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MUON SPIN ROTATION STUDIES OF URU2SI2 AND DICHALCOGENIDE SUPERCONDUCTORSWilson, Murray Neff 14 June 2018 (has links)
This dissertation details studies of two different material classes: isoelectronically doped URu2Si2, and dichalcogenide superconductors, both of which are primarily studied with the muon spin rotation (μSR) experimental technique.
The objective of the work on URu2Si2 was to probe how the low temperature "hidden order" state, which transitions into antiferromagnetism under hydrostatic pressure, evolves when perturbed by isoelectronic chemical doping. μSR measurements of iron doped URu2Si2, which produces positive chemical pressure, show long range magnetic order. Neutron diffraction measurements demonstrate that this magnetic order is antiferromagnetism, and both muon spin rotation and neutron scattering suggest that the magnetic moment increases with increasing doping in contrast to the pressure independent moment seen in the pressure induced antiferromagnetic state of URu2Si2. Inelastic neutron scattering measurements show a significantly larger commensurate gap at the (1 0 0) position compared to that seen in the pressure induced antiferromagnetic phase. Osmium doping, which gives negative effective chemical pressure, shows similar behaviour in μSR measurements to the iron doped samples. This suggests that these samples are also antiferromagnetic and that the evolution from hidden order to antiferromagnetism is not solely caused by changes in the lattice size. This is further supported by μSR measurements on germanium doped samples that do not show magnetic order despite giving similar negative chemical pressure to the osmium doped samples.
Work on the dichalcogenide superconductors involved using transverse field μSR to measure the temperature dependence of the magnetic penetration depth of two different materials, Pt0.05Ir0.95Te2 and PbTaSe2. The μSR data on Pt0.05Ir0.95Te2 were supplemented by magnetometry measurements of the penetration depth. Zero field μSR measurements were also performed on PbTaSe2, and rule out any time reversal symmetry breaking field greater than 0.05 G. These measurements all suggest that both materials are fully gapped superconductors. / Thesis / Doctor of Philosophy (PhD)
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An ARPES study of correlated electron materials on the verge of cooperative orderTrinckauf, Jan 08 January 2015 (has links) (PDF)
In this thesis the charge dynamics of correlated electron systems, in which a metallic phase lies in close proximity to an ordered phase, are investigated by means of angle resolved photoemission spectroscopy (ARPES). The analysis of the experimental data is complemented by electronic structure calculations within the framework of density functional theory (DFT).
First the charge dynamics of the colossal magnetoresistant bilayer manganites are studied. The analysis of the ARPES spectra based on DFT calculations and a Peierls type charge density wave model, suggests that charge, orbital, spin and lattice degrees of freedom conspire to form a fluctuating two dimensional local order that produces a large pseudo gap of about 450 meV in the ferromagnetic metallic phase and that reduces the expected bilayer splitting.
Next, the interplay of Kondo physics and (magnetic) order in the heavy fermion superconductor URu2Si2 is investigated. The low energy electronic structure undergoes strong changes at 17.5 K, where a second order phase transition occurs whose phenomenology is well characterized, but whose order parameter could not yet be unambigeously identified. Below THO, non-dispersive quasi particles with a large scattering rate suddenly acquire dispersion and start to hybridize with the conduction band electrons. Simultaniously the scattering rate drops sinificantly and a large portion of the Fermi surface vanishes due to the opening of a gap within the band of heavy quasi particles. The observed behaviour is in stark contrast to conventional heavy fermion systems where the onset of hybridization between localized and itinerant carriers happens in a crossover type transition rather than abruptly. These experimental results suggest that Kondo screening and the hidden order parameter work together to produce the unusual thermodynamic signatures observed in this compound.
Finally, the influence of charge doping and impurity scattering on the superconducting porperties of the transition metal substituted iron pnictide superconductor Ba(Fe1-xTMx)2As2 (TM = Co, Ni) is studied. Here, resonant soft X-ray ARPES is applied to see element selective the contribution of the 3d states of the TM substitute to the Fe 3d host bands.
