Die transversale,negative, differentielle Leitfähigkeit von n-Germanium: Zwei 2-Tal-Modelle ihrer statischen und dynamischen Eigenschaften

Taking into account two of the four 111-energy minima of the Ge-conduction band, expressions for the field and frequency dependent transverse differential conductivityσ _yy(ω, E _x) have been derived. Forω=0, one obtains the static properties, especially information concerning the critical fieldsE _xc defined byσ _yy(0,E _xc)=0. Forω≠0, the plasma frequenciesω _p have been calculated fromσ _yy(ω, E _x). One of these goes to zero forE _x→E _xc (soft mode). The optical quantities: refractive index, extinction and reflection coefficient have also been calculated. They show anomalies in the regionE _x=E _xc. To evaluate the equations quantitatively, the intervalley and the momentum scattering rate have been derived in the last part of this work using known distribution functions.     

New aspects of the rotational model in25Mg

Spin assignments have been made to the²⁵Mg levels in theE _x=5–6 MeV region fromγ-ray angular distributions measured in the²²Ne(α, n y) reaction atE _α=8.0 and 8.8 MeV and fromp-γ angular correlations measured in the²⁴Mg(d, p γ) reaction atE _d=6 MeV. Unique spin assignments ofJ=7/2, 5/2, 5/2, and 9/2 could be made to the levels atE _x=5005, 5511, 5851, and 5967 keV, respectively. Ambigious assignments have been made to the levels atE _x=5245, 5524, 5785, 6032 keV (J=11/2, 7/2), 5455 keVJ=13/2, 9/2), and 5738 keV (J=3/2, 5/2). The present data confirm previous assignments ofJ=1/2 to the levels atE _x=5108 and 5466 keV, respectively. Lifetime estimates have been obtained, using the Doppler-shift attenuation method, for the levels atE _x=5245 keV (τ=30–60 fs), 5785 keV (τ=50–100 fs), 5967 keV (τ=50–100 fs), and 5455 keV (τ>1ps). All other levels in theE _x=5–6 MeV region haveτ<60 fs. A breakdown of theK-selection rule has been observed in theγ-decay of some high spin states, indicating a deviation from the strong coupling model.     

Structural and elastic properties of Ni2+xMn1−xGa alloys

The structural parameters and the energetics of the Ni_2+xMn_1−xGa alloys have been investigated by the first-principles Exact Muffin Tin Orbital-Coherent Potential Approximation (EMTO-CPA) for 0.10<x<0.30. The difference in total energies (δE) between the low-temperature tetragonal phase and the high-temperature cubic phase has been considered as a qualitative indicator of the martensitic transformation temperature T_m. The qualitative behavior of δE with variation of x has been found to be in agreement with the experimentally observed variation of T_m with x. The elastic constants for the entire range of x have also been calculated and the determination of a relationship between δE and the elastic shear modulus has been attempted. It is seen that δE varies linearly with elastic shear modulus C′, qualitatively similar to the relation between T_m and C′. The energetics calculated with the EMTO method agrees quite well with the all-electron full-potential results ensuring the accuracy of the method. These results show that the EMTO-CPA method is one of the most reliable and accurate first-principles methods, in the context of off-stoichiometric alloys which undergo martensitic phase transformations.     

Compositional dependence of the optical properties of the amorphous GexTe1−x system

Optical absorption at room temperature of vacuum evaporated Ge_xTe_1−x (0.125?x?0.225) amorphous thin films has been studied as a function of composition. It was found that the optical absorption is due to a direct transition. The real and imaginary parts of the dielectric constant were determined. The single-oscillator energy (E_o) and the energy dispersion parameter (E_d) have been calculated and discussed in terms of the Wemple and Di-Domenico model. The relationship between the optical gap, the average heat of atomization and the coordination numbers has been determined. The optical properties of the amorphous thin films do not change monotonically with increasing Ge content. The observed behavior of the optical properties in the range of compositions studied are explained on the basis of the Bond Constraint Theory (BCT) and rigidity theory, which provide a powerful framework for understanding the structure and properties of amorphous materials. The non-monotonic variation of the optical properties indicates that a transition from floppy to rigid occurs in the Ge_xTe_1−x films.     

Valence and inner proton hole states in119in via the (d,3He) reaction

Proton holes states have been studied up toE _x=17 MeV andE _x=3.5 MeV in the¹¹⁹In nucleus via the¹²⁰Sn(d,³He)¹¹⁹In reaction respectively atE _d=108.4 MeV andE _d=51 MeV. DWBA analysis of angular distributions has allowedl attributions for a large number of new levels and the determination of valence and inner hole strength distributions. The first 1g _9/2, 2p _1/2 and 2p _3/2 levels only exhaust 40%, 60% and 32% of their respective sum rule limits. The missing strengths are shared among several low lying levels and significant higher lying contributions. The 1f strength, not identified in the previous experiments is spread fromE _x=1 MeV to about 17 MeV. The low lying levels aroundE _x=2.4 MeV could exhaust some 40% of the 1f _5/2 sum rule. The higher lying strength with a flat maximum aroundE _x=7.5 MeV could account for the 1f _7/2inner hole strength and the missing 1f _5/2 valence strength. The experimental strength functions compare rather well with the predictions of the quasiparticle-phonon model.     

