Cette thèse se concentre sur l'étude de composés magnétiques géométriquement frustrés où les ions magnétiques se situent aux sommets d'un réseau de tétraèdres partageant leurs sommets: les composés pyrochlores. Deux familles de formule chimique R2M2O7, où R est un lanthanide et M= Ti, Sn, sont particulièrement intéressantes puisqu'elles présentent une grande variété d'états magnétiques exotiques. Premièrement, nous avons étudié le champ cristallin agissant au site de la terre rare dans l'approximation de Stevens où uniquement le terme fondamental est considéré. Un jeu unique de paramètres a été déterminé pour chaque famille considérée grâce à une analyse globale incluant des spectres de neutrons inélastiques de plusieurs composés. Ensuite, nous avons caractérisé avec un large éventail de techniques les propriétés physiques à basse température de Nd2Sn2O7. En dessous de la température de transition Tc=0.91 K, ce composé possède un ordre magnétique à longue portée dans la configuration de spins dite "all-in-all-out". Une persistance de la dynamique de spins a été révélée dans la phase ordonnée, attribuée à des excitations unidimensionnelles de spins. Une dynamique de spins anormalement lente est également reportée dans la phase paramagnétique. Enfin, nous avons apporté quelques informations sur les deux états fondamentaux proposés pour le composé très étudié Tb2Ti2O7: premièrement, l'apparition d'une transition Jahn-Teller à basse température est suggérée mais l'absence d'élargissement des pics de Bragg réfute la présence d'une transition structurale. Enfin ce composé pourrait être un exemple d'une glace de spin quantique mais l'existence d'un plateau d'aimantation n'est pas évident jusqu'à T=20 mK. / This Phd thesis focuses on the study of magnetically frustrated compounds where magnetic ions lie at the vertices of a corner-sharing tetrahedra network: the pyrochlore compounds. The two series of chemical formula R2M2O7, where R is a lanthanide and M=Ti, Sn, are of peculiar interest since they display a large variety of exotic magnetic ground states. First, we have studied the crystal-electric-field acting at the rare earth within the Stevens approximation where only the ground state multiplet is considered. A single set of parameters for each families of interest has been determined through a global analysis including several inelastic neutron scattering spectra of various compounds. Then, we have characterised with a large panel of techniques the low temperature physical properties of Nd2Sn2O7. This compound enters a long-range magnetic order at transition temperature Tc=0.91 K with an ``all-in-all-out'' spin configuration. A persistence of spin dynamics has been found in the ordered phase, ascribed to one-dimensional spin loops excitations. Anomalously slow paramagnetic spin fluctuations are also reported. Finally, we have brought information on the two proposed ground states of the widely studied compound Tb2Ti2O7: first, a Jahn-Teller transition is claimed to occur at low temperatures but no broadening of the Bragg peaks is seen down to T=4 K precluding premises of a structural transition. Secondly, this compound could be a realisation of a quantum spin-ice but no definitive evidence of a magnetisation plateau is found down to T=20 mK.

Cette thèse se concentre sur l'étude de composés magnétiques géométriquement frustrés où les ions magnétiques se situent aux sommets d'un réseau de tétraèdres partageant leurs sommets: les composés pyrochlores. Deux familles de formule chimique R2M2O7, où R est un lanthanide et M= Ti, Sn, sont particulièrement intéressantes puisqu'elles présentent une grande variété d'états magnétiques exotiques. Premièrement, nous avons étudié le champ cristallin agissant au site de la terre rare dans l'approximation de Stevens où uniquement le terme fondamental est considéré. Un jeu unique de paramètres a été déterminé pour chaque famille considérée grâce à une analyse globale incluant des spectres de neutrons inélastiques de plusieurs composés. Ensuite, nous avons caractérisé avec un large éventail de techniques les propriétés physiques à basse température de Nd2Sn2O7. En dessous de la température de transition Tc=0.91 K, ce composé possède un ordre magnétique à longue portée dans la configuration de spins dite "all-in-all-out". Une persistance de la dynamique de spins a été révélée dans la phase ordonnée, attribuée à des excitations unidimensionnelles de spins. Une dynamique de spins anormalement lente est également reportée dans la phase paramagnétique. Enfin, nous avons apporté quelques informations sur les deux états fondamentaux proposés pour le composé très étudié Tb2Ti2O7: premièrement, l'apparition d'une transition Jahn-Teller à basse température est suggérée mais l'absence d'élargissement des pics de Bragg réfute la présence d'une transition structurale. Enfin ce composé pourrait être un exemple d'une glace de spin quantique mais l'existence d'un plateau d'aimantation n'est pas évident jusqu'à T=20 mK. / This Phd thesis focuses on the study of magnetically frustrated compounds where magnetic ions lie at the vertices of a corner-sharing tetrahedra network: the pyrochlore compounds. The two series of chemical formula R2M2O7, where R is a lanthanide and M=Ti, Sn, are of peculiar interest since they display a large variety of exotic magnetic ground states. First, we have studied the crystal-electric-field acting at the rare earth within the Stevens approximation where only the ground state multiplet is considered. A single set of parameters for each families of interest has been determined through a global analysis including several inelastic neutron scattering spectra of various compounds. Then, we have characterised with a large panel of techniques the low temperature physical properties of Nd2Sn2O7. This compound enters a long-range magnetic order at transition temperature Tc=0.91 K with an ``all-in-all-out'' spin configuration. A persistence of spin dynamics has been found in the ordered phase, ascribed to one-dimensional spin loops excitations. Anomalously slow paramagnetic spin fluctuations are also reported. Finally, we have brought information on the two proposed ground states of the widely studied compound Tb2Ti2O7: first, a Jahn-Teller transition is claimed to occur at low temperatures but no broadening of the Bragg peaks is seen down to T=4 K precluding premises of a structural transition. Secondly, this compound could be a realisation of a quantum spin-ice but no definitive evidence of a magnetisation plateau is found down to T=20 mK.

