Charge Neutrality Research Articles

Dirac spectroscopy of strongly correlated phases in twisted trilayer graphene.

Magic-angle twisted trilayer graphene (MATTG) hosts flat electronic bands, and exhibits correlated quantum phases with electrical tunability. In this work, we demonstrate a spectroscopy technique that allows for dissociation of intertwined bands and quantification of the energy gaps and Chern numbers C of the correlated states in MATTG by driving band crossings between Dirac cone Landau levels and energy gaps in the flat bands. We uncover hard correlated gaps with C = 0 at integer moiré unit cell fillings of ν = 2 and 3 and reveal charge density wave states originating from van Hove singularities at fractional fillings ν = 5/3 and 11/3. In addition, we demonstrate displacement-field-driven first-order phase transitions at charge neutrality and ν = 2, which are consistent with a theoretical strong-coupling analysis, implying C2T symmetry breaking. Overall, these properties establish a diverse electrically tunable phase diagram of MATTG and provide an avenue for investigating other related systems hosting both steep and flat bands.

A phase-field study of stainless-steel oxidation from high-temperature carbon dioxide exposure

An electrochemical phase-field model has been developed to investigate the oxidation mechanisms of the 21-2N valve stainless steel alloy exposed to carbon dioxide at 973 K. Three oxide phases observed in oxidation experiments are included in the model: Mn3O4, Cr2O3, and MnCr2O4. Local charge neutrality and the conserved current condition are assumed to include the impact of the electric potential on oxidation, though it is found to be negligible when the system is electrically isolated. The sensitivity of the oxidation processes to the values of the diffusion mobilities is examined, and the oxidation rate is calibrated against experimental data by modifying the sensitive mobilities. We find that both inward oxygen and outward metal diffusion are important for oxidation. By investigating the impact of the order of the initial oxide layers, we find that Cr2O3 serves as a barrier to outward Mn diffusion. Moreover, controlling the initial oxide layer order can be used to limit oxidation.

Local Kekulé distortion turns twisted bilayer graphene into topological Mott insulators and superconductors

Magic-angle twisted bilayer graphene displays at different fillings of the four flat bands lying around the charge neutrality point a wealth of notable phases that include magnetic Chern insulators, whose magnetization is mostly of an orbital nature and contiguous superconducting domes. Such a rich phase diagram is explained through the positive interplay of Coulomb repulsion and the electron coupling to a twofold optical mode that corresponds to Kekul\'e distortions localized into the small AA stacked regions of the moir\'e supercells. A static distortion stabilizes, at any integer filling of the flat bands, valence-bond insulators that carry finite Chern number away from charge neutrality. Similarly, a dynamic distortion that resonates between the two lattice vibrations leads to resonating-valence-bond topological insulators with built-in chiral d-wave pairs that have finite Chern number equal to the angular momentum, and thus are prone to turn superconducting upon doping away from integer filling.

Intrinsic localized modes in polymers and hyperconductors

The history of the experimental study of nonlinear lattice excitations in layered silicate materials, when exposed to swift particles of appreciable momentum is described briefly. For brevity, and because of the difficulty of studying the structure of the lattice excitations, the term quodon was adopted to reflect their ballistic and quasione-dimensional propagative nature. Quodons in muscovite were observed experimentally. Eventually, it was deduced that the lattice excitations were carrying an electric charge. This led to the prediction of hyperconductivity (HC) in which charge is carried ballistically by neutral, mobile lattice excitations in absence of a driving electromotive force and at any temperature. HC was later observed experimentally. For practical applications of HC, it is necessary to encase the HC material in an insulating sheath. This focused attention on the behavior of insulating materials in the presence of quodons. These could enter the sheath by direct contact with the HC material or by the impact of the swift particles. It was found that quodons can exist and propagate in many different materials, perhaps all, but their behavior can vary dramatically. This universality and charge neutrality, together with their unexpected existence in the excellent insulator polytetrafluoroethylene, probably accounts for the delay in finding evidence of their existence.

Signatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe5

The quantum limit (QL) of an electron liquid, realised at strong magnetic fields, has long been proposed to host a wealth of strongly correlated states of matter. Electronic states in the QL are, for example, quasi-one dimensional (1D), which implies perfectly nested Fermi surfaces prone to instabilities. Whereas the QL typically requires unreachably strong magnetic fields, the topological semimetal ZrTe5 has been shown to reach the QL at fields of only a few Tesla. Here, we characterize the QL of ZrTe5 at fields up to 64 T by a combination of electrical-transport and ultrasound measurements. We find that the Zeeman effect in ZrTe5 enables an efficient tuning of the 1D Landau band structure with magnetic field. This results in a Lifshitz transition to a 1D Weyl regime in which perfect charge neutrality can be achieved. Since no instability-driven phase transitions destabilise the 1D electron liquid for the investigated field strengths and temperatures, our analysis establishes ZrTe5 as a thoroughly understood platform for potentially inducing more exotic interaction-driven phases at lower temperatures.

Investigation on structural, dielectric, and impedance characteristics of Zr-modified NaNbO3 ceramics at elevated temperature

NaNb 1 − x Zr x O 3 (NZr) ceramics with 0 ≤ x ≤ 0.15 are prepared via solid-state reaction route. Structural, microstructural, and electronic state investigations of NZr samples were performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS), respectively. Rietveld refinement reveals that NZr samples crystallize in the orthorhombic phase similar to room temperature P b m a phase of NaNbO 3 . Minor quantities of ZrO 2 are detected for higher NZr-composition, which suppress the grain growth of NZr ceramics. XPS reveals that small amount of Zr substitution in NaNbO 3 also creates an ambiance of unbalanced charge neutrality. The dielectric characteristics of NZr samples are investigated from -190 to 450 °C temperature range in frequency window 1 KHz–1 MHz for both the heating and cooling cycles. A clear thermal hysteresis loop is obtained in each composition and the observed anomaly corresponds to P (AFE) - R (AFE) polymorphic phase transition. As x grows, ϵ ′ (T) becomes more dispersive and strongly depend on applied field. The activation energy obtained from the analysis of Arrhenius relation indicates that the oxygen vacancies are responsible for conduction process of the material. Impedance analysis confirms the NTCR behavior and non-Debye relaxation of the material. • The polycrystalline NZr ceramics are prepared by solid state reaction method. • Small amount of Zr dopant creates an ambiance of unbalanced charge neutrality. • The AFE (P) to AFE (R) phase transition is confirmed by ɛ ’ (T) analysis. • Impedance spectroscopic studies reveal the NTCR behavior of NZr ceramics. • The non-Debye type nature of material is verified by Nyquist analysis.

A Self‐Sustained Charge Neutrality Intracloud Lightning Parameterization Containing Channel Decay and Reactivation

AbstractA self‐sustained charge neutrality parameterization scheme of intracloud (IC) lightning is presented in this paper. The scheme contains the following features: simultaneous development of multiple branches, polarity asymmetry of positive and negative leaders, nonlinear electrical parameters, complete charge neutrality, and channel decay and reactivation. Nonlinear electrical parameters and charge neutrality are the key factors that determinate channel decay and reactivation. To demonstrate the simulation capacity of this scheme, two simulated intracloud flashes by this scheme under the tripole charge structure are chosen for analysis. These two IC flashes show clearly the above features, where reactivation processes that initiated from different branches and the real‐time changes in nonlinear electrical parameters are mainly analyzed. The simulation results are in line with the current knowledge on lightning, which also suggests that the new scheme may become a useful tool to deeply explore the discharge phenomena involving channel decay and reactivation.

Hydrodynamic magnetoresistance in graphene Corbino devices

We study hydrodynamic electron magnetotransport in graphene devices. We show that in these systems a distinct mechanism of magnetoresistance appears which is absent in systems with Galilean-invariant electron liquid. The resulting magnetoresistance depends on the intrinsic conductivity and viscosity of the electron liquid, and becomes especially pronounced near charge neutrality. We obtain analytic expressions for magnetotransport coefficients of Corbino devices and obtain estimates for the electrical and thermal magnetoresistances for monolayer and bilayer systems at charge neutrality. Magnetoresistance becomes strong (of order $100\phantom{\rule{0.16em}{0ex}}%$) at relatively weak fields, at which the kinetic coefficients of the electron liquid are practically unaffected by the magnetic field.

