Search NASA⌕ Search

SEARCH · Search NASA

Results for “Photoemission spectroscopy”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Electronic and vibrational properties of bulk Cr 2 Ge 2 Te 6 from first-principles calculations

Here we use density functional theory based ab initio calculations to investigate the structural, vibrational, magnetic, and electronic properties of the layered ferromagnet Cr 2 Ge 2 Te 6 (CGT) that has attracted attention for potential spintronic applications. We optimize the structure for a fixed c/a ratio. Our results are in excellent agreement with experimental data on structure, phonons, and electronic properties. The use of van der Waals interactions and relativistic spin-orbit coupling yields accurate lattice constants and interlayer distances. Simultaneously, the adopted theoretical methods lead to a rigorous description of the vibrational normal modes, as well as the valence bands, that are in excellent agreement with Raman spectroscopy and angle-resolved photoemission spectroscopy spectra, respectively. The magnetic moment is slightly overestimated, and the magnetic anisotropy has the correct sign but is greater in magnitude than that in experiment. We find that inclusion of the on-site Coulomb repulsion on Cr−3d orbitals worsens agreement with experiment, in contrast to previous studies. Our results provide a path toward ab initio analysis of magnetic heterostructures based on CGT.

36 MATERIALS SCIENCE↗
DOE OSTI2023

EuCd 2 ⁢As 2 : A Magnetic Semiconductor

EuCd 2 ⁢As 2 is now widely accepted as a topological semimetal in which a Weyl phase is induced by an external magnetic field. Here, we challenge this view through firm experimental evidence using a combination of electronic transport, optical spectroscopy, and excited-state photoemission spectroscopy. We show that the EuCd 2 ⁢As 2 is in fact a semiconductor with a gap of 0.77 eV. We show that the externally applied magnetic field has a profound impact on the electronic band structure of this system. This is manifested by a huge decrease of the observed band gap, as large as 125 meV at 2 T, and, consequently, by a giant redshift of the interband absorption edge. However, the semiconductor nature of the material remains preserved. EuCd 2 ⁢As 2 is therefore a magnetic semiconductor rather than a Dirac or Weyl semimetal, as suggested by ab initio computations carried out within the local spin-density approximation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrathin epitaxial MgB 2 on SiC: Substrate surface-polarity-dependent properties

High-quality, ultrathin superconducting films are required for advanced devices such as hot-electron bolometers, superconducting nanowire single-photon detectors, and quantum applications. Using hybrid physical-chemical vapor deposition, we show that MgB 2 films as thin as 4 nm can be fabricated on the carbon-terminated 6⁢H-SiC (0001) surface with a superconducting transition temperature above 33 K and a rms roughness of 0.7 nm. Remarkably, the film quality is a function of the SiC surface termination, with the C-terminated surface preferred to the Si-terminated surface. To understand the MgB 2 thin film/SiC substrate interactions giving rise to this difference, we characterized the interfacial structures using Rutherford backscattering spectroscopy/channeling, electron-energy-loss spectroscopy, and x-ray photoemission spectroscopy. The MgB 2 /SiC interface structure is complex and different for the two terminations. Both terminations incorporate substantial unintentional oxide layers influencing MgB 2 growth and morphology, but with a different extent, intermixing, and interface chemistry. In this paper, we report measurements of transport, resistivity, and the critical superconducting temperature of MgB 2 /SiC that are different for the two terminations, and they link interfacial structure variations to observed differences. The result shows that the C face of SiC is a preferred substrate for the deposition of ultrathin superconducting MgB 2 films.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Exploring Nontrivial Topological Superconductivity in 2M-WS2 for Topological Quantum Computation

