Fermi Energy Changes Research Articles

Bi-functional dynamic tunable absorber and highly robust polarisation converter for graphene-based metasurface

In this study, a graphene-based metasurface structure with bi-functional dynamically tunable absorber and highly robust polarisation converter is proposed. The device primarily consists of two functional tiers. The top tier of cyclically patterned graphene and the back of a fully reflective gold mirror, which are separated by an intermediate tier of SiO2 dielectric to form a typical sandwich structure. The proposed device is numerically calculated by finite element method (FEM), and the equivalent R-L-C circuit model is built to validate the calculated results. The proposed reflective-type circularly to circularly polarisation converter (RCCPC) has a wide operating band of 0.8 THz (2.12 THz to 2.92 THz) and can obtain near unity polarisation conversion rate (χPCR). It is worth noting that the device is also immune to structural parameter deviations. In addition, due to the excited charge of monolayer graphene has the cyclotron property in a static magnetic field, the cross-polarised component of the polarisation converter can be increased by applying a vertical magnetic field, which provides a feasible method to improve the energy conversion rate of the device. At the same time, the device can also serve as a narrowband dual channel absorber, with 95 % absorption effect. In addition, the tunability of graphene is based on the correlation between conductivity and Fermi energy. As the Fermi energy changes, the working frequency of the device is shifted, realizing the dynamic tunable absorber and polarisation converter. It is believed that these findings provide a way to implement a variety of novel multifunctional photonic device, including polarisation imaging, electromagnetic energy absorption, and integrated photonic devices.

The electronic properties of C2N/antimonene heterostructure regulated by the horizontal and vertical strain, external electric field and interlayer twist

The electronic properties of C2N/antimonene (Sb) van der Waals (vdW) heterostructure are investigated based on the density functional theory. The results show that the C2N/Sb vdW heterostructure behaves type-II band alignment with a direct Eg of 0.35 eV. The electronic structure can be adjusted by horizontal (vertical) strain, electric field, interlayer twist, and vertical strained-twisted cases, respectively. The horizontal strain shows a more extensive controlling range of Eg than the vertical strain, but the vertical strain adjusting shows better linear features than the horizontal strain. When applying an electric field, the maximum Eg can reach 1.4 eV with Type-I band alignment, which has potential in transistors, solar cells, Light Emitting Diode(LED), and other applications. The heterostructure keeps robust type-II band alignment with different twist angles between C2N and Sb monolayer, which contributes to efficient separation of photo-generated electron-hole pairs as photovoltaic and optical detector devices. When the vertical strain is applied on the twisted heterostructure, the band alignment, Eg, and energy states near the Fermi energy (EF) change obviously due to orbital hybridization strengthened. Our results suggest that there are various means to regulate the electronic properties of heterostructures.

Charge states of nitrogen-vacancy centers in Fermi level controlled diamond n-i-n junctions

Control of the charge state of the nitrogen-vacancy (NV) center is crucial because of its instability and its transitions between the negative (NV–) and neutral (NV0) NV charge states under laser irradiation In this study, we fabricated an n-i-n junction, with an i-layer sandwiched between two phosphorus-doped n-layers; then, we measured the charge state of NV centers under steady state and laser irradiation in a known band structure where the Fermi energy changes gradually. The steady-state charge state measured by a nondestructive single shot exhibited stable NV– and NV0 signals when the Fermi level was even slightly above and below the transition level, respectively. This result indicates that the charge state can be significantly stabilized through band engineering. Both charge-state populations were observed only when the Fermi level was close to the transition level. Under continuous green laser irradiation, the ratio of NV– measured by the photoluminescence spectra changed gradually with the Fermi level in the depletion layer because of the balance between excitation from the laser and the supply of charge from the band. This outcome agrees reasonably with the calculated bands. Furthermore, we measured the PL spectra of the ensemble NV centers and discovered that their charge state can be well-controlled, as in the single NV center. The charge state of the i-layer at the interface can be stabilized by depositing a thin n-layer on the surface. These results would contribute significantly to improve sensor performance.

