Composition dependence of the energy gap and thermal diffusivity in bulk As-Se glasses

We have measured the composition dependence of the optical energy gap and thermal diffusivity in bulk As _x Se_1−x (0.10⩽x⩽0.50) glasses using photoacoustic technique. The energy gap shows a threshold minimum value and thermal diffusivity has a threshold maximum value at the stoichiometric composition As₂Se₃ corresponding tox=0.40. The decrease in energy gap is explained on the basis of chemical bonding. It is argued that the threshold percolation of rigidity in the random network is responsible for the peaking of the thermal diffusivity at the stoichiometric composition.     

Tuning of the open‐circuit voltage by wide band‐gap absorber and doped layers in thin film silicon solar cells

We present an experimental study combined with computer simulations on the effects of wide band‐gap absorber and window layers on the open‐circuit voltage (V_oc) in single junction thin film silicon solar cells. The quantity ΔE_p, taking as the difference between the band gap and the activation energy in ?p? layer, is treated as a measure of the p‐layer properties and shows a linear relation with V_oc over a range of 100 mV with a positive slope of around 430 mV/eV. Two limiting mechanisms of V_oc are identified: the built‐in potential at lower ΔE_p and the band gap of the absorber layer at higher ΔE_p. The results of the experimental findings are confirmed by computer simulations. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

High-Tc superconductor near the S–I transition

We report on the density of states measurements of Bi2212 (Bi_2+xSr_xCaCu₂O_8+δ) near the superconductivity-insulator transition using a low temperature scanning tunneling microscope. We prepared highly underdoped Bi rich Bi2212 single crystals (T_c  32 K). The energy gap distribution did not provide an energy scale proportional to T_c. Averaged tunnel spectra with various doping levels were scaled into a single line if energy was normalized by their respective gap values. This indicated there was no crossover energy, which separates a pseudogap and a superconducting gap.     

Far-infrared probe of energy gap in LaO1−xFxFeGe compound

We report on the electrical resistivity and far-infrared reflectance measurements of LaO_1?xF_xFeGe samples. Furthermore, we introduce a new method to probe the energy gap and determine its value. The onset transition temperature was 22.8 K for x = 0.13, and a clear anomaly was observed at 90 K in the ρ(T) curve for x = 0.11 with T_c = 20.6 K. We clearly observed the phonon-suppressed feature in reflectance spectra where F-doping caused a strong suppression of a peak at 200 cm^?1. The energy gap above T_c, 2Δ = 2.10 meV, was determined from the measured spectra based on the changes in reflectivity by F-doping.     

First-principles study of the optical properties of PbFX (X = Cl,Br, I) compounds in its matlockite-type structure

We present the results of the ab initio theoretical study of the optical properties for PbFX (X = Cl, Br, I) compounds in its matlockite-type structure using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Z resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε( ω) and its zero-frequency limit ε₁ ( 0 ). We find that the values of ε₁ ( 0 ) increases with decreasing the energy gap. The reflectivity spectra and absorption coefficient has been calculated and compared with the available experimental data. The optical properties are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure.     

DFT study on size-dependent geometries, stabilities, and electronic properties of AunM2 (M = Si, P; n = 1–8) clusters

The ab initio method based on density functional theory at the PW91PW91 level has been employed to systematically study the structures, stabilities, electronic, and magnetic properties of gold clusters with or without silicon/phosphorus doping. The optimized geometries show that the most stable isomers for Au _n Si₂ and Au _n P₂ (n = 1–8) clusters prefer a three-dimensional structure when n = 2 and n = 3 upwards, respectively, and they can be viewed as grown from the already observed Au _n−1M₂ (M = Si, P). The relative stabilities of calculated Au _n M₂ (M = Si, P) clusters have been analyzed through the atomic average binding energy, fragmentation energy, second-order difference of energy, and HOMO-LUMO gap. A pronounced odd-even alternative phenomenon indicates that the clusters with even-numbered valence electrons possess a higher stability than their neighboring ones. For both systems, natural population analysis reveals that electronic properties of dopant atoms in the corresponding configuration are mainly related to s and p states. We also investigated magnetic effects of clusters as a function of cluster size, however, their oscillatory magnetic moments were found to vary inversely to the fragmentation energy, second-order difference of energy, and HOMO-LUMO gap.     

Ultra-low temperature study of the even-denominator FQHE state

We present experimental data showing unambiguously an even-denominator fractional quantum Hall effect (FQHE) state at . At a bath temperature T_b=8 mK, we observe a Hall plateau quantized to a value of 2h/5e² with an uncertainty smaller than 2 parts in 10⁶ and a vanishing R_xx (R_xx=1.7±1.7 Ω). The thermal activation energy gaps Δ at Landau level filling factors , and are 0.11, 0.10, and 0.055 K, respectively. Adding a disorder broadening (typically 2 K) to these values, we deduce that all three FQHE states have probably very similar energy gaps. The electron heating experiment shows that the 2D electrons are efficiently cooled to the bath temperature for T_b8 mK. We also explore the density dependence of the activation gap at . Preliminary results at T_b25 mK show that the state is very sensitive to disorder.     

