Fermi Energy EF Research Articles

The Effect of Adsorbed Molecules on Electronic Structure and Magnetic Properties of Nanographites

The results of investigation of changes in electronic structure and magnetic properties of multilayer graphene nanoclusters (nanographites) occurring during their interaction with adsorbed chlorine molecules are presented. The found reversible decrease in the density of states of current carriers D(EF) at the Fermi energy EF can be explained by the spin-splitting of edge π-electron states in nanographites induced by the enhancement of electron-electron interactions due to increase of the D(EF) at partial transfer of the electron density from nanographites to chlorine adatoms. The revealed irreversible decrease in the concentration of localized spins indicates that the electron spins of 3p-orbitals of chlorine and unpaired (dundling) σ-orbitals of edge carbon atoms are coupled also at this interaction, i.e. the edge covalent compound of nanographite with chlorine forms. Character of changes in the spin-relaxation rate of π-electrons depending on the amount of adsorbed chlorine molecules and on temperature in chlorinated samples are also consistent with the above model of nanographite-chlorine interaction.

Optical Response of Mixed Molybdenum Dichalcogenides for Solar Cell Applications Using the Modified Becke–Johnson Potential

Abstract Energy bands and density of states (DOS) of mixed molybdenum dichalcogenides like MoS2, MoSeS, MoSe2, MoTe2, MoTeS, and MoTe0.5S1.5 are reported for the first time using the Tran–Blaha modified Becke–Johnson potential within full potential-linearised augmented plane wave technique. From the partial DOS, a strong hybridisation between the Mo-d and chalcogen-p states is observed below the Fermi energy E F. In addition, the dielectric constants, absorption coefficients, and refractivity spectra of these compounds have also been deduced. The integrated absorption coefficients derived from the frequency-dependent absorption spectra within the energy range of 0–4.5 eV show a possibility of using molybdenum dichalcogenides, particularly MoTe0.5S1.5, in solar cell applications. Birefringence and degree of anisotropy are also discussed using the data on refractivity and imaginary components of the dielectric constant.

First-principles study of the electronic structure of a superstoichiometric rare earth dihydride GdH2.25

AbstractWe performed ab initio calculations of electronic structure and equilibrium properties for the rare earth superstoichiometric dihydride GdH2.25 (with the space group Pm−3m (No. 221)) using the full-potential linearized augmented plane wave method (FP-LAPW) approach within the density functional theory (DFT) in the generalized gradient approximation (GGA) and local density approximation (LDA) as implemented in the WIEN2k simulation code at 0 K. The equilibrium properties were determined, the density of states, electronic density and the energy band structures were studied in details. It was concluded that the GGA optimized lattice parameter agreed much better with the experimental findings than the LDA one. The non negligible electronic density of states at the Fermi level confirmed that the GdH2.25 had a metallic character. The Fermi energy EF fell at a level where most of the states were rare-earth 5d-eg conduction states while negligible contributions of both interstitial (tetrahedral and octahedral) H s-states were observed near EF. From electronic density, the bonding between Gd and tetrahedral H atoms was of prominent covalency, while was ionic between Gd and octahedral H atoms.

Calculation of the Schottky barrier height at the contact between a metal and (SiC)1–x (AlN) x semiconductor solid solution

A simple model of the metal–semiconductor contact that is nonlinear in the concentration of defects when the Schottky barrier is formed by the surface defect states Ei localized at the interface has been proposed. It has been shown that taking into account the nonlinear dependence of the Fermi energy EF on the concentration of defects results in an increase in the Schottky barrier height by 15 to 25%. The calculated values of the Schottky barrier height have been used for the analysis of the current-voltage characteristics of M/(SiC)1–x(AlN)x structures. The theoretical results have been compared with the experimental data.

Frequency dependence of optical third-harmonic generation from doped graphene

In connection with the controversial question about the frequency dependence of the optical third-harmonic generation (THG) from doped graphene, which has recently been discussed in the literature, we develop an analytical theory for the THG susceptibility of doped graphene by using the original Genkin–Mednis nonlinear-conductivity-theory formalism including mixed intra- and interband terms. The theory is free of any nonphysical divergences at zero frequency, and it predicts the main resonant peak in the THG spectrum to be located at the photon energy ħω equal to two thirds of the Fermi energy EF of charge carriers in doped graphene.

