The order parameter-entropy relation in some universal classes: experimental evidence

The asymptotic behaviour near phase transitions can be suitably characterized by the scaling of $\Delta s/Q^2$ with $\epsilon=1-T/T_c$, where $\Delta s$ is the excess entropy and $Q$ is the order parameter. As $\Delta s$ is obtained by integration of the experimental excess specific heat of the transition $\Delta c$, it displays little experimental noise so that the curve $\log(\Delta s/Q^2)$ versus $\log\epsilon$ is better constrained than, say, $\log\Delta c$ versus $\log\epsilon$. The behaviour of $\Delta s/Q^2$ for different universality classes is presented and compared. In all cases, it clearly deviates from being a constant. The determination of this function can then be an effective method to distinguish asymptotic critical behaviour. For comparison, experimental data for three very different systems, Rb2CoF4, Rb2ZnCl4 and SrTiO3, are analysed under this approach. In SrTiO3, the function $\Delta s/Q^2$ does not deviate within experimental resolution from a straight line so that, although Q can be fitted with a non mean-field exponent, the data can be explained by a classical Landau mean-field behaviour. In contrast, the behaviour of $\Delta s/Q^2$ for the antiferromagnetic transition in Rb2CoF4 and the normal-incommensurate phase transition in Rb2ZCl4 is fully consistent with the asymptotic critical behaviour of the universality class corresponding to each case. This analysis supports, therefore, the claim that incommensurate phase transitions in general, and the A$_2$BX$_4$ compounds in particular, in contrast with most structural phase transitions, have critical regions large enough to be observable.Comment: 13 pp. 9 ff. 2 tab. RevTeX. Submitted to J. Phys.: Cond. Matte

Influence of the Grzmiąca catchment geological structure on groundwater outflow from Quaternary aquifers [central Poland]

The author concentrated on characteristic elements of the Grzmiąca basin geology determining its groundwater outflow. The study area is a small part of the Bzura drainage basin located NE of Łódź, in the northern margin of the Łódź Heights, which were formed during theWartanian glaciation. Large thickness [50–110 m] of the fluvioglacially originated Pleistocene sands and gravels is the most important feature of this basin.. They are surrounded from N,W, and S with clays of the Odranian andWartanian glaciations origin. The clays form a ring coinciding with the watershed zone of the basin. This structure is responsible for the high underground water capacity indicated by the analysis of the groundwater outflow changeability. Below the Pleistocene sediments the basin is sealed by Pliocene silts and clays, in practice precluding percolation of groundwaters below the drainage level. It was confirmed by the equalized water balance for 1996–1997 derived from field studies. High permeability of the surface sedimentsn facilitates rainwater infiltration. It also concerns thawing waters, as shown by a comparing the river groundwater supply after and before thawing [March 1996]. Fluctuations of the groundwater outflow are considerable but at the same time they show a very high runoff trend stability. The Grzmiąca catchment fills up with water for a long time and then is slowly drained. The other feature of the studied basin is its high spring yield which together with intensive linear drainage along the rivers results in a very high percentage of groundwater outflow in the total runoff [88.5%]. This is due to the fact that the springs area and along the main river course there is a thick water-bearing horizon consisting of sands of the Odranian and Wartanian age which are continuous, not separated with clays