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Magnetism in Quantum Materials probed by X-ray and Neutron Scattering
Full text linkIn 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
Full text linkPrBa2Cu3O6+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
Her Dark Materials: exploration of magnetic phenomena in quantum materials using neutron scattering
Full text linkIn this thesis I present experimental investigations of four different quantum materials, each of which showcases the strongly correlated magnetism of 3d transition metal ions.
The topological semimetal candidate YbMnSb2 has a high magnetic ordering temperature of TN ≈ 345 K. I performed unpolarised and polarised neutron diffraction, as well as triple-axis spectroscopy on YbMnSb2. From these results, I refined the magnetic and crystal structure of YbMnSb2. Similar to other square net antimonides YbMnSb2 displays C-type antiferromagnetism with the moments oriented along the c axis. I interpreted the spin wave spectrum using linear spin wave theory, where the predominant exchange interactions are in-plane; however, the out-of-plane exchange along the c axis is surprisingly large. This interaction acts through the Sb square net and so may influence the charge transport properties of the material. Our magnetic model of YbMnSb2 is consistent with this compound being a gapped Dirac semimetal.
The topological insulator MnSb4Te7 is closely related to the much-studied van der Waals crystal MnBi2Te4. MnSb4Te7 had been identified as a possible axion insulator candidate. However, the ground state magnetic structure in the bulk had not been determined. I utilised single crystal neutron diffraction for this task and found that there is a strong degree of site-mixing between Mn and Sb sites. This leads to a magnetic structure where Mn2+ moments are coupled ferrimagnetically within the site-mixed magnetic layers but antiferromagnetically between magnetic layers with propagation vector q = (0, 0, 1/2). The application of an external magnetic field of 0.2 T drives a spin-flip transition, removing the interlayer antiferromagnetic coupling. The magnetism in MnSb4Te7 is increasingly 2D at low temperatures.
The charge-doped antiferromagnetic Mott insulator La(2–x)Ba(x)CoO4 is a structural analogue of the high temperature cuprate superconductors, which have properties that are also sensitive to dopant concentration x. The cuprates show a signature hour-glass shaped spin wave spectrum, which has also been observed in other similar insulators and explained using a disordered charge stripe model. I studied this material through magnetometry, time-of-flight neutron spectroscopy and single crystal diffraction. I discovered that the magnetic order in La(2-x)Ba(x)CoO4 is very short range with a correlation length of ≈ 15 Å and is described by the incommensurate propagation vector q = (0.5±0.23, 0.5±0.23) in the (h, h) plane for x = 0.5. The spin wave spectrum has the characteristic hour-glass shape of the cuprates. These results are in contrast to the equivalent Sr-doped compound La1.5Sr0.5CoO4, which has long-range collinear magnetic order and does not feature the hour-glass.
Finally, the rare earth iron garnets are of prime interest for spintronic applications due to their very coherent, spin-polarised magnons. I present a study of the novel cobalt-doped rare earth iron garnet Lu3Co0.5Si0.5Fe4O12, which utilised magnetometry and two different inelastic neutron scattering techniques. This material orders ferrimagnetically at TC ≈ 480 K. Introducing cobalt generates a high degree of magnetocrystalline anisotropy in the system at low temperatures, comparable with the magnetic rare earths. However, at room temperature, Lu3Co0.5Si0.5Fe4O12 is a soft magnet, similar to YIG and LuIG. This is an indication that the Co2+ moments are disordered by 300 K. The spin wave spectrum is gapped at low temperatures. The gap between the first acoustic and optical magnon modes is temperature-dependent and comparable with thermal energies at 300 K
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Magnetism in topological materials
Full text linkNotions of topology are of considerable interest currently in physics, especially in the way topology influences the electronic properties of crystalline solids. This thesis concerns the experimental investigation of four different magnetic materials, which have been predicted to display a strong interplay between magnetism and the topology of the electronic band structure.