Fermi states and anisotropy of Brillouin zone scattering in the decagonal Al-Ni-Co quasicrystal.

Quasicrystals (QCs) are intermetallic alloys that have excellent long-range order but lack translational symmetry in at least one dimension. The valence band electronic structure near the Fermi energy EF in such materials is of special interest since it has a direct relation to their unusual physical properties. However, the Fermi surface (FS) topology as well as the mechanism of QC structure stabilization are still under debate. Here we report the first observation of the three-dimensional FS and valence band dispersions near EF in decagonal Al70Ni20Co10 (d-AlNiCo) QCs using soft X-ray angle-resolved photoemission spectroscopy. We show that the FS, formed by dispersive Al sp-states, has a multicomponent character due to a large contribution from high-order bands. Moreover, we discover that the magnitude of the gap at the FS related to the interaction with Brillouin zone boundary (Hume–Rothery gap) critically differs for the periodic and quasiperiodic directions.

NMR evidence for an intimate relationship between antiferromagnetic spin fluctuations and extendeds-wave superconductivity in monocrystallineSrFe2(As1−xPx)2

We report systematic 31P-NMR study on iron (Fe)-based superconductors SrFe2(As_{1-x}P_{x})2 (Sr122AsP), in which a superconducting (SC) transition temperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an as-grown mono-crystalline sample. The present NMR study has unraveled that Tc reaches a highest value of 33 K at x=0.35 around a quantum critical point at which antiferromagnetic (AFM) order disappears. When noting that the SC transition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no longer present, we remark that the onset and increase of Tc are apparently associated with the emergence and enhancement of AFM-SFs, respectively. In the SC state, the residual density of state (RDOS) at the Fermi energy EF in the SC state becomes much smaller for the annealed sample than for the as-grown one, suggesting that some inhomogeneity and/or imperfection for the latter increases RDOS as expected for unconventional SC state with nodal gap. These findings in Sr122AsP are consistent with the unconventional s(+-)-wave Cooper pairing state that is mediated by AFM-SFs. We also discuss other key-ingredients besides the AFM-SFs to increase Tc further.

Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function.

We report the results of an extensive investigation of metal-molecule-metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M1-S-(C6H4)n-S-M2, with 1 ≤ n ≤ 4 and M1,2 = Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = ΦSAM - Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment εh = EF - EHOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy EF of the junction. Where photoelectron spectroscopy data are available, the εh values agree very well with those determined by TVS. Using a single-level model, which we justify via ab initio quantum chemical calculations at post-density functional theory level and additional UV-visible absorption measurements, we are able to quantitatively reproduce the I-V measurements in the whole bias range investigated (∼1.0-1.5 V) and to understand the behavior of εh and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal-S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in εh exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (R). Correlation among Φ, ΔΦ, R, transition voltage (Vt), and εh and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions.

Topological nature and the multiple Dirac cones hidden in Bismuth high-Tc superconductors.

Recent theoretical studies employing density-functional theory have predicted BaBiO3 (when doped with electrons) and YBiO3 to become a topological insulator (TI) with a large topological gap (~0.7 eV). This, together with the natural stability against surface oxidation, makes the Bismuth-Oxide family of special interest for possible applications in quantum information and spintronics. The central question, we study here, is whether the hole-doped Bismuth Oxides, i.e. Ba1-xKxBiO3 and BaPb1-xBixO3, which are “high-Tc” bulk superconducting near 30 K, additionally display in the further vicinity of their Fermi energy EF a topological gap with a Dirac-type of topological surface state. Our electronic structure calculations predict the K-doped family to emerge as a TI, with a topological gap above EF. Thus, these compounds can become superconductors with hole-doping and potential TIs with additional electron doping. Furthermore, we predict the Bismuth-Oxide family to contain an additional Dirac cone below EF for further hole doping, which manifests these systems to be candidates for both electron- and hole-doped topological insulators.

