Research

We do research nanoelectronics in the cryolab and with molecules.

Nanoelectronics in the cryolab

One of our cryostats.

In the cryolab we investigate electronic transport phenomena in nanometer scaled devices at cryogenic temperatures, typically in the range between 50 mK and 4 K. In order to obtain such low temperatures for macroscopic samples we are equipped with two dilution refrigerators, two 3He and one 4He cryostat, see picture gallery and the photo to the left. The small devices and low temperatures allow us to investigate new and exciting phenomena where individual electrons are manipulated and the classical description has to be replaced by quantum mechanics.

We are also specialised in fabricating samples ourselves. Below you can find some examples. All our devices consist of a central structure with metallic contacts by which we measure the electrical conductance through the structure. The latter often consists of a material with reduced dimensions, i.e. where the electrons are strongly confined in at least one spatial direction and the corresponding degree of freedom is “frozen out”, i.e. the excited states are not accessible to the electrons by thermal excitation. We use classical materials like metals and semiconductors, but our focus lies on one-dimensional carbon nanotubes, semiconducting nanowires (e.g. InAs) and on two-dimensional graphene. By electron beam lithography and various metal evaporation techniques we deposit the electrical contacts. These contacts can be used to further reduce the electron confinement to prepare zero-dimensional objects, so-called quantum dots. These structures have well-defined discrete energy levels and properties that are to some degree independent of the host material. In contrast to single atoms in nature, which have similar properties, our structures have multiple electrical contacts which allow us to investigate a single nano-object by transport spectroscopy – and of course this opens the way to future electronics applications.

A carbon nanotube contacted by four gold electrodes.

Single-wall carbon nanotubes can be thought of as a sheet of carbon atoms wrapped up into a tube. They are unique because they carry only two transport channels at the Fermi energy, which makes them almost ideal one-dimensional conductors. InAs nanowires are useful because InAs is a semiconductor, which allows us to locally tune the Femi energy by electrical gating. Since InAs is optically active, InAs nanowires are promising for future interfaces between electronic and optical applications. Graphene is a true single atomic layer of carbon atoms and its existence in itself is an intriguing topic. However, we are more interested in its unique electronic properties, e.g. that the band structure is linear near the Fermi energy, which leads to the electrons behaving like mass-less fermions.

The miniaturization of electronics devices with carbon nanotubes, or ultra-high mobility transistors based on graphene are only the first steps in the development. More interesting for physicists is that due to the small spin-orbit coupling both materials are expected to be suitable to carry spin information, possibly independent of charge transport. To investigate spin effects in our structures we developed ferromagnetic contacts to all these materials, which allows us to inject a spin-polarized current and to detect potential differences due to spin accumulation. On a more fundamental level we investigate how the proximity to ferromagnetically ordered metals affects the electronic properties of our mesoscopic devices.

Another source of electrons in a well-defined quantum state are superconductors, where Cooper pairs form spin singlet states. The injection of Cooper pairs from a superconducting contact into graphene, carbon nanotubes, nanowires and quantum dots leads to the discovery of new physics, like the proximity effect in low-dimensional structures and novel materials, and opens the opportunity to investigate the competition of different many-particle interactions, e.g. between Kondo correlations and superconductivity. Cooper pairs might also serve as a source of entangled electrons and we investigate effects related to such Cooper pair splitting.

A InAs nanowire contacted with three aluminium electrodes and three gold top gates.

A graphene ribbon contacted with several aluminium electrodes and one big iron electrode.

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