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DOE OSTI2025

Magnet-superconductor hybrid quantum systems: a materials platform for topological superconductivity

Magnet–superconductor hybrid (MSH) systems have recently emerged as one of the most significant developments in condensed matter physics. This has generated, in the last decade, a steadily rising interest in the understanding of their unique properties. They have been proposed as one of the most promising platforms for the establishment of topological superconductivity, which holds high potential for application in future quantum information technologies. Their emergent electronic properties stem from the exchange interaction between the magnetic moments and the superconducting condensate. Given the atomic-level origin of such interaction, it is of paramount importance to investigate new magnet–superconductor hybrids at the atomic scale. In this regard, scanning tunneling microscopy (STM) and spectroscopy are playing a crucial role in the race to unveil the fundamental origin of the unique properties of MSH systems, with the aim to discover new hybrid quantum materials capable of hosting topologically non-trivial unconventional superconducting phases. In particular, the combination of STM studies with tight-binding model calculations have represented, so far, the most successful approach to unveil and explain the emergent electronic properties of MSHs. The scope of this review is to offer a broad perspective on the field of MSHs from an atomic-level investigation point-of-view. The focus is on discussing the link between the magnetic ground state hosted by the hybrid system and the corresponding emergent superconducting phase. This is done for MSHs with both one-dimensional (atomic chains) and two-dimensional (atomic lattices and thin films) magnetic systems proximitized to conventional s-wave superconductors. We present a systematic categorization of the experimentally investigated systems with respect to defined experimentally accessible criteria to verify or falsify the presence of topological superconductivity and Majorana edge modes. The discussion will start with an introduction to the physics of Yu–Shiba–Rusinov bound states at magnetic impurities on superconducting surfaces. This will be used as a base for the discussion of magnetic atomic chains on superconductors, distinguishing between ferromagnetic, antiferromagnetic and non-collinear magnetic ground states. A similar approach will be used for the discussion of magnetic thin film islands on superconductors. Given the vast number of publications on the topic, we limit ourselves to discuss works which are most relevant to the search for topological superconductivity.

Majorana states↗

DOE OSTI2020

Aperiodically ordered nano-graphene on the quasicrystalline substrate

Designing exotic structures in low dimensions is key in today's quest to tailor novel quantum states in materials with unique symmetries. Particularly intriguing materials in this regard are low dimensional aperiodic structures with non-conventional symmetries that are otherwise forbidden in translation symmetric crystals. In our work, we focus on the link between the structural and electronic properties of aperiodically ordered aromatic molecules on a quasicrystalline surface, which has largely been neglected so far. As an exemplary case, we investigate the self-assembly and the interfacial electronic properties of the nano-graphene-like molecule coronene on the bulk truncated icosahedral (i) Al–Pd–Mn quasicrystalline surface using multiple surface sensitive techniques. We find an aperiodically ordered coronene monolayer (ML) film on the i-Al–Pd–Mn surface that is characterized by the same local motifs of the P1 Penrose tiling model as the bare i-Al–Pd–Mn surface. The electronic valence band structure of the coronene/i-Al–Pd–Mn system is characterized by the pseudogap of thebare i-Al–Pd–Mn, which persists the adsorption of coronene confirming the quasiperiodic nature of the interface. In addition, we find a newly formed interface state of partial molecular character that suggests an at least partial chemical interaction between the molecule and the quasicrystalline surface. We propose that this partial chemical molecule–surface interaction is responsible for imprinting the quasicrystalline order of the surface onto the molecular film.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

Chanakian, Sevan↗

DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

36 MATERIALS SCIENCE↗

DOE OSTI2023

The role of native defects and their diffusivity in CdTe

CdTe is a leading thin-film technology for solar-cell applications. Basic features such as the optimum band gap of 1.5 eV and simple manufacturing process have been driving the research on improving the current record efficiency of 22% for CdTe solar cells toward the theoretical limit of 32%. Low p-type doping and short carrier lifetimes in the CdTe absorber have been suggested as the main limiting factors and attributed to defect compensation and carrier recombination. Understanding the role of point defects and impurities in CdTe is crucial to solving these problems. Despite many years of research, the sources of compensation and the microscopic recombination mechanisms are still unclear. By using hybrid density functional calculations with spin-orbit coupling and large supercells, we investigate the electronic properties, the formation energies, and the diffusivities of the native defects in CdTe. As possible sources of compensation for p-type conductivity, we discuss the structural and electronic properties of Cd interstitial, Te vacancies, and TeCd antisites. The stability of each defect is discussed in terms of both the defect formation energy and the migration barrier. We also address possible mechanisms that lead to non-radiative recombination.

14 SOLAR ENERGY↗

DOE OSTI2025

Magnetism in Moiré Materials

This project focused on the theory of the electronic properties of magnetic moire materials. Moire materials can be formed by overlaying two or more two-dimensional crystals that are either semiconductors or semimetals and establishing a moire pattern. Low-energy electronic properties are then described by a continuum model with crystalline periodicity. Moire materials are therefore artificial two-dimensional crys- tals whose periodicity is at the tens of nanometers scale, increasing the unit cell area by a factor of about 10,000 compared to real crystals. They are important because the main tuning knob of chemistry, the number of electrons per effective atom or molecule (the analog of the number of electrons per unit cell in real crystals), can be shifted by more than one simply by using electrical gates. The research covered two diffeerent classes of moire materials, graphene multilayers twisted to a rotation angle at which correlations are strong, and transition metal dichalcogenide semiconductors. The proposal identifed one project in each area both related very broadly to magnetism: i) Advancing understanding of broken spin-valley and sublattice symmetries in Magic Angle Twisted Bilayer Graphene and ii) Developing the theory of itinerant electron magnetism in Gamma-valley and K-valley transition metal dichalcogenide moire materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Delocalized polaron and Burstein-Moss shift induced by Li in α–V 2 O 5 : A DFT + DMFT study

For this work, we performed density functional theory (DFT)+U and dynamical mean field theory (DMFT) calculations with a continuous-time quantum Monte Carlo impurity solver to investigate the electronic properties of V 2 O 5 and Li x V 2 O 5 (x = 0.125 and 0.25). Pristine V 2 O 5 is a charge-transfer insulator with strong O p–V d hybridization, and it exhibits a large band gap (E gap ) as well as a nonzero conduction-band (CB) gap.We show that the band gap, the number of d electrons of vanadium, N d , and the CB gap for V 2 O 5 obtained from our DMFT calculations are in excellent agreement with the experimental values. While the DFT +U approach replicates the experimental band gap, it overestimates the value of N d and underestimates the CB gap. In the presence of low Li doping, the electronic properties of V 2 O 5 are mainly driven by a polaronic mechanism, and electron spin resonance and electron nuclear double resonance spectroscopies observed the coexistence of free and bound polarons. Notably, our DMFT results identify both polaron types, with the bound polaron being energetically preferred, while the DFT +U method only predicts the free polaron. Our DMFT analysis also reveals that increased Li doping leads to electron filling in the conduction band, shifting the Fermi level. This result is consistent with the observed Burstein-Moss shift upon enhanced Li doping, and we thus demonstrate that the DFT + DMFT approach can be used for an accurate and realistic description of strongly correlated materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Processable High Electron Mobility {pi}-Copolymers via Mesoscale Backbone Conformational Ordering

The synthesis and experimental/theoretical characterization of a new series of electron-transporting copolymers based on the naphthalene bis(4,8-diamino-1,5-dicarboxyl)amide (NBA) building block are reported. Comonomers are designed to test the emergent effects of manipulating backbone torsional characteristics, and density functional theory (DFT) analysis reveals the key role of backbone conformation in optimizing electronic delocalization and transport. The NBA copolymer conformational and electronic properties are characterized using a broad array of molecular/macromolecular, thermal, optical, electrochemical, and charge transport techniques. All NBA copolymers exhibit strongly aggregated morphologies with significant nanoscale order. Copolymer charge transport properties are investigated in thin-film transistors and exhibit excellent electron mobilities ranging from 0.4 to 4.5 cm(2) V-1 s(-1). Importantly, the electron transport efficiency correlates with the film mesoscale order, which emerges from comonomer-dependent backbone planarity and extension. These results illuminate the key NBA building block structure-morphology-bulk property design relationships essential for processable, electronics-applicable high-performance polymeric semiconductors.

