Fermi Energy Research Articles

2D g-C3N5 p-Doping of Donor Material for High-Efficiency Organic Solar Cells.

Molecular doping of organic semiconductor is a great strategy for significantly regulating the electronic band structure of organic semiconductor while increasing charge mobility and carrier concentration. Here, a facile strategy is presented by introducing 2D g-C3N5 as a p-dopant into PM6, improving the charge mobility and hole carrier concentration of PM6. Moreover, the electron transfer between PM6 and g-C3N5 can effectively downshift the Fermi energy level and highest occupied molecular orbital (HOMO) energy level of PM6, which leads to the increase the built-in electric field of organic solar cells (OSCs). The addition of g-C3N5 also effectively enhances the crystallization of active layer, thereby improving the stability of OSCs. As a result, a champion bulk-heterojunction (BHJ) and layer-by-layer (LbL) structure OSCs are successfully achieved featuring a high-power conversion efficiency of 18.10%/18.25%, simultaneously having excellent device stability. This work shows that introducing a low concentration dopant into organic donor is an effective method for improving the electrical performance of organic donor and the efficiency of OSCs.

Intrinsic anomalous, spin and valley Hall effects in ’ex-so-tic’ van-der-Waals structures

We consider the anomalous, spin, valley, and valley spin Hall effects in a pristine graphene-based van-der-Waals (vdW) heterostructure consisting of a bilayer graphene (BLG) sandwiched between a semiconducting van-der-Waals material with strong spin-orbit coupling (e.g., WS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {WS}_2$$\\end{document}) and a ferromagnetic insulating vdW material (e.g. Cr2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Cr}_2$$\\end{document}Ge2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ge}_2$$\\end{document}Te6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Te}_6$$\\end{document}). Due to the exchange proximity effect from one side and spin-orbit proximity effect from the other side of graphene, such a structure is referred to as graphene based ’ex-so-tic’ structure. First, we derive an effective Hamiltonian describing the low-energy states of the structure. Then, using the Green’s function formalism, we obtain analytical results for the Hall conductivities as a function of the Fermi energy and gate voltage. For specific values of these parameters, we find a quantized valley Hall conductivity.

Searching accretion-enhanced dark matter annihilation signals in the Galactic Centre

This study reanalyzes the detection prospects of dark matter (DM) annihilation signals in the Galactic Center, focusing on velocity-dependent dynamics within a spike density near the supermassive black hole (Sgr A⋆). We investigate three annihilation processes — p-wave, resonance, and forbidden annihilation — under semi-relativistic velocities, leveraging gamma-ray data from Fermi and DAMPE telescopes. Our analysis integrates a fermionic DM model with an electroweak axion-like particle (ALP) portal, exploring annihilation into two or four photons. Employing a comprehensive six-dimensional integration, we precisely calculate DM-induced gamma-ray fluxes near Sgr A⋆, incorporating velocity and positional dependencies in the annihilation cross-section and photon yield spectra. Our findings highlight scenarios of resonance and forbidden annihilation, where the larger ALP-DM-DM coupling constant Caχχ can affect spike density, potentially yielding detectable gamma-ray line spectra within Fermi and DAMPE energy resolution. We set upper limits for Caχχ across these scenarios, offering insights into the detectability and spectral characteristics of DM annihilation signals from the Galactic Center.

Local Electronic Structure Regulation Enabling Fluorophosphates Cathode with Improved Redox Potential and Reversible Capacity for Sodium-Ion Batteries.

Mn-based fluorophosphates have attracted much attention as cathodes for sodium-ion batteries owing to their high cost effectiveness, considerable capacity, and stable framework. However, the fascinating Mn3+/2+ redox couple suffers from inadequate activation due to the Mn-O covalent character and poor electronic conductivity, impeding its further applications. Herein, a local electronic structure regulation strategy is proposed to improve the Mn3+/2+ redox potential and reversible capacity simultaneously through introducing elements with low-energy 3d orbitals to expand the energy gap between the eg orbitals and Fermi energy of Na. Moreover, the 3d element substitution serves to narrow the band gap toward the improved intrinsic electronic conductivity. In comparison with pristine Na2Fe0.45Mn0.55PO4F, the as-prepared Na2Fe0.45Mn0.4V0.1PO4F cathode achieves an increase from 3.5 to 3.6 V in the high-voltage platform and an improvement in energy density from 330 to 361 Wh kg-1. This work inspires new ideas in adjusting the redox potential of polyanionic cathodes through deliberate regulation of the local electronic structure.

