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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 6. |
- Harrysson Rodrigues, Isabel, 1993
(författare)
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Cryogenic InP High Electron Mobility Transistors in a Magnetic Field
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The InGaAs-InAlAs-InP high electron mobility transistor (InP HEMT) is the preferred active device used in a cryogenic low noise amplifier (LNA) for sensitive detection of microwave signals. In this thesis it is demonstrated that the InP HEMT, when placed in a magnetic field, has a strong angular dependence in its output current. This has important implications for the alignment of cryogenic InP HEMT LNAs in microwave detection experiments involving magnetic fields. InP HEMTs with various gate lengths and gate widths have been fabricated and characterized. The output current for the InP HEMTs was measured in static magnetic fields up to 14 T at different angles in an ambient temperature of 2 K. The results showed that for all InP HEMT devices placed in a perpendicular arrangement, the output current is drastically suppressed. It is shown that the reduction in output current is negligible once placed parallel to the applied field. Furthermore, it was found that the output current strongly depends on the angle between the current direction and the magnetic field. In the investigated device geometry, the output current in the InP HEMT is limited by geometrical magnetoresistance. This was expressed in an output current equation which showed excellent agreement with measured data as a function of angle and magnetic field. Device parameters such as transconductance and on-resistance were found to be significantly affected even at small angles and magnetic fields. A 0.3-14 GHz cryogenic LNA module based on the same transistor technology used in device experiments was measured in a perpendicular magnetic field at 2 K. The LNA was heavily degraded in gain and average noise temperature already up to 1.5 T. In comparison with previous work reported for GaAs single-heterojunction HEMT LNAs, it is shown here that the effect is much stronger for InP HEMT cryogenic LNAs.
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| 7. |
- Armakavicius, Nerijus
(författare)
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Study of novel electronic materials by mid-infrared and terahertz optical Hall effect
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Development of silicon based electronics have revolutionized our every day life during the last three decades. Nowadays Si based devices operate close to their theoretical limits that is becoming a bottleneck for further progress. In particular, for the growing field of high frequency and high power electronics, Si cannot offer the required properties. Development of materials capable of providing high current densities, carrier mobilities and high breakdown fields is crucial for a progress in state of the art electronics.Epitaxial graphene grown on semi-insulating silicon carbide substrates has a high potential to be integrated in the current planar device technologies. High electron mobilities and sheet carrier densities make graphene extremely attractive for high frequency analog applications. One of the remaining challenges is the interaction of epitaxial graphene with the substrate. Typically, much lower free charge carrier mobilities, compared to free standing graphene, and doping, due to charge transfer from the substrate, is reported. Thus, a good understanding of the intrinsic free charge carriers properties and the factors affecting them is very important for further development of epitaxial graphene.III-group nitrides have been extensively studied and already have proven their high efficiency as light sources for short wavelengths. High carrier mobilities and breakdown electric fields were demonstrated for III-group nitrides, making them attractive for high frequency and high power applications. Currently, In-rich InGaN alloys and AlGaN/GaN high electron mobility structures are of high interest for the research community due to open fundamental questions.Electrical characterization techniques, commonly used for the determination of free charge carrier properties, require good ohmic and Schottky contacts, which in certain cases can be difficult to achieve. Access to electrical properties of buried conductive channels in multilayered structures requires modification of samples and good knowledge of the electrical properties of all electrical contact within the structure. Moreover, the use of electrical contacts to electrically characterize two-dimensional electronic materials, such as graphene, can alter their