The spectroscopic signatures of the substitution are found to be well reproduced by DFT supercell and model impurity calculations. Namely, the hybridization of the dopant with the host decreases with increasing impurity potential and the electronic states of the impurtiy become increasingly localized. Simultaniously, in all simulated cases a shift of the Fermi level due to electron doping is observed. The magnitude of the shift in the chemical potential that accurs in BaFe2As2, however, is in stark contrast to the marginal doping values obtained for the impurity model, where the shift of the chemical potential is largely compensated by the influence of the increasing impurity potential. This suggests that the rigid band behaviour of TM substituded BaFe2As2 is a peculiarity of the compound, which has strong implications for the developement of superconductivity. / In dieser Arbeit wird die Ladungstraegerdynamik in korrelierten Elektronensystemen, in denen eine metallische Phase in direkter Nachbarschaft zu einer geordneten Phase liegt, mit Hilfe von winkelaufgeloester Photoelektronenspektroskopie (ARPES) untersucht. Die Analyse der experimentellen Daten wird ergaenzt durch lektronenstrukturrechnungen im Rahmen der Dichtefunktionaltheorie (DFT).
Zuerst wird die Ladungstraegerdynamik in gemischtvalenten zweischichtmanganaten mit kolossalem Magnetiwiderstand studiert. Die Analyse der Photoemissionsspektren basierend auf DFT Rechnungen und einem Peierls artigem Ladungsdichtewellenmodell, legt nahe, dass die Freiheitsgrade von Ladung, Orbitalen, Spin und des Ionengitters konspirieren, um eine fluktuierende zweidimensionale lokale Ordnung zu bilden, die verantwortlich ist fuer die beobachtete Pseudobandluecke von 450 meV, und die zur Reduktion der erwarteten Zweischichtaufspaltung beitraegt. Als naechstes wird das Zusammenspiel von Kondo Physik und (magnetischer) Ordung im Schwerfermionensupraleiter URu2Si2 untersucht. Die iedrigenergetische elektronische Struktur zeigt starke Veraenderungen bei 17.5 K, wo ein Phasenuebergang zweiter Ordnungstattfindet, der phenomenologisch gut charakterisiert ist, aber dessen Ordungsparameter nocht nicht eindeutig identifiziert werden konnte. Unterhalb von THOerlangen nicht dispergierende Quasiteilchen mit gro en Streuraten abrupt Dispersion und hybridisieren mit den Leitungselektronen. Gleichzeitig sinkt die Streurate und ein gro er Teil der Fermiflaeche verschwindet durch das Oeffnen einer Bandluecke innehalb des Bandes schwerer Quasiteilchen.
Das beobachtete Verhalten steht in starkem Kontrast zu dem von konventionellen Schwerfermionensystemen, in denen die Hybridisierung zwischen lokalisierten und itineranten Ladungstraegern in einem kontinuierlichen Uebergang ablaeuft, anstatt abrubt. Diese experimentellen Befunde lassen den Schluss zu, dass das zusammenspiel zwischen Kondo Abschirmung und dem unbekannten Ordnungsparameter die ungewoehnlichen thermodynamischen Signaturen in dieser Verbindung hervorruft.