Energy spectrum of excitations in quasi-one-dimensional conductors with a charge-density wave

A well-formed energy gap Δ is observed in the energy spectrum of the quasi-one-dimensional orthorhombic conductor TaS₃ at temperatures much lower than the Peierls transition temperature T _P . As the temperature increases, in the region T>T _P /2 there is a growth of the density of states in the gap and a relative decrease of the density at energies greater than Δ. In addition, absorption lines which probably correspond to soliton states in a charge-density wave are observed in the gap. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 246–250 (25 February 1996)     

B$lt;sub$gt;$lt;i$gt;n$lt;/i$gt;$lt;/sub$gt;($lt;i$gt;n$lt;/i$gt;=2—15)团簇的几何结构和电子性质

应用密度泛函理论中的B3LYP方法计算并分析了不同生长模式下B_n（n=2—15）团簇的几何结构及电子性质. 同时,比较和讨论了不同生长模式下硼团簇的原子束缚能、能级间隙和第一电离势. 研究表明：直线构型稳定性最低,金属性较强,尤其在n=8时能隙仅有0.061 eV,说明该团簇已具有金属特征. 平面或准平面构型稳定性最高,非金属性强. 立体构型的稳定性与金属性介于直线和平面构型之间. 另外,还讨论了基态团簇的束缚能、能量二阶差分、能级间隙和第一电离势随团簇尺寸的变化,结果表明B₁₂与B₁₄是幻数团簇. 关键词： n团簇')" href="#">B_n团簇 密度泛函理论 几何结构 电子性质     

Band mixing in narrow gap semiconductors

The interaction of conduction and valence bands in narrow gap semiconductors such as InSb and HgCdTe influences the position and width of subband energy levels in space-charge layers. While a nonzero width can only occur if electrons from the conduction band can tunnel into approximately degenerate states of the valence band the level shifts due to band mixing are always present. We present a Green's function treatment which allows in a simple way to discuss the dependence of band mixing effects on the parameters of thek·p-Hamiltonian in particular the band gap. The essential qualitative feature of the level shifts is adecrease of subband energy separation withdecreasing effective mass. This agrees with recent experimental results for Hg_1-x Cd _x Te.     

Optical and electrical properties of modulation-doped n and p-type Ga x In1-x N y As1-y /GaAs quantum wells for 1.3 μm laser applications

We present a comprehensive study of spectral photoluminescence (PL), photoconductivity and Hall mobility in undoped, n and p-type modulation-doped quantum wells of Ga_1-x In _x N _y As_1-y /GaAs with varying nitrogen concentration. We show that the increasing nitrogen composition red shifts the energy gap and this red shift is accompanied with a reduction of the 2D electron mobility in the quantum wells. True temperature dependence of the band gap, free from errors associated with nitrogen induced exciton trapping effects, is observed because in the modulation doped QW samples PL emission is dominated by band-to-band recombination and the S-shape temperature dependence is eliminated. Excellent fit to semi-experimental Varshni equation is obtained and the temperature dependence of the band gap in the linear regime (dE/dT) is tabulated as a function of nitrogen concentration and the type of dopant.     

Pressure dependence of the absorption edge at the E0 gap of mocvd - Grown ZnTe films

Abstract

We have investigated the direct gap absorption of 1μm thick ZnTe-epilayers grown on GaAs substrates by metalorganic chemical vapour deposition (MOCVD). Free ZnTe-layers were obtained by selective etching. The absorption coefficient was measured up to about 50000 cm^?1 in a diamand anvil cell in the temperature range from 115–300 K. The spectra near the direct gap E₀ are dominated by a sharp excitonic structure. Its change with pressure is evaluated by a model which allows to determine the pressure shift of the gap energy dE₀/dP and the change of the Rydberg energies of the excitons dR*/dP.^[1]     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Excitonic polarons in quasi-two-dimensional structures

A theory of excitonic polarons in semiconductor quantum wells is presented. Using a unitary transformation, we have diagonalized the exciton-phonon interaction operator in a quasi-two-dimensional system partially and then calculated the ground-state energy of an excitonic polaron. We have numerically evaluated the energy gap shift and effective mass of an excitonic polaron. We have numerically evaluated the energy gap shift and effective mass of an excitonic polaron in GaAs-Al _x Ga_1–x As systems. The results obtained here indicate that the polaronic effect is significant in the case of the light hole excitons in quantum wells of small well widths.     

Investigations of the exciton condensate

Recent experiments on quantum Hall bilayers in the vicinity of total filling factor 1 (ν_T=1) have revealed many exciting observations characteristic of a superfluidic exciton condensate. We report on our experimental work involving the ν_T=1 exciton condensate in independently contacted bilayer two-dimensional electron systems. We observe previously reported phenomena as a zero-bias resonant tunneling peak, a quantized Hall drag resistivity, and in counter-flow configuration, the near vanishing of both ρ_xx and ρ_xy resistivity components. At balanced electron densities in the layers, we find for both drag and counter-flow current configurations, thermally activated transport with a monotonic increase of the activation energy for d/ℓ_B<1.65 with activation energies up to 0.4 K. In the imbalanced system the activation energies show a striking asymmetry around the balance point, implying that the gap to charge excitations is considerably different in the separate layers that form the bilayer condensate. This indicates that the measured activation energy is neither the binding energy of the excitons, nor their condensation energy.     

