Showing papers on "Electronic structure published in 2021"

Recent progress on 2D magnets: Fundamental mechanism, structural design and modification

Abstract: The two-dimensional (2D) magnet, a long-standing missing member in the family of 2D functional materials, is promising for next-generation information technology. The recent experimental discovery of 2D magnetic ordering in CrI3, Cr2Ge2Te6, VSe2, and Fe3GeTe2 has stimulated intense research activities to expand the scope of 2D magnets. This review covers the essential progress on 2D magnets, with an emphasis on the current understanding of the magnetic exchange interaction, the databases of 2D magnets, and the modification strategies for modulation of magnetism. We will address a large number of 2D intrinsic magnetic materials, including binary transition metal halogenides; chalogenides; carbides; nitrides; oxides; borides; silicides; MXene; ternary transition metal compounds CrXTe3, MPX3, Fe-Ge-Te, MBi2Te4, and MXY (M = transition metal; X = O, S, Se, Te, N; Y = Cl, Br, I); f-state magnets; p-state magnets; and organic magnets. Their electronic structure, magnetic moment, Curie temperature, and magnetic anisotropy energy will be presented. According to the specific 2D magnets, the underlying direct, superexchange, double exchange, super-superexchange, extended superexchange, and multi-intermediate double exchange interactions will be described. In addition, we will also highlight the effective strategies to manipulate the interatomic exchange mechanism to improve the Curie temperature of 2D magnets, such as chemical functionalization, isoelectronic substitution, alloying, strain engineering, defect engineering, applying electronic/magnetic field, interlayer coupling, carrier doping, optical controlling, and intercalation. We hope this review will contribute to understanding the magnetic exchange interaction of existing 2D magnets, developing unprecedented 2D magnets with desired properties, and offering new perspectives in this rapidly expanding field.

Two-dimensional superconductivity and anisotropic transport at KTaO3 (111) interfaces.

Abstract: The distinctive electronic structure found at interfaces between materials can allow unconventional quantum states to emerge. Here we report on the discovery of superconductivity in electron gases formed at interfaces between (111)-oriented KTaO3 and insulating overlayers of either EuO or LaAlO3 The superconducting transition temperature, as high as 2.2 kelvin, is about one order of magnitude higher than that of the LaAlO3/SrTiO3 system. Notably, similar electron gases at KTaO3 (001) interfaces remain normal down to 25 millikelvin. The critical field and current-voltage measurements indicate that the superconductivity is two-dimensional. In EuO/KTaO3 (111) samples, a spontaneous in-plane transport anisotropy is observed before the onset of superconductivity, suggesting the emergence of a distinct "stripe"-like phase, which is also revealed near the critical field.

Understanding Doping of Quantum Materials.

Abstract: Doping mobile carriers into ordinary semiconductors such as Si, GaAs, and ZnO was the enabling step in the electronic and optoelectronic revolutions. The recent emergence of a class of "quantum materials", where uniquely quantum interactions between the components produce specific behaviors such as topological insulation, unusual magnetism, superconductivity, spin-orbit-induced and magnetically induced spin splitting, polaron formation, and transparency of electrical conductors, pointed attention to a range of doping-related phenomena associated with chemical classes that differ from the traditional semiconductors. These include wide-gap oxides, compounds containing open-shell d electrons, and compounds made of heavy elements yet having significant band gaps. The atomistic electronic structure theory of doping that has been developed over the past two decades in the subfield of semiconductor physics has recently been extended and applied to quantum materials. The present review focuses on explaining the main concepts needed for a basic understanding of the doping phenomenology and indeed peculiarities in quantum materials from the perspective of condensed matter theory, with the hope of forging bridges to the chemists that have enabled the synthesis of some of the most interesting compounds in this field.

Chiral Photocurrent in Parity-Violating Magnet and Enhanced Response in Topological Antiferromagnet

Abstract: A new classification of photocurrent responses in light of symmetry violations in the presence of magnetic order unveils two new types of photocurrent that can be readily tuned and enhanced by topological electronic structure in solids.

