Identification of intrinsic gapped behavior in spin-1/2 ladder with staggered dimerization

The zero- and low-temperature behaviors of spin-1/2 two-leg ladder with staggered dimerization are investigated by the Green’s function theory. At zero temperature, the ground state phase diagram is explored, wherein the leg-dimer and rung-singlet phases are revealed, which reflect two different intrinsic gapped behaviors. The former is attributed to the bond alternation along the legs, while the latter is due to the strong rung coupling. It is found that the quantum phase transition from one to another is of the first order, which can be clearly signaled by the rung entanglement entropy. At finite temperatures, the temperature dependence of thermodynamic quantities such as the magnetic susceptibility, specific heat, thermal Drude weight and rung entanglement entropy are calculated to characterize the corresponding quantum phases. It is shown that the magnetic behaviors clearly manifest a typical antiferromagnetism at low temperature, which is in accordance with the experimental results. It is also found that the intrinsic gapped low-lying excitations are responsible for the observed thermodynamic behaviors.