Excitation spectrum of the homogeneous spin liquid

We discuss the excitation spectrum of a disordered, isotropic and translationally invariant spin state in the 2D Heisenberg antiferromagnet. The starting point is the nearest-neighbor RVB state which plays the role of the vacuum of the theory, in a similar sense as the Neel state is the vacuum for antiferromagnetic spin wave theory. We discuss the elementary excitations of this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1 excited dimers which must be modeled by bond Bosons. We derive an effective Hamiltonian describing the excited dimers which is formally analogous to spin wave theory. Condensation of the bond-Bosons at zero temperature into the state with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering. The latter is a key ingredient for a microscopic interpretation of Zhang's SO(5) theory of cuprate superconductivityComment: RevTex-file, 16 PRB pages with 13 embedded eps figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: [email protected]

Electron momentum distribution in underdoped cuprates

We investigate the electron momentum distribution function (EMD) in a weakly doped two-dimensional quantum antiferromagnet (AFM) as described by the t-J model. Our analytical results for a single hole in an AFM based on the self-consistent Born approximation (SCBA) indicate an anomalous momentum dependence of EMD showing 'hole pockets' coexisting with a signature of an emerging large Fermi surface. The position of the incipient Fermi surface and the structure of the EMD is determined by the momentum of the ground state. Our analysis shows that this result remains robust in the presence of next-nearest neighbor hopping terms in the model. Exact diagonalization results for small clusters are with the SCBA reproduced quantitatively.Comment: 5 pages, submitted to PR

Kulturwissenschaftliche Bildtheorien: ihre Potentiale und Grenzen für die historische Diskursanalyse

In der historischen Diskursanalyse wird zunehmend versucht, Bilder und Bildlichkeit einzubeziehen und die Geschichte des Sagbaren mit der des Zeigbaren zu verflechten. Dabei ergeben sich Fragen nach den Spezifika der Bildkommunikation, der Beziehung von Sprache und Bildlichkeit, nach Bildwahrnehmung und Bildpraktiken. Der vorliegende Text gibt einen Überblick über Positionen der jüngeren Kulturwissenschaften aus semiotischen wie hermeneutischen Denktraditionen. Diskutiert wird, ob diese Positionen in theoretischer Hinsicht Bildlichkeit fassbar machen, ohne in anthropologische und ontologische Essenzialisierungen zu verfallen. Gefragt wird außerdem, welche methodologischen Anknüpfungspunkte sich für die historische Diskursanalyse bieten, um multimodale Diskurse zu erschließen