferromagnetism and offer a pathway for engineering higher TC in 2D materials for future spintronic devices.

Park, Kyoungree↗
DOE OSTI2020

High-Quality SnSe 2 Single Crystals: Electronic and Thermoelectric Properties

In this work, high-quality SnSe 2 single crystals were successfully synthesized using a temperature gradient method. N-type characteristics and strong anisotropic transport properties of SnSe 2 single crystals were exhibited between the ab plane and the c-axis. At 673 K, the power factor (PF) value along the ab plane is 3.43 μW cm -1 K -2 , while it is 0.92 μW cm -1 K -2 along the c-axis. The ratio between thermal conductivities along the ab plane (κ ab ) and c-axis (κ c ) is on the order of 7.6 at 300 K, while this value is about 5.6 at 673 K. The thermoelectric figure of merit (ZT) in the c-axis (0.15) is higher than that (0.1) along the ab plane, according to the ultralow out-of-plane thermal conductivity. The electronic band structure results, which were examined by angle-resolved photoemission spectroscopy (ARPES) predicted the potential of improving the thermoelectric performance of SnSe 2 single crystals by electron doping.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Achieving High-Yield Conversion of Janus Transition Metal Dichalcogenides on Diverse Substrates

Janus transition metal dichalcogenides (TMDCs) with intrinsic broken mirror symmetry and vertical dipole moment provide an additional degree of freedom to manipulate material symmetry down to atomic-layer thickness. However, despite advances in synthesis strategies, fundamental understanding of this atomic substitution process remains limited, which has impeded their implementation in advanced devices. Here, by using a room-temperature atomic-layer substitution (RT-ALS) strategy, we systematically investigate the critical factors facilitating the high-yield conversion of Janus TMDCs on diverse substrates. Combining Raman spectroscopy probes, X-ray photoelectron spectroscopy (XPS) measurements, and density functional theory (DFT) calculations, we demonstrate that substrates with enhanced electron doping or larger surface polarity substantially benefit the conversion of Janus TMDCs reaching a near-unity yield. Intriguingly, the strong affinity between Janus TMDCs and substrates (e.g., Au) brings about abnormal Raman spectroscopic phenomena. These findings highlight the significance of substrates in achieving the reliable synthesis of Janus two-dimensional materials with improved homogeneity on various substrates. In addition, this takes us one step closer to utilizing Janus TMDCs as a versatile platform in next-generation optoelectronic devices, sensors, and quantum technologies.

Doping↗
DOE OSTI2021

Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe 2 . These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites' photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Quadrupling the depairing current density in the iron-based superconductor SmFeAsO1–xHx

Abstract Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densities J c . The typical approach to increasing J c is to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximum J c of only ∼30% of the depairing current density J d , which depends on the coherence length and the penetration depth. Here we dramatically boost J c in SmFeAsO 1– x H x films using a thermodynamic approach aimed at increasing J d and incorporating vortex pinning centres. Specifically, we reduce the penetration depth, coherence length and critical field anisotropy by increasing the carrier density through high electron doping using H substitution. Remarkably, the quadrupled J d reaches 415 MA cm –2 , a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high J c values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Signatures of fractional quantum anomalous Hall states in twisted MoTe 2

The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) state, which exhibits an integer quantum Hall effect at zero magnetic field owing to intrinsic ferromagnetism. In the presence of strong electron–electron interactions, fractional QAH (FQAH) states at zero magnetic field can emerge. These states could host fractional excitations, including non-Abelian anyons—crucial building blocks for topological quantum computation9. Here we report experimental signatures of FQAH states in a twisted molybdenum ditelluride (MoTe 2 ) bilayer. Magnetic circular dichroism measurements reveal robust ferromagnetic states at fractionally hole-filled moiré minibands. Using trion photoluminescence as a sensor, we obtain a Landau fan diagram showing linear shifts in carrier densities corresponding to filling factor v = −2/3 and v = −3/5 ferromagnetic states with applied magnetic field. These shifts match the Streda formula dispersion of FQAH states with fractionally quantized Hall conductance of ${\sigma }_{xy}=-\,\frac{2}{3}\frac{{e}^{2}}{h}$ and ${\sigma }_{xy}=-\,\frac{3}{5}\frac{{e}^{2}}{h}$ , respectively. Moreover, the v = −1 state exhibits a dispersion corresponding to Chern number −1, consistent with the predicted QAH state. In comparison, several non-ferromagnetic states on the electron-doping side do not disperse, that is, they are trivial correlated insulators. The observed topological states can be electrically driven into topologically trivial states. Our findings provide evidence of the long-sought FQAH states, demonstrating MoTe 2 moiré superlattices as a platform for exploring fractional excitations.

