Details

Autor(en) / Beteiligte

• Husmann, Anke

Titel

Interactions of spins and electrons in highly correlated systems

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1997

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• In this thesis, we have been interested in two particular systems. NiS$\sb2$ is a Mott-Hubbard system in which strong electron-electron interactions split the half-filled conduction band and open up a gap, which can be suppressed by substituting Se for S and/or by applying hydrostatic pressure. At finite temperature, it is well established that the metal-insulator transition in Mott-Hubbard systems is of first order, as can be seen in many systems. However, when the transition temperature is suppressed to zero (the so-called quantum phase transition), the nature of the phase transition as a function of the electron-electron interactions is not known. The challenge in characterising this quantum phase transition lies in the difficulty of finding a suitable experimental system. The constraints on the system are rather stringent, and Ni(S,Se)$\sb2$ is (to date) the only known Mott-Hubbard system which can be used for the study of critical behaviour. The perovskite manganites, such as LaMnO$\sb3,$ are the prototype of materials where the double exchange mechanism couples the conduction electrons strongly to the magnetic ordering of the background lattice. This leads to a variety of differently ordered ground states, such as ferromagnetic metals and insulators, antiferromagnetic insulators, and also charge ordered antiferromagnets. These materials can be tuned in many ways. We are interested in only a small subset of this rich phase space. Pr$\sb{0.5}$Sr$\sb{0.5}$MnO$\sb3$ and Nd$\sb{0.5}$Sr$\sb{0.5}$MnO$\sb3$ undergo two magnetic phase transitions as a function of decreasing temperature: the second order Curie (ferromagnetic ordering) temperature, T$\sb{\rm C},$ is followed by the first order Neel (antiferromagnetic ordering) temperature, T$\sb{\rm N},$ which coincides in Nd$\sb{0.5}$Sr$\sb{0.5}$MnO$\sb3$ with the charge ordering temperature, T$\sb{\rm CO}.$ The peculiarity in these two materials is that T$\sb{\rm N}$ can be suppressed to zero by applying a magnetic field. The nature of the charge degrees of freedom at low temperatures as a function of magnetic field is an open issue which we investigated further.