| 1. |
- Fan, Qunping, 1989, et al.
(författare)
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Over 14% efficiency all-polymer solar cells enabled by a low bandgap polymer acceptor with low energy loss and efficient charge separation
- 2020
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Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 13:12, s. 5017-5027
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Tidskriftsartikel (refereegranskat)abstract
- Obtaining both high open-circuit voltage (V-oc) and short-circuit current density (J(sc)) has been a major challenge for efficient all-polymer solar cells (all-PSCs). Herein, we developed a polymer acceptor PF5-Y5 with excellent optical absorption capability (onset extending to similar to 880 nm and maximum absorption coefficient exceeding 105 cm(-1) in a film), high electron mobility (3.18 x 10(3) cm(2) V-1 s(-1)) and high LUMO level (-3.84 eV) to address such a challenge. As a result, the PBDB-T:PF5-Y5-based all-PSCs achieved a high power conversion efficiency of up to 14.45% with both a high Voc (0.946 V) and a high Jsc (20.65 mA cm(-2)), due to the high and broad absorption coverage, small energy loss (0.57 eV) and efficient charge separation and transport in the device, which are among the best values in the all-PSC field. In addition, the all-PSC shows a similar to 15% improvement in PCE compared to its counterpart small molecule acceptor (Y5)-based device. Our results suggest that PF5-Y5 is a very promising polymer acceptor candidate for applications in efficient all-PSCs.
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| 2. |
- Manzetti, Sergio, et al.
(författare)
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A Korteweg-DeVries type model for helical soliton solutions for quantum and continuum phenomena
- 2021
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Ingår i: International Journal of Modern Physics C. - : World Scientific. - 0129-1831. ; 32:03
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Tidskriftsartikel (refereegranskat)abstract
- Quantum mechanical states are normally described by the Schrödinger equation, which generates real eigenvalues and quantizable solutions which form a basis for the estimation of quantum mechanical observables, such as momentum and kinetic energy. Studying transition in the realm of quantum physics and continuum physics is however more difficult and requires different models. We present here a new equation which bears similarities to the Korteweg–DeVries (KdV) equation and we generate a description of transitions in physics. We describe here the two- and three-dimensional form of the KdV like model dependent on the Plank constant ℏ and generate soliton solutions. The results suggest that transitions are represented by soliton solutions which arrange in a spiral-fashion. By helicity, we propose a conserved pattern of transition at all levels of physics, from quantum physics to macroscopic continuum physics.
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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
-
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. |
- Frigerio, Jacopo, et al.
(författare)
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Mid-infrared second harmonic generation in Ge/SiGe coupled quantum wells
- 2020
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Ingår i: 2020 IEEE Photonics Conference, IPC 2020 - Proceedings. ; September 2020
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Konferensbidrag (refereegranskat)abstract
- We present the theoretical investigation and the experimental demonstration of second harmonic generation in the mid-infrared by hole-doped Ge/SiGe asymmetric quantum wells.
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| 5. |
- Catena, Riccardo, 1978, et al.
(författare)
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Dark matter-electron interactions in materials beyond the dark photon model
- 2023
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 2023:3
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Tidskriftsartikel (refereegranskat)abstract
- The search for sub-GeV dark matter (DM) particles via electronic transitions in underground detectors attracted much theoretical and experimental interest in the past few years. A still open question in this field is whether experimental results can in general be interpreted in a framework where the response of detector materials to an external DM probe is described by a single ionisation or crystal form factor, as expected for the so-called dark photon model. Here, ionisation and crystal form factors are examples of material response functions: interaction-specific integrals of the initial and final state electron wave functions. In this work, we address this question through a systematic classification of the material response functions induced by a wide range of models for spin-0, spin-1/2 and spin-1 DM. We find several examples for which an accurate description of the electronic transition rate at DM direct detection experiments requires material response functions that go beyond those expected for the dark photon model. This concretely illustrates the limitations of a framework that is entirely based on the standard ionisation and crystal form factors, and points towards the need for the general response-function-based formalism we pushed forward recently [1,2]. For the models that require non-standard atomic and crystal response functions, we use the response functions of [1,2] to calculate the DM-induced electronic transition rate in atomic and crystal detectors, and to present 90% confidence level exclusion limits on the strength of the DM-electron interaction from the null results reported by XENON10, XENON1T, EDELWEISS and SENSEI.
