1. Experimental access to the density of Lee-Yang zeros from magnetization data of a 2d Ising-ferromagnet (L.J. de Jongh, A.R. Miedema Adv. Phys. 23 (1974)) 0,3579 nm 1,7536 nm

2.  Ising – hamiltonian s i =  1: Lee - Yang theorem h:= g  B  0 H magn. field  := 1/k B T g: Landé factor  B : Bohr´s magneton  partition function Z is a polynomial in Z determined by its zeros thermodynamics  circle- theorem The zeros of the partition function of an Ising-ferromagnet are distributed on the unit circle in the complex z-plane. T.D. Lee und C.N. Yang 1952

3.  Example: 2 interacting Ising-spins Partition function: ! complex zeros: J < 0 AF

4. X 1/4 x 1/2 T=99K T=53K T=52K T=51K T=50K T=49K  zeros of Z determine free energy according to isothermal magnetization experimental access to zero distribution with T=49,...,53K T=34K T=4.5K T=99K Padé-type ansatz function: Ch. Binek, PRL 81 5644 (1998)