Cette thèse de doctorat a pour cadre l’expérience CMS auprès du grand collisionneur de protons du CERN, le LHC. Le LHC, qui a permis la découverte en 2012 du boson de Brout-Englert-Higgs, est destiné à fonctionner pour encore 20 ans, avec une luminosité qui croîtra progressivement pour atteindre d’ici 2025 la valeur de 7.5 x 10^34 cm^-2 s^-1, c'est à dire environ cinq fois la valeur initialement prévue. Ceci a pour conséquence que les expériences doivent s’adapter et mettre à niveau une série de leurs composants et détecteurs. Une des prochaines mises à niveau de l’expérience CMS concerne les détecteurs Triple Gas Electron Multiplier (GEM) qui sont actuellement en développement pour la partie avant du spectromètre à muons de l’expérience. Ces détecteurs seront installés dans CMS durant le deuxième long arrêt du LHC, en 2018-2019, appelé LS2. Cette mise à niveau a pour but de contrôler les taux de déclenchement d’événements pour la détection de muons, grâce à la haute performance de ces détecteurs Triple GEM en présence de taux de particules extrêmement élevés (>1 kHz/cm^2). De plus, grâce à sa très bonne résolution spatiale (~250 um), la technologie GEM peut améliorer la reconstruction des traces de muons et la capacité d’identification du détecteur avant.Le but de mon travail de recherche est d’estimer la sensitivité des Triple GEMs à l’environnement de radiation hostile dans CMS, essentiellement composé de neutrons et de photons produits lors des interactions entre les particules et les détecteurs constituant l’expérience CMS. L’estimation précise de cette sensitivité est très importante, car une sous-estimation pourrait avoir des effets désastreux pour l’efficacité des Triple GEMs une fois installés dans CMS. Pour valider mes simulations, j’ai également reproduit des résultats expérimentaux obtenus avec d’autres détecteurs similaires déjà installés dans CMS, tels que les Resistive Plate Chambers (RPC).La deuxième partie de mon travail concerne l’étude de la capacité de l’expérience CMS à discerner différents modèles de nouvelle physique prédisant l’existence de bosons vecteurs, appelés Z'. Ces modèles font partie des extensions plausibles du Modèle Standard. En particulier, l’analyse se concentre sur des simulations dans lesquelles le Z' se désintègre en deux muons, et sur l’impact que les mises à niveau avec les détecteurs Triple GEM apporteront à ces mesures tout le long de la phase de haute intensité du LHC. Mes simulations montrent que plus de 20% des événements simulés comptent au moins un muon dans la région en pseudo-rapidité (eta) de CMS couverte par les détecteurs Triple GEM. Les résultats préliminaires démontrent que, dans le case de modèles à 3 TeV/c^2, il sera possible dès la fin de la Phase I de distinguer un Z'I d'un Z'SSM avec un niveau de signification alpha > 3 sigma. / This PhD thesis takes place in the CMS experiment at CERN's Large Hadron Collider (LHC). The LHC allowed the discovery of the Brout-Englert-Higgs boson in 2012, and is designed to run for at least 20 years, with an increasing luminosity that will reach by 2025 a value of 7.5 x 10^34 cm^-2 s^-1, that is a yield five times greater than the one initially intended. As a consequence, the experiments must adapt and upgrade many of their components and particle detectors. One of the foreseen upgrades of the CMS experiment concerns the Triple Gas Electron Multiplier (GEM) detectors, currently in development for the forward muon spectrometer. These detectors will be installed in CMS during the second long LHC shutdown (LS2), in 2018-2019. The aim of this upgrade is to better control the event trigger rate at Level 1 for muon detection, thanks to the high performance of these Triple GEM detectors, in presence of very high particle rates (>1 kHz/cm^2). Moreover, thanks to its excellent spatial resolution (~250 um), the GEM technology can improve the muon track reconstruction and the identification capability of the forward detector.The goal of my research is to estimate the sensitivity of Triple GEMs to the hostile background radiation in CMS, essentially made of neutron and photons generated by the interaction between the particles and CMS detectors. The accurate evaluation of this sensitivity is very important, as an underestimation could have ruinous effects of the Triple GEMs efficiency, once they are installed in CMS. To validate my simulations, I have reproduced experimental results obtained with similar detectors already installed in CMS, such as the Resistive Plate Chambers (RPC).The second part of my work regards the study of the CMS experiment capability to discriminate between different models of new physics predicting the existence of neutral vector bosons called Z'. These models belong to plausible extensions of the Standard Model. In particular, the analysis is focused on simulated samples in which the Z' decays in two muons, and on the impact that the Triple GEM detectors upgrades will bring to these measurements during the high luminosity phase of the LHC, called Phase II. My simulations prove that more than 20% of the simulated events see at least one muon in the CMS pseudo-rapidity (eta) region covered by Triple GEM detectors. Preliminary results show that, in the case of 3 TeV/c^2 models, it will be possible already at the end of Phase I to discriminate a Z'I from a Z'SSM with a significance level alpha > 3 sigma. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

