| 1. |
- Carlström, Stefanos, et al.
(författare)
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Spatially and spectrally resolved quantum path interference with chirped driving pulses
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:12
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Tidskriftsartikel (refereegranskat)abstract
- We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short (${\alpha }_{{\rm{s}}}$) and long (${\alpha }_{{\rm{l}}}$) trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure.
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| 2. |
- Kanski, Janusz, 1946, et al.
(författare)
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Electronic structure of (Ga,Mn)As revisited
- 2017
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Ingår i: New Journal of Physics. - : Institute of Physics (IOP). - 1367-2630. ; 19:2, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- The detailed nature of electronic states mediating ferromagnetic coupling in dilute magnetic semiconductors, specifically (Ga,Mn)As, has been an issue of long debate. Two confronting models have been discussed emphasizing host band vs. impurity band carriers. Using angle resolved photoemission we show that the electronic structure of the (Ga,Mn)As system is significantly modified from that of GaAs throughout the valence band. Close to the Fermi energy, the presence of Mn induces a strong mixing of the bulk bands of GaAs, which results in the appearance of a highly dispersive band in the gap region of GaAs.For Mn concentrations above 1% the band reaches the Fermi level, and can thus host the delocalized holes needed for ferromagnetic coupling. Overall, our data provide a firm evidence of delocalized carriers belonging to the modified host valence band.
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| 3. |
- Konopik, Michael, et al.
(författare)
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Solving the subset sum problem with a nonideal biological computer
- 2021
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:9
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Tidskriftsartikel (refereegranskat)abstract
- We consider the solution of the subset sum problem based on a parallel computer consisting of self-propelled biological agents moving in a nanostructured network that encodes the computing task in its geometry. We develop an approximate analytical method to analyze the effects of small errors in the nonideal junctions composing the computing network by using a Gaussian confidence interval approximation of the multinomial distribution. We concretely evaluate the probability distribution for error-induced paths and determine the minimal number of agents required to obtain a proper solution. We finally validate our theoretical results with exact numerical simulations of the subset sum problem for different set sizes and error probabilities, and discuss the scalability of the nonideal problem using realistic experimental error probabilities.
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| 4. |
- Small, Lara S.R., et al.
(författare)
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The bar-hinge motor : A synthetic protein design exploiting conformational switching to achieve directional motility
- 2019
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 21:1
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Tidskriftsartikel (refereegranskat)abstract
- One challenge to synthetic biology is to design functional machines from natural building blocks, from individual amino acids up to larger motifs such as the coiled coil. Here we investigate a novel bipedal motor concept, the Bar-Hinge Motor (BHM), a peptide-based motor capable of executing directed motion via externally controlled conformational switching between a straight bar and a V-shaped hinged form. Incorporating ligand-regulated binding to a DNA track and periodic control of ligand supply makes the BHM an example of a 'clocked walker'. Here, we employ a coarse-grained computational model for the BHM to assess the feasibility of a proposed experimental realization, with conformational switching regulated through the photoisomerization of peptide-bound azobenzene molecules. The results of numerical simulations using the model show that the incorporation of this conformational switch is necessary for the BHM to execute directional, rather than random, motion on a one-dimensional track. The power-stroke-driven directed motion is seen in the model even under conditions that underestimate the level of control we expect to be able to produce in the experimental realisation, demonstrating that this type of design should be an excellent vehicle for exploring the physics behind protein motion. By investigating its force-dependent dynamics, we show that the BHM is capable of directional motion against an applied load, even in the more relaxed conformational switching regimes. Thus, BHM appears to be an excellent candidate for a motor design incorporating a power stroke, enabling us to explore the ability of switchable coiled-coil designs to deliver power strokes within synthetic biology.
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| 5. |
- Morrison, S, et al.
(författare)
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Physical replicas and the Bose glass in cold atomic gases
- 2008
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10:7
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Tidskriftsartikel (refereegranskat)abstract
- We study cold atomic gases in a disorder potential and analyse the correlations between different systems subjected to the same disorder landscape. Such independent copies with the same disorder landscape are known as replicas. While, in general, these are not accessible experimentally in condensed matter systems, they can be realized using standard tools for controlling cold atomic gases in an optical lattice. Of special interest is the overlap function which represents a natural order parameter for disordered systems and is a correlation function between the atoms of two independent replicas with the same disorder. We demonstrate an efficient measurement scheme for the determination of this disorder-induced correlation function. As an application, we focus on the disordered Bose–Hubbard model and determine the overlap function within the perturbation theory and a numerical analysis. We find that the measurement of the overlap function allows for the identification of the Bose-glass phase in certain parameter regimes.
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| 6. |
- Alegret, Joan, 1977, et al.
(författare)
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Green's tensor calculations of plasmon resonances of single holes and hole pairs in thin gold films
- 2008
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Ingår i: New Journal of Physics. - Bristol, UK : IOP Publishing. - 1367-2630. ; 10, s. 13-
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Tidskriftsartikel (refereegranskat)abstract
- We present numerical calculations of the plasmon properties of single-hole and hole-pair structures in optically thin gold films obtained with the Green's tensor formalism for stratified media. The method can be used to obtain the optical properties of a given hole system, without problems associated with the truncation of the infinite metal film. The calculations are compared with previously published experimental data and an excellent agreement is found. In particular, the calculations are shown to reproduce the evolution of the hole plasmon resonance spectrum as a function of hole diameter, film thickness and hole separation.
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| 7. |
- Aurand, Bastian, et al.
(författare)
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Radiation pressure-assisted acceleration of ions using multi-component foils in high-intensity laser-matter interactions
- 2013
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 15
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Tidskriftsartikel (refereegranskat)abstract
- Experimental results on the acceleration of protons and carbon ions from ultra-thin polymer foils at intensities of up to 6x10(19)Wcm(-2) are presented revealing quasi-monoenergetic spectral characteristics for different ion species at the same time. For carbon ions and protons, a linear correlation between the cutoff energy and the peak energy is observed when the laser intensity is increased. Particle-in-cell simulations supporting the experimental results imply an ion acceleration mechanism driven by the radiation pressure as predicted for multi-component foils at these intensities.
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| 8. |
- Favre-Nicolin, V., et al.
(författare)
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Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging
- 2010
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this paper, we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. We also discuss the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.
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| 9. |
- Foerster, Heidi, et al.
