Browsing by Author "Göres, Jörn"

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    Correlation effects in 2-dimensional electron systems : composite fermions and electron liquid crystals
    (2004) Göres, Jörn; von Klitzing, Klaus (Prof. Dr.)
    The discovery of the Fractional Quantum Hall Effect (FQHE) and its explanation brought the importance of electron-electron correlation effects in 2-dimensional electron systems into focus. Correlations often dominate the physics of clean 2DES subjected to a strong perpendicular magnetic field, because the kinetic degrees of freedom are effectively frozen out. Up to this day, the complicated collective phenomena that arise from electron correlations remain a challenging subject of research throughout physics. This thesis mainly deals with two prominent examples of correlation phenomena in 2DES, namely Composite Fermions and Electron Liquid Crystal phases. Composite Fermions are quasi-particles each assembled from one electron and two elementary magnetic flux quanta. Within the Composite Fermion framework, the FQHE of electrons is naturally understood as the IQHE of Composite Fermions. It was shown that Composite Fermions exist independently of the IQHE and behave as weakly interacting particles subjected to an effective magnetic field which is substantially reduced from the external magnetic field. Guided by the analogy with electron systems at low magnetic fields, one can devise mesoscopic transport experiments for Composite Fermions. The outcome of such experiments would have never been predictable without resorting to Composite Fermions. In our thesis, we mainly study the particle-like behavior of Composite Fermions in transport through well defined and tunable geometries. Ultimately, the fascinating question poses whether it is also possible to observe wave-like properties of Composite Fermions. As an important first step towards this goal, we study special device geometries which show pronounced interference signatures for electrons at low magnetic fields. At intermediate magnetic fields, outside the realm of the FQHE observed at high fields, electron correlations lead to qualitatively new experimental signatures, namely strongly anisotropic transport and a reentrant IQHE (RIQHE). These effects arise from an instability of the 2DES towards the formation of long range ordered electron density modulations. The modulation is triggered by a competition between repulsive and attractive interaction components, and it bears a strong similarity to the charge modulation in conventional Charge Density Wave (CDW) conductors. Depending on the morphology of the charge density modulation of the 2DES, one distinguishes between anisotropic stripe phases and isotropic bubble phases. Transitions between the different types of phases are induced by simply tuning the applied magnetic field. The bubble phase in the CDW picture is the many-electron analogue of the well known Wigner crystal. A more sophisticated model includes quantum fluctuations for a realistic description of the correlated phases. The resulting phase diagram is very similar to that of classical molecular liquid crystals. Therefore, the model is known as the Electron Liquid Crystal (ELC) picture. In our own study of the ELC phases in high quality 2DES, we use offset currents to induce a non-equilibrium situation within the sample and destabilize the correlated phases in a very controlled way. This provides us with an additional parameter which we use to investigate the ELC phases in more detail. We find regimes of strong negative differential resistance (NDR) and indications of a connection between the stripe and the bubble phase that has not been noticed so dar. Furthermore the bubble phase in non-equilibrium shows clear signs of anisotropic behaviour which can not understood with current models. We believe that an improved understanding of the properties of Composite Fermions and Electron Liquid Crystal phases will not only contribute to the field of electron correlations in semiconductor 2DES. Likely, it will also have an impact on related areas of physics like high-Tc superconductivity, the physics of ultra-cold atoms in optical lattices, quantum phase transitions, and quantum field theory in general.