In this work, the electronic phase diagrams of Ca1−xNaxFe2As2 and Fe1+yTe were investigated using muon spin relaxation and Mössbauer spectroscopy. Single crystals of Ca1−xNaxFe2As2 with x = 0.00, 0.35, 0.50, and 0.67 were examined. The undoped 122 parent compound CaFe2As2 is a semi metal and shows antiferromagnetic commensurate spin density wave order below 167 K. By hole doping via Na substitution, the magnetic order is suppressed and superconductivity emerges including a Na-substitution level region, where both phases coexist. Upon Na substitution, a tilting of the magnetic moments out of the ab-plane is found. The interaction of the magnetic and superconducting order parameter in this coexistence region was studied and a nanoscopic coexistence of both order parameters is found. This is proven by a reduction of the magnetic order parameter of 7 % in x = 0.50 below the superconducting transition temperature. This reduction was analysed using Landau theory and a systematic correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures, Tc/TN, for the 122 family of the iron pnictides is presented. The magnetic phase transition is accompanied by a tetragonal-to-orthorhombic phase transition. The lattice dynamics at temperatures above and below this magneto-structural phase transition were studied and no change in the lattice dynamics were found. However, the lattice for finite x is softer than for the undoped compound. For x = 0.67, diluted magnetic order is found. Therefore, the magnetism in Ca1−xNaxFe2As2 is persistent even at optimal doping. The superconducting state is investigated by measuring the temperature dependence of the magnetic penetration depth, where two superconducting gaps with a weighting of nearly 50:50 are obtained. A temperature independent anisotropy of the magnetic penetration depth γ_λ = 1.5(4) is obtained, which is much smaller compared to other 122 compounds indicating a more three-dimensional behaviour of Ca1−xNaxFe2As2. Powder samples of Fe1+yTe with y = 0.06, 0.12, 0.13, and 0.15 were examined. Fluctuating paramagnetic moments at room temperature were found, which are independent of the excess iron level y. Below 100 K, a magnetic precursor phase is observed, which is independent of y. Fe1.06Te shows a commensurate spin density wave phase below TN, while for y ≥ 0.13 an incommensurate spin density wave phase below TN is found. However, a slowing down of the magnetic fluctuations with decreasing temperature and static magnetic order at lowest temperature are observed. / In dieser Arbeit wurden die elektronischen Phasendiagramme von Ca1−xNaxFe2As2 and Fe1+yTe mit Hilfe der Myonspinrelaxations- und Mössbauerspektroskopie untersucht. Einkristalle von Ca1−xNaxFe2As2 mit x = 0.00, 0.35, 0.50 und 0.67 wurden untersucht. Das undorierte 122-System CaFe2As2 ist ein Halbmetal und zeigt eine antiferromagnetische Spindichtewelle unterhalb von 167 K. Substituiert man Ca durch Na, werden Löcher in das System eingebracht. Die magnetische Ordnung wird mit steigendem Na-Anteil unterdrückt und Supraleitung tritt auf. Dabei existiert ein Na-Substitutionslevelbereich, in welchem Magnetismus und Supraleitung koexistieren. Desweiteren wurde ein herausdrehen der magnetischen Momente aus der ab-Ebene als Funktion von x beobachtet. Die Wechselwirkung des magnetischen mit dem supraleitenden Ordnungsparameter in der Koexistenzregion wurde untersucht und nanoskopische Koexistenz der beiden Ordnungsparameter wurde gefunden. Dies konnte durch eine Reduktion des magnetischen Ordnungsparameteres um 7 % in x = 0.50 unterhalb der supraleitenden Ordnungstemperatur gezeigt werden. Diese Reduktion wurde mit Hilfe der Landautheorie untersucht und es wurden systematische Korrelationen zwischen der Reduktion des magnetischen Ordnungsparamteres und dem Verhältnis der Übergangstemperaturen, Tc/TN, in der 122-Familie der Eisenpniktide gefunden. Der magnetische Phasenübergang wird von einem strukturellen Phasenübergang begleitet. Die Gitterdynamik wurde bei Temperaturen oberhalb und unterhalb dieses magneto-elastischen Phasenübergangs untersucht. Es wurden keine Änderungen in der Gitterdynamik festgestellt. Jedoch konnte festgestellt werden, dass das Gitter für endliche x weicher ist als für das undotierte System. Für x = 0.67 wurde festgestellt, dass der Magnetismus im Ca1−xNaxFe2As2-System auch noch bei optimaler Dotierung zu finden ist. In der supraleitenden Phase wurde die Temperaturabghängigkeit der magnetischen Eindringtiefe untersucht und es wurden zwei supraleitende Bandlücken gefunden. Die Anisotropie der magnetischen Eindringtiefe ist temperaturunabhängig und mit γ_λ = 1.5(4) wesentlich kleiner als in anderen 122- Verbindungen, was für eine erhöhte Dreidimensionalität in Ca1−xNaxFe2As2 spricht. Pulverproben von Fe1+yTe mit y = 0.06, 0.12, 0.13 und 0.15 wurden untersucht. Es wurden fluktuierende paramagnetische Momente bei Raumtemperatur gefunden, welche unabhängig vom Überschusseisenlevel y sind. Unterhalb von 100 K wurde eine magnetische Vorgängerphase gefunden, welche unabhängig von y ist. Mit fallender Temperatur wurde eine Verlangsamung der magnetischen Fluktuationen festgestellt, welche in einer statischen magnetischen Ordnung bei tiefen Temperaturen münden.