n‐↗

NASA NTRS1982

Electronics materials research

The electronic materials and is aimed at the establishment of quantitative relationships underlying crystal growth parameters, materials properties, electronic characteristics and device applications. The overall program evolves about the following main thrust areas: (1) crystal growth novel approaches to engineering of semiconductor materials; (2) investigation of materials properties and electronic characteristics on a macro and microscale; (3) surface properties and surface interactions with the bulk and ambients; (4) electronic properties controlling device applications and device performance.

Source record↗

DOE OSTI2022

From p- to n-Type Mixed Conduction in Isoindigo-Based Polymers through Molecular Design

Organic mixed ionic and electronic conductors are of significant interest for bioelectronic applications. Here, we use three different isoindigoid building blocks to obtain polymeric mixed conductors with vastly different structural and electronic properties which can be further fine-tuned through the choice of comonomer unit. We show how careful design of the isoindigoid scaffold can afford highly planar polymer structures with high degrees of electronic delocalization, while subtle structural modifications can control the dominant charge carrier (hole or electron) when probed in organic electrochemical transistors. We employ a combination of experimental and computational techniques to probe electrochemical, structural and mixed ionic and electronic properties of the polymer series which in turn allows us to derive important structure-property relations for this promising class of materials in the context of organic bioelectronics. Ultimately, we use these findings to outline robust molecular design strategies for isoindigo-based mixed conductors that can support efficient p-type, n-type and ambipolar transistor operation in an aqueous environment.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Electronic Structure-Based Descriptors for Oxide Properties and Functions

The transition from fossil fuels to renewable energy requires the development of efficient and cost-effective energy storage technologies. A promising way forward is to harness the energy of intermittent renewable sources, such as solar and wind, to perform (electro)catalytic reactions to generate fuels, thus storing energy in the form of chemical bonds. Furthermore, current catalysts rely on the use of expensive, rare, or geographically localized elements, such as platinum. Widespread adoption of new (electro)catalytic technologies hinges on the discovery and development of materials containing earth-abundant elements, which can efficiently catalyze an array of (electro)chemical reactions.

Adsorption↗

NASA NTRS2009

Measurements of Lunar Dust Charging Properties by Electron Impact

Dust grains in the lunar environment are believed to be electrostatically charged predominantly by photoelectric emissions resulting from solar UV radiation on the dayside, and on the nightside by interaction with electrons in the solar wind plasma. In the high vacuum environment on the lunar surface with virtually no atmosphere, the positive and negative charge states of micron/submicron dust grains lead to some unusual physical and dynamical dust phenomena. Knowledge of the electrostatic charging properties of dust grains in the lunar environment is required for addressing their hazardous effect on the humans and mechanical systems. It is well recognized that the charging properties of individual small micron size dust grains are substantially different from the measurements on bulk materials. In this paper we present the results of measurements on charging of individual Apollo 11 and Apollo 17 dust grains by exposing them to mono-energetic electron beams in the 10-100 eV energy range. The charging/discharging rates of positively and negatively charged particles of approx. 0.1 to 5 micron radii are discussed in terms of the sticking efficiencies and secondary electron yields. The secondary electron emission process is found to be a complex and effective charging/discharging mechanism for incident electron energies as low as 10-25 eV, with a strong dependence on particle size. Implications of the laboratory measurements on the nature of dust grain charging in the lunar environment are discussed.