Impact of composition on the properties of full-Heusler Ti2FexMn1-xAl alloys in spintronics

This study explores the structural, electronic, and magnetic characteristics of full-Heusler Ti2FexMn1-xAl alloys for spintronic applications. The regular Heusler structure is identified as the most stable across all x concentrations. The inverse Heusler structure exhibits half-metallic behavior with a finite energy band gap in the spin-up states, while the regular structure shows metallic behavior for both spin directions. Dirac-like points along the M→Γ direction are observed, particularly in alloys with x = 0 and 0.25 (inverse structure) and x = 0.5, 0.75, and 1 (regular structure), indicating advanced electronic properties. Magnetic analysis reveals that Ti atoms' local magnetic moments are antiparallel to those of Mn and Fe atoms. The total magnetic moment is highest for x = 1 (Ti2MnAl) and nearly zero for x = 0 (Ti2FeAl). Additionally, the inverse Heusler structure achieves 100 % spin polarization at the Fermi energy, underscoring its suitability for spintronic applications. This study highlights the potential of Ti2FexMn1-xAl alloys for future spintronic devices.

The magicity, the radii of neutron orbits 1<i>f</i><sub>7/2</sub>, 2<i>p</i><sub>3/2</sub> and halo-like structure of <sup>52,54</sup>Ca nuclei

The evolution of neutron single-particle spectra of isotones with N = 32 and 34 new magic neutron numbers in the region 16 ≤ Z ≤ 32 was calculated in the dispersive optical model. It was shown that the minimum of the difference between the Fermi energy and the half-sum of the energy levels of the last predominantly occupied state and the first predominantly unoccupied state is achieved in the magic isotones with N = 32 and 34. The calculated root-mean-square radius of the neutron halo-like state 2p3/2 in double magic 52Ca nucleus exceeded the radius of the underlying 1f7/2 state by 0.8 fm. It is consistent with the recent experimental data and theoretical predictions that explain ‟unexpectedly” large root-mean-square charge radius of this nucleus.

Tunable graphene plasmonic metadevice for infrared polarization resolved spectroscopy detection

Polarization-resolved spectroscopy imaging holds significant promise in various fields. However, conventional polarization-resolved spectroscopy imaging systems often reply on separate dispersion elements, polarization splitting components, and mechanical scanning or rotating structures, resulting in bulky systems with intricate designs. The pursuit of lightweight and seamlessly integrated polarization spectroscopy imaging systems presents a critical challenge. Here, we propose an integrated polarization spectroscopy detector operating in the mid-infrared range based on a tunable graphene metasurface. Within the graphene metasurface, plasmonic resonances of graphene are excited by a gold nano-grating, allowing simultaneous modulation of spectral and polarization transmission characteristics for infrared light by tuning the Fermi energy of graphene. With the help of neural network based recovery algorithms, this system achieves pixel-level spectral detection (with a resolution below 200 nm), multi-color polarization Stokes vector analysis (with a monochromatic angular error of less than 0.1 rad), and concurrent polarization and spectroscopy detection. This research paves the way for the development of the next generation integrated infrared polarization resolved spectroscopy imaging systems.

