Spontaneous ferromagnetism in High-Tc superconductors

Abstract

We propose a model based on the charge disorder of the high-Tc superconductors to explain the weak ferromagnetic signal observed in compounds such as YBa2Cu3O6+x. In order to set the framework, we develop the basic elements of second quantization for fermions, and introduce important elements of both the microscopic and phenomenological theory of superconductivity. Our basic model relies on an electronic phase separation with the formation of low and high density domains. At low temperatures this system may act as a granular superconductor where the grains are formed by isolated regions with varying charge density and superconducting amplitudes. These isolated regions may have different local critical temperatures (Tc), that is, not the same onset of superconducting amplitudes, forming a Josephson network and the whole system undergoes the superconducting or resistivity transition through phase coherence among the grains. The main point of this dissertation is that the low electronic density of the grains in conjunction with intrinsic electronic disorder, two well known properties of cuprates, may produce a negative Josephson coupling that provokes spontaneous frustration. Spontaneous current loops are produced that are responsible for the overall observed weak ferromagnetic order. We show that this model provides a novel explanation and reproduces the experimental data of the observed magnetic signal