This project has two main research goals: (1) growing the high-quality two-dimensional (2D) 2M-pahse WS 2 (2M-WS 2 ) single crystals and identifying clear signatures of the unconventional superconductivity in the 2M-WS 2 ; and (2) establishing the layer-dependence of the Majorana zero mode in the 2M-WS 2 down to the monoatomic layer limit. These goals were planned to be achieved by growing high-quality and large-scale 2M-WS 2 single crystals and transferring their thin layers onto different substrates for the proposed measurements. The layer-dependent unconventional superconductivity in 2M-WS 2 were systematically studied by different techniques, including transport measurements (charge, thermal and spin), scanning tunneling microscopy and spectroscopy (STM/S), angle-resolved photoemission spectroscopy (ARPES), and theoretical calculations. The research team is comprised of researchers from University of Wyoming (UW) and three DOE National Laboratories (DOE NLs), including Argonne National Laboratory (ANL), Lawrence Berkeley National Laboratory (LBNL) and Sandia National Laboratories (SNL), with complete and complementary expertise: PI Tian: Handling 2D materials, nanofabrication, nanodevices, and quantum transport; Co-Is: Ackerman and Leonard: van der Waals material crystal growth and handling; Chien: Nanoimaging with STM/S; and Tang: Magnetic measurements and charge and thermal transport; National lab collaborators (NLs): Guisinger (ANL): STM/S and nanoimaging; Mo and Rotenberg (LBNL): ARPES and nano ARPES (nARPES); Lu (SNL): Quantum information science, quantum transport, and nanofabrication; and Baczewski (SNL): Theoretical modeling and calculations.

36 MATERIALS SCIENCE↗
NASA NTRS1985

Size and shape dependence of CO adsorption sites on sapphire supported Fe microcrystals

The surface structure and stoichiometry of alumina substrates, as well as the size, growth characteristics, and shape of Fe deposits on sapphire substrates have been investigated by low energy electron diffraction (LEED), Auger electron spectroscopy, electron energy loss spectroscopy, and X-ray photoemission spectroscopy (XPS), as well as work function measurements, in conjunction with transition electron microscopy observations. The substrates used in this work were the following: (1) new, clean Al2O3; (2) same surface amorphized by Ar ion bombardment; (3) same surface regenerated by 650 C annealing; (4) amorphous alumina films on Ta slab; and (5) polycrystal alumina films, obtained by heating amorphous films to 600 C. Substrate cleaning was found to be most effective in producing a reproducible surface upon oxygen RF plasma treatment. The Fe nucleation and growth process was found to depend strongly on the type of substrate surface and deposition conditions. Ar ion bombardment under beam flooding, and subsequent annealing at 650 C was found an effective means to restore the original Al2O3 (1102) surface for renewed Fe deposition.

Papageorgopoulos, C.↗
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 OSTI2026

Ohmic contact formation at the Au/Sr 3 Ir 2 O 7 interface

Here, using X-ray photoelectron spectroscopy (XPS), we show that Au forms an Ohmic contact with the surface of the narrow-bandgap Mott insulator Sr 3 Ir 2 O 7 thin films. XPS reveals no significant binding energy shift with increasing Au overlayer thickness on epitaxial Sr 3 Ir 2 O 7 (001) thin films, consistent with the linear current-voltage (I-V) behavior and indicative of negligible band bending. The absence of Schottky barrier formation at the Au/Sr 3 Ir 2 O 7 interface corroborates the metallic surface character of the Sr 3 Ir 2 O 7 thin film, as observed in angle-resolved photoemission spectroscopy (ARPES) studies, suggesting that the interface behaves effectively as a metal-on-metal contact. Furthermore, XPS points to the presence of distinct surface/interface and bulk electronic states for Sr 3 Ir 2 O 7 . This study provides critical material information for designing iridate-based electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Competition between magnetic order and charge localization in Na 2 IrO 3 thin crystal devices thin crystal devices

