2024/03/08 by Monalisa Chatterjee, Chatterjee, Monalisa, Manoranjan Kumar +3
Physics and Astronomy · Computer Science · #Black Holes and Theoretical Physics #Physics of Superconductivity and Magnetism #Computational Physics and Python Applications
paper · pdf · doi:10.48550/arxiv.2403.05238
The frustrated ladder with alternate ferromagnetic(F) exchange -JF and AF exchange JA to first neighbors and F exchange -JL to second neighbors is studied by exact diagonalization (ED) and density matrix renormalization group (DMRG) calculations in systems of 2N spins-1/2 with periodic boundary conditions. The ground state is a singlet (S = 0) and the singlet-triplet gap εT is finite for the exchanges considered. Spin-1/2 string correlation functions g1(N) and g2(N) are defined for an even number N of consecutive spins in systems with two spins per unit cell; the ladder has string order g2(∞)> 0 and g1(∞) = 0. The minimum N^* of g2(N) is related to the range of ground-state spin correlations. Convergence to g2(∞) is from below, and g1(N) decreases exponentially for N ≥ N^*. Singlet valence bond (VB) diagrams account for the size dependencies. The frustrated ladder at special values of JF, JL and JA reduces to well-known models such as the spin-1 Heisenberg antiferromagnet and the J1-J2 model, among others. Numerical analysis of ladders matches previous results for spin-1 gaps or string correlation functions and extends them to spin-1/2 systems. The nondegenerate singlet ground state of ladder is a bond-order wave, a Kekulé VB diagrams at JL = JF/2 ≤ JA, that is reversed on interchanging -JF and JA. Inversion symmetry is spontaneously broken in the dimer phase of the J1-J2 model where the Kekulé diagrams are the doubly degenerate ground states at J2/J1 = 1/2.