Frustration-Induced Magnetic Properties of the Spin-1/2 Heisenberg Antiferromagnet on the Cairo Pentagon Lattice

The magnetic phase diagram under a magnetic field of the spin-1/2 Heisenberg antiferromagnet on the Cairo pentagon lattice has been derived from a magnetization process and correlation functions calculated by numerical diagonalization for small clusters with up to 36 sites. The phase transition between two types of 1/3 magnetization states takes place, which continues to the orthogonal dimer state and ferrimagnetic state in the limits of x = 0 and x = infinity, respectively, where x is the ratio between two types of nearest-neighbor exchange interactions, in the absence of a magnetic field. A peculiar magnetization jump has been found at the lower- or higher-field edge of the 1/3 plateau, when x is larger or smaller than its critical value of similar to 0.8, respectively. The temperature dependence of the specific heat shows a double-peak structure.