intrinsic properties. Furthermore, the determination of effective mass parameters commonly employs cyclotron resonance and Shubnikov-de Haas oscillations measurements, which require long scattering times of free charge carriers, high magnetic fields and low temperatures.The optical Hall effect is an external magnetic field induced optical anisotropy in conductive layers due to the motion of the free charge carriers under the influence of the Lorentz force, and is equivalent to the electrical Hall effect at optical frequencies. The optical Hall effect can be measured by generalized ellipsometry and provides a powerful method for the determination of free charge carrier properties in a non-destructive and contactless manner. In principle, a single optical Hall effect measurement can provide quantitative information about free charge carrier types, concentrations, mobilities and effective mass parameters at temperatures ranging from few kelvins to room temperature and above. Further, it was demonstrated that for transparent samples, a backside cavity can be employed to enhance the optical Hall effect.Measurement of the optical Hall effect by generalized ellipsometry is an indirect technique requiring subsequent data analysis. Parameterized optical models are fitted to match experimentally measured ellipsometric data by varying physically significant parameters. Analysis of the optical response of samples, containing free charge carriers, employing optical models based on the classical Drude model, which is augmented with an external magnetic field contribution, provide access to the free charge carrier properties.The main research results of the graduate studies presented in this licentiate thesis are summarized in the five scientific papers.Paper I. Description of the custom-built terahertz frequency-domain spectroscopic ellipsometer at Linköping University. The terahertz ellipsometer capabilities are demonstrated by an accurate determination of the isotropic and anisotropic refractive indices of silicon and m-plane sapphire, respectively. Further, terahertz optical Hall effect measurements of an AlGaN/GaN high electron mobility structures were employed to extract the two-dimensional electron gas sheet density, mobility and effective mass parameters. Last, in-situ optical Hall effect measurement on epitaxial graphene in a gas cell with controllable environment, were used to study the effects of environmental doping on the mobility and carrier concentration.Paper II. Presents terahertz cavity-enhanced optical Hall measurements of the monolayer and multilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed p-type doping for monolayer graphene with a carrier density in the low 1012 cm−2 range and a carrier mobility of 1550 cm2/V·s. For the multilayer epitaxial graphene, n-type doping with a carrier density in the low 1013 cm−2 range, a mobility of 470 cm2/V·s and an effective mass of (0.14 ± 0.03) m0 were extracted. The measurements demonstrate that cavity-enhanced optical Hall effect measurements can be applied to study electronic properties of two-dimensional materials.Paper III. Terahertz cavity-enhanced optical Hall effect measurements are employed to study anisotropic transport in as-grown monolayer, quasi free-standing monolayer and quasi free-standing bilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed a strong anisotropy in the carrier mobilities of the quasi freestanding bilayer graphene. The anisotropy is demonstrated to be induced by carriers scattering at the step edges of the SiC, by showing that the mobility is higher along the step than across them. The scattering mechanism is discussed based on the results of the optical Hall effect, low-energy electron microscopy, low-energy electron diffraction and Raman measurements.Paper IV. Mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect measurements are employed to determine the electron effective mass in an In0.33Ga0.67N epitaxial layer. The data analysis reveals slightly anisotropic effective mass and carrier mobility parameters together with the optical phonon frequencies and broadenings.Paper V. Terahertz cavity-enhanced optical Hall measurements are employed to study the free charge carrier properties in a set of AlGaN/GaN high electron mobility structures with modified interfaces. The results show that the interface structure has a significant effect on the free charge carrier mobility and that the sample with a sharp interface between an AlGaN barrier and a GaN buffer layers exhibits a record mobility of 2332±73 cm2/V·s. The determined effective mass parameters showed an increase compared to the GaN value, that is attributed the the penetration of the electron wavefunction into the AlGaN barrier layer.
