Heating process in the pre-Breakdown regime of the Quantum Hall Efect : a size dependent effect

Our study presents experimental measurements of the contact and longitudinal voltage drops in Hall bars, as a function of the current amplitude. We are interested in the heating phenomenon which takes place before the breakdown of the quantum Hall effect, i.e. the pre-breakdown regime. Two types of samples has been investigated, at low temperature (4.2 and 1.5K) and high magnetic field (up to 13 T). The Hall bars have several different widths, and our observations clearly demonstrate that the size of the sample influences the heating phenomenon. By measuring the critical currents of both contact and longitudinal voltages, as a function of the filling factor (around $i=2$), we highlight the presence of a high electric field domain near the source contact, which is observable only in samples whose width is smaller than 400 microns.Comment: 4 pages, 5 igures, 7th International Symposium of Research in High Magnetic Fields, to be published in physica

Electronic Processes at the Breakdown of the Quantum Hall Effect

Microscopic processes giving the energy gain and loss of a two-dimensional electron system in long-range potential fluctuations are studied theoretically at the breakdown of the quantum Hall effect in the case of even-integer filling factors. The Coulomb scattering within a broadened Landau level is proposed to give the gain, while the phonon scattering to give the loss. The energy balance equation shows that the electron temperature T_e and the diagonal conductivity sigma_{xx} exhibit a bistability above the lower critical electric field E_{c1}. Calculated values of E_{c1} as well as T_e and sigma_{xx} at E_{c1} are in agreement with the observed values in their orders of magnitude.Comment: 4 pages, 2 Postscript figures, submitted to the Journal of the Physical Society of Japa

Electric-Field Breakdown of Absolute Negative Conductivity and Supersonic Streams in Two-Dimensional Electron Systems with Zero Resistance/Conductance States

We calculate the current-voltage characteristic of a two-dimensional electron system (2DES) subjected to a magnetic field at strong electric fields. The interaction of electrons with piezoelectric acoustic phonons is considered as a major scattering mechanism governing the current-voltage characteristic. It is shown that at a sufficiently strong electric field corresponding to the Hall drift velocity exceeding the velocity of sound, the dissipative current exhibits an overshoot. The overshoot of the dissipative current can result in a breakdown of the absolute negative conductivity caused by microwave irradiation and, therefore, substantially effect the formation of the domain structures with the zero-resistance and zero-conductance states and supersonic electron streams.Comment: 5 pages, 4 figure

Nonlinear effects in microwave photoconductivity of two-dimensional electron systems

We present a model for microwave photoconductivity of two-dimensional electron systems in a magnetic field which describes the effects of strong microwave and steady-state electric fields. Using this model, we derive an analytical formula for the photoconductivity associated with photon- and multi-photon-assisted impurity scattering as a function of the frequency and power of microwave radiation. According to the developed model, the microwave conductivity is an oscillatory function of the frequency of microwave radiation and the cyclotron frequency which turns zero at the cyclotron resonance and its harmonics. It exhibits maxima and minima (with absolute negative conductivity) at the microwave frequencies somewhat different from the resonant frequencies. The calculated power dependence of the amplitude of the microwave photoconductivity oscillations exhibits pronounced sublinear behavior similar to a logarithmic function. The height of the microwave photoconductivity maxima and the depth of its minima are nonmonotonic functions of the electric field. It is pointed to the possibility of a strong widening of the maxima and minima due to a strong sensitivity of their parameters on the electric field and the presence of strong long-range electric-field fluctuations. The obtained dependences are consistent with the results of the experimental observations.Comment: 9 pages, 6 figures Labeling of the curves in Fig.3 correcte

Application of the TRL calibration technique for HEMT's microwave characterization at temperatures down to 77 K.

For the possible application to the cryogenically cooled low noise HEMT amplifier, this paper presents scattering parameter measurements of a HEMT chip at room and liquid nitrogen temperatures and in the 1-20 GHz frequency range. The measurement method which uses the well known TRL calibration technique will be fully discussed. S parameters and noise parameters measured at room temperature by means of this technique have been used to design a single stage low noise amplifier for the K band. Without any tuning this amplifier has an overall noise figure value of 1.6 dB with a 7 dB associated gain at 18 GHz which is in good agreements with predictions

ELECTRICAL PROPERTIES OF ALUMINA SCALES DEVELOPED ON β NiAl AT 1100°C

La variation de la conductivité électrique dans l'épaisseur d'une couche oxydée a été étudiée par une méthode électrochimique dans le cas de l'alumine formée par oxydation à 1100 °C d'un alliage NiAl β. Les contributions respectives de la diffusion des espèces, dans le volume de l'alumine dopée par le nickel et le long des joints de grains, sur la conductivité de la couche sont discutées . Il est ainsi montré que la conduction électronique est une propriété du volume de l'alumine, alors que la conduction ionique est intergranulaire . Par ailleurs, la croissance de la couche d'alumine est assurée principalement par la diffusion d'espèces chargées le long des joints de grains.The variation of electrical conductivity in the thickness of an oside scale was determined by an electrochemical method applied to alumina scale formed by osidation at 1100 °C of a β NiAl alloy . Respective contributions of the charged species diffusing in the volume of nickel doped alumina and of that diffusing along grain boundaries are discussed . So, it is shown that electronic conduction is a volume conduction, whereas ionic conductivity is intergranular . Moreover, the growth of alumina scale is mainly ensured by charged species diffusion along grain boundaries of the scale

The Use of Thermosensitive Quartz Sensor for Thermal Regulation at Cryogenic Temperatures: Application to Microwave Sapphire Resonator References

We demonstrated the use of themosensitive quartz resonator oscillator as thermal sensor for temperature control at the liquid nitrogen temperature. The high sensitivity of the quartz enables an efficient thermal regulation at ambient temperature. LC-cut quartz oscillator phase noise measurements show that the temperature measurement resolution is not limited by the intrinsic noise of the sensor and that a resolution of 10 µK can be achieved. This thermal regulation is applied to control a microwave temperature compensated sapphire resonator oscillator at a temperature above 77 K, enabling the achievement of a flicker floor of 9.10-13 at 9 GH

Thermal stabilisation of a high-Q sapphire microwave resonator using a thermosensitive Quartz sensor

The authors demonstrate the use of a thermosensitive quartz resonator oscillator as a thermal sensor to measure and control the temperature of a sapphire whispering gallery mode resonator. The high sensitivity of the quartz sensor allows an efficient thermal regulation of the sapphire resonator at ambient temperature as well as at 77K. At 300K, the measured frequency stability of the 9.4 GHz sapphire resonator oscillator is better than 10-9 for integration times up to 1000s