| 1. |
- Alegret, Joan, 1977
(författare)
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Numerical Simulations of Plasmonic Nanostructures
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the study of metallic nanostructures that support plasmons. Special emphasis is devoted to two specific numerical methods that allow us to predict plasmon characteristics: the discrete dipole approximation (DDA) and the Green's tensor (GT) method.DDA is an approximate method that produces fast and accurate results, but it can only be applied to systems in which the nanostructure is situated in a homogeneous background. In this thesis, DDA has been applied to predict the field enhancement and field decay around nano-rings, showing that the structure is well suited for biosensing; to obtain the spectral characteristics of silver trimers, showing that the actual plasmon modes are closely related to symmetry-adapted coordinates derived from group-theory; and to calculate the optical forces between two spherical particles illuminated by a plane wave, showing that the illumination wavelength determines the separation between the particles.The GT method, on the other hand, is an exact method, in the sense that the system can be solved to arbitrary precision depending on the size of the discretization elements. Its major drawback is the long time it takes to perform the calculations. To tis end, this thesis introduces a novel algorithm, called the top-down extended meshing algorithm (TEMA), that speeds up GT calculations by reducing the number of elements in the discretization process. This decreases the total time needed to perform the calculations, while keeping the precision of the result essentially unaltered. The GT method with TEMA meshes has successfully been used to study single holes of different sizes and shapes (circular and ellipsoidal) in the near- and far-field regime, as well as hole pairs as a function of their separation distance. The results compare very well with experiments, demonstration that the GT method is well suited for predicting the behavior of nano-holes.
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| 2. |
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| 3. |
- Warren, Christopher, 1992
(författare)
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Benchmarking and Metrology of Scaled Superconducting Quantum Processors
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ultimate goal of quantum computing is to develop quantum algorithms and hardware that outperform any classical methods. However, noise in quantum systems hinders their direct implementation. Achieving universal quantum computing necessitates a fault-tolerant quantum computer, which requires thousands of physical qubits. This thesis explores whether our architecture can overcome these challenges and scale to the required number of qubits. Superconducting quantum circuits are a highly developed platform for building quantum computers, leveraging advanced device design and fabrication technology that can scale rapidly to hundreds or thousands of qubits. Our architecture features fixed-frequency qubits connected by tunable couplers, operating at very low temperatures (∼10 mK). Qubits are controlled using radio-frequency electromagnetic fields, while magnetic fields parametrically modulate the couplers to enable interactions between qubits. There are many axes along which one can scale to larger system sizes. The most commonly approached axis is by developing high-coherence quantum hardware. Coherence times determines the memory/operational lifetime of quantum information. Our fabrication has allowed us to achieve multi-qubit processors with coherence times over 100 µs. However, coherence times are not without a context, as we also require fast gate times. The control of quantum hardware is a second direction towards scaling; minimizing the time to implement a logical operation relative to the coherence times of the device. In our processors, we are able to implement two-qubit operations with < 1% error in 250 ns, with which we implemented two quantum algorithms to infer the performance of our architecture. Moreover we improve the readout accuracy in our architecture by artificially extending the lifetime of the qubit during measurement through a state shelving scheme. A third, often overlooked axis for scaling quantum hardware is expanding the native logical gate set. Typically, quantum processors use a limited set of operations. We developed a technique to implement a native three-qubit gate by simultaneously applying our two-qubit operations, expanding the gate set without altering the architecture. This demonstrated coherence-limited performance and enabled faster generation of highly entangled states compared to using only two-qubit operations. Although our parametric architecture offers advantages for scaling, significant challenges remain, particularly in maintaining coherence, minimizing crosstalk, and ensuring device yield as qubit numbers increase. This thesis explores the limitations and obstacles in scaling superconducting quantum processors, using experimental data and theoretical models. We address key issues with the parametric gate, such as frequency crowding and crosstalk, and discuss the fabrication tolerances needed to scale to a 100-qubit system.
