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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Coexistence of Bulk-Nodal and Surface-Nodeless Cooper Pairings in a Superconducting Dirac Semimetal

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below $T_{c}$ ~ 4.5 K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc. In conclusion, these observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electronic and elemental properties of the Cu{sub 2}ZnSn(S,Se){sub 4} surface and grain boundaries

X-ray and femtosecond UV photoelectron spectroscopy, secondary ion mass spectrometry and photoluminescence imaging were used to investigate the electronic and elemental properties of the CZTS,Se surface and its oxides. Oxide removal reveals a very Cu poor and Zn rich surface relative to bulk composition. O and Na are observed at the surface and throughout the bulk. Upward bending of the valence bands indicates the presence of negative charge in the surface region and the Fermi level is found near the band gap center. The presence of point defects and the impact of these findings on grain boundary properties will be described.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy

The electronic structure of GaN(1–100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1 eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3 eV, i.e., below the bulk band gap. These results indicate that the GaN(1–100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Schottky barrier heights at the interfaces between pure-phase InAs nanowires and metal contacts

Understanding of the Schottky barriers formed at metal contact-InAs nanowire interfaces is of great importance for the development of high-performance InAs nanowire nanoelectronic and quantum devices. Here, we report a systematical study of InAs nanowire field-effect transistors (FETs) and the Schottky barrier heights formed at the contact-nanowire interfaces. The InAs nanowires employed are grown by molecular beam epitaxy and are high material quality single crystals, and the devices are made by directly contacting the nanowires with a series of metals of different work functions. The fabricated InAs nanowire FET devices are characterized by electrical measurements at different temperatures and the Schottky barrier heights are extracted from the measured temperature and gate-voltage dependences of the channel current. We show that although the work functions of the contact metals are widely spread, the Schottky barrier heights are determined to be distributed over 35–55 meV, showing a weak but not negligible dependence on the metals. The deduced Fermi level in the InAs nanowire channels is found to be in the band gap and very close to the conduction band. The physical origin of the results is discussed in terms of Fermi level pinning by the surface states of the InAs nanowires and a shift in pinned Fermi level induced by the metal-related interface states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

The Role of Metal-Semiconductor Interface in Hybrid Perovskite Devices for High-Performance Radiation Photon Counting

Halide perovskites are promising optoelectronic semiconductors. For applications in solid-state detectors that operate in low photon flux counting mode, blocking interfaces are essential to minimize the dark current noise. Here, we investigate the interface between methylammonium lead tri-iodide (MAPbI 3 ) single crystals and commonly used high and low work function metals to achieve photon counting capabilities in a solid-state detector. Using scanning photocurrent microscopy, we observe a large Schottky barrier at the MAPbI 3 /Pb interface, which efficiently blocks dark current. Moreover, the shape of the photocurrent profile indicates that the MAPbI 3 single-crystal surface has a deep fermi level close to that of Au. Rationalized by first-principle calculations, we attribute this observation to the defects due to excess iodine on the surface underpinning emergence of deep band-edge states. The photocurrent decay profile yields a charge carrier diffusion length of 10–25 μm. Using this knowledge, we demonstrate a single-crystal MAPbI 3 detector that can count single γ-ray photons by producing sharp electrical pulses with a fast rise time of <2 μs. Overall, our study indicates that the interface plays a crucial role in solid-state detectors operating in photon counting mode.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Gapless Dirac surface states in the antiferromagnetic topological insulator MnBi 2 Te 4

We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic properties of MnBi 2 Te 4 , a material that was predicted to be an intrinsic antiferromagnetic (AFM) topological insulator. In striking contrast to earlier literature showing a full gap opening between two surface band manifolds on the (0001) surface, we observed a gapless Dirac surface state with a Dirac point sitting at E B = -280 meV . Furthermore, our ARPES data revealed the existence of a second Dirac cone sitting closer to the Fermi level. Surprisingly, these surface states remain intact across the AFM transition. The presence of gapless Dirac states in this material may be caused by different ordering at the surface from the bulk or weaker magnetic coupling between the bulk and surface. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering most likely due to weak coupling to magnetism, we did observe a splitting of the bulk band accompanying the AFM transition. Finally, with a moderately high ordering temperature and interesting gapless Dirac surface states, MnBi 2 Te 4 provides a unique platform for studying the interplay between magnetic ordering and topology.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Orbital control of Rashba spin orbit coupling in noble metal surfaces

