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A measure of the average depression between Lyman absorption lines in the spectrum of the faint quasar BR1202-0725 ($z_{em}=4.695$) is presented. The relatively high resolution of the spectrum ($\sim 40$ km s$^{-1}$) allows the selection of regions free of strong absorption lines in the Lyman alpha forest. A reliable evaluation of the continuum shape is based on the careful flux calibration and on the large wavelength interval covered (5000--9300 ~\AA). A best estimate of $\tau _{_{GP}}\leq 0.02\pm 0.03$ has been found for the Gunn-Peterson optical depth at the highest absorption redshift observed at this resolution, $z \simeq 4.3$. The derived baryon density of the intergalactic medium is $\Omega _{IGM}\mincir 0.01$ if the observed quasars are the major contributor to the ionizing UV background flux. This limit, when compared with the total baryon density deduced from the nucleosynthesis, could imply that most of the baryons are already in bound systems at $z\sim 5$.

When the electric field in certain compound semiconductors exceeds a well-defined threshold value, the electrical current through the material becomes unstable. These instabilities may take the form of coherent Oscillations at microwave frequencies. The oscillations are not greatly affected by external circuit conditions. The effect was first discovered in gallium arsenide and indium phosphide by J. B. Gunn in 1963, and is now referred to as the \"Gunn effect\".This paper discusses some of the practical aspects of the Gunn effect in gallium arsenide. A brief theoretical explanation is given, of the intervalley electron energy transfer mechanism which is now generally accepted as the mechanism through which the effect occurs. The preparation of samples is described, and the nature of the electrical contacts to the material discussed. Ohmic contacts are necessary to prevent junction effects or carrier injection from masking the oscillations. Mounting techniques are described. Circuit considerations are discussed, and the results of various measurements reported. The value of the Gunn effect lies in its potential to directly convert pulse or DC power into microwave power, at frequencies and power levels higher than is possible with other semiconductor devices, and at reasonable efficiencies.

A general asymptotic analysis of the Gunn effect in n-GaAs under general boundary conditions for metal-semiconductor contacts is presented. Depending on the parameter values in the boundary condition of the injecting contact, different types of waves mediate the Gunn effect. The periodic current oscillation typical of the Gunn effect may be caused by moving charge-monopole accumulation or depletion layers, or by low or high-field charge-dipole solitary waves. A new instability caused by multiple shedding of (low field) dipole waves is found. In all cases the shape of the current oscillation is described in detail: we show the direct relationship between its major features (maxima, minima, plateau's, ...) and several critical currents (which depend on the values of the contact parameters). Our results open the possibility of measuring contact parameters from the analysis of the shape of the current oscillation.

We present an asymptotic analysis of the Gunn effect in a drift-diffusion model---including electric-field-dependent generation-recombination processes---for long samples of strongly compensated p-type Ge at low temperature and under dc voltage bias. During each Gunn oscillation, there are different stages corresponding to the generation, motion and annihilation of solitary waves. Each stage may be described by one evolution equation for only one degree of freedom (the current density), except for the generation of each new wave. The wave generation is a faster process that may be described by solving a semiinfinite canonical problem. As a result of our study we have found that (depending on the boundary condition) one or several solitary waves may be shed during each period of the oscillation. Examples of numerical simulations validating our analysis are included.

Electron transport in heterostructure with two tunnel-coupled quantum well has been numerically investigated under condition of strong non-one-dimensionality of electric field in the structure. It is shown for the first time an ability of the effect in such heterostructures which is similar to the Gunn effect in bulk semiconductor. The effect is conditioned by the electron tunneling transition between quantum wells with different mobility and is characterized by formation of the strong field domain and the negative differential conductivity region on current-voltage characteristic.

Electron transport in heterostructure with two tunnel-coupled quantum well has been numerically investigated under condition of strong non-one-dimensionality of electric field in the structure. It is shown for the first time an ability of the effect in such heterostructures which is similar to the Gunn effect in bulk semiconductor. The effect is conditioned by the electron tunneling transition between quantum wells with different mobility and is characterized by formation of the strong field domain and the negative differential conductivity region on current-voltage characteristic.

Electron transport in heterostructure with two tunnel-coupled quantum well has been numerically investigated under condition of strong non-one-dimensionality of electric field in the structure. It is shown for the first time an ability of the effect in such heterostructures which is similar to the Gunn effect in bulk semiconductor. The effect is conditioned by the electron tunneling transition between quantum wells with different mobility and is characterized by formation of the strong field domain and the negative differential conductivity region on current-voltage characteristic.