Spin orbit assisted Mott insulators such as sodium iridate (Na 2 IrO 3 ) have been an important subject of study in recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects, and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of Na 2 IrO 3 has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic transport measurements supported by angle-resolved photoemission spectroscopy (ARPES) experiments, that electronic transport in Na 2 IrO 3 is ruled by variable range hopping and it is strongly dependent on the magnetic ordering transition known for bulk Na 2 IrO 3 , as well as on external electric fields. Furthermore, electronic transport measurements allow us to deduce a value for the localization length and the density of states in our Na 2 IrO 3 thin crystal devices, and offer an alternative approach to study insulating 2D-materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Spin-Dependent $\pi$$\pi$* Gap in Graphene on a Magnetic Substrate

Here, we present a detailed analysis of the electronic properties of graphene/Eu/Ni(111). By using angle- and spin-resolved photoemission spectroscopy and ab initio calculations, we show that the intercalation of Eu in the graphene/Ni(111) interface gives rise to a gapped freestanding dispersion of the $\pi$$\pi$* Dirac cones at the K point with an additional lifting of the spin degeneracy due to the mixing of graphene and Eu states. The interaction with the magnetic substrate results in a large spin-dependent gap in the Dirac cones with a topological nature characterized by a large Berry curvature and a spin-polarized Van Hove singularity, whose closeness to the Fermi level gives rise to a polaronic band.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Intercalation induced quasi-freestanding layer in TiSe 2

Angle-resolved photoemission spectroscopy is employed to study the electronic structure of bulk TiSe 2 before and after doping with potassium impurities. A splitting in the conduction band into two branches is observed after room-temperature deposition. The splitting energy increases to approximately 130 meV when the sample is cooled to 40 K. One branch exhibits a nondispersive two-dimensional feature, while the other shows the characteristics of three-dimensional bulk band dispersion. Core-level spectroscopy suggests that the K impurities predominantly occupy the intercalated sites within the van der Waals gap. Furthermore, the results indicate the formation of a quasi-freestanding TiSe 2 layer. Additionally, doping completely suppresses the periodic lattice distortion in the surface region. These findings are further supported by density functional theory calculations, which compare the band structure of monolayer and bulk TiSe 2 with experimental data. Thus, the dimensional and intrinsic electronic properties of 1⁢𝑇−TiSe 2 can be controlled through the intercalation procedure used in this work.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Constraints on the two-dimensional pseudospin-$\frac{1}{2}$Mott insulator description of $\mathrm{Sr_2IrO_4}$

Sr 2 IrO 4 has often been described via a simple, one-band pseudospin-$\frac{1}{2}$ model subject to electron-electron interactions on a square lattice, fostering analogies with cuprate superconductors believed to be well described by a similar model. In this work we argue - based on a detailed study of the low-energy electronic structure by circularly polarized spin and angle-resolved photoemission spectroscopy combined with dynamical mean-field theory calculations - that a pseudospin-$\frac{1}{2}$ model fails to capture the full complexity of the system. We show instead that a realistic multiband Hubbard Hamiltonian, accounting for the full correlated t 2g manifold, provides a detailed description of the interplay between spin-orbital entanglement and electron-electron interactions and yields quantitative agreement with experiments. Our analysis establishes that the j 3/2 states make up a substantial percentage of the low-energy spectral weight, i.e., approximately 74% as determined from the integration of the j-resolved spectral function in the 0 to -1.64eV energy range. Therefore, the results in our work are of relevance not only to Ir-based materials but also more generally to multiorbital materials with closely spaced energy scales.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. In this work, we combine equilibrium and time-resolved ARPES to track the temperature-dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and bi-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Two-step electronic response to magnetic ordering in a van der Waals ferromagnet

The two-dimensional material Cr 2 ⁢Ge 2 ⁢Te 6 is a member of the class of insulating van der Waals (vdW) magnets. Here, using high resolution angle-resolved photoemission spectroscopy in a detailed temperature dependence study, we identify a clear response of the electronic structure to a dimensional crossover in the form of two distinct temperature scales marking onsets of modifications in the electronic structure. Specifically, we observe Te 𝑝-orbital-dominated bands to undergo changes at the Curie transition temperature 𝑇 𝐶 while the Cr 𝑑-orbital-dominated bands begin evolving at a higher temperature scale. Combined with neutron scattering, density functional theory calculations, and Monte Carlo simulations, we find that the electronic system can be consistently understood to respond sequentially to the distinct temperatures at which in-plane and out-of-plane spin correlations exceed a characteristic length scale. Finally, our findings reveal the sensitivity of the orbital-selective electronic structure for probing the dynamical evolution of local moment correlations in vdW insulating magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Nonmonotonic Band Flattening near the Magic Angle of Twisted Bilayer MoTe 2

