H. Capellmann

Spin-phonon coupling in intermediate valency: exactly solvable models

Two exactly solvable models are presented to describe the dynamic coupling of magnetic transitions within Kramers degenerate multiplets and lattice vibrations. The magnetic susceptibilities are calculated. It is shown that the “spin-boson” coupling can reduce the single ion Curic constant at low temperature. For special symmetries the Curie constant may vanish, yielding a temperature independent susceptibility at lowT.

Structural and magnetic investigations of a La2CuO4 single crystal with polarization analysis

Neutron scattering results on magnetic and structural ordering phenomena in a La2CuO4 single crystal are presented. Polarized neutrons and polarization analysis were used. A general discussion of the possibilities of this method in the study of single crystals is given. Precise information on magnetic structure, ordered moment as a function of temperature, and a lowering of crystalline symmetry at low temperature are obtained.

Crystallographic investigations and comparison of superconducting and nonsuperconducting CeCu2Si2

Two samples of CeCuxSi2 withx=1.8 (non superconducting) andx=2.2 (superconducting) have been investigated by neutron powder diffraction. Both samples were characterized crystallographically and then their impurity content and lattice site occupation were determined. Anisotropic thermal vibrations of the Cu and Si atoms is detected at low temperatures. A relationship between the structural parameterz (defining the distance Ce to Si) and the occurance of superconductivity is suggested.

A Polarized Neutron Scattering Study of the Normal Phase in YBa2Cu3O6.95

Polarized neutron scattering with polarization analysis has revealed negligible magnetic scattering up to 30 meV in the normal phase of YBa2Cu3O6.95 indicating the absence of local moments.

A microscopic model for the coupling of spin fluctuations and charge fluctuation in intermediate valency

A microscopic model is presented to show explicitly how dynamical charge and lattice fluctuations (at zero temperature the zero point motion) will-through spin orbit coupling-induce spin-flip terms off-electrons in intermediate valent and heavy fermion systems. A simple model Hamiltonian coupling charge, lattice, and spin degrees of freedom is derived and diagonalized exactly.

Quantum antiferromagnetism in 2 dimensions

We study the spin dynamics of the 2-dimensional spin 1/2 Heisenberg model at finite temperatureT, in particular the frequency region ω<T≪J (the coupling constant). The main conclusion reached is that the spin fluctuations have relaxational character, no propagating spin waves exist for ω<T. Comparison with experimental results on La2CuO4 give good agreement. Similarities and differences to previous theoretical treatments are discussed.

Energy of electrons on a two-dimensional lattice in a magnetic field: perturbation theory versus “Peierls substitution”

Energy levels and total energy of electrons in a magnetic field moving in a two dimensional periodic potential are obtained. Two limiting cases are treated: the tight binding limit and the weak potential. In these cases different direct perturbation theories are applicable. The energy difference with and without magnetic field is compared with calculations based on the “Peierls substitution”. The latter is shown to lead to severe qualitative and quantitative errors. The sources of these errors are clarified. The spontaneous generation of a magnetic field is shown to be impossible.