(författare)
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Current-voltage correlations in interferometers
- 2007
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- We investigate correlations of current at contacts and voltage fluctuations at voltage probes coupled to interferometers. The results are compared with correlations of current and occupation number fluctuations at dephasing probes. We use a quantum Langevin approach for the average quantities and their fluctuations. For higher order correlations we develop a stochastic path integral approach and find the generating functions of voltage or occupation number fluctuations. We also derive a generating function for the joint distribution of voltage or occupation number at the probe and current fluctuations at a terminal of a conductor. For energy independent scattering we found earlier that the generating function of current cumulants in interferometers with a one-channel dephasing or voltage probe are identical. Nevertheless, the distribution function for voltage and the distribution function for occupation number fluctuations differ, the latter being broader than that of the former in all examples considered here.
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| 10. |
- Helsing, Johan, et al.
(författare)
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Spectral super-resolution in metamaterial composites
- 2011
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 13:11
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the optical properties of periodic composites containing metamaterial inclusions in a normal material matrix. We consider the case when these inclusions have sharp corners and, following Hetherington and Thorpe, use analytic results to argue that it is then possible to deduce the shape of the corner (its included angle) by measurements of the absorptance of such composites when the scale size of the inclusions and period cell is much finer than the wavelength. These analytic arguments are supported by highly accurate numerical results for the effective permittivity function of such composites as a function of the permittivity ratio of inclusions to the matrix. The results show that this function has a continuous spectral component with limits independent of the area fraction of inclusions, and with the same limits for both square and staggered square arrays. For staggered arrays where the squares are almost touching, the absorption spectrum is an extremely sensitive probe of the inclusion separation distance and acts like a Vernier scale.
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| 11. |
- Lindblad, Andreas, et al.
(författare)
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Vibrational scattering anisotropy in O-2-dynamics beyond the Born-Oppenheimer approximation
- 2012
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 14, s. 113018-
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Tidskriftsartikel (refereegranskat)abstract
- Born-Oppenheimer and Franck-Condon approximations are two major concepts in the interpretation of electronic excitations and modeling of spectroscopic data in the gas and condensed phases. We report large variations of the anisotropy parameter (beta) for the fully resolved vibrational sub-states of the X-2 Pi(g) electronic ground state of O-2(+) populated by participator resonant Auger decay following excitations of K-shell electrons into the sigma(star) resonance by monochromatic x-rays. Decay spectra for light polarization directions parallel and perpendicular to the electron detection axis recorded at four different excitation energies in the vicinity of the O 1s -> sigma(star) transition are presented. Breakdown of the Born-Oppenheimer approximation is for the first time selectively observed for the lower vibrational sub-states, where two quantum paths-resonant and direct-leading to the same final cationic state exist. The higher vibrational sub-states can only be populated by resonant photoemission; hence no interference between these channels can occur.
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| 12. |
- Mansten, Erik, et al.
(författare)
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Spectral shaping of attosecond pulses using two-colour laser fields
- 2008
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
-
Tidskriftsartikel (refereegranskat)abstract
- We use a strong two-colour laser field composed of the fundamental (800 nm) and the second harmonic (400 nm) of an infrared (IR) laser field to generate attosecond pulses with controlled spectral and temporal properties. With a second-harmonic intensity equal to 15% of the IR intensity the second-harmonic field is strong enough to significantly alter and control the electron trajectories in the generation process. This enables us to tune the central photon energy of the attosecond pulses by changing the phase difference between the IR and the second-harmonic fields. In the time domain the radiation is emitted as a sequence of pulses separated by a full IR cycle. We also perform calculations showing that the effect of even stronger second-harmonic fields leads to an extended tunable range under conditions that are experimentally feasible.
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| 13. |
- Reimann, Stephanie, et al.
(författare)
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Rotating quantum liquids crystallize
- 2006
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Small crystallites form when finite quantal systems are set highly rotating. This crystallization is independent of the statistics of the particles, and occurs for both trapped bosons and fermions. The spin degree of freedom does not change the tendency for localization. In a highly rotating state, the strongly correlated bosonic and fermionic systems approach to that of classical particles.
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| 14. |
- Stierle, A., et al.
(författare)
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Real time observation of ultrathin epitaxial oxide growth during alloy oxidation
- 2007
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 9, s. 331-331
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the thermal oxidation of the intermetallic alloy CoGa in situ, in real time on the atomic scale, during the growth of an ultrathin, epitaxial Ga oxide layer. On the basis of an extended set of surface x-ray diffraction data, density functional theory calculations and core level spectroscopy data, we find that the oxide film consists of an oxygen ion double layer, which contains the basic building block of bulk beta- Ga2O3. The oxide formation takes place via the nucleation of two- dimensional, anisotropic oxide islands which laterally grow and coalesce. A dramatic increase of the oxide island size is observed for low O-2 pressures in the 10(-8) mbar regime, which we interpret as the onset of a step flow like growth mode. This allows us to conclude that thermal oxidation can be considered as a hetero- epitaxial growth process, that follows similar atomistic growth principles to molecular beam epitaxy. As a consequence, the structural perfection of the oxide layer can be tailored by the appropriate choice of oxygen pressure and temperature.
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| 15. |
- Söderström, Johan, 1978-, et al.
(författare)
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Angle-resolved electron spectroscopy of the resonant Auger decay in xenon with meV energy resolution
- 2011
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 13, s. 073014-
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Tidskriftsartikel (refereegranskat)abstract
- The angle-resolved resonant Auger spectrum of Xe is investigated with a record high meV energy resolution in the kinetic energy region of 34.45-39.20 eV at hv = 65.110 eV, corresponding to the resonant excitation of the Auger Xe* 4d(5/2)(-1)6p state. New lines have been observed and assigned in the spectra. The results of previous measurements concerning energies, intensities and angular distribution asymmetry parameters have been refined, complemented and, for some of the lines, corrected.
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| 16. |
- Baba, S., et al.
(författare)
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Cooper-pair splitting in two parallel InAs nanowires
- 2018
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 20:6
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Tidskriftsartikel (refereegranskat)abstract
- We report on the fabrication and electrical characterization of an InAs double-nanowire (NW) device consisting of two closely placed parallel NWs coupled to a common superconducting electrode on one side and individual normal metal leads on the other. In this new type of device we detect Cooper-pair splitting (CPS) with a sizeable efficiency of correlated currents in both NWs. In contrast to earlier experiments, where CPS was realized in a single NW, demonstrating an intrawire electron pairing mediated by the superconductor (SC), our experiment demonstrates an interwire interaction mediated by the common SC. The latter is the key for the realization of zero-magnetic field Majorana bound states, or Parafermions; in NWs and therefore constitutes a milestone towards topological superconductivity. In addition, we observe transport resonances that occur only in the superconducting state, which we tentatively attribute to Andreev bound states and/or Yu-Shiba resonances that form in the proximitized section of one NW.