Quantum Hall↗
DOE OSTI2024

Interlayer engineering of Fe 3 GeTe 2 : From 3D superlattice to 2D monolayer

The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546 , 265–269 (2017) and B. Huang et al., Nature 546 , 270–273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14 , 408–419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe 3 GeTe 2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature ( T C ), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Correlated topological flat bands in rhombohedral graphite

Flat bands and nontrivial topological physics are two important topics of condensed matter physics. With a unique stacking configuration analogous to the Su–Schrieffer–Heeger model, rhombohedral graphite (RG) is a potential candidate for realizing both flat bands and nontrivial topological physics. Here, in this study, we report experimental evidence of topological flat bands (TFBs) on the surface of bulk RG, which are topologically protected by bulk helical Dirac nodal lines via the bulk-boundary correspondence. Moreover, upon in situ electron doping, the surface TFBs show a splitting with exotic doping evolution, with an order-of-magnitude increase in the bandwidth of the lower split band, and pinning of the upper band near the Fermi level. These experimental observations together with Hartree–Fock calculations suggest that correlation effects are important in this system. Our results demonstrate RG as a platform for investigating the rich interplay between nontrivial band topology, correlation effects, and interaction-driven symmetry-broken states.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Flat-band tuning and emergent itinerant magnetism in Sr(Co 1− x Pd x ) 2 As 2

The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi energy (E F ). Here, in this study, we investigate this phenomenon in the Pauli paramagnet SrCo 2 As 2 , where an FB associated with Co e g orbitals lies close to E F . Remarkably, a minute substitution of the nonmagnetic element Pd onto the Co site (~2%) induces antiferromagnetic order with a transition temperature as high as T N = 25 K. Temperature- and magnetic-field-dependent magnetic and transport measurements, complemented by zero-field neutron diffraction, reveal a helical magnetic order for x ≤ 0.10 in Sr(Co 1−x Pd x ) 2 As 2 , transitioning to a complex ferromagnetic state at higher Pd concentrations. Spectroscopic evidence and theoretical band structure calculations demonstrate that electron doping shifts the flat band closer to E F , significantly enhancing the Stoner parameter. This enhancement drives a strong ferromagnetic instability, leading to helical magnetic ordering dominated by in-plane ferromagnetic interactions. The emergence of robust magnetic ordering through substitution with nonmagnetic elements is a unique phenomenon that underscores the pivotal role of flat-band instability in tuning magnetism in itinerant systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Superconducting properties of MoTe 2 from ab initio anisotropic Migdal-Eliashberg theory

Molybdenum ditelluride (MoTe 2 ) is attracting considerable interest since it is the archetypal type-II Weyl semimetal and a candidate for topological superconductivity. In this work, we investigate the superconducting phase diagram of two MoTe 2 polymorphs using the ab initio anisotropic Migdal-Eliashberg theory, and we show that the superconducting dome originates from the synergistic contribution of the density of states at the Fermi level and the transverse acoustic Te modes in the 1$\textit{T}’$ phase. We find that the electron and hole pockets carry trivial $\textit{s}$-wave order parameters of slightly different magnitude, reminiscent of a two-gap structure as suggested by recent experiments. We suggest that a possible route for enhancing the superconducting critical temperature, and realizing $s_{+ –}$ pairing, in the $T_d$ phase is to exploit its nontrivial band topology via electron doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Realizing gapped surface states in the magnetic topological insulator Mn Bi 2 - x Sb x Te 4

The interplay between magnetism and nontrivial topology in magnetic topological insulators (MTIs) is expected to give rise to exotic topological quantum phenomena like the quantum anomalous Hall effect and the topological axion states. A key to assessing these novel properties is to realize gapped topological surface sates. MnBi 2 Te 4 possesses nontrivial band topology with an intrinsic antiferromagnetic state. However, the highly electron-doped nature of the MnBi 2 Te 4 crystals obstructs the exhibition of the surface band gap. Here, we tailor the material through Sb substitution to reveal the gapped surface states in MnBi 2-x Sb x Te 4 . By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the Néel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. Finally, the realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Superconductivity in the bilayer Hubbard model: Two Fermi surfaces are better than one

Fully occupied or unoccupied bands in a solid are often considered inert and irrelevant to a material's low-energy properties. But the discovery of enhanced superconductivity in heavily electron-doped FeSe-derived superconductors poses questions about the possible role of incipient bands (those laying close to but not crossing the Fermi level) in pairing. To answer this question, researchers have studied pairing correlations in the bilayer Hubbard model, which has an incipient band for large interlayer hopping t⊥, using many-body perturbation theory and variational methods. They have generally found that superconductivity is enhanced as one of the bands approaches the Lifshitz transition and even when it becomes incipient. Here we address this question using the nonperturbative quantum Monte Carlo (QMC) dynamical cluster approximation (DCA) to study the bilayer Hubbard model's pairing correlations. We find that the model has robust s± pairing correlations in the large t⊥ limit, which can become stronger as one band is made incipient. While this behavior is linked to changes in the effective interaction, we further find that it is counteracted by a suppression of the intrinsic pair-field susceptibility and does not translate to an increased Tc. Our results demonstrate that the highest achievable transition temperatures in the bilayer Hubbard model occur when the system has two bands crossing the Fermi level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