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| 6. |
- Jönsson, Mattias
(författare)
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Theory for superconducting few-photon detectors
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-performance photon detectors are essential for fiber communication, which is the foundation of the modern internet. Emerging quantum information technologies, such as quantum key distribution, impose new requirements on the photon detectors used. Superconducting single-photon detectors (SSPDs) exhibit high efficiencies, low dark count rates and fast recovery times, which makes them commonly used for quantum applications.The ability to resolve photon numbers in a wave packet is useful in applications like imaging, characterization of light sources, and in optical quantum computation. Ordinary single-photon detectors like SSPDs are not photon-number resolving (PNR), and are only capable of determining if light is present or not. However, photon-number resolution may be achieved by combining multiple single-photon detectors in an array and split the input over them.In this thesis, we introduce and model PNR detectors based on multiplexing single-photon detectors. Using these models, we investigate the requirements on the single-photon detectors when they are used in a multiplexed scheme and we investigate how a PNR detector may be used in imaging applications. We experimentally realize a temporally multiplexed PNR detector based on SSPDs and show that it is capable of accurately determining the mean photon number for a series of wave packets.We also model a SSPD using the generalized time-dependent Ginzburg-Landau model to investigate how the geometry of the SSPD affects the performance of the detector. We show that the geometric reduction of the critical current in turnarounds is less pronounced than previously reported, which relaxes design restrictions.
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| 7. |
- Lundberg, Petter, 1988-
(författare)
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Light for a brighter morrow : paving the way for sustainable light-emitting devices
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We live in an artificially lit world, where light enhances our productivity and improves our quality of life. Today our appetite for light is stronger than ever, and emerging light-emitting technologies do not just replace the classical incandescent light bulb, they also open up for a new world of applications. The problem is that our environment does not cope with the increased energy demand during fabrication and usage, and the insufficient recycling that currently follows this rapid technological development. We must therefore adapt, and from here on out consider the entire environmental footprint and the necessity of our devices. Organic electronics has the potential to become sustainable. It allows for cheap and energy-efficient fabrication methods, using abundant materials, mainly carbon. Such sophisticated conductive plastics can be made thin and flexible, and they are thereby very versatile. It is in this context that we find the light-emitting electrochemical cell (LEC)—a strong contender for affordable and sustainable light. The LEC has a simple device design that is fit for solution based fabrication and new useful applications in, for example, medicine. The simple LEC design is enabled by its operational mechanism, where mobile ions aid electronic charge injection and improves electric conductivity by electrochemical doping. However, this dynamic nature complicates the attainment of devices that are efficient, bright, and retain a long lifetime. Herein, we face these challenges with sustainability as the beacon. We find that careful design of the active material, and selection of its constituents, can lead to LECs that are both efficient and bright. Importantly we show that this is attainable with entirely organic active materials, via thermally activated delayed fluorescence; thereby moving away from unsustainable phosphorescent emitters that contain problematic rare metals. With large-scale manufacturing in mind, we introduce a tool that identifies environmentally benign and functional solvents. Furthermore we design and validate a realistic optical model that unveils the common optical loss mechanisms in LECs. The insights gained guide the optical design of highly efficient LECs in the transition towards an upscaled production.I hope that the progress made will contribute to a road map for the design of sustainable light-emitting devices. It is then our responsibility, as a society, to make use of them where needed.
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| 8. |
- Hooton, M.J., et al.
