Brane order and quantum magnetism in modulated anisotropic ladders

TL;DR

This paper analyzes modulated anisotropic two-leg spin ladders, revealing a rich phase diagram with conventional and non-local brane orders, using dualities and exact solutions to understand quantum phase transitions.

Contribution

It introduces an analytical approach to study the phase diagram of modulated anisotropic ladders, identifying multiple phases and calculating order parameters using dualities and exact results.

Findings

Nine phases in staggered ladders, including four antiferromagnets and five with non-local brane orders.

Three phases in columnar ladders with no magnetic long-range order.

Analytical expressions for magnetizations and brane order parameters as functions of effective couplings.

Abstract

Two-leg spin-$\frac12$ ladders with anisotropy and two different dimerization patterns are analyzed at zero temperature. This model is equivalent to a modulated interacting (Kitaev) ladder. The Hartree-Fock mean-field approximation reduces the model to a sum of two quadratic effective Majorana Hamiltonians, which are dual to a sum of two (even/odd) XY quantum chains in the alternating transverse fields. The mapping between the effective Hamiltonian of the ladder and the pair of the dual XY chains considerably simplifies calculations the order parameters and analysis of the hidden symmetry breaking. The ground-state phase diagram of the staggered ladder contains nine phases, four of them are conventional antiferromagnets, while the other five possess non-local brane orders. Using the dualities and the newly found exact results for the local and string order parameters of the transverse XY chains, we were able to find analytically all the magnetizations and the brane order parameters for the staggered case, as functions of the renormalized couplings of the effective Hamiltonian. The columnar ladder has three ground-state phases and does not possess magnetic long-ranged order. The brane order parameters for these three phases are calculated numerically from the Toeplitz determinants. We expect this study to motivate the search for the real spin-Peierls anisotropic ladder compounds which can undergo the predicted quantum phase transitions with gap closures and distinct brane orders.