Comment on "Anomalous Thermal Conductivity of Frustrated Heisenberg Spin Chains and Ladders"

In a recent letter [Phys. Rev. Lett. 89, 156603 (2002); cond-mat/0201300], Alvarez and Gros have numerically analyzed the Drude weight for thermal transport in spin ladders and frustrated chains of up to 14 sites and have proposed that it remains finite in the thermodynamic limit. In this comment, we argue that this conclusion cannot be sustained if the finite-size analysis is taken to larger system sizes.Comment: One page REVTeX4, 1 figure. Published version (minor changes

Thermal transport of the XXZ chain in a magnetic field

We study the heat conduction of the spin-1/2 XXZ chain in finite magnetic fields where magnetothermal effects arise. Due to the integrability of this model, all transport coefficients diverge, signaled by finite Drude weights. Using exact diagonalization and mean-field theory, we analyze the temperature and field dependence of the thermal Drude weight for various exchange anisotropies under the condition of zero magnetization-current flow. First, we find a strong magnetic field dependence of the Drude weight, including a suppression of its magnitude with increasing field strength and a non-monotonic field-dependence of the peak position. Second, for small exchange anisotropies and magnetic fields in the massless as well as in the fully polarized regime the mean-field approach is in excellent agreement with the exact diagonalization data. Third, at the field-induced quantum critical line between the para- and ferromagnetic region we propose a universal low-temperature behavior of the thermal Drude weight.Comment: 9 pages REVTeX4 including 5 figures, revised version, refs. added, typos correcte

Techniques to Update a Land Management Information System with Landsat

The Minnesota State Planning Agency has developed a geographically referenced Land Management Information System which is being used extensively for planning purposes. Land use categories in the system were originally coded from aerial photographs; this method is inefficient for updating the large-area data base. Landsat data and many computer-assisted techniques are available to analyze the classification system and to update the land use data base. The data derived from a Landsat analysis could be used to supplement the existing data base and to complement detailed interpretations of aerial photographs. This study had as its primary objective an evaluation of computer manipulation, classification, and accuracy assessment techniques for use in updating land use data in the Land Management Information System. Four approaches to statistical computer manipulation (polygons selected from cathode ray tube displays, unsupervised clustering, polygons selected from aerial photographs and data extracted from the existing land use data base) were attempted. The resulting statistics were applied to the image data by three pattern-recognition algorithms: minimum distance to the mean, maximum likelihood, and canonical analysis with minimum distance to the mean. Twelve output images were compared to photo interpreted samples, ground-verified samples, and the current land use data base for accuracy assessment. The results of this study indicate that for a reconnaissance inventory, statistical computer manipulation via polygons selected from aerial photographs applied with the canonical analysis and minimum distance algorithm is the most accurate and efficient approach. Crosstabulation with the accuracy samples indicated classification accuracies between 20 to 40 percent. These accuracy levels could probably be increased with the availability of appropriate seasonal coverage and the collection of more timely multidate supporting data

Frustrated ferromagnetic spin-1/2 chain in a magnetic field: The phase diagram and thermodynamic properties

The frustrated ferromagnetic spin-1/2 Heisenberg chain is studied by means of a low-energy field theory as well as the density-matrix renormalization group and exact diagonalization methods. Firstly, we study the ground-state phase diagram in a magnetic field and find an `even-odd' (EO) phase characterized by bound pairs of magnons in the region of two weakly coupled antiferromagnetic chains. A jump in the magnetization curves signals a first-order transition at the boundary of the EO phase, but otherwise the curves are smooth. Secondly, we discuss thermodynamic properties at zero field, where we confirm a double-peak structure in the specific heat for moderate frustrating next-nearest neighbor interactions.Comment: 4 pages RevTex4, 4 figures. Minor changes, title modified. Additional material is available here: http://www.theorie.physik.uni-goettingen.de/~honecker/j1j2-td