(författare)
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Spi-OPS: Spitzer and CHEOPS confirm the near-polar orbit of MASCARA-1 b and reveal a hint of dayside reflection
- 2022
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The light curves of tidally locked hot Jupiters transiting fast-rotating, early-type stars are a rich source of information about both the planet and star, with full-phase coverage enabling a detailed atmospheric characterisation of the planet. Although it is possible to determine the true spin-orbit angle ψ-a notoriously difficult parameter to measure-from any transit asymmetry resulting from gravity darkening induced by the stellar rotation, the correlations that exist between the transit parameters have led to large disagreements in published values of ψ for some systems. Aims. We aimed to study these phenomena in the light curves of the ultra-hot Jupiter MASCARA-1 b, which is characteristically similar to well-studied contemporaries such as KELT-9 b and WASP-33 b. Methods. We obtained optical CHaracterising ExOPlanet Satellite (CHEOPS) transit and occultation light curves of MASCARA-1 b, and analysed them jointly with a Spitzer/IRAC 4.5 μm full-phase curve to model the asymmetric transits, occultations, and phase-dependent flux modulation. For the latter, we employed a novel physics-driven approach to jointly fit the phase modulation by generating a single 2D temperature map and integrating it over the two bandpasses as a function of phase to account for the differing planet-star flux contrasts. The reflected light component was modelled using the general ab initio solution for a semi-infinite atmosphere. Results. When fitting the CHEOPS and Spitzer transits together, the degeneracies are greatly diminished and return results consistent with previously published Doppler tomography. Placing priors informed by the tomography achieves even better precision, allowing a determination of ψ = 72.1-2.4+2.5 deg. From the occultations and phase variations, we derived dayside and nightside temperatures of 3062-68+66 K and 1720 ± 330 K, respectively.Our retrieval suggests that the dayside emission spectrum closely follows that of a blackbody. As the CHEOPS occultation is too deep to be attributed to blackbody flux alone, we could separately derive geometric albedo Ag = 0.171-0.068+0.066 and spherical albedo As = 0.266-0.100+0.097 from the CHEOPS data, and Bond albedoAB = 0.057-0.101+0.083 from the Spitzer phase curve.Although small, the Ag and As indicate that MASCARA-1 b is more reflective than most other ultra-hot Jupiters, where H- absorption is expected to dominate. Conclusions. Where possible, priors informed by Doppler tomography should be used when fitting transits of fast-rotating stars, though multi-colour photometry may also unlock an accurate measurement of ψ. Our approach to modelling the phase variations at different wavelengths provides a template for how to separate thermal emission from reflected light in spectrally resolved James Webb Space Telescope phase curve data.
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| 9. |
- Szabó, G.M., et al.
(författare)
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The changing face of AU Mic b: Stellar spots, spin-orbit commensurability, and transit timing variations as seen by CHEOPS and TESS
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 654
-
Tidskriftsartikel (refereegranskat)abstract
- AU Mic is a young planetary system with a resolved debris disc showing signs of planet formation and two transiting warm Neptunes near mean-motion resonances. Here we analyse three transits of AU Mic b observed with the CHaracterising ExOPlanet Satellite (CHEOPS), supplemented with sector 1 and 27 Transiting Exoplanet Survey Satellite (TESS) photometry, and the All-Sky Automated Survey from the ground. The refined orbital period of AU Mic b is 8.462995 ± 0.000003 d, whereas the stellar rotational period is Prot = 4.8367 ± 0.0006 d. The two periods indicate a 7:4 spin-orbit commensurability at a precision of 0.1%. Therefore, all transits are observed in front of one of the four possible stellar central longitudes. This is strongly supported by the observation that the same complex star-spot pattern is seen in the second and third CHEOPS visits that were separated by four orbits (and seven stellar rotations). Using a bootstrap analysis we find that flares and star spots reduce the accuracy of transit parameters by up to 10% in the planet-to-star radius ratio and the accuracy on transit time by 3-4 min. Nevertheless, occulted stellar spot features independently confirm the presence of transit timing variations (TTVs) with an amplitude of at least 4 min. We find that the outer companion, AU Mic c, may cause the observed TTVs.
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| 10. |
- Andrén, Daniel, 1991, et al.
(författare)
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Microscopic metavehicles powered and steered by embedded optical metasurfaces
- 2021
-
Ingår i: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 16:9, s. 970-974
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructured dielectric metasurfaces offer unprecedented opportunities to manipulate light by imprinting an arbitrary phase gradient on an impinging wavefront(1). This has resulted in the realization of a range of flat analogues to classical optical components, such as lenses, waveplates and axicons(2-6). However, the change in linear and angular optical momentum(7) associated with phase manipulation also results in previously unexploited forces and torques that act on the metasurface itself. Here we show that these optomechanical effects can be utilized to construct optical metavehicles-microscopic particles that can travel long distances under low-intensity plane-wave illumination while being steered by the polarization of the incident light. We demonstrate movement in complex patterns, self-correcting motion and an application as transport vehicles for microscopic cargoes, which include unicellular organisms. The abundance of possible optical metasurfaces attests to the prospect of developing a wide variety of metavehicles with specialized functional behaviours.
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| 11. |
- Bahrova, O. M., et al.