Abbas, Mian M.↗

DOE OSTI2021

Correlation Between Corrugation-Induced Flexoelectric Polarization and Conductivity of Low-Dimensional Transition Metal Dichalcogenides

The tunability of polar and semiconducting properties of low-dimensional transition metal dichalcogenides (TMDs) have propelled them to the forefront of fundamental and applied physical research. These materials can vary their electrophysical properties from nonpolar to ferroelectric, and from direct-band semiconducting to metallic. In addition to classical controlling factors, such as field effect, composition, and doping, new degrees of freedom emerge in TMDs due to the curvature-induced electron redistribution and the associated changes in electronic properties. Here we theoretically explore the elastic and electric fields, flexoelectric polarization and free charge density for a TMD nanoflake placed on a rough substrate with a sinusoidal corrugation profile. Finite element modelling results for different flake thickness and corrugation depth yield insights into the flexoelectric nature of the out-of-plane electric polarization and establish the unambiguous correlation between the polarization and static conductivity modulation. The modulation is caused by the coupling between the deformation potential and inhomogeneous elastic strains, which evolve in the TMD nanoflake due to the adhesion between the flake surface and corrugated substrate. We reveal a pronounced maximum in the thickness dependences of the electron and hole conductivity of MoS 2 and MoTe 2 nanoflakes placed on a corrugated substrate, which opens the way for the optimization of their geometry towards significant improvement in their polar and electronic properties, necessary for advanced applications in nanoelectronics and memory devices. Specifically, the obtained results can be useful for the development of nanoscale straintronic devices based on the bended MoS 2 , MoTe 2 , and MoSTe nanoflakes, such as diodes and bipolar transistors with a bending-controllable sharpness of p-n junctions.

42 ENGINEERING↗

DOE OSTI2021

Measuring Electronic Structure of Multiply Charged Anions to Understand their Chemistry: A Case Study on Gaseous Polyhedral closo-Borate Dianions

Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the last decades, mainly to understand fundamental molecular physics phenomena, e.g., intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs’ chemistry appears often vague. Here, we discuss how insights on the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision induced dissociation can provide fundamental understanding on the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified on our studies on polyhedral closo-borate dianions [BnXn]2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are either tuned to bind noble gases effectively or to activate small molecules like CO and N2. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [BnXn]2-, and gas phase NIPES provides insights on the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions which cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are discussed. J.W is grateful to the Volkswagen foundation for a Freigeist Fellowship. X. B. W acknowledges support from the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, the Division of Chemical Sciences, Geosciences and Biosciences. We are grateful to our colleagues who support our work on gas phase ion chemistry for many years. In particular we acknowledge the members of our groups, the Asmis group (Leipzig), Laskin Group (West Lafayette), the Jenne group (Wuppertal), the Finze group (Würzburg), the Grabowsky group (Bern), and the Sun group (Shanghai). We are grateful to Edoardo Apra for the many theoretical investigations on closo-borate anions. JW acknowledges support of Harald Knorke and Markus Rohdenburg with the preparation of graphical material, and XBW thanks Qinqin Yuan for the help in organizing references.

Warneke, Jonas↗

DOE OSTI2023

Substrate-Driven Catalyst Reducibility for Oxygen Evolution and Its Effect on the Operation of Proton Exchange Membrane Water Electrolyzers

To increase the efficiency of hydrogen production by proton exchange membrane water electrolyzers (PEMWEs), the relationships between the specific activity and stability of the membrane–electrode assembly (MEA) must be clarified. Ir oxide electrodeposited on Ti substrate is used as an oxygen electrode, and its electronic properties and electrochemical behavior in PEMWE operation are observed. The electrode, fabricated through a facile strategy based on annealing in the air atmosphere, enhances the specific oxygen evolution reaction (OER) activity and stability in PEMWE operation. Furthermore, the electronic catalyst–substrate interactions associated with different reducibilities in the heteroatom system are studied. The morphology, electronic properties, and chemical state of the oxygen electrode are investigated through X-ray spectroscopy, microscopy techniques, and computational analyses. Thermal treatment of the catalyst-coated substrate decreases the bulk oxidation state of Ir and increases the surface oxidation state. According to the electrochemical and physical behavior analyses of PEMWEs, the oxygen content in the Ir oxide structure, which defines the OER activity and its stability, is influenced by the crystalline structure and formation of a stable interface between the catalyst and substrate. The outcomes can facilitate the development of strategies for enhancing the performance of PEMWEs and designing rational MEAs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Naphthodithiophene‐Fused Porphyrins: Synthesis, Characterization, and Impact of Extended Conjugation on Aromaticity

