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Polarised neutron diffraction measurements of PrBa2Cu3O6+x and the Bayesian statistical analysis of such data
Get PDFThe physics of the series PryY1-yBa2Cu3O6+x, and ability of Pr to suppress superconductivity, has been a subject of frequent discussions in the literature for more than a decade. This thesis describes a polarised neutron diffraction (PND) experiment performed on PrBa2Cu3O6.24 designed to find out something about the electron structure. This experiment pushed the limits of what can be done using the PND technique. The problem is one of a limited number of measured Fourier components that need to be inverted to form a real space image. To accomplish this inversion the maximum entropy technique has been employed. In some cases, the maximum entropy technique has the ability to increase the resolution of ‘inverted’ data immensely, but this ability is found to depend critically on the choice of constants used in the method. To investigate this a Bayesian robustness analysis of the maximum entropy method is carried out, resulting in an improvement of the maximum entropy technique for analysing PND data. Some results for nickel in the literature have been re-analysed and a comparison is made with different maximum entropy algorithms. Equipped with an improved data analysis technique and carefully measured PND data for PrBa2Cu3O6.24 a number of new interesting features are observed, putting constraints on existing theoretical models of PryY1-yBa2Cu3O6+x and leaving room for more questions to be answered
Cooperative Spin Excitations in Quantum Materials Studied by Neutron Spectroscopy
Get PDFThis thesis describes the experimental investigation of three different strongly correlated transition-metal oxide systems. The magnetic behaviour of each has been probed using inelastic neutron spectroscopy. A distinctive hour-glass excitation spectrum has been observed in the layered cobaltate La1.75Sr0.25CoO4. This spectrum is similar to that measured in a related cobaltate La1.67Sr0.33CoO4, although it appears broader. The spectrum has been reproduced using a spin wave model derived from a disordered cluster spin glass ground state. Signatures of spin glass behaviour have also been observed in bulk magnetisation measurements of La1.75Sr0.25CoO4. These findings, once more, demonstrate the emergence of an hour-glass spectrum from a ground state that combines quasi-one dimensional magnetic correlations and disorder. Additionally, this study shows that charge and magnetic stripe order persists to lower dopings in La2-xSrxCoO4 than previously thought. The complete magnetic excitation spectrum of the multiferroic compound CuO has been measured for the first time. A high energy, one-dimensional magnetic spectrum is observed and modelled using the Muller ansatz derived for the S=1/2 Heisenberg antiferromagnetic chain. At lower energies, a three-dimension spectrum is observed. The measured spectrum is inconsistent with all previous theoretical estimates of the dominant inter-chain exchange interactions in CuO. The inter-chain dispersion is successfully described by a phenomenological model based on linear spin wave theory. The third material investigated, LuFe2O4 demonstrates complex charge and magnetic order, the precise nature of which is still under debate. The full spectrum of in-plane excitations in LuFe2O4 has been measured and a complicated dispersion consistent with six magnetic modes is observed. These findings are compatible with structures described by a magnetic unit cell containing six spins. The dispersion can be described by a spin wave model derived from a bilayer structure comprised of charge-rich and charge-poor monolayers. This structure is consistent with the original site-specific model for the 3D magnetic ordering in LuFe2O4
Neutron and x-ray scattering studies of strongly correlated electron systems
Get PDFIn this thesis results of x-ray scattering and neutron scattering experiments on several strongly correlated transition metal oxides are presented. The prototypical charge ordered cuprate La1.48Nd0.4Sr0.12CuO4 was investigated using polarised neutron scattering. The results show that several proposed schemes for the magnetic order in this class of materials may be ruled out, however the data are consistent with one-dimensional stripe-like magnetic order. X-ray diffraction was used to show that the charge order is insensitive to an applied magnetic field, but might be affected by the existence of superconductivity. The magnetic excitations were also studied, and at low energies a gap in the magnetic fluctuations was observed and there is tentative evidence that this is related to magnetic anisotropy. The spin state transition in LaCoO3 was investigated using neutron inelastic scattering, and excitations reminiscent of those observed in ferromagnets above their critical temperatures were observed. The debate surrounding the nature of the excited spin state, S=1 or S=2, could not be resolved, however. The nature of the spin excitations in La0.82Sr0.18CoO3 was investigated using polarised neutrons and it was found that at low energies the excitations take the form of spin-waves. At higher energies this mode becomes heavily damped, and several possible damping mechanisms for this are discussed. Finally, the multiferroic material DyMn2O5 was studied using x-ray resonant scattering. A complex, temperature dependent, magnetic structure was found using a Dy resonance, which reflects an underlying order of the Mn ions. The measurements were in agreement with a theory of multiferroics based on acentric spin-density waves
Magnetism in Quantum Materials probed by X-ray and Neutron Scattering
Get PDFIn his programmatic article More Is Different (1972), Nobel laureate P. W. Anderson captured the
fundamental interest in quantum matter in a nutshell. The central motive in this field
is emergence. In the inaugural volume of the homonymous journal, J. Goldstein defined this
as âthe arising of novel and coherent structures, patterns and properties during the process of
self-organization in complex systems". Famously, the idea that the "the whole is greater
than the sum of its parts" goes back to Aristotle's metaphysics, and it has served as a stimulating
concept in 19th century biology, economics and philosophy.