A study of the 230Th (α, α′f) reaction at 50 MeV

The total kinetic energy release (TKE) of the fissioning nucleus ²³⁰Th is measured as a function of the excitation energy E_x, for various mass splittings. A small rise in the TKE(E_x) of 0.2 ± 0.1 MeV per 1 MeV rise in E_x is observed. The mass-yield distributions show fine structure for E_x around 6.7 MeV. The mass yield is presented as function of the TKE.     

Temperature shift of the CdxHg1−xTe energy gap

The temperature coefficient of the Cd_xHg_1?xTe energy gap dE_g/dT as a sum of lattice dilatation and the phonon-electron interaction terms has been calculated as the function of molar composition x, for 0?x?0.3, in the temperature range 4.2–300 K. A non-linear dependence of $\text{d}\text{E}{}_{\mathrm{g}}\text{d}\text{T}$ vs x and a strong effect of temperature on $\text{d}\text{E}{}_{\mathrm{g}}\text{d}\text{T}$ values have been obtained and a comparison with experimental data is discussed.     

Nanoindentation of amorphous Ge-As-Se films

Changes in the nanohardness H and Young’s modulus E of Ge _x As _y Se_{100 ? x ? y} films have been studied as a function of the penetration depth of the Berkovich indenter. The values of H and E have been measured in the regime of harmonic modulation of a linearly increasing indenter load. It has been shown that the changes in E and H of the films under study during nanoindentation arise due to the peculiarities of their elastoplastic behavior, the formation of deformation zones near the nanocontact, and also size effects.     

An EXAFS study on Bi2Sr2Ca1−xPrxCu2O8−δ single crystal using polarized synchrotron radiation

We have made polarization dependent Extended X-ray Absorption Fine Structure (EXAFS) measurements on a Bi₂Sr₂Ca_1?xPr_xCu₂O_8?δ single crystal in the E//a, E//b and E//a^45° orientations. The results on Cu-K edge and Pr L_III edge EXAFS have been discussed to explain the quenching of superconductivity due to the presence of Pr cation as it was known to destruct the superconductivity by disturbing the local environment around Ca.     

Raman scattering study of IrxRu1−xO2 mixed metal oxide nanowires (0 ≤ x ≤ 1)

Recent interests in mixed metal oxide nanostructured materials especially Ir_xRu_1−xO₂ compounds have been mainly driven by the technological application as electrocatalyst and electrode materials. We present room temperature Raman scattering results of single crystalline Ir_xRu_1−xO₂ (0 ≤ x ≤ 1) nanowires grown by atmospheric pressure chemical vapor deposition. We observed that the E_g, the A_1g, and the B_2g phonon modes of a single Ir_xRu_1−xO₂ nanowire are blue-shifted linearly with respect to the Ir contents from which we could get stoichiometry information. We also observed that the asymmetric lineshape and the broadening of the full width at half maximum of the E_g mode that involves the out-of-plane oxygen vibration. The unusual asymmetric broadening of the E_g phonon can be explained by the activation of the non-zone-center phonons due to substitutional disorder present in the system. We also found that there is a mixed mode of the A_1g and the B_2g phonons due to the substitutional disorder, in the range of 630–750 cm⁻¹.     

Electroreflectance study of the dilure magnetic semiconductor alloy Hg1−xMnxTe

We report the first electroreflectance study of the dilute magnetic semiconductor Hg_1?xMn_xTe in the photon energy region of 0.7–6 eV. The composition dependence of the E₀ + Δ₀, E₁, E₁ + Δ₁, E₀, E₂, E₂ + δ optical features has been determined for 0.015 ≤ × ≤ 0.23. We find that at room temperature the semimetal-semiconductor transition (Γ₆ – Γ₈ crossing) occurs at x = 0.05 ± 0.01. Discontinuities in the composition dependence of all the observed spectral features are seen at this value of x.     

Anomalous phonon intensities in the Raman spectrum of disordered CsMg1−xCoxCl3

The five Raman-active $\text{k}\text{= 0}$ phonons have been measured at low temperatures and for a range of x in the disordered lattice CsMg_1-xCo_xCl₃. While the E_2g^a, E_2g^b, E_2g^c and A_1g modes at 55.0, 132.0, 189.0 and 255.0 cm^?1 for x = 0 exhibit normal one-mode behaviour, the intensity of the E_1g phonon at 127.5 cm^?1 has a most unusual concentration dependence which requires a new theory.     

Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model

Raman and Fourier transform infrared (FTIR) spectroscopies have been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. Pseudo unit cell (PUC) model was applied to investigate the phonons frequency, mode number, static dielectric constant, and high frequency dielectric constant of the Al_xIn_yGa_1−x−yN mixed crystals. The theoretical results were compared with the experimental results obtained from the quaternary samples by using Raman and FTIR spectroscopies. The experimental results indicated that the Al_xIn_yGa_1−x−yN alloy had two-mode behavior, which includes A₁(LO), E₁(TO), and E₂(H). Thus, these results are in agreement with the theoretical results of PUC model, which also revealed a two-mode behavior for the quaternary nitride. We also obtained new values of E₁(TO) and E₂(H) for the quaternary nitride samples that have not yet been reported in the literature.     

Exchange bias in bulk Ni-Mn-In-based Heusler alloys

The crystal structure and exchange bias of the bulk Heusler alloys Ni₅₀Mn_50−xIn_x with 14.5?x?15.2 have been investigated using X-ray diffraction and magnetization measurements, respectively. Magnetic measurements were performed with SQUID magnetometry after samples were zero-field cooled and field cooled (FC) in positive magnetic fields up to H=50 kOe, from a temperature T=380 K. Three temperatures of the phase transitions, T₁<T_M<T_C, and a shift of the FC (50 kOe) magnetic hysteresis loops up to 120 Oe at 5 K have been detected for all samples. The exchange bias field (H_E) was almost constant for intermediate In concentrations 14.8<x<15.2, and sharply decreased to about 20 Oe on the borders of this concentration interval (x?14.5; 15.2?x). The changes of H_E have been related to changes in the ratio of T₁ to T_M: the overlapping of transitions at T₁ and T_M (for x=14.8, and 15.2) results in a decrease in H_E.     

Detectors for nuclear resonant scattering experiments

Ferrite samples of Zn_0.5Cu_0.5Al _x Fe_2?x O₄, 0≤x≤1 were studied using Mössbauer, X-ray and infrared spectra. Mössbauer spectra taken at room temperature show signs of relaxation for all samples. The obtained spectra were analysed into two Zeeman magnetic patterns assigned to the tetrahedral A- and octahedral B-sites and a paramagnetic phase C (quadrupole doublet ΔE). ΔE and the doublet area, which increase with x and the oxygen parameter u are studied. The quadrupole shift is small and may be ignored within experimental error. The isomer shift of the B-sites increases, with x while that of the A-sites does not change. The hyperfine magnetic field of the A-site (H _A) is higher than that of the B-sites (H _B). H _B decreases for x≤0.8 and increases at x=1 while H _A decreases for x≤0.4 and increases for x>0.4. This behaviour is discussed. The cation distribution has been estimated. The linewidth of the outermost A and B-sites and the calculated magnetization have been studied as functions of x for all samples. The B-sites show a composite pattern that has been successfully analysed into separate components. The obtained hyperfine parameters are discussed. The oxygen parameter, the ionic radius and the metal-oxygen bonds of the A- and B-sites were calculated for all samples. The infrared spectra show three vibration bands υ₁, υ₂ and υ₄. The bands υ₁ and υ₂ show a shoulder, splitting and increasing overlap. Their behaviour has been discussed as a function of x. They show the same behaviour of magnetization and area ratio as the B- to A-sites. All parameters show a different behaviour at x=0.4, which is assigned to higher fraction of the Cu²⁺ ions in the A-site at this composition.     

Fundamental reflectivity spectra of Hg1−xMnxTe mixed crystals in the 1.7 to 3.5 eV energy range

The room temperature reflectivity coefficient R(E) for the mixed crystals Hg_1?xMn_xTe (x up to 0.57) in the energy range 1.7 to 3.5 eV was investigated. Two distinct maxima E₁ and E₁ + Δ₁ connected with the transitions in the critical points on the [111] direction of the Brillouin zone for the samples with x up to 0.3 and the more diffused structure of R(E) for the samples with x > 0.3 was observed. A quadratic dependence of E₁ and E₁ + Δ₁ transition energy vs alloy composition with x up to 0.3 was found, with bowing coefficient c = 1.21 ± 0.02 and 1.06 ± 0.02 respectively. The energy variation of an additional shoulder probably connected with the e₁ transitions at L point of the Brillouin zone is also reported.     

Isoelectronic trap transitions in GaAs1−xPx:N in the region of resonant enhancement (ENN ∼ EΓ, 0.3<x<0.4)

For GaAs_1?xP_x:N crystal composition x < 0.8, the distinction between NN₁ and NN₃ transitions becomes blurred. Photoluminescence measurements on N-doped GaAs_1?xP_x layers (0.3 < x < 0.4) and, for comparison, on the same vapor epitaxial crystals with the N-doped layer removed, indicate that the NN₃ transition reappears (and thus also the NN₁ transition) as E_NN3 approaches E_Γ. This is attributed to the resonant enhancement that occurs when E_NN ～ E_Γ.     

Calculation of the density of states of NbNx by the recursion method

The density of states, n(E), of NbN_x has been calculated by means of the Recursion Method for x ? 1. The results have been compared with experiment, obtaining that the most important trends of the density of states are reproduced fairly well, even if the variation of the density of states at the Fermi level, as a function of the concentration of N-impurities cannot be predicted.