| 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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| 4. |
- Tornsö, Marcus, 1993
(författare)
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Holographic descriptions of collective modes in strongly correlated media
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solving the puzzle of high temperature superconductivity may be one of the most desired scientific breakthroughs of our time, as access to room temperature superconductivity could revolutionize society as we know it. In this thesis, we strive to increase the theoretical understanding of such matter, by studying the phase above, in temperature, the superconducting phase - the "strange metal". The strange metal phase is a phase characterized by the absence of a quasi-particle description. The electrons in this phase are strongly coupled, which means that conventional methods, such as perturbation theory in quantum field theory and Monte Carlo methods fall short of being able to describe their dynamics. Perhaps surprisingly, string theory provides a different method, capable of describing precisely such systems - the holographic duality. Whereas there has been significant effort devoted to the applications of the duality since its inception in 1997, and even more so in the last decade after it was observed that it worked remarkably well for condensed matter theory, it wasn't until our project that the dynamical polarization of such strongly coupled systems where properly treated. In this thesis, we introduce the minimal constraints required for a sensible description of a polarizing medium, and convert those to boundary conditions to the equations of motion provided by the holographic dual. These boundary conditions deviate from previous holographic studies, and we contrast the quasinormal modes previously studied with the emergent collective modes we find for some different models. We find novel 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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| 5. |
- Laraña Aragón, Jorge, 1993-
(författare)
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Linear response theory : from black hole thermalization to Weyl semimetals
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Linear response theory is an incredibly powerful calculation tool. We apply this framework in quantum field theory to a variety of models originated from distinct areas in theoretical physics and for different reasons. In the context of black hole holography, we consider a quench model where we investigate effective thermalization as well as the boundary signal of the so called evanescent modes which indicate the presence of a black hole like object in the bulk. The problem of quantum thermalization plays a central role within the holographic duality between thermal states in the boundary field theory and black hole like objects in the bulk. However, quantum thermalization is also an interesting question in itself from a fundamental point of view and with that motivation we continue to explore this phenomenon further. Inspired by recent progress in understanding how operators in quantum field theories thermalize, which occurs even when considering integrable models, we investigate the so called operator thermalization hypothesis. We focus on gauge theories at finite temperature with a large number of fields which present a phase transition between the low-temperature and high-temperature regimes. In particular, these theories are the so called vector model and the adjoint matrix model. Last, within the common background of linear response theory we investigate transport properties in a family of Weyl semimetal systems. Concretely, we develop a general analytic method to compute the magneto-optical conductivity of these systems in the presence of an external magnetic field aligned with the tilt of the spectrum.
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| 6. |
- 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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| 7. |
- 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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| 8. |
- Holmvall, Patric, 1988
(författare)
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Modeling mesoscopic unconventional superconductors
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-temperature superconducting materials are often experimentally realized as thin films that can be patterned into devices operating in the mesoscopic regime. On this length scale, various finite-size and surface effects heavily influence the nature of the superconducting state, and can induce new ground states with spontaneously broken symmetries. Motivated by the wide technological application of such mesoscopic devices and the many open questions regarding the new emergent ground states, this thesis sets out to study mesoscopic grains. In particular, a recently discovered phase which spontaneously breaks translational and time-reversal symmetries will be studied, referred here to as the "loop-current phase". The aim is to study how this phase responds to magnetic and geometric perturbations. The quasiclassical theory of superconductivity is used to simulate mesoscopic thin-film grains in equilibrium, with a strong emphasis on d-wave superconductors, e.g. the cuprates. The properties of the loop-current phase are cataloged, with an explanation of how and why it occurs. Various phase diagrams are produced, and the magnetic-field dependent thermodynamics is studied.In conclusion, the loop-current phase occurs at pairbreaking interfaces that host quasiparticle midgap states. The phase is associated with a spontaneous superfluid momentum which drives circulating current loops that break continuous translational symmetry, providing an energetically favorable Doppler shift of the midgap states. The phase is found to be robust against external fields in the whole Meissner state, but not against very high fields in the mixed state. The phase is lost when there is a competing effect which significantly broadens the spectrum, e.g. a strong external vector potential. The phase transition is associated with a large jump in the heat capacity, serving as a hallmark for the phase to be observed experimentally. It is predicted that the phase leads to a broadening of the spectrum which is consistent with experimental findings.
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| 9. |
- 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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| 10. |
- 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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