On-chip electron spin resonance for quantum device applications

Electron Spin Resonance (ESR) is an essential technique for characterising materials with unpaired electrons. Improving the sensitivity of ESR measurements is a major research goal to bring the benefits of such characterisation to ever smaller or more dilute samples. This has been given added prominence by the rise in solid state quantum information processing technology with the concurrent investigation and manipulation of materials and surfaces in solid state devices at ever diminishing length scales. We establish a testbed system for the development of high-sensitivity ESR techniques for small samples at millikelvin temperatures. Our system is centred on a high Q niobium nitride planar superconducting resonator designed to have a concentrated mode volume to couple to a small amount of paramagnetic material, with resilience to magnetic fields of up to 400 mT. In our first set of measurements on our resonator `chip' we demonstrate high-cooperativity coupling between an organic radical microcrystal containing 10^{12} spins in a pico-litre volume, and our resonator mode, at 65 mK. Conventional ESR spectrometers by contrast ordinarily measure sample volumes a million times larger. We detect a saturation recovery relaxation rate via the dispersive frequency shift of the resonator. Techniques such as these could be suitable for reading out the quantum state of the spin ensemble in quantum information memory protocols. The second set of measurements presented here demonstrate the capability of the pulsed ESR spectrometer developed in this thesis. We use the custom-built framework to characterise a sample of rare earth-doped crystalline solid, potentially of interest as a quantum information storage medium, over 10 to 400 mK. This experiment also reaches the high-cooperativity regime, mediated by the high Q superconducting resonator. ESR measurements with even further enhanced coupling might eventually allow for pulsed ESR interrogation of very few spins and provide insights into the surface chemistry of, for example, material defects in superconducting quantum processors. As such, the framework is of interest for developing on-chip ESR techniques of the kind that could, in the future, enhance our understanding of the materials of solid state quantum devices.Open Acces

Direct visualization of magnetic vortex pinning in superconductors

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high resolution imaging techniques: Bitter decoration and scanning Hall probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field in agreement with the results of simulations. Real space vortex imaging has revealed that, in the onion state, the corners of the square rings act as effective pinning centers for vortices.Comment: To be published in Phys. Rev.

Aharonov-Bohm oscillations of a tunable quantum ring

With an atomic force microscope a ring geometry with self-aligned in-plane gates was directly written into a GaAs/AlGaAs-heterostructure. Transport measurements in the open regime show only one transmitting mode and Aharonov-Bohm oscillations with more than 50% modulation are observed in the conductance. The tuning via in-plane gates allows to study the Aharonov-Bohm effect in the whole range from the open ring to the Coulomb-blockade regime.Comment: 3 pages, 3 figure

Interacting electrons on a quantum ring: exact and variational approach

We study a system of interacting electrons on a one-dimensional quantum ring using exact diagonalization and the variational quantum Monte Carlo method. We examine the accuracy of the Slater-Jastrow -type many-body wave function and compare energies and pair distribution functions obtained from the two approaches. Our results show that this wave function captures most correlation effects. We then study the smooth transition to a regime where the electrons localize in the rotating frame, which for the ultrathin quantum ring system happens at quite high electron density.Comment: 19 pages, 10 figures. Accepted for publication in the New Journal of Physic

The detection of ultra-relativistic electrons in low Earth orbit

Aims. To better understand the radiation environment in low Earth orbit (LEO), the analysis of in-situ observations of a variety of particles, at different atmospheric heights, and in a wide range of energies, is needed. Methods. We present an analysis of energetic particles, indirectly detected by the Large Yield RAdiometer (LYRA) instrument on board ESA's Project for On-board Autonomy 2 (PROBA2) satellite as background signal. Combining Energetic Particle Telescope (EPT) observations with LYRA data for an overlapping period of time, we identified these particles as electrons with an energy range of 2 to 8 MeV. Results. The observed events are strongly correlated to geo-magnetic activity and appear even during modest disturbances. They are also well confined geographically within the L=4-6 McIlwain zone, which makes it possible to identify their source. Conclusions. Although highly energetic particles are commonly perturbing data acquisition of space instruments, we show in this work that ultra-relativistic electrons with energies in the range of 2-8 MeV are detected only at high latitudes, while not present in the South Atlantic Anomaly region.Comment: Topical Issue: Flares, CMEs and SEPs and their space weather impacts; 20 pages; 7 figures; Presented during 13th European Space Weather Week, 201

Lipid-bilayer-spanning DNA nanopores with a bifunctional porphyrin anchor

Holding tight: An artificial nanopore assembled from DNA oligonucleotides carries porphyrin tags (red), which anchor the nanostructure into the lipid bilayer. The porphyrin moieties also act as fluorescent dyes to aid the microscopic visualization of the DNA nanopore

“Service Encounter 2.0” : an investigation into the roles of technology, employees and customers

The service encounter – one of the foundational concepts in service research – is fundamentally changing due to rapid evolutions in technology. In this paper, we offer an updated perspective on what we label the “Service Encounter 2.0”. To this end, we develop a conceptual framework that captures the essence of the Service Encounter 2.0 and provides a synthesis of the changing interdependent roles of technology, employees, and customers. We find that technology either augments or substitutes service employees, and can foster network connections. In turn, employees and customers are taking on the role of enabler, innovator, coordinator and differentiator. In addition, we identify critical areas for future research on this important topic

DNA origami based assembly of gold nanoparticle dimers for surface-enhanced Raman scattering.

Plasmonic sensors are extremely promising candidates for label-free single-molecule analysis but require exquisite control over the physical arrangement of metallic nanostructures. Here we employ self-assembly based on the DNA origami technique for accurate positioning of individual gold nanoparticles. Our innovative design leads to strong plasmonic coupling between two 40 nm gold nanoparticles reproducibly held with gaps of 3.3 ± 1 nm. This is confirmed through far field scattering measurements on individual dimers which reveal a significant red shift in the plasmonic resonance peaks, consistent with the high dielectric environment due to the surrounding DNA. We use surface-enhanced Raman scattering (SERS) to demonstrate local field enhancements of several orders of magnitude through detection of a small number of dye molecules as well as short single-stranded DNA oligonucleotides. This demonstrates that DNA origami is a powerful tool for the high-yield creation of SERS-active nanoparticle assemblies with reliable sub-5 nm gap sizes