Scanning tunneling optical spectroscopy in mercury cadmium telluride and related compounds

We consider II–VI narrow gap semiconducting alloys: mercury cadmium telluride, Hg_(1−x)Cd_(x)Te (MCT), mercury zinc telluride, Hg_(1−x)Zn_(x)Te (MZT), and mercury zinc selenide, Hg_(1−x)Zn_(x)Se (MZS). MCT is emphasized for actual calculations, but a table of values needed in all calculations is presented. These materials are of interest because of their application to infrared detectors and related devices, and because they are candidates for low gravity crystal growth to improve uniformity. We present new calculations of the scanning tunneling optical spectroscopy (STOS) current from which the local energy gap, a function of x, and hence the stoichiometry (x) can be determined as a function of position with presumably high spatial resolution. The low temperature tunneling current (vs. photon frequency) has a sharper onset at the band gap than the low temperature optical absorption. This sharp onset originates from the rapid increase in the integrated transmission probabilities and is greatly enhanced by large diffusion lengths. Thus, STOS should be a competitive technique, compared to optical absorption, for determining the local stoichiometry, a property that is important for characterizing crystals.     

Band structure characteristics for CdP2 in the tetragonal modification

We have investigated the density of states for deep local centers and special points of the M₀ type in β-CdP₂ using amplitude modulation laser spectroscopy. We used impurity absorption of laser and probe radiation for these studies. We observed deep local centers with energy levels in the forbidden gap at a depth of 0.34 (d ₁ ), 0.43 (d ₂ ), 0.86 (d ₃ ), 1.45 (a ₁ ) from the bottom of the conduction band and M₀ points at a depth of 0.46 and 1.03 eV in the conduction band, and we also confirmed the presence of M₀ points at a depth of 0.35 eV in the valence band. Ukrainian State Pedagogical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 64–68, April, 1998.     

Extended -wave superconductivity. Flat nodes in the gap and the low-temperature asymptotic properties of high- superconductors

Remarkable anisotropic structures have been recently observed in the order parameter of the underdoped superconductor Bi₂Sr₂CaCu₂O . Such findings are strongly suggestive of deviations from a simple d _{x2 - y2} -wave picture of high- superconductivity, i.e. . In particular, flatter nodes in are observed along the directions in -space, than within this simple model for a d-wave gap. We argue that nonlinear corrections in the -dependence of near the nodes introduce new energy scales, which would lead to deviations in the predicted power-law asymptotic behaviour of several measurable quantities, at low or intermediate temperatures. We evaluate such deviations, either analytically or numerically, within the interlayer pair-tunneling model, and within yet another phenomenological model for a d-wave order parameter. We find that such deviations are expected to be of different sign in the two cases. Moreover, the doping dependence of the flatness of the gap near the nodes is also attributable to Fermi surface effects, in addition to possible screening effects modifying the in-plane pairing kernel, as recently proposed. Received 19 November 1999     

Effect of antiferromagnetic ordering on temperature dependent superconducting gap in ErNi2B2C: Laser-photoemission spectroscopy

We have performed temperature (T)-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi₂B₂C to study the electronic structure, especially the effect of AF ordering in T-dependent superconducting (SC) gap. To estimate the values of T-dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s-wave SC gap. From the fitting results, we find a sudden deviation from the BCS prediction just below T_N. This observation can be well explained by the theoretical model, indicating that the origin of anomalous T-dependence is competition between rapid evolution of AF molecular field and SC condensation energy.     

Disordering of the electronic structure of YBaCuO amorphous films upon incorporation of crystalline clusters formed in laser-induced plasma into their composition

The effect of crystalline clusters formed in a laser-induced plasma on the optical properties of YBa₂Cu₃O_{6 + x} amorphous films prepared by pulsed laser deposition has been investigated. It has been demonstrated that an increase in the number of clusters leads to a gradual disappearance of interference fringes inherent in optically homogeneous media. Simultaneously, the incorporation of metallic and insulating clusters into the amorphous medium results in a decrease in the optical band gap E ₀ of the YBaCuO amorphous matrix from 1.28 to 1.06 eV and a considerable decrease in the probability of interband optical transitions with charge transfer O 2p → Cu 3d due to the loosening of the structure and generation of local stresses. It has been revealed that there is an additional band gap E ₁, which decreases from 0.25–0.30 eV to zero values with a decrease in the optical band gap E ₀. The additional gap has been interpreted as an energy gap between localized states that belong to the valence and conduction bands. A decrease in the density of electronic states in the narrow 3d band leads to the overlap of tails of the density of states, so that the band gap E ₁ becomes negative.