Magnetization plateaus in antiferromagnetic-(ferromagnetic), polymerized S=1/2 XXZ chains

The plateau-non-plateau transition in the antiferromagnetic-(ferromagnetic)(n) polymerized S = 1/2 XXZ chains under the magnetic field is investigated. The universality class of this transition belongs to the Brezinskii-Kosterlitz-Thouless (BKT) type. The critical points are determined by level spectroscopy analysis of the numerical diagonalization data for 4 less than or equal to p less than or equal to 13 where p(equivalent to n + 1) is the size of a unit cell. It is found that the critical strength of ferromagnetic coupling decreases with p for small p but increases for large enough p. It is also found that the plateau for large p is wide enough for moderate values of exchange coupling so that it should be easily observed experimentally. This is in contrast to the plateaus for p = 3 chains which are narrow for a wide range of exchange coupling even away from the critical point.