Field theoretical methods for strongly correlated electron systems

Physics, Condensed Matter

Cassanello, Carlos Raul

关键词: Physics, Condensed Matter;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/23057/9712215.pdf?sequence=2&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Large N-slave fermion techniques have been established as a powerful computational tool to study quantum many-body systems. They have been applied successfully to the study of various systems of strongly correlated electrons in condensed matter physics, particularly to problems where a non-perturbative treatment is required.In the first part of this thesis we use this approach to investigate the frustrated quantum antiferromagnet. We study the chiral spin liquid state of the antiferromagnet at zero temperature, which has a spin-gap generated by the spontaneous breakdown of time-reversal-symmetry. This state is known as the chiral spin state. We determine conditions for the restoration of time-reversal-symmetry in a bilayer antiferromagnet with an interlayer exchange interaction. We show the energetically favored ground state has chiral spin liquids with opposite chiralities on each layer. This constitutes a dynamic mechanism which prevents the observation of broken time-reversal-symmetry in a bilayer system, even though this breakdown may be present on each layer independently.In the second part of this thesis we use the slave-fermion technique to study the physics of magnetic impurities both in quantum antiferromagnets (in a flux phase) and in d-wave superconductors. This is a system consisting of a magnetic impurity coupled to either the low lying excitations of a flux phase, or to the normal quasi-particles close to the Fermi surface of a superconductor whose gap function exhibits nodes. We show that both systems display a zero temperature quantum phase transition in the large N approximation. The transition takes place between a weak coupling regime, in which the magnetic impurity is effectively decoupled from the quasi-particles, to a strong coupling phase which exhibits a Kondo effect and the magnetic impurities are screened. The ground state is a singlet. If the system has particle-hole symmetry the impurity appears to be overscreened. This effect is not due to a multichannel character in the system. The physical origin of the quantum phase transition is directly related to the fact that this is a non-marginal Kondo system. This follows from having a quasi-particle band with a density of states that vanishes linearly with the energy in the proximity of the Fermi surface.The susceptibility and the specific heat are investigated both at zero temperature, zero field limit and at low temperature, low field regimes. The results are consistent with the existence of perfect screening and a singlet ground state in the strong coupling phase. When particle-hole is an exact symmetry of the system, the impurity appears to be overscreened with a vanishing paramagnetic susceptibility. (Abstract shortened by UMI.)