research projects

Nano-electromechanical systems (NEMS) are nanostructures which combine transport of electrical charge with mechanical motion. They often take the shape of suspended nano-scale beams, either attached (clamped) to the chip substrate on one or on both ends. So far, classical continuum mechanics of solids has been rather successful in modelling these devices. Top-down lithographic fabrication of beam resonators from metals or semiconductor materials as e.g. GaAs or silicon has led to resonator frequencies in the MHz regime. However, miniaturization has been accompanied with high dissipation losses - due to e.g. surface, clamping, or intrinsic friction effects.

Several research groups worldwide have already demonstrated that single-wall carbon nanotubes can also be used as nanomechanical resonators, as e.g. a vibrating violin string [1-3]. Since the nanotubes are very light and mechanically very strong at the same time, they vibrate at very high frequencies (in so-far published works, the fundamental resonance mode reaches ca. 100-400 MHz). In addition, the low mass leads to comparatively large quantum effects. For example, even if the nanotube in its ground state of motion, i.e. at lowermost kinetical energy, one expects a zero point position fluctuation of approximately 1pm.

Recently, a new fabrication technique has been developed where contamination and thereby also mechanical damping is minimized [4,5]. In addition, we have realized a new measurement scheme that enables motion detection at true millikelvin resonator temperatures [6-8]. As a consequence, nanotube resonators have reached mechanical quality factors of up to 10⁵ [6], and strong coupling between the mechanical motion and charge tunneling in a suspended nanotube has been observed [7,8].

We plan to extend the techniques of Refs. [6-8] far into the GHz regime, where the vibrational energy quantum exceeds k_BT in our millikelvin setup. Here, we hope to be able to demonstrate a truly quantum-mechanical NEMS system.

Although not superconducting by themselves, short single-wall carbon nanotube segments can carry a proximity-induced supercurrent when contacted with superconducting leads [9-11]. This means that many concepts, tools, and devices from superconducting nanoelectronics can be directly transferred to circuits integrating carbon nanotubes. One recent high-profile example from literature is the nanotube SQUID [12], where a single nanotube forms the two weak links in the loop of the superconducting quantum interference device.

We intend to make use of this and probe both the properties of the nanotube using the superconductivity of the leads and vice versa. As an example, although this is already subject of theoretical considerations [13], no experimental knowledge exists so far on the detailed dynamics of a mobile Josephson junction weak link, where both current and device geometry may be time-dependent.

Based on existing experience with superconducting nanotube electrodes (see graphics, from [11]), we target processes involving type-I superconductors with comparatively high T_c, as e.g. niobium (T_c=9.25K) or lead (T_c=7.2K). These materials promise larger critical currents and more robust superconductivity with respect to temperature and magnetic field.

The department for physics of micro- and nanostructures runs a nanofabrication and characterization facility in the basement of our physics institute. In this cleanroom and in additional lab space we have access to a wide spectrum of equipment. This ranges e.g. on the fabrication side from optical and electron beam lithography to dry etching and wire bonding; in the framework of the Collaborative Research Centre (SFB) 689, the purchase of an additional third electron beam system with included focused ion beam (FIB) has already been approved.

Concerning low-temperature measurement setups, several crostats the group Prof. C. Strunk, which we will share, are up and running (including a ³He system, and two ³He/⁴He dilution refrigerators reaching T=30mK). With the recent installation of a new helium liquefier at the department, optimal conditions for future extension of our low-temperature activities exist, and the acquisition of a measurement setup specifically optimized for nano-electromechanical systems is in the planning stage.