Charge transfer between photosynthetic proteins and hematite in bio-hybrid photoelectrodes for solar water splitting cells

Abstract Functionalization of the hematite photoanode with the photosynthetic light antenna protein C-phycocyanin (PC) can yield substantial enhancement of the photocurrent density. Photoelectrochemical cells with bio-hybrid electrodes from photosynthetic proteins and inorganic semiconductors have thus potential for the use in artificial photosynthesis. We investigate here processing routes for the functionalization of hematite photoanodes with PC, including in situ co-polymerization of PC with enzymatically-produced melanin, and using a recombinant PC genetically engineered to carry a hexa-histidine tag (αHisPC). First, the effect of the immobilisation of PC on the electrode morphology and photocurrent production is evaluated. Then, the electronic charge transfer in dark and light conditions is assessed with electrochemical impedance spectroscopy and valence band (VB) X-ray photoemission spectroscopy. The relative shift of the VB spectrum towards the Fermi energy EF upon illumination is smaller for the more complex processed coating and virtually disappears for αHisPC immobilised with a melanin film. Optimal conditions for protein immobilisation are determined and the dark currents benefit most from the most advanced protein coating processes.

FORMULA FOR MAXIMUM SECONDARY ELECTRON YIELD FROM METALS

Based on free-electron model, the calculated inelastic mean escape depth of secondary electrons, experimental one, the energy band of metal, the characteristics and processes of secondary electron emission, maximum number of secondary electrons released per primary electron δ(Φ,EF)PEm as a function of parameter Km, work function Φ and Fermi energy EF was deduced, where Km is a constant for a given metal in the energy range 100–800 eV. According to the relationship between maximum secondary electron yield from metal δ(Φ,EF)m and δ(Φ,EF)PEm, the formula for δ(Φ,EF)m as a function of atomic number Z, parameter Km, Φ and EF was deduced. Using the deduced formula for δ(Φ,EF)m, Z, experimental δ(Φ,EF)m, Φ and EF, Km relative to alkali metals, Km relative to earth-alkali metals and the mean value of Km were computed, respectively. And the formulae for maximum secondary electron yield from alkali metals, earth-alkali metals and metals were obtained and proved to be true, respectively. On the basis of the deduced formula for δ(Φ,EF)m and the empirical relation that high Φ are connected with high EF, it can be concluded that high δ(Φ,EF)m are connected with high Φ and vice versa.

On the Origin of Mass and Angular Momentum of Stellar Objects

The consequence of the 5D projection theory [1] is extended beyond the Gell-Mann Standard Model for hadrons to cover astronomical objects and galaxies. The proof of Poincare conjecture by Pe-relman’s differential geometrical techniques led us to the consequence that charged massless spinors reside in a 5D void of a galactic core, represented by either an open 5D core or a closed, time frozen, 3D × 1D space structure, embedded in massive structural stellar objects such as stars and planets. The open galactic core is obtained from Ricci Flow mapping. There exist in phase, in plane rotating massless spinors within these void cores, and are responsible for 1) the outward spiral motion of stars in the galaxy in the open core, and 2) self rotations of the massive stellar objects. It is noted that another set of eigen states pertaining to the massless charged spinor pairs rotating out of phase in 1D (out of the 5D manifold) also exist and will generate a relatively weak magnetic field out of the void core. For stars and planets, it forms the intrinsic dipole field. Due to the existence of a homogeneous 5D manifold from which we believe the universe evolves, the angular momentum arising from the rotation of the in-phase spinor pairs is proposed to be counter-balanced by the rotation of the matter in the surrounding Lorentz domain, so as to conserve net zero angular momentum. Explicit expression for this total angular momentum in terms of a number of convergent series is derived for the totally enclosed void case/core, forming in general the structure of a star or a planet. It is shown that the variables/parameters in the Lorentz space-time domain for these stellar objects involve the object’s mass M, the object’s Radius R, period of rotation P, and the 5D void radius Ro, together with the Fermi energy Ef and temperature T of the massless charged spinors residing in the void. We discovered three laws governing the relationships between Ro/R, T, Ef and the angular momentum Iω of such astronomical object of interest, from which we established two distinct regions, which we define as the First and Second Laws for the evolution of the stellar object. The Fermi energy Ef was found to be that of the electron mass, as it is the lightest massive elementary particle that could be created from pure energy in the core. In fact the mid-temperature of the transition region between the First and Second Law regions for this Ef value is 5.3 × 109 K, just about that of the Bethe fusion temperature. We then apply our theory to analyse observed data of magnetars, pulsars, pre-main-sequence stars, the NGC 6819 group, a number of low-to-mid mass main sequence stars, the M35 members, the NGC 2516 group, brown dwarfs, white dwarfs, magnetic white dwarfs, and members of the solar system. The ρ = (Ro/R) versus T, and ρ versus P relations for each representative object are analysed, with reference to the general process of stellar evolution. Our analysis leads us to the following age sequence of stellar evolution: pulsars, pre-main-sequence stars, matured stars, brown dwarfs, white dwarfs/magnetic white dwarfs, and finally neutron stars. For every group, we found that there is an increasing average mass density during their evolution.