Abschliessend wird das Zusammenwirken von Ladungstraegerdotierung und Streuung an Stoeratomen auf die Supraleitung uebergangsmetalldotierter Eisenpniktid Supraleiter Ba(Fe1-xTMx)2As2 (TM = Co, Ni) untersucht. Mit Hilfe von resonantem Weichenroentgen ARPES gelingt es, elementselektiv den Beitrag der 3d Zustaende des TM Substituenten zu den Eisen 3d Wirtsbaendern zu beobachten. Die spektroskopischen Signaturen der Substitution sind mit Hilfe von DFT Rechnungen und Modelrechnungen mit zufaellig verteilten Stoeratomen gut zu reproduzieren. Insbesondere nimmt die Hybridisierung des dotierten Uebergangsmetalls und der Eisenbaender mit zunehmender Kernladungszahl ab und die elektronischen Zustaende der Stoeratome werden zunehmen lokalisiert. Gleichzeitig wird in allen gerechneten Faellen eine Verschiebung des Fermi Niveaus durch Elektronendotierung beobachtet. Der Betrag der Verschiebung des chemischen Potentials in BaFe2As2 steht allerdings in starkem Kontrast zu den Werten, die man im Falle der Modellrechnungen erhaelt, wo die Verschiebung des Fermi Niveaus durch den Einfluss des Potentials der Stoeratome groesstenteils kompensiert wird. Dies legt nahe, dass das beobachtete "rigid band" Verhalten von TM substituiertem BaFe2As2 eine Besonderheit dieser Verbindung ist, welches starke Auswirkungen auf die Ausbildung von Supraleitung hat.
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Theoretical Studies of Magnetism and Electron Correlation in SolidsGrånäs, Oscar January 2012 (has links)
This work presents new development and applications of ab-initio simulation tools for material science. Focus lies on materials with strong electronic correlation and strong spin-orbit coupling. Improvements on methods for solving the impurity problem in LDA+DMFT is presented, as well as a reliant method for charge self-consistency in a LMTO based electronic structure code. A new adaptive scheme for Brillouin zone integration is developed, where we show a strong reduction of numerical noise compared to standard techniques. A reformulation of the standard LDA+U method aiming to reduce the number of free parameters is introduced. Fast and realistic reduction of the number of free parameters provides the possibility of high throughput calculations and enabled us to study a large number of compounds. An analysis method for polarization in terms of coupled multipoles, and their corresponding energy contributions is developed and applied. This led to the formulation of Katt's rules, a set of rules complementary to Hund's rules. Katt's rules applies for occupying the orbitals of an electronic shell with strong spin-orbit coupling. The analysis is also used to investigate the unconventional Uranium based superconductors URu2Si2, UPt3, UPd2Al3 and UNi2Al3, as well as the high temperature superconductor LaOFeAs. We also investigate the non-magnetic delta-phase of Plutonium, providing insight to the electronic structure and the branching ratios of 4d to 5f transitions seen in photo emission spectra.The influence of surface reconstruction on the magneto crystalline anisotropy is investigated in multilayer Fe/ZnSe, showing that Fe deposited on an unreconstructed interface strongly reduces the uniaxial component of the MAE. We provide a detailed understanding of the magnetic properties of Fe2P, opening possible routes for enhancing the MAE in this system. A general route to strong MAE in nano-laminates is presented, we apply this to propose a candidate with extremely strong anisotropy energy density, 5Fe/2W1-xReX for x=[0.6-0.8].
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Study of the electronic states in heavy fermion compound URu2Si2 / 重い電子系化合物URu2Si2における電子状態の解明Tonegawa, Sho 24 March 2014 (has links)
京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18055号 / 理博第3933号 / 新制||理||1567(附属図書館) / 30913 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 芝内 孝禎, 教授 石田 憲二, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM
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An ARPES study of correlated electron materials on the verge of cooperative orderTrinckauf, Jan 30 June 2014 (has links)
In this thesis the charge dynamics of correlated electron systems, in which a metallic phase lies in close proximity to an ordered phase, are investigated by means of angle resolved photoemission spectroscopy (ARPES). The analysis of the experimental data is complemented by electronic structure calculations within the framework of density functional theory (DFT).
First the charge dynamics of the colossal magnetoresistant bilayer manganites are studied. The analysis of the ARPES spectra based on DFT calculations and a Peierls type charge density wave model, suggests that charge, orbital, spin and lattice degrees of freedom conspire to form a fluctuating two dimensional local order that produces a large pseudo gap of about 450 meV in the ferromagnetic metallic phase and that reduces the expected bilayer splitting.
