Abstract. Self-generated chaotic current fluctuations in the post-breakdown regime of a n-GaAs layer at 4.2 K have been analyzed in detail. Without an external magnetic field only regular oscillations were observed. Increasing the magnetic field strength up to 100mT generates a sequence of quasiperiodic and frequency-locking current oscillations and finally a Ruelle-Takens-Newhouse scenario with chaos. This may be understood by assuming two coupled oscillatory processes caused by dielectric relaxation and energy relaxation in the distribution of free carriers. 05.40., 72.70., 72.20J High-purity semiconductors at low temperature show highly nonlinear current-voltage characteristics. For small electric fields almost all carriers are bound to shallow impurities yielding a low conductance of the sample. At a critical field of a few volts per cm the impact ionization rate of shallow impurities exceeds the capture rate for low carrier concentration resulting in a rapid increase of the current. The steady-state properties of the transition from the low-conducting state to the high-conducting state have been analyzed in terms of nonequilibrium phase transformations 1-14]. In the course of the transition, spontaneous oscillations and chaotic current fluctuations have been observed in several semiconductor materials 1,5-17]. Different types of, and routes to, chaos were recognized and discussed in terms of nonlinear dynamics [18][19][20][21][22][23][24][25][26][27][28]. Current fluctuations in semiconductors may occur spontaneously I-5-13] or be induced by an external periodic driving force 1-13-17]. PACS:In the present paper we report on a detailed study of self-generated current fluctuations in high purity n-GaAs epitaxial layers which occur within a limited bias voltage interval in the post-breakdown regime of the material. The observed phenomena depend critically on the strength of an external magnetic field. At zero field, B = 0, only regular oscillations were found. Increasing B up to not more than 100mT causes a sequence of quasiperiodic and frequency-locking current phenomena, finally undergoing a Ruelle-TakensNewhouse scenario to chaos. This behavior may be attributed to the coupling of two oscillatory processes, in the present case dielectric relaxation and an oscillation of the nonequilibrium electron distribution. The experimental results are in excellent agreement with the predictions of the circle-map theory. The coupling strength, frequencies and amplitudes of both selfsustained processes depend strongly on the magnetic field strength.

Abstract. The formation of current filaments in n-type GaAs due to the low temperature impurity breakdown has been investigated in crossed electric and magnetic fields with a laser scanning microscope. In a highly compensated, low mobility, epitaxial layer the lateral displacement and stretching of a filament due to the Lorentz force has been observed at currents slightly larger than those required to form a filament between point contacts. The transverse elongation of the filament increases linearly with rising magnetic field strength until the filament is disrupted. In this case periodic current oscillations are generated by a repetitive filament ignition and extinction. The elongation decreases with rising current and finally vanishes even at high magnetic fields. This stiffening of a current filament is attributed to an increased electron temperature and mobility in the filament, which cause the charges of the Hall field to be accumulated at the filament borders rather than at the edges of the sample.

This work evaluated the experiences of 45 patients of children with cancer and 101 cancer patients with their home pastors and hospital chaplains. The sttisfactions and difficulties enountered in these interactions are detailed, and recommendations are offered to aid persons in acute distress. Individuals in these circumstances appear to desire spiritual support more than evident psychologial aid, though the skillful use of the latter to understant the religious desires of the patients and parents is appreciated. It is also evident that pastors dealing which problems related to potential terminality are themselves under considerble stress.

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