Rashba spin orbit coupling (SOC) in noble metal surfaces is of great importance for the application of metal films in spintronic devices. By combining the density-functional theory calculations with our recently developed orbital selective external potential method, we investigate the Rashba SOC in the Shockley surface states of Au(111) and Ag(111). We find that the large Rashba SOC in the sp-character surface states of Au(111) is mainly contributed by the minor d-orbitals in the surface states. While for the sd-character surface states, although they are dominated by the d-orbitals, Rashba splitting is found to be rather small. Band structure analysis reveals that this is mainly because the sd-character surface states are well below the Fermi level and can be less influenced by the asymmetric surface potential. We demonstrate that the Rashba SOC in noble metal surfaces can be effectively manipulated by shifting the d-orbitals in the surface states, which can be physically implemented through surface decoration. Our investigation provides a deep understanding on Rashba SOC in noble metal surfaces and could be helpful to their applications in spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Secondary particle emission from a ferromagnetic binary compound

Secondary ion emission from an ion-bombarded binary compound in the ferro- and paramagnetic states has been studied using experimental methods and molecular dynamics simulations. The experiments were performed with a widely used NiPd binary compound, which was bombarded by obliquely incident 10-keV Ar ions. It is established that the intensity of Ni{sup +} and Pd{sup +} ion emission from a polycrystalline NiPd sample decreases significantly when it passes from the ferromagnetic to paramagnetic state. This effect is explained by a change in the surface binding energy and density of surface states at the Fermi level and by a cumulative process related to sputtering. The energies and directions of emission of secondary particles from poly- and single-crystalline NiPd samples in the ferromagnetic state have been jointly studied. It is established that the maximum of the polar angular distribution of secondary particles deviates toward the normal to the irradiated surface with a decrease in their energy. A difference in the azimuthal distribution of emitted Ni and Pd particles has been observed during the bombardment of a (001)NiPd crystal face and explained by specific features of correlated collisions. The best agreement of experimental data with the results of molecular dynamics simulations is obtained if the calculations are performed with allowance for an experimentally established modified composition of the three uppermost surface layers [39].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Key role of antibonding electron transfer in bonding on solid surfaces

The description of the chemical bond between a solid surface and an atom or a molecule is the fundamental basis for understanding surface reactivity and catalysis. Despite considerable research efforts, the physics that rules the strength of such chemical bonds remains elusive, especially on semiconductor surfaces. Widespread understandings are mostly based on the degree of filling of antibonding surface-adsorbate states that weaken the surface adsorption. The unoccupied antibonding surface-adsorbate states are often considered to have no effects on surface bonding. Here in this paper, we show that the energy levels of unoccupied antibonding surface-adsorbate states relative to the Fermi-level play a critical role in determining the trends in variations of surface adsorption energies. The electrons that would occupy those high-energy antibonding states are transferred to the Fermi-level, leading to an energy gain that largely controls surface bonding. To illustrate this picture, as a validating case, we study the hydrogen evolution reaction (HER) catalyzed by MoS 2 from density functional theory calculations. We find that the majority of antibonding surface-hydrogen states are positioned well above the Fermi-energy. A clear linear relationship between the energy gain from antibonding electron transfer and the adsorption energy is identified for hydrogen binds to either molybdenum or sulfur atoms at different sites. The antibonding-electron transfer energy can thus serve as a primary catalytic activity descriptor. The emerging picture identifies the origin of HER on MoS 2 , which is related to the empty in-gap states induced by sulfur vacancies or edges. Under this picture, the effects of surface inhomogeneity (e.g., defects, step edges) on surface bonding strength can be understood. This antibonding electron transfer picture also offers a physically different explanation for the well-known d-band theory for hydrogen adsorption on transition metal surfaces. The results provide guidelines for understanding and optimizing catalyst performance and designing new solid catalysts.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Unconventional surface state pairs in a high-symmetry lattice with anti-ferromagnetic band-folding

