Magnetic Vortices in High Temperature Superconductors

It is suggested that modes, observed in recent neutron scattering experiments by Lake {\it et al.}, on La$_{2-x}$Sr$_x$CuO$_4$ in strong magnetic fields ($\approx$ 7 T), are due to the existence of antiferromagnetic moments associated with the cores of vortices generated by the field. These moments form one-dimensional chains along the $c$-axis (the vortex axis), which at finite temperatures are disordered. At temperatures higher than 10 K the correlation length gets shorter than the lattice parameter, resulting in no scattering from coherent spin-waves above that temperature. The bandwidth of the spin-waves is estimated to be $\approx$ 4 meV in accordance with the observations.Comment: 3 pages, 1 figur

Spin dynamics in the stripe phase of the cuprates

Within a model that supports stripe spin and charge order coexisting with a d$_{x^2-y^2}$-wave superconducting phase, we study the self-consistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particle-hole continuum can strongly damp parts of the low-energy spin wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the low-energy collective magnetic modes of lanthanum based cuprate superconductors.Comment: 4 pages, 3 figure

Dynamic rotor mode in antiferromagnetic nanoparticles

We present experimental, numerical, and theoretical evidence for a new mode of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering experiments on 8 nm particles of hematite display a loss of diffraction intensity with temperature, the intensity vanishing around 150 K. However, the signal from inelastic neutron scattering remains above that temperature, indicating a magnetic system in constant motion. In addition, the precession frequency of the inelastic magnetic signal shows an increase above 100 K. Numerical Langevin simulations of spin dynamics reproduce all measured neutron data and reveal that thermally activated spin canting gives rise to a new type of coherent magnetic precession mode. This "rotor" mode can be seen as a high-temperature version of superparamagnetism and is driven by exchange interactions between the two magnetic sublattices. The frequency of the rotor mode behaves in fair agreement with a simple analytical model, based on a high temperature approximation of the generally accepted Hamiltonian of the system. The extracted model parameters, as the magnetic interaction and the axial anisotropy, are in excellent agreement with results from Mossbauer spectroscopy

Dissipative quantum systems modeled by a two level reservoir coupling

The coupling between a quantum dynamical system and a two-level system reservoir is analysed within the framework of the Feynman-Vernon theory. We stress the differences between this new reservoir and the well-known bath of oscillators and show that, in order to obtain the Langevin equation for the system of interest in the high temperature regime, we have to choose a spectral distribution function $J(\omega)$ which is finite for $\omega=0$.Comment: 6 pages, RevteX, preprint UNICAM

Magnetoresistance of Two-Dimensional Fermions in a Random Magnetic Field

We perform a semiclassical calculation of the magnetoresistance of spinless two-dimensional fermions in a long-range correlated random magnetic field. In the regime relevant for the problem of the half filled Landau level the perturbative Born approximation fails and we develop a new method of solving the Boltzmann equation beyond the relaxation time approximation. In absence of interactions, electron density modulations, in-plane fields, and Fermi surface anisotropy we obtain a quadratic negative magnetoresistance in the weak field limit.Comment: 12 pages, Latex, no figures, Nordita repor