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Static and dynamical properties of frustrated spin models
Full text linkThis thesis is devoted to the numerical investigation of quantum spin models which describe the low-energy physics of frustrated magnets. At extremely low temperatures, these
systems can host the so-called spin liquid phase, an unconventional state of matter characterized by a high degree of quantum entanglement and the absence of magnetic order.
The experimental identification of the spin liquid phase in actual materials relies on the detection of its distinctive excitations (named spinons), which possess fractional quantum
numbers and can be probed by inelastic neutron scattering experiments.
From the theoretical point of view, variational methods have been largely employed to tackle ground state properties of frustrated spin models. In particular, variational Monte Carlo techniques based on Gutzwiller-projected fermionic wave functions have been shown to provide accurate results for several frustrated systems. In this thesis, we pursue an extension of this variational scheme to target dynamical spectra, which are directly measured by inelastic neutron scattering experiments. Specifically, we compute the dynamical structure factor by constructing approximate excited states, which are obtained by applying two-spinon operators to the ground state wave function. Our results prove that this variational method can accurately describe the spectral features of different spin systems. Focusing on prototypical frustrated models on the square and triangular lattices, we observe how the dynamical structure factor reflects the phase transition between a magnetically ordered phase with spin wave excitations, to a spin liquid state with fractional degrees of freedom.
In addition to spectral properties, we also explore new directions to improve the accuracy of Gutzwiller-projected states by the application of a neural network correlator, in the form of a restricted Boltzmann machine. While this hybrid variational scheme provides a considerable improvement of the variational energy in the case of unfrustrated spin models, less satisfactory results are obtained for frustrated systems, which call for further refinements of the neural network variational Ansätze
Some Notes on the Role of the Identity of Indiscernibles in Metaphysics
No full textThe Principle of the Identity of Indiscernibles (PII) asserts that if putative objects x and y shareall properties P, then they must be one and the same entity. Since the usual formal rendering ofthe PII has the same formal structure as the Leibniz Identity, it may be unclear whether it canbe used to define identity and objectuality. As identity and objectuality are closely related, thisstudy aims to examine their relationship within the framework of formal ontology. Crucialfor the discussion are issues about type and range of quantification and the invariance ofthe identity predicate coourring in the PII. Ultimately, the analysis reveals that the appeal toPII is insufficient for providing both identity and objectuality. Some further considerationsabout how the PII sensitively constrains the range of available ontologies or metaphysics areformulated.Fil: Ferrari, Francesco Maria. Universidad de Buenos Aires. Facultad de Filosofía y Letras. Instituto de Filosofía "Dr. Alejandro Korn"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
CONSIDERAZIONI IN TEMA DI RESPONSABILITÀ PROFESSIONALE DEGLI OPERATORI NEGLI OSPEDALI: NUOVI PARADIGMI?
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