Many complex magnetic structures in a high-symmetry lattice can arise from a superposition of well-defined magnetic wave vectors. These “multi-q” structures have garnered much attention because of interesting real-space spin textures such as skyrmions. However, the role multi-q structures play in the topology of electronic bands in momentum space has remained rather elusive. Here we show that the type-I anti-ferromagnetic 1q, 2q and 3q structures in an face-centered cubic sublattice with band inversion, such as NdBi, can induce unconventional surface state pairs inside the band-folding hybridization bulk gap. Our density functional theory calculations match well with the recent experimental observation of unconventional surface states with hole Fermi arc-like features and electron pockets below the Neel temperature. We further show that these multi-q structures have Dirac and Weyl nodes. Our work reveals the special role that band-folding from anti-ferromagnetism and multi-q structures can play in developing new types of surface states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Exploration of two surfaces observed in Weyl semimetal BaMnSb 2

Single crystalline BaMnSb 2 is considered as a 3D Weyl semimetal with the 2D electronic structure containing Dirac cones from the Sb sheet. We report experimental investigation of low-temperature cleaved BaMnSb 2 surfaces using scanning tunneling microscopy/spectroscopy and low energy electron diffraction. By natural cleavage, we find two terminations: one is Ba (above the orthorhombically distorted Sb sheet) and another Sb2 (at the surface of the Sb/Mn/Sb sandwich layer). Both terminations show the 2×1 surface reconstructions, with drastically different morphologies and electronic properties, however. The reconstructed structures, defect types and nature of the electronic structures of the two terminations are extensively studied. The quasiparticle interference (QPI) analysis is conducted at the energy range between -2 V and 2 V, although no interesting states are observed near the Fermi level, the surface-projected electronic band structures strongly depend on the surface termination above 1.6 V. The existence of defects can greatly modify the local density of states to create electronic phase separations on the surface in the order of tens of nm scale. Our observation on the atomic structures of the terminations and the corresponding electronic structures provides critical information towards an understanding of topological properties of BaMnSb 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2019

Control of MXenes’ electronic properties through termination and intercalation

MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes’ electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes’ electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thermoelectric properties of topological insulator Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3}, and Bi{sub 2}Se{sub 3} thin film quantum wells

The thermoelectric (TE) figure of merit ZT of topological insulator Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3}, and Bi{sub 2}Se{sub 3} thin film quantum wells is calculated for thicknesses below 10 nm, for which hybridization of the surface states as well as quantum confinement in the bulk are individually predicted to enhance ZT. Here, the question is addressed what ZT can be expected from coexisting surface and bulk states in such quantum wells. It is demonstrated that the parallel contributing bulk and surface channels tend to cancel each other out. This is because the surface-to-volume ratios of the thin films prevent the domination of transport through a single channel and because the individual bulk and surface ZTs are optimized at different Fermi levels.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

MLCT excited states at interfaces and in rigid media: Applications in energy conversion

In this proposal for continued support from the DOE, we propose to build on our understanding of light-driven electron transfer by molecules and molecular assemblies on oxide surfaces. We will conduct a systematic investigation of injection and back electron transfer dynamics on the surfaces of thin (1-10 μm), mesoporous, transparent conducting oxide films consisting of 10-20 nm tin-doped indium oxide (Sn:In2O3, ITO)) nanoparticles. These materials are unique because they mimic the morphology, surface area, and oxide nature of conventional mesoporous, nanocrystalline metal oxide thin films such as TiO 2 , SnO2, and NiO used in DSSC and DSPEC applications but they are conductive with high electron mobilities (0.1-10 cm2 V-1 s-1) and conductivities (101-104 Ω-1 cm-1) over a wide range of applied potentials. Their high conductivities and metallic behavior allow the Fermi level at nanoparticle surfaces to be tuned systematically by varying an applied bias. We will investigate injection, intra-assembly, and back electron transfer dynamics on nanoITO by transient absorption measurements on the picosecond to millisecond timescales. One series of experiments will build on initial results from DOE-supported research and focus on the bias- and medium-dependence of interfacial electron transfer by surface-bound molecular chromophores to investigate: i) the bias dependence of injection and back electron transfer, ii) injection by short-lived, otherwise inaccessible excited-states, and iii) medium effects. In parallel, we will investigate the dynamic interfacial properties of nanoITO-bound molecular assemblies designed for photogalvanic solar energy conversion. These studies will focus on surface-prepared, layer-by-layer donor-chromophore-acceptor assemblies in which inter-molecular excited-state quenching leads to redox separation as opposed to interfacial electron transfer quenching by the semiconductor material. Transient absorption measurements performed on nanoITO|-acceptor-chromophore-donor (photogalvanic anode) and nanoITO|-donor-chromophore-acceptor (photogalvanic cathode) assemblies will be used to explore: i) light-induced intermolecular electron transfer in layer-by-layer assemblies and ii) bias dependence on transient redox energy storage and long range (>5 nm) electron transfer reactions.