(författare)
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Cooling of nanomechanical vibrations by Andreev injection
- 2022
-
Ingår i: Low Temperature Physics. - : AIP Publishing. - 1063-777X .- 1090-6517. ; 48:6, s. 476-482
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Tidskriftsartikel (refereegranskat)abstract
- A nanoelectromechanical weak link composed of a carbon nanotube suspended between two normal electrodes in a gap between two superconducting leads is considered. The nanotube is treated as a movable single level quantum dot in which the position-dependent superconducting order parameter is induced due to the Cooper pair tunneling. We show that electron tunneling processes significantly affect the state of the mechanical subsystem. We found that at a given direction of the applied voltage between the electrodes, the stationary state of the mechanical subsystem has a Boltzmann form with an effective temperature dependent on the parameters of the device. As this takes place, the effective temperature can reach significantly small values (cooling effect). We also demonstrate that nanotube fluctuations strongly affect the dc current through the system. The latter can be used to probe the predicted effects in an experiment. Published under an exclusive license by AIP Publishing.
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| 12. |
- Brookes, Paul, et al.
(författare)
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Critical slowing down in circuit quantum electrodynamics
- 2021
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Ingår i: Science advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 7:21
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Tidskriftsartikel (refereegranskat)abstract
- Critical slowing down of the time it takes a system to reach equilibrium is a key signature of bistability in dissipative first-order phase transitions. Understanding and characterizing this process can shed light on the underlying many-body dynamics that occur close to such a transition. Here, we explore the rich quantum activation dynamics and the appearance of critical slowing down in an engineered superconducting quantum circuit. Specifically, we investigate the intermediate bistable regime of the generalized Jaynes-Cummings Hamiltonian (GJC), realized by a circuit quantum electrodynamics (cQED) system consisting of a transmon qubit coupled to a microwave cavity. We find a previously unidentified regime of quantum activation in which the critical slowing down reaches saturation and, by comparing our experimental results with a range of models, we shed light on the fundamental role played by the qubit in this regime.
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| 13. |
- Gran, Ulf, 1973, et al.
(författare)
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Plasmons in holographic graphene
- 2020
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Ingår i: SciPost Physics. - : SCIPOST FOUNDATION. - 2542-4653. ; 8:6
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how self-sourced collective modes - of which the plasmon is a prominent example due to its relevance in modern technological applications - are identified in strongly correlated systems described by holographic Maxwell theories. The characteristic ω ∝ pk plasmon dispersion for 2D materials, such as graphene, naturally emerges from this formalism. We also demonstrate this by constructing the first holographic model containing this feature. This provides new insight into modeling such systems from a holographic point of view, bottom-up and top-down alike. Beyond that, this method provides a general framework to compute the dynamical charge response of strange metals, which has recently become experimentally accessible due to the novel technique of momentum-resolved electron energy-loss spectroscopy (M-EELS). This framework therefore opens up the exciting possibility of testing holographic models for strange metals against actual experimental data.
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| 14. |
- Holmvall, Patric, 1988, et al.
(författare)
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Phase crystals
- 2020
-
Ingår i: Physical Review Research. - 2643-1564. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- Superconductivity owes its properties to the phase of the electron pair condensate that breaks the U(1) symmetry. In the most traditional ground state, the phase is uniform and rigid. The normal state can be unstable towards special inhomogeneous superconducting states: the Abrikosov vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state. Here we show that the phase-uniform superconducting state can go into a fundamentally different and more ordered nonuniform ground state, which we refer to as a phase crystal. This state breaks translational invariance through formation of a spatially periodic modulation of the phase, manifested by unusual superflow patterns and circulating currents, that also break time-reversal symmetry. We list the general conditions needed for realization of phase crystals. Using microscopic theory, we then derive an analytic expression for the superfluid density tensor for the case of a nonuniform environment in a semi-infinite superconductor. We demonstrate how the surface quasiparticle states enter the superfluid density and identify phase crystallization as the main player in several previous numerical observations in unconventional superconductors, and predict the existence of a similar phenomenon in superconductor-ferromagnetic structures. This analytic approach provides a unifying aspect for the exploration of boundary-induced quasiparticles and collective excitations in superconductors. More generally, we trace the origin of phase crystallization to nonlocal properties of the gradient energy, which implies the existence of similar pattern-forming instabilities in many other contexts.
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| 15. |
- Muralidhar, Shreyas, et al.