No description available.

<p>This thesis is the result of several experiments designed to probe the low temperature physics underlying the 1D-Ising-like behaviour of chains of spins in the structure of Cobalt Niobate, CoNb2O6. A collection of prior work has been done by several groups prior to this, focusing on mapping the phase diagram above 0.5K. Interest in this material was renewed recently based upon theoretical work and experimental confirmation of the unique structure of the spins in the system. The bulk of this work was done at temperatures below the previously investigated range to probe the unique properties of this system.</p> <p>The material was grown at McMaster University using the optical floating zone technique from oxide powders. The crystal was examined and oriented using single crystal and Laue diffraction and was cut for use in further experiments. Squid magnetometry was used to confirm the material properties and phase transition temperatures, and was compared to literature values.</p> <p>Heat capacity measurements were performed locally down to 2K, and by collaborators at Waterloo in the range from 330mK to 1K. The heat capacity measurement confirmed the 2.9K transition and explored the relaxation time of the material. Cobalt niobate was found to have an exceptionally long relaxation time at low temperatures indicating strong spin-spin interactions. A sharp transition with zero applied field was found to become a broad, smooth feature at 2.9K when a small field was applied.</p> <p>We performed muSR measurements in zero, longitudinal and transverse field. The muSR results confirmed the long relaxation time found by the heat capacity measurements, which may reflect the coupling of the spin system to the lattice. Additionally, the material was never seen to statically order in zero or longitudinal field down to 700mK and up to 1T. The material was found to behave dynamically throughout all the field ranges.</p> / Master of Science (MSc)

