A new type of carbon resistance thermometer with excellent thermal contact at millikelvin temperatures

Using a new brand of commercially available carbon resistor we built a cryogenic thermometer with an extremely good thermal contact to its thermal environment. Because of its superior thermal contact the thermometer is insensitive to low levels of spurious radio frequency heating. We calibrated our thermometer down to 5mK using a quartz tuning fork He-3 viscometer and measured its thermal resistance and thermal response time.Comment: 5 pages, 4 figure

Influence of parallel magnetic fields on a single-layer two-dimensional electron system with a hopping mechanism of conductivity

Large positive (P) magnetoresistance (MR) has been observed in parallel magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs heterostructure with a variable-range-hopping (VRH) mechanism of conductivity. Effect of large PMR is accompanied in strong magnetic fields by a substantial change in the character of the temperature dependence of the conductivity. This implies that spins play an important role in 2D VRH conductivity because the processes of orbital origin are not relevant to the observed effect. A possible explanation involves hopping via double occupied states in the upper Hubbard band, where the intra-state correlation of spins is important.Comment: 10 pages, 4 jpeg figure

Thermal impedance between a thick-film resistor liquid helium below 1 K

We have investigated the temperature dependence of the thermal impedance between a thick-film resistor used as a thermometer and liquid helium at temperatures between 1 K and 15 mK. Above 100 mK the impedance is found to vary as T−2, whereas at lower temperatures a T−3 law is obtained. The prefactors are unsually large when compared with Kapitza resistance or thermal conductivities

On the Unobservability of a Trust Relation in Mobile Ad Hoc Networks

Abstract. More and more mobile devices feature wireless communication capabilities. They can self-organize in a mobile ad hoc network in order to communicate and maintain connectivity without any infrastructure component. In this context, some devices may benefit from established trust relations in order to communicate private data. Various solutions already exist for establishing and detecting such trust relations. But is it still possible to detect a trust relation in an unobservable manner? That is, in a way that an attacker cannot understand whether devices share a trust relation or not. We exhibit a solution to this problem. Our solution guaranties the anonymity and the unobservability of participants against passive and active attackers. The security properties of the solution are machine checked with the AVISPA framework [2] and the SPAN tool [5]. The main applications could be found in mobile ad hoc networks and in vehicular networks [6,7] where anonymity and unobservability contribute to a better privacy.

Low-temperature electronic transport measurements on a gated delta -doped GaAs sample: magnetoresistance, quantum Hall effect and conductivity fluctuations

We present magnetotransport measurements (up to 7 T) performed at very low temperatures (down to 20 mK) on a GaAs sample containing two parallel delta -doped layers whose carrier concentration can be varied by means of a gate electrode. With increasing negative gate voltage the resistance becomes more strongly temperature-dependent, indicating a more localized electron system. The magnetoresistance is found to be strongly anisotropic. When the field is parallel to the layers we find a large positive magnetoresistance which we attribute to orbital shrinking of the strongly localized donor wavefunction. In contrast, in the perpendicular orientation, we observe a strong negative magnetoresistance at low fields whose origin remains unclear, and the quantum Hall effect at larger fields. At low gate voltages both delta -layers are in the quantum Hall state whereas at larger negative voltages the layer adjacent to the gate becomes insulating. In the case of strong depletion the high-ohmic sample shows reproducible conductivity fluctuations as a function of either the gate voltage or the magnetic field. The fluctuations diminish at higher temperatures and larger measuring currents