(författare)
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Femtosecond Laser Pulse Driven Caustic Spin Wave Beams
- 2021
-
Ingår i: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 126:3
-
Tidskriftsartikel (refereegranskat)abstract
- © 2021 authors. Controlling the directionality of spin waves is a key ingredient in wave-based computing methods such as magnonics. In this Letter, we demonstrate this particular aspect by using an all-optical pointlike source of continuous spin waves based on frequency comb rapid demagnetization. The emitted spin waves contain a range of k vectors and by detuning the applied magnetic field slightly off the ferromagnetic resonance (FMR), we observe X-shaped caustic spin wave patterns at 70° propagation angles as predicted by theory. When the harmonic of the light source approaches the FMR, the caustic pattern gives way to uniaxial spin wave propagation perpendicular to the in-plane component of the applied field. This field-controlled propagation pattern and directionality of optically emitted short-wavelength spin waves provide additional degrees of freedom when designing magnonic devices.
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| 16. |
- Parafilo, A. V., et al.
(författare)
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Pumping and Cooling of Nanomechanical Vibrations Generated by Cooper-Pair Exchange
- 2022
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Ingår i: Journal of Low Temperature Physics. - : Springer Science and Business Media LLC. - 0022-2291 .- 1573-7357. ; 210:1-2, s. 150-65
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Tidskriftsartikel (refereegranskat)abstract
- We consider a nanoelectromechanical system composed of a carbon nanotube suspended between two normal leads and coupled to a superconducting scanning tunneling microscope (STM) tip via vacuum tunnel barrier. Treating the nanotube as a single-level quantum dot, it is shown that an applied voltage between the superconducting STM tip and normal leads gives rise to a pumping or a cooling of the mechanical subsystem depending on the direction of the electronic flow. It is also demonstrated that the transition between these two regimes is controlled by the strength of the tunnel coupling between the nanotube and superconducting STM tip and the relative position of the electronic level. Such phenomena are realized due to a specific electromechanical coupling that is fully governed by the quantum dynamics of the Cooper pairs. The amplitude of the self-sustained oscillations in the pumping regime is analyzed numerically, and the effective temperature of the mechanical subsystem in the cooling regime is obtained.
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| 17. |
- Tello Marmolejo, Javier, 1995, et al.
(författare)
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Fano Combs in the Directional Mie Scattering of a Water Droplet
- 2023
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 130:4
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Tidskriftsartikel (refereegranskat)abstract
- When light scatters off a sphere, it produces a rich Mie spectrum full of overlapping resonances. Single resonances can be explained with a quantum analogy and result in Fano profiles. However, the full spectrum is so complex that recognizable patterns have not been found, and is only understood by comparing to numerical simulations. Here we show the directional Mie spectrum of evaporating water droplets arranged in consecutive Fano Combs. We then fully explain it by expanding the quantum analogy. This turns the droplet into an "optical atom"with angular momentum, tunneling, and excited states.
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| 18. |
- Zhong, Shiyang, et al.
(författare)
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Attosecond electron–spin dynamics in Xe 4d photoionization
- 2020
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- The photoionization of xenon atoms in the 70–100 eV range reveals several fascinating physical phenomena such as a giant resonance induced by the dynamic rearrangement of the electron cloud after photon absorption, an anomalous branching ratio between intermediate Xe+ states separated by the spin-orbit interaction and multiple Auger decay processes. These phenomena have been studied in the past, using in particular synchrotron radiation, but without access to real-time dynamics. Here, we study the dynamics of Xe 4d photoionization on its natural time scale combining attosecond interferometry and coincidence spectroscopy. A time-frequency analysis of the involved transitions allows us to identify two interfering ionization mechanisms: the broad giant dipole resonance with a fast decay time less than 50 as, and a narrow resonance at threshold induced by spin-flip transitions, with much longer decay times of several hundred as. Our results provide insight into the complex electron-spin dynamics of photo-induced phenomena.
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| 19. |
- Flygare, Mattias, 1978-
(författare)
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The influence of crystallinity on the properties of carbon nanotubes
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon nanotubes have been advertised as a material with quite extraordinary properties, both mechanically and electrically. The truth is that carbon nanotubes is not one material, but several different. Depending on the method used to produce them, and consequently the quality of the atomic structure within their walls, their physical properties can also differ drastically. In this doctoral thesis a method was developed for quantifying the degree of order within the tubes' walls, namely their crystallinity, by using transmission electron microscopy. The method enables the characterization of the inherent properties of the tubes such as electrical conductivity and bending stiffness, alongside the determination of crystallinity, making it possible to quantify the influence of tube crystallinity on these critical properties. Furthermore, a model for electrical conduction in the outermost wall of multi-walled carbon nanotubes is suggested, enabling the determination of intrinsic quantities like the sheet resistance of individual crystallite grains within the walls and the boundaries in-between them. The studies reveal a profound shift in both mechanical and electrical behavior at a critical crystallite size, with large differences connected to production method, and even between individual tubes from the same production batch. These findings successfully explain previously seen differences and highlight the need for well-defined characterization techniques with protocols and classification systems, in order to successfully exploit the promising properties of carbon nanotubes in the future.