Finite temperature fermionic condensate and energy–momentum tensor in cosmic string spacetime

Here we analyze the expectation value of the fermionic condensate and the energy–momentum tensor associated with a massive charged fermionic quantum field with a nonzero chemical potential propagating in a magnetic-flux-carrying cosmic string in thermal equilibrium at finite temperature T. The expectation values of the fermionic condensate and the energy–momentum tensor are expressed as the sum of vacuum expectation values and the finite temperature contributions coming from the particles and antiparticles excitation. The thermal expectations values of the fermionic condensate and the energy–momentum tensor are even periodic functions of the magnetic flux with period being the quantum flux, and also even functions of the chemical potential. Because the analyses of vacuum expectation of the fermionic condensate and energy–momentum tensor have been developed in literature, here we are mainly interested in the investigation of the thermal corrections. In this way we explicitly study how these observable behaves in the limits of low and high temperatures, and also for points near the string. Besides the analytical discussions, we included some graphs that exhibit the behavior of these observable for different values of the physical parameters of the model.

Exploring optoelectronic, optical thin films, mechanical and thermal transport properties of bromide double perovskites Rb2Ag(Ga/In)Br6 for photovoltaic and thermoelectric applications

Lead-free double halide perovskites like Rb2Ag(Ga/In)Br6 have demonstrated themselves potential candidates in solar cell research owing to their environmental friendliness, stability, and exceptional performance. This study comprehensively analyzes the structural, mechanical, optoelectronic and optical coating features, as well as thermodynamic and thermoelectric properties of two Rb2AgGaBr6 and Rb2AgInBr6 compounds. Using the Wien2k code with GGA + mBJ exchange-correlation potentials, we confirm their structural stability in cubic phase Fm-3m and identifying them as direct band gap semiconductors (Γ → Γ) of 0.38 eV and 1.0644 eV, respectively. Then, optical analysis reveals broad absorption bands across visible and ultraviolet wavelengths, making them suitable for photovoltaic absorbers. Finally, the thermoelectric investigations under varying temperatures show favourable properties, such as a high Seebeck coefficient with poor electronic thermal conductivity. This also yields exceptional value (0.96 and 0.994 for Rb2AgGaBr6, Rb2AgInBr6, respectively) of figure of merit (ZT) at room temperature and chemical potential μ-μ0 = - 0.09eV near the Fermi energy level, enhancing their potential for thermoelectric applications. These findings underscore the versatility and promising future of Rb2Ag(Ga/In)Br6 as important semiconductors processing for optoelectronic, thermoelectric, and mechanical devices.

Singular photon spin Hall effect in chiral-graphene heterostructures and its application in chirality detection.

In this paper, we consider the singular photonic spin Hall effect (PSHE) in a chiral-graphene-chiral (CGC) heterostructure in the THz band. We investigate the impact of a chiral medium on reflectance spectra and the modulation of the Fermi energy on the surface conductivity of graphene. Our study shows that placing a chiral medium on both sides of a monolayer of graphene results in an enhanced transverse shift (TS) compared to placing a non-chiral medium. Moreover, the direction of the TS of the PSHE can be altered by adjusting the sign of the chirality parameter and the Fermi energy of graphene. Finally, we establish a quantitative relationship between the PSHE and the chirality parameter and the Fermi energy of graphene. By dynamically modulating the PSHE in graphene, it is possible to flexibly detect chirality parameters. This work opens up new avenues for chiral molecular detection and graphene-PSHE dynamic modulation.

Analogical atomic effects of graphene nanostructure induced by nonlinearity.

A scheme to construct analogical atoms using graphene nanostructures via quantum nonlinearity is proposed. Due to the strong field localization capability of graphene plasmons and the significant intrinsic nonlinearity of graphene, a strong nonlinear optical response can be realized even with single-photon excitation. In this process, the quantum vacuum localized plasmonic mode plays a crucial role in achieving the third-order multi-photon nonlinear effect. The eigenfrequency of the nanostructure can shift by an amount tens of times larger than the decay rate, causing the nanostructure cavity to exhibit atomic-like behaviors. Furthermore, multilevel atoms can be constructed through the interaction of composite graphene nanostructures. The parameters of these atoms can be manipulated by adjusting the nanostructure size, Fermi energy, and other parameters. This research holds significant potential for applications in highly integrated, controllable nanophotonics.

Quasi-One-Dimensional Spin Transport in Altermagnetic Z^{3} Nodal Net Metals.

