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Quantenphasenübergänge in den Schwere-Fermionen-Systemen Yb(Rh_{1-x}M_x)_2Si_2 und CePd_{1-x}Rh_xWesterkamp, Tanja 06 April 2009 (has links)
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.
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Electronic States of Heavy Fermion Metals in High Magnetic FieldsRourke, Patrick Michael Carl 25 September 2009 (has links)
Heavy fermion metals often exhibit novel electronic states at low temperatures, due to competing interactions and energy scales. In order to characterize these states, precise determination of material electronic properties, such as the Fermi surface topology, is necessary. Magnetic field is a particularly powerful tool, since it can be used as both a tuning parameter and probe of the fundamental physics of heavy fermion compounds.
In CePb3, I measured magnetoresistance and torque for 23 mK ≤ T ≤ 400 mK, 0 T ≤ H ≤ 18 T, and magnetic field rotated between the (100), (110), and (111) directions. For H||(111), my magnetoresistance results show a decreasing Fermi liquid temperature range near Hc, and a T^2 coefficient that diverges as A(H) ∝ |H −Hc|^−α, with Hc ~ 6 T and α ~ 1. The torque exhibits a complicated dependence on magnetic field strength and angle. By comparison to numerical spin models, I find that the “spin-flop” scenario previously thought to describe the physics of CePb3 does not provide a good explanation of the experimental results.
Using novel data acquisition software that exceeds the capabilities of a traditional measurement set-up, I measured de Haas–van Alphen oscillations in YbRh2Si2 for 30 mK ≤ T ≤ 600 mK, 8 T ≤ H ≤ 16 T, and magnetic field rotated between the (100), (110), and (001) directions. The measured frequencies smoothly increase as the field is decreased through H0 ≈ 10 T. I compared my measurements to 4f-itinerant and 4f-localized electronic structure calculations, using a new algorithm for extracting quantum oscillation information from calculated band energies, and conclude that the Yb 4f quasi-hole remains itinerant over the entire measured field range, with the behaviour at H0 caused by a Fermi surface Lifshitz transition. My measurements are the first to directly track the Fermi surface of YbRh2Si2 across this field range, and rule out the 4f localization transition/crossover that was previously proposed to occur at H0.
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Electronic States of Heavy Fermion Metals in High Magnetic FieldsRourke, Patrick Michael Carl 25 September 2009 (has links)
Heavy fermion metals often exhibit novel electronic states at low temperatures, due to competing interactions and energy scales. In order to characterize these states, precise determination of material electronic properties, such as the Fermi surface topology, is necessary. Magnetic field is a particularly powerful tool, since it can be used as both a tuning parameter and probe of the fundamental physics of heavy fermion compounds.
In CePb3, I measured magnetoresistance and torque for 23 mK ≤ T ≤ 400 mK, 0 T ≤ H ≤ 18 T, and magnetic field rotated between the (100), (110), and (111) directions. For H||(111), my magnetoresistance results show a decreasing Fermi liquid temperature range near Hc, and a T^2 coefficient that diverges as A(H) ∝ |H −Hc|^−α, with Hc ~ 6 T and α ~ 1. The torque exhibits a complicated dependence on magnetic field strength and angle. By comparison to numerical spin models, I find that the “spin-flop” scenario previously thought to describe the physics of CePb3 does not provide a good explanation of the experimental results.
Using novel data acquisition software that exceeds the capabilities of a traditional measurement set-up, I measured de Haas–van Alphen oscillations in YbRh2Si2 for 30 mK ≤ T ≤ 600 mK, 8 T ≤ H ≤ 16 T, and magnetic field rotated between the (100), (110), and (001) directions. The measured frequencies smoothly increase as the field is decreased through H0 ≈ 10 T. I compared my measurements to 4f-itinerant and 4f-localized electronic structure calculations, using a new algorithm for extracting quantum oscillation information from calculated band energies, and conclude that the Yb 4f quasi-hole remains itinerant over the entire measured field range, with the behaviour at H0 caused by a Fermi surface Lifshitz transition. My measurements are the first to directly track the Fermi surface of YbRh2Si2 across this field range, and rule out the 4f localization transition/crossover that was previously proposed to occur at H0.