Twisted bilayer MoTe 2 (tMoTe 2 ) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy measurements of tMoTe 2 , revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the 𝐾 point exhibits a nonmonotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe 2 as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.

2D materials↗
DOE OSTI2026

Evolution of Electronic Properties of Graphene Nanoribbons with Progressive Carving: From Straight to Porous to Chevron Ribbons

Graphene nanoribbons (GNRs) are highly versatile materials due to their unique electronic, magnetic, and optical properties, which can be precisely tuned by controlling their width, edge structure, and topology. Here, we report the on-surface synthesis and characterization of a straight N = 15 armchair GNR with periodic annulene nanopores (15-pGNR). It serves as a structural link between two well-established GNRs: the pristine N = 15 armchair GNR without pores (15-AGNR) and the chevron GNR (cGNR). With the addition of the 15-pGNR reported in this study, these three GNRs form a rare experimentally accessible series of ribbons, in which the evolution of electronic properties can be tracked upon progressive carving of a basic 15-AGNR: first, by creating periodic nanopores to form 15-pGNR and then by extending the pore area and producing meandering cGNR. We have designed a molecular precursor for the 15-pGNR and grown the nanoribbons on single-crystal gold substrates by on-surface synthesis in ultra-high vacuum (UHV) conditions. The atomically precise structure of 15-pGNR was confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). The band gap of 15-pGNR was studied by scanning tunneling spectroscopy (STS) and dI/dV mapping, and the occupied electronic levels were investigated by angle-resolved photoemission spectroscopy (ARPES). A theoretical and experimental comparison of 15-pGNRs, 15-AGNRs, and cGNRs demonstrates that the introduction of periodic nanopores into 15-AGNR leads to a more than 2-fold increase in its band gap. In contrast, the band gaps of 15-pGNR and cGNR differ only by about 15%. Such band gap increase can be qualitatively understood to arise from two combined effects, the periodic perforation of the graphene lattice and the confinement effect induced by the GNR width.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Polarization dependent photoemission as a probe of the magnetic ground state in the van der Waals ferromagnet VI 3

Van der Waals ferromagnets are thrilling materials from both a fundamental and technological point of view. VI 3 is an interesting example, with a complex magnetism that differentiates it from the first reported Cr based layered ferromagnets. In this work, we show in an indirect way through angle resolved photoemission spectroscopy experiments, the importance of spin–orbit coupling setting the electronic properties of this material. Our light polarized photoemission measurements point to a ground state with a half-filled $\mathcal{e}_{±}^{'}$ doublet, where a gap opening is triggered by spin–orbit coupling enhanced by electronic correlations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Chapter 12: Photoelectron Spectroscopy Methods in Solar Cell Research

Photoelectron spectroscopy (PES), also referred to as photoemission spectroscopy, is a direct experimental method for assessing the chemical and electronic properties of materials. The technique is becoming increasingly important in the research of photovoltaic (PV) devices--where, more specifically, X-ray photoelectron spectroscopy (XPS) is used primarily to measure the chemical properties such as composition and contamination of solar cell materials, whereas ultraviolet photoelectron spectroscopy (UPS) reveals key electronic properties such as work function and electronic energy-level positions. PES is a surface-sensitive technique ideally suited for the analysis of thin films and interfaces, either completed ones or during their formation process. Because the new generation of PV devices comprise a multitude of complex interfaces--each of which plays a critical role for performance and functionality--PES analysis of functional cell components has gained even more relevance.

photoelectron spectroscopy↗