Electronic interaction between transition metal single-atoms and anatase TiO2 boosts CO2 photoreduction with H2O

Abstract: Single-atom catalysts are playing a pivotal-role in understanding atomic-level photocatalytic processes. However, single-atoms are typically non-uniformly distributed on photocatalyst surfaces, hindering the systematic investigation of structure–property correlation at atomic precision. Herein, by combining material design, spectroscopic analyses, and theoretical studies, we investigate the atomic-level CO2 photoreduction process on TiO2 photocatalysts with uniformly stabilized transition metal single-atoms. First, the electronic interaction between single Cu atoms and the surrounding TiO2 affects the reducibility of the TiO2 surface, leading to spontaneous O vacancy formation near Cu atoms. The coexistence of Cu atoms and O vacancies cooperatively stabilizes CO2 intermediates on the TiO2 surface. Second, our approach allows us to control the spatial distribution of uniform single Cu atoms on TiO2, and demonstrate that neighboring Cu atoms simultaneously engage in the interaction with CO2 intermediates by controlling the charge localization. Optimized Cu1/TiO2 photocatalysts exhibit 66-fold enhancement in CO2 photoreduction performance compared to the pristine TiO2.

Chirality-driven topological electronic structure of DNA-like materials.

Abstract: Topological aspects of the geometry of DNA and similar chiral molecules have received a lot of attention, but the topology of their electronic structure is less explored. Previous experiments revealed that DNA can efficiently filter spin-polarized electrons between metal contacts, a process called chiral-induced spin selectivity. However, the underlying correlation between chiral structure and electronic spin remains elusive. In this work, we reveal an orbital texture in the band structure, a topological characteristic induced by the chirality. We found that this orbital texture enables the chiral molecule to polarize the quantum orbital. This orbital polarization effect (OPE) induces spin polarization assisted by the spin–orbit interaction of a metal contact and leads to magnetoresistance and chiral separation. The orbital angular momentum of photoelectrons also plays an essential role in related photoemission experiments. Beyond chiral-induced spin selectivity, we predict that the orbital polarization effect could induce spin-selective phenomena even in achiral but inversion-breaking materials. An orbital polarization effect is proposed to understand chiral-induced spin selectivity.

Fluorine-Driven Enhancement of Birefringence in the Fluorooxosulfate: A Deep Evaluation from a Joint Experimental and Computational Study

Abstract: Understanding and exploring the functional modules (FMs) consisting of local atomic groups can promote the development of the materials with functional performances. Oxygen-containing tetrahedral modules are popular in deep-ultraviolet (DUV) optical materials, but their weak optical anisotropy is adverse to birefringence. Here, the fluorooxosulfate group is proved as a new birefringence-enhanced FM for the first time. The birefringence of fluorooxosulfates can be 4.8-15.5 times that of sulfates with the same metal cations while maintaining a DUV band gap. The polarizing microscope measurement confirms the birefringence enhancement by using the millimeter crystals experimentally. The theoretical studies from micro and macro levels further reveal a novel universal strategy that the fluorine induced anisotropic electronic distribution in fluorooxo-tetrahedral group is responsible for the enhancement of birefringence. This study will guide the future discovery of DUV optical materials with enlarged birefringence.

First Principles Calculations of Electronic, Structural and Optical Properties of (PMMA–ZrO 2 –Au) and (PMMA–Al 2 O 3 –Au) Nanocomposites for Optoelectronics Applications

Abstract: This study focuses on the quantum mechanical treatment of the geometrical optimization and the electronic structure problems of a nanomaterial PMMA and nanocomposites. The hybrid functional B3LYP/6-31G level of DFT is used to investigate four molecules divided into two groups, they are PMMA as an original basis molecule and (PMMA–Au), (PMMA–Al2O3–Au), (PMMA–ZrO2–Au) nanocomposites as the two group. The DFT calculations have been performed using Gaussian 09 package of programs. The geometrical optimization included both bonds in °A and angles in deg. The calculated electronic properties included the total energy, HOMO and LUMO energies, energy gap, ionization energy, electron affinity, electronegativity, electrochemical hardness, electronic softness and Electrophilic index. The geometrical optimization of PMMA and nanocomposites has been found in good agreement with the experimental data because of its relaxed geometrical parameters. One of the important results was obtain in this study, is the decreasing of the energy gap. This states that these nanocomposites are the nearest to semiconductor due to the both HOMO and LUMO levels become more adjacent. These consequences mention to construct new structures with new electronic properties. All nanocomposites need small energy to become cationdue to ionization potential is decrease with addition nanoparticles to the pure PMMA, but the electronic affinity is an increase with with addition nanoparticles to the pure PMMA. The total ground state energy of the PMMA have largest value of total energy compared for other nanocomposites, where ET decreased with addition nanoparticles to pure PMMA. The hardness decrease with addition nanoparticles to the pure PMMA, therefore all the nanocomposites are softer, and this reduces the resistance of a species to lose electrons. Good relax for the structures of the studied PMMA was obtained theoretically, in which, the angles C–C, C=O and C–H in pure PMMA are remain in the same ranges for other nanocomposites. In general, most of the studied nonocomposites direct electronic transition from the valence to conduction band with wave length lies in the range of solar spectrum. The obtained results showed that the (PMMA–ZrO2–Au) and (PMMA–Al2O3–Au) nanocomposites have huge applications in electronics and photo-electronics fields.