Computational Study of the pH-Dependent Ionic Environment around β-Lactoglobulin.

Ions are involved in multiple biological processes and may exist bound to biomolecules or may be associated with their surface. Although the presence of ions in nucleic acids has traditionally gained more interest, ion-protein interactions, often with a marked dependency on pH, are beginning to gather attention. Here we present a detailed analysis on the binding and distribution of ions around β-lactoglobulin using a constant-pH MD (CpHMD) method, at a pH range 3-8, and compare it with the more traditional Poisson-Boltzmann (PB) model and the existing experimental data. Most analyses used ion concentration maps built around the protein, obtained from either the CpHMD simulations or PB calculations. The requirements of approximate charge neutrality and ionic strength equal to bulk, imposed on the MD box, imply that the absolute value of the ion excess should be half the protein charge, which is in agreement with experimental observation on other proteins ( Proc. Natl. Acad. Sci. U.S.A. 2021, 118, e2015879118) and lends support to this protocol. In addition, the protein total charge (including territorially bound ions) estimated with MD is in excellent agreement with electrophoretic measurements. Overall, the CpHMD simulations show good agreement with the nonlinear form of the PB (NLPB) model but not with its linear form, which involves a theoretical inconsistency in the calculation of the concentration maps. In several analyses, the observed pH-dependent trends for the counterions and co-ions are those generally expected, and the ion concentration maps correctly converge to the bulk ionic strength as one moves away from the protein. Despite the overall similarity, the CpHMD and NLPB approaches show some discrepancies when analyzed in more detail, which may be related to an apparent overestimation of counterion excess and underestimation of co-ion exclusion by the NLPB model, particularly at short distances from the protein.

Multifactor theoretical study on ionic transport numbers of mixed oxygen ionic-electronic conducting oxides determined by the electromotive force method

The electromotive force (EMF) method is widely applied to measure the ionic transport numbers (ti) of mixed oxygen-ionic and electronic conducting oxides. However, the results determined by the EMF method are the average/apparent ti (tiapp), subjected to a given gradient of oxygen partial pressure (PO2), rather than ti at a specific PO2, making it challenging to reveal the accurate properties of materials. In this study, to clarify the impact of this issue, a precise defect distribution model of Gd doped CeO2 (GDC), considering defect equilibrium and local charge neutrality, is built. A multifactor theoretical analysis is conducted to compare tiapp and ti of the membrane at different sides. The modeling results reveal that a thick membrane is recommended for avoiding the influence of membrane thickness on tiapp. When there is a large difference of PO2 between two sides of the GDC membrane (PO2highPO2PO2lowPO2>1025), tiapp is inclined to ti at the side of high PO2. Only when both sides of the GDC membrane are reducing atmosphere, tiapp of the modified EMF method is approximately the average of ti at both sides. The findings of this study are instructive for the rational application of EMF methods.

Integer quantum Hall effect and enhanced g factor in quantum-confined Cd3As2 films

We investigate the integer quantum Hall effect in Cd3As2 thin films under conditions of strong to moderate quantum confinement (thicknesses of 10 nm, 12 nm, 15 nm). In all the films, we observe the integer quantum Hall effect in the spin-polarized lowest Landau level (filling factor {\nu} = 1) and at spin-degenerate higher index Landau levels with even filling factors ({\nu} = 2,4,6). With increasing quantum confinement, we also observe a lifting of the Landau level spin degeneracy at {\nu} = 3, manifest as the emergence of an anomaly in the longitudinal and Hall resistivity. Tight-binding calculations show that the enhanced g-factor likely arises from a combination of quantum confinement and corrections from nearby subbands. We also comment on the magnetic field induced transition from an insulator to a quantum Hall liquid when the chemical potential is near the charge neutrality point.

