arXiv subjects

M. Christensen

Publications and source records attributed to M. Christensen.

11 recordsLinked to original sources

arXiv2023

In-depth investigations of size and occupancies in cobalt ferrite nanoparticles by joint Rietveld refinements of X-ray and neutron powder diffraction data

Powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD) have been used to investigate the crystal structure of CoFe2O4 nanoparticles prepared via different hydrothermal synthesis routes, with particular attention given to accurately determining the spinel inversion degrees. The study is divided into four parts. In the first part, the investigations focus on the influence of using different diffraction pattern combinations (NPD, Cu-source PXRD and Co-source PXRD) for the structural modelling. It is found that combining PXRD data from a Co source with NPD data offers a robust structural model. The second part of the study evaluates the reproducibility of the employed multipattern Rietveld refinement procedure using different data sets collected on the same sample, as well as on equivalently prepared samples. The refinement procedure gives reproducible results and reveals that the synthesis method is likewise reproducible since only minor differences are noted between the samples. The third part focuses on the structural consequences of (i) the employed heating rate (achieved using three different hydrothermal reactor types) and (ii) changing the cobalt salt in the precursors [aqueous salt solutions of Co(CH3COOH)2, Co(NO3)2 and CoCl2] in the synthesis. It is found that increasing the heating rate causes a change in the crystal structure (unit cell and crystallite sizes) while the Co/Fe occupancy and magnetic parameters remain similar in all cases. Also, changing the type of cobalt salt does not alter the final crystal/magnetic structure of the CoFe2O4 nanoparticles. The last part of this study is a consideration of the chemicals and parameters used in the synthesis of the different samples.

cond-mat.mtrl-sci↗

arXiv2019

Performance study of the Jalousie detector baseline design for the ESS thermal powder diffractometer HEIMDAL through GEANT4 simulations

HEIMDAL is a thermal powder diffractometer designed to operate at the European Spallation Source, world's most intense neutron source. The detailed design of the instrument, which is expected to enter user operation in 2024/2025, assumes that the neutrons scattered by the powder under investigation will be collected with hundreds of large-area Multi-Wire Proportional Counters employing a $^{10}$B$_4$C-solid converter. The gas counters will consists of large active volumes and tapered trapezoidal shapes that allow for close packing into a cylindrical shell with high solid angle coverage. The whole detector will operate in an air environment within the shielding cave and provide signals with sensitivity for locating detection in three dimensions. This paper presents the results of a GEANT4 study of the baseline design for the HEIMDAL powder diffraction detector. The detector model was used to study key performance parameters such as detection efficiency and spatial resolution. The contribution of the detector to the resolving power of the instrument, one of the key figures-of-merit for powder diffractometers, was also investigated. Most of the simulation results reported in this work cannot be validated against a sufficiently similar physical reference until the first segment or module are constructed and tested with neutron beam. However, these results can help identify possible ways of optimising the detector design and provide the first glimpse into the expected performance of this technological approach.

physics.ins-det↗

arXiv2016

Neutron detectors for the ESS diffractometers

The ambitious instrument suite for the future European Spallation Source whose civil construction started recently in Lund, Sweden, demands a set of diverse and challenging requirements for the neutron detectors. For instance, the unprecedented high flux expected on the samples to be investigated in neutron diffraction or reflectometry experiments requires detectors that can handle high counting rates, while the investigation of sub-millimeter protein crystals will only be possible with large-area detectors that can achieve a position resolution as low as 200 μm. This has motivated an extensive research and development campaign to advance the state-of-the-art detector and to find new technologies that can reach maturity by the time the ESS will operate at full potential. This paper presents the key detector requirements for three of the Time-of-Flight diffraction instrument concepts selected by the Scientific Advisory Committee to advance into the phase of preliminary engineering design. We discuss the available detector technologies suitable for this particular instrument class and their major challenges. The detector technologies selected by the instrument teams to collect the diffraction patterns are briefly discussed. Analytical calculations, Monte-Carlo simulations, and real experimental data are used to develop a generic method to esti- mate the event rate in the diffraction detectors. The proposed approach is based upon conservative assumptions that use information and input parameters that reflect our current level of knowledge and understanding of the ESS project. We apply this method to make predictions for the future diffraction instruments, and thus provide additional information that can help the instrument teams with the optimisation of the detector designs.

physics.ins-det↗

arXiv2013

Formation of Nickel-Platinum Silicides on a Silicon Substrate: Structure, Phase Stability, and Diffusion from Ab initio Computations

The formation of Ni(Pt)silicides on a Si(001) surface is investigated using an ab initio approach. After deposition of a Ni overlayer alloyed with Pt, the calculations reveal fast diffusion of Ni atoms into the Si lattice, which leads initially to the formation of Ni2Si. At the same time Si atoms are found to diffuse into the metallic overlayer. The transformation of Ni2Si into NiSi is likely to proceed via a vacancy-assisted diffusion mechanism. Silicon atoms are the main diffusing species in this transformation, migrating from the Si substrate through the growing NiSi layer into the Ni2Si. Pt atoms have a low solubility in Ni2Si and prefer Si-sites in the NiSi lattice, thereby stabilizing the NiSi phase. The diffusivity of Pt is lower than that of Ni. Furthermore, Pt atoms have a tendency to segregate to interfaces, thereby acting as diffusion barriers.