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| 20. |
- Gil-Escrig, Lidon, et al.
(författare)
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Efficient Wide-Bandgap Mixed-Cation and Mixed-Halide Perovskite Solar Cells by Vacuum Deposition
- 2021
-
Ingår i: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; 6:2, s. 827-836
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Tidskriftsartikel (refereegranskat)abstract
- Vacuum deposition methods are increasingly applied to the preparation of perovskite films and devices, in view of the possibility to prepare multilayer structures at low temperature. Vacuum-deposited, wide-bandgap solar cells based on mixed-cation and mixed-anion perovskites have been scarcely reported, due to the challenges associated with the multiple-source processing of perovskite thin films. In this work, we describe a four-source vacuum deposition process to prepare wide-bandgap perovskites of the type FA(1-n)Cs(n)Pb-(I1-xBrx)(3) with a tunable bandgap and controlled morphology, using FAI, CsI, PbI2, and PbBr2 as the precursors. The simultaneous sublimation of PbI2 and PbBr2 allows the relative Br/Cs content to be decoupled and controlled, resulting in homogeneous perovskite films with a bandgap in the 1.7-1.8 eV range and no detectable halide segregation. Solar cells based on 1.75 eV bandgap perovskites show efficiency up to 16.8% and promising stability, maintaining 90% of the initial efficiency after 2 weeks of operation.
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| 21. |
- Jin, Wentao, et al.
(författare)
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Absolute surface energies of wurtzite (10 1 over bar 1) surfaces and the instability of the cation-adsorbed surfaces of II-VI semiconductors
- 2021
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 119:20, s. 1-6
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the wurtzite (10 1 over bar 1) planes of five semiconductors, AlN, GaN, GaAs, ZnO, and ZnS. The absolute surface energies are obtained by using a series of wedge nanowire structures. A cation-adsorbed surface reconstruction, (1 x 1)X (X is the electropositive element of the semiconductor) adlayer, is found to have dramatically low energy under the cation-rich condition for AlN and GaN. A p electron draining mechanism is proposed to explain these results. We also developed a framework to analyze the stabilization mechanism of the unneutral surfaces. It suggests that the cation-adsorbed surfaces of II-VI semiconductors should be more unstable than the anion-adsorbed surfaces.
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| 22. |
- Khansili, Akash, 1997-
(författare)
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Probing quantum criticality in heavy fermion CeCoIn5
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the low-temperature properties of strongly correlated materials requires accurate measurement of the physical properties of these systems. Specific heat and nuclear spin-lattice relaxation are two such properties that allow the investigation of the electronic behavior of the system. In this thesis, nanocalorimetry is used to measure specific heat, but also as basis for new experimental approach, developed to disentangle the different contributions to specific heat at low temperatures. The technique, that we call Thermal Impedance Spectroscopy (TISP) allows independent measurement of the electronic and nuclear specific heat at low temperatures based on the frequency response of the calorimeter-sample assembly. The method also enables simultaneous measurements of the nuclear spin-lattice relaxation time (T1). The nuclear spin lattice relaxation, as 1/T1T, and electronic specific heat, as C/T, provide information about the same quantity, electronic density of states, in the system. By comparing these properties in strongly correlated systems, we can obtain insights of electronic interactions. Metallic indium is studied using thermal impedance spectroscopy from 0.3 K to 7 K at 35 T. The magnetic field dependence of nuclear spin-lattice relaxation rate is measured. Indium is a simple metallic system and the expected behavior of the nuclear spin-lattice relaxation is similar to that of the electronic specific heat. The results of the measurement are matched with the expectation from a simple metallic system and Nuclear Magnetic Resonance (NMR) measurements. This demonstrates the effectiveness of the new technique. The heavy-fermion superconductor CeCoIn5 is studied using thermal impedance spectroscopy and ac-calorimetry. This material is located near a quantum critical point (QCP) bordering antiferromagnetism, as evidenced by doping studies. The nature of its quantum criticality and unconventional superconductivity is still elusive. Contrasting specific heat and nuclear spin-lattice relaxation in this correlated system helps to reveal the character of its quantum criticality. The quantum criticality in CeCoIn5 is also studied using X-ray Absorption Spectroscopy (XAS) across the superconducting transition and X-ray Magnetic Circular Dichroism (XMCD) at 0.1 K and 6 T. The element-specific probe zooming in on cerium in this material indicates two things, a mixed valence of Ce in the superconducting state and a very small magnetic moment, that implies resonance-bond like antiferromagnetic local ordering in the system.