The vacancy defects and oxygen atoms occupation effects on mechanical and electronic properties of Mo5Si3 silicides

Abstract: Improving brittle behavior and mechanical properties is still a big challenge for high-temperature structural materials. By means of first-principles calculations, in this paper, we systematically investigate the effect of vacancy and oxygen occupation on the elastic properties and brittle-or-ductile behavior on Mo5Si3. Four vacancies (Si–Va1, Si–Va2, Mo–Va1, Mo–Va2) and oxygen occupation models (O–Mo1, O–Mo2, O–Si1, O–Si2) are selected for research. It is found that Mo–Va2 vacancy has the stronger structural stability in the ground state in comparison with other vacancies. Besides, the deformation resistance and hardness of the parent Mo5Si3 are weakened due to the introduction of different vacancy defects and oxygen occupation. The ratio of B/G indicates that oxygen atoms occupation and vacancy defects result in brittle-to-ductile transition for Mo5Si3. These vacancies and the oxygen atoms occupation change the localized hybridization between Mo–Si and Mo–Mo atoms. The weaker O–Mo bond is a contributing factor for the excellent ductile behavior in the O-Si2 model for Mo5Si3.

Rich Nature of Van Hove Singularities in Kagome Superconductor CsV$_3$Sb$_5$

Abstract: The recently discovered layered kagome metals AV$_3$Sb$_5$ (A=K, Rb, Cs) exhibit diverse correlated phenomena, which are intertwined with a topological electronic structure with multiple van Hove singularities (VHSs) in the vicinity of the Fermi level. As the VHSs with their large density of states enhance correlation effects, it is of crucial importance to determine their nature and properties. Here, we combine polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to directly reveal the sublattice properties of 3d-orbital VHSs in CsV$_3$Sb$_5$. Four VHSs are identified around the M point and three of them are close to the Fermi level, with two having sublattice-pure and one sublattice-mixed nature. Remarkably, the VHS just below the Fermi level displays an extremely flat dispersion along MK, establishing the experimental discovery of higher-order VHS. The characteristic intensity modulation of Dirac cones around K further demonstrates the sublattice interference embedded in the electronic structure. The crucial insights into the electronic structure, revealed by our work, provide a solid starting point for the understanding of the intriguing correlation phenomena in the kagome metals AV$_3$Sb$_5$.

Interfacial electronic structure engineering on molybdenum sulfide for robust dual-pH hydrogen evolution.

Abstract: Molybdenum disulfide, as an electronic highly-adjustable catalysts material, tuning its electronic structure is crucial to enhance its intrinsic hydrogen evolution reaction (HER) activity. Nevertheless, there are yet huge challenges to the understanding and regulation of the surface electronic structure of molybdenum disulfide-based catalysts. Here we address these challenges by tuning its electronic structure of phase modulation synergistic with interfacial chemistry and defects from phosphorus or sulfur implantation, and we then successfully design and synthesize electrocatalysts with the multi-heterojunction interfaces (e.g., 1T0.81-MoS2@Ni2P), demonstrating superior HER activities and good stabilities with a small overpotentials of 38.9 and 95 mV at 10 mA/cm2, a low Tafel slopes of 41 and 42 mV/dec in acidic as well as alkaline surroundings, outperforming commercial Pt/C catalyst and other reported Mo-based catalysts. Theoretical calculation verified that the incorporation of metallic-phase and intrinsic HER-active Ni-based materials into molybdenum disulfide could effectively regulate its electronic structure for making the bandgap narrower. Additionally, X-ray absorption spectroscopy indicate that reduced nickel possesses empty orbitals, which is helpful for additional H binding ability. All these factors can decrease Mo-H bond strength, greatly improving the HER catalytic activity of these materials. The understanding and regulation of the surface electronic structure of molybdenum disulfide-based catalysts for hydrogen evolution reaction (HER) remains a challenges. Here, the authors design and synthesize electrocatalysts with multi-heterojunction interfaces showing enhanced HER activities and stabilities.

Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet.

Abstract: The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCo0.8Ru0.2Al, reaching 23 microvolts per kelvin. Uranium’s 5f electrons provide strong electronic correlations that lead to narrow bands, a known route to producing a large thermoelectric response. In addition, uranium’s strong spin-orbit coupling produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that in UCo0.8Ru0.2Al at least 148 Weyl nodes, and two nodal lines, exist within 60 millielectron volt of the Fermi level. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Concept of Orbital Entanglement and Correlation in Quantum Chemistry.

Abstract: A recent development in quantum chemistry has established the quantum mutual information between orbitals as a major descriptor of electronic structure. This has already facilitated remarkable improvements in numerical methods and may lead to a more comprehensive foundation for chemical bonding theory. Building on this promising development, our work provides a refined discussion of quantum information theoretical concepts by introducing the physical correlation and its separation into classical and quantum parts as distinctive quantifiers of electronic structure. In particular, we succeed in quantifying the entanglement. Intriguingly, our results for different molecules reveal that the total correlation between orbitals is mainly classical, raising questions about the general significance of entanglement in chemical bonding. Our work also shows that implementing the fundamental particle number superselection rule, so far not accounted for in quantum chemistry, removes a major part of correlation and entanglement seen previously. In that respect, realizing quantum information processing tasks with molecular systems might be more challenging than anticipated.

High-Throughput Screening and Automated Data-Driven Analysis of the Triplet Photophysical Properties of Structurally Diverse, Heteroleptic Iridium(III) Complexes.

Abstract: Steady state emission spectra and excited state lifetimes were measured for 1440 distinct heteroleptic [Ir(C^N)2(N^N)]+ complexes prepared via combinatorial parallelized synthesis; 72% of the complexes were found to be luminescent, and the emission maxima of the library spanned the visible spectrum (652-459 nm). Spectral profiles ranged from broad structureless bands to narrow emissions exhibiting vibrational substructure. Measured excited state lifetimes ranged between ∼0.1-14 μs. Automated emission spectral fitting with successive Gaussian functions revealed four distinct measured classes of excited states; in addition to well understood metal-ligand to ligand-charge transfer (3MLLCT) and ligand-centered (3LC) excited states, our classification also identified photophysical characteristics of less explored mixed 3MLLCT/3LC states. Electronic structure features obtained from DFT calculations performed on a large subset of these Ir(III) chromophores offered clear insights into the excited state properties and allowed the prediction of structure/luminescence relationships in this class of commonly used photocatalysts. Models with high prediction accuracy (R2 = 0.89) for emission color were developed on the basis of experimental data. Furthermore, different degrees of nuclear reorganization in the excited state were shown to significantly impact emission energy and excited state lifetimes.

Synthesis and Isolation of a Kinetically Stabilized Crystalline Triangulene.

Abstract: The synthesis and isolation of hydrocarbons with a triplet ground state in crystalline forms have been sought in materials science. Triangulene is one of the most famous triplet-ground-state benzenoid hydrocarbons. Its unique electronic structure and highly symmetric structure have prompted many scientists to synthesize and isolate triangulene and its derivatives, but all attempts so far to isolate them as crystals have been unsuccessful. Herein we report the synthesis and isolation of a kinetically stabilized crystalline triangulene for the first time. The key to success is the introduction of bulky substituents onto the reactive zigzag edges. Its highly symmetric structure was confirmed by X-ray crystallography, and its fundamental properties, including the triplet ground state, were revealed. The achievement here will open the door for the synthesis and isolation of other hydrocarbons with higher spin multiplicity.

Luminescence, chiroptical, magnetic and ab initio crystal-field characterizations of an enantiopure helicoidal Yb(III) complex

Abstract: The electronic structure of a chiral Yb(III)-based complex is fully determined by taking advantage of experimental magnetic, luminescence, and chiroptical (NIR-ECD and CPL) characterizations in combination with ab initio wavefunction calculations. The combined use of these techniques allows determining with high resolution the electronic structure diagram as well as the nature of the different states involved in the magnetic and chiroptical properties of the investigated complex. The crystal-field picture deduced from spectroscopic measurements (absorption and emission) is used to reproduce the magnetic properties. Subsequently, advanced ab initio calculations demonstrate that global chiroptical spectra correspond to the sum of entangled transitions with similar or opposite polarizations.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation.

