Density Of Available States Research Articles

[formula omitted] electronic structure and spin EPR-shift of [formula omitted] ion in diluted magnetic p-type [formula omitted][formula omitted]

In this correspondence, we deduce the non-parabolic band dispersion within the framework of effective mass representation (EMR) using k→⋅π→ perturbation theory in the presence of s.o interaction, magnetic impurity and external magnetic field. We measure the electronic parameters like Fermi energy, density of states, effective g factor and effective mass etc. in p-type Sn1−xEuxTe by taking into account a 6-level band structure with normal and bumped band edges. In the study, we use the experimental and simulated band gap values as functions of Eu+2 impurity and temperature. Apart from the general trend, the electronic parameters exhibit extreme behaviour near the band inversion point x=0.020 at T=300 K (Nayak et. al.2023), but in the range of impurity concentration and temperature considered; from x=0.0005 to x=0.001 and T=1.5 K to T=300 K, the behaviour is normal. Again, we follow the expression for spin-electron paramagnetic resonance (EPR) shift (Ps) where the perturbation expansion of Green’s function is used in the presence of the above-mentioned interactions. To get around the problem of loss of translational symmetry in the presence of the magnetic field, Green’s function is redefined in terms of Peierl’s phase factor. Here we have made an extensive study of the shift of Eu+2 ion in p-type Sn1−xEuxTe for several impurity concentrations from x=0.0005 to x=0.001 in the temperature range T=1.4 K to T=300 K. Lastly, we analyze the EPR shift and other electronic parameters and their anisotropies in p-type Sn1−xEuxTe as functions of carrier concentration and Eu impurity from T=1.5K to T=300K. We report the EPR shift is 2.97×10−3 at T=300 K and 3.31×10−3 at T=1.5 K with anisotropy at p=5×1020cm−3 in contrast to the experimental observation of +(3.4±0.4)×10−3 in the same system. The discrepancy is attributed to the reduction in the density of available states at Fermi energy at the L-band caused by the simultaneous rise in that at the Σ− band noticeable particularly at/above high hole concentrations p≥5×1020cm−3.

Reformation of La0.7Sr0.3MnO3 properties by using ZnO in La0.7Sr0.3MnO3-ZnO heterostructures grown on (001) oriented Si.

Study of the available density of states (DOS) close-to-zero bias for conduction in strongly correlated electron systems, such as half-metallic La0.7Sr0.3MnO3 (LSMO) and its heterostructures, is important for fundamental and application reasons. As the DOS is proportional to the differential conductance (dI/dV), the dI/dV of a 120 Å LSMO film and its reformation in LSMO/ZnO heterostructures was investigated for different ZnO thicknesses. Unlike in conventional metals, the dI/dV of LSMO exhibits a power-law dependent zero-bias anomaly, i.e., dI/dV ∝ Vm (m ∼ 1) near zero bias in the ferromagnetic metallic state at 10 K. The growth of ZnO on LSMO reforms the linear dI/dVvs. V of LSMO near zero bias to non-linear. The exponent 'm' becomes ∼0.5 for a higher ZnO thickness, revealing increased electron-electron interactions and suppression of Kondo-like, double and superexchange interactions, which are responsible for the depression of the DOS of LSMO near zero bias. In a magnetically disordered state, i.e., around the Curie temperature, ZnO reforms the linear V-shaped dI/dV vs. V of LSMO to parabolic U-shaped dI/dVvs.V and controls the electron concentrations in the t2g-orbitals of Mn realized from the DOS simulations. Additionally, ZnO introduces a peak in the dI/dV vs. V due to Fowler-Nordheim tunnelling, and the peak voltage can be tuned by varying the ZnO thickness or temperature from 300 K to 360 K. Such functions of ZnO yield major perspectives for novel applications in thin-film-based devices.

