Lorenz Lechner is working as a research scientist and is currently pursuing his PhD a TKK´s Low Temperature Laboratory. His research concentrates on high frequency phenomena in carbon nano materials. In addition he assists in coordinating the European ISP5 project CARDEQ  and is the designated focused ion beam expert for Micronova.
Qualification for these jobs is the Diploma (M.Sc.) in physics he received in 2005 from the University of Regensburg, Germany.

To keep a good health-work balance he is a dedicated bicycle commuter, is into all kinds of outdoor activities, engages in Aikido, and enjoys cooking for himself and friends.

"Already back in Germany my work and Diploma Thesis was about transport phenomena in carbon nanotubes. When I was asked to continue with my PhD in Regensburg I hesitated because I felt wanderlust. My professor told me about this excellent group at the Low Temperature Laboratory working in the same area of research. Even though I knew little about Finland at that time I was very happy that my application there was accepted. After all, going to Helsinki to work in low temperature physics seemed just natural to me."

Nano Electromechanical Resonators - Towards the Quantum Limit

Miniaturization of transistors has lead to enormous performance improvements in computers over the past decades. As a consequence more and more of the (mobile) appliances that surround us become intelligent. However, in order to be useful without constant human interaction they have to rely on other sources of input. Fortunately miniaturization has also lead to a quantum leap in sensor design. What started out as big electromechanical devices has been transformed to silicon technology and shrunk to MEMS size. These micro-electromechanical systems can now be found as force, pressure, and acceleration sensors in cars, cellphones, notebooks, and even pacemakers. But microscale is not the limit: researchers are now striving for nano-electromechanical systems (NEMS).

What is the gain from shrinking devices further? The answer seems simple: a smaller - thus lighter - resonator is much more sensitive to tiny forces. However, there is a problem. The lighter the resonator the more sensitive it becomes to distortions. Eventually, the thermal movement of the system’s molecules would make accurate measurements impossible.

Figure 1. Schematic of one kind of NEMS: Carbon nanotubes are single sheets of graphite (graphene) wrapped to a cylinder. Depending on the wrapping angle with respect to the grapene’s honeycomb lattice they can be either metallic or semiconducting. Clamped between electrodes and coupled to a gate they can form ultra-sensitive force detectors. (Image courtesy of CARDEQ, ENS)

This inherent problem cannot be overcome in a world governed by the laws of classical mechanics. Fortunately, by shrinking our system to a nanoscale we enter the realms of quantum mechanics. There, neither electric charge nor energy is a continuous variable. Instead they have to be treated as a discrete number of packets or quanta. The electron is the quantum of charge, the photon is the quantum of the electro-magnetic field, and the phonon is a quantized lattice vibration.

This quantization of vibration has an important effect on our resonator. It can only vibrate at frequencies that are integer multiples of the ground state frequency. For a string this resonance frequency is inversely proportional to the length; essentially it is the higher the shorter the string. The energy of the vibration is in turn proportional to the frequency. Thus, we can regard the ground state of our system being equivalent to a phonon with energy E=hf where h denotes Planck's constant. The string can only be excited by other phonons of the same energy as or integer multiple of this ground state.

In this behavior also lies the solution to our noise problem. If the string is short and stiff enough the ground state energy of the resonator becomes so high that it cannot be excited by the thermal phonons or photons of the surrounding material. For a device that could operate at room temperature and atmospheric pressure the frequency would have to be larger than 1 GHz. However, to achieve this “holy grail” of ultrahigh-frequency operation a special string material with remarkable characteristics is needed.

Carbon nanotubes have been found to be the ideal candidate. They are lightweight but yet a thousand times stronger than steel. In fact, they posses the highest known specific strength of all materials. The cylindrical shape and the perfect crystal lattice make them very stiff. Their electrical properties are also outstanding: depending on how they are rolled up with respect to the graphene's honeycomb lattice they can be either metallic or semiconducting.

These special properties can be utilized by clamping a nanotube between two contacts above a gate electrode.  A high frequency voltage applied between the gate and the tube is used to apply a driving force to the resonator. The resulting vibrations modulate a current flowing through the tube. With a clever readout scheme the apparatus can detect a force pulling or pushing the string.

Figure 2. Artificially coloured scanning electron micrograph of a nanotube resonator: The tube is suspended in the middle between to gold contacts. A small gold strip that runs underneath the tube is used as a gate electrode. An HF etching technique was used to created the elevated structures on a silicon wafer with oxide cover. (Image courtesy of CARDEQ, A. Bachtold)

Applications for this type of self-detecting gigahertz resonator are obvious. The exceptionally low intrinsic mass of a carbon nanotube suggests the use as an ultrasensitive mass and force detector. Such a detector would be capable of discriminating mass on molecular level. This could open up new ways for detecting substances at very low concentrations, e.g. identifying chemical warfare agents or in medical diagnosis. Having a device that acts as detector and amplifier in one also makes highly integrated solutions with existing silicon technology possible. That could result in improved optical modulators for fiber communications and enhanced performance clocks for high frequency electronics.

In cooperation with our CARDEQ partners we have been working on fabricating nanotube resonators and developing corresponding readout schemes. Over the past year we together with BlueFors Cryogenics have developed a radio frequency measurement setup in a dry dilution refrigerator. Since last week the system is up and running and we could already confirm a base temperature of below 20mK. With a measurement frequency range of up to 18GHz we now posses the ideal research tool for investigating ultra-high frequency mechanical resonators.

Figure 3. Image of the cryogenic circuitry: Even though the goal for most applications is room temperature operation, the quantum limit of sensitivity will first be achieved at very low temperatures. Creating good rf connections from the room temperature measurement equipment to the cold enourmous design challenge. Both CAD and FES software were used to optimize signal characteristics while retaining the refrigerators minimum operation temperature.

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Attendance lists (Updated 24.11)

Colloquium on New Materials - Fall 2008

Colloquium on New Materials - Spring 2008

Colloquium on New Materials - Fall 2007

Colloquium on New Materials - Spring 2007

Colloquium on New Materials - Fall 2006

Colloquium on New Materials - Spring 2006

Colloquium on New Materials - Fall 2005