Die Gewinner der SPG Preise 2013

Titus Neupert is awarded with the SPS 2013 Prize in General Physics for his pioneering PhD work, especially for his theoretical discovery of "Fractional quantum Hall states at zero magnetic field".

The integer and fractional quantum Hall effects were experimentally detected in 1981 and 1982, respectively, at cryogenic temperatures. The discovery of graphene in 2005 established that the integer quantum Hall effect could be achieved at room temperature. Theorists predicted that the integer quantum Hall effect was one out of many examples of a larger family of semiconducting states supporting quantized susceptibilities in materials called topological band insulators. Titus Neupert gave the first quantitative answer to the question whether strong interactions could drive a fractional topological insulator in very much the same way as interactions drive a fractional quantum Hall insulator. One of the most remarkable prediction made by the award winner is that, by taking advantage of materials with strong spin-orbit coupling, it might become possible to achieve a fractional quantum Hall effect that is robust at room temperature and this without the use of any laboratory magnetic field.

Fractional quantum Hall states at zero magnetic field

A central theme of condensed matter physics is to classify and understand phases of matter. The Landau theory of symmetry breaking has been the long-standing paradigm for this classification: Two phases are distinct if they have different symmetries. In recent years, the study of topological phases showed that a second paradigm must be considered on equal footing: Two phases are distinct if they have different topological character, even if they share the same symmetries. Topological properties cannot be changed smoothly, thus endowing a topological state with a natural universality and protection against perturbations.
Topological phases are understood and classified in the limit of small electron–electron interactions. The opposite limit, in contrast, is at the frontier of current research. Strong electron–electron interactions can be responsible for the emergence of correlated topological states with excitations that have a fraction of the electron's charge, so-called fractional topological insulators (FTIs). The first example of an FTI that is well studied both experimentally and theoretically is the fractional quantum Hall effect of electrons in partially filled Landau levels. Recently, we discovered another type of FTIs, the fractional Chern insulator [1]. These states arise in lattice models in two spatial dimensions, if a nearly dispersionless band with a nonzero Chern number is partially filled with repulsively interacting electrons. Fractional Chern insulators share many universal and topological properties with the fractional quantum Hall effect in Landau levels, where the role of the strong magnetic field is replaced by time-reversal symmetry breaking electronic hopping integrals on the lattice. Comparing and contrasting the fractional Chern insulators with the fractional quantum Hall effect allows us to better understand what are the core ingredients for a fractional topological state to emerge.
In a combination of numerical and analytical work, we have studied several aspects of FTIs in two spatial dimensions. For example, we found that if a topological insulator, as is realized in HgTe quantum wells, has a sufficiently small bandwidth, repulsive electron–electron interactions can favor a spontaneous breaking of time-reversal symmetry along with the formation of an anomalous quantum Hall effect or a fractional Chern insulator state [2].

[1] T. Neupert, L. Santos, C. Chamon, and C. Mudry, Phys. Rev. Lett. 106, 236804 (2011).
[2] T. Neupert, L. Santos, S. Ryu, C. Chamon, and C. Mudry, Phys. Rev. B 84, 165107 (2011).

Iris Crassee is awarded with the SPS 2013 Prize in Applied Physics for her excellent PhD work on magneto-optical properties of graphene and the discovery of giant Faraday rotation in the terahertz range in single and multilayer graphene epitaxially grown on silicon carbide. She pioneered the application of the infrared Hall spectroscopy, where both transmission and Faraday rotation are measured in a broad frequency range, to graphene, which allowed her to distinguish Landau-level transitions that stem from different graphene layers.

Giant Faraday rotation and magneto-plasmonic effects in graphene

We explored the complex structure of the Landau levels and cyclotron resonances in graphene epitaxially grown on silicon carbide. Epitaxial graphene is of particular practical importance because of a possibility for scalable growth. However, its properties are rather complicated because of a strong coupling of the carbon layers to the substrate, their unusual stacking and variation of doping across the layers. We were the first to apply the technique of the infrared Hall effect in monolayer and multilayer graphene, where both transmission and the Faraday rotation are measured. This allowed us to distinguish and classify spectroscopically various Landau-level transitions that stem from different graphene layers, including the buried ones, which are not accessible by other techniques. In particular, a multicomponent cyclotron resonance structure was observed in multilayer graphene grown on the C-face of silicon carbide.
In our study of doped monolayer graphene on Si-face of SiC, we discovered a giant Faraday rotation in the terahertz range that originates from the cyclotron resonance of free carriers [1]. The unprecedented value of the rotation angles (about 6 degrees at 7 T) by the thinnest material in condensed matter physics, contradicts common sense, since the Faraday effect is known to be proportional to the thickness. Nevertheless, we explained our observation by the extremely small mass of Dirac fermions and their high mobility, and numerically reproduced the experimental curves. In a follow-up publication [2], we found that due to the presence of substrate terraces, a strong terahertz plasmonic peak appears in epitaxial graphene. Furthermore, we found that the plasmon peak splits in magnetic field into bulk and edge magnetoplasmon branches, similar to the ones observed in some classical experiments on GaAs based 2D electron gases. This was actually the first observation of magnetoplasmons in graphene. Importantly, the plasmonic effects found by us modify dramatically the wavelengths dependence of the Faraday rotation. All these results are of a high practical importance as they suggest that graphene can be used in ultrathin and ultrafast switchable magneto-optical devices.

[1] I. Crassee, J. Levallois, A. L. Walter, M. Ostler, A. Bostwick, E. Rotenberg, Th. Seyller, D. van der Marel, and A. B. Kuzmenko, Giant Faraday rotation in single- and multilayer graphene, Nature Physics 7, 48–51, 2011.
[2] I. Crassee, M. Orlita, M. Potemski, A. L. Walter, M. Ostler, Th. Seyller, I. Gaponenko, J. Chen, and A. B. Kuzmenko, Intrinsic Terahertz Plasmons and Magnetoplasmons in Large Scale Monolayer Graphene, Nano Letters 12, 2470-2474, 2012.