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| 23. |
- Su, Wenyan, et al.
(författare)
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Nonconjugated Terpolymer Acceptors with Two Different Fused-Ring Electron-Deficient Building Blocks for Efficient All-Polymer Solar Cells
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:5, s. 6442-6449
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Tidskriftsartikel (refereegranskat)abstract
- The ternary polymerization strategy of incorporating different donor and acceptor units forming terpolymers as photovoltaic materials has been proven advantageous in improving power conversion efficiencies (PCEs) of polymer solar cells (PSCs). Herein, a series of low band gap nonconjugated terpolymer acceptors based on two different fused-ring electron-deficient building blocks (IDIC16 and ITIC) with adjustable photoelectric properties were developed. As the third component, ITIC building blocks with a larger pi-conjugation structure, shorter solubilizing side chains, and red-shifted absorption spectrum were incorporated into an IDIC16-based nonconjugated copolymer acceptor PF1-TS4, which built up the terpolymers with two conjugated building blocks linked by flexible thioalkyl chain-thiophene segments. With the increasing ITIC content, terpolymers show gradually broadened absorption spectra and slightly down-shifted lowest unoccupied molecular orbital levels. The active layer based on terpolymer PF1-TS4-60 with a 60% ITIC unit presents more balanced hole and electron mobilities, higher photoluminescence quenching efficiency, and improved morphology compared to those based on PF1-TS4. In all-polymer solar cells (all-PSCs), PF1-TS4-60, matched with a wide band gap polymer donor PM6, achieved a similar open-circuit voltage (V-oc) of 0.99 V, a dramatically increased short-circuit current density (J(sc)) of 15.30 mA cm(-2), and fill factor (FF) of 61.4% compared to PF1-TS4 = 0.99 V, J(sc) = 11.21 mA cm(-2), and FF = 55.6%). As a result, the PF1-TS4-60-based all-PSCs achieved a PCE of 9.31%, which is similar to 50% higher than the PF1-TS4-based ones (6.17%). The results demonstrate a promising approach to develop high-performance nonconjugated terpolymer acceptors for efficient all-PSCs by means of ternary polymerization using two different A-D-A-structured fused-ring electron-deficient building blocks.
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| 24. |
- Susic, Isidora, et al.
(författare)
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Intrinsic Organic Semiconductors as Hole Transport Layers in p–i–n Perovskite Solar Cells
- 2022
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Ingår i: Solar RRL. - : John Wiley & Sons. - 2367-198X. ; 6:4
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Tidskriftsartikel (refereegranskat)abstract
- Thin polymeric and small-molecular-weight organic semiconductors are widely employed as hole transport layers (HTLs) in perovskite solar cells. To ensure ohmic contact with the electrodes, the use of doping or additional high work function (WF) interlayer is common. In some cases, however, intrinsic organic semiconductors can be used without any additive or buffer layers, although their thickness must be tuned to ensure selective and ohmic hole transport. Herein, the characteristics of thin HTLs in vacuum-deposited perovskite solar cells are studied, and it is found that only very thin (<5 nm) HTLs readily result inhigh-performing devices, as the HTL acts as a WF enhancer while still ensuring selective hole transfer, as suggested by ultraviolet photoemission spectroscopy and Kelvin probe measurements. For thicker films (>= 5 nm), a dynamic behavior for consecutive electrical measurements is observed, a phenomenon which is also common to other widely used HTLs. Finally, it is found that despite their glass transition temperature, small-molecule HTLs lead to thermally unstable solar cells, asopposed to polymeric materials. The origin of the degradation is still not clear, but might be related to chemical reactions/diffusion at the HTL/perovskite interface, in detriment of the device stability
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| 25. |
- 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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