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| 4. |
- Eriksson, Martin, 1989
(författare)
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There's Plenty of Room in Higher Dimensions - Nonlinear Dynamics of Nanoelectromechanical Systems
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanoelectromechanical systems (NEMS) couple the dynamics of electrons to vibrating nanostructures such as suspended beams or membranes. These resonators can be used in for instance nanoelectronics and sensor applications. NEMS are also of fundamental interest since electrons exhibit strong quantum effects when confined in nanoobjects. Furthermore, NEMS such as graphene resonators are strongly nonlinear, which opens the door for complex dynamical response. The operation of nanoresonators often rely on actuation of mechanical vibrations driven by an electric ac-field. The first part of this thesis theoretically investigates high-frequency nonresonant actuation relying on electromechanical back action (Papers I-II). The nonresonant phenomenon can be utilized to study nonlinear dissipation and to selectively actuate different vibrational modes, also asymmetric ones, even though the driving field is homogeneous (Paper III). Another nonresonant actuation mechanism converts heat into mechanical energy and relies on electron-electron interaction in a movable quantum dot (Paper IV).Furthermore, parametric actuation of a nanoresonator can be used to generate a supercurrent through a superconducting weak link even though the superconducting phase difference across the link is zero (Paper V). The excitation leads to a spontaneous symmetry breaking, which allows for a new possibility to switch between the two current directions.Actuation of mechanical vibrations is also used to study nonlinear dynamics and mode coupling in nanoresonators. The strength of nonlinearities and vibrational frequencies can be tuned by electrostatic means (Paper VI). This tunability and the low dissipation in nanoresonators make it possible to selectively address individual or combinations of modes. Coupled modes allow for much richer nonlinear dynamics, such as internal resonances (Paper VII), due to the increased dimensionality of the relevant phase space. Furthermore, exotic dynamical regions may be hidden and not observed in standard experiments. However, bifurcation theory can help to construct maps which reveal the hidden regions. A lot more is therefore to be expected from coupled mode dynamics, since there’s plenty of room in higher dimensions.
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| 5. |
- Schmidt, Falko, 1992
(författare)
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Active Matter in a Critical State: From passive building blocks to active molecules, engines, and active droplets
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The motion of microscopic objects is strongly affected by their surrounding environment. In quiescent liquids, motion is reduced to random fluctuations known as Brownian motion. Nevertheless, microorganisms have been able to develop mechanisms to generate active motion. This has inspired researchers to understand and artificially replicate active motion. Now, the field of active matter has developed into a multi-disciplinary field, with researchers developing artificial microswimmers, producing miniaturized versions of heat engines and showing that individual colloids self-assemble into larger microstructures. This thesis taps into the development of artificial microscopic and nanoscopic systems and demonstrates that passive building blocks such as colloids are transformed into active molecules, engines and active droplets that display a rich set of motions. This is achieved by combining optical manipulation with a phase-separating environment consisting of a critical binary mixture. I first show how simple absorbing particles are transformed into fast rotating microengines using optical tweezers, and how this principle can be scaled down to nanoscopic particles. Transitioning then from single particles to self-assembled modular swimmers, such colloidal molecules exhibit diverse behaviour such as propulsion, orbital rotation and spinning, and whose formation process I can control with periodic illumination. To characterize the molecules dynamics better, I introduce a machine-learning algorithm to determine the anomalous exponent of trajectories and to identify changes in a trajectory’s behaviour. Towards understanding the behaviour of larger microstructures, I then investigate the interaction of colloidal molecules with their phase-separating environment and observe a two-fold coupling between the induced liquid droplets and their immersed colloids. With the help of simulations I gain a better physical picture and can further analyse the molecules’ and droplets’ emergence and growth dynamics. At last, I show that fluctuation-induced forces can solve current limitations in microfabrication due to stiction, enabling a further development of the field towards smaller and more stable nanostructures required for nowadays adaptive functional materials. The insights gained from this research mark the path towards a new generation of design principles, e.g., for the construction of flexible micromotors, tunable micromembranes and drug delivery in health care applications.
