Direct Observation of Interband Spin-Orbit Coupling in a Two-Dimensional Electron System

We report the direct observation of interband spin-orbit (SO) coupling in a two-dimensional (2D) surface electron system, in addition to the anticipated Rashba spin splitting. Using angle-resolved photoemission experiments and first-principles calculations on Bi/Ag/Au heterostructures we show that the effect strongly modifies the dispersion as well as the orbital and spin character of the 2D electronic states, thus giving rise to considerable deviations from the Rashba model. The strength of the interband SO coupling is tuned by the thickness of the thin film structures

Towards lateral dots on few-layer transition metal dichalcogenides (TMDCs)

Trabajo Fin de Máster en Física de la Materia Condensada y de los Sistemas BiológicosOwing to their unique properties, two-dimensional materials are a field of intense research activity since the discovery of graphene in 2004 [1]. Among these, semiconductor transition metal dichalcogenides (TMDCs) are of interest for applications that require a bandgap, such as field-effect transistors or quantum dots (QDs). Interestingly, theoretical work suggests that TMDC QDs have potential as an active element in quantum technologies, e. g. as valley filters or spin-valley qubits [2]. However, the experimental realization of controllable QDs in these materials still remains as a challenge. This project ultimately aims to contribute in this direction by experimentally realizing electrically tunable QDs in TMDCs. In this work, we take the first steps towards this goal. On the one hand, we start developing the technology to fabricate the envisioned devices. Parallelly, we direct efforts towards optimizing the device geometry by means of electrostatic simulations using the software COMSOL Multiphysics [3]. In particular, we address the impacts of the gate geometry and of the thicknesses of the different 2D materials. We conclude that these two parameters are crucial in order to confine quantum dots with small enough dimensions to ensure a quantized energy spectrum. We also report on the experimental progress obtained with respect to the fabrication of TMDC device

Piezoelectric and Magnetoelastic Strain in the Transduction and Frequency Control of Nanomechanical Resonators

Stress and strain play a central role in semiconductors, and are strongly manifested at the nanometer-scale regime. Piezoelectricity and magnetostriction produce internal strains that are anisotropic and addressable via a remote electric or magnetic field. These properties could greatly benefit the nascent field of nanoelectromechanical systems (NEMS), which promises to impact a variety of sensor and actuator applications. The piezoelectric semiconductor GaAs is used as a platform for probing novel implementations of resonant nanomechanical actuation and frequency control. GaAs/AlGaAs heterostructures can be grown epitaxially, are easily amenable to suspended nanostructure fabrication, have a modest piezoelectric coefficient roughly twice that of quartz, and if appropriately doped with manganese, can form dilute magnetic compounds. In ordinary piezoelectric transducers there is a clear distinction between the metal electrodes and piezoelectric insulator. But this distinction is blurred in semiconductors. An integrated piezoelectric actuation mechanism is demonstrated in a series of suspended anisotype GaAs junctions, notably pin diodes. A dc bias was found to alter the resonance amplitude and frequency in such devices. The results are in good agreement with a model of strain based actuation encompassing the diode’s voltage-dependent carrier depletion width and impedance. A bandstructure engineering approach is employed to control the actuation efficiency by appropriately designing the doping level and thickness of the GaAs structure. Actuation and frequency are also sensitively dependent on the device’s crystallographic orientation. This combined tuning behavior represents a novel type of depletion-mediated electromechanical coupling in piezoelectric semiconductor nanostructures. All devices are actuated piezoelectrically, whereas three techniques are demonstrated for sensing: optical interferometry, piezoresistance and piezoelectricity. Finally, a nanoelectromechanical GaMnAs resonator is used to obtain the first measurement of magnetostriction in a dilute magnetic semiconductor. Resonance frequency shifts induced by field-dependent magnetoelastic stress are used to simultaneously map the magnetostriction and magnetic anisotropy constants over a wide range of temperatures. Owing to the central role of carriers in controlling ferromagnetic interactions in this material, the results appear to provide insight into a unique form of magnetoelastic behavior mediated by holes