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| 8. |
- Brem, Samuel, 1991
(författare)
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Microscopic Mechanisms of the Formation, Relaxation and Recombination of Excitons in Two-Dimensional Semiconductors
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Monolayers of Transition Metal Dichalcogenides (TMDs) present a giant leap forward towards the realization of semiconductor devices with atomic scale thickness. As a natural consequence of their two-dimensional character TMDs exhibit a reduced dielectric screening, leading to the formation of unusually stable excitons, i.e. Coulomb-bound electron-hole pairs. Excitons dominate the optical response as well as the ultrafast dynamics in TMDs. As a result, a microscopic understanding of excitons, their formation, relaxation and decay dynamics becomes crucial for a technological application of TMDs. A detailed theoretical picture of the internal structure of excitons and their scattering channels allows for a controlled manipulation of TMD properties enabling an entire new class of light emitters and detectors. The aim of this thesis is to investigate the many-particle processes governing the ultrafast dynamics of excitons. The focus is to provide a sophisticated picture of exciton-phonon and exciton-photon interaction mechanisms and the impact of dark exciton states starting from the formation of bound excitons out of a free electron-hole gas up to the eventual radiative decay of bright and dark exciton populations. Based on an equations-of-motion approach for the density matrix of an interacting electron, phonon and photon system, we simulate the dynamics of excitons in TMDs across the full Rydberg-like series of bright and dark states. Our theoretical model allows us to predict fundamental relaxation time scales as well as spectral features accessible in multiple spectroscopic experiments, such as absorption, photoluminescence and ultrafast pump-probe. In particular we predict intriguing features appearing in the terahertz absorption spectrum during the formation of excitons as well as distinct -so far unexplained- low temperature luminescence features stemming from phonon-assisted recombinations of dark excitons.
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| 9. |
- Dashti, Nastaran, 1988
(författare)
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Charge and Energy Noise from On-demand Electron Sources
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- On-demand single electron sources (SES) are of key importance for future electronic applications such as metrology or quantum optics with electron. They allow for achieving a controlled, low-fluctuations flow of particles in a coherent mesoscopic conductor. One way to characterize the precision and spectrum of the injected single-particle state from these sources is to study correlations of charge- and energy currents. We analyze a prominent example for such single-electron sources which is the emission of single electrons from a driven mesoscopic capacitor in the quantum- Hall regime. By employing the Floquet scattering approach, we study the features of this source in charge- and energy-current noise. Whereas the charge-current noise is proportional to the number of emitted particles, the energy-current noise is sensitive to properties of the driving potential. When the mesoscopic capacitor is driven slowly, we compare its features with the application of a Lorentzian- shaped, time-dependent potential to a coherent conductor. Both sources emit exactly the same pulse but with different type and number of particles. In contrast to charge currents, energy currents and their fluctuations are more difficult to access experimentally. We theoretically propose a setup for the detection of fluctuating charge and energy currents, as well as their correlations, generated by an arbitrary time-dependently driven electronic source. Employing the Boltzmann-Langevin approach, we show that these fluctuations are detectable through a read-out of frequency-dependent temperature and electrochemical-potential fluctuations. We discuss the feasibility of our detection scheme for a concrete example of the mesoscopic capacitor setup in the quantum Hall regime. Finally, we review different, experimental-related aspects that should be taken into account when optimizing the proposed detection scheme.
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| 10. |
- Ferreira, Beatriz, 1996
(författare)
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Microscopic Modeling of Exciton-Polaritons in Two-Dimensional Materials
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Integrating 2D materials into high-quality optical microcavities opens the door to fascinating many-particle phenomena including the formation of exciton-polaritons. These are hybrid quasi-particles inheriting properties of both the constituent photons and excitons. The corresponding change in the dispersion relation has a large impact on the optics, dynamics and transport behaviour of the materials. In this thesis, we aim to microscopically understand the optical response and propagation of exciton-polaritons in transition metal dichalcogenides (TMDs). The theoretical method is based on the density matrix formalism combined with the Hopfield approach. In particular, we investigate how the diffusion of exciton changes in the strong coupling regime, i.e. when exciton-polaritons are formed. Furthermore, we study the impact of dark excitons on the optical response of upper and lower polariton branches in absorption spectra of molybdenum- and tungsten-based TMDs, which are known to be direct and indirect semiconductors, respectively. Furthermore, we show how different experimentally accessible quantities, such as temperature or mirror reflectance, can be exploited to tune the optical response of polaritons. Our study contributes to an improved microscopic understanding of exciton-polaritons and their interaction with phonons, potentially suggesting experiments that could determine the energy of dark exciton states via momentum-resolved polariton absorption.
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