Simple mechanisms that impede the Berry phase identification from magneto-oscillations

The phase of quantum magneto-oscillations is often associated with the Berry phase and is widely used to argue in favor of topological nontriviality of the system (Berry phase $2\pi n+\pi$). Nevertheless, the experimentally determined value may deviate from $2\pi n+\pi$ arbitrarily, therefore more care should be made analyzing the phase of magneto-oscillations to distinguish trivial systems from nontrivial. In this paper we suggest two simple mechanisms dramatically affecting the experimentally observed value of the phase in three-dimensional topological insulators: (i) magnetic field dependence of the chemical potential, and (ii) possible nonuniformity of the system. These mechanisms are not limited to topological insulators and can be extended to other topologically trivial and non-trivial systems.Comment: 9 pages, 4 figures, in published version the title was change

Laser-pump-resistive-probe technique to study nanosecond-scale relaxation processes

Standard optical pump-probe techniques allow to study a temporal response of a system to a laser pulse starting from the sub-femtoseconds to several nanoseconds, limited by the length of the optical delay line. Resistance is a sensitive detector of a plethora of phenomena in solid state, nanostructure physics, chemistry, and biology. However, resistance measurements are typically rather slow (> microsecond) because they require stabilization of current or voltage and analog-to-digital conversion. Here we develop a time-resolved pump-probe technique, where the "Pump" is an optical pulse from the laser and the "Probe" is a rectangular electrical pulse passing through the sample under study. Transmission of the probe pulse through the sample is measured in the 50 Ohm matched circuit with the digital oscilloscope. The delay can be electrically driven from nanoseconds to seconds. We demonstrate the usability of the technique to study heat-induced changes in a thin amorphous VO$_x$ film and carrier relaxation in a CdS photoresistor. The technique could be further explored to study heat transfer, biochemical reactions, and slow electronic transformations