| 1. |
- Eriksson, Anton, et al.
(författare)
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How choosing random-walk model and network representation matters for flow-based community detection in hypergraphs
- 2021
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Ingår i: Communications Physics. - : Nature Publishing Group. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Hypergraphs offer an explicit formalism to describe multibody interactions in complex systems. To connect dynamics and function in systems with these higher-order interactions, network scientists have generalised random-walk models to hypergraphs and studied the multibody effects on flow-based centrality measures. Mapping the large-scale structure of those flows requires effective community detection methods applied to cogent network representations. For different hypergraph data and research questions, which combination of random-walk model and network representation is best? We define unipartite, bipartite, and multilayer network representations of hypergraph flows and explore how they and the underlying random-walk model change the number, size, depth, and overlap of identified multilevel communities. These results help researchers choose the appropriate modelling approach when mapping flows on hypergraphs.
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| 2. |
- Fonseca, Nelson J. G., et al.
(författare)
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A solution to the complement of the generalized Luneburg lens problem
- 2021
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Lenses are of interest for the design of directive antennas and multi-optics instruments in the microwave, terahertz and optical domains. Here, we introduce an optical problem defined as the complement of the well-known generalized Luneburg lens problem. The spherically symmetric inhomogeneous lenses obtained as solutions of this problem transform a given sphere in the homogeneous region outside of the lens into a virtual conjugate sphere, forming a virtual image from a real source. An analytical solution is proposed for the equivalent geodesic lens using the analogy between classical mechanics and geometrical optics. The refractive index profile of the corresponding inhomogeneous lens is then obtained using transformation optics. The focusing properties of this family of lenses are validated using ray-tracing models, further corroborated with full-wave simulations. The numerical results agree well with the predictions over the analyzed frequency bandwidth (10-30 GHz). This virtual focusing property may further benefit from recent developments in the fields of metamaterials and transformation optics. Spherically-symmetric lenses can create sharp virtual images, but a general expression for their refractive index profiles had not yet been developed. Here, this expression is provided via analogy between classical mechanics and geometrical optics, yielding solutions complementary to existing lenses obtained from the generalized Luneburg lens problem.
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| 3. |
- He, Yuchi, et al.
(författare)
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Charge density waves and their transitions in anisotropic quantum Hall systems
- 2021
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- In recent experiments, external anisotropy has been a useful tool to tune different phases and study their competitions. In this paper, we look at the quantum Hall charge density wave states in the N=2 Landau level. Without anisotropy, there are two first-order phase transitions between the Wigner crystal, the 2-electron bubble phase, and the stripe phase. By adding mass anisotropy, our analytical and numerical studies show that the 2-electron bubble phase disappears and the stripe phase significantly enlarges its domain in the phase diagram. Meanwhile, a regime of stripe crystals that may be observed experimentally is unveiled after the bubble phase gets out. Upon increase of the anisotropy, the energy of the phases at the transitions becomes progressively smooth as a function of the filling. We conclude that all first-order phase transitions are replaced by continuous phase transitions, providing a possible realisation of continuous quantum crystalline phase transitions. The quantum Hall effect can be used to study the physics of correlated systems and can reveal different features of the associated phase transitions. Here, the authors consider mass anisotropy and the impact on the phase transitions of quantum Hall charge density wave states in a 2D electron gas.
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| 4. |
- He, Y. T., et al.
(författare)
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Dominance of gamma-gamma electron-positron pair creation in a plasma driven by high-intensity lasers
- 2021
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Creation of electrons and positrons from light alone is a basic prediction of quantum electrodynamics, but yet to be observed. Our simulations show that the required conditions are achievable using a high-intensity two-beam laser facility and an advanced target design. Dual laser irradiation of a structured target produces high-density gamma rays that then create > 10(8) positrons at intensities of 2 x 10(22) Wcm(-2). The unique feature of this setup is that the pair creation is primarily driven by the linear Breit-Wheeler process (gamma gamma -> e(+)e(-)), which dominates over the nonlinear Breit-Wheeler and Bethe-Heitler processes. The favorable scaling with laser intensity of the linear process prompts reconsideration of its neglect in simulation studies and also permits positron jet formation at experimentally feasible intensities. Simulations show that the positrons, confined by a quasistatic plasma magnetic field, may be accelerated by the lasers to energies >200 MeV. Electron-positron pair generation from nonlinear quantum electrodynamics is predicted at high intensities that are, so far, beyond experimental capabilities. Here, simulations predict a high yield of positrons can be obtained from gamma-gamma photon collisions in the linear regime, using counter-propagating pulses and a microstructured target.
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| 5. |
- Hoque, Anamul Md, 1988, et al.