30 DIRECT ENERGY CONVERSION↗
NASA NTRS1986

The effect of CO chemisorption on the metal surface CO/Ni(100)

The chemisorption of CO on Ni(100) is studied with a cluster model. The calculations suggest that chemisorption changes the nature of the Ni 3d orbitals. The open shell Ni 3d accepts charge from the Ni valence 4s4p orbitals, reducing the open shell 3d character on the Ni. The closed shell Ni 3d orbitals also mix with the Ni valence 4s4p orbitals and donate charge to the CO; thus helping to maintain the Ni 3d population near 9. These changes should affect the density of 3d states near the Fermi level and the surface magnetic moment. The variation in the bonding with the size of cluster is also discussed.

Bauschlicher, Charles, W., jr.↗
DOE OSTI2018

Facet-Dependent Kinetics and Energetics of Hematite for Solar Water Oxidation Reactions

Currently, the performance of a photoelectrochemical (PEC) system is highly dependent on the charge separation, transport and transfer characteristics at the photoelectrode|electrolyte interface. Of the factors that influence the charge behaviors, the crystalline facets of the semiconductor in contact with the electrolyte play an important role but has been poorly studied previously. Here, we present a study aimed at understanding how the different facets of hematite affect the charge separation and transfer behaviors in a solar water oxidation reaction. Specifically, hematite crystallites with predominantly {012} and {001} facets exposed were synthesized. Density functional theory (DFT) calculations revealed that hematite {012} surfaces feature higher OH coverage, which was confirmed by X-ray photoelectron spectroscopy (XPS). These surface OH groups act as active sites to mediate water oxidation reactions, which plays a positive role for the PEC system. These surface OH groups also facilitate charge recombination, which compromises the charge separation capabilities of hematite. Moreover, intensity modulated photocurrent spectroscopy (IMPS) confirmed that hematite {012} surfaces exhibit higher rate constants for both charge transfer and recombination. Open circuit potential (OCP) measurements revealed that the hematite {012} surface reflects a greater degree of Fermi level pinning effect. Our results shed light on how different surface crystal structures may change surface kinetics and energetics. The information is anticipated to contribute to efforts on optimizing PEC performance for practical solar fuel synthesis.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Emergent mystery in the Kondo insulator samarium hexaboride

Samarium hexaboride (SmB 6 ) is an example of a Kondo insulator, in which strong electron correlations cause a band gap to open. SmB 6 hosts both a bulk insulating state and a conductive surface state. Within a Fermi-liquid framework, the strongly correlated ground-state electronic structure can be mapped to a simple state resembling a topological insulator. Although uncertainties remain, many experiments provide compelling evidence that the conductive surface states have a topological origin. However, the bulk behavior is less well understood and some experiments indicate bulk in-gap states. This has inspired the development of many theories that predict the emergence of new bulk quantum phases beyond Landau’s Fermi-liquid model. We review the current progress on understanding both the surface and the bulk states, especially the experimental evidence for each. A mystery centres on the existence of the bulk in-gap states and why they appear in some experiments but not others. Adding to the mystery is why quantum oscillations in SmB6 appear only in magnetization but not in resistivity. Here, we conclude by elaborating on three questions: why SmB 6 is worth studying, what can be done to move forwards and what other correlated insulators could give additional insight.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