Weak Threshold Research Articles

Photoacoustic spectroscopy of diluted magnetic semiconductors

The absorption coefficient of several diluted magnetic semiconductors was measured below the fundamental absorption edge of the pure host compound at room temperature using the photoacoustic technique. Several absorption bands have been detected and assigned to electronic transitions between the crystal field split 3d levels of the transition metal ion. In addition, weak absorption thresholds have been observed and assigned to the ionization of the transition metal d states. An energy level diagram of the d states has been discussed.

Single weak boson production at TeV e +e − colliders

We present exact tree level matrix elements for all the single weak boson production processes to order e 3 in very high energy e +e − collisions; e +e − → e ± v (−) c W ∓, e +e − Z, v c v c Z and γγZ. The electron mass is kept finite so that the differential and total cross sections are accurate up to terms of order m e 2/ s. The weak boson distributions are compared with those obtained by the equivalent photon/electron approximation (EPA), which works reasonably well at high vector boson transverse momenta but fails at small transverse momenta for the processes e +e −→ e +e −Z and γγZ. A simple modification of the EPA formulae is found to give a reasonable overall description of the distributions for all the single weak boson production processes. Electron helicity-flip amplitudes are found to contribute at the percent level at most, which can be reproduced accurately by a novel collinear approximation with equivalent real photon or electron distributions in an electron. Interference effects between the annihilation and non-annihilation diagrams are found to be significant only below the weak boson pair production threshold. Also explained is a straightforward method to incorporate the leptonic and parton level hadronic decays of the weak bosons into the production amplitudes.

CMOS 1/f noise: n-channel versus p-channel

This note reports on the noise of CMOS devices. It is shown experimentally that a weak boron threshold implant (∼1012 cm−3) can influence the 1/f noise levels. For wafers with threshold adjustment the p-channel noise decreases whilst the n-channel noise increases. The changes in the n/p noise ratio with/without threshold implantation are predicted using a simple model in conjunction with carrier profile simulations.

Valence-level photon-stimulated desorption of H+ from solid Si(CH3)4

Results of photon-stimulated desorption (PSD) measurements of H+ from solid Si(CH3)4 in the photon energy range 15-40 eV are presented. The H+ PSD spectrum has a weak threshold at 17(±0.2) eV and a stronger threshold at 21.5(±0.5) eV. The H+ desorption threshold at 17 eV is interpreted in terms of excitation of an inner valence 4a1 (Si 3s/C 2s) electron to an unoccupied orbital (u). This excited state decays to 1t1−2u or 5t2−2u final states resulting in C-H and Si-C bond breaking, respectively. The desorption threshold at 21.5 eV is due to excitation of an inner valence 3t2 (C 2s) electron to an unoccupied orbital, which decays to a 1t1−2u final state, resulting in C-H bond breaking.

Photon-stimulated ion desorption from condensed SiF4 and SiH4

Results of photon-stimulated ion desorption (PSID) experiments on condensed SiF4 and SiH4 are presented. Data on condensed SiF4 are presented in the photon energy range 100–170 eV. The major product is F+. PSID yield spectra are interpreted in terms of excitation of a Si(2p) electron and are compared to previous gas-phase and condensed SiF4 experiments. H+ was found to be the only product desorbing in significant quantities from condensed SiH4 in the photon energy range 15–30 eV. A weak threshold is observed at 16.5 eV and a strong threshold appears at 19 eV. These data are interpreted in terms of excitation of a Si(3s) electron, and comparisons are made to previous electron-stimulated desorption experiments on condensed Si(CH3)4 and Si(111):SiH3.

Theoretical study of long-wavelength laser operation in Ga1−xAlxAsySb1−y

A theoretical study is done on laser operation in Ga1−xAlxAsySb1−y which is lattice-matched to GaSb. The strongly x-dependent intervalence-band absorption, the almost degenerate conduction-band minima, and the nonradiative Auger recombination are major factors deteriorating the laser operation. The first two factors explain the observation that the threshold current rapidly increases with x. It is shown that the Ga1−xAlxAsySb1−y laser with an adequate value of x is promising as a long-wavelength light source characterized by a weak temperature-dependent and low threshold.

Cell cycle kinetics of regenerating urothelial cells of the rat urinary bladder after partial cystectomy.

The cell cycle kinetics of bladder urothelial cells regenerating after partial cystectomy were investigated in 96 female Wistar rats using the percentage labelled mitoses method. In the area of resection a mean cell cycle time (TC) of 15 h was determined. The DNA synthesis phase (TS) lasted 6 h and the premitotic-postsynthetic phase together with the mitosis phase (TG2 + M) 1.5 h, thus giving a presynthetic-postmiotic phase (TG1) of 7.5 h. Similar values were found for the urothelial cells in the stump: the mean cycle time measured 14 h, the TS-phase 6 h, the TG6 + M-phase 2 h and the TG1-phase 6 h. These data are discussed with respect to known cell cycle parameters of bladder urothelium regenerating in response to cytotoxic agents and of neoplastic urothelial cells. The reported findings provide a basis for further investigations using weak carcinogens and threshold doses of potent carcinogens to test the working hypothesis that stimulation of proliferation following partial cystectomy is capable of initiating, accelerating and/or potentiating carcinogenic cell transformation in the urinary bladder.

Thermomodulation experiments at the cubic-to-tetragonal phase transition in SrTiO 3

After a brief comment on the important features of strontium titanate and a critical review of recent optical work in this material, thermoabsorption results at the cubic-to-tetragonal second-order phase transition are reported. Use of insulating samples has been possible by means of indirect heating. Optical evidence of the phase change is seen for the momentum-forbidden transitions starting at the indirect gap energy. This is discussed in terms of the relaxation of the momentum condition brought about by structural fluctuations near the transition temperature T 0. Part of the effect may be related to the increasing static distortion of the lattice in the tetragonal state. The results presented also confirm earlier pictures of the absorption threshold as due to LO-phonon-assisted Γ 15→X 3 transitions (with phonon energy equal to 52 meV). Furthermore, in the vicinity of T 0, an extra thermomodulation signal, corresponding to a weak absorption threshold ≅ 20 meV higher than the gap, becomes visible. This critical point is also particularly sensitive to the application of electric fields, as found by electroabsorption results obtained at liquid nitrogen temperature.

Statistical Criteria for the Detection of Pulsed Carriers in Noise. I

A theory for the detection of pulsed carriers (of constant amplitude) in narrow-band random noise is described, based on several types of optimum tests of a statistical hypothesis against an alternative (i.e., signal and noise vs noise). Siegert's concept of the betting curve is introduced, whereby, for a finite integration time, the minimum detectable signal is uniquely defined. Three types of observer are next considered: the Neyman-Pearson, the Ideal, and the Sequential observer, whose properties are determined by the manner in which the test is carried out. For each observer it is verified that the best second detector is a logI0-rectifier, which in practice is closely approximated by the usual half-wave linear envelope-tracer. In Part II specific betting curves are calculated and the performance of the three observers is analyzed for both the weak (threshold) and strong-signal cases.