Low-lying excitations and thermodynamics of an antiferromagnetic Heisenberg fractal system of a dimension between one and two

We investigate a frustrated Heisenberg spin-1/2 antiferromagnet on a fractal lattice of dimension d=ln3/ln2 (Sierpinski gasket). Calculations were performed using (a) exact diagonalization of all eigenstates and eigenvectors for systems up to N=15 and (b) the Decoupled-Cell Quantum-Monte-Carlo method for systems up to N=366. We present the low-lying spectrum and the specific heat. The specific heat shows a second maximum in the low-temperature region. This behavior is similar to the behavior of the quantum Heisenberg antiferromagnet on a kagome lattice and suggests a disordered ground state and a spin gap in the considered system.Comment: 2 pages, LaTeX, 3 eps figures, to appear in JMM

Quasi-periodic Variations in the Hard X-ray emission of a Large Arcade Flare

Quasi-periodic oscillations of the hard X-ray (HXR) emission of the large flare of 2 November 1991 have been investigated using HXR light curves and soft X-ray and HXR images recorded by the {\sl Yohkoh} X-ray telescopes. The results of the analysis of these observations are the following: i) The observations confirm that electrons are accelerated in oscillating magnetic traps which are contained within the cusp magnetic structure. ii) The amplitude of the HXR pulses increase due to the increase in the amplitude of the magnetic trap oscillations and the increase in the density within the traps caused by the chromospheric evaporation upflow. iii) The increase in the amplitude of the HXR pulses terminates when further increase in the density inside the traps inhibits the acceleration of electrons. iv) The model of oscillating magnetic traps is able to explain time variation of the electron precipitation, strong asymmetry in precipitation of accelerated electrons, and systematic differences in the precipitation of 15 and 25 keV electrons. v) We have obtained a direct observational evidence that strong HXR pulses are the result of the inflow of dense plasma coming from the chromospheric evaporation, into the acceleration volume.Comment: 18 pages, 12 figures, accepted by Solar Physic

Flare Hybrids

Svestka (Solar Phys. 1989, 121, 399) on the basis of the Solar Maximum Mission observations introduced a new class of flares, the so-called flare hybrids. When they start, they look as typical compact flares (phase 1), but later on they look like flares with arcades of magnetic loops (phase 2). We summarize the features of flare hybrids in soft and hard X-rays as well as in extreme-ultraviolet; these allow us to distinguish them from other flares. Additional energy release or long plasma cooling timescales have been suggested as possible cause of phase 2. Estimations of frequency of flare hybrids have been given. Magnetic configurations supporting their origin have been presented. In our opinion, flare hybrids are quite frequent and a difference between lengths of two interacting systems of magnetic loops is a crucial parameter for recognizing their features.Comment: 15 pages, 4 figures, to appear in Solar Physic

On the existence conditions of surface spin wave modes in (Ga,Mn)As thin films

Spin-wave resonance (SWR) is a newly emerged method for studying surface magnetic anisotropy and surface spin-wave modes (SSWMs) in (Ga,Mn)As thin films. The existence of SSWMs in (Ga,Mn)As thin films has recently been reported in the literature; SSWMs have been observed in the in-plane configuration (with variable azimuth angle $\phi_M$ between the in-plane magnetization of the film and the surface [100] crystal axis), in the azimuth angle range between two in-plane critical angles $\phi_{c1}$ and $\phi_{c2}$. We show here that cubic surface anisotropy is an essential factor determining the existence conditions of the above-mentioned SSWMs: conditions favorable for the occurrence of surface spin-wave modes in a (Ga,Mn)As thin film in the in-plane configuration are fulfilled for those azimuth orientations of the magnetization of the sample that lie around the hard axes of cubic magnetic anisotropy. This implies that a hard cubic anisotropy axis can be regarded in (Ga,Mn)As thin films as an easy axis for surface spin pinning