Abstract: Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y2Ru2O7 alternative through partial A-site substitution of Sr2+ for Y3+, efficiently improving its intrinsic OER activity. The optimized Y1.7Sr0.3Ru2O7 candidate observes a highly intrinsic mass activity of 1018 A gRu-1 at an overpotential of 300 mV with excellent durability in 0.5 M H2SO4 electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr2+ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

Synthesis, characterization, and theoretical studies of the photovoltaic properties of novel reactive azonitrobenzaldehyde derivatives

Abstract: All dyes conduct but at different degrees of absorption; it is interesting to study the degree of conductivity and absorptivity of novel reactive azo-dyes in respect to dye-sensitized solar cells (DSSCs) to ascertain their viability for such applications. In this study, four novel reactive azo-dyes were experimentally synthesized from p-aminobenzaldehyde, 4-amino-3-nitrobenzaldehyde, and aniline through series of condensation and coupling reactions. The various functional groups, molecular connectivities, and molecular weight of the various fragments of the synthesized dyes were elucidated using the GC-MS, FT-IR, UV-vis, and NMR respectively. The experimentally determined structures were modeled and investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches to computationally compute the electronic structure properties, reactivity, absorption and solvatochromism in four different phases: gas, ethanol, acetone, and water, and the photovoltaic properties for possible applications in dye-sensitized solar cells (DSSCs). By comparing the HOMO (EH) and the LUMO (EL) energies from the results obtained demonstrates that dye D has the highest EL energy value of −2.48 eV with a relatively lowest EH energy value of −5.63 eV such that it lies underneath the conduction band edge of TiO2 which is necessary to enable charge regeneration. Pi-electron delocalization was observed from the natural bond orbital (NBO) calculations between the different aromatic rings with dye B and A having the relatively highest and least second-order stabilization energies between σ* → σ* and LP* → LP interacting orbitals respectively. It is also observed in all the solvents that the Gibbs free energy of injection (ΔGinject) is greater than 0.2 eV and hence, all the studied azo structures in the four phases provided efficient electron injection and light harvesting efficiency (LHE), however, the value of ΔGinject for dyes B and D is greatest in all the four phases and thus, provided the highest electron injection of all the dyes. From the fact-findings of quantum theory of atoms-in-molecules (QTAIM), dyes A and C have extra-stability due to their relatively high numbers of intramolecular H-bond interactions along with some additional intra-atomic bonding between atoms within the studied compounds. Hence, all the four dyes are good for DSSCs applications.

A state-averaged orbital-optimized hybrid quantum–classical algorithm for a democratic description of ground and excited states

Abstract: In the noisy intermediate-scale quantum (NISQ) era, solving the electronic structure problem from chemistry is considered as the ‘killer application’ for near-term quantum devices. In spite of the success of variational hybrid quantum/classical algorithms in providing accurate energy profiles for small molecules, careful considerations are still required for the description of complicated features of potential energy surfaces. Because the current quantum resources are very limited, it is common to focus on a restricted part of the Hilbert space (determined by the set of active orbitals). While physically motivated, this approximation can severely impact the description of these complicated features. A perfect example is that of conical intersections (i.e. a singular point of degeneracy between electronic states), which are of primary importance to understand many prominent reactions. Designing active spaces so that the improved accuracy from a quantum computer is not rendered useless is key to finding useful applications of these promising devices within the field of chemistry. To answer this issue, we introduce a NISQ-friendly method called ‘state-averaged orbital-optimized variational quantum eigensolver’ which combines two algorithms: (1) a state-averaged orbital-optimizer, and (2) a state-averaged VQE. To demonstrate the success of the method, we classically simulate it on a minimal Schiff base model (namely the formaldimine molecule CH2NH) relevant also for the photoisomerization in rhodopsin—a crucial step in the process of vision mediated by the presence of a conical intersection. We show that merging both algorithms fulfil the necessary condition to describe the molecule’s conical intersection, i.e. the ability to treat degenerate (or quasi-degenerate) states on the same footing.

