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Project

Interplay between magnetism and band topology in Dirac/Weyl semimetals

The quantum Hall effect (QHE), in which the transverse conductance of a two-dimensional (2D) electron gas in an applied magnetic field becomes quantized to multiples of e²/h, has fascinated physicists for over forty years. In 2013, the quantum anomalous Hall effect (QAHE) was realized in magnetic topological insulators. The hallmark of the QAHE is its dissipationless chiral 1D state running along the edge of the quasi-2D thin film, resulting in a vanishing longitudinal resistance and quantized Hall resistance similar to the QHE at filling factor one. However, the QAHE does not require the application of an external magnetic field, giving it a distinct advantage over the QHE.

When reducing the device dimensions or increasing the current density, an abrupt breakdown of the QAHE occurs with a relatively small critical current. In the first part of the thesis, the mechanism of this breakdown is studied in multi-terminal devices and the electric field created between opposing chiral edge states is identified as the driving force. Namely, the electric-field-driven percolation of 2D charge puddles in the gapped surface states of the compensated topological-insulator films is proposed as the most likely cause of the breakdown.

Although generally it is desirable to avoid breakdown, the chiral nature of the edge states gives rise to rectification (nonreciprocal) effects in the longitudinal resistance when the ideal zero-resistance state is lost. In the second part of the thesis, the nonreciprocal charge transport associated with the broken-down QAHE is studied over a large parameter space of different temperatures, applied magnetic fields, electrostatic gate-potentials, and probe currents. Two distinct regimes are identified. At high currents and/or temperatures where Coulomb disorder only plays a minor role, the current-voltage relation follows the well-known quadratic current-dependence of nonreciprocal systems. On the other hand, at ultra-low temperatures when the current amplitude is decreased to only slightly exceed the critical current for breakdown, the description of the nonreciprocal charge transport becomes more complex. In this regime, the finite nonreciprocal response when the chemical potential lies inside the exchange gap is argued to be determined by the majority 2D charge puddles (either n- or p-type) resulting from an imperfect charge compensation.

The PhD project received support in the form of a PhD scholarship from the Fonds Wetenschappelijk Onderzoek (FWO, File No. 27531 and No. 52751).

Date:21 Sep 2015 →  12 May 2023
Keywords:Dirac, semimetals
Disciplines:Electronic (transport) properties, Magnetism and superconductivity, Nanophysics and nanosystems, Surfaces, interfaces, 2D materials, Quantum information, computation and communication
Project type:PhD project