Electronic Phenomena Research Articles

Ferro-ionic states and domains morphology in HfxZr1−xO2 nanoparticles

Unique polar properties of nanoscale hafnia-zirconia oxides (HfxZr1−xO2) are of great interest for condensed matter physics, nanophysics, and advanced applications. These properties are connected (at least partially) to the ionic–electronic and electrochemical phenomena at the surface, interfaces, and/or internal grain boundaries. Here, we calculated the phase diagrams, dielectric permittivity, spontaneous polar, and antipolar ordering, as well as the domain structure morphology in HfxZr1−xO2 nanoparticles covered by ionic–electronic charge originating from surface electrochemical adsorption. We revealed that the ferro-ionic coupling supports the polar long-range order in nanoscale HfxZr1−xO2, induces, and/or enlarges the stability region of the labyrinthine domains toward smaller sizes and smaller environmental dielectric constant at low concentrations of the surface ions. The ferro-ionic coupling causes the transition to the single-domain ferro-ionic state at high concentrations of the surface ions. We predict that the labyrinthine domain states, being multiple-degenerated, may significantly affect the emergence of the negative differential capacitance state in the nanograined/nanocrystalline HfxZr1−xO2 films.

On-chip cryogenic low-pass filters based on finite ground-plane coplanar waveguides for quantum measurements.

Quantum technology exploits fragile quantum electronic phenomena whose energy scales demand ultra-low electron temperature operation. The lack of electron-phonon coupling at cryogenic temperatures makes cooling the electrons down to a few tens of millikelvin a non-trivial task, requiring extensive efforts on thermalization and filtering high-frequency noise. Existing techniques employ bulky and heavy cryogenic metal-powder filters, which prove ineffective at sub-GHz frequency regimes and unsuitable for high-density quantum circuits such as spin qubits. In this work, we realize ultra-compact and lightweight on-chip cryogenic filters based on the attenuation characteristics of finite ground-plane coplanar waveguides. These filters are made of aluminum on sapphire substrates using standard microfabrication techniques. The attenuation characteristics are measured down to a temperature of 500 mK in a dilution refrigerator in a wide frequency range of a few hundred kHz to 8.5GHz. We find their performance is superior by many orders compared to the existing filtering schemes, especially in the sub-GHz regime, negating the use of any lumped-element low-pass filters. The compact and scalable nature makes these filters a suitable choice for high-density quantum circuits such as quantum processors based on quantum dot spin qubits.

Pulse Dynamics in Reduced Graphene Oxide Electrolyte‐Gated Transistors: Charge Memory Effects and Mechanisms Governing the Ion‐To‐Electron Transduction

AbstractElectrolyte‐gated transistors (EGTs) are widely employed in bioelectronics due to their ability to bridge ionic and electronic phenomena in a single device. Among potential materials, reduced graphene oxide (rGO) has gained significant attention due to its ambipolar current response, quantum capacitance, and tunable conductivity. However, the rGO EGT dynamic behavior remains significantly unexplored. Here, the time‐dependent response of rGO EGTs is systematically investigated under gate voltage pulsing across different time scales (10 ms to 40 s) and amplitudes (up to |±0.8 V|). Significant charge memory is observed, particularly for long (40 s) pulses at 0.8 V, with effects also evident for shorter (1 s) and weaker stimuli (0.6 V). Multiple low‐level (0.1 V) fast pulsing (100 ms) further demonstrate charge retention post‐stimulation. All these characteristics are attributed to a complex interplay between ion entrapment within the rGO film, electrical double‐layer formation, and charge transfer processes. The stability of rGO EGTs under prolonged bias stress is also examined, aiming to contribute to the development of more robust devices. These findings revealed the complex role of electrolyte ions and electronic carriers governing the ion‐to‐electron transduction and charge memory effects in rGO EGTs, contributing to the advancement of the next‐generation bioelectronic devices.

The Importance of Catalytic Effects in Hot-Electron-Driven Chemical Reactions.