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| 17. |
- Bender, P., et al.
(författare)
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Distribution functions of magnetic nanoparticles determined by a numerical inversion method
- 2017
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 19:7
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, we applied a regularized inversion method to extract the particle size, magnetic moment and relaxation-time distribution of magnetic nanoparticles from small-angle x-ray scattering (SAXS), DC magnetization (DCM) and AC susceptibility (ACS) measurements. For the measurements the particles were colloidally dispersed in water. At first approximation the particles could be assumed to be spherically shaped and homogeneously magnetized single-domain particles. As model functions for the inversion, we used the particle form factor of a sphere (SAXS), the Langevin function (DCM) and the Debye model (ACS). The extracted distributions exhibited features/peaks that could be distinctly attributed to the individually dispersed and non-interacting nanoparticles. Further analysis of these peaks enabled, in combination with a prior characterization of the particle ensemble by electron microscopy and dynamic light scattering, a detailed structural and magnetic characterization of the particles. Additionally, all three extracted distributions featured peaks, which indicated deviations of the scattering (SAXS), magnetization (DCM) or relaxation (ACS) behavior from the one expected for individually dispersed, homogeneously magnetized nanoparticles. These deviations could be mainly attributed to partial agglomeration (SAXS, DCM, ACS), uncorrelated surface spins (DCM) and/or intra-well relaxation processes (ACS). The main advantage of the numerical inversion method is that no ad hoc assumptions regarding the line shape of the extracted distribution functions are required, which enabled the detection of these contributions. We highlighted this by comparing the results with the results obtained by standard model fits, where the functional form of the distributions was a priori assumed to be log-normal shaped.
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| 18. |
- Boeyens, Julia, et al.
(författare)
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Probe thermometry with continuous measurements
- 2023
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Ingår i: New Journal of Physics. - 1367-2630. ; 25:12
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Tidskriftsartikel (refereegranskat)abstract
- Temperature estimation plays a vital role across natural sciences. A standard approach is provided by probe thermometry, where a probe is brought into contact with the sample and examined after a certain amount of time has passed. In situations where, for example, preparation of the probe is non-trivial or total measurement time of the experiment is the main resource that must be optimized, continuously monitoring the probe may be preferred. Here, we consider a minimal model, where the probe is provided by a two-level system coupled to a thermal reservoir. Monitoring thermally activated transitions enables real-time estimation of temperature with increasing accuracy over time. Within this framework we comprehensively investigate thermometry in both bosonic and fermionic environments employing a Bayesian approach. Furthermore, we explore adaptive strategies and find a significant improvement on the precision. Additionally, we examine the impact of noise and find that adaptive strategies may suffer more than non-adaptive ones for short observation times. While our main focus is on thermometry, our results are easily extended to the estimation of other environmental parameters, such as chemical potentials and transition rates.
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| 19. |
- Dorsch, Sven, et al.
(författare)
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Characterization of electrostatically defined bottom-heated InAs nanowire quantum dot systems
- 2021
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:12
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Tidskriftsartikel (refereegranskat)abstract
- Conversion of temperature gradients to charge currents in quantum dot systems enables probing various concepts from highly efficient energy harvesting and fundamental thermodynamics to spectroscopic possibilities complementary to conventional bias device characterization. In this work, we present a proof-of-concept study of a device architecture where bottom-gates are capacitively coupled to an InAs nanowire and double function as local joule heaters. The device design combines the ability to heat locally at different locations on the device with the electrostatic definition of various quantum dot and barrier configurations. We demonstrate the versatility of this combined gating- and heating approach by studying, as a function of the heater location and bias, the Seebeck effect across the barrier-free nanowire, fit thermocurrents through quantum dots for thermometry and detect the phonon energy using a serial double quantum dot. The results indicate symmetric heating effects when the device is heated with different gates and we present detection schemes for the electronic and phononic heat transfer contribution across the nanowire. Based on this proof-of-principle work, we propose a variety of future experiments.
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| 20. |
- Düsterer, S, et al.
(författare)
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Two-color XUV+NIR femtosecond photoionization of neon in the near-threshold region
- 2019
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 21
-
Tidskriftsartikel (refereegranskat)abstract
- Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low kinetic energy (few eV) sidebands, the photoelectron energy varies as a function of the emission angle. Such behavior has not yet been observed in sideband creation and has not been predicted in commonly used theoretical descriptions such as strong field approximation and soft photon approach. Describing the photoionization with a time-dependent Schrödinger equation allows a qualitative description of the observed effect, as well as the prediction of fine structure in the sideband distribution.
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| 21. |
- Dzhigaev, D, et al.
(författare)
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Three-dimensional coherent x-ray diffraction imaging of ferroelastic domains in single CsPbBr3 perovskite nanoparticles
- 2021
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:6
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Tidskriftsartikel (refereegranskat)abstract
- Metal halide perovskites attract significant interest due to their remarkable performance in optoelectronic devices. However, the gap in understanding the relationship between their nanoscale structure and properties limits their application towards novel devices. In this work, twinned ferroelastic domains in single 500 nm CsPbBr3 particles are studied with 3D Bragg coherent x-ray diffraction imaging. A preferential double-domain structure is revealed in four identical particles, with one domain oriented along the [110] and the other along the [002] direction. The particles exhibit similar scattering volume ratios of 0.12 0.026 between twin phases, suggesting the possibility of a deterministic formation process. The domains exhibit a difference in lattice tilt of 0.59 degrees, in excellent agreement with calculations of the lattice mismatch at the (112) twin boundary. These results provide important insights both for the fundamental understanding of ferroelastic nanoscale materials and for the performance improvement of perovskite-based devices. Moreover, this work paves the way towards real-time imaging of the domain dynamics in ferroic systems.
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| 22. |
- Francesco Diotallevi, Giovanni, et al.