Next, the interplay of Kondo physics and (magnetic) order in the heavy fermion superconductor URu2Si2 is investigated. The low energy electronic structure undergoes strong changes at 17.5 K, where a second order phase transition occurs whose phenomenology is well characterized, but whose order parameter could not yet be unambigeously identified. Below THO, non-dispersive quasi particles with a large scattering rate suddenly acquire dispersion and start to hybridize with the conduction band electrons. Simultaniously the scattering rate drops sinificantly and a large portion of the Fermi surface vanishes due to the opening of a gap within the band of heavy quasi particles. The observed behaviour is in stark contrast to conventional heavy fermion systems where the onset of hybridization between localized and itinerant carriers happens in a crossover type transition rather than abruptly. These experimental results suggest that Kondo screening and the hidden order parameter work together to produce the unusual thermodynamic signatures observed in this compound.
Finally, the influence of charge doping and impurity scattering on the superconducting porperties of the transition metal substituted iron pnictide superconductor Ba(Fe1-xTMx)2As2 (TM = Co, Ni) is studied. Here, resonant soft X-ray ARPES is applied to see element selective the contribution of the 3d states of the TM substitute to the Fe 3d host bands.
The spectroscopic signatures of the substitution are found to be well reproduced by DFT supercell and model impurity calculations. Namely, the hybridization of the dopant with the host decreases with increasing impurity potential and the electronic states of the impurtiy become increasingly localized. Simultaniously, in all simulated cases a shift of the Fermi level due to electron doping is observed. The magnitude of the shift in the chemical potential that accurs in BaFe2As2, however, is in stark contrast to the marginal doping values obtained for the impurity model, where the shift of the chemical potential is largely compensated by the influence of the increasing impurity potential. This suggests that the rigid band behaviour of TM substituded BaFe2As2 is a peculiarity of the compound, which has strong implications for the developement of superconductivity. / In dieser Arbeit wird die Ladungstraegerdynamik in korrelierten Elektronensystemen, in denen eine metallische Phase in direkter Nachbarschaft zu einer geordneten Phase liegt, mit Hilfe von winkelaufgeloester Photoelektronenspektroskopie (ARPES) untersucht. Die Analyse der experimentellen Daten wird ergaenzt durch lektronenstrukturrechnungen im Rahmen der Dichtefunktionaltheorie (DFT).
Zuerst wird die Ladungstraegerdynamik in gemischtvalenten zweischichtmanganaten mit kolossalem Magnetiwiderstand studiert. Die Analyse der Photoemissionsspektren basierend auf DFT Rechnungen und einem Peierls artigem Ladungsdichtewellenmodell, legt nahe, dass die Freiheitsgrade von Ladung, Orbitalen, Spin und des Ionengitters konspirieren, um eine fluktuierende zweidimensionale lokale Ordnung zu bilden, die verantwortlich ist fuer die beobachtete Pseudobandluecke von 450 meV, und die zur Reduktion der erwarteten Zweischichtaufspaltung beitraegt. Als naechstes wird das Zusammenspiel von Kondo Physik und (magnetischer) Ordung im Schwerfermionensupraleiter URu2Si2 untersucht. Die iedrigenergetische elektronische Struktur zeigt starke Veraenderungen bei 17.5 K, wo ein Phasenuebergang zweiter Ordnungstattfindet, der phenomenologisch gut charakterisiert ist, aber dessen Ordungsparameter nocht nicht eindeutig identifiziert werden konnte. Unterhalb von THOerlangen nicht dispergierende Quasiteilchen mit gro en Streuraten abrupt Dispersion und hybridisieren mit den Leitungselektronen. Gleichzeitig sinkt die Streurate und ein gro er Teil der Fermiflaeche verschwindet durch das Oeffnen einer Bandluecke innehalb des Bandes schwerer Quasiteilchen.
Das beobachtete Verhalten steht in starkem Kontrast zu dem von konventionellen Schwerfermionensystemen, in denen die Hybridisierung zwischen lokalisierten und itineranten Ladungstraegern in einem kontinuierlichen Uebergang ablaeuft, anstatt abrubt. Diese experimentellen Befunde lassen den Schluss zu, dass das zusammenspiel zwischen Kondo Abschirmung und dem unbekannten Ordnungsparameter die ungewoehnlichen thermodynamischen Signaturen in dieser Verbindung hervorruft.
