| 31. |
- Johannesson, Henrik, 1953, et al.
(författare)
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Conformal field theory approach to the two-channel Anderson model
- 2004
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 272-276:SUPPL. 1
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Tidskriftsartikel (refereegranskat)abstract
- The two-channel Anderson impurity model serves as a prototype for describing heavy-fermion materials with a possible mixed-valent regime with both quadrupolar and magnetic character. We report on the low-energy physics of the model, using a conformal field theory approach with exact Bethe Ansatz results as input.
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| 32. |
- 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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| 33. |
- 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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| 34. |
- Kuttruff, Joel, et al.
(författare)
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Magneto-Optical Activity in Nonmagnetic Hyperbolic Nanoparticles
- 2021
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 127
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Tidskriftsartikel (refereegranskat)abstract
- Active nanophotonics can be realized by controlling the optical properties of materials with external magnetic fields. Here, we explore the influence of optical anisotropy on the magneto-optical activity in nonmagnetic hyperbolic nanoparticles. We demonstrate that the magneto-optical response is driven by the hyperbolic dispersion via the coupling of metallic-induced electric and dielectric-induced magnetic dipolar optical modes with static magnetic fields. Magnetic circular dichroism experiments confirm the theoretical predictions and reveal tunable magneto-optical activity across the visible and near infrared spectral range.
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| 35. |
- Ali, M., et al.
(författare)
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Reduced photoluminescence from InGaN/GaN multiple quantum well structures following 40 MeV iodine ion irradiation
- 2009
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Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 404:23-24, s. 4925-4928
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Tidskriftsartikel (refereegranskat)abstract
- The effects following ion irradiation of GaN-based devices are still limited. Here we present data on the photoluminescence (PL) emitted from InGaN/GaN multiple quantum well (MQW) structures, which have been exposed to 40 MeV I ion irradiation. The PL is reduced as a function of applied ion fluence, with essentially no PL signal left above 1011 ions/cm2. It is observed that even the ion fluences in the 109 ions/cm2 range have a pronounced effect on the photoluminescence properties of the MQW structures. This may have consequences concerning application of InGaN/GaN MQW’s in radiation-rich environments, in addition to defect build-up during ion beam analysis.
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| 36. |
- Alizadehheidari, Mohammadreza, 1987, et al.
(författare)
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Nanoconfined Circular and Linear DNA: Equilibrium Conformations and Unfolding Kinetics
- 2015
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 48:3, s. 871-878
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Tidskriftsartikel (refereegranskat)abstract
- Studies of circular DNA confined to nanofluidic channels are relevant both from a fundamental polymer-physics perspective and due to the importance of circular DNA molecules in vivo. We here observe the unfolding of confined DNA from the circular to linear configuration as a light-induced double-strand break occurs, characterize the dynamics, and compare the equilibrium conformational statistics of linear and circular configurations. This is important because it allows us to determine to what extent existing statistical theories describe the extension of confined circular DNA. We find that the ratio of the extensions of confined linear and circular DNA configurations increases as the buffer concentration decreases. The experimental results fall between theoretical predictions for the extended de Gennes regime at weaker confinement and the Odijk regime at stronger confinement. We show that it is possible to directly distinguish between circular and linear DNA molecules by measuring the emission intensity from the DNA. Finally, we determine the rate of unfolding and show that this rate is larger for more confined DNA, possibly reflecting the corresponding larger difference in entropy between the circular and linear configurations.
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| 37. |
- Anderson, Johan, 1973, et al.
(författare)
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The probability density function tail of the Kardar–Parisi–Zhang equation in the strongly nonlinear regime
- 2016
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Ingår i: Journal of Physics A: Mathematical and Theoretical. - : IOP Publishing. - 1751-8121 .- 1751-8113. ; 49:50, s. 505001-
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Tidskriftsartikel (refereegranskat)abstract
- An analytical derivation of the probability density function (PDF) tail describing the strongly correlated interface growth governed by the nonlinear Kardar–Parisi–Zhang equation is provided. The PDF tail exactly coincides with a Tracy–Widom distribution i.e. a PDF tail proportional to exp(-cw_2^(3/2)), where w_2 is the the width of the interface. The PDF tail is computed by the instanton method in the strongly non-linear regime within the Martin–Siggia– Rose framework using a careful treatment of the non-linear interactions. Inaddition, the effect of spatial dimensions on the PDF tail scaling is discussed. This gives a novel approach to understand the rightmost PDF tail of the interface width distribution and the analysis suggests that there is no uppercritical dimension.
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| 38. |
- Balabanov, Oleksandr, 1991, et al.
(författare)
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Topology of critical chiral phases: Multiband insulators and superconductors
- 2021
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Ingår i: Physical Review Research. - 2643-1564. ; 3:4
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Tidskriftsartikel (refereegranskat)abstract
- Recent works have proved the existence of symmetry-protected edge states in certain one-dimensional topological band insulators and superconductors at the gap-closing points which define quantum phase transitions between two topologically nontrivial phases. We show how this picture generalizes to multiband critical models belonging to any of the chiral symmetry classes AIII, BDI, or CII of noninteracting fermions in one dimension.
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| 39. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Electrochemical etching of AlGaN for the realization of thin-film devices
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 115:18, s. 182103-
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Tidskriftsartikel (refereegranskat)abstract
- Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 µm x 1 µm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.
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| 40. |
- Bergqvist, Lars, et al.
(författare)
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Atomistic spin dynamics of low-dimensional magnets
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the magnetic properties of a range of low-dimensional ferromagnets using a combination of first-principles calculations and atomistic spin dynamics simulations. This approach allows us to evaluate the ground state and finite temperature properties of experimentally well characterized systems such as Co/Cu(111), Co/Cu(001), Fe/Cu(001) and Fe/W(110), for different thicknesses of the magnetic layer. We compare our calculated spin wave spectra with experimental data available in the literature, and find a good quantitative agreement. We also predict magnon spectra for systems for which no experimental data exist at the moment, and estimate the role of temperature effects.
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