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| 6. |
- Tornsö, Marcus, 1993
(författare)
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Plasma Oscillations in Holographic Quantum Matter
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we explore strongly correlated matter in the framework of holographic duality. Specifically, we examine the quasinormal modes of such systems, and we extend the current framework to efficiently and naturally cover plasmons and other collective modes that may be found within strongly correlated matter. The interest in strongly correlated matter is motivated by the presence of a “strange metal” phase both in high temperature superconductors and in near charge neutral graphene, both being materials of immense scientific interest. The strange metal phase is a phase characterized by the absence of quasi-particles. This implies that conventional methods, such as perturbation theory in quantum field theory and Monte Carlo methods fall short of being able to describe the dynamics. Perhaps surprisingly, string theory provides a novel method, capable of precisely describing such systems - the holographic duality. With the holographic duality, strongly coupled matter is mapped onto a weakly coupled gravity theory in one additional dimension, allowing for a conventional treatment of the dual system. In this thesis, we extend the existing framework to also describe polarizing media. This is explicitly done in the form of new boundary conditions on the holographic dual, which deviate from previous holographic studies, and we contrast the quasinormal modes previously studied with the emergent collective modes we find for some studied models. We find new results, as well as confirm the predictions of less general models in their respective regions of validity and pave the way for more complex future models.
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| 7. |
- Urdshals, Einar, 1995
(författare)
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Dark matter electron interactions in detector materials
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dark Matter (DM) makes up 85% of the matter content of the universe, and its gravitational effects are seen on scales ranging from that of cosmology to that of galactic astrophysics. The nature of DM is, however, unknown. Study- ing DM in the lab with a class of experiments called direct detection (DD) experiments is key to understanding its properties. For decades, experiments have been attempting to do this through searches for DM induced nuclear recoils. These have not been found, and a possible reason for this is that the hypothetical DM particle is too light to induce nuclear recoils. Therefore, in the last decade experiments have been built to study DM through electron recoils instead. As the electron is 4 orders of magnitude lighter than the nu- cleus, electron recoils can be induced by DM down to 4 orders of magnitude lighter than the lightest DM particle probeable with nuclear recoils. In order to understand current and upcoming results from experiments searching for DM induced electron recoils, a theoretical understanding of DM electron scatterings in detector materials is needed. When modelling such electron interactions, one need input both from DM and material physics. This thesis improves the theoretical understanding by both improving the material description using density functional theory (DFT), and by extending the DM description using non-relativistic effective theory (NR-EFT) tools. The improvement gives not only a more accurate description of the DM- electron interactions that the experiments are expected to see; it also vastly extends the forms of DM that can be studied in direct detection experiments. Before this extension, one typically focused on a benchmark case of DM, the so called dark photon model. With this extension, one can cover all forms of gravitationally bound DM with spins of 0, 1/2 or 1. In the included works, advances are made in the description of DM-electron interactions in common detector materials such as liquid xenon, silicon and germanium, as well as to materials in the research and development phase, such as graphene and carbon nanotubes (CNTs).
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| 8. |
- Zema, Vanessa, 1991
(författare)
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Unveiling the nature of dark matter with direct detection experiments
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The desire of discovery is an anthropic need which characterises and connects the human being over the eras. In particular, observing the sky is an instinctive drive exerted by the curiosity of the mysteries which it retains. At the present time, the tremendous advances in the exploration of space have opened even more challenges than back in the days. One of the most urgent question is unveiling the nature of dark matter (DM). As stated by Neta A. Bahcall (Professor at Princeton University), "Cosmology has revealed an amazing universe, filled with a "dark sector" that composes 95% of the energy density of our cosmos [...]" ( Dark matter universe , PNAS, 2015). About one-third of this dark sector is associated to an invisible and still undetected form of matter, the so-called dark matter, whose gravitational effect manifests at all cosmological scales. Both theoretical and experimental observations based on ordinary gravity reinforced the evidences for the existence of DM, since its first appearance in the pioneering calculations of F. Zwicky (1933). This PhD project explores the hypothesis that DM is made of new particles beyond the standard model. More specifically, it focuses on those DM particles which are trapped into the galactic gravitational field and populate the galactic halo. If DM interacts with ordinary particles, extremely sensitive detectors operating in very low-background environments, are expected to detect galactic DM particles scattering off their target material. This widely employed experimental technique is known as DM direct detection and it is the focus of my studies, where I consider the further hypothesis that DM interacts with atomic nuclei. The research I conducted during my PhD program consists of two main parts: the first part focused on purely phenomenology aspects of the DM direct detection (namely on the DM annual modulation treated using a non-relativistic effective theory and on the scattering of spin-1 DM particles off polarised nuclei) and the second one is more closely connected to experimental applications. The latter has been strongly stimulated by my collaboration with the two DM direct detection experiments CRESST and COSINUS. For CRESST, I compute the DM-nucleus cross-section for the conventional spin-dependent interactions, used to analyse the data collected with a prototype Li-based detector module, and I derive some prospects for a time dependent analysis of CRESST-III data, using a statistical frequentist approach based on Monte Carlo simulations. For COSINUS, I provide a significant extension of the pulse shape model currently used by CRESST and COSINUS in order to explain experimental observations related to the COSINUS detector response. Finally, I contribute to ongoing studies on the phonon propagation in NaI crystals based on solid state physics. This PhD thesis has been oriented to fill the gap between theoretical and experimental efforts in the DM field. This approach has facilitated the exchange of expertise, has driven the trend of my research and has stimulated the development of the ideas and methods described in this PhD thesis.