Molecular Spintronics: Destructive Quantum Interference Controlled by a Gate

The ability to control the spin-transport properties of a molecule bridging conducting electrodes is of paramount importance to molecular spintronics. Quantum interference can play an important role in allowing or forbidding electrons from passing through a system. In this work, the spin-transport properties of a polyacetylene chain bridging zigzag graphene nanoribbons (ZGNRs) are studied with nonequilibrium Green's function calculations performed within the density functional theory framework (NEGF-DFT). ZGNR electrodes have inherent spin polarization along their edges, which causes a splitting between the properties of spin-up and spin-down electrons in these systems. Upon adding an imidazole donor group and a pyridine acceptor group to the polyacetylene chain, this causes destructive interference features in the electron transmission spectrum. Particularly, the donor group causes a large antiresonance dip in transmission at the Fermi energy EF of the electrodes. The application of a gate is investigated and found to provide control over the energy position of this feature making it possible to turn this phenomenon on and off. The current-voltage (I-V) characteristics of this system are also calculated, showing near ohmic scaling for spin-up but negative differential resistance (NDR) for spin-down.

Graphene with adatoms: Tuning the magnetic moment with an applied voltage

We show that, in graphene with a small concentration of adatoms, the total magnetic moment μT can be switched on and off by varying the Fermi energy EF, either by applying a gate voltage or by suitable chemical doping. Our calculation is carried out using a simple tight-binding model described previously, combined with a mean-field treatment of the electron-electron interaction on the adatom. The values of EF at which the moment is turned on or off are controlled by the strength of the hopping between the graphene sheet and the adatom, the on-site energy of the adatom, and the strength of the electron-electron correlation energy U. Our result is qualitatively consistent with recent experiments [R. R. Nair et al., Nat. Commun. 4, 2010 (2013)].

Electronic Structure and Properties of Novel Layered Superconductors

The electronic energy structures and magnetic properties of layered superconductors RNi2B2C, RFe4Al8, and FeSe are systematically studied, by using the density functional theory (DFT). The calculations allowed us to reveal a number of features of the electronic structure, which can cause the manifestation of peculiar structural, magnetic and superconducting properties of these systems. It is demonstrated that the Fermi energy EF is located close to the pronounced peaks of the electronic density of states (DOS). The main contribution to DOS at the Fermi level arises from 3d-electrons. The calculations of the pressure-dependent electronic structure and the magnetic susceptibility in the normal state indicate that the novel superconductors are very close to a magnetic instability with dominating spin paramagnetism. It is shown that experimental data on the pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of DOS at EF as a function of the pressure.

Characterization of the quantized Hall insulator phase in the quantum critical regime

The conductivity σ and resistivity ρ tensors of the disordered Hofstadter model are mapped as functions of Fermi energy EF and temperature T in the quantum critical regime of the plateau-insulator transition (PIT). The finite-size errors are eliminated by using the non-commutative Kubo formula. The results reproduce all the key experimental characteristics of this transition in integer quantum Hall effect (IQHE) systems. In particular, the quantized Hall insulator (QHI) phase is detected and analyzed. The presently accepted characterization of the QHI phase in the quantum critical regime, based entirely on experimental data, is fully supported by our theoretical investigation.

