Magnetic order in orbital models of the iron pnictides

We examine the appearance of the experimentally-observed stripe spin-density-wave magnetic order in five different orbital models of the iron pnictide parent compounds. A restricted mean-field ansatz is used to determine the magnetic phase diagram of each model. Using the random phase approximation, we then check this phase diagram by evaluating the static spin susceptibility in the paramagnetic state close to the mean-field phase boundaries. The momenta for which the susceptibility is peaked indicate in an unbiased way the actual ordering vector of the nearby mean-field state. The dominant orbitally resolved contributions to the spin susceptibility are also examined to determine the origin of the magnetic instability. We find that the observed stripe magnetic order is possible in four of the models, but it is extremely sensitive to the degree of the nesting between the electron and hole Fermi pockets. In the more realistic five-orbital models, this order competes with a strong-coupling incommensurate state which appears to be controlled by details of the electronic structure below the Fermi energy. We conclude by discussing the implications of our work for the origin of the magnetic order in the pnictides.Comment: 19 pages, 19 figures; published version, typos corrected, references adde

Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides

Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a 5-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$) symmetry ($s^\pm$). In particular we focus on the role of the hole pocket at the $(\pi,\pi)$ point of the unfolded Brillouin zone identified as crucial to the pairing by Kuroki {\it et al.}, and show that its presence leads to additional nesting of hole and electron pockets which stabilizes the isotropic $s^\pm$ state. The pocket's contribution to the pairing can be tuned by doping, surface effects, and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the RPA fluctuation exchange approximation allow us to draw connections between aspects of electronic structure, interaction parameters, and the form of the superconducting gap

Gap symmetry and structure of Fe-based superconductors

The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s\pm) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.Comment: 86 pages, revie

Festvortrag bei der Jahrestagung der Arbeitsgemeinschaft für Medizinisches Bibliothekswesen (AGMB e. V.) im Großen Hörsaal des Mannheimer Universitätsklinikums am 28. September 2004

The German physician Rudolf Virchow (1821-1902) , is world-famous not only due to the fact that he was the most distinguished chairman in Pathological Anatomy, a new discipline at the German Medical Schools in the second half of 19th century. Virchow became well-known also by his further activities, especially his endeavours for reforming medicine on a scientific basis and his political commitment to healthcare and social politics. As a member of the liberal party Deutsche Fortschrittspartei he became a representative of the Prussian State Parliament Landtag and of the German Federal Parliament Reichstag. As a gifted author and as an untiring editor he founded and ran two different journals: In 1848/49 he published the weekly paper The Medical Reform in which he gave his support to a scientific reform of medicine and to a democratic change in society, writing that politics were "nothing else but medicine in the broader perspective". In 1847 he founded the Archives for Pathological Anatomy and Physiology and for Clinical Medicine which is published yet today under the name of Virchow Archiv. In this journal many different schools of scientific medicine were invited to publish the results of their research. Occasionally, Virchow himself wrote articles on medical epistemology or on social politics. When he died in 1902, Rudolf Virchow had edited 169 volumes of his journal.Der Mediziner Rudolf Virchow (1821-1902) , ist nicht nur durch seine Tätigkeit als der bedeutendste deutsche Lehrstuhlinhaber für das in der zweiten Hälfte des 19. Jahrhunderts an den Universitäten etablierte Fachgebiet Pathologische Anatomie bekannt geworden, sondern mindestens ebenso durch seine weiteren Aktivitäten. Zu diesen gehörten sowohl das Bemühen um eine Reform der gesamten Medizin, die Virchow auf eine naturwissenschaftliche Grundlage stellen wollte, als auch sein politisches Engagement im linksliberalen Spektrum der Gesundheits- und Sozialpolitik, das ihn als Abgeordneten bis in den Preußischen Landtag und den Deutschen Reichstag führte. Während seines ganzen Lebens war Virchow aber zugleich auf mehreren Ebenen publizistisch tätig. Als stilistisch begabter Autor sowie als überragend fleißiger Redakteur und Herausgeber führte er zwei Zeitschriften zu großer Blüte: Die 1848/49 erschienene Wochenschrift Die medicinische Reform diente Virchow als kämpferisches Organ für seinen Wunsch nach einer gleichzeitigen Reform der Medizin und der Gesellschaft, der in seinem Ausspruch, die Politik sei "weiter nichts als Medizin im Großen" kulminierte. Das 1847 gegründete und bis heute bestehende Archiv für Pathologische Anatomie und Physiologie und für klinische Medicin war dem gegenüber eine Fachzeitschrift, in der alle Richtungen der naturwissenschaftlich fundierten Medizin zu Wort kamen. Zugleich nutzte Virchow auch dieses Journal mitunter für allgemeine Reflexionen zur Theorie der Medizin sowie, speziell in den Anfangsjahren, als Bühne für gesundheits- und sozialpolitische Grundsatzfragen. Bis zu Rudolf Virchows Tod im Jahre 1902 waren 169 Bände dieser Zeitschrift erschienen und von ihm persönlich redigiert worden