NON-HERMITIAN TRICRITICALITY IN THE BLUME-CAPEL MODEL WITH IMAGINARY FIELD

Using finite-size-scaling methods, we study the quantum chain version of the spin-1-Blume-Capel model coupled to an imaginary field. The aim is to realize simple non-unitary minimal conformal field theories in an Ising-type spin model. We find that the first ground-state level cr ossing in the high-temperature phase leads to a second-order phase tra nsition of the Yang-Lee universality class (central charge c = 22/5). The Yang-Lee transition region ends at a line of a new type of tricrit icality, where the three lowest energy levels become degenerate. The a nalysis of the spectrum at two points on this line gives good evidence that this line realizes the universality class of the M2, 7-conformal theory with c = -68/7.