Coherent Plasma in a Lattice

We present a fully second-quantized calculation showing the emergence of spontaneous coherent configurations of the electromagnetic field interacting with charged bosons in a regular lattice. The bosons tend to oscillate at their plasma frequency, and in addition are subjected to electrostatic forces which keep them confined close to the lattice sites while causing a frequency shift in the oscillation. Under certain conditions upon these frequencies, we find that a suitably defined set of coherent states (coherent both in the field and matter degrees of freedom) exhibit a negative energy gap with respect to the perturbative ground state. This is true in the RWA approximation and for position-independent fields to both the first and second order in the interaction Hamiltonian. We compare this result with other recent findings from cavity QED, and note that (1) consideration of full 3D wavefunctions and a careful definition of the coherent states are essential for obtaining the energy gap, and (2) although our calculation is made in reference to bosons, it may apply to protons bound in a crystal matrix as well if their density is very low compared to the density of available states.

Valence Alignment of Mixed Ni-Fe Hydroxide Electrocatalysts through Preferential Templating on Graphene Edges for Enhanced Oxygen Evolution.

Engineering the metal-carbon heterointerface has become an increasingly important route toward achieving cost-effective and high-performing electrocatalysts. The specific properties of graphene edge sites, such as the high available density of states and extended unpaired π-bonding, make it a promising candidate to tune the electronic properties of metal catalysts. However, to date, understanding and leveraging graphene edge-metal catalysts for improved electrocatalytic performance remains largely elusive. Herein, edge-rich vertical graphene (er-VG) was synthesized and used as a catalyst support for Ni-Fe hydroxides for the oxygen evolution reaction (OER). The hybrid Ni-Fe/er-VG catalyst exhibits excellent OER performance with a mass current of 4051 A g-1 (at overpotential η = 300 mV) and turnover frequency (TOF) of 4.8 s-1 (η = 400 mV), outperforming Ni-Fe deposited on pristine VG and other metal foam supports. Angle-dependent X-ray absorption spectroscopy shows that the edge-rich VG support can preferentially template Fe-O units with a specific valence orbital alignment interacting with the unoccupied density of states on the graphene edges. This graphene edge-metal interaction was shown to facilitate the formation of undersaturated and strained Fe-sites with high valence states, while promoting the formation of redox-activated Ni species, thus improving OER performance. These findings demonstrate rational design of the graphene edge-metal interface in electrocatalysts which can be used for various energy conversion and chemical synthesis reactions.

Bandgap engineering in CuO nanostructures: Dual-band, broadband, and UV-C photodetectors

In this work, the bandgap of CuO (p-type semiconductor) has been engineered from an indirect bandgap of ∼1 eV to a direct bandgap of 4 eV just by tuning the nanostructure morphology and midgap defect states. The absorption in near-infrared (NIR) and visible regions is ordinarily suppressed by controlling the growth parameters. Considering the increasing scope and demand of varying spectral range (UV-C to NIR) photodetectors, the systematic variation of the available density of states (DOS) at a particular energy level in CuO nanostructures has been utilized to fabricate dual-band (250 nm and 900 nm), broadband (250 nm–900 nm), and UV-C (250 nm) photodetectors. The sensitivity and detectivity of the photodetector for broadband detectors were ∼103 and 2.24 × 1011 Jones for the wavelengths of 900 nm and 122 and 2.74 × 1010 Jones for 250 nm wavelength light, respectively. The UV-C detector showed a sensitivity of 1.8 and a detectivity of 4 × 109 Jones for 250 nm wavelength light. A plausible mechanism for the photoconduction has been proposed for explaining the device operation and the effect of variation in available DOS. The obtained photodetectors are the potential candidates for future optoelectronic applications.

Role of the Metal-Oxide Work Function on Photocurrent Generation in Hybrid Solar Cells

ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.

Strong Size Dependency on the Carrier and Photon Dynamics in InGaN/GaN Single Nanowalls Determined Using Photoluminescence and Ultrafast Transient Absorption Spectroscopy.