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| 9. |
- Bergmann, Michael Alexander, 1989
(författare)
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Thin-film ultraviolet light-emitting diodes realized by electrochemical etching of AlGaN
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ultraviolet (UV) light sources have a direct impact on everyone’s life. They are used to sterilize surfaces as well as for water purification. In addition, they are used in green houses to enhance health-promoting substances in plants, for phototherapy to treat skin diseases, for sensing and material curing. Today, most of these applications use mercury lamps that are fragile, bulky and toxic. AlGaN-based UV light-emitting diodes (LEDs) have the potential to solve all these issues, but their implementation has been limited due to their low electrical to optical power conversion efficiency (PCE) being below 10%. Blue-emitting GaN-based LEDs have already found their way into everyone’s home through general lighting. This was made possible by the tremendous performance improvements, reaching PCEs close to 90%. Unfortunately, the device concepts for achieving highly efficient GaN-based LEDs, such as the thin-film flip-chip (TFFC) design that can greatly improve light-extraction efficiency, are not easily transferred to AlGaN-based UV LEDs. In this work, we demonstrate a new device platform to realize UV LEDs with a TFFC design based on electrochemical etching to remove the substrate. In the first part of this work, electrochemical (EC) etching of AlGaN layers with a high Al content up to 50% was demonstrated, which enabled the separation of epitaxial LED layers from their substrate while maintaining the high quality of the active region. The second key technological step was the integration of EC etching in a standard UV LED fabrication process, which required protection schemes to prevent parasitic electrochemical etching of the LED structure and the development of a device design compatible with flip-chip bonding. Finally, this work was completed by the first demonstration of a TFFC UVB LED using electrochemical etching.
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| 10. |
- Dashti, Nastaran, 1988
(författare)
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Heat Transport from On-demand Single-Electron Sources
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The controlled injection of quantized charge excitations from single-electron emitters into nanoscopic conductors sets the basis for many important applications ranging from metrology to the emerging field of quantum optics with electrons. Successful implementation of these applications relies not only on achieving control on the precision of the particle emission, but also on the energetic properties of the injected particles. These fundamental properties are reflected in transport observables such as time-resolved charge and energy current, as well as the spectral, i.e. energy-resolved current, or the zero-frequency correlators of charge and energy currents, thereby providing a tool for transport spectroscopy. This thesis deals with two important aspects of the characterization of different time-dependently driven single-electron sources (SES): it provides (i) a detailed analysis of the aforementioned observables and (ii) proposals for the readout of such transport properties. First, we analyze in detail the transport observables in three different SESs. The SESs differ by the characteristics of the applied time-dependent driving voltage and by the degree of particle confinement in the driven conductor; their common feature is that pulses of quantized charge are produced going along with a minimal excitation of the Fermi sea. We point out the impact of the device design and of tunable external pa- rameters, such as temperature, on the transport observables. Second, we the- oretically propose ways to experimentally access the transport observables. Charge transport observables are standardly detected for different kinds of sources. In contrast, energy transport–particularly energy-current noise– is more difficult to access experimentally. We propose a setup for the detection of fluctuating charge and energy currents, as well as their correlations, generated by a SES, via reading out frequency-dependent electrochemical- potential and temperature fluctuations in a probe contact. Furthermore, in a second proposal, we investigate how to access the spectral current, giving access to the particles’ energy distribution, in an energy-selective detector setup. More specifically, we propose to readout modifications of thermoelectric response coefficients due to the time-dependent driving as a measure of the spectral current. However, importantly, this type of setup also opens completely novel routes: we find that SESs can be used as probes to sense until now unexplored quantum screening effects in thermoelectric transport.
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