I demonstrate that EuCd2Sb2, below TN= 7.4 K, displays an A-type anti-ferromagnetic order on the Eu sub-lattice with a magnetic propagation vector of k=(0,0,1/2). I also establish that the C3 rotational symmetry along the crystal c axis (essential for the stabilisation of band crossings called Dirac nodes) is broken by the in-plane orientation of the Eu moments.
Following the initial discoveries of crystalline solids which can host topologically-protected band crossings called Weyl nodes, there is now a need for better material realisations, ideally comprising a single pair of nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. I propose that EuCd2As2, in a magnetic field of B >1.6 T along the c axis, to be such a system. This material is of general interest as it represents the simplest possible Weyl semimetal, and is therefore a model system for fundamental investigations of Weyl physics.
I refute the proposition that semimetallic YbMnBi2 hosts Weyl nodes induced by the time-reversal symmetry breaking mechanism. Furthermore, the full magnetic excitation spectrum of YbMnBi2, which has been mapped for the first time, demonstrates that the magnetic order of the Mn sub-lattice is weakly coupled to the charge carriers in the Bi square net.
Finally, the complex magnetic order of the Mn sub-lattice in Mn3Ge has been elucidated for the first time. This will shed light on the origin of the large anomalous Hall effect in Mn3Ge, which is unusual for an antiferromagnet
Variations on the Author
Full text link“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
Experimental studies of electronic correlations in quantum materials
Full text linkIn this thesis I present experimental studies of three quantum materials, resolving an important unanswered question about the novel microscopic physics in each case.
I first present inelastic neutron scattering measurements on a powder sample of the unconventional, high-temperature superconductor lithium iron selenide hydroxide, Li1-xFexODFe1-ySe (x = 0.16; y = 0.02, Tc = 41 K). The spectrum shows an enhanced intensity below Tc at energy transfers below the superconducting pair breaking energy, with broad maxima at two different wavevectors. The behaviour of this feature is consistent
with the spin resonance mode found in other unconventional superconductors, and strongly resembles the spin resonance observed in the spectrum of the molecular intercalated iron selenide, Li0.6(ND2)0.2(ND3)0.8Fe2Se2. The signal can be described with a characteristic two-dimensional wave vector which is consistent with the nesting vector between electron Fermi sheets. These results place a strong constraint on the pairing symmetry in this unconventional superconductor and may have relevance to the high temperature superconductor monolayer FeSe due to structural and electronic similarities
between the two systems.
In Na2Ti2Pn2O (Pn = As, Sb), two important members of the titanium oxypnictide family of superconductors, I then present single crystal x-ray diffraction data which reveal a charge superstructure that appears below density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements I am able to completely determine the symmetry and all atomic positions in the distorted phase. I
also analyse angle resolved photoemission spectroscopy (ARPES) data, which show band folding and back bending consistent with a density wave with the same symmetry as the
lattice distortion as well as evidence for Fermi surface nesting which may help drive the density wave transition. The results provide direct evidence for phonon-assisted charge
density wave order in Na2Ti2Pn2O. This represents the first complete characterisation of the density wave phase in a titanium oxypnictide, information which is crucial in microscopic modelling of the superconductivity in this family.
Finally I present a detailed study of the pyrochlore osmate Y2Os2O7 via a wide variety of experimental techniques. I observe a small, non-zero paramagnetic moment and spin freezing at temperature Tf = 5 K, consistent with previous results, and show based on high-field magnetisation measurements that the paramagnetic moment is very likely to be due to large effective moments located on a small fraction ~ 2% of Os sites.
Comparison of single-ion energy level calculations with resonant inelastic x-ray scattering (RIXS) data yields a non-magnetic Jeff = 0 ground state on the majority Os4+ sites with the spin-orbit interaction, Hund's coupling and trigonal distortion of OsO6 octahedra all important in modelling the experimentally observed spectra. These results fully explain the unexpected magnetism in Y2Os2O7, and the single-ion spectrum may prove useful to inform the search for novel excitonic magnetism in other, related 5d materials.</p
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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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