(författare)
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All-electrical creation and control of spin-galvanic signal in graphene and molybdenum ditelluride heterostructures at room temperature
- 2021
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- The ability to engineer new states of matter and control their spintronic properties by electric fields is at the heart of future information technology. Here, we report a gate-tunable spin-galvanic effect in van der Waals heterostructures of graphene with a semimetal of molybdenum ditelluride at room temperature due to an efficient spin-charge conversion process. Measurements in different device geometries with control over the spin orientations exhibit spin-switch and Hanle spin precession behavior, confirming the spin origin of the signal. The control experiments with the pristine graphene channels do not show any such signals. We explain the experimental spin-galvanic signals by theoretical calculations considering the spin-orbit induced spin-splitting in the bands of the graphene in the heterostructure. The calculations also reveal an unusual spin texture in graphene heterostructure with an anisotropic out-of-plane and in-plane spin polarization. These findings open opportunities to utilize graphene-based heterostructures for gate-controlled spintronic devices.
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| 6. |
- K. Manikandan, Sreekanth, et al.
(författare)
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Quantitative analysis of non-equilibrium systems from short-time experimental data
- 2021
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Estimating entropy production directly from experimental trajectories is of great current interest but often requires a large amount of data or knowledge of the underlying dynamics. In this paper, we propose a minimal strategy using the short-time Thermodynamic Uncertainty Relation (TUR) by means of which we can simultaneously and quantitatively infer the thermodynamic force field acting on the system and the (potentially exact) rate of entropy production from experimental short-time trajectory data. We benchmark this scheme first for an experimental study of a colloidal particle system where exact analytical results are known, prior to studying the case of a colloidal particle in a hydrodynamical flow field, where neither analytical nor numerical results are available. In the latter case, we build an effective model of the system based on our results. In both cases, we also demonstrate that our results match with those obtained from another recently introduced scheme. Thermal fluctuations play a crucial role in non-equilibrium phenomena at microscopic length scales, making it challenging to analyse and interpret experimental data. Here, the authors demonstrate that the short-time thermodynamic uncertainty relation inference scheme can estimate the entropy production rate for a colloidal particle in time-varying potentials and with background flows determined by the presence of a microbubble.
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| 7. |
- Seibold, Goetz, et al.
(författare)
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Strange metal behaviour from charge density fluctuations in cuprates
- 2021
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Besides the mechanism responsible for high critical temperature superconductivity, the grand unresolved issue of the cuprates is the occurrence of a strange metallic state above the so-called pseudogap temperature T-*. Even though such state has been successfully described within a phenomenological scheme, the so-called Marginal Fermi-Liquid theory, a microscopic explanation is still missing. However, recent resonant X-ray scattering experiments identified a new class of charge density fluctuations characterized by low characteristic energies and short correlation lengths, which are related to the well-known charge density waves. These fluctuations are present over a wide region of the temperature-vs-doping phase diagram and extend well above T-*. Here we investigate the consequences of charge density fluctuations on the electron and transport properties and find that they can explain the strange metal phenomenology. Therefore, charge density fluctuations are likely the long-sought microscopic mechanism underlying the peculiarities of the metallic state of cuprates. The strange metallic state of cuprates occurring in a broad region of their phase diagram outside the superconducting and pseudogapped regions remains a mystery. Here the authors consider the charge density fluctuations recently discovered in resonant X-ray experiments as a possible source of scattering and show that these fluctuations can account for the strange metallic behavior.
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| 8. |
- Shaker, Kian, et al.
(författare)
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Phase-contrast X-ray tomography resolves the terminal bronchioles in free-breathing mice
- 2021
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Mechanical ventilation of living animals is routinely used to achieve high-resolution pulmonary imaging, but this can damage the subject. Here, an alternative, free-breathing method enables X-ray tomography with 30 mu m resolution. Phase-contrast X-ray lung imaging has broken new ground in preclinical respiratory research by improving contrast at air/tissue interfaces. To minimize blur from respiratory motion, intubation and mechanical ventilation is commonly employed for end-inspiration gated imaging at synchrotrons and in the laboratory. Inevitably, the prospect of ventilation induced lung injury (VILI) renders mechanical ventilation a confounding factor in respiratory studies of animal models. Here we demonstrate proof-of-principle 3D imaging of the tracheobronchial tree in free-breathing mice without mechanical ventilation at radiation levels compatible with longitudinal studies. We use a prospective gating approach for end-expiration propagation-based phase-contrast X-ray imaging where the natural breathing of the mouse dictates the acquisition flow. We achieve intrapulmonary spatial resolution in the 30-mu m-range, sufficient for resolving terminal bronchioles in the 60-mu m-range distinguished from the surrounding lung parenchyma. These results should enable non-invasive longitudinal studies of native state murine airways for translational lung disease research in the laboratory.
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