Abschliessend wird das Zusammenwirken von Ladungstraegerdotierung und Streuung an Stoeratomen auf die Supraleitung uebergangsmetalldotierter Eisenpniktid Supraleiter Ba(Fe1-xTMx)2As2 (TM = Co, Ni) untersucht. Mit Hilfe von resonantem Weichenroentgen ARPES gelingt es, elementselektiv den Beitrag der 3d Zustaende des TM Substituenten zu den Eisen 3d Wirtsbaendern zu beobachten. Die spektroskopischen Signaturen der Substitution sind mit Hilfe von DFT Rechnungen und Modelrechnungen mit zufaellig verteilten Stoeratomen gut zu reproduzieren. Insbesondere nimmt die Hybridisierung des dotierten Uebergangsmetalls und der Eisenbaender mit zunehmender Kernladungszahl ab und die elektronischen Zustaende der Stoeratome werden zunehmen lokalisiert. Gleichzeitig wird in allen gerechneten Faellen eine Verschiebung des Fermi Niveaus durch Elektronendotierung beobachtet. Der Betrag der Verschiebung des chemischen Potentials in BaFe2As2 steht allerdings in starkem Kontrast zu den Werten, die man im Falle der Modellrechnungen erhaelt, wo die Verschiebung des Fermi Niveaus durch den Einfluss des Potentials der Stoeratome groesstenteils kompensiert wird. Dies legt nahe, dass das beobachtete "rigid band" Verhalten von TM substituiertem BaFe2As2 eine Besonderheit dieser Verbindung ist, welches starke Auswirkungen auf die Ausbildung von Supraleitung hat.
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Compétition d'états fondamentaux dans URu2Si2 et UCoGeHassinger, Elena 26 October 2010 (has links) (PDF)
Dans cette thèse, deux composés à fermions lourds ont été étudiés sous pression. L'état fondamental en dessous de T0 = 17.5 K dans URu2Si2 est appelé "ordre caché (HO), parce que le paramètre d'ordre n'a pas encore été trouvé. A 1.5 K, le système devient en plus supraconducteur. Sous pression, le système devient antiferromagnetique (AF) au dessus d'une pression critique. Des mesures Shubnikov-de Haas sous pression montre, que la surface de Fermi ne change pas entre les deux phases. Dans la phase AF, le dédoublement de la maille implique une reconstruction de la surface de Fermi. Vu que celle-ci ne change pas sous pression, ce dédoublement doit avoir lieu déjà dans l'HO. Nos mesures de la dépendance angulaire des fréquences d'oscillation supportent des nouveau calcules de bande dans le cas d'électrons plutôt itinérant. Comme deuxième partie de ma thèse, j'ai étudié le diagramme de phase sous pression du supraconducteur (SC) ferromagnétique (FM) UCoGe (TC = 2.8 K,Tsc = 0.6 K). Les mesures de résistivité, ac calorimétrie et ac susceptibilité montrent que la phase FM est supprimé a environs 1 GPa mais la phase SC existe jusque dans la phase paramagnétique induite par la pression. Ce diagramme de phase est unique dans la classe des supraconducteurs ferromagnétiques.
