Ferromagnetism in the Two-Dimensional Periodic Anderson Model

Using the constrained-path Monte Carlo method, we studied the magnetic properties of the two-dimensional periodic Anderson model for electron fillings between 1/4 and 1/2. We also derived two effective low energy theories to assist in interpreting the numerical results. For 1/4 filling we found that the system can be a Mott or a charge transfer insulator, depending on the relative values of the Coulomb interaction and the charge transfer gap between the two non-interacting bands. The insulator may be a paramagnet or antiferromagnet. We concentrated on the effect of electron doping on these insulating phases. Upon doping we obtained a partially saturated ferromagnetic phase for low concentrations of conduction electrons. If the system were a charge transfer insulator, we would find that the ferromagnetism is induced by the well-known RKKY interaction. However, we found a novel correlated hopping mechanism inducing the ferromagnetism in the region where the non-doped system is a Mott insulator. Our regions of ferromagnetism spanned a much smaller doping range than suggested by recent slave boson and dynamical mean field theory calculations, but they were consistent with that obtained by density matrix renormalization group calculations of the one-dimensional periodic Anderson model

Itinerant Ferromagnetism in the Periodic Anderson Model

We introduce a novel mechanism for itinerant ferromagnetism, based on a simple two-band model. The model includes an uncorrelated and dispersive band hybridized with a second band which is narrow and correlated. The simplest Hamiltonian containing these ingredients is the Periodic Anderson Model (PAM). Using quantum Monte Carlo and analytical methods, we show that the PAM and an extension of it contain the new mechanism and exhibit a non-saturated ferromagnetic ground state in the intermediate valence regime. We propose that the mechanism, which does not assume an intra atomic Hund's coupling, is present in both the iron group and in some f electron compounds like Ce(Rh_{1-x} Ru_x)_3 B_2, La_x Ce_{1-x} Rh_3 B_2 and the uranium monochalcogenides US, USe, and UTe

Numerical Analysis of Gas Flow Instabilities in Simplified Vertical HVPE GaN Reactors

This paper investigates the gas flow and the mass transport in simplified axial-symmetric vertical HVPE reactors for the growth of GaN bulk crystals through numerical simulations. We evaluate the relative significance of different flow and transport phenomena in dependence on the direction of gravity. The performed simulations show that buoyancy effects due to density differences between neighboring gas lines are the main factor causing the deformation of laminar flow patterns and the formation of recirculation cells within the growth zone. Baroclinic instabilities have been identified as the source for these phenomena. In contrast, typical vertical temperature gradients show only a minor impact on the stability of the gas flow within the growth zone in the vicinity of the growing crystal. Based on these results, major differences of the species transport in vertical HVPE reactors, where the flow is parallel or anti-parallel to the direction of gravity, referred to as down-flow and up-flow, respectively, are summarized. The performed analysis of the interplay and relative significance of different flow effects in the HVPE environment allows a general recommendation for reactor design and scaling with respect to stable gas flow conditions within the growth zone