(författare)
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Steady-state entanglement production in a quantum thermal machine with continuous feedback control
- 2024
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Ingår i: New Journal of Physics. - 1367-2630. ; 26:5
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Tidskriftsartikel (refereegranskat)abstract
- Quantum thermal machines can generate steady-state entanglement by harvesting spontaneous interactions with local environments. However, using minimal resources and control, the entanglement is typically weak. Here, we study entanglement generation in a two-qubit quantum thermal machine in the presence of a continuous feedback protocol. Each qubit is measured continuously and the outcomes are used for real-time feedback to control the local system-environment interactions. We show that there exists an ideal operation regime where the quality of entanglement is significantly improved, to the extent that it can violate standard Bell inequalities and uphold quantum teleportation. In agreement with (Khandelwal et al 2020 New J. Phys. 22 073039), we also find, for ideal operation, that the heat current across the system is proportional to the entanglement concurrence. Finally, we investigate the robustness of entanglement production when the machine operates away from the ideal conditions.
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| 23. |
- Gallardo González, I., et al.
(författare)
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Effects of the dopant concentration in laser wakefield and direct laser acceleration of electrons
- 2018
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 20:5
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we experimentally study the effects of the nitrogen concentration in laser wakefield acceleration of electrons in a gas mixture of hydrogen and nitrogen. A 15 TW peak power laser pulse is focused to ionize the gas, excite a plasma wave and accelerate electrons up to 230 MeV. We find that at dopant concentrations above 2% the total divergence of the electrons is increased and the high energy electrons are emitted preferentially with an angle of ±6 mrad, leading to a forked spatio-spectral distribution associated to direct laser acceleration (DLA). However, electrons can gain more energy and have a divergence lower than 4 mrad for concentrations below 0.5% and the same laser and plasma conditions. Particle-in-cell simulations show that for dopant concentrations above 2%, the amount of trapped charge is large enough to significantly perturb the plasma wave, reducing the amplitude of the longitudinal wakefield and suppressing other trapping mechanisms. At high concentrations the number of trapped electrons overlapping with the laser fields is increased, which rises the amount of charge affected by DLA. We conclude that the dopant concentration affects the quantity of electrons that experience significant DLA and the beam loading of the plasma wave driven by the laser pulse. These two mechanisms influence the electrons final energy, and thus the dopant concentration should be considered as a factor for the optimization of the electron beam parameters.
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| 24. |
- Gower, Artur, et al.
(författare)
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Effective waves for random three-dimensional particulate materials
- 2021
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:6
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Tidskriftsartikel (refereegranskat)abstract
- How do you take a reliable measurement of a material whose microstructure is random? When using wave scattering, the answer is often to take an ensemble average (average over time or space). By ensemble averaging we can calculate the average scattered wave and the effective wavenumber. To date, the literature has focused on calculating the effective wavenumber for a plate filled with particles. One clear unanswered question was how to extend this approach to a material of any geometry and for any source. For example, does the effective wavenumber depend on only the microstructure, or also on the material geometry? In this work, we demonstrate that the effective wavenumbers depend on only microstructure, though beyond the long wavelength limit there are multiple effective wavenumbers for one fixed incident frequency. We show how to calculate the average wave scattered from a random particulate material of any shape, and for broad frequency ranges. As an example, we show how to calculate the average wave scattered from a sphere filled with particles.
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| 25. |
- Gustafsson, Mats, et al.
(författare)
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Upper bounds on absorption and scattering
- 2020
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 22:7
-
Tidskriftsartikel (refereegranskat)abstract
- A general framework for determining fundamental bounds in nanophotonics is introduced in this paper. The theory is based on convex optimization of dual problems constructed from operators generated by electromagnetic integral equations. The optimized variable is a contrast current defined within a prescribed region of a given material constitutive relations. Two power conservation constraints analogous to the optical theorem are utilized to tighten the bounds and to prescribe either losses or material properties. Thanks to the utilization of matrix rank-1 updates, modal decompositions, and model order reduction techniques, the optimization procedure is computationally efficient even for complicated scenarios. No dual gaps are observed. The method is well-suited to accommodate material anisotropy and inhomogeneity. To demonstrate the validity of the method, bounds on scattering, absorption, and extinction cross sections are derived first and evaluated for several canonical regions. The tightness of the bounds is verified by comparison to optimized spherical nanoparticles and shells. The next metric investigated is bi-directional scattering studied closely on a particular example of an electrically thin slab. Finally, the bounds are established for Purcell's factor and local field enhancement where a dimer is used as a practical example.
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| 26. |
- Korten, Till, et al.
(författare)
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Design of network-based biocomputation circuits for the exact cover problem
- 2021
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:8
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Tidskriftsartikel (refereegranskat)abstract
- Exact cover is a non-deterministic polynomial time (NP)-complete problem that is central to optimization challenges such as airline fleet planning and allocation of cloud computing resources. Solving exact cover requires the exploration of a solution space that increases exponentially with cardinality. Hence, it is time- and energy consuming to solve large instances of exact cover by serial computers. One approach to address these challenges is to utilize the inherent parallelism and high energy efficiency of biological systems in a network-based biocomputation (NBC) device. NBC is a parallel computing paradigm in which a given combinatorial problem is encoded into a graphical, modular network that is embedded in a nanofabricated planar device. The network is then explored in parallel using a large number of biological agents, such as molecular-motor-propelled protein filaments. The answer to the combinatorial problem can then be inferred by measuring the positions through which the agents exit the network. Here, we (i) show how exact cover can be encoded and solved in an NBC device, (ii) define a formalization that allows to prove the correctness of our approach and provides a mathematical basis for further studying NBC, and (iii) demonstrate various optimizations that significantly improve the computing performance of NBC. This work lays the ground for fabricating and scaling NBC devices to solve significantly larger combinatorial problems than have been demonstrated so far.
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| 27. |
- Maitrallain, A., et al.
(författare)
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Parametric study of high-energy ring-shaped electron beams from a laser wakefield accelerator
- 2022
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 24:1
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Tidskriftsartikel (refereegranskat)abstract
- Laser wakefield accelerators commonly produce on-axis, low-divergence, high-energy electron beams. However, a high charge, annular shaped beam can be trapped outside the bubble and accelerated to high energies. Here we present a parametric study on the production of low-energy-spread, ultra-relativistic electron ring beams in a two-stage gas cell. Ring-shaped beams with energies higher than 750 MeV are observed simultaneously with on axis, continuously injected electrons. Often multiple ring shaped beams with different energies are produced and parametric studies to control the generation and properties of these structures were conducted. Particle tracking and particle-in-cell simulations are used to determine properties of these beams and investigate how they are formed and trapped outside the bubble by the wake produced by on-axis injected electrons. These unusual femtosecond duration, high-charge, high-energy, ring electron beams may find use in beam driven plasma wakefield accelerators and radiation sources.
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| 28. |
- Maroju, P. K., et al.
(författare)
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Analysis of two-color photoelectron spectroscopy for attosecond metrology at seeded free-electron lasers
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:4
-
Tidskriftsartikel (refereegranskat)abstract
- The generation of attosecond pulse trains at free-electron lasers opens new opportunities in ultrafast science, as it gives access, for the first time, to reproducible, programmable, extreme ultraviolet (XUV) waveforms with high intensity. In this work, we present a detailed analysis of the theoretical model underlying the temporal characterization of the attosecond pulse trains recently generated at the free-electron laser FERMI. In particular, the validity of the approximations used for the correlated analysis of the photoelectron spectra generated in the two-color photoionization experiments are thoroughly discussed. The ranges of validity of the assumptions, in connection with the main experimental parameters, are derived.
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| 29. |
- Miranda, M. Sanchez, et al.
(författare)
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Prospects for single-molecule electrostatic detection in molecular motor gliding motility assays
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:6
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular motor gliding motility assays based on myosin/actin or kinesin/microtubules are of interest for nanotechnology applications ranging from cargo-trafficking in lab-on-a-chip devices to novel biocomputation strategies. Prototype systems are typically monitored by expensive and bulky fluorescence microscopy systems. The development of integrated, direct electric detection of single filaments would strongly benefit applications and scale-up. We present estimates for the viability of such a detector by calculating the electrostatic potential change generated at a carbon nanotube transistor by a motile actin filament or microtubule under realistic gliding assay conditions. We combine this with detection limits based on previous state-of-the-art experiments using carbon nanotube transistors to detect catalysis by a bound lysozyme molecule and melting of a bound short-strand DNA molecule. Our results show that detection should be possible for both actin and microtubules using existing low ionic strength buffers given good device design, e.g., by raising the transistor slightly above the guiding channel floor. We perform studies as a function of buffer ionic strength, height of the transistor above the guiding channel floor, presence/absence of the casein surface passivation layer for microtubule assays and the linear charge density of the actin filaments/microtubules. We show that detection of microtubules is a more likely prospect given their smaller height of travel above the surface, higher negative charge density and the casein passivation, and may possibly be achieved with the nanoscale transistor sitting directly on the guiding channel floor.
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| 30. |
- Nicolaï, L., et al.
(författare)
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Bi ultra-thin crystalline films on InAs(1 1 1)A and B substrates : A combined core-level and valence-band angle-resolved and dichroic photoemission study
- 2019
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 21:12
-
Tidskriftsartikel (refereegranskat)abstract
- The growth of Bi on both the In-terminated (A) face and the As-terminated (B) face of InAs(1 1 1) has been investigated by low-energy electron diffraction, scanning tunnelling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d5/2 photoelectron signals allow to get a comprehensive picture of the Bi/InAs(1 1 1) interface. From the early stage the Bi growth on the A face is epitaxial, contrary to that on the B face that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a Bi deposit of ≈10 bi-layers on the A face is identical to that of bulk Bi, while more than ≈30 bi-layers are needed for the B face. Both bulk and surface electronic states observed are well accounted for by fully relativistic ab initio calculations performed using the one-step model of photoemission. These calculations are used to analyse the dichroic photoemission data recorded in the vicinity of the Fermi level around the Γ&bar; point of the Brillouin zone.
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| 31. |
- Potts, Patrick, et al.
(författare)
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A thermodynamically consistent Markovian master equation beyond the secular approximation
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23
-
Tidskriftsartikel (refereegranskat)abstract
- Markovian master equations provide a versatile tool for describing open quantum systems whenmemory effects of the environment may be neglected. As these equations are of an approximatenature, they often do not respect the laws of thermodynamics when no secular approximation isperformed in their derivation. Here we introduce a Markovian master equation that isthermodynamically consistent and provides an accurate description whenever memory effects canbe neglected. The thermodynamic consistency is obtained through a rescaled Hamiltonian for thethermodynamic bookkeeping, exploiting the fact that a Markovian description implies a limitedresolution for heat. Our results enable a thermodynamically consistent description of a variety ofsystems where the secular approximation breaks down.
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| 32. |
- Pozas-Kerstjens, Alejandro, et al.
(författare)
-
Post-quantum nonlocality in the minimal triangle scenario
- 2023
-
Ingår i: New Journal of Physics. - 1367-2630. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate network nonlocality in the triangle scenario when all three parties have no input and binary outputs. Through an explicit example, we prove that this minimal scenario supports nonlocal correlations compatible with no-signaling and independence of the three sources, but not with realisations based on independent quantum or classical sources. This nonlocality is robust to noise. Moreover, we identify the equivalent to a Popescu-Rohrlich box in the minimal triangle scenario.
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| 33. |
- Reuther, Cordula, et al.
(författare)
-
Molecular motor-driven filament transport across three-dimensional, polymeric micro-junctions
- 2021
-
Ingår i: New Journal of Physics. - : Institute of Physics Publishing (IOPP). - 1367-2630. ; 23:12
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular motor-driven filament systems have been extensively explored for biomedical and nanotechnological applications such as lab-on-chip molecular detection or network-based biocomputation. In these applications, filament transport conventionally occurs in two dimensions (2D), often guided along open, topographically and/or chemically structured channels which are coated by molecular motors. However, at crossing points of different channels the filament direction is less well determined and, though crucial to many applications, reliable guiding across the junction can often not be guaranteed. We here present a three-dimensional (3D) approach that eliminates the possibility for filaments to take wrong turns at junctions by spatially separating the channels crossing each other. Specifically, 3D junctions with tunnels and overpasses were manufactured on glass substrates by two-photon polymerization, a 3D fabrication technology where a tightly focused, femtosecond-pulsed laser is scanned in a layer-to-layer fashion across a photo-polymerizable inorganic-organic hybrid polymer (ORMOCER(R)) with mu m resolution. Solidification of the polymer was confined to the focal volume, enabling the manufacturing of arbitrary 3D microstructures according to computer-aided design data. Successful realization of the 3D junction design was verified by optical and electron microscopy. Most importantly, we demonstrated the reliable transport of filaments, namely microtubules propelled by kinesin-1 motors, across these 3D junctions without junction errors. Our results open up new possibilities for 3D functional elements in biomolecular transport systems, in particular their implementation in biocomputational networks.
