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2008 Europhysics Condensed Matter Prize awarded to Geim and Novoselov

The 2008 Europhysics Condensed Matter Prize has been awarded to Andre Geim and Kostya Novoselov of the University of Manchester for their pioneering work on carbon films with a thickness of one atom, otherwise known as graphene. Electrons in graphene are strictly two-dimensional and have very special properties. Geim and Novoselov were first both to discover how to fabricate graphene films, and also to demonstrate several of their key novel properties. Their work has provided inspiration for intensive experimental and theoretical research all over the world.

The fabrication method that Geim and Novoselov developed is surprisingly simple and, enabled by the details they provided, could be repeated by others in a very short time. It is called micromechanical cleavage of graphite. It consists of putting sticky tape to a graphite surface, peeling off one single monolayer and depositing this monolayer onto a substrate. An essential part is the identification of those parts of the sample where the thickness is just one single atom, distinguishing those areas from the thicker ones. For that purpose, Geim and Novoselov used optical microscopy, scanning electron microscopy and atomic force microscopy. They found that graphene is visible in an optical microscope, despite its extreme thinness if it is placed onto a specially oxidized silicon substrate. The sticky tape method provided the breakthrough momentum that opened this new field of experimental research. The technique has recently been scaled up to allow mass production of graphene suspension and powder. In the longer term epitaxial techniques will likely provide reliable, large area graphene films.

What makes graphene so special is that the energy of electrons at the Fermi surface depends linearly on wave vector, similar to the dispersion relation for photons. The electron density can be changed with a gate potential and by shifting the Fermi level one can have electrons or holes with very similar properties (except of course for the effective charge). The mobility of these charge carriers is extremely high, even at room temperature. The electrons and holes in this two-dimensional solid behave as massless relativistic fermions and are best described with the Dirac equation with the effective ‘speed of light’ having a value around 10⁶ m/s. Graphene has a honeycomb lattice with two atoms per unit cell. Electronic states are associated with the two sublattices, and quasiparticles have to be described with a two-component wavefunction similar to the wavefunction for electron spin. The quasiparticles possess a chirality that is positive for electrons and negative for holes. Pseudospin and chirality tend to be conserved in interaction processes. A clear example of the special nature of electrons in graphene is the quantum Hall effect. Usually, quantum Hall plateaus are observed at transverse conductivity values n(Me²/h), with n an integer not equal to zero and M the number of different types of quasiparticles (M =4 in graphene, for 2 spins and 2 pseudospins). In contrast, in graphene the plateaus are found at (n+½)(4e²/h). This is called the half-integer quantum Hall effect and is a consequence of the massless nature of graphene’s quasiparticles. A very special feature of electron transport in graphene is the minimum conductivity. With careful tuning of the gate voltage to the turnaround point between electrons and holes, no metal-insulator transition is observed but instead a value of the conductivity of about 4e²/h. A large number of other effects have been demonstrated in the last few years. These include the observation of proximity-induced superconductivity, Klein tunneling between electron- and hole-doped areas in graphene, and the universal optical opacity described by the fine structure constant.

Geim and Novoselov published their first results with six co-authors from Manchester and Chernogolovka in a paper titled ‘Electric field effect in atomically thin carbon films’ in 2004 [1]. This paper describes the fabrication method and the observation of gate-induced carriers of opposite polarity with very high mobility. The first observations of the quantum Hall effect were published simultaneously by Geim, Novoselov and co-workers [2] and by Zhang and co-workers from Columbia University [3]. Geim and Novoselov give a clear description of the work on graphene in a review paper titled ‘The rise of graphene’ in 2007 [4]. The upward trend suggested by this title is an understatement, rather than an exaggeration. For the years up to 2004 the ISI Web of Science lists around 100 papers on the topic ‘graphene’. For the first half of 2008 the number is higher than 600. A large number of experimental groups have shifted their attention to this material, enabled by the method of fabrication discovered by Geim and Novoselov and inspired by their demonstrations of the special properties. Although the band structure of graphene was theoretically known previously, theoreticians are now also very active in this field, motivated by the unusual properties found in new experiments carried out worldwide. It is clear that Geim and Novoselov have opened the gate to a new, productive and very stimulating field of research.

[1] KS Novoselov, AK Geim, SV Morozov, D Jiang, Y Zhang, SV Dubonos, IV Grigorieva and AA Firsov, Science 306, 666 (2004)

[2] KS Novoselov, AK Geim, SV Morozov, D Jiang, MI Katsnelson, IV Grigorieva, SV Dubonos and AA Firsov, Nature 438, 197 (2005)

[3] Y Zhang, Y-W Tan, HL Stormer and P Kim, Nature 438, 201 (2005)

[4] AK Geim and KS Novoselov, Nature Materials 6, 183 (2007)