Re-Examining the Doping Effect on the Performance of Quantum Well Infrared Photodetectors

This paper investigates the dependence of background limited performance (BLIP) temperature of quantum well infrared photodetectors (QWIPs) on doping density. In contrast to the generally accepted optimal doping condition E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> =k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BLIP</sub> , our theoretical prediction shows that lower doping densities should slightly increase the BLIP temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BLIP</sub> taking into account the temperature dependence of the Fermi energy E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> , a factor neglected in the previous analyses. Numerical modeling results are used to reinterpret the reported T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BLIP</sub> measurements for a series of QWIPs of typical design for 9 μm peak wavelength with different doping values. In addition, based on the same general expression for the Fermi energy, the optimized sheet doping concentration to achieve maximum detectivity is given by E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> =1.37 kBT, a revision to the previous E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> =2 k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> T condition.

4,4′-Dithiodipyridine on Au(111): A Combined STM, STS, and DFT Study

We studied the adsorption of 4,4′-dithiodipyridine (PySSPy) on Au(111) under ultrahigh vacuum conditions and at low-temperature both experimentally by means of scanning tunneling microscopy and spectroscopy (STM and STS) and theoretically by density functional theory (DFT). We find PySSPy molecules, characterized by their elongated appearance in STM, assembled in islands that reside exclusively in fcc regions of the herringbone reconstructed Au (HB) terraces. A triangular structure motif dominates the local arrangement of the PySSPy. DFT calculations reveal a virtually planar adsorption geometry of the PySSPy with the S–S bond of the molecule almost parallel to the Au–Au bonds of the substrate underneath. Though van der Waals forces dominate the stability of the adsorbed PySSPy, there is also a covalent contribution to the PySSPy/Au interaction. As a consequence, the PySSPy structures are relatively unstable. As found by STS, the highest occupied molecular orbital (HOMO) is located at around −0.7 eV below the Fermi energy (Ef), in good agreement with the HOMO provided by DFT. The position of the lowest unoccupied molecular orbital (LUMO) is about 3.2 eV above Ef, leaving the HOMO–LUMO gap practically unchanged compared to a free molecule. The molecules exhibit a strong propensity to dissociate into monomers forming pyridine thiolates (PyS) at step edges or elbow sites of the HB. At room temperature, all PySSPy on the substrate dissociate resulting in linear chains of PyS characterized by a drop of the LUMO by about −0.9 eV.

Effect of contact preparation on the profile of the electric field in CdZnTe detectors

We used the transient-current technique to delineate the effects of different contact metals on the formation of space charge and on the profiles of the electric field in CdZnTe radiation detectors. In contrast to existing results on the polarization of semiconductor radiation detectors, we find that detectors with ohmic (Au/Au) contacts may experience a larger distortion of the internal electric field than those having Schottky (Au/In) contacts. We explain this difference by postulating the presence of a deep hole trap, ET, below the Fermi energy EF, which captures holes generated by the weakly injecting Au anode. The observed behaviour was described successfully by a numerical model relating the energy difference between ET and EF and the band bending at the contacts. We also present results on aging effects that limit the detector's performance and stability over long times.

Analysis of energy states of two-dimensional electron gas taking into account the nonparabolicity of the conduction band

We determined the energy states of a two-dimensional electron gas (2DEG) in high electron mobility transistors (HEMTs) with a pseudomorphically strained InAs channel (InAs PHEMTs) rigorously, taking into account the nonparabolicity of the conduction band for InAs. To consider this nonparabolicity in the Schrodinger equation, we have used the energy-dependent effective mass based on Kane's k · p perturbation theory. The Schrodinger equation was solved according to a standard perturbation theory by regarding the terms including the nonparabolicity parameter as a perturbed Hamiltonian. Numerical simulation was carried out by solving the Schrodinger and Poisson equations self-consistently. Since the effective mass of electrons increases with their energy, the Fermi energy EF and the first and second subband energies lie within the InAs well and hence most of the electrons are strongly confined in the InAs well. When the same calculation was done for conventional In0.53Ga0.47As HEMTs for comparison, there was no noticeable difference between the results for the parabolic and nonparabolic conduction-band cases. In addition, it was found from the ns dependence of EF that the threshold voltage VTH for InAs PHEMTs is about 0.21 V lower than that for conventional In0.53Ga0.47As HEMTs. This VTH shift corresponds to the effective conduction-band offset energy between the In0.52Al0.48As and InAs layers. The theoretical result for VTH agrees fairly well the experimental ones reported so far.