Thermal and ground-state entanglement in Heisenberg XX qubit rings

We study the entanglement of thermal and ground states in Heisernberg $XX$ qubit rings with a magnetic field. A general result is found that for even-number rings pairwise entanglement between nearest-neighbor qubits is independent on both the sign of exchange interaction constants and the sign of magnetic fields. As an example we study the entanglement in the four-qubit model and find that the ground state of this model without magnetic fields is shown to be a four-body maximally entangled state measured by the $N$-tangle.Comment: Four pages and one figure, small change

Magnon-magnon interactions in the Spin-Peierls compound CuGeO_3

In a magnetic substance the gap in the Raman spectrum, Delta_R, is approximatively twice the value of the neutron scattering gap, Delta_S, if the the magnetic excitations (magnons) are only weakly interacting. But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in the quasi-1D Spin-Peierls compound CuGe_3. We present numerical estimates for Delta_R/Delta_S from exact diagonalization studies for finite chains and find agreement with experiment for intermediate values of the frustration parameter alpha. An analysis of the numerical Raman intensity leads us to postulate a continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in detail the numerical method used, the dependence of the results on the model parameters and a novel matrix-element effect due to the dimerization of the Raman-operator in the Spin-Peierls phase.Comment: submitted to PRB, Phys. Rev. B, in pres

Spin and charge dynamics of chromium alloys

Both the spin- and charge-density waves of Cr alloys are produced by the Coulomb attraction between electrons and holes on nearly nested Fermi surfaces. Driven by quasi-particle transitions, transverse spin- wave and longitudinal phason modes are associated with rotational and translational symmetries of pure Cr and its dilute alloys. At low frequencies, both spin and charge phasons have a nearly linear dispersion with a mode velocity which approaches the spin-wave velocity as T approaches T{sub N} or as the mismatch between the Fermi surfaces increases