Control and imaging of excitons, spins and spin dynamics in Van der Waals semiconductors and superconductor-magnet hybrids

Abstract

Magnetic phenomena are at the foundation of modern-day information storage and processing technologies. In this dissertation,we study three phenomena that hold promise for the next generation of information technology: excitons in two-dimensional semiconductors, two-dimensional magnets and magnetic excitations. First, we introduce the reader to magnetism, its role in society (Chapter 1) and how magnetic phenomena can be imaged with high resolution using the nitrogen vacancy (NV) center in diamond (Chapter 2). Before presenting research results, we introduce the aforementioned phenomena, along with two experimental setups that we constructed ground-up to probe them using spectroscopy and cryogenic NV magnetometry (Chapter 2).

Valleytronics aims to encode information using valleys in the band structure of crystalline solids. In group-IV transition metal dichalcogenides such as WS2, MoS2, WSe2 and MoSe2, optical selection rules enable selective valley-excitation of excitons (electron-hole pairs), making them attractive for valleytronic applications. We use polarization sensitive spectroscopy to demonstrate that chemical doping ofWS2 using anisole changes the balance between neutral and charged excitons, and thereby the degree of valley polarization under optical excitation. We capture this behaviour by developing a rate equation model and show that optical quenching can increase valley polarization at the cost of exciton lifetimes (Chapter 3). These experiments are an important step towards NV-based detection of the magnetic moment associated with the valley index (Chapter 6).

A central challenge in NV magnetometry is minimizing the distance between sample and the sensor spins. We address this challenge by fabricating small 50 × 50 µm² diamond membranes that facilitate direct contact between sample and sensor (Chapter 2). We demonstrate their sensing potential by imaging uncompensatedmonolayer stray fields of Van der Waals interlayer antiferromagnet CrSBr (Chapter 4). Using the quantitative nature of NV measurements, we extract the CrSBr monolayer magnetization and Néel temperature. These results are an important stepping stone towards detecting spin waves, oscillations of the magnetic order, in Van derWaals magnets (Chapter 6).

Spin waves and their quanta, magnons, are promising signal carriers for next generation information devices that exploit their wave nature, non-reciprocal transport properties, and low intrinsic damping. An outstanding challenge is the efficient gating of spin-wave transport, which might be achieved by shaping their magnetic environment using diamagnetism. We employ diamond membranes to image spin waves in an yttrium iron garnet thin film as they travel underneath an optically opaque superconductor, to reveal hybridization of Meissner currents and spin wave modes (Chapter 5). We show that the superconductor modulates the spin-wave dispersion, enabling tuning of spin-wave transport using temperature, magnetic field and lasers. Finally, we suggest the use of superconductors to create magnonic cavities, crystals and spin-wave optics (Chapter 6).