Exploring the dielectric and conduction characteristics of iodine substituted CaCu3Ti4O12-xIx

AC electrical measurements were carried out on iodine doped, CaCu3Ti4O12-xIx (x = 0, 0.005, 0.05 and 0.2) ceramic specimens sintered at 1100 °C for 12 h in the temperature (300–450 K) and frequency (20 Hz - 1 MHz). The crystalline structure, microstructure, and oxidation state of various ions of pure and iodine-substituted sintered specimens were carried out through XRD, SEM, and XPS, respectively. The dielectric relaxations in grains, grain boundaries, and sample-electrode interfaces for different compositions were investigated through impedance and modulus spectroscopic techniques and were reported in the first part of the investigation. It revealed the involvement of similar types of charge carriers in conduction and relaxation processes in grains and grain boundaries, respectively. This second section emphasizes the dielectric and conduction properties of CaCu3Ti4O12 (CCTO) with iodine doping at the anion site. The XPS study reveals that the concentration [VO••] is reduced, and [VM″] gets increased with increasing concentration of iodine from x = 0.05 to 0.2. This observation also supports the point defect model as reported earlier for charge compensation of impurity defect IO• i.e., with increasing substitutions of iodine, the charge on IO• change from electronic to cationic vacancies, VTi//// or copper VCu// or both to keep electrical charge neutrality. The metal ions get segregated at grain boundaries. This, in turn, will minimize hopping due to electrons between Ti4+ and Ti3+ or Cu2+ and Cu1+. As a result, it increases resistivity and minimizes dielectric losses for the higher concentration of iodine doping. Thus, a high concentration of iodine doping at the oxygen site promotes the formation of a barrier layer capacitor.

Dual-gated hBN/bilayer-graphene superlattices and the transitions between the insulating phases at the charge neutrality point

We report on transport properties in dual-gated hexagonal boron nitride (hBN)/bilayer-graphene (BLG) superlattices. Here, BLG is nontwisted, i.e., plain. This paper focuses on the charge neutrality point (CNP) for a plain BLG. Under a perpendicular magnetic field, transitions between two insulating phases at the CNP are detected by varying a displacement field with the study on the resistance-temperature characteristics and the magnetoresistance. This work opens avenues for exploring the global phase diagram of the hBN/BLG superlattices beyond the CNP.

Impact of Membrane Modification and Surface Immobilization Techniques on the Hemocompatibility of Hemodialysis Membranes: A Critical Review.

Despite significant research efforts, hemodialysis patients have poor survival rates and low quality of life. Ultrafiltration (UF) membranes are the core of hemodialysis treatment, acting as a barrier for metabolic waste removal and supplying vital nutrients. So, developing a durable and suitable membrane that may be employed for therapeutic purposes is crucial. Surface modificationis a useful solution to boostmembrane characteristics like roughness, charge neutrality, wettability, hemocompatibility, and functionality, which are important in dialysis efficiency. The modification techniques can be classified as follows: (i) physical modification techniques (thermal treatment, polishing and grinding, blending, and coating), (ii) chemical modification (chemical methods, ozone treatment, ultraviolet-induced grafting, plasma treatment, high energy radiation, and enzymatic treatment); and (iii) combination methods (physicochemical). Despite the fact that each strategy has its own set of benefits and drawbacks, all of these methods yielded noteworthy outcomes, even if quantifying the enhanced performance is difficult. A hemodialysis membrane with outstanding hydrophilicity and hemocompatibility can be achieved by employing the right surface modification and immobilization technique. Modified membranes pave the way for more advancement in hemodialysis membrane hemocompatibility. Therefore, this critical review focused on the impact of the modification method used on the hemocompatibility of dialysis membranes while covering some possible modifications and basic research beyond clinical applications.

Synchronously recovering different nutrient ions from wastewater by using selective electrodialysis.