In three dimensions, quasi-one-dimensional (Q1D) transport has traditionally been associated with systems featuring a Q1D chain structure. Here, based on first-principle calculations, we go beyond this understanding to show that the Q1D transport can also be realized in certain three-dimensional (3D) altermagnetic (AM) metals with a topological nodal net in momentum space but lacking Q1D chain structure in real space, including the existing compounds β-Fe_{2}(PO_{4})O, Co_{2}(PO_{4})O, and LiTi_{2}O_{4}. These materials exhibit an AM ground state and feature an ideal crossed Z^{3} Weyl nodal line in each spin channel around Fermi level, formed by three straight and flat nodal lines traversing the entire Brillouin zone. These nodal lines eventually lead to an AM Z^{3} nodal net. Surprisingly, the electronic conductivity σ_{xx} in these topological nodal net metals is dozens of times larger than σ_{yy} and σ_{zz} in the up-spin channel, while σ_{yy} dominates transport in the down-spin channel. This suggests a distinctive Q1D transport signature in each spin channel, and the principal moving directions for the two spin channels are orthogonal, resulting in Q1D direction-dependent spin transport. This novel phenomenon cannot be found in both conventional 3D bulk materials and Q1D chain materials. In particular, the Q1D spin transport gradually disappears as the Fermi energy moves away from the nodal net, further confirming its topological origin. Our Letter not only enhances the comprehension of topological physics in altermagnets but also opens a new direction for the exploration of topological spintronics.

Delta-doping modulation of three quantum wells under the influence of an electric field

The impact of delta-doping modulation and electric field influence within a nanostructure consisting of three GaAs quantum wells (QWs) separated by AlGaAs barriers was investigated. The quantized energy levels, Fermi energy, wavefunctions, self-consistent potential, and electron density distribution were evaluated by a self-consistent solution of the Schrödinger and Poisson equations using the finite element method within the FEniCS project in Python. The results indicate that increasing the electric field reduces energy levels and Fermi energy. In addition, the delta-doping position and electric field significantly affect the self-consistent potential and electron density distribution. This study provides a possibility to tailor optical properties, such as the linear absorption coefficient and photoluminescence, by adjusting geometrical and non-geometrical parameters, including donor density, enhancing the functionality of doped QWs in designing high electron mobility transistors.

Coincidence detection probability of (γ,2e) photoemission measurement

In the study of strongly correlated electrons, one of the challenging core tasks is to develop the potential techniques for direct detection of the many-body correlations of strongly correlated electrons. The(γ,2e)photoemission technique has been developed to investigate the two-body correlations of the target correlated electrons. In this article, we will focus on this technique for the correlated electrons near the Fermi energy in condensed matter. The coincidence detection probability of the two emitted electrons in the(γ,2e)photoemission measurement is shown to be relevant to a two-body Bethe-Salpeter wave function, which describes the dynamical two-body correlations of the target correlated electrons near the Fermi energy. As the coincidence detection probability involves an electron-electron interaction matrix element, the arbitrary momentum and/or energy transfer due to this electron-electron interaction makes the(γ,2e)photoemission technique fail to reveal the inner-pair structures of the two-body Bethe-Salpeter wave function. However, the center-of-mass momentum and energy of the two-body Bethe-Salpeter wave function can be distinctly resolved. Thus, the(γ,2e)photoemission technique can provide the center-of-mass physics of the two-body correlations of the target correlated electrons. It will be one potential technique to study the center-of-mass physics of the Cooper pairs in superconductor.

