Chemical Potential Derivative as Hallmark for Phase Transitions

M. Pietruszka Faculty of Biology and Environmental Protection, University of Silesia, Jagiellońska 28, 40-032 Katowice, Poland and M. Matlak Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland

Received: July 27, 1999; in final form October 15, 1999

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We study antiferromagnetic properties of the two-band extended s-f model with fluctuating valence in the context of two mutually bound new effects of chemical potential critical behaviour, as well as of critical electron redistribution. In order to exemplify both phenomena we build phase diagrams of the system displaying the dependence of the critical Néel temperatures (TN) of the system versus 4f (5f) level positions. The phase diagram consists of two different areas corresponding to antiferromagnetic and paramagnetic phases. We plot the magnetizations and the correlation functions of the system as functions of temperature. Next, we investigate the temperature dependence of the relative average occupation numbers Δnf(d) and the chemical potential Δμ for a given 4f (5f) level position Ef. Plotting this quantities along the Ef cross-section lines we observe small (of the order of 10-4-10-3) but well localized kinks exactly at the Néel temperature TN. Last but not least, we plot the first derivative of the chemical potential dμ/dT which, as it shows clearly visible jumps at TN, may turn out to be very accurate and sensitive (auxiliary) tool to find critical temperatures of the considered system. Moreover, we plot the difference μAF-μPARA where we subtract a chemical potential value of a reference paramagnetic sample from the actual value of the antiferromagnetic system. Also in this case we report the observation of discontinuous change in slope at TN. Our observations can be extended to point out to a new practical possibility of how to find experimentally the critical temperatures of the antiferromagnetic systems exclusively from the chemical potential measurements. We expect that the same type of measurement, according to our recent and present results, would also apply to all types of critical phenomena in real solids.

DOI: 10.12693/APhysPolA.96.725 PACS numbers: 64.60.-i, 75.40.Cx, 75.50.Ee