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)

Quantum phase transitions (QPT) on the border of magnetism have provided a fertile hunting ground for the discovery of new states of matter, for example; the marginal Fermi Liquid and non Fermi Liquid states as well high T$_C$ cuprate and magnetically mediated superconductivity. In this thesis I present work on three materials in which it may be possible to tune the system through a magnetic QPT with the application of hydrostatic pressure. Although the details of the underlying physics are different in each of the materials, they are linked by the possibility of finding new states on the border of magnetism. Applying hydrostatic pressure, we have suppressed the ferromagnetic (FM) transition in metallic Fe$_2$P to very low temperature and to a potential QPT. Counter-intuitive broadening of the magnetic hysteresis leading up to the FM-AFM QPT may well be a crucial clue as to the nature of the model needed to understand this phase transition. A sharp increase in the quasi-particle scattering cross-section as well as the residual resistivity accompany a departure from the quadratic temperature dependence of the resistivity. This possible deviation from Fermi liquid behaviour is stable over a significant range of temperature. The unexplained upturn in the resistivity of CeGe that accompanies the AFM transition was studied under pressure. Pressure increased the residual resistivity as well as decreasing the relative size of the upturn, but had a moderate effect on the Neel temperature. The insensitivity of the N$\acute e$el temperature to pressure has been compared to its relative sensitivity to applied feld. The existence of the upturn and its evolution with pressure and applied feld can reasonably be argued to be due to the details of the electron band structure in the system. By applying pressure we have drastically reduced the resistivity of the insulating antiferromagnet NiPS$_3$. Concurrent work on FePS$_3$ has shown metallisation under pressure. It seems reasonable to speculate that NiPS$_3$ may also metallise at higher pressure. The energy gap is narrowed in both materials as pressure is increased. Magnetisation measurements have revealed a low temperature upturn indicating some possible ferromagnetic component or proximity to another magnetic state. A peak in the magnetisation is also seen at 45K in zero-feld cooled measurements. Both of these features point to a system with a complex magnetic ground state.

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Seltenerd-Eisenpniktide ziehen gegenwärtig großes wissenschaftliches Interesse auf sich, da sie wegen der bei ihnen beobachteten Hochtemperatursupraleitung eine vielversprechende Alternative zu den herkömmlichen Kuprat-Supraleitern darstellen. Neben der Supraleitung weisen diese Systeme ein breites Spektrum magnetischer Eigenschaften auf, die auf dem Wechselspiel von Eisen 3d- und Seltenerd-4f-Elektronen beruhen und teils in Konkurrenz zu, teils aber auch in Koexistenz mit der Supraleitung auftreten. Das theoretische Verständnis dieser Phänomene lässt noch viele Fragen offen, da sich vor allem die 4f-Elektronen infolge ihrer starken räumlichen Lokalisierung und der damit verbundenen hohen Coulomb-Korrelationsenergien einer einfachen Behandlung im Rahmen von Bandstrukturtheorien weitgehend entziehen. Die vorliegende Arbeit beschäftigt sich mit winkelaufgelösten Photoemissionsuntersuchungen an CeFe2P2, CeFePO und CeFeP0.3As0.7O, bei denen Schwer-Fermionen-Verhalten bzw. Ferromagnetismus beobachtet werden kann. Die Wechselwirkung der 4f-Elektronen mit dem überwiegend von Fe 3d-Orbitalen abgeleiteten Valenzband spiegelt sich in den Spektren durch das Auftreten dispergierender Strukturen im Bereich der Kondoresonanz wider. Diese werden in der Arbeit auf der Grundlage von LDA-Bandstrukturrechnungen und dem Periodischen Andersonmodell, welches für einfache Fallbeispiele mittels der DMFT (Dynamical Mean Field Theory) gelöst wird, diskutiert. Anhand der experimentellen Beobachtungen wird ein Mechanismus für die in der Verbindungsklasse beobachteten magnetischen Übergänge, basierend auf charakteristischen Unterschieden in der Bandstruktur, vorgeschlagen sowie ein möglicher Zusammenhang mit der Supraleitung diskutiert.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22989号 / 理博第4666号 / 新制||理||1669(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 柳瀬 陽一, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

We have studied two different systems of compounds, YPd2Al3 and CenTmIn3n+2m. Polycrystalline YPd2Al3, a new member of the REPd2Al3 system, was prepared by arc melting. The X-ray diffraction confirmed that YPd2Al3 crystallizes in the hexagonal PrNi2Al3-type structure as the other REPd2Al3 compounds. Magnetization, AC susceptibility, specific heat and resistivity measurements revealed superconductivity below Ts ≈ 2.2 K. The second part of the work was focused on studies of Ce2PdIn8 and Pd-doped CenRhIn3n+2. Single crystals of Ce2Rh1-xPdxIn8 with x = 0, 0.10, 0.15, 0.30, 0.45, 0.85, 1 and CeRh1-xPdxIn5 with x = 0, 0.1, 0.25 were prepared by solution growth method. The quality of crystals was confirmed by microprobe analysis and X-ray diffraction. The effect of Pd doping on magnetism of CenRh1-xPdxIn3n+2 was studied by specific heat, magnetization and resistivity measurements. The Pd doping gradually suppresses the Néel temperature in both systems, however the effect is stronger in Ce2Rh1-xPdxIn8. Temperature dependence of resistivity of CeRh0.75Pd0.25In5 was studied in pressure up to 2.2 GPa. Similar to CeRhIn5, the antiferromagnetism is gradually suppressed by the applied pressure, while superconductivity is induced and coexists with antiferromagnetism.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21548号 / 理博第4455号 / 新制||理||1640(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 川上 則雄, 教授 田中 耕一郎, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18052号 / 理博第3930号 / 新制||理||1567(附属図書館) / 30910 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 准教授 芝内 孝禎, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20164号 / 理博第4249号 / 新制||理||1611(附属図書館) / 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 池田 隆介, 教授 石田 憲二, 教授 川上 則雄 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM