Magnetic plateaus and jumps in a spin-1/2 ladder with alternate Ising-Heisenberg rungs: a field dependent study

Abstract: We study a frustrated two-leg spin-1/2 ladder with alternate Ising and isotropic Heisenberg rung exchange interactions, whereas, interactions along legs and diagonals are Ising type. The ground-state (GS) of this model has four exotic phases: (i) the stripe rung ferromagnet (SRFM), (ii) the anisotropic anti-ferromagnet (AAFM), (iii) the Dimer, and (iv) the stripe leg ferromagnet (SLFM) in absence of any external magnetic field. In this work, we study the effect of externally applied longitudinal and transverse fields on GS phases and note that there are two plateaus with per-site magnetization $1/4$ and $1/2$. There is another plateau at zero magnetization due to a finite spin gap in the presence of a longitudinal field. The exact diagonalization (ED) and the transfer matrix (TM) methods are used to solve the model Hamiltonian and the mechanism of plateau formation is analyzed using spin density, quantum fidelity, and quantum concurrence. In the (i) SRFM phase, Ising exchanges are dominant for all spins but the Heisenberg rungs are weak, and therefore, the magnetization shows a continuous transition as a function of the transverse field. In the other three phases [(ii)-(iv)], the Ising dimer rungs are weak and those are broken first to reach a plateau with per-site magnetization $1/4$, having a large gap which is closed by further application of the transverse field.