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| 34. |
- Salhotra, Aseem, et al.
(författare)
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Prolonged function and optimization of actomyosin motility for upscaled network-based biocomputation
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:8
-
Tidskriftsartikel (refereegranskat)abstract
- Significant advancements have been made towards exploitation of naturally available molecular motors and their associated cytoskeletal filaments in nanotechnological applications. For instance, myosin motors and actin filaments from muscle have been used with the aims to establish new approaches in biosensing and network-based biocomputation. The basis for these developments is a version of the in vitro motility assay (IVMA) where surface-adsorbed myosin motors propel the actin filaments along suitably derivatized nano-scale channels on nanostructured chips. These chips are generally assembled into custom-made microfluidic flow cells. For effective applications, particularly in biocomputation, it is important to appreciably prolong function of the biological system. Here, we systematically investigated potentially critical factors necessary to achieve this, such as biocompatibility of different components of the flow cell, the degree of air exposure, assay solution composition and nanofabrication methods. After optimizing these factors we prolonged the function of actin and myosin in nanodevices for biocomputation from 60 min. In addition, we demonstrated that further optimizations could increase motility run times to >20 h. Of great importance for the latter development was a switch of glucose oxidase in the chemical oxygen scavenger system (glucose oxidase-glucose-catalase) to pyranose oxidase, combined with the use of blocking actin (non-fluorescent filaments that block dead motors). To allow effective testing of these approaches we adapted commercially available microfluidic channel slides, for the first time demonstrating their usefulness in the IVMA. As part of our study, we also demonstrate that myosin motor fragments can be stored at -80 degrees C for more than 10 years before use for nanotechnological purposes. This extended shelf-life is important for the sustainability of network-based biocomputation.
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| 35. |
- Savelyev, Evgeny, et al.
(författare)
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Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser
- 2017
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 19:4
-
Tidskriftsartikel (refereegranskat)abstract
- In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.
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| 36. |
- Shen, L., et al.
(författare)
-
Magnetic phase diagram of the quantum spin chain compound SrCo2V2O8 : A single-crystal neutron diffraction study
- 2019
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 21:7
-
Tidskriftsartikel (refereegranskat)abstract
- We explore magnetic order in the quantum spin chain compound SrCo2V2O8 up to 14.9 T and down to 50 mK, using single-crystal neutron diffraction. Upon cooling in zero-field, commensurate antiferromagnetic (C-AFM) order with modulation vector=(0, 0, 1) develops below T N≃5.0K. Applying an external magnetic field (H∥c axis) destabilizes this C-AFM order, leading to an order-disorder transition between T N and ∼1.5 K. Below 1.5 K, a commensurate to incommensurate (IC-AFM) transition occurs at 3.9 T, above which the magnetic reflections can be indexed by=(0, 0, 1 ±δl). The incommensurability δl scales monotonically with H until the IC-AFM order disappears around 7.0 T. Magnetic reflections modulated by emerge again at higher fields. While the characters of the C-AFM, IC-AFM and the emergent AFM order in SrCo2V2O8 appear to fit the descriptions of the Néel, longitudinal spin density wave and transverse AFM order observed in the related compound BaCo2V2O8, our results also reveal several unique signatures that are not present in the latter, highlighting the inadequacy of mean-field theory in addressing the complex magnetic order in systems of this class.
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| 37. |
- Solon, Alexandre P., et al.
(författare)
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Generalized thermodynamics of motility-induced phase separation : Phase equilibria, Laplace pressure, and change of ensembles
- 2018
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 20:7
-
Tidskriftsartikel (refereegranskat)abstract
- Motility-induced phase separation leads to cohesive active matter in the absence of cohesive forces. We present, extend and illustrate a recent generalized thermodynamic formalism which accounts for its binodal curve. Using this formalism, we identify both a generalized surface tension, that controls finite-size corrections to coexisting densities, and generalized forces, that can be used to construct new thermodynamic ensembles. Our framework is based on a non-equilibrium generalization of the Cahn-Hilliard equation and we discuss its application to active particles interacting either via quorum-sensing interactions or directly through pairwise forces.
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| 38. |
- Stenqvist, B.
(författare)
-
Electrostatic pair-potentials based on the Poisson equation
- 2019
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 21:6
-
Tidskriftsartikel (refereegranskat)abstract
- Electrostatic pair-potentials within molecular simulations are often based on empirical data, cancellation of derivatives or moments, or statistical distributions of image-particles. In this work we start with the fundamental Poisson equation and show that no truncated Coulomb pair-potential, unsurprisingly, can solve the Poisson equation. For any such pair-potential the Poisson equation gives two incompatible constraints, yet we find a single unique expression which, pending two physically connected smoothness parameters, can obey either one of these. This expression has a general form which covers several recently published pair-potentials. For sufficiently large degree of smoothness we find that the solution implies a Gaussian distribution of the charge, a feature which is frequently assumed in pair-potential theory. We end up by recommending a single pair-potential based both on theoretical arguments and empirical evaluations of non-thermal lattice- and thermal water-systems. The same derivations have also been made for the screened Poisson equation, i.e. for Yukawa potentials, with a similar solution.
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| 39. |
- Veyrinas, K., et al.
(författare)
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Chromatic aberrations correction of attosecond high-order harmonic beams by flat-top spatial shaping of the fundamental beam
- 2023
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 25:2
-
Tidskriftsartikel (refereegranskat)abstract
- Attosecond pulses created by high-order harmonic generation in gases often exhibit strong chromatic aberrations, arising from the broad bandwidth and wavelength-dependent nonlinear light-matter interaction. When the driving laser intensity varies spatially, as for Gaussian driving beams, the apparent source position of the harmonics differs significantly from one order to the next, thus affecting the achievable intensity and duration of the attosecond pulses when they are focused on a target. We show that these chromatic aberrations can be reduced by spatially shaping the fundamental beam to generate high-order harmonics with a driver having a flat-top profile inside the gas medium. By measuring both the intensity profile and wavefront for each harmonic in a plane, we access the extreme ultra-violet (XUV) beam properties and investigate these properties near focus. We observe that controlling chromatic aberrations by flat-top spatial shaping strongly reduces the variation of the XUV spectrum on the beam axis during propagation and, in return, the longitudinal sensitivity of both the temporal profiles and the temporal shifts of the focused attosecond pulses.
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| 40. |
- Wittkowski, Raphael, et al.