Hot electrons (HEs) represent out-of-equilibrium carriers that are capable of facilitating reactions which are inaccessible under conventional conditions. Despite the similarity of the HE process to catalysis, optimization strategies such as orbital alignment and adsorption kinetics have not received significant attention in enhancing the HE-driven reaction yield. Here, we investigate catalytic effects in HE-driven reactions using a compositional catalyst modification (CCM) approach. Through a top-down alloying process and systematic characterization, using electrochemical, photodegradation, and ultrafast spectroscopy, we are able to disentangle chemical effects from competing electronic phenomena. Correlation between reactant energetics and the HE reaction yield demonstrates the crucial role of orbital alignment in HE catalytic efficiency. Optimization of this parameter was found to enhance HE reaction efficiency 5-fold, paving the way for tailored design of HE-based catalysts for sustainable chemistry applications. Finally, our study unveils an emergent ordering effect in photocatalytic HE processes that imparts the catalyst with an unexpected polarization dependence.

Exploring Quantum Materials with Resonant Inelastic X-Ray Scattering

Understanding quantum materials—solids in which interactions among constituent electrons yield a great variety of novel emergent quantum phenomena—is a forefront challenge in modern condensed matter physics. This goal has driven the invention and refinement of several experimental methods, which can spectroscopically determine the elementary excitations and correlation functions that determine material properties. Here we focus on the future experimental and theoretical trends of resonant inelastic x-ray scattering (RIXS), which is a remarkably versatile and rapidly growing technique for probing different charge, lattice, spin, and orbital excitations in quantum materials. We provide a forward-looking introduction to RIXS and outline how this technique is poised to deepen our insight into the nature of quantum materials and of their emergent electronic phenomena. Published by the American Physical Society 2024

Spontaneous emergence of straintronics effects and striped stacking domains in untwisted three-layer epitaxial graphene

Emergent electronic phenomena, from superconductivity to ferroelectricity, magnetism, and correlated many-body band gaps, have been observed in domains created by stacking and twisting atomic layers of Van der Waals materials. In graphene, emergent properties have been observed in ABC stacking domains obtained by exfoliation followed by expert mechanical twisting and alignment with the desired orientation, a process very challenging and nonscalable. Here, conductive atomic force microscopy shows in untwisted epitaxial graphene grown on SiC the surprising presence of striped domains with dissimilar conductance, a contrast that demonstrates the presence of ABA and ABC domains since it matches exactly the conductivity difference observed in ABA/ABC domains in twisted exfoliated graphene and calculated by density functional theory. The size and geometry of the stacking domains depend on the interplay between strain, solitons crossing, and shape of the three-layer regions. Interestingly, we demonstrate the growth of three-layer regions in which the ABA/ABC stacking domains self-organize in stable stripes of a few tens of nanometers. The growth-controlled production of isolated and stripe-shaped ABA/ABC domains open the path to fabricate quantum devices on these domains. These findings on self-assembly formation of ABA/ABC epitaxial graphene stripes on SiC without the need of time-consuming and nonscalable graphene exfoliation, alignment, and twisting provide different potential applications of graphene in electronic devices.

Universal moiré buckling of freestanding 2D bilayers

The physics of membranes, a classic subject, acquires new momentum from two-dimensional (2D) materials multilayers. This work reports the surprising results emerged during a theoretical study of equilibrium geometry of bilayers as freestanding membranes. While ordinary membranes are prone to buckle around compressive impurities, we predict that all 2D material freestanding bilayers universally undergo, even if impurity-free, a spontaneous out-of-plane buckling. The moiré network nodes here play the role of internal impurities, the dislocations that join them giving rise to a stress pattern, purely shear in homobilayers and mixed compressive/shear in heterobilayers. That intrinsic stress is, theory and simulations show, generally capable to cause all freestanding 2D bilayers to undergo distortive bucklings with large amplitudes and a rich predicted phase transition scenario. Realistic simulations predict quantitative parameters expected for these phenomena as expected in heterobilayers such as graphene/hBN, [Formula: see text] heterobilayers, and for twisted homobilayers such as graphene, hBN, [Formula: see text]. Buckling then entails a variety of predicted consequences. Mechanically, a critical drop of bending stiffness is expected at all buckling transitions. Thermally, the average buckling corrugation decreases with temperature, with buckling-unbuckling phase transitions expected in some cases, and the buckled state often persisting even above room temperature. Buckling will be suppressed by deposition on hard attractive substrates, and survives in reduced form on soft ones. Frictional, electronic, and other associated phenomena are also highlighted. The universality and richness of these predicted phenomena strongly encourages an experimental search, which is possible but still missing.