Digestive slurry normally contains various nutrient ions with high concentrations, including NH4+, PO43-, K+, Mg2+, Ca2+ and SO42-, which is a resource pool for nutrient recovery. In this study, a synchronously cationic and anionic selective electrodialysis (SCAE) was developed to recover anionic and cationic nutrient ions. Results showed that SCAE could synchronously recover more than 85.0%, 90.2% and 97.8% of PO43-, SO42- and other cations (including NH4+, K+, Ca2+, Mg2+) from the simulated digestive slurry, respectively. The ionic permeation sequence, NH4+ > K+ > Ca2+ > Mg2+ for cations, and SO42- > PO43- for anions, was affected by hydrated radius and hydration numbers, and did not alter despite the variation in electric field. High electrolyte concentration in the product streams would promote the recovery efficiency of both divalent cations and anions due to the ionic replacement effect and the demand for charge neutrality. Under continuous operation, the maximum concentrations of PO43-, SO42-, Mg2+, Ca2+, NH4+ and K+ in product streams reached 231.9, 496.6, 180.7, 604.3, 9,648.4 and 4,571.4 mg·L-1, respectively. By directly mixing different streams, the feasibility of producing mineral fertilizers without dosing externally precipitating chemicals was proved. Struvite, NH4HSO4 and potassium chloride minerals were produced successfully. The outcome provided an optional method for nutrient recovery from wastewater.

Spin-triplet superconducting pairing in doped MoS2

Searching for triplet superconductivity has been pursued intensively in a broad field of material science and quantum information for decades. Nevertheless, these novel states remain rare. Within a simplified effective three-orbital model, we reveal a spin triplet pairing in doped MoS$_2$ by employing both the finite temperature determinant quantum Monte Carlo approach and the ground state constrained-phase quantum Monte Carlo method. In a wide filling region of $\avg{n}=0.60-0.80$ around charge neutrality $\avg{n}=2/3$, the $f$-wave pairing dominates over other symmetries. The pairing susceptibility strongly increases as the on-site Coulomb interaction increases, and it is insensitive to spin-orbit coupling.

Zero-temperature thermodynamics of dense asymmetric strong-interaction matter

Employing constraints derived from the microscopic theory of the strong interaction, we estimate the zero-temperature phase structure of dense isospin-asymmetric matter with two quark flavors. We find indications that strong-interaction matter along trajectories relevant for astrophysical applications undergoes a first-order phase transition from a color-superconducting phase to an ungapped quark-matter phase when the density is increased. Such a phase transition is found to be absent in isospin-symmetric matter. Moreover, by taking into account constraints from $\ensuremath{\beta}$ equilibrium, charge neutrality, and color neutrality, we provide an estimate for the speed of sound in neutron-star matter. Notably, we observe that the speed of sound in neutron-star matter exceeds the asymptotic value associated with the noninteracting quark gas and even increases toward lower densities across a wide range, in agreement with recent results for isospin-symmetric matter. Considering results from studies based on chiral effective field theory at low densities, our findings suggest the existence of a maximum in the speed of sound for $n/{n}_{0}\ensuremath{\lesssim}10$, where ${n}_{0}$ is the nuclear saturation density.

Effect of charge and anisotropic diffusivity on ion exchange kinetics in nuclear waste form materials

Motifs of radionuclide absorption particles (absorbers or fillers) for the nuclear waste treatment usually have crystal structures with nano-sized tunnels for fast ion diffusion. On the other hand, the accumulation of ions generates an electric field which affects the ion diffusion. Therefore, the strong anisotropic kinetic properties and the electric field affect ion exchange kinetics, especially in an assembly of these particles which may block the fast diffusion tunnels for each other. In this work, we developed a mesoscale model to describe the effect of anisotropic kinetic properties and electric field on ion exchange kinetics. With the model we simulated the effect of anisotropic diffusivity, electrochemical potential, particle morphology, size and aggregation on ion exchange kinetics during batch experiments. It is found that (1) ion exchange results in charge accumulation inside the particle because different ions have different diffusivities and different diffusion driving forces. The electric neutrality is not preserved; (2) the electric field speeds up the slower diffusion ions (either uptake or release) and slows down the faster diffusion ions to reduce charge accumulation and maintains charge neutrality; (3) the ion exchange becomes very slow at the later stages because (a) diffusion driving force (electrochemical gradient, Cs+ concentration in the solution and Na+ inside the particle) continuously decreases, as observed in batch experiments; and (b) the diffusivity decreases due to the Cs+ concentration dependence of ion diffusivity. This can explain the observed experimental phenomena that the radionuclide capacity usually cannot be reached; (4) for needle-like particle and their clusters, both surface area and diffusion distance along the direction with the fastest diffusivities have significant impact on the ion exchange kinetics.