Here, we have demonstrated strong size dependency of quasi-equilibrium and nonequilibrium carrier and photon dynamics in InGaN/GaN single nanowalls using photoluminescence and transient absorption spectroscopy. We demonstrate that two-dimensional carrier confinement, strain relaxation, and modified density of states lead to a reduced Stokes shift, smaller full width at half-maxima, increased exciton binding energy, and reduced nonradiative recombination. The ultrafast transient spectroscopy shows that carrier capture is a two-step process dominated by optical phonons and carrier-carrier scattering in succession. The carrier capture is a strongly size-dependent process and becomes slower due to modulation of the density of available states for progressively decreasing nanowall sizes. The slowest process is the electron-hole recombination, which is also extremely size-dependent and the rate increases by almost an order of magnitude in comparison to that of quantum-well structures. Electron-hole wave function overlap and modified density of states are among the key aspects in determining all the properties of these structures.

Temperature Sensitivity of Electrical Resistivity of Graphene/Copper Hybrid Nano Ribbon Interconnect: A First Principle Study

Recent studies have shown superior thermal transport of graphene on copper as a potential candidate for the next generation interconnects. Using density function theory (DFT) we have studied the current transport of graphene/copper (G/Cu) hybrid-nano wire interconnect system and compared electrical characteristics with other two dimensional counterparts along with graphene. From the first principle calculation, band structure and density of states have been calculated. Using Landauer-Buttiker (LB) formalism, electrical transport is calculated. We explained why G/Cu hybrid interconnect shows more conductivity than graphene only interconnects with the help of phase space argument. As graphene on copper system has more available density of states near the Fermi level it offers more states than graphene for conduction.

Effect of asymmetric molecule–electrode coupling and molecular bias on rectification in molecular junctions

In this research work, we compare the rectification trends of two symmetrical and one asymmetrical molecular junction formed with gold and silver electrodes bridging benzenedithiol molecule. The origin of rectification is attributed to both molecular bias drop and asymmetric molecule–electrode coupling. The electronic transport properties are computed by using semi-empirical extended Huckel method combined with non-equilibrium Green’s function framework. The results are fully rationalized by analysing the distribution of molecular orbitals with changing bias voltage, available density of states and area of transmission spectra spanned within bias window, transmission eigenstates and transmission pathways. We deduce through this work that the molecular rectification is not only the property of asymmetric molecule–metal coupling, but molecular bias also plays vital role in stemming asymmetric I–V characteristics. Our results suggest how to realize molecular rectification by using different electrode materials which act as Schottky barriers in molecular junctions that emulate p–n junction diode in semiconductor electronics.

Quantum revivals in conformal field theories in higher dimensions

We investigate the behavior of the return amplitude following a quantum quench in a conformal field theory (CFT) on a compact spatial manifold of dimension and linear size , from a state of extensive energy with short-range correlations. After an initial gaussian decay reaches a plateau value related to the density of available states at the initial energy. However for this value is attained from below after a single oscillation. For a holographic CFT the plateau persists up to times at least , where is the dimensionless Stefan–Boltzmann constant. On the other hand for a free field theory on manifolds with high symmetry there are typically revivals at times . In particular, on a sphere of circumference , there is an action of the modular group on implying structure near all rational values of t/L, similar to what happens for rational CFTs in .

Influence of Photonic Crystal on Fluorescence Resonance Energy Transfer Efficiency between Laser Dyes

Spontaneous emission by an excited molecule strongly depends upon the available density of states into which the molecule can decay. In a photonic crystal, the allowed local density of states depletes within the stop band and enhances at the band edge of the crystal. As a result, an emitter implanted in a photonic crystal is forced to redistribute its fluorescence energy within its emission spectral range, and its spontaneous emission spectrum thus gets modified. Here, we studied the influence of change in local density of states on energy transfer efficiency between a donor–acceptor pair embedded in a colloidal photonic crystal. Rhodamine-B and Rhodamine-800 dyes were chosen as the energy donor and energy acceptor, respectively. We observed an angle-dependent quenching in the emission intensity of the donor accompanied by enhancement in acceptor emission when both dyes were in the photonic crystal. This occurred owing to depletion in the allowed local density of states available to the donor. Reduction in the fluorescence lifetime of the donor in the presence of the acceptor confirmed fluorescence resonance energy transfer between the chromophores. In spite of marginal overlapping between the emission band of the donor and the absorption band of the acceptor, the energy transfer efficiency between the dyes was ∼80% in the photonic crystal environment. This enhancement resulted from forced proximity and hence reduced intermolecular distance between the donor and acceptor on being physically restricted within nanovoids of the photonic crystal.