The study of emergence in condensed matter physics is unique in that the underlying complex
systems are sufficiently "simple" to be modelled from first principles. Notably, the emergent
phenomena discovered in this field, such as high-temperature superconductivity, giant magnetoresistance, and strong permanent magnetism have had an enormous impact on
technology, and thus, society. Historically, there has been a distinction between materials with localized,
strongly interacting (or correlated) electrons â and non-interacting, itinerant electronic
states. In the last decade, several new states of matter have been discovered, which emerge not
from correlations, but from peculiar symmetries (or topology) of itinerant electronic states.
The term quantum materials has therefore become popular to subsume these two strands of condensed
matter physics: Electronic correlations and topology.
In this thesis, I report investigations of four quantum materials which each illustrate present key
interests in the field: The mechanism of high temperature superconductivity, the search
for materials that combine both electronic correlations and non-trivial topology and novel
emergent phenomena that arise from the synergy of electronic correlations and a strong coupling
of spin- and orbital degrees of freedom. The common factor and potential key to understanding
these materials is magnetism. My experimental work is focused on neutron and x-ray
scattering techniques, which are able to determine both order and dynamics of magnetic states at
the atomic scale.
I illustrate the full scope of these methods with experimental studies at neutron and synchrotron
radiation facilities. This includes both diffraction and spectroscopy, of either single- or polycrystalline
samples. My in-depth analysis of each dataset is aided by structural, magnetic and charge
transport experiments. Thus, I provide a quantitative characterization of magnetic fluctuations
in an iron-based superconductor and in two Dirac materials, and determine the magnetic
order in a Dirac semimetal candidate and a complex oxide. As a whole, these results
demonstrate the elegant complementarity of modern scattering techniques. Although such methods
have a venerable history, they are presently developing at a rapid pace. Several results of this
thesis have only been enabled by very recent instrumental advances.</p
Neutron scattering and praseodymium suppression of superconductivity
Get PDFPrBa2Cu3O6+x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa2Cu3O6+x:it is the only compound which has the same structure as YBa2Cu3O6+x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa2Cu3O6+x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O2 planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa2Cu3O6+x, I have used instead the compounds PrO2 and Y0.3Pr0.7Ba2Cu4O8, which contain praseodymium in a chemically similar environment. Y0.3Pr0.7Ba2Cu4O8 shows the same broadening effects as PrBa2Cu3O6+x; PrO2 shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail
Magnetic dynamics in iron-based superconductors probed by neutron spectroscopy
Get PDFThis thesis describes inelastic neutron scattering (INS) experiments on several iron-based materials. The experiments were primarily designed to investigate the link between magnetic dynamics and superconductivity. The work contributes to evidence that magnetic fluctuations influence or are influenced by superconductivity. It is demonstrated that the INS response of a material, in conjunction with theoretical models, can provide valuable information about both superconductivity and magnetism. I measured the magnetically ordered parent-compound SrFe2As2 to investigate the nature of magnetism in iron-based systems. Comparison of the data to models based on both itinerant and localised magnetism showed that an itinerant model offers the best description of the data. LiFeAs is a superconductor that shows no magnetic order, however I was able to distinguish a magnetic signal in its INS spectrum. The signal is consistent with the magnetic resonance observed in several other iron-based superconductors. This indicates that LiFeAs likely hosts an s± gap symmetry. I investigated two iron-phosphide systems, LaFePO and Sr2ScO3FeP, and in this case I was unable to identify any magnetic scattering. Comparison to LiFeAs showed that any signal in LaFePO is at least 7 times weaker. These results suggest that magnetic fluctuations are not as influential to the electronic properties of iron-phosphide systems as they are in other iron-based superconductors. In CsxFe2−ySe2 I found two independent signals that appear to be related to phase-separated magnetic and superconducting regions of the sample. I showed that fluctuations associated with the magnetically ordered phase are consistent with localised magnetism, and do not respond to superconductivity. The second signal, however, increases in intensity below the superconducting transition temperature Tc = 27K, consistent with a magnetic resonance. This could be indicative of a pairing symmetry in CsxFe2−ySe2 that is distinct from most other iron-based superconductors. Finally, the molecular intercalated FeSe compound Li0.6(ND2)0.2(ND3)0.8Fe2Se2 revealed strong magnetic fluctuations. Again the signal was consistent with a magnetic resonance responding to Tc = 43 K. The results suggest that Lix(ND2)y(ND3)1−yFe2Se2 is similar to the superconducting phase of CsxFe2−ySe2, placing constraints on theoretical models to describe the molecular intercalated FeSe compounds
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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