Spin transport of a doped Mott insulator in moiré heterostructures

Moiré superlattices of semiconducting transition metal dichalcogenide heterobilayers are model systems for investigating strongly correlated electronic phenomena. Specifically, WSe2/WS2 moiré superlattices have emerged as a quantum simulator for the two-dimensional extended Hubbard model. Experimental studies of charge transport have revealed correlated Mott insulator and generalized Wigner crystal states, but spin transport of the moiré heterostructure has not yet been sufficiently explored. Here, we use spatially and temporally resolved circular dichroism spectroscopy to directly image the spin transport as a function of carrier doping and temperature in WSe2/WS2 moiré heterostructures. We observe diffusive spin transport at all hole concentrations at 11 Kelvin — including the Mott insulator at one hole per moiré unit cell — where charge transport is strongly suppressed. At elevated temperatures the spin diffusion constant remains unchanged in the Mott insulator state, but it increases significantly at finite doping away from the Mott state. The doping- and temperature-dependent spin transport can be qualitatively understood using a t–J model, where spins can move via the hopping of spin-carrying charges and via the exchange interaction.

(Invited) Suppression of Electronic Leakage in Proton-Conducting Cells

Solid oxide cells (SOC) utilizing proton-conducting metal oxides as electrolytes offer the advantage of reduced operating temperatures compared to conventional SOCs. When used as fuel cells, they exhibit high fuel utilization rates, and when applied to steam electrolysis, they offer the benefit of producing hydrogen without steam. However, there are still several challenges to address, and the electronic leakage is most crucial. Proton-conducting oxides generally exhibit characteristics of hole conduction in oxidative atmospheres. Hole conduction affects the entire cell, even if one side of the cell is in a reducing atmosphere, resulting in decreased Faraday efficiency during steam electrolysis for hydrogen production and extra fuel consumption without contributing to power generation in fuel cell applications. Therefore, electronic leakage phenomena lead to a decrease in energy efficiency in both types of devices.In this presentation, it is demonstrated how electronic leakage phenomena in proton-conducting solid oxide cells appear experimentally in both steam electrolysis and fuel cell modes, and discussed on the possible methods to suppress this phenomenon.In steam electrolysis, electronic leakage is observed as a decrease in Faraday efficiency. Ideally, with the electrolyte being fully ionically conductive, the amount of hydrogen generated by a charge of electrical quantity Q [C] should be Q/2F [mol], where F is the Faraday constant. However, actual hydrogen generation is reduced due to electronic leakage. Depending on the electrolyte, Faradaic efficiency can be from 20% to 30%, resulting in a significant portion of input power dissipating as Joule heat. Even achieving 80% to 90% Faraday efficiency leaves a considerable loss of 10% to 20%. In the fuel cell mode, electronic leakage is observed as excessive fuel consumption. Even in an open-circuit state with hydrogen and air introduced into the cell, water formation is observed. This is due to internal short circuits caused by electronic leakage in the electrolyte.One proposed method to suppress this phenomenon is to find materials that do not exhibit hole conduction (or exhibit it to a lesser extent). Is this feasible? The mechanism by which proton conduction occurs in metal oxides is explained by the hydration of oxygen vacancies. When the temperature is sufficiently low, around 600°C or below, for chemical equilibrium hydration to progress in a humid atmosphere, oxygen vacancies are hydrated, and protons (hydrogen ions) appear as conducting species within the lattice. On the other hand, oxygen vacancies can also accept oxygen from the atmosphere, resulting in hole conduction that causes the aforementioned leakage phenomenon. In other words, the equilibrium reactions that produce protons and holes are very similar, both involving the filling of oxygen vacancies with oxygen. Therefore, as long as materials are designed to achieve proton conduction through this mechanism, eliminating only hole conduction seems theoretically difficult.As an alternative approach to suppress electronic leakage in proton-conducting cells, the authors are investigating the control of transport phenomena involved in electronic leakage reactions. The figure depicts reactions around the anode (oxygen-evolving electrode) in steam electrolysis, where a mixed conductor of oxide ions and electrons is used as the electrode. The correct reaction involves water splitting into protons and oxide ions at the electrolyte-electrode interface, but leakage current flows when holes and electrons are generated at the same interface. To mitigate this, an improvement in Faraday efficiency was observed by inserting gadolinium-doped ceria (GDC) as an electron-blocking layer in part of the electrode. Thus, it seems possible to suppress electronic leakage by controlling the flow of specific substances through the implementation of layers with specific functions in the electrolyte or electrodes and utilizing this control effectively.AcknowledgementThis paper is partially based on results obtained from a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Reductive mechanochemical synthesis of alkali molybdenum bronze nanoparticles