(Digital Presentation) Bipolar Electrochemical Impedance Spectroscopy for Corrosion Monitoring of Steel Reinforced Cementitious Structural Components

Bipolar electrochemical impedance spectroscopy (BPEIS) was previously explored as a potential nondestructive method for detecting corrosion of steel in post-tensioned tendons which are used as reinforcing structural elements in segmental bridge construction. The tendons are composed of a grout-filled cylindrical plastic duct that contains high-strength steel strands. A proof of concept was demonstrated by experiments performed on tendons with passive and corroded strands and through finite-element simulations of the indirect impedance response1. Further work showed that bipolar impedance was sensitive to local corrosion in field tendons if the measurement is obtained near the corroding region2. This work further scrutinizes BPEIS as a corrosion monitoring method for reinforced cementitious structural components with direct comparison between corrosion damage estimates and actual metal loss.Bi-polar electrochemistry is a noncontact method in which a conducting material placed within an electric field induced by two feeder electrodes is indirectly polarized while maintaining charge neutrality such that the conducting material forms two poles, a positively charged pole which acts as the anode and negatively charge one that acts as the cathode. If the electric field is applied between the feeder electrodes under alternating current, the poles fluctuate between anode and cathode extremes corresponding to the direction of the induced field. Under such polarization conditions, the opportunity to assess the electrochemical impedance between the feeder electrodes is provided. The measured impedance includes the interfacial impedance of the feeder electrodes, the conducting material interface impedance, and the impedance of the conductor itself which can be assumed to be negligible. By placing two reference electrodes between the feeder electrodes, the interfacial impedance of the feeder electrodes can be eliminated from the measurement, thus allowing one to obtain an impedance reflective of the electrolyte impedance between the reference electrodes and the impedance of the conducting material interface.A finite element model is used to relate the measured impedance response to the impedance of the steel and cement interface and therefore the corrosion rate. The ability to accurately monitor the corrosion rate will be assessed by comparing the estimated corrosion damage obtained by integration of periodic estimates of corrosion rate to the actual damage. The role of a corrosion product layer, reinforcement geometry, and electrode configuration to the sensitivity of the measurement to corrosion rate will be described.[1] Alexander, C. L., & Orazem, M. E. (2020). Indirect electrochemical impedance spectroscopy for corrosion detection in external post-tensioned tendons: 1. Proof of concept. Corrosion Science, 164, 108331.[2] Alexander, C. L., & Orazem, M. E. (2020). Indirect impedance for corrosion detection of external post-tensioned tendons: 2. Multiple steel strands. Corrosion Science, 164, 108330.

Noise spectroscopy based numerical modelling of chemisorption on SnO2 surface for CO gas sensing applications

The gas sensor response of the tin oxide (SnO2) semiconductor toward carbon monoxide (CO) gas has been simulated using the noise spectroscopy technique. The Wolkenstein gas adsorption model is used to calculate the surface coverage of environmental oxygen ions and target gas molecules. The interaction of CO gas molecules with the negatively charged oxygen ions has been incorporated in the numerical model. The equilibrium charge neutrality condition of the semiconductor material has been solved as a function of gas adsorption parameters like surface coverage, temperature, pressure and surface potential. Eventually, the power density spectrum (PDS) of the electrical conductance of the sensing layer was calculated before and after the gas adsorption conditions. It has been observed that the semiconductor band bending increases sharply at a certain concentration of environmental oxygen gas concentration (10−4 to 10−3 atm). At a temperature of 480 K, a steep decrease in surface potential was observed. This temperature range of maximum sensor response was optimized for calculating noise spectrum of electrical conductivity fluctuations at equilibrium condition of adsorption-desorption. The cutoff frequency (109 Hz) was calculated using normalized PDS of electrical conductance. The gas sensor response was calculated as a ratio of film conductance before and after the gas adsorption as a function of gas pressure and surface temperature at the calculated cutoff frequency. The increase in sensor response was observed with the increase in CO partial pressure as a function of temperature for a specific cut off frequency. The comparison of sensor response of the presented model has been demonstrated with another published numerical model. The presented model shows improved sensor response for higher (>10−8 atm) CO partial pressure.