Tunable light trapping in the graphene metasurface.

Graphene metasurfaces based on surface plasmon resonance can greatly enhance the interaction between light and matter at the nanoscale. At present, the resonance of graphene metasurfaces is widely used to enhance the absorption of atomic layer graphene, but little work has focused on the light field trapping capabilities it brings. In this paper, we numerically study the light trapping and manipulation of an asymmetric graphene metasurface. The designed device supports two resonant modes, and the multipole decomposition confirms that the electric dipole response dominates them. The calculated average electric field enhancement factor (EF) can reach 1206 and 1779, respectively. The near-field distribution indicates that the electric field is mainly localized in the graphene nanodisks. When the Fermi energy changes, the intensity and peak position of EF can be effectively regulated. In addition, when the polarization of the incident light is adjusted, the light field capture of the two modes is independently regulated. These results reveal that the graphene metasurface has significant light field capture and regulation ability, which provides a new idea for the realization of active regulation of high-performance low-dimensional optical devices.

Design and prediction of PIT devices through deep learning.

Graphene material has excellent performance and unique variable carrier density characteristics, making it an excellent mid-infrared material. And deep learning makes it possible to quickly design mid-infrared band devices with good performance. A graphene nano-ring-symmetric sector-shaped disk array structure based on the PIT principle is proposed here for sensing. The influence of structural parameters and Fermi energy changes are studied. And its FOM (Figure Of Merit) can reach 28.7; the sensitivity is 574 cm-1 / RIU (Refractive Index Unit). At the same time, we designed a six-layer deep learning network that can predict structural parameters and curve predictions. When predicting structural parameters, its MAPE (Mean Absolute Percentage Error) converges to 0.5. In curve prediction, MSE (Mean Square Error) converges to 1.2. It shows that predictions can be made very well. This paper proposes a symmetrical sector disk array structure and a 6-layer deep learning network. And the deep neural network designed based on the device data has good prediction accuracy under the premise of ensuring the network is simple. This will lay a good foundation for future sensor design and device acceleration optimization design.

Charge doping zirconium nitride halide monolayers

The observation of superconducting phases in transition metal nitride halides has sparked interest due to its unconventional and not yet fully understood mechanism. These materials have a layered structure, and can be exfoliated into quasi-two dimensional systems. The thinnest form of these materials are of particular interest for electric-double-layer field-effect transistor configurations. Here, we have studied the effect of charge doping on the structural and electronic structure of Zirconium-based nitride halides. We find that upon electron doping, the effective masses and the bands close to the Fermi energy change considerably. The effect of considering quasiparticles, with self energies within the G0W0 approximation, has also been addressed. This work aims to help build better models for understanding the superconducting properties of ZrNXs, and TMNXs in general.

Tunable Terahertz Plasmon-Induced Transparency in Resonator-Coupled Dirac Semimetal Waveguides

The bulk Dirac semimetal (BDS) is an interesting material, similar to graphene, which can dynamically adjust its optical properties via a variation in its Fermi energy or electrical voltage. In this work, a BDS-based plasmonic device, which enables tunable terahertz plasmon-induced transparency, was proposed and designed. By using the finite element method, the surface plasmon polariton and plasmon-induced transparency of this device were systematically investigated. The results demonstrate that the plasmon-induced transparency of such device can be dynamically tuned by varying its Fermi energy. When the Fermi energy changes from 55 meV to 95 meV, the maximum group delay time of the device increases from 13.2 ps to 21 ps. In the case of a cascading device, the maximum group delay time can be further pushed up to 44.57 ps. The influence of the ambient refractive index on the optical properties of the proposed device was also considered and investigated.