The synthesis of two mixed-valent oxides of Mo using mechanochemical methods was presented. The reactions proceed via two-steps including ball milling and an annealing step. Two alkali molybdenum oxides bronzes of special interest, K0·3MoO3 and Na0.9Mo6O17 were considered. The structures and stoichiometry are supported by powder X-ray diffraction and energy dispersive X-ray spectroscopy measurements. It was found that the effects of ball milling significantly reduce the time required (from 12 or more to 4 h) to complete the reaction by creating solid solutions which dramatically lower the melting point of MoO3 from around 800 °C–400 °C. Additionally, we demonstrate the use of ball milling to synthesize nanoparticles of these bronzes with controlled size distributions, with average length down to 7.7 ± 2.5 nm; shown by pXRD and transmission electron microscopy measurements. The results demonstrate the possibilities for reducing total reaction times necessary for the formation of complex oxides by combining mechanochemical methods with additional synthetic routes. This methodology, as well as the ability to control product size, enables further investigation of size-dependent electronic phenomena in these systems.

Ultra-flatbands in twisted penta-hexa-CB bilayer with large twist angles

We report the discovery of ultra-flatbands in twisted penta-hexa-carbon boron (PH-CB) bilayers, a finding with profound implications for condensed matter physics. Using first-principles calculations, we show that PH-CB bilayers exhibit an exceptionally narrow bandwidth of 0.13 meV at a distortion angle of 9.6°, indicating the robust electron correlation effects. This observation contrasts with the need for tiny twist angles in graphene to achieve similar effects. The PH-CB system, with its indirect bandgap of 2.11 eV, represents a versatile platform for exploring novel electronic phenomena that may lead to advances in quantum materials. Our results highlight the inverse relationship between wave function localization and bandwidth and provide a new perspective for the design of two-dimensional materials with tailored electronic properties.

Layer-dependent evolution of electronic structures and correlations in rhombohedral multilayer graphene.

The recent discovery of superconductivity and magnetism in trilayer rhombohedral graphene (RG) establishes an ideal, untwisted platform to study strong correlation electronic phenomena. However, the correlated effects in multilayer RG have received limited attention, and, particularly, the evolution of the correlations with increasing layer number remains an unresolved question. Here we show the observation of layer-dependent electronic structures and correlations-under surprising liquid nitrogen temperature-in RG multilayers from 3 to 9 layers by using scanning tunnelling microscopy and spectroscopy. We explicitly determine layer-enhanced low-energy flat bands and interlayer coupling strengths. The former directly demonstrates the further flattening of low-energy bands in thicker RG, and the latter indicates the presence of varying interlayer interactions in RG multilayers. Moreover, we find significant splittings of the flat bands, ranging from ~50 meV to 80 meV, at 77 K when they are partially filled, indicating the emergence of interaction-induced strongly correlated states. Particularly, the strength of the correlated states is notably enhanced in thicker RG and reaches its maximum in the six-layer, validating directly theoretical predictions and establishing abundant new candidates for strongly correlated systems. Our results provide valuable insights into the layer dependence of the electronic properties in RG and demonstrate it as a suitable system for investigating robust and highly accessible correlated phases.

Single-Molecule Mechanoresistivity by Intermetallic Bonding.

The metal-electrode interface is key to unlocking emergent behaviour in all organic electrified systems, from battery technology to molecular electronics. In the latter, interfacial engineering has enabled efficient transport, higher device stability, and novel functionality. Mechanoresistivity - the change in electrical behaviour in response to a mechanical stimulus and a pathway to extremely sensitive force sensors - is amongst the most studied phenomena in molecular electronics, and the molecule-electrode interface plays a pivotal role in its emergence, reproducibility, and magnitude. In this contribution, we show that organometallic molecular wires incorporating a Pt(II) cation show mechanoresistive behaviour of exceptional magnitude, with conductance modulations of more than three orders of magnitude upon compression by as little as 1 nm. We synthesised series of cyclometalated Pt(II) molecular wires, and used scanning tunnelling microscopy - break junction techniques to characterise their electromechanical behaviour. Mechanoresistivity arises from an interaction between the Pt(II) cation and the Au electrode triggered by mechanical compression of the single-molecule device, and theoretical modelling confirms this hypothesis. Our study provides a new tool for the design of functional molecular wires by exploiting previously unreported ion-metal interactions in single-molecule devices, and develops a new framework for the development of mechanoresistive molecular junctions.

Single‐Molecule Mechanoresistivity by Intermetallic Bonding

AbstractThe metal‐electrode interface is key to unlocking emergent behaviour in all organic electrified systems, from battery technology to molecular electronics. In the latter, interfacial engineering has enabled efficient transport, higher device stability, and novel functionality. Mechanoresistivity – the change in electrical behaviour in response to a mechanical stimulus and a pathway to extremely sensitive force sensors – is amongst the most studied phenomena in molecular electronics, and the molecule‐electrode interface plays a pivotal role in its emergence, reproducibility, and magnitude. In this contribution, we show that organometallic molecular wires incorporating a Pt(II) cation show mechanoresistive behaviour of exceptional magnitude, with conductance modulations of more than three orders of magnitude upon compression by as little as 1 nm. We synthesised series of cyclometalated Pt(II) molecular wires, and used scanning tunnelling microscopy – break junction techniques to characterise their electromechanical behaviour. Mechanoresistivity arises from an interaction between the Pt(II) cation and the Au electrode triggered by mechanical compression of the single‐molecule device, and theoretical modelling confirms this hypothesis. Our study provides a new tool for the design of functional molecular wires by exploiting previously unreported ion‐metal interactions in single‐molecule devices, and develops a new framework for the development of mechanoresistive molecular junctions.

Spin-orbit proximity in MoS2/bilayer graphene heterostructures

Van der Waals heterostructures provide a versatile platform for tailoring electronic properties through the integration of two-dimensional materials. Among these combinations, the interaction between bilayer graphene and transition metal dichalcogenides (TMDs) stands out due to its potential for inducing spin–orbit coupling (SOC) in graphene. Future devices concepts require the understanding of the precise nature of SOC in TMD/bilayer graphene heterostructures and its influence on electronic transport phenomena. Here, we experimentally confirm the presence of two distinct types of SOC – Ising (ΔI = 1.55 meV) and Rashba (ΔR = 2.5 meV) – in bilayer graphene when interfaced with molybdenum disulfide. Furthermore, we reveal a non-monotonic trend in conductivity with respect to the electric displacement field at charge neutrality. This phenomenon is ascribed to the existence of single-particle gaps induced by the Ising SOC, which can be closed by a critical displacement field. Our findings also unveil sharp peaks in the magnetoconductivity around the critical displacement field, challenging existing theoretical models.

Optical properties of CVD-grown multilayer graphene on nickel using spectroscopic ellipsometry

Incorporating graphene into relevant technologies requires its integration with commercially suitable substrates. Understanding the interactions between graphene and these substrates is crucial, as graphene serves as an ideal model system for investigating electronic phenomena. In this work, we report the optical properties of multilayer graphene on nickel substrates using spectroscopic ellipsometry. We provide information on the spectral dependence of optical properties of multilayer graphene, such as the complex dielectric constant, refractive index, and optical conductivity in the energy range of 1.6–5.0 eV. The optical conductivity profile obtained from SE analysis showed a symmetrical peak at 4.38 eV, suggesting an interband transition from the π to π∗ orbital at the M point. The graphene/Ni interaction generated changes in the number of available states below the Fermi level, leading to significant changes in electron density. Our result provides the information essential for understanding relevant research and developing graphene-based optoelectronic applications.

