A NEW THERMODYNAMICAL EXPRESSION FOR CALCULATING THE CORRELATION LENGTH L. DI6SI and B. LUKACS

It was originally revealed by Einstein [ 1 ] that equilibrium thermodynamics can provide information on the magnitude of statistical fluctuations in thermodynamical systems. In this paper we shall propose a thermodynamical expression for the correlation length 6 of spatial fluctuations of local thermodynamic characteristics. The correlation length is an important quantity of systems, especially in second order phase transitions [2]. The size ,$ may probably characterize the formation of the new phase in first order transitions of classical thermodynamic systems as well as of the dense nuclear matter or the continuum of the very early universe [3]. Let us consider an inftitely large homogeneous equilibrium system characterized by the set of n independent extensive densities x0 = (x:, xi, . . . . $). In a given subsystem of volume V the density xr will fluctuate around its average value xi with the following correlation [ 1,4] :