Graphene terahertz modulators by ionic liquid gating.

Graphene based THz modulators are promising due to the conical band structure and high carrier mobility of graphene. Here, we tune the Fermi level of graphene via electrical gating with the help of ionic liquid to control the THz transmittance. It is found that, in the THz range, both the absorbance and reflectance of the device increase proportionately to the available density of states due to intraband transitions. Compact, stable, and repeatable THz transmittance modulation up to 93% (or 99%) for a single (or stacked) device has been demonstrated in a broad frequency range from 0.1 to 2.5 THz, with an applied voltage of only 3 V at room temperature.

Fundamental Conductance <inline-formula> <tex-math notation="TeX">\(\div \) </tex-math></inline-formula> Voltage Limit in Low Voltage Tunnel Switches

There is a fundamental conductance ÷ voltage limit in low voltage (<;4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T/q) tunneling switches that turn off by relying upon the band edges to cutoff the available density of states. The Fermi occupation probabilities are thermally broadened by 4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T. However, current is only allowed to flow in a narrow energy range limited by the applied voltage, V. This means that if a voltage less than 4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T/q is applied, the conductance will be reduced by at least qV/4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T. Even with a perfect tunneling probability of 1 in a perfect quantum channel, the conductance quantum would be diminished by qV/4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T. Attempts at lowering the operating voltage below <;4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T/q must come at the expense of smaller conductance.

Interaction of hydrogen in carbon matrix with impurities of nickel. Effects of spin fluctuation

This work aims to define general criteria to allow theoretical and experimental design of new materials with high hydrogen content, with a view to their potential application as moderators in reactors at high temperatures and hydrogen storage materials.To this end we study the effects of nickel impurities on the properties of pure carbon (slabs as well as nanoparticles and gels) in order to analyze the thermodynamical characteristics, and improve the behavior of alloys for Ni-metal hydride rechargeable batteries. These elements are chosen because they have a wide range of solubility of hydrogen, which from the technological point of view makes them important candidates in the search for new materials. Our results show that this kind of carbon material could be used as support for hydrogen storage improving the screening charge density and the density of available states, as analyzed by the authors in previous works.

Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber

A stable mode-locked fiber laser (MLFL) employing multi-layer graphene as saturable absorber (SA) is presented. The multi-layer graphene were grown by chemical vapor deposition (CVD) on Ni close to A-A stacking. Linear absorbance spectrum of multi-layer graphene was observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of different atomic-layers (7-, 11-, 14-, and 21- layers) graphene based SA are investigated and compared. The results found that the thicker 21-layer graphene based SA exhibited a smaller modulation depth (MD) value of 2.93% due to more available density of states in the band structure of multi-layer graphene and favored SA nonlinearity. A stable MLFL of 21-layer graphene based SA showed a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a time-bandwidth product (TBP) of 0.323 at fundamental soliton-like operation. This study demonstrates that the atomic-layer structure of graphene from CVD process may provide a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.

Tuning Electron Transfer Rates via Systematic Shifts in the Acceptor State Density Using Size-Selected ZnO Colloids

We report direct measurements of the influence of the available density of acceptor states on the rate of near-barrierless electron transfer between a dye sensitizer and an oxide semiconductor. The electron donor was the excited state of a zinc porphyrin, and the acceptors were a series of size-selected ZnO nanocrystals. The available density of states was tuned by controlling the relative position of the ZnO band edge using quantum confinement. The resulting change in the rate was consistent with a simple model of the state density as a function of energy above the ZnO band edge.