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Quantização de Landau e efeitos associados para átomos ultrafrios do tipo tripod na presença de uma campo magnético artificialSilva, Bruno Farias da 27 February 2015 (has links)
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Previous issue date: 2015-02-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this thesis, we propose an experimental setup for the study of Landau quantization and
associated effects in a two-dimensional ultracold atomic gas. Gauge fields can emerge in
the equation of motion for the optically addressed ultracold atoms. To this end, spatially
dependent dark states are necessary for the internal states of the atoms. A tripod level
scheme yields two degenerate dark states which can leads to either an Abelian U(1) U(1)
gauge field or a non-Abelian SU(2) gauge field. Using a suitable laser configuration, we
obtain a uniform U(1) U(1) magnetic field which causes the atoms organize themselves
in Landau levels. The strength of the effective magnetic field depends on the relative intensity
of the lasers beams at the atomic cloud. We estimate the degeneracy of the energy
levels for an atomic gas formed by atoms of 87Rb. In addition, we establish the experimental
conditions to reach the lowest Landau level regime. In the zero-temperature limit,
we realize the emergence of magnetic oscillations in the atomic energy and its derivative
as function of the inverse of the effective magnetic field (de Haas van Alphen effect). The
period of the de Haas van Alphen oscillation allow us to determine area of the Fermi circle
for the atomic gas via an Onsager-like relation. We also show that detuning the a laser
from the two-photon resonance we generate a parabolic scalar potential that laterally
confines the atoms. As a consequence, the Landau levels degeneracy is removed, since
the energy spectrum depends explicitly on the transverse atomic momentum. We show
that the Landau levels presents a reminiscent degeneracy when the boundaries conditions
are considered. The residual degeneracy occurs when different energy levels overlap. We
map the residual degeneracy points as a function of the effective magnetic field. Finally,
we present an experimental scheme for observing the spin Hall effect for ultracold atoms
in a tripod configuration. / Nesta tese, propomos um arranjo experimental para o estudo da quantização de Landau
e efeitos associados em um gás atômico ultrafrio bidimensional. Campos de calibre podem
surgir na equação de movimento para átomos ultrafrios oticamente vestidos. Para
que isto ocorra, estados escuros espacialmente dependentes são necessários a partir dos
estados internos dos átomos. Átomos numa configuração de níveis de energia do tipo
tripod produzem dois estados escuros degenerados, que podem levar a campos de calibre
Abelianos U(1) U(1) ou não-Abelianos SU(2). Utilizando uma configuração adequada
de lasers, mostramos que é possível se produzir um campo magnético sintético uniforme
U(1) U(1) que atua nos átomos neutros fazendo-os se organizarem em níveis de Landau.
A intensidade do campo efetivo depende da intensidade relativa dos feixes de luz
na nuvem atômica. Estimamos a degenerescência dos níveis de energia para um gás atômico
formado por átomos de 87Rb e estabelecemos as condições experimentais para que
seja atingido o regime em que todos os átomos populam unicamente o nível de Landau
menos energético. Considerando o limite de temperatura nula, verificamos o surgimento
de oscilações magnéticas na energia e em sua derivada como uma função do inverso do
campo magnético efetivo (efeito de Haas van Alphen). O período da oscilação magnética
nos permite determinar a área do círculo de Fermi para o gás atômico através de uma
expressão similar a de Onsager para sistemas eletrônicos. Mostramos também que dessintonizando
um dos lasers em relação à ressonância de dois fótons geramos um potencial
escalar parabólico que faz com os átomos sejam lateralmente confinados. Isto resulta na
remoção da degenerescência dos níveis de Landau, uma vez que a energia depende explicitamente
do momento atômico transverso. Demonstramos que, aplicando condições
periódicas de contorno ao sistema, temos o surgimento de uma degenerescência residual.
A degenerescência remanescente ocorre quando diferentes níveis de energia se superpõem.
Mapeamos os pontos de degenerescência como uma função do campo magnético efetivo.
Por fim, apresentamos um esquema experimental para a observação do efeito spin Hall
para átomos ultrafrios em uma configuração tripod.
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Characterization of the electronic properties of LaIrIn5: calculations, transport-, heat capacity- and de Haas-van Alphen-experiments / Bestimmung der elektronischen Eigenschaften von LaIrIn5: Rechnungen, Transport-, Wärmekapazitäts- und de Haas-van Alphen-ExperimenteForzani, Eugenio Angelo 12 January 2007 (has links)
No description available.
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