Design and simulation of the optical amplitude-shift keying modulator using Fabry–Perot resonator coupled with graphene at near-infrared frequencies

An optical amplitude modulator operating at near-infrared frequencies is presented. The proposed modulator structure consists of a Fabry–Perot (FP) cavity coupled with graphene located in the middle of two dielectric Bragg mirrors. Graphene’s presence as an active material with adjustable optical conductivity, as well as its effective interaction with powerful standing waves confined in the cavity, enables spectral tuning of the transmission spectrum. Since the resonator has a high quality factor, a small resonance frequency shift due to graphene Fermi energy change results in a huge change of the input light transmission value of the resonator, which makes it possible to achieve high modulation depth (MD). The results demonstrate that the proposed optical modulator has insertion loss lower than 0.1 and MD larger than 90%. It is believed that our study may be useful for optical wireless communication acting at near-infrared region.

Exploring Bandgap Engineering on Photovoltaics and their Possible Economic implications

Bandgap an intrinsic property of molecules on nanoscale, has capability of making alterations in the characteristics of the material through cataclysmic changes from interaction with light. Ultraviolet spectrographs of three organic extracts were explored for their optical frequency and possible use as communication organic optical fiber. The bandgaps of these dye extracts were determined and tailored by varying their absorptivity of ions in electrolytic solvents. A distinctive absorption power, transmittance and amplitude promoted vital regional shifts in wavelength. Interaction with radiation absorbed, influenced a Fermi energy change within a large band gap ≈ 4eV, which is a hypothetical line of symmetry. Results revealed that all the bands below that energy level were entirely filled, whereas all higher-lying bands were completely or nearly empty, with the exception of minimal thermal excitation which is induced at high temperatures.

Tunable terahertz metamaterial absorber based on electricity and light modulation modes

In this paper, a metamaterial absorber that achieved absorption tuning by electricity and light control has been proposed in the terahertz (THz) regime. The THz absorber exhibits an absorbance of 97.5% at a resonant frequency of 0.245 THz. First, we simulated the absorption spectra under different structural parameters. Then the absorption characteristics are analyzed under different Fermi energies and pump fluences. When the Fermi energy changes from 0 to 1 eV, the peak absorbance decreases from 97.5% to 56.2%. As the fluence of the pump beam increases from 0 to 100 µJ/cm2, the peak absorbance decreases from 97.5% to 42.8%. The amplitude modulation depth T of our designed absorber is approximately 0.55. Electric and magnetic resonances are proposed in this article, which allows for nearly perfect absorption. Finally, the absorption for both transverse electric and transverse magnetic modes were investigated under different incident angles, from 0° to 75° with a step-width of 15°. The absorber can be potentially applied to THz detection, imaging, and sensing.

Generation of intense and coherent sub-femtosecond X-ray pulses in electron storage rings

Temporally short X-ray pulses are an indispensable tool for the study of electron transitions close to the Fermi energy and structural changes in molecules undergoing chemical reactions which take place on a time-scale of hundreds of femtoseconds. The time resolution of experiments at 3rd generation light sources which produce intense synchrotron radiation is limited fundamentally by the electron-bunch length in the range of tens of picoseconds. Here we propose a new scheme for the generation of intense and coherent sub-femtoseconds soft X-ray pulses in storage rings by applying the Echo-Enabled Harmonic Generation (EEHG) method. Many issues for obtaining the EEHG structure such as two modulators and a radiator are solved by a paradigm shift in an achromatic storage ring cell. Numerical demonstration of the feasibility of the scheme for the BESSY II beam parameters is presented.

Weak anti-localization in spin–orbit coupled lattice systems

The quantum correction to electrical conductivity is studied on the basis of two-dimensional Wolff Hamiltonian, which is an effective model for a spin–orbit coupled (SOC) lattice system. It is shown that weak anti-localization (WAL) arises in SOC lattices, although its mechanism and properties are different from the conventional WAL in normal metals with SOC impurities. The interband SOC effect induces the contribution from the interband singlet Cooperon, which plays a crucial role for WAL in the SOC lattice. It is also shown that there is a crossover from WAL to weak localization in SOC lattices when the Fermi energy or band gap changes. The implications of the present results to Bi–Sb alloys and PbTe under pressure are discussed.

