Zero-energy peak of the density of states and localization properties of a one-dimensional Frenkel exciton: Off-diagonal disorder

We study a one-dimensional Frenkel Hamiltonian with off-diagonal disorder, focusing our attention on the physical nature of the zero-energy peak of the density of states. The character of excitonic states (localized or delocalized) is also examined in the vicinity of this peak. It is shown that the state being responsible for the peak is localized. A detailed comparison of the nearest-neighbor approach with the long-range dipole-dipole coupling is performed.Comment: 15 pages with 7 figures (REVTeX). To appear in Physical Review

A projection method for general form linear least-squares problems

One of the possible approaches for the solution of underdetermined linear least-squares problems in general form, for a chosen regularization operator L, projects the problem in the null space of L and in its orthogonal complement. In this paper, we show that the projected problem cannot be solved by the generalized singular value decomposition, and propose some approaches to overcome this issue. Numerical experiments ascertain the stability of the new procedures

Intermittent quakes and record dynamics in the thermoremanent magnetization of a spin-glass

A novel method for analyzing the intermittent behavior of linear response data in aging systems is presented and applied to spin-glass thermoremanent magnetization (TRM) (Rodriguez et al. Phys. Rev. Lett. 91, 037203, 2003). The probability density function (PDF) of magnetic fluctuations is shown to have an asymmetric exponential tail, demonstrating that the demagnetization process is carried by intermittent, significant, spin rearrangements or \emph{quakes}. These quakes are most pronounced shortly after the field removal, $t/t_w \approx 1$ and in the non-equilibrium aging regime $t/t_w >>1$. For a broad temperature range, we study the dependence of the TRM decay rate on $t$, the time since the initial quench and on $t_w$, the time at which the magnetic field is cut. The $t$ and $t_w$ dependence of the rate is extracted numerically from the data and described analytically using the assumption that the linear response is subordinated to the intermittent process which spasmodically release the initial imbalances created by the quench.Comment: 8 pages, 9 figures. The paper has been expanded and restructured, the figures have been enlarged and improved. Final version, to appear in Phy. Rev.

Universality Classes of Diagonal Quantum Spin Ladders

We find the classification of diagonal spin ladders depending on a characteristic integer $N_p$ in terms of ferrimagnetic, gapped and critical phases. We use the finite algorithm DMRG, non-linear sigma model and bosonization techniques to prove our results. We find stoichiometric contents in cuprate $CuO_2$ planes that allow for the existence of weakly interacting diagonal ladders.Comment: REVTEX4 file, 3 color figures, 1 tabl