Se p 20 08 epl draft Effects of phase transitions in devices actuated by the electro - magnetic vacuum force

We study the influence of the electromagnetic vacuum force on the behaviour of a model device based on materials, like germanium tellurides, that undergo fast and reversible metalinsulator transitions on passing from the crystalline to the amorphous phase. The calculations are performed at finite temperature and fully accounting for the behaviour of the material dielectric functions. The results show that the transition can be exploited to extend the distance and energy ranges under which the device can be operated without undergoing stiction phenomena. We discuss the approximation involved in adopting the Casimir expression in simulating nanoand microdevices at finite temperature. Introduction. – With the development of modern technology towards smaller structures an increasing attention has been addressed to the role of electromagnetic vacuum fluctuation forces (dispersion or van der Waals and Casimir forces) in microand nano-electromechanical systems [1–7]. These forces vary typically with the third or fourth inverse power of the distance between the surfaces of the interacting bodies and therefore can be very intense in the sub-micrometer regime. When the moving parts of a device come to such a close distance, dispersion forces affect the dynamics and the operation of the device and, under certain circumstances, they can cause adhesion or stiction between the surfaces, thus limiting the device lifetime. Devices actuated by Casimir force have been designed and demonstrated [8–10]. Since these forces depend upon the reflectivity, the size and the geometry of the interacting parts [11, 12], several possibilities can be explored to devise structures that exploit such dependence in order to improve the performance of existing devices or to design new types of devices [13]. In this paper we investigate the possibility of using materials, that undergo fast and reversible phase transitions, to extend the travel range of a device. Thermally induced phase changes between the amorphous and the polycrystalline state in a thin film have been observed since a Fig. 1: (a) Three layers model of equation (1); (b) Simple representation of the dispersion forces based device. long time in tellurides containing Ge and Sb (GST materials) [14–21]. The transition is accompanied by significant changes in optical and transport properties, a feature that is exploited in optical data storage [22] and could be useful in new nanoscale memories [23, 24]. Previous theoretical studies have shown that metal insulator transitions may cause significant changes in the Casimir force behaviour [25, 26]. In this paper we propose to use this transition to modify the force between the components of a simple device. We show that the locations of the extrema in the total energy potential curve of the device are