This thesis presents a study on several series of unconventional Fe-based superconductors; namely, derivatives of NaFeAs and LiFeAs, as well as molecular-intercalated FeSe. Primarily using muon spin rotation (SR), but also x-ray diffraction (XRD) and magnetic susceptibility measurements, the nature of both magnetic and superconducting phases within these systems is studied. Particular attention is focussed on how these states compete or coexist with one another. The aforementioned experimental techniques are first used to explore the phase diagram of NaFe1xCoxAs. This phase diagram includes regions of long-range antiferromagnetism and short-range order, that both coexist with superconductivity. Magnetism is gradually destroyed, primarily through a diminishment of the size of the ordered moment, as superconductivity is enhanced by Co substitution. This interplay is explored in detail. By contrast, superconductivity in LiFeAs cannot be enhanced by transition metal substitution, suggesting that it is intrinsically optimally-doped. I investigate this conclusion by studying the evolution of the penetration depth in superconducting compositions of LiFe1xCoxAs and LiFe1xNixAs, and comparing these data to those from other electron-doped systems. I also study an unusual and emergent magnetic phase in Li1yFe1+yAs. This work suggests that LiFeAs supports a superconducting phase that resembles those in other Fe-pnictides, but is uniquely close to an additional magnetic instability. I then move on to the study of a recently discovered series, based on FeSe intercalated with ammonia and various metals. I study both the penetration depth and the intrinsic magnetic phases in these systems using SR and compare them with other compounds based on FeSe. I find that these intercalated systems support spacially separated regions of dynamic magnetism and superconductivity and I discuss how much these phases depend on the precise chemical details of the intercalated layer. Finally, I return to the experimental study of NaFe1xCoxAs, extending the range of techniques employed by using high-field magnetometry and high-pressure SR. These studies reveal new features of this system that were not accessible using low-field and ambient-pressure methods. In particular, I show that the magnetic moment size in NaFeAs unexpectedly increases with pressure, suggesting that the electronic structure of this compound may be unique amongst known Fe-based superconductors.

This thesis presents the design qualification and construction of the Beam Scintillator Counter (BSC) for the CMS Collaboration at CERN in 2007 - 2008. The BSC detector is designed to aid in the commissioning of the Compact Muon Solenoid (CMS) during the first 2 years of operation and provide technical triggering for beam halo and minimum-bias events. Using plastic scintillator tiles mounted at both ends of CMS, it will detect minimum ionizing particles through the low-to-mid luminosity phases of the Large Hadron Collider (LHC) commissioning. During these early phases, the BSC will provide probably the most interesting and widely used data of any of the CMS sub-detectors and will be employed in the track based alignment procedure of the central tracker and commissioning of the Forward Hadron Calorimeter.

No description available.

This thesis is concerned with how the physical properties of transition metal oxides change due to chemical substitution or intercalation. Experiments using the muon-spin relaxation and rotation (μSR) techniques were carried out at the ISIS Facility (UK) and the Paul Scherrer Institute (CH). In conjunction with the μSR results, the results of heat capacity and magnetic susceptibility experiments are used to provide complementary information on the same samples. Investigations of the properties of the layered triangular lattice magnets NaNiO2 and LiNiO2 are presented. For NaNiO2, all three experimental techniques are used to provide a full survey of the thermodynamic and magnetic properties of this compound. For LiNiO2, μSR studies of notionally stoichiometric and Mg-doped samples were carried out. These showed that Mg doping causes a significant change in the magnetic dynamics of the material, but neither sample exhibits long-range magnetic order. The magnetic ordering of the extensively studied perovskite compounds LaTiO3 and YTiO3 is investigated using μSR. The results were in agreement with previous neutron diffraction studies of the two compounds, but clarified the orientation of the magnetic moments in LaTiO3. It was also possible to make a detailed comparison between the μSR results and those of dipole field calculations of the magnetic field at possible muon stopping sites, allowing these to be deduced and compared with results in other well characterized transition metal oxides. The two titanium chain compounds NaTiSi2O6 and TiOCl exhibit spin gap formation at unusually high temperatures due to unconventional dimerization mechanisms. A model allowing the comparison of X-ray diffraction data, dimerization, and the magnitude of the spin gap is proposed. This is tested against both magnetic susceptibility and μSR data for both compounds. For NaTiSi2O6 both experimental techniques are in reasonable agreement, whereas in TiOCl the results are conclusively different. The origin of this disparity in TiOCl is explored. The intercalation of organic chain molecules into Bi based high-temperature superconductors has previously been demonstrated to extend the interlayer spacing by a factor of up to three without changing the superconducting transition temperature. μSR is used to investigate the London penetration depth, as a function of interlayer spacing, of two series of such samples. The results show a simple trend corresponding to a constant density of superconducting electron pairs in each layer. The consequences of this result are discussed in the context of previously identified scaling relations between superconducting parameters. Results of experiments excluding the possibility of magnetic order and muon-organic radical formation in these samples are presented, as well a preliminary study of the field distributions in a mosaic of intercalated crystallites.