Intensive γ -ray light sources based on oriented single crystals

The feasibility of gamma-ray light sources based on the channeling phenomenon of ultrarelativistic electrons and positrons in oriented single crystals is demonstrated using rigorous numerical modeling. Case studies are presented for 10 GeV and sub-GeV e−/e+ beams incident on 10−1−100 mm thick diamond and silicon crystals. It is shown that for moderate values of the beam average current (≲10 μA), the average photon flux in the energy range of 100–102 MeV emitted within the 101–103 μrad cone and 1% bandwidth can be on the level of 1010 photon/s for electrons and 1010–1012 photon/s for positrons. These values are higher than the fluxes available at modern laser-Compton gamma-ray light sources. Published by the American Physical Society 2024

A THEORETICAL STUDY OF THE PHYSICAL PROPERTIES OF HEXAGONAL GALLIUM NITRATE USING DENSITY FUNCTIONAL THEORY

First-principles calculations based on density functional theory (DFT) have been used to investigate structural, electronic, optical and thermodynamic aspects of the α-GaN crystal. Based on local density approximations (LDA), Generalized gradient approximation (GGA) and meta-generalized gradient approximation (m-GGA) functional methods, the band gap energies of α-GaN crystals have been estimated as 1.962 eV, 2.069 eV and 2.354 eV. The band gap energies that are presented in these studies are in accordance with the ones from the other experimental and theoretical studies. Besides, our findings give us knowledge about the electronic and optical properties of α-GaN crystal. The band gap energies in the α-GaN crystal are the key factors that define its electrical and optical characteristics. They are the energy range in which the electrons can be exited from the valence band to the conduction band, which in turn affects the material's conductivity and the ability of the material to absorb and emit light. The approximate of our results with the previous researches indicates the reliability of our findings and thus increases our knowledge of α-GaN's electronic and optical phenomena. The orbital characteristics of the Ga and N atoms were found by simulating the density of state and the partial density of state for α-GaN. Besides the analysis of the band structure, density of states and optical properties of the compound we also included. The results show that α-GaN has a direct bandgap, which is at the G point in the Brillouin zone. This is the reason for its great potential for the development of optoelectronic devices. Also, we use the three approximations that are given earlier to find the optical characteristics (the absorption coefficient) of the compound. In addition to that, the thermodynamic properties that can be calculated like Debye temperature, enthalpy, free energy, entropy and heat capacity enable us to understand the thermal behavior of the compound better. The heat capacity of α-GaN is detected to be 17.3 Jmole-1K-1, with a Debye temperature of 824.6K. This research will offer a detailed interpretation of α- G-N, covering all its basic properties and the possible applications in optoelectronic and electronic devices. The results of this study are very important and the new technologies that will be developed based on the α-GaN research will be very beneficial.

X-ray spectromicroscopy of single NiO antiferromagnetic nanoparticles

The chemical and magnetic properties of NiO nanoparticles (NP) have been studied with single-particle sensitivity by means of synchrotron-based, polarization-dependent X-ray absorption spectroscopy using photoemission electron microscopy around the Ni L3,2 edges. Three samples of NP in a size range of 40-120 nm were synthesized by thermal decomposition and subsequent calcination processes. The analysis of the local X-ray absorption spectra of tens of individual NP indicates a strong dependence of their Ni oxidation state with the calcination protocol of each sample. Additional electron-microscopy-based images and spectra of a few individual NP as well as other standard macroscopic data are in very good agreement with these experimental findings. These results showcase the relevance of combining standard and advanced single-particle studies to gain further insight into the understanding and control of electronic and magnetic phenomena at the nanoscale.

3D-Mapping and Manipulation of Photocurrent in an Optoelectronic Diamond Device.

Establishing connections between material impurities and charge transport properties in emerging electronic and quantum materials, such as wide-bandgap semiconductors, demands new diagnostic methods tailored to these unique systems. Many such materials host optically-active defect centers which offer a powerful in situ characterization system, but one that typically relies on the weak spin-electric field coupling to measure electronic phenomena. In this work, charge-state sensitive optical microscopy is combined with photoelectric detection of an array of nitrogen-vacancy (NV) centers to directly image the flow of charge carriers inside a diamond optoelectronic device, in 3D and with temporal resolution. Optical control is used to change the charge state of background impurities inside the diamond on-demand, resulting in drastically different current flow such as filamentary channels nucleating from specific, defective regions of the device. Conducting channels that control carrier flow, key steps toward optically reconfigurable, wide-bandgap optoelectronics are then engineered using light. This work might be extended to probe other wide-bandgap semiconductors (SiC, GaN) relevant to present and emerging electronic and quantumtechnologies.