cond-mat.mtrl-sci↗

arXiv2002

Classification of String-like Solutions in Dilaton Gravity

The static string-like solutions of the Abelian Higgs model coupled to dilaton gravity are analyzed and compared to the non-dilatonic case. Except for a special coupling between the Higgs Lagrangian and the dilaton, the solutions are flux tubes that generate a non-asymptotically flat geometry. Any point in parameter space corresponds to two branches of solutions with two different asymptotic behaviors. Unlike the non-dilatonic case, where one branch is always asymptotically conic, in the present case the asymptotic behavior changes continuously along each branch.

gr-qc↗

arXiv2001

Initial magnetic field configurations for 3-dimensional simulations of astrophysical jets

We solve, and provide analytical expressions, for current-free magnetic configurations in the context of initial setups of 3-dimensional simulations of astrophysical jets involving an accretion disk corona in hydrostatic balance around a central object. These configurations which thread through the accretion disk and its corona preserve the initial hydrostatic state. This work sets stage for future 3-dimensional jet simulations (including disk rotation and mass-load) where launching, acceleration and collimation mechanisms can be investigated.

astro-ph↗

arXiv2000

Comment on "Formation of primordial black holes by cosmic strings"

We show that in a pioneering paper by Polnarev and Zembowicz, some conclusions concerning the characteristics of the Turok-strings are generally not correct. In addition we show that the probability of string collapse given there, is off by a large prefactor (~1000).

gr-qc↗

arXiv2000

Cosmic String Loops Collapsing to Black Holes

We examine the question of collapse of Turok's two-parameter family of cosmic strings. We first perform a classification of the strings according to the specific time(s) at which the minimal string size is reached during one period. We then obtain an exact analytical expression for the probability of collapse to black holes for the Turok strings. Our result has the same general behavior as previously obtained in the literature but we find, in addition, a numerical prefactor that changes the result by three orders of magnitude. Finally we show that our careful computation of the prefactor helps to understand the discrepancy between previously obtained results and, in particular, that for "large" values of $Gμ$, there may not even be a discrepancy. We also give a simple physical argument that can immediately rule out some of the previously obtained results.

gr-qc↗

arXiv1999

Complete Classification of the String-like Solutions of the Gravitating Abelian Higgs Model

The static cylindrically symmetric solutions of the gravitating Abelian Higgs model form a two parameter family. In this paper we give a complete classification of the string-like solutions of this system. We show that the parameter plane is composed of two different regions with the following characteristics: One region contains the standard asymptotically conic cosmic string solutions together with a second kind of solutions with Melvin-like asymptotic behavior. The other region contains two types of solutions with bounded radial extension. The border between the two regions is the curve of maximal angular deficit of $2π$.

gr-qc↗

arXiv1998

Domain wall interacting with a black hole: A new example of critical phenomena

We study a simple system that comprises all main features of critical gravitational collapse, originally discovered by Choptuik and discussed in many subsequent publications. These features include universality of phenomena, mass-scaling relations, self-similarity, symmetry between super-critical and sub-critical solutions, etc. The system we consider is a stationary membrane (representing a domain wall) in a static gravitational field of a black hole. For a membrane that spreads to infinity, the induced 2+1 geometry is asymptotically flat. Besides solutions with Minkowski topology there exists also solutions with the induced metric and topology of a 2+1 dimensional black hole. By changing boundary conditions at infinity, one finds that there is a transition between these two families. This transition is critical and it possesses all the above-mentioned properties of critical gravitational collapse. It is remarkable that characteristics of this transition can be obtained analytically. In particular, we find exact analytical expressions for scaling exponents and wiggle-periods. Our results imply that black hole formation as a critical phenomenon is far more general than one might expect.

hep-th↗

arXiv1998

Soap Bubbles in Outer Space: Interaction of a Domain Wall with a Black Hole

We discuss the generalized Plateau problem in the 3+1 dimensional Schwarzschild background. This represents the physical situation, which could for instance have appeared in the early universe, where a cosmic membrane (thin domain wall) is located near a black hole. Considering stationary axially symmetric membranes, three different membrane-topologies are possible depending on the boundary conditions at infinity: 2+1 Minkowski topology, 2+1 wormhole topology and 2+1 black hole topology. Interestingly, we find that the different membrane-topologies are connected via phase transitions of the form first discussed by Choptuik in investigations of scalar field collapse. More precisely, we find a first order phase transition (finite mass gap) between wormhole topology and black hole topology; the intermediate membrane being an unstable wormhole collapsing to a black hole. Moreover, we find a second order phase transition (no mass gap) between Minkowski topology and black hole topology; the intermediate membrane being a naked singularity. For the membranes of black hole topology, we find a mass scaling relation analogous to that originally found by Choptuik. However, in our case the parameter $p$ is replaced by a 2-vector $\vec{p}$ parametrizing the solutions. We find that $Mass\propto|\vec{p}-\vec{p}_*|^γ$ where $γ\approx 0.66$. We also find a periodic wiggle in the scaling relation. Our results show that black hole formation as a critical phenomenon is far more general than expected.

hep-th↗