Modification of Ag(111) surface electronic structure via weak molecular adsorption of adenine measured with low temperature scanning tunneling microscopy and spectroscopy

Low temperature scanning tunneling microscopy and spectroscopy have been used to resolve modifications to the Ag(111) surface electronic structure due to the weak adsorption of the nucleobase adenine. Differential conductance spectroscopy recorded at 15 K reveals an upward energetic shift of the surface state native to Ag(111) from a band edge of -67 meV on the clean surface to +82.5 meV recorded over adenine islands. Differential conductance images show the impact of adenine domains on the density of available states as a function of energy relative to the uncovered Ag terraces as well as free-electron-like scattering in the adenine domains. Dispersion of the parallel wave vector of scattered electrons in the adenine domains is compared with the dispersion for electron scattering in bare silver and the ratio of effective masses for electrons in those bands is 1.1+/-0.2. It is hypothesized that this shift occurs due to a combination of effects brought on by the adsorption of adenine including dielectric screening of the first image potential.

Influence of resonant Raman scattering in the elemental analysis using X-ray emission based techniques

A tabulation of characteristic X-ray energies across the periodic table are provided where those X-rays are expected to result in a significant fractional resonant Raman scattering (RRS) contribution to the X-ray attenuation from a particular shell/subshell of the same or another element. The tabulations can be considered as guideline so as to know what can be expected due to RRS in typical photon- and particle-induced X-ray emission spectrometry. The RRS contribution is not included in the available theoretical attenuation coefficients, which are generally used in estimation of the matrix corrections in routine quantitative elemental analysis based on various X-ray emission techniques. The radiative RRS peaks can also interfere with normal X-ray spectrum and influence the elemental analysis. The RRS cross-section depends upon the energy difference of the X-ray energy and the shell/subshell ionization threshold taken in the units of the shell/subshell energy width, density of available states near the Fermi level, and the band structure in case the element is in the solid form. Some aspects of the dependence of the RRS contribution on the chemical forms of the elements are also discussed.

Propagation of electromagnetic waves in 1D finite photonic crystals for the investigation of linear properties

In present work, the transmittance, Bloch wavevector, group velocity, group index, photon delay and effective gain has been analyzed by computer simulation. The group velocity vanishes at the edges of forbidden band gaps at normalized frequencies 0.2922, 0.4199, 0.6608 and 0.7822, where photons are localized. This results in the superluminal group velocity being increased and causes a large decrement in group index. We have obtained a small value of group velocity at the photonic band edges which represents the available density of states of photon localization. Photons percolating through the layers make many passes (being reflected between the layers) and slow down as they pass through the active layers, which increase the optical path length and optical gain. The photon delay factor has been observed to be dominant as the group velocity approaches zero. Our analysis explores the applicability of 1D photonic crystal for threshold-less lasing.

Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Using a nonequilibrium Green's function approach, quantum simulations are performed to assess the device characteristics for cross-plane transport in Si/Ge/Si-superlattice thin films. The effect of quantum confinement on the Seebeck coefficient and electrical transport and its impact on the power factor of superlattices are studied. In this case, decreasing well width leads to an increased subband spacing causing the Seebeck coefficient of the superlattice to decrease. Electron confinement also causes a drastic reduction in the overall available density of states. Results show that confinement effects in the silicon barrier are responsible for a 40% decrease in the electrical conductivity of superlattices where barrier films are thinner by a factor of 1.5. In the same two devices, there is a negligible change in the Seebeck coefficient, which results in a decrease in the power factor corresponding to the decrease in conductivity. This decrease in the electrical performance for superlattices with thinner layers may offset the previously hypothesized gains of highly scaled superlattice structures resulting from reduced thermal conductivity. Simulations of the present superlattice structure at varying doping levels show a decrease in power factor with a decrease in device size parameters.