(författare)
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Nonequilibrium dynamics of mixtures of active and passive colloidal particles
- 2017
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 19:10
-
Tidskriftsartikel (refereegranskat)abstract
- We develop a mesoscopic field theory for the collective nonequilibrium dynamics of multicomponent mixtures of interacting active (i.e., motile) and passive (i.e., nonmotile) colloidal particles with isometric shape in two spatial dimensions. By a stability analysis of the field theory, we obtain equations for the spinodal that describes the onset of a motility-induced instability leading to cluster formation in such mixtures. The prediction for the spinodal is found to be in good agreement with particle-resolved computer simulations. Furthermore, we show that in active-passive mixtures the spinodal instability can be of two different types. One type is associated with a stationary bifurcation and occurs also in one-component active systems, whereas the other type is associated with a Hopf bifurcation and can occur only in active-passive mixtures. Remarkably, the Hopf bifurcation leads to moving clusters. This explains recent results from simulations of active-passive particle mixtures, where moving clusters and interfaces that are not seen in the corresponding one-component systems have been observed.
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| 41. |
- Zhu, Jingyuan, et al.
(författare)
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Physical requirements for scaling up network-based biocomputation
- 2021
-
Ingår i: New Journal of Physics. - : Institute of Physics (IOP). - 1367-2630. ; 23:10
-
Tidskriftsartikel (refereegranskat)abstract
- The high energy consumption of electronic data processors, together with physical challenges limiting their further improvement, has triggered intensive interest in alternative computation paradigms. Here we focus on network-based biocomputation (NBC), a massively parallel approach where computational problems are encoded in planar networks implemented with nanoscale channels. These networks are explored by biological agents, such as biological molecular motor systems and bacteria, benefitting from their energy efficiency and availability in large numbers. We analyse and define the fundamental requirements that need to be fulfilled to scale up NBC computers to become a viable technology that can solve large NP-complete problem instances faster or with less energy consumption than electronic computers. Our work can serve as a guide for further efforts to contribute to elements of future NBC devices, and as the theoretical basis for a detailed NBC roadmap.
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| 42. |
- Bengtson, Charlotta, et al.
(författare)
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The role of quantum coherence in dimer and trimer excitation energy transfer
- 2017
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 19
-
Tidskriftsartikel (refereegranskat)abstract
- Recent progress in resource theory of quantum coherence has resulted in measures to quantify coherence in quantum systems. Especially, the l1-norm and relative entropy of coherence have been shown to be proper quantifiers of coherence and have been used to investigate coherence properties in different operational tasks. Since long-lasting quantum coherence has been experimentally confirmed in a number of photosynthetic complexes, it has been debated if and how coherence is connected to the known efficiency of population transfer in such systems. In this study, we investigate quantitatively the relationship between coherence, as quantified by l1 norm and relative entropy of coherence, and efficiency, as quantified by fidelity, for population transfer between end-sites in a network of two-level quantum systems. In particular, we use the coherence averaged over the duration of the population transfer in order to carry out a quantitative comparision between coherence and fidelity. Our results show that although coherence is a necessary requirement for population transfer, there is no unique relation between coherence and the efficiency of the transfer process.
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| 43. |
- Blackburn, Thomas, 1989, et al.
(författare)
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From local to nonlocal: higher fidelity simulations of photon emission in intense laser pulses
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:8
-
Tidskriftsartikel (refereegranskat)abstract
- State-of-the-art numerical simulations of quantum electrodynamical (QED) processes in strong laser fields rely on a semiclassical combination of classical equations of motion and QED rates, which are calculated in the locally constant field approximation. However, the latter approximation is unreliable if the amplitude of the fields, a (0), is comparable to unity. Furthermore, it cannot, by definition, capture interference effects that give rise to harmonic structure. Here we present an alternative numerical approach, which resolves these two issues by combining cycle-averaged equations of motion and QED rates calculated in the locally monochromatic approximation. We demonstrate that it significantly improves the accuracy of simulations of photon emission across the full range of photon energies and laser intensities, in plane-wave, chirped and focused background fields.
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| 44. |
- He, Y., et al.
(författare)
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A single-laser scheme for observation of linear Breit-Wheeler electron-positron pair creation
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:11
-
Tidskriftsartikel (refereegranskat)abstract
- We show that a single laser pulse, traveling through a dense plasma, produces a population of MeV photons of sufficient density to generate a large number of electron-positron pairs via the linear Breit-Wheeler process. While it may be expected that the photons are emitted predominantly in the forward direction, parallel to the laser propagation, we find that a longitudinal plasma electric field drives the emission of photons in the backwards direction. This enables the collision of oppositely directed, MeV-level photons necessary to overcome the mass threshold for the linear Breit-Wheeler process. Our calculations predict the production of 10(7) electron-positron pairs, per shot, by a laser with peak intensity of just 3 x 10(22) W cm(-2). By using only a single laser pulse, the scheme sidesteps the practical difficulties associated with the multiple-laser schemes previously investigated.
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| 45. |
- Leo, Naëmi, et al.
(författare)
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Chiral switching and dynamic barrier reductions in artificial square ice
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:3
-
Tidskriftsartikel (refereegranskat)abstract
- Collective dynamics in lithographically-defined artificial spin ices offer profound insights into emergent correlations and phase transitions of geometrically-frustrated Ising spin systems. Their temporal and spatial evolution are often simulated using kinetic Monte Carlo (kMC) simulations, which rely on the precise knowledge of the switching barriers to obtain predictive results in agreement with experimental observations. In many cases, however, the barriers are derived from simplified assumptions only, and do not take into account the full physical picture of nanomagnetic switching. Here we describe how the immediate magnetic square- or kagome-ice environment of a nanomagnet reversing via quasi-coherent rotation can induce clockwise and counter-clockwise switching channels with different barrier energies. This energy splitting for chiral reversal channels can be sizeable and, as string-method micromagnetic simulations show, is relevant for artificial spin ice systems made of both exchange- as well as magnetostatically-dominated units. Due to the barrier splitting and further reductions due to non-uniform reversal, transition rates can be exponentially enhanced by several orders of magnitude compared to mean-field predictions, especially in the limit of rare switching events where thermal excitation is less likely. This leads to significantly faster relaxation time scales and modified spatial correlations. Our findings are thus of integral importance to achieve realistic kMC simulations of emergent correlations in artificial spin systems, magnonic crystals, or the evolution of nanomagnetic logic circuits.
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| 46. |
- Löfgren, R, et al.
