Amorphous Fe–Mg alloy thin films: magnetic properties and atomic vibrational dynamics

Amorphous (a-) Fe _x Mg_1?x alloys are interesting materials for the investigation of non-Debye-like low-energy vibrational excitations. We have prepared a-Fe _x Mg_1?x alloy thin films (0.3 ≤ × ≤0.7) by vapour quenching. The amorphous state was confirmed by conversion electron Mössbauer spectroscopy between 4.2–300 K, and the x- and temperature-dependence of the isomer shift and hyperfine magnetic field was measured. For x= 0.6 and 0.7, magnetic ordering occurs below ~150 K. The atomic vibrational density of states, g(E), was determined by nuclear resonant inelastic scattering, providing clear evidence for the non-Debye-like low-energy vibrational excitations.     

On the absolute matrix summability of Fourier series and some associated series

The object of the paper is to study the absolute matrix summability problem of Fourier series, conjugate series and some associated series under a new set of conditions on matrix methods, generalising many known results in the literature.     

High responsivity, LWIR dots-in-a-well quantum dot infrared photodetectors

In this paper we report studies on normal incidence, InAs/In_0.15Ga_0.85As quantum dot infrared photodetectors (QDIPs) in the dots-in-a-well (DWELL) configuration. Three QDIP structures with similar dot and well dimensions were grown and devices were fabricated from each wafer. Of the three devices studied, the first served as the control, the second was grown with an additional 400 Å AlGaAs blocking layer, and the third was grown on a GaAs n+ substrate with the intention of testing a single pass geometry. Spectral measurements on all three devices show one main peak in the long-wave IR (≈8 μm). The absorption was attributed to the bound-to-bound transition between the ground state of the InAs quantum dot and the ground state of the In_0.15Ga_0.85As well. Calibrated peak responsivity and peak detectivity measurements were performed on each device at 40, 60, and 80 K. For the same temperatures, frequency response measurements from 20 Hz to 4 kHz at a bias of V_b=−1 V were also performed. The addition of the blocking layer was shown to slightly enhance responsivity, which peaked at 2.4 A/W at 77 K, V_b=−1 V and responsivity was observed to be significantly reduced in the single pass (n+ substrate) sample. The rolloff of the frequency response was observed to be heavily dependent on temperature, bias, and irradiance. The results from the characterization of each sample are reported and discussed.     

Stability analysis of structurally unstable man–machine system involving time delays

In this paper, a structurally unstable man–machine system is considered and a rigorous mathematical analysis is performed to study the influence of time delays in the control force on stability of the equilibrium solution of this system. Results on stability, instability and conditions for the preservation of stability (instability), besides the existence of bifurcation are presented.     

The sojourn time and related characteristics of the age-dependent branching process

An age-dependent branching process where disasters occur as a renewal process leading to annihilation or survival ofall the cells, is considered. For such a process, the total mean sojourn time of all the cells in the system is analysed using the regeneration point technique. The mean number of cells which die in timet and its asymptotic behaviour are discussed. When the disasters arrival as a Poisson process and the lifetime of the cells follows exponential distribution, elegant inter-relationships are found among the means of (i) the total number of cells which die in timet (ii) the total sojourn time of all cells in the system upto timet and (iii) the number of living cells at timet. Some of the existing results are deduced as special cases for related processes.     

Precise measurement of hyperfine structure in the state of Rb

We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the 5 P _3/2 state of ⁸⁵Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant A = 25.038(5) MHz and the electric-quadrupole coupling constant B = 26.011(22) MHz. These values are a significant improvement over previous results. Received 6 March 2003 Published online 15 April 2003     

The change of sign of E2 and M1 matrix elements,and of δ-values of166Er at high spins

Calculations of the Dynamic Deformation Model for the E2/M1 mixing ratios for ψ-band to ground-band transitions in¹⁶⁶Er have been extended to higher spins (up to I=20). Previous comparisons with the experimental values and with the IBA-1 predictions, available at present up to spins 8 only, are also included for the sake of completeness. Additional comparison with the Frankfurt Model is also given. While all three models give reasonable magnitudes of the mixing ratios, only the DDM gives the sign changes at the correct spin values. Predictions are presented for additional sign changes of the mixing ratios, as well as those of the E2 and the M1 transition moments.     

Spherically symmetric static inhomogeneous cosmological models

Spherically symmetric static cosmological models filled with black-body radiation are considered. The models are isotropic about a central observer but inhomogeneous. It is suggested that the energy density of the free gravitational field, which is coupled to the isotropic radiation energy density, might play an important role in generating sufficient field (vacuum) energy (when converted into thermal energy) and initiate processes like inflation. On the central world line the energy density of the free gravitational field vanishes whereas the proper pressure and density of the isotropic black-body radiation are constants. Further, it is shown that the cosmological constant is no more arbitrary but given in terms of the central pressure and density. Also, at its maximum value the energy density of the free gravitational field is proved to be equal to one third of the combined value of radiation pressure and density.     

Preparation of (CdO)1−x(PbO)x and (CdS)1−x(PbS)x thin films by spray pyrolysis technique and their characterization

Mixed thin films of (CdO)_1−x(PbO)_x and (CdS)_1−x(PbS)_x (x=0.25) were prepared on glass substrates by spray pyrolysis technique for various substrate temperatures 300, 320 and 340 °C. Structural and optical properties were studied. XRD studies reveal the formation of mixed films. The substrate temperature of 340 °C seems to be critical for the formation of CdO-PbO mixed films. It is observed that (CdS)_1−x(PbS)_x mixed films were formed at all the three substrate temperatures. The direct band gap value of (CdO)_1−x(PbO)_x and (CdS)_1−x(PbS)_x mixed films is about 2.6 and 2.37 eV, respectively.