Investigating the efficacy of Palladium Doped Ceria for the Photodegradation of Azo Dyes

Ceria (CeO2) was prepared by microwave assisted precipitation method while sonothermal method was utilized for the synthesis of palladium doped ceria (Pd-CeO2) photocatalyst. HR-TEM analysis confirms the face centered cubic geometry (FCC) of CeO2. The particle size of Pd-CeO2 calculated from HR-TEM analysis (7.6±2.18 nm) is in close relation with XRD (8.25±0.25 nm). The SAED pattern of Pd-CeO2 shows the poly crystallinity where the d-spacing was 0.31 and 0.30 nm which corresponds to CeO2 and Pd crystal facets 111 and 100 respectively. The amount of dopant and existence of Pd, Ce and O was confirmed from EDX spectra and elemental mapping. BET surface area analysis of CeO2 (64.45 m2/g), Pd-CeO2 (60.20 m²/g) revealed that the decrease in the surface area of CeO2 due to Pd loading which is confirmed by pore volume (0.079 cm³/g) for CeO2 and pore volume for Pd-CeO2 (0.073 cm³/g). The photocatalytic activity of Pd-CeO2 was screened against a model azo dye Acid red-4 (AR-4). The fluctuation of the rate of photodegradation (PD) of AR-4 was observed under different reaction conditions. Therefore, the reaction parameters were optimized to achieve best catalytic activity up to PD; 98%. The kinetic study suggests that the PD of AR-4 follows 1st order kinetics with regression coefficient (R2=0.973) and rate constant (k=0.024/min). Recycling study revealed the prolong use of catalyst without significant loss of activity. DFT study and experiments revealed the synergism of Pd on the photo response of CeO2. Perhaps this synergistic effect due to metal support interaction, causes redistribution of charges and Fermi energy levels. The synthesized catalyst showed potential application for the treatment of textile industrial effluents.

A dynamically switchable and tunable metamaterial device based on vanadium dioxide and Dirac semimetal

In this paper, a dual-function metamaterial device based on vanadium dioxide (VO2) and Dirac semimetal (DS) operating in terahertz (THz) frequency range is designed. The reversible phase transition properties of VO2 are exploited to achieve the dynamic switch between the function of broadband absorption and broadband polarization conversion. It is demonstrated that when VO2 is in the metallic state and the Fermi energy level () of DS is set to 30 the broadband absorption in the operating frequency range of 1.42-3.40 THz can be achieved, and its absorbance exceeds 90% with a relative bandwidth of 82.16%. When VO2 is in the insulating state and the of DS is set at 180 , the high efficient broadband polarization conversion can be obtained in the operating frequency range of 2.08-4.88 THz and the polarization conversion ratio (PCR) is greater than 90% with a relative bandwidth of up to 80.46%. Therefore, the proposed switchable dual-functional THz device can be extensively utilized in absorption, polarization conversion and other potential fields.

First-principles study on the mechanism of Hf and Ti doping promoting hydrogen release in ZrCoH3

This paper reveals the mechanism of Hf and Ti doping promoting hydrogen release in ZrCoH3. Different from previous experimental methods, this study employs the plane wave pseudopotential method and density functional theory to predict the relationship between the formation energy and electronic structure of the ZrCoH3-Hf/Ti system, and uses theoretical calculations to guide the modification design of ZrCoH3. Firstly, by calculating the thermodynamic properties of the ZrCoH3-Hf/Ti system and combining the density of states theory, the bonding characteristics and dehydrogenation preference between H atoms and neighboring metal atoms in the ZrCoH3-Hf/Ti system are determined. Additionally, the electronic structure of the ZrCoH3-Hf/Ti system is calculated to elucidate the hybridization of bonding and antibonding orbitals between atoms in the ZrCoH3-Hf/Ti system. Moreover, DCo-d is utilized to measure the strength of antibonding interactions in the ZrCoH3-Hf/Ti system. The main cause of antibonding interactions in the ZrCoH3-Hf/Ti system is attributed to the variation in formation energy, and the change in antibonding interactions is sensitive to the variation in formation energy. Finally, a feasible method to improve the properties of ZrCoH3 is proposed, which involves modifying the electronic structure to increase the partial density of states at the Fermi energy level, making the formation energy of hydrides less negative and forcing the structure to be more unstable. This method has potential application value in the modification of ZrCoH3.