Absorption due to HeII Ly-alpha has been detected at low resolution in the spectra of four quasars between redshifts z = 2.74-3.29. We assess these observations, giving particular attention to the radiative transfer of the ionizing background radiation, cloud diffuse emission and ionization physics, and statistical fluctuations. We use high-resolution observations of HI absorption towards quasars to derive improved models for the opacity of the intergalactic medium (IGM) and use these models to calculate the HI and HeII photoionization history in both optically-thin and self-shielded clouds, and the average line-blanketing contribution to HeII absorption. The derived ionization rate at z = 2-4, Gamma(HI) = (1-3)x10^-12 s^-1, is consistent with the ionizing background intensity inferred from the proximity effect, but it remains larger than that inferred by N-body hydrodynamical simulations of the Lya absorber distribution. The HeII observations are con- sistent with line blanketing from clouds with N(HI) > 10^12 cm^-2, although a contribution from a more diffuse IGM would help to explain the observed opacity. We compute the expected HeII optical depth and examine the sizable fluc- tuations that arise from variations in the cloud numbers and ionizing radiation field. We assess how HeII absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources (quasars and starburst galaxies). Finally, we demonstrate how high-resolution UV observations can distinguish between absorption from the diffuse IGM and the Lya forest clouds and determine the source of the ionizing background.

AD800479

A general formulation of boundary conditions for semiconductor-metal contacts follows from a phenomenological procedure sketched here. The resulting boundary conditions, which incorporate only physically well-defined parameters, are used to study the classical unipolar drift-diffusion model for the Gunn effect. The analysis of its stationary solutions reveals the presence of bistability and hysteresis for a certain range of contact parameters. Several types of Gunn effect are predicted to occur in the model, when no stable stationary solution exists, depending on the value of the parameters of the injecting contact appearing in the boundary condition. In this way, the critical role played by contacts in the Gunn effect is clearly stablished.

Attempts to measure the optical depth due to a diffuse intergalactic medium (IGM) in the spectra of high redshift quasars (GP test) within the ESO key program on the intergalactic medium are reviewed. It is shown that there is no evidence for any Gunn-Peterson effect up to the highest redshifts observable in quasar spectra. The Ly$\alpha$ line statistics consistent with this limit is not able to explain the strong absorption observed at the HeII forest in one QSO, leaving room for a true HeII GP effect. The HI/HeII GP ratio implies a steep ionizing UV background at $z\sim 5$ and ionizing sources of stellar origin.

Attempts to measure the optical depth due to a diffuse intergalactic medium (IGM) in the spectra of high redshift quasars (GP test) within the ESO key program on the intergalactic medium are reviewed. It is shown that there is no evidence for any Gunn-Peterson effect up to the highest redshifts observable in quasar spectra. The Ly$\alpha$ line statistics consistent with this limit is not able to explain the strong absorption observed at the HeII forest in one QSO, leaving room for a true HeII GP effect. The HI/HeII GP ratio implies a steep ionizing UV background at $z\sim 5$ and ionizing sources of stellar origin.

Many experimental evidences indicate the presence of a ionizing background radiation flux at large redshifts; in some previous works I suggested the possibility that this ionizing flux comes from the quantum evaporation of primordial black holes (PBHs). Here I discuss the constraints that the experimental measurements put upon the free parameters of this reionization model and I study the absorption of the ionizing background due to $\lia$ clouds: in particular, I discuss this phenomenon in presence of different absorption levels and I calculate the HI Gunn Peterson optical depth $\tau_{GP}(z)$; from a comparison with the experimental data I obtain a constraint on the intergalactic medium density parameter, namely $\Omega_{IGM}$ $

In order to carry out a systematic and thorough measurement of the HI Gunn-Peterson effect at high redshift, a quantitatively testable and repeatable procedure, in particular, a robust statistical weighting technique, is developed. It is applied to an echelle spectrum of resolution 15 km s$^{-1}$ of PKS 1937-101 with z=3.787. A weighted intensity distribution which is derived overwhelmingly from pixels close to the continuum level in the Ly alpha forest region is constructed by the evaluation of how closely correlated each pixel is with its neighboring pixels. The merit of the distribution is its stronger and narrower peak compared to the unweighted, as well as its smaller dependence on uncertainty of strong absorption lines and noise spikes. By comparison to the weighted intensity distribution of synthetic Ly alpha forest spectrum with various chosen diffuse HI opacities, a chi square fit is performed. In addition to a weak line population with power law N_H distribution $\beta =1.7$ extrapolated down to 10$^{12}$ cm$^{-2}$, a best chi square fit requires a GP opacity of $0.115 \pm 0.025$ at average z=3.4 with estimation of the contribution from the variance of $\beta$. Although no evidence of more than 1-2\% error is seen in the continuum extrapolation, the possible systematic overestimation due to the slope can be as high as the level of the $\chi^2$ fit, which is investigated by splitting the \Lya forest region into subsamples to check the continuum drop's dependence on absorber redshift.

