Selfconsistent Approximations in Mori's Theory

The constitutive quantities in Mori's theory, the residual forces, are expanded in terms of time dependent correlation functions and products of operators at $t=0$, where it is assumed that the time derivatives of the observables are given by products of them. As a first consequence the Heisenberg dynamics of the observables are obtained as an expansion of the same type. The dynamic equations for correlation functions result to be selfconsistent nonlinear equations of the type known from mode-mode coupling approximations. The approach yields a neccessary condition for the validity of the presented equations. As a third consequence the static correlations can be calculated from fluctuation-dissipation theorems, if the observables obey a Lie algebra. For a simple spin model the convergence of the expansion is studied. As a further test, dynamic and static correlations are calculated for a Heisenberg ferromagnet at low temperatures, where the results are compared to those of a Holstein Primakoff treatment.Comment: 51 pages, Latex, 3 eps figures included, elsart and epsf style files included, also available at http://athene.fkp.physik.th-darmstadt.de/public/wolfram.html and ftp://athene.fkp.physik.th-darmstadt.de/pub/publications/wolfram

Electronic and magnetic properties of orthorhombic iron selenide (FeSe)

Iron orbitals in orthorhombic iron selenide (FeSe) can produce charge-like multipoles that are polar (parity-odd). Orbitals in question include Fe(3d), Fe(4p) and p-type ligands that participate in transport properties and bonding. The polar multipoles may contribute weak, space-group forbidden Bragg spots to diffraction patterns collected with x-rays tuned in energy to a Fe atomic resonance (Templeton & Templeton scattering). Ordering of conventional, axial magnetic dipoles does not accompany the tetragonal-orthorhombic structural phase transition in FeSe, unlike other known iron-based superconductors. We initiate a new line of inquiry for this puzzling property of orthorhombic FeSe, using a hidden magnetic-order that belongs to the m'm'm' magnetic crystal-class. It is epitomized by the absence of ferromagnetism and axial magnetic dipoles, and the appearance of magnetic monopoles and magneto-electric quadrupoles. A similar magnetic order occurs in cuprate superconductors, YBCO & Hg1201, where it was unveiled with the Kerr effect and in Bragg diffraction patterns revealed by polarized neutrons. PACS number 75.25.-

Strange magnetic multipoles and neutron diffraction by an iridate perovskite (Sr2IrO4)

A theoretical investigation of a plausible construct for electronic structure in iridate perovskites demonstrates the existence of magnetic multipoles hitherto not identified. The strange multipoles, which are parity-even, time-odd and even rank tensors, are absent from the so-called jeff = 1/2 model. We prove that the strange multipoles contribute to magnetic neutron diffraction, and we estimate their contribution to intensities of Bragg spots for Sr2IrO4. The construct encompasses the jeff = 1/2 model, and it is consistent with the known magnetic structure, ordered magnetic moment, and published resonant x-ray Bragg diffraction data. Over and above time-odd quadrupoles and hexadecapoles, whose contribution changes neutron Bragg intensities by an order of magnitude, according to our estimates, are relatively small triakontadipoles recently proposed as the primary magnetic order-parameter of Sr2IrO4

Anapole Correlations in Sr2IrO4 Defy the jeff = 1/2 Model

Zel'dovich (spin) anapole correlations in Sr2IrO4 unveiled by magnetic neutron diffraction contravene the spin-orbit coupled ground state used by the jeff = 1/2 (pseudo-spin) model. Specifically, spin and space know inextricable knots which bind each to the other in the iridate. The diffraction property studied in the Letter is enforced by strict requirements from quantum mechanics and magnetic symmetry. It has not been exploited in the past, whereas neutron diffraction by anapole moments is established. Entanglement of the electronic degrees of freedom is captured by binary correlations of the anapole and position operators, and hallmarked in the diffraction amplitude by axial atomic multipoles with an even rank

High-order Dy multipole motifs observed in DyB2C2 with resonant soft x-ray Bragg diffraction

Resonant soft x-ray Bragg diffraction at the Dy M4,5 edges has been exploited to study Dy multipole motifs in DyB2C2. Our results are explained introducing the intra-atomic quadrupolar interaction between the core 3d and valence 4f shell. This allows us to determine for the first time higher order multipole moments of dysprosium $4f$ electrons and to draw their precise charge density. The Dy hexadecapole and hexacontatetrapole moment have been estimated at -20% and +30% of the quadrupolar moment, respectively. No evidence for the lock-in of the orbitals at T_N has been observed, in contrast to earlier suggestions. The multipolar interaction and the structural transition cooperate along c but they compete in the basal plane explaining the canted structure along [110].Comment: 4 pages, 3 figure

The Clerical Character in George Eliot\u27s Fiction

Writing to Blackwood in November 1856 to introduce Scenes of Clerical Life, G.H. Lewes promised, tales and sketches illustrative of the actual life of our country clergy about a quarter of a century ago; but solely in its \u27human\u27 and \u27not at all\u27 in its \u27theological\u27 aspect; the object being to do what has never yet been done in our Literature, for we have abundant religious stories polemical and doctrinal, but since the Vicar and Miss Austen, no stories representing the clergy like any other class with the humours, sorrows, and troubles of other men. Oliver Lovesey\u27s new study is an overdue attempt to examine the value of Lewes\u27s claim, placing Eliot\u27s presentation of the clergy in the context of contemporary social history and prevalent literary practice, whilst also throwing in some examination of fictional theory as a sop to those not content with the works of Eliot per se