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]     

用高压显微光谱系统研究(Zn0.85Cd0.15)S:Cu,Al磷光体在流体静压力下施主-受主对的发光

用金刚石对顶砧高压显微光谱系统在室温和1bar—66kbar的流体静压力范围内研究了(Zn_0.85Cd_0.15)S:Cu,Al磷光体的发光峰位置和相对发光强度随压力而变化的规律。随着压力的增加,发射峰值波长迅速移向短波方向,而发射峰值对应的光子能量随压力增加的速率为4.7meV/kbar(38cm^-1/kbar)。这个值比该材料的吸收边随压力增加的速率要小。随着压力的增加,该磷光体的发光峰值相对强度急骤下降,当压力从常压升到66kbar时,发光峰值相对强度下降到原值的6％。这些结果可以用Al³⁺-Cu⁺的施主-受主对模型来解释。本文还估计施主(Al³⁺)和受主(Cu⁺)的激活能之和随压力增加的速率为3.7meV/kbar(30cm^-1/kbar)。 关键词：     

Excitation spectra of self-activated luminescence in ZnSe crystals under pressure

A narrow excitation band observed for S-A luminescence in ZnSe crystal is attributed to free exciton absorption. Some overlap with a higher characteristic band of low intensity is considered for low temperature spectra.This band shifts under pressure toward higher energies with a coefficient dE/dP = (7.5±0.3) meV Kb^-1 at 300 K and (7.4±0.5) meV Kb^-1 at 85 K.The pressure shift of the excitation due to edge absorption is (7.0± 0.5) meV Kb^-1.     

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO_0.88F_0.12FeAs samples with bulk critical temperature T _c = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are Δ _L = 10.5 ± 2 meV and Δ _s = 2.3 ± 0.4 meV, respectively.     

The pressure dependence of the superconducting transition temperature of LaT4P12(T = Fe,Ru, Os)

The superconducting transition temperature, T_c, of the LaT₄P₁₂ compounds with T = Fe, Ru, or Os has been measured under hydrostatic pressure P up to 1.8 GPa. The T = Fe compound exhibits a substantial increase of T_c from T_c (P = 0) = 4.1 K at a rate (dT_c/dP)_P=0= +7.2 x 10^-1 K/GPa. In contrast, the Ru and Os compounds exhibit only weak decreases of T_c from T_c (P = 0) = 7.2 K and 1.8 K with (dT_c/dP)_P=0= -1.6 x 10^-1 K/GPa and -9.5 x 10^-2 K/GPa, respectively. An analysis of this strikingly divergent behavior of T_c(P) in terms of the structural characteristics of the RT₄X₁₂ class of compounds where R = rare earth element, T = Fe, Ru, or Os, and X = P, As, or Sb suggests that T_c(P) for these materials consists of two competing contributions: a depression of T_c due to the compression of the lattice (i.e., decrease in volume), and an enhancement of T_c due to the effect of pressure on La itself.     

Optical absorption of zinc selenide doped with cobalt (Zn1−xCoxSe) under hydrostatic pressure

Abstract

The optical absorption of the diluted magnetic semiconductor Zn_1?xCO_xSe (x = 0.02) has been measured at room temperature under hydrostatic pressure up to 14GPa in a membrane diamond-anvil cell. We found two absorption features: (i) an absorption structure in the energy range 1.6?1.8eV, with a negligible pressure shift (i.e., 0.45 ± 0.05 meV/GPa) which we have identified as the Co²⁺(3d⁷) internal transition ⁴A₂(F)→+⁴T₁(P) and (ii) an onset in the energy range 2?2.7eV which redshifts with pressure (?8.1±0.6meV/GPa). We have attributed such absorption edge to charge transfer between the ZnSe valence band and the Co²⁺(3d⁷) levels.     

Indirect exciton dispersion in III–V semiconductors: “Camel's back” in GaP

A new perturbation approach to exciton dispersion in indirect gap semiconductors is developed. For GaP and AlSb existence of the “camel's back” in exciton dispersion is confirmed, and a precise value of the “camel's back” parameter for X^c₁-minima in GaP is reported: E(X^c₁)?E_min(Δ^c₁)=3.5±0.3 meV. At the X-point the 21.44 and 19.48 meV exciton binding energies in GaP are obtained. The corresponding valley-anisotropy splitting is 1.96 meV.     

Reevaluation of bandgap and free exciton binding energy of GaP

Very sensitive measurements on the spectral behaviour of the free-to-bound excitation σ_pl^°(hν) from the valence band to the deep 0 donor in GaP at low temperatures are presented. Evaluation of the threshold energy for the electronic transition, together with the known value of the 0 binding energy, provides a simple and accurate way to determine the indirect bandgap E_g of GaP. Our new value is E_g = 2.3525 ± 0.003 eV at 1.5 K, which gives an exciton binding energy E_x = 24 ± 3 meV, considerably larger than previously used values. These data also imply an upward revision of acceptor binding energies in GaP with 10 ± 2 meV.     

Pressure dependence of the E1 gap in GaSb: Resonant Raman technique

A new method to determine the pressure dependence of energy gaps with the help of the diamond anvil cell is reported. The method is based on resonance Raman scattering observed as the gap energies cross the photon energies of several laser line. The technique is illustrated by means of measurements of the pressure dependence of the E₁ gap of GaSb up to the phase transition (7.7 Gpa). A sublinear dependence of the E₁ gap on pressure and a linear dependence on lattice constant (dE₁/dlnV = 4.67 ± 0.1 eV) is obtained. These results are well explained by pseudopotential calculations.     