Assessing conformer energies using electronic structure and machine learning methods

Abstract: We have performed a large-scale evaluation of current computational methods, including conventional small-molecule force fields, semiempirical, density functional, ab initio electronic structure methods, and current machine learning (ML) techniques to evaluate relative single-point energies. Using up to 10 local minima geometries across ~700 molecules, each optimized by B3LYP-D3BJ with single-point DLPNO-CCSD(T) triple-zeta energies, we consider over 6,500 single points to compare the correlation between different methods for both relative energies and ordered rankings of minima. We find promise from current ML methods and recommend methods at each tier of the accuracy-time tradeoff, particularly the recent GFN2 semiempirical method, the B97-3c density functional approximation, and RI-MP2 for accurate conformer energies. The ANI family of ML methods shows promise, particularly the ANI-1ccx variant trained in part on coupled-cluster energies. Multiple methods suggest continued improvements should be expected in both performance and accuracy.

Two-dimensional carbon nitride C6N nanosheet with egg-comb-like structure and electronic properties of a semimetal.

Abstract: In this study, the structural, electronic and optical properties of theoretically predicted C6N monolayer structure are investigated by means of Density Functional Theory-based First-Principles Calculations. Phonon band dispersion calculations and molecular dynamics simulations reveal the dynamical and thermal stability of C6N single-layer structure. We found out that the C6N monolayer has large negative in-plane Poissons ratios along both X and Y direction and the both values are almost four times that of the famous-pentagraphene. The electronic structure shows that C6N monolayer is a semi-metal and has a Dirac-point in the BZ. The optical analysis using the RPA method constructed over HSE06 illustrates that the first peak of absorption coefficient of the C6N monolayer along all polarizations is located in the IR range of spectrum, while the second absorption peak occurs in the visible range, which suggests its potential applications in optical and electronic devices. Interestingly, optically anisotropic character of this system is highly desirable for the design of polarization-sensitive photodetectors. Thermoelectric properties such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity and power factor are investigated as a function of carrier doping at temperatures 300 K, 400 K, and 500 K. In general, we predict that the C6N monolayer could be a new platform for study of novel physical properties in two-dimensional semi-metal materials, which may provide new opportunities to realize high-speed low-dissipation.

Enhanced Superconductivity in Monolayer Td-MoTe2

Abstract: Crystalline two-dimensional (2D) superconductors (SCs) with low carrier density are an exciting new class of materials in which electrostatic gating can tune superconductivity, electronic interactions play a prominent role, and electrical transport properties may directly reflect the topology of the Fermi surface. Here, we report the dramatic enhancement of superconductivity with decreasing thickness in semimetallic Td-MoTe2, with critical temperature (Tc) increasing up to 7.6 K for monolayers, a 60-fold increase with respect to the bulk Tc. We show that monolayers possess a similar electronic structure and density of states (DOS) as the bulk, implying that electronic interactions play a strong role in the enhanced superconductivity. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. The response to high in-plane magnetic fields is distinct from that of other 2D SCs and reflects the canted spin texture of the electron pockets.

Electronic Nematic States Tuned by Isoelectronic Substitution in Bulk FeSe1−xSx

Abstract: Isoelectronic substitution is an ideal tuning parameter to alter electronic states and correlations in iron-based superconductors. As this substitution takes place outside the conducting Fe planes, the electronic behaviour is less affected by the impurity scattering experimentally and relevant key electronic parameters can be accessed. In this short review, I present the experimental progress made in understanding the electronic behaviour of the nematic electronic superconductors, FeSe 1−x S x . A direct signature of the nematic electronic state is in-plane anisotropic distortion of the Fermi surface triggered by orbital ordering effects and electronic interactions that result in multi-band shifts detected by ARPES. Upon sulphur substitution, the electronic correlations and the Fermi velocities decrease in the tetragonal phase. Quantum oscillations are observed for the whole series in ultra-high magnetic fields and show a complex spectra due to the presence of many small orbits. Effective masses associated to the largest orbit display non-divergent behaviour at the nematic end point (x ∼ 0.175(5)), as opposed to critical spin-fluctuations in other iron pnictides. Magnetotransport behaviour has a strong deviation from the Fermi liquid behaviour and linear T resistivity is detected at low temperatures inside the nematic phase, where scattering from low energy spin-fluctuations are likely to be present. The superconductivity is not enhanced in FeSe1−xSx and there are no divergent electronic correlations at the nematic end point. These manifestations indicate a strong coupling with the lattice in FeSe1−xSx and a pairing mechanism likely promoted by spin fluctuations.

