Mohamed Azzouz

Quantum criticality in dimerized spin ladders

We analyze a possibility of quantum criticality (gaplessness) in dimerized antiferromagnetic two- and three-leg spin-1/2 ladders. Contrary to earlier studies of these models, we examine different dimerization patterns in the ladder. We find that ladders with the columnar dimerization order have lower zero-temperature energies and they are always gapped. For the staggered dimerization order, we find the quantum critical lines, in agreement with earlier analyses. The bond mean-field theory we apply, demonstrates its quantitative accuracy and agrees with available numerical results. We conclude that unless some mechanism for locking dimerization into the energetically less favorable staggered configuration is provided, the dimerized ladders do not order into the phase where the quantum criticality occurs.

Magnetic Transport in Spin Antiferromagnets for Spintronics Applications

Had magnetic monopoles been ubiquitous as electrons are, we would probably have had a different form of matter, and power plants based on currents of these magnetic charges would have been a familiar scene of modern technology. Magnetic dipoles do exist, however, and in principle one could wonder if we can use them to generate magnetic currents. In the present work, we address the issue of generating magnetic currents and magnetic thermal currents in electrically-insulating low-dimensional Heisenberg antiferromagnets by invoking the (broken) electricity-magnetism duality symmetry. The ground state of these materials is a spin-liquid state that can be described well via the Jordan–Wigner fermions, which permit an easy definition of the magnetic particle and thermal currents. The magnetic and magnetic thermal conductivities are calculated in the present work using the bond–mean field theory. The spin-liquid states in these antiferromagnets are either gapless or gapped liquids of spinless fermions whose flow defines a current just as the one defined for electrons in a Fermi liquid. The driving force for the magnetic current is a magnetic field with a gradient along the magnetic conductor. We predict the generation of a magneto-motive force and realization of magnetic circuits using low-dimensional Heisenberg antiferromagnets. The present work is also about claiming that what the experiments in spintronics attempt to do is trying to treat the magnetic degrees of freedoms on the same footing as the electronic ones.

Theory of the magnetic resonance for the high-TC cuprate superconductors

Abstract The magnetic response expected from a state characterized by rotating antiferromagnetism in a neutron-scattering experiment is calculated. We predict the occurrence of a peak at the frequency of the rotation of the rotating antiferromagnetic order parameter. The doping dependence trends of this frequency are very similar to those of the frequency of the magnetic resonance observed in neutron-scattering experiments for the hole-doped high- T C cuprates. This leads us to propose the rotating antiferromagnetism as a possible mechanism for this magnetic resonance. Also, the interpretation of the rotating order parameter as a probability for a spin flip process to occur allows us to argue that the unusual zero momentum antiferromagnetic order observed by Fauque et al. is equivalent to the rotating antiferromagnetism phenomenon. We conclude that while the magnitude of the rotating antiferromagnetic order parameter was previously proposed to be responsible for the pseudogap and the unusual thermodynamic and transport properties, the phase of the rotating order parameter is proposed here to be responsible for the unusual magnetic properties of the high- T C cuprate superconductors.

Effect of the Fermi surface reconstruction on the self-energy of the copper-oxide superconductors

Abstract We calculated the self-energy corrections beyond the mean-field solution of the rotating antiferromagnetism theory using the functional integral approach. The frequency dependence of the scattering rate 1 / τ is evaluated for different temperatures and doping levels, and is compared with other approaches. The general trends we found are fairly consistent with the nearly antiferromagnetic Fermi liquid as far as the k -anisotropy and some aspects of the marginal-Fermi liquid behavior are concerned. The present approach provides the justification from the microscopic point of view for the phenomenology of the marginal Fermi liquid ansatz, which was used in the calculation of several physical properties of the high- T C cuprates within the rotating antiferromagnetism theory. In addition, the expression of self-energy we calculated takes into account the two currently hot issues of the high- T C cuprate superconductors, namely the Fermi surface reconstruction and the hidden symmetry, which are closely related to the pseudogap.

Fermi Surface Reconstruction due to Hidden Rotating Antiferromagnetism in N and P-Type High-TC Cuprates

The Fermi surface calculated within the rotating antiferromagnetism theory undergoes a topological change when doping changes from p-type to n-type, in qualitative agreement with experimental data for n-type cuprate Nd2−xCexCuO4 and p-type La2−xSrxCuO4. Also, the reconstruction of the Fermi surface, observed experimentally close to optimal doping in p-type cuprates, and slightly higher than optimal doping in the overdoped regime for this n-type high-TC cuprate, is well accounted for in this theory. This reconstruction is a consequence of the quantum criticality caused by the disappearance of rotating antiferromagnetism. The present results are in qualitative agreement with recently observed quantum oscillations in some high-TC cuprates. This paper presents new results about the application of the rotating antiferromagnetism theory to the study of the electronic structure for n-type materials.