Effective indirect exchange interaction in p -doped MoS2 nanoribbons in the presence of intrinsic spin-orbit interaction

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between localized magnetic impurities in the ${\text{MoS}}_{2}$ nanoribbons. Our calculations are based on a three-orbital tight-binding model for a ${\text{MoS}}_{2}$ finite nanoribbon system with periodic boundary conditions. We consider impurities hybridized to different d orbitals of the host transition metal. The large intrinsic spin-orbit coupling leads to the spatial anisotropy in the RKKY interaction amplitude and appearance of collinear and noncollinear terms. We find that the RKKY interaction is sensitive to the Fermi energy values and changes dramatically in doped systems. We analyze the exponent decay behavior of the RKKY interaction envelope for doping levels inside the band gap of the infinite layer.

A semiconductor-electrochemistry model for design of high-rate Li ion battery

Abstract For designing batteries with high-rate and long-life, electronic/ionic transport and reaction must be unified for metal oxide electrodes. However, it remains challenging for effectively integrating the whole substrate/active materials/electrolyte interfaces. Herein by taking Li ion battery as example, we propose a semiconductor-electrochemistry model by which a general but novel insight has been gained into interfacial effect in batteries. Different from those traditional viewpoints, this derived model lies across from physics to electrochemistry. A reaction driving force can be expressed in terms of Fermi energy change, based on the tradeoff between electronic and ionic concentration at the reaction interfacial region. Therefore, at thermodynamic-controlled interface I of substrate/electrode, increasing contact areas can afford higher activity for active materials. Whereas at kinetically-governed interface II of electrode/electrolyte or inside active materials, it is crucial to guarantee high-reaction Li ionic concentration, with which some sufficient reaction degrees can reach.

Dynamically tunable electromagnetically induced transparency analogy in terahertz metamaterial

A metamaterial analogy of tunable electromagnetically induced transparency (EIT) is theoretically investigated in terahertz regime. The proposed metamaterial consists of vertical gold strips and horizontal graphene wires, which perform as bright elements and dark elements, respectively. The EIT-like phenomenon can be induced by bright–dark mode coupling on condition of structural lateral displacement. Numerical result reveals that the EIT-like effect remains noticeable with a wide range of incidence polarization angles. Most importantly, by manipulating gate voltages, the EIT window can be dynamically controlled without refabricating the structure. The amplitude modulation depth can reach 81%, 79%, and 68% respectively at three characteristic frequencies as Fermi energy changes in the scope of 0.8–1.0 eV. Furthermore, a sensitivity of 0.95 THz per refractive index unit (RIU) is realized varying the refractive index in the surrounding medium. This structure provides potential applications for detectors, sensors, and modulators.

Impact of Phosphorus superlattices on charge and spin dependent transport properties of zigzag silicene nanoribbons

To investigate charge and spin dependent conductance properties of Phosphorus doped zigzag silicene nanoribbons (ZSiNRs), we utilize recursive Green's function method and Landauer-Büttiker formalism. Our calculations are performed in the absence and presence of exchange magnetic fields with both parallel and antiparallel configurations. Considering a supperlattice of Phosphorus substituents in a periodic distribution at the edge of nanoribbon, the effect of increasing number of dopants and period of the distribution on transport properties are studied. It is found that transport properties of doped ZSiNRs vary with doping concentration according to being odd or even of number of dopants. For parallel configuration, doped ZSiNR with various concentrations works as a controllable spin filter with Fermi energy. Increasing doping concentration leads to increasing size of conductance gap and improvement of controlling quality of spin-filtering property while increasing period of Phosphorus atomic distribution has destructive effect on size of conductance gap and destroys spin-filtering property. Moreover, we show that although the same results are obtained for transport properties of doped ZSiNR with various concentrations of Phosphorus atoms in presence of antiparallel exchange magnetic fields, a completely controllable spin-filtering property cannot be achieved by Fermi energy changes.