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Competing orders in Uru2Si2: from ordered magnetism to spin liquid phases / Ordres en comp?tition dans URu2Si2: de l?ordre magn?tique aux phases de liquide de spinFarias, Carlene Paula Silva de 10 April 2017 (has links)
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Previous issue date: 2017-04-10 / The main objective of this thesis is to investigate the competing ordered
phases in the metallic heavy fermion compound URu2Si2, which displays a bodycentered
tetragonal lattice. We first provide a study case of the competition between
antiferromagnetic (AF) and spin liquid phases. The antiferromagnetic state is study
with spin-wave theory. Whereas the spin liquid analysis has been carried out in an
algebraic spin liquid representation. In the second part, we describe an effective theory
for Raman scattering experiments at these particular phases. We provide insight
about the hidden order phase displayed by the heavy fermion compound URu2Si2. / L?objectif central de cette th?se est d??tudier des phases ordonn?es en comp?tition
dans des mat?riaux magn?tiques pr?sentant une structure cristalline t?tragonale
centr?e. Ce travail est divis? en deux parties principales. Dans la premi?re,
nous pr?sentons les r?sultats de notre ?tude de la comp?tition entre des ?tats ordonn?s
antiferromagn?tiques et des phases liquides de spin. Nous montrons comment ces
derni?res peuvent ?tre stabilis?es par la frustration g?om?trique et par une g?n?ralisation
de la sym?trie de spin au groupe SU(n). Les ?tats antiferromagn?tiques sont
d?crits par une th?orie d?onde de spin et l?analyse de liquide de spin est effectu?e par
une repr?sentation fermionique des op?rateurs de spin. Dans la deuxi?me partie, nous
d?crivons une th?orie effective pour d?rcrire des exp?riences de diffusion Raman. Nous
fournissons un aper?u de la phase d?ordre cach? affich?e par le compos? de fermions
lourds URu2Si2.
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Electronic & Magnetic Properties of Ba(Fe,Co)2As2 & URu2Si2Williams, Travis J. 04 1900 (has links)
<p>This thesis details a collection of experiments performed on two condensed matter systems, Co-doped BaFe<sub>2</sub>As<sub>2</sub> and URu<sub>2</sub>Si<sub>2</sub>. These two materials are related by their structural type (<em>ThCr<sub>2</sub>Si<sub>2</sub></em>-type) serving as great examples of the diversity of material properties present in this family. They are also both superconducting materials and belong to the collection of strongly-correlated electron systems. The interest in studying the Ba(Fe,Co)<sub>2</sub>As<sub>2</sub> group of materials is due to the high superconducting transition temperature in these (and related) materials, while the compound URu<sub>2</sub>Si<sub>2</sub> was studied due to the presence of a poorly-understood 'hidden order' phase.</p> <p>Muon spin relaxation/rotation/resonance (µSR) was used to measure several single crystals of the series Ba(Fe<sub>2-<em>x</em></sub>Co<em><sub>x</sub></em>)<sub>2</sub>As<sub>2</sub> with Cobalt concentrations <em>x </em>= 0.038, 0.047, 0.061, 0.074, 0.107 and 0.114, and a single crystal of Sr(Fe<sub>0.87</sub>Co<sub>0.13</sub>)<sub>2</sub>As<sub>2</sub>. The two samples with the lowest doping, <em>x </em>= 0.038 and <em>x </em>= 0.047, showed strong c-axis magnetism occurring below the magnetic transition, T<sub>SDW</sub>. The measurements suggest that the local magnetic field is increasingly disordered as the concentration of Co increases. These samples were shown to exhibit both superconductivity and magnetism, but that the entire sample contains non-zero local magnetic fields, meaning that superconductivity exists in or near regions of strong magnetic order.</p> <p>The remaining compounds (with <em>x </em>= 0.061, 0.074, 0.107, 0.114 and Sr(Fe<sub>0.87</sub>Co<sub>0.13</sub>)<sub>2</sub>As<sub>2</sub>) were measured with zero-field (ZF)-µSR and no magnetic ordering was found down to T = 1.65 K. An analytic Ginzburg-Landau model was used to fit the data and obtain absolute values for the penetration depth, λ. A model for the temperature dependence of the density of superconducting carriers, n<sub>s</sub> ≈ λ<sup>2</sup>, based on two <em>s</em>-wave gaps describes the data well. Below T<sub>SC</sub>, a paramagnetic frequency shift was observed indicative of field-induced magnetism along the c crystallographic direction.