Incorporation of SnSe2/SnO2 heterostructures in carbon nanotubes for excellent lithium storage performance

As a new type of negative electrode material, tin-based material exhibits a high theoretical capacity. However, tin-based anode materials always undergo severe volume change upon alloying reaction during lithium insertion and extraction, which leads to the pulverization of the active materials and poor electrochemical performance, seriously hampering its practical application. In this work, oxygen-containing group functionalized carbon nanotubes (FCNTs) were facilely obtained, and the three-dimensional crosslinked SnSe2-SnO2@FCNTs multiphase materials were prepared by solvothermal and high-temperature heat treatment. The formation of Sn-O-C bonds from oxygen-containing functional groups of FCNTs combined with Sn can promote the electric connection and integrity of the composites. The implanted defects in FCNTs can not only be beneficial to anchoring SnO2 and SnSe2 nanoparticles, but also helpful to the penetration of Li+ ions into the electrodes from electrolyte. In addition, the large Fermi energy difference between SnSe2 and SnO2 nanoparticles results in the formation of a built-in electric field at the interface of the multiphase materials, accelerating the diffusion of ions. The smaller size of the multiphase materials leads to significant pseudocapacitive behavior, which facilitates the highly reversible surface/interface adsorption. Under the multiple effects of FCNTs, SnSe2, and SnO2, the SnSe2-SnO2@FCNTs multiphase materials exhibited excellent electrochemical performance. At a current density of 0.1A·g-1, a discharge specific capacity of 898.0mAh·g-1 was achieved. When the current density was increased to 2A·g-1, it still exhibited a discharge specific capacity of 443.9mAh·g-1. After a long charge/discharge cycling at 1A·g-1 over 450 cycles, it still showed a specific capacity of ~ 400mAh·g-1. This work significantly demonstrated the enhanced Li-storage performance of SnSe2-SnO2 heterostructures incorporated in the functionalized multi-walled CNTs. This opens a new way to synthesize advanced tin-based materials as high-performance anodes for high-energy density lithium-ion batteries.

Preparation of CuCo2O4@NFs nanoflower material with abundant VO/VCo: An excellent catalyst for oxidative breakage of the β-O-4 bond of lignin

A hollow nanoflower material CuCo2O4@NFs with abundant oxygen vacancies (VO) and cobalt vacancies (VCo) was prepared by a simple method. The novel material CuCo2O4@NFs showed excellent oxidation activity, cleaving the β-O-4 bond in lignin model compounds. Conversion of the substrate was as high as 99.02%, and the yield of the main product, phenol, reached 49.50%. Structural analysis showed that the petal and cavity form of the material increased its specific surface area, which facilitated its full contact with O2 molecules. Moreover, the VO generated by lattice oxygen detachment during annealing favor the adsorption of O2, and the VCo generated through lattice distortion induced by Cu-metal doping promoted the detachment of oxygen intermediates. The synergy of these two defects improved the rate of the rate-limiting step. Density functional theory calculations revealed that the synergistic effect of VO and VCo narrowed the gap between the Fermi energy levels and the d-band center, thereby enhancing electron mobility. Finally, excellent recyclability of CuCo2O4@NFs was confirmed through cycling experiments. The application of this material provided technical support for the efficient cracking of lignin into high-value-added chemicals and realized the effective utilization of woody biomass resources.

Effect of Atomic Vacancy Defect in h – BN Monolayer on Electronic Properties: A DFT Study

Abstract h-BN is a 2D material which also has considerable development potential in several fields. 2D materials such as h-BN are basically insulating materials but have unique properties and it would be good if they could be developed, especially to meet future energy needs or in the development of electronic devices. Therefore, the properties of h-BN are certainly needed to support this, one of which is the magnetic properties of h-BN. In this research, we modified the h-BN structure by providing defects in the form of vacancy defects by removing atoms in the h-BN layer. We modeled a monolayer h-BN with a supercell size of 4 x 4 x 1, and our calculations were carried out with Quantum ESPRESSO, which is based on density functional theory. The results we obtained showed the emergence of magnetic moment values in h-BN due to defects in the form of atomic vacancies. Furthermore, this research successfully illustrates how vacancy defects in h-BN can modify the band structure, leading to changes in electron behavior around the Fermi energy level.