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<p> The de Haas-van Alphen effect has been used to measure the Fermi surface areas, cyclotron masses and Dingle temperatures in antimony and its alloys containing less than 0.3 percent tin. The Fermi surface of each alloy was similar to the pure antimony surface. However the hole surface increased in size and the electron surface shrunk since tin removes electrons from the alloy. The cyclotron masses increased and decreased for holes and electrons respectively, giving a definite indication of nonparabolic conduction and valence bands. The cyclotron masses were found from the temperature dependence of the dHvA amplitude after interfering dHvA frequency components were removed by a Fourier analysis technique. The Dingle temperature increased roughly linearily with tin concentration.</p> <p> A comparison of the hole and electron Fermi surface volumes with the number of tin atoms added to the alloys shows that one tin atom removes one electron from the alloy as expected from the unit valence difference between antimony and tin. This value is higher than that found by other workers using different techniques.</p> <p> The shapes of the energy bands along with the cyclotron masses have been compared with several band models. An ellipsoidal band provides a rough overall description of both holes and electrons while an ellipsoidal nonparabolic band describes the mass behaviour on alloying more accurately. A pseudopotential band calculated using the method and potential of Falicov and Lin (1967) was also compared with the data.</p> <p> The observed relative frequency changes were used to compare the data with the rigid band model of alloying. The bands are rigid for low concentrations. At higher concentrations there are deviations apparently caused by the cyclotron mass change and an axial ratio change in the hole Fermi surface.</p> / Thesis / Doctor of Philosophy (PhD)

A digital program has been developed to Fourier analyse the data obtained in experiments to study the de Haas-van Alphen effect. A physical account of the origin of this effect is given and the problem of analysing the data is explained. An account is given of the development of the program together with a consideration of the mathematical properties of the Fourier transform. General questions concerning the resolving power and accuracy of this method of analysis are discussed. The program was tested using artificial synthetic data of known analytic form and later applied to the analysis of data from a single crystal of mercury. / Thesis / Master of Science (MSc)

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The interplay of partially filled d- or f-electron shells with conduction-band electrons is a key ingredient in new rare-earth transition-metal compounds for the emergence of unusual electronic and magnetic properties. Among which unconventional superconductivity is one of the most studied. Despite many years of intensive experimental investigations and plenty promising theoretical models, unconventional superconductivity still remains hotly debated a very rich topic. One of the fundamental unsolved problems for condensed-matter physicists is the mechanism that causes the electrons to form anisotropic superconductivity. Since electrons in the vicinity of the Fermi level are primarily responsible for superconductivity, in order to better understand the mechanism giving rise to this phenomenon and the origin of complex forces between correlated electrons, knowledge of the Fermi surface and band selective effective mass is essential. Of the many techniques used to study electronic band-structure properties, measurements of quantum oscillations in the magnetization, so-called de Haas-van Alphen (dHvA) effect, in combination with band-structure calculations is the traditional proven tool for studying Fermi-surface topology and quasiparticle effective mass. In the present work, electronic structure and Fermi-surface properties of Ybsubstituted heavy fermion superconductor CeCoIn5 and iron based ternary phosphides LaFe2P2 and CeFe2P2 have been investigated by means of dHvA measurements. For these measurements, capacitive cantilever-torque magnetometry was utilized. In Ce1−xYbxCoIn5, the evolution of the Fermi surface and effective mass was studied as a function of Yb concentration. The observed topology change is consistent with what is expected from the band-structure calculations. For a small Yb concentration, x = 0.1, the band-structure topology and the effective masses remain nearly unchanged compared to CeCoIn5. This contrasts clearly modified Fermi surfaces and light, almost unrenormalized effective masses for x = 0.2 and above. For LaFe2P2 and CeFe2P2, the obtained effective masses are light. Good agreement between the calculated and measured dHvA frequencies was identified only for LaFe2P2. However, for CeFe2P2 strong disagreement was observed. Moreover, different CeFe2P2 single crystals reveal different experimental results. In order to reconcile the results of the dHvA measurements and density-functional-theory calculations more work is necessary.

The interplay of partially filled d- or f-electron shells with conduction-band electrons is a key ingredient in new rare-earth transition-metal compounds for the emergence of unusual electronic and magnetic properties. Among which unconventional superconductivity is one of the most studied. Despite many years of intensive experimental investigations and plenty promising theoretical models, unconventional superconductivity still remains hotly debated a very rich topic. One of the fundamental unsolved problems for condensed-matter physicists is the mechanism that causes the electrons to form anisotropic superconductivity. Since electrons in the vicinity of the Fermi level are primarily responsible for superconductivity, in order to better understand the mechanism giving rise to this phenomenon and the origin of complex forces between correlated electrons, knowledge of the Fermi surface and band selective effective mass is essential. Of the many techniques used to study electronic band-structure properties, measurements of quantum oscillations in the magnetization, so-called de Haas-van Alphen (dHvA) effect, in combination with band-structure calculations is the traditional proven tool for studying Fermi-surface topology and quasiparticle effective mass. In the present work, electronic structure and Fermi-surface properties of Ybsubstituted heavy fermion superconductor CeCoIn5 and iron based ternary phosphides LaFe2P2 and CeFe2P2 have been investigated by means of dHvA measurements. For these measurements, capacitive cantilever-torque magnetometry was utilized. In Ce1−xYbxCoIn5, the evolution of the Fermi surface and effective mass was studied as a function of Yb concentration. The observed topology change is consistent with what is expected from the band-structure calculations. For a small Yb concentration, x = 0.1, the band-structure topology and the effective masses remain nearly unchanged compared to CeCoIn5. This contrasts clearly modified Fermi surfaces and light, almost unrenormalized effective masses for x = 0.2 and above. For LaFe2P2 and CeFe2P2, the obtained effective masses are light. Good agreement between the calculated and measured dHvA frequencies was identified only for LaFe2P2. However, for CeFe2P2 strong disagreement was observed. Moreover, different CeFe2P2 single crystals reveal different experimental results. In order to reconcile the results of the dHvA measurements and density-functional-theory calculations more work is necessary.

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Die Betrachtung von Schwere-Fermionen-Systemen stellt ein wichtiges Themengebiet im Bereich der Festkörperphysik dar. Das Verhalten von Schwere-Fermionen-Systemen wird durch die starken Korrelationen der magnetischen Momente der ungepaarten Spins der f-Elektronen bestimmt. Experimentell zugängliche Messgrößen sind dadurch bei tiefen Temperaturen stark erhöht, so dass sich diese Systeme besonders gut zur Untersuchung von Grundzustandseigenschaften eignen. Zentrales Thema dieser Arbeit ist die Untersuchung zweier intermetallischer Seltenerd-Verbindungen in Bezug auf Quantenphasenübergänge. Diese treten am absoluten Nullpunkt der Temperatur als Funktion eines anderen Parameters wie Magnetfeld, Druck oder chemischer Substitution auf und sind bei endlicher Temperatur durch Abweichungen physikalischer Messgrößen von der durch L. D. Landau aufgestellten Theorie der Fermi-Flüssigkeiten nachzuweisen. Zu diesem Zweck wurden Tieftemperaturexperimente bis hinab zu 20mK und in Magnetfeldern bis zu 18T durchgeführt. Es wurden elektrischer Widerstand, magnetische Wechselfeldsuszeptibilität, Magnetostriktion und thermische Ausdehnung gemessen. / The investigation of heavy-fermion systems marks an important subject in the research field of solid state physics. The behaviour of heavy-fermion systems is dominated by the strong correlations of the magnetic moments of the unpaired f-electron spins. At low temperatures, experimentally accessible variables are strongly enhanced so that these systems are especially suited to analyse ground state properties. The central topic of this thesis is the investigation of two intermetallic rare-earth compounds with regard to quantum phase transitions. The latter occur at zero temperature as a function of parameters such as magnetic field, pressure or chemical substitution. They are traceable at finite temperature due to deviations of physical variables from the theory of Fermi liquids established by L. D. Landau. For this purpose, low-temperature experiments were performed down to 20mK and in magnetic fields up to 18T. Electrical resistivity, magnetic ac susceptibility, magnetostriction and thermal expansion were measured.