</p> <p>Measurements of URu<sub>2</sub>Si<sub>2</sub> under chemical and hydrostatic pressure have focused on measuring the spin correlations that are present in the hidden order phase. The chemical pressure that is induced by 5% Re doping perturbs, but does not destroy, the commensurate spin excitations. The spin gap that is present in the parent material is also present under this chemical doping. The hidden order phase survives at least halfway to the quantum critical point to ferromagnetism, but is weakened by the Re substitution.</p> <p>Under hydrostatic pressure of 10.1 kbar, URu<sub>2</sub>Si<sub>2</sub> becomes antiferromagnetic, but the spin correlations are found to be qualitatively similar to those of the hidden order phase. The width in reciprocal space (Q-width) of the excitations and their gapped nature remains unchanged upon entering the antiferromagnetic phase. Quantitatively, there is an increase in the magnitude of the gap at Q = (1.4 0 0). This may be a result of the increase in the transition temperature preceding the entry to the antiferromagnetic phase.</p> <p>Due to the large difference in their properties, and hence the motivation for studying Ba(Fe<sub>1-<em>x</em></sub>Co<em><sub>x</sub></em>)<sub>2</sub>As<sub>2</sub> and URu<sub>2</sub>Si<sub>2</sub>, they will be introduced and presented separately. Chapter 1 will provide the necessary background material on Ba(Fe,Co)<sub>2</sub>As<sub>2</sub>, while Chapter 2 will provide the background for the work on URu<sub>2</sub>Si<sub>2</sub>. Chapter 3 will describe the experimental techniques that were used to study these systems.</p> <p>Original research results on Ba(Fe,Co)<sub>2</sub>As<sub>2</sub> are presented in Chapter 4. This is mainly focused on µSR measurements of dopings that display superconductivity. Samples that did not order magnetically were measured in the mixed state to measure the vortex lattice to extract the various properties, including the superconducting pairing symmetry. Samples that did order magnetically were measured to analyze the amount of magnetic disorder and discover the extent of coexistence or phase separation between magnetism and superconductivity.</p> <p>Chapter 5 details the original research results on URu<sub>2</sub>Si<sub>2</sub>. This involved crystal growth of these compounds, and two neutron scattering experiments to measure the spin correlations while perturbing the hidden order state. The first experiment was done on a Re-doped crystal, URu<sub>1.9</sub>Re<sub>0.1</sub>Si<sub>2</sub>. Doping with Re suppresses the hidden order, eventually leading to ferromagnetism at higher dopings. This work showed that the spin correlations are also suppressed, but not as quickly as the hidden order. The second experiment was on pure URu<sub>2</sub>Si<sub>2</sub> under hydrostatic pressure. Applied pressure increases the hidden order transition, but eventually leads to antiferromagnetism, the phase in which the experiment was performed.</p> / Doctor of Philosophy (PhD)
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Multipoles in Correlated Electron MaterialsCricchio, Francesco January 2010 (has links)
Electronic structure calculations constitute a valuable tool to predict the properties of materials. In this study we propose an efficient scheme to study correlated electron systems with essentially only one free parameter, the screening length of the Coulomb potential. A general reformulation of the exchange energy of the correlated electron shell is combined with this method in order to analyze the calculations. The results are interpreted in terms of different polarization channels, due to different multipoles. The method is applied to various actinide compounds, in order to increase the understanding of the complicate behaviour of 5f electrons in these systems. We studied the non-magnetic phase of δ-Pu, where the spin polarization is taken over by a spin-orbit-like term that does not break the time reversal symmetry. We also find that a non-trivial high multipole of the magnetization density, the triakontadipole, constitutes the ordering parameter in the mysterious hidden order phase of the heavy-fermion superconductor URu2Si2. This type of multipolar ordering is also found to play an essential role in the hexagonal-based superconductors UPd2Al3, UNi2Al3 and UPt3 and in the dioxide insulators UO2, NpO2 and PuO2. The triakontadipole moments are also present in all magnetic actinides we considered, except for Cm. These results led us to formulate a new set of rules for the ground state of a system, that are valid in presence of strong spin-orbit coupling interaction instead of those of Hund; the Katt's rules. Finally, we applied our method to a new class of high-Tc superconductors, the Fe-pnictides, where the Fe 3d electrons are moderately correlated. In these materials we obtain the stabilization of a low spin moment solution, in agreement with experiment, over a large moment solution, due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 705
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Calor específico de um modelo teórico para o composto uru2si2 / Specific heat of a theoretical model for compoundPedrolo, Bruna Garcia 29 January 2016 (has links)
The Heavy fermion URu2Si2 compound have attracted much interest in the last
decades due to its unusual behaviors and still an unsolved problem in Condensed Matter
Physics. Specific heat measurements in URu2Si2 compounds show that when the
temperature is decreased the system undergoes a phase transition at 17,5K and another
phase transition at 1,5K. The phase transition at 1,5K leads the system to a superconducting
state. On the other hand, the phase transition at 17,5K leads the system to a
unclear order known as Hidden Order. In the last three decades, URu2Si2 the system has
been intensively experimental and theoretical investigated, nevertheless, the nature of the
hidden order is still unclear. In presence of an external magnetic field Hz, the URu2Si2
system presents a interesting behavior. In particular, there are experimental results for
the specific heat showing that above 25T, the specific heat anomaly becomes very sharp
and narrow and loses its second-order character.Recentily proposed a theory based in
the underscreened Anderson Lattice Model. Such theory has been also used to study
a minimal model proposed to describe the hidden order in the URu2Si2 system. In the
present work, the effects of an external magnetic field Hz on the specific heat of a minimal
model are investigated. The specific heat has been calculated from the second-derivative
of the Helmholtz Free-energy with respect to the temperature. For magnetic fields below
approximately 28T, the present results show a second order character for the specific heat.
However, for magnetic fields above 28T, the specific heat becomes very sharp and narrow
indicating that the system reached a region of first-order transition. It is also important
to note that these results are, at least, in qualitatively agreement with experimental results. / O composto férmion pesado URu2Si2 tem atraído muito interesse nas últimas décadas
devido ao seu comportamento incomun, sendo ainda um problema não resolvido
em Física da Matéria Condensada. Medidas de calor específico em compostos URu2Si2
mostram que, quando a temperatura é diminuída, o sistema sofre uma transição de fase
a 17,5K e outra transição de fase a 1,5K. A transição de fase a 1,5 K leva o sistema para
um estado supercondutor. Por outro lado, a transição de fase a 17,5 K leva o sistema para
um estado conhecido como Ordem Oculta. Nas últimas três décadas, o sistema URu2Si2
tem sido intensamente investigado tanto experimental, como teoricamente, no entanto,
a natureza da Ordem Oculta ainda não é clara. Na presença de um campo magnético
externo Hz, o sistema URu2Si2 apresenta um comportamento interessante. Em particular,
existem resultados experimentais para o calor específico, mostrando que, acima de
25T, o pico do calor específico torna-se muito intenso e estreito, perdendo o seu carácter
de segunda ordem. Recentemente foi proposta uma teoria baseada no Modelo de Anderson
Underscreened . Tal teoria foi também utilizada para estudar um modelo mínimo
proposto para descrever a Ordem Oculta no sistema URu2Si2. No presente trabalho, os
efeitos de um campo magnético externo, Hz, no calor específico de um modelo mínimo,
são investigados. O calor específico foi calculado a partir da derivada segunda da energia
livre de Helmholtz em relação à temperatura. Para campos magnéticos abaixo de aproximadamente
28T, os presentes resultados mostram uma característica de segunda ordem
para o calor específico. No entanto, para campos magnéticos acima de 28T, o pico do
calor específico torna-se muito intenso e estreito, o que indica que o sistema atingiu uma
região de transição de primeira ordem. É importante ressaltar que estes resultados estão,
pelo menos, qualitativamente de acordo com os resultados experimentais.
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