Effect of pressure on the properties of DH diode lasers in AlxGa1-xAsySb1-y/GaSb system

Abstract

A helium pressure appparatus for diode laser studies up to 1.4 GPa at 77–300 K has been developed. DH lasers with Al_xGa_1-xAs_ySb_1-y active layers (x=0-0.05) lattice-matched to GaSb substrates have been investigated. It has been shown that in lasers with x,y=0 pressure dependences of the threshold current density (J_th) and the average electron lifetime at the threshold (τ) measured at 80 K depend strongly on the quadratic recombination of L^c ₆ electrons, the characteristic coefficient being 1.5×10^?11 cm³s^?1. The pressure-composition equivalence coefficient dx/dP=2.2×10^?10 Pa^?1 has been obtained for the lowest temperatures used.     

Spectroscopic and structural investigations of the ferrous and ferric high-spin state of a “picket—fence” porphyrinato acetato iron complex. A refined model for the P460 center of hydroxylamine oxidoreductase

Mössbauer investigations were performed in the ferrous and ferric form of the ‘picket-fence’ porphyrinato acetato iron complex |Fe(CH₃CO₂) (TP_pivP)|^1?,0 at temperatures varying from 1.5–200K and in fields of 0–6.2T. The ferrous complex has an unusually large quadrupole splitting, ΔE_Q=+4.25mms^?1. The quadrupole splitting in the ferric species, ΔE_Q=+1.1mms^?1, is as normally found in ferric high-spin iron porphyrins. Spin-Hamiltonian analysis of the magnetic spectra yields zero-field parameters D=?0.9mms^?1, E/D=0.33 and magnetic hyperfine parameters A_x,y=?17T, A_z=?13.3T in the ferrous high-spin (S=2) complex, and D=7.5cm^?1, E/D≈0 and A_x,y,z=20T in the ferric high-spin (S=5/2) species.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Photoluminescence of GaSb under hydrostatic pressure

Photoluminescence measurements on GaSb samples were carried out at low temperatures (2 – 5 K) and high pressures (0 – 9 kbar). The energy shift of the direct gap was determined: $\text{d}\text{E}{}^{\mathrm{\Gamma }}\text{/dP= 14.5± 0.3 meV/kbar}$. At pressures above 8 kbar the spectra showed additional structure from the indirect L-conduction band minima. The energy shift of the L-conduction bands were determined: $\text{d}\text{E}\text{L/d}\text{P}\text{= 5.5± 1. meV/kbar.}$ From these data the critical pressure for the inversion of the two conduction bands was calculated: $\text{p}{}^{\mathrm{h}}{}_{\mathrm{c}}\text{= 10.5 ± 1. kbar}$.     

Gd1-xCaxBa2Cu3O7-y高温超导体压力效应的研究

研究了Ｇｄ_1-xＣａ_xＢａ₂Ｃｕ₃Ｏ_7-y（０．０≤Ｘ≤０．２０）高温超导体在常压和高压下的超导电性在１－３００Ｋ温度范围内，利用Ｂｒｉｄｇｍａｎ对顶砧获得压力达９．０ＧＰａ，测量了（Ｘ＝０．１０，０．１５，０．２０）样品的ｄＴ_c／ｄｐ分别为７．６８，７．８和４．４６Ｋ／ＧＰａ。发现Ｔ_c的压力导数随着ｃａ²⁺含量的增加而下降，分析了氧含量对Ｔ_c和ｄＴ_c／ｄＰ的影响．利用常压下晶格参数精修值和阳离子与氧离子间距随压力的改变，说明ＣｕＯ₂面在超导电性上的作用，用ＣｕＯ₂面之间耦合解释Ｔ_c（Ｐ）曲线的非线性关系。 关键词：     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

Magnetization behaviour of DyAl2 single crystals

We report the theoretical interpretation of the magnetization and the magnetocrystalline anisotropy of ferromagnetic DyAl₂ single crystals between 4.2 and 60 K and magnetic fields up to 15 T. Good agreement between theory and experiment is obtained by using three temperature independent parameters: the two crystal field parameters B₄ = (?0.50 ± 0.05) × 10^?4 meV, B₆ = ? (0.51 ± 0.05) × 10^?6 meV and the Curie temperature T_c = (62 ± 2) K.     

Singlett-triplett splitting of the free A-exciton in ZnO

By comparing the absorption and emission spectra (4 K) of ZnO single crystals the positions of the free A-exciton lines in emission can be declared. These transitions for E ⊥ c and E ∥ c were produced by excitons having a parallel or antiparallel spin orientation of the electron and the hole. This fine structure splitting is 0.20 meV.     

Superconductivity at 108 K in YBa2Cu4O8 at pressures up to 12 Gpa

Abstract

Resistive measurements in a cryogenic diamond anvil cell show that the T, of YBa₂Cu₄O₈ can be increased by almost 30 K by applying a pressure of 12 GPa. The pressure derivative is, however, not constant. At p < 5 GPa, dT_c/dp ～5 K/GPa. At higher pressures dT_c/dp decreases gradually and T_c saturates at ～108 K. This behavior can be reproduced by a phenomenological model where T, depends only on the number of holes in the CuO₂-planes.     

Piezoreflectance measurements on GaxIn1−x Sb alloys

Piezoreflective experiments between 77 and 300°K are performed on Ga_x, In_1?x Sb alloys. E_o and E_o + Δ_o transition energies, vs composition are determined. Band gap E_o variations, are obtained in good agreement with the two band dielectric model. The Δ_o splitting is observed to be linear with composition.The fundamental edge temperature coefficient β is also determined, it is found to vary linearly with composition.E_o and E_o + Δ_o energy determination permit one to deduce effective mass and g_e value variation with composition.     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.