The fusion of tetrapyrroles with aromatic heterocycles constitutes a useful tool for manipulating their opto-electronic properties. In this work, the synthesis of naphthodithiophene-fused porphyrins was achieved through a Heck reaction-based cascade of steps followed by the Scholl reaction. The naphthodithiophene-fused porphyrins display a unique set of optical and electronic properties. Fusion of the naphtho[2,1-b:3,4-b’]dithiophene to porphyrin (F2VTP) leads to a ~20% increase in the fluorescence lifetime, which is accompanied, unexpectedly, by a more than two-fold drop in the emission quantum yield (ϕ=0.018). In contrast, fusion of the isomeric naphtho[1,2-b:4,3-b’]dithiophene to porphyrin (F3VPT) results in a ~1.5-fold increase in the fluorescence quantum yield (ϕ=0.13) with a concomitant ~30 % increase in the fluorescence lifetime. This behavior suggests that fusion of the porphyrin with the naphthodithiopheno-system mainly affects the radiative rate constant in the Q-state deactivation pathway, where the effects of the isomeric naphtho[2,1-b:3,4-b’]dithiophene- versus naphtho[1,2-b:4,3-b’]dithiophene-fusion are essentially the opposite. Interestingly, nucleus-independent chemical shifts analysis revealed a considerable difference between the aromaticities of these two isomeric systems. Furthermore, our results demonstrate that subtle structural differences in the fused components of the porphyrin can be reflected in rather significant differences between the photophysical properties of the resulting systems.

Chemistry↗

DOE OSTI2022

Suppressing Charged Cation Antisites via Se Vapor Annealing Enables p-Type Dopability in AgBiSe 2 –SnSe Thermoelectrics

Cation disordering is commonly found in multinary cubic compounds, but its effect on electronic properties has been neglected because of difficulties in determining the ordered structure and defect energetics. An absence of rational understanding of the point defects present has led to poor reproducibility and uncontrolled conduction type. AgBiSe 2 is a representative compound that suffers from poor reproducibility of thermoelectric properties, while the origins of its intrinsic n-type conductivity remain speculative. Here, it is demonstrated that cation disordering is facilitated by Bi Ag charged antisite defects in cubic AgBiSe 2 which also act as a principal donor defect that greatly controls the electronic properties. Using density functional theory calculations and in situ Raman spectroscopy, how saturation annealing with selenium vapor can stabilize p-type conductivity in cubic AgBiSe 2 alloyed with SnSe at high temperatures is elucidated. With stable and controlled hole concentration, a peak is observed in the weighted mobility and the density-of-states effective mass in AgBiSnSe 3 , implying an increased valley degeneracy in this system. These findings corroborate the importance of considering the defect energetics for exploring the dopability of ternary thermoelectric chalcogenides and engineering electronic bands by controlling self-doping.

36 MATERIALS SCIENCE↗

NASA NTRS1985

Crystal growth of device quality GaAs in space

The present program has been aimed at solving the fundamental and technological problems associated with Crystal Growth of Device Quality in Space. The initial stage of the program was devoted strictly to ground-based research. The unsolved problems associated with the growth of bulk GaAs in the presence of gravitational forces were explored. Reliable chemical, structural and electronic characterization methods were developed which would permit the direct relation of the salient materials parameters (particularly those affected by zero gravity conditions) to the electronic characteristics of single crystal GaAs, in turn to device performance. These relationships are essential for the development of optimum approaches and techniques. It was concluded that the findings on elemental semiconductors Ge and Si regarding crystal growth, segregation, chemical composition, defect interactions, and materials properties-electronic properties relationships are not necessarily applicable to GaAs (and to other semiconductor compounds). In many instances totally unexpected relationships were found to prevail.

Gatos, H. C.↗