The optical reflectance of BiSI has been measured with photon energy, from 2.2 to 3.6 eV. The band structures of BiSBr, BiSCl, BiSeBr, BiSeCl and BiSeI have been calculated by using the self-consistent pseudopotential method. These results show that: (i) they are indirect gap semiconductors; (ii) the top of the valence band is located at some k-point along rZ (the z-direction) with finite curvature (small effective mass); (iii) the highest valence band at r (the center of the Brillouin zone) is only a few tenths of an eV lower than the energy of the top of the valence band and is flat (large effective mass) especially in the directions perpendicular to z. (iv) in addition to the above listed features, the minimum of the conduction band for BiSCl and BiSeCl is located at some k-point along rX, the x direction and is a few tenths of an eV lower than the lowest conduction band at r. These results suggest that: All the six compounds studied in this project will exhibit the Gunn effect if they are properly doped. They are potential candidates for microwave oscillators. (Author)

Bibliography: p. 60-61

We report new limits on the Gunn-Peterson effect at a redshift near 5 using spectroscopic observations of the z = 5 Sloan Digital Sky Survey quasar, J033829.31+002156.3, made with the LRIS and HIRES spectrographs on the Keck telescopes. Lower resolution spectrophotometrically calibrated observations made with LRIS over the wavelength region 4500-9600 angstroms were used to obtain a continuum shape and to flux calibrate much higher resolution (R = 36,000) observations made with HIRES. The LRIS data show an Oke D_A index of 0.75. Portions of the HIRES spectrum return to near the extrapolated continuum level. Including both statistical and systematic errors we place an upper limit of tau = 0.1 on the regions of minimum opacity. We argue that, even if this opacity arises in underdense regions of the intergalactic gas, we require a high value of the metagalactic ionizing flux at these redshifts ($J_{\nu} \gg 4 \times 10^{-23}~{\rm ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1}}$ at $z\sim4.72$) to produce a solution which is consistent with even minimum nucleosynthesis estimates of the baryon density. We also report the presence of a Mg II absorption system of extremely high equivalent width ($W_{\lambda,rest}(2796)=1.73$ \AA) at z=2.304.

We consider a charged Brownian gas under the influence of external, static and uniform electric and magnetic fields, immersed in a uniform bath temperature. We obtain the solution for the associated Langevin equation, and thereafter the evolution of the nonequilibrium temperature towards a nonequilibrium (hot) steady state. We apply our results to a simple yet relevant Brownian model for carrier transport in GaAs. We obtain a negative differential conductivity regime (Gunn effect) and discuss and compare our results with experimental results.

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The Gunn effect consists of time-periodic oscillations of the current flowing through an external purely resistive circuit mediated by solitary wave dynamics of the electric field on an attached appropriate semiconductor. By means of a new asymptotic analysis, it is argued that Gunn-like behavior occurs in specific classes of model equations. As an illustration, an example related to the constrained Cahn-Allen equation is analyzed.

Although it has been known for many years that under the proper conditions an oscillator can lock to and follow an external signal of much smaller amplitude, this phenomena has not had widespread usage. The development of negative resistance solid state oscillator diodes, such as the Gunn diode, has brought about renewed interest in the locked oscillator, however. The locking characteristics of an X-Band Gunn-effect oscillator have been investigated. The theory of injection locking is discussed and the experimental work performed is described. Amplifier performance of 20 db gain with 40 MHz bandwidth and 30 db gain with 13 MHz bandwidth is reported. The advantages of the locked Gunn-effect oscillator as an amplifier for frequency modulated signals are its minimal power supply requirements, small size, low weight, and simplicity.

We present an HST STIS spectrum of the HeII Gunn-Peterson effect towards HE2347-4342. Compared to the previous HST GHRS data obtained by Reimers et al. (1997), the STIS spectrum has a much improved resolution. The 2-D detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the HeII G-P opacity in the high opacity regions. We use the STIS spectrum together with a Keck--HIRES spectrum covering the corresponding HI Lya forest to calculate a 1-D map of the softness S of the ionization radiation along the line of sight towards HE 2347-4342, where S is the ratio of the HI to HeII photoionization rates. We find that S is generally large but presents important variations, from S ~ 30 in the opacity gaps to a 1 sigma lower limit of 2300 at z~2.86, in a region which shows an extremely low HI opacity over a 6.5 A range. We note that a large S naturally accounts for most of the large SiIV to CIV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. (2000) to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al. (1997), we propose that the large softness observed at z~2.86 is due to the presence of bright soft sources close to the line of sight, i.e. for which the ratio between the number of HI to HeII ionizing photons reaching the IGM is large. We discuss these two models and suggest ways to discriminate between them.

We present and numerically solve a model of the photorefractive Gunn effect. We find that high field domains can be triggered by phase-locked interference fringes, as it has been recently predicted on the basis of linear stability considerations. Since the Gunn effect is intrinsically nonlinear, we find that such considerations give at best order-of-magnitude estimations of the parameters critical to the photorefractive Gunn effect. The response of the system is much more complex including multiple wave shedding from the injecting contact, wave suppression and chaos with spatial structure.

We show that spatial correlations in a stochastic large scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow, may cause a `line-like' structure in QSO spectra similar to the population of unsaturated Ly-alpha forest lines which usually are attributed to individual clouds with 10^{11}

The Gunn Effect - IBM