Magnetic and f-electron effects in LaNiO2 and NdNiO2 nickelates with cuprate-like $$3{d}_{{x}^{2}-{y}^{2}}$$ 3 d x 2 − y 2 band

Abstract: Recent discovery of superconductivity in the doped infinite-layer nickelates has renewed interest in understanding the nature of high-temperature superconductivity more generally. The low-energy electronic structure of the parent compound NdNiO2, the role of electronic correlations in driving superconductivity, and the possible relationship between the cuprates and the nickelates are still open questions. Here, by comparing LaNiO2 and NdNiO2 systematically within a parameter-free, all-electron first-principles density-functional theory framework, we reveal the role of Nd 4f electrons in shaping the ground state of pristine NdNiO2. Strong similarities are found between the electronic structures of LaNiO2 and NdNiO2, except for the effects of the 4f electrons. Hybridization between the Nd 4f and Ni 3d orbitals is shown to significantly modify the Fermi surfaces of various magnetic states. In contrast, the competition between the magnetically ordered phases depends mainly on the gaps in the Ni $$3{d}_{{x}^{2}-{y}^{2}}$$ band. Our estimated value of the on-site Hubbard U in the nickelates is similar to that in the cuprates, but the value of the Hund’s coupling JH is found to be sensitive to the Nd magnetic moment. In contrast with the cuprates, NdNiO2 presents 3D magnetism with competing antiferromagnetic and (interlayer) ferromagnetic exchange, which may explain why the Tc is lower in the nickelates. The recent discovery of superconducting nickelates has reignited interest in these materials and whether they can shed light on the mechanism of unconventional superconductivity in the cuprates. Here, the authors use first principles calculations to investigate the f electrons and magnetic ordering effects in the infinite layer nickelates and elaborate on the role of the cuprate-like 3dx2-y2 band.

Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet

Abstract: Analogous to the Hall effect, the Nernst effect is the generation of a transverse voltage due to a temperature gradient in the presence of a perpendicular magnetic field. The Nernst effect has promise for thermoelectric applications and as a probe of electronic structure. In magnetic materials, a so-called anomalous Nernst effect (ANE) is possible in zero magnetic field. Here we report a colossal ANE reaching 23 $\mu$V/K in the ferromagnetic metal UCo$_{0.8}$Ru$_{0.2}$Al. Uranium's $5f$ electrons provide strong electronic correlations that lead to narrow bands, which are a known route to producing a large thermoelectric response. Additionally, the large nuclear charge of uranium generates strong spin-orbit coupling, which produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that at least 148 Weyl nodes and two nodal lines exist within $\pm$ 60 meV of the Fermi level in UCo$_{0.8}$Ru$_{0.2}$Al. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Applications of Carbon Dots in Optoelectronics.

Abstract: Carbon dots (CDs) are an attractive class of nanomaterials due to the ease of their synthesis, biocompatibility, and superior optical properties. The electronic structure of CDs and hence their optical transitions can be controlled and tuned over a wide spectral range via the choice of precursors, adjustment of the synthetic conditions, and post-synthetic treatment. We summarize recent progress in the synthesis of CDs emitting in different colors in terms of morphology and optical properties of the resulting nanoparticles, with a focus on the synthetic approaches allowing to shift their emission to longer wavelengths. We further consider formation of CD-based composite materials, and review approaches used to prevent aggregation and self-quenching of their emission. We then provide examples of applications of CDs in optoelectronic devices, such as solar cells and light-emitting diodes (LEDs) with a focus on white LEDs.

Electric gating and interlayer coupling controllable electronic structure and Schottky contact of graphene/BiI 3 van der Waals heterostructure

Abstract: Graphene-based van der Waals heterostructures have received tremendous interest from both fundamental and experimental studies because they can enhance the properties and expand the possibility of applications of both graphene and two-dimensional materials. Motivated by the successful synthesis of the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure [Chang et al., Adv. Funct. Mater. 28, 1800179 (2018)]., here, we systematically investigate the electronic structure and interfacial characteristics of this material using first-principles simulations. We find that the structure of the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure is mainly characterized by weak van der Waals interactions, which keeps the heterostructure feasible. In the ground state, the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure forms the $n$-type Schottky contact with a barrier of 0.53 eV. The barriers of the Schottky contact can be adjusted by various factors, including interlayer coupling and electric gating. Both the interlayer coupling and electric gating lead to the transformation from the $n$-type Schottky contact to the $p$-type one or to the $n$-type Ohmic contact. These findings demonstrate that graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ can be considered a promising building block for high-performance photoresponsive optoelectronic devices.