(författare)
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A theoretical study of de-charging excitations of the NV-center in diamond involving a nitrogen donor
- 2020
-
Ingår i: New Journal of Physics. - : Institute of Physics (IOP). - 1367-2630. ; 22:12
-
Tidskriftsartikel (refereegranskat)abstract
- The negatively charged nitrogen vacancy centre in diamond is a promising candidate for future nanoscale quantum applications. For its operation it is important to have control of the centres charge state, and to avoid temporary disappearance of the NV-center's functionality, termed photo-blinking. In this work, we use density functional theory simulations to investigate excitations that result in loss of an electron from NV− to a nearby nitrogen donor (donor-N+), leading to NV0 and donor-N0 charge state, and the corresponding deexcitation. Since these processes involve two different localized defect centres in the diamond lattice (the NV-center and the donor-N) they are non-local excitations. We have studied the de-charging both as a one-photon process and through a sequential two-photon process via the NV-center excited state. We propose de-charging directly from the NV-center to the donor-N as a possible mechanism for photo-blinking of the NV-center that involve an additional electron spin resonance active defect, the donor-N0. We have found that the excitation energies are converged when the distance between the two is larger than 10.4 Å. We also compute excitations to the conduction band edge from NV− (to NV0) and from donor-N0 (to donor-N+) using G0W0 + BSE.
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| 47. |
- Löfgren, Robin, et al.
(författare)
-
Charged dopants in neutral supercells through substitutional donor (acceptor) : nitrogen donor charging of the nitrogen-vacancy center in diamond
- 2018
-
Ingår i: New Journal of Physics. - : Institute of Physics (IOP). - 1367-2630. ; 20
-
Tidskriftsartikel (refereegranskat)abstract
- Charged defects are traditionally computed by adding (subtracting) electrons for negative (positive) impurities. When using periodic boundary conditions this results in artificially charged supercells that also require a compensating background charge of the opposite sign, which makes slab supercells problematic because of an arbitrary dependence on the vacuum thickness. In this work, we test the method of using neutral supercells through the use of a substitutional electron donor (acceptor) to describe charged systems. We use density functional theory (DFT) to compare the effects of charging the well-studied NV-center in diamond by a substitutional donor nitrogen. We investigate the influence of the donor-N on the NV-center properties as a function of the distance between them, and find that they converge toward those obtained when adding an electron. We analyze the spin density and conclude that the donor-N has a zero magnetic moment, and thus, will not be seen in electron spin resonance. We validate our DFT energies through comparison to GW simulations. Charging the NV-center with a substitutional donor-N enables accurate calculations of slabs, without the ambiguity of using charged supercells. Implantation of donor-N atoms opens up the possibility to engineer NV-centers with the desired charge state for future ICT and sensor applications.
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| 48. |
- Peng, Xubiao, et al.
(författare)
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Rotation by deformation and time-crystalline dynamics of cyclopropane molecule
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:7
-
Tidskriftsartikel (refereegranskat)abstract
- A deformable body can rotate even with no angular momentum simply by changing its shape. Here the first all-atom level molecular dynamics example of this phenomenon is presented. For this the thermal vibrations of individual atoms in an isolated cyclopropane molecule are simulated in vacuum and at ultra-low internal temperature values. When the molecule is observed stroboscopically, at discrete equidistant time steps, the random thermal vibrations of the individual atoms become self-organized into a collective oscillatory motion of the entire molecule. The period of oscillation is emergent and intrinsic to the molecule so that this self-organization bears resemblance to a driven time crystal. The oscillation period increases in a self-similar manner when the length of the stroboscopic time step is increased. In the limit of very long stroboscopic time steps the entire molecule can then rotate in an apparent uniform fashion, but with no angular momentum. It is proposed that the observed behavior is universal in the case of triangular molecules. Moreover, it is shown that the emergent uniform rotation without any angular momentum, can be described in an effective theory approach as an autonomous Hamiltonian time crystal. The emergent oscillatory motion appears to be highly sensitive to temperature. This proposes that potential applications could be found from the development of molecular level detector to sensor and control technologies.
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| 49. |
- Roussou, Alexandra, et al.
(författare)
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Excitation spectrum of a mixture of two Bose gases confined in a ring potential with interaction asymmetry
- 2018
-
Ingår i: New Journal of Physics. - : Institute of Physics (IOP). - 1367-2630. ; 20
-
Tidskriftsartikel (refereegranskat)abstract
- We study the rotational properties of a two-component Bose-Einstein condensed gas of distinguishable atoms which are confined in a ring potential using both the mean-field approximation, as well as the method of diagonalization of the many-body Hamiltonian. We demonstrate that the angular momentum may be given to the system either via single-particle, or "collective" excitation. Furthermore, despite the complexity of this problem, under rather typical conditions the dispersion relation takes a remarkably simple and regular form. Finally, we argue that under certain conditions the dispersion relation is determined via collective excitation. The corresponding many-body state, which, in addition to the interaction energy minimizes also the kinetic energy, is dictated by elementary number theory.
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| 50. |
- Pertsova, Anna, et al.
(författare)
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Probing the wavefunction of the surface states in Bi2Se3 topological insulator : a realistic tight-binding approach
- 2014
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 16, s. Article ID: 063022-
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Tidskriftsartikel (refereegranskat)abstract
- We report on microscopic tight-binding modeling of surfacestates in Bi$_2$Se$_3$ three-dimensional topological insulator, based on a\textit{sp}$^3$ Slater-Koster Hamiltonian, with parameters calculated fromdensity functional theory. The effect of spin-orbit interaction on theelectronic structure of the bulk and of a slab with finite thickness isinvestigated. In particular, a phenomenological criterion of band inversion isformulated for both bulk and slab, based on the calculated atomic- andorbital-projections of the wavefunctions, associated with valence and conductionband extrema at the center of the Brillouin zone. We carry out athorough analysis of the calculated bandstructures of slabs with varyingthickness, where surface states are identified using a quantitative criterionaccording to their spatial distribution. The thickness-dependent energy gap,attributed to inter-surface interaction, and the emergence of gapless surfacestates for slabs above a critical thickness are investigated. We map out thetransition to the infinite-thickness limit by calculating explicitly themodifications in the spatial distribution and spin-character of the surfacestates wavefunction with increasing the slab thickness. Our numerical analysisshows that the system must be approximately forty quintuple-layers thick toexhibit completely decoupled surface states, localized on the oppositesurfaces. These results have implications on the effect of external perturbationson the surface states near the Dirac point.
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