Synthesis, Transport and Magnetic Properties of Ba-Co-Ge Clathrates

Ba-Co-Ge intermetallic clathrates were synthesized in both type-I and chiral-type crystal structures to investigate thermoelectric and physical properties. Seebeck coefficients, thermal conductivities, electrical resistivities, and Hall coefficients were measured, as well as specific heat and magnetic susceptibility. Type-I Ba8Co x Ge46−y was formed with a large number of spontaneous vacancies, similar to a Zintl condition, but without the vacancy ordered superstructure of Ba8Ge43. However, the vacancies for this composition do not moderate the carrier density as expected from Zintl electron balancing. Instead, the physical properties point to a complex Fermi surface property with a large effective carrier density, a behavior consistent with other materials close to Ba8Ge43. In the case of chiral samples, Co substitution strongly suppresses the temperature of only the lower of the two structural transformations. Susceptibility and specific heat measurements, coupled with the measured transport properties, demonstrate that the electron densities of states near the Fermi energy change very little in the transformations, a significant reduction in the effect of these transitions compared to the case of unsubstituted Ba8Ge25.

GaN/AlN constant total effective radius multi-wells quantum rings: Physical properties under well number variation effects

In this study, effect of the number of wells and quantum ring thickness on subband electronic structure, ground state wave function, Fermi energy and linear refractive index changes of an AlN/GaN constant total effective radius multi-wells quantum ring have been studied. It is shown that, effect of increasing of the inner quantum ring radius R0 on the ground state energy and other physical properties are larger than increasing of the number of wells. This fact helps the simplification of the experiments, since making quantum ring with larger R0 is easier than making multiple-wells quantum rings. However, effect of the increasing of the R0 values in systems with larger number of wells is also larger. Increase of the R0 also leads to increase of the refractive index changes amplitudes. Resonant energy undergo a red-shift by going from single quantum well system to double quantum well system, and further increasing of the numbers of wells leads to a blue-shift. Finally, we propose number of wells as a tuning tool to select the position of the maximum probability of finding free carriers within the system.

Ultrasensitive terahertz/infrared waveguide modulators based on multilayer graphene metamaterials

AbstractThis paper studies and classifies the electromagnetic regimes of multilayer graphene‐dielectric artificial metamaterials in the terahertz/infrared range. The employment of such composites for waveguide‐integrated modulators is analysed and three examples of novel tunable devices are presented. The first one is a modulator with excellent ON‐state transmission and very high modulation depth: >38 dB at 70 meV graphene's electrochemical potential (Fermi energy) change. The second one is a modulator with extreme sensitivity towards graphene's Fermi energy ‐ a minute 1 meV variation of the latter leads to >13.2 dB modulation depth. The third one is a tunable waveguide‐based passband filter. The narrow‐band cut‐off conditions around the ON‐state allow the latter to shift its central frequency by 1.25% per every meV graphene's Fermi energy change.

Indirect exchange interaction between magnetic adatoms in monolayer MoS2

We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in a monolayer MoS${}_{2}$. We show that the rotation of the itinerant electron spin due to the spin-orbit coupling causes a twisted interaction between two magnetic adatoms, which consists of different RKKY coupling terms: the Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions. We find that the interaction terms are very sensitive to the Fermi energy values and change dramatically from doped to undoped systems. A finite doping causes all parts of the interaction to oscillate with the distance of two magnetic impurities $R$, and the interaction behaves like ${R}^{\ensuremath{-}2}$ for a long distance between two localized spins. We explore a beating pattern of oscillations of the RKKY interaction that occurs for the doped system.