Regularly alternating spin-1/2 anisotropic XY chains: The ground-state and thermodynamic properties

TL;DR

This paper rigorously analyzes the ground-state and thermodynamic properties of periodically alternating spin-1/2 XY chains, revealing multiple quantum phase transitions, critical behavior variations, and unique magnetization and specific heat features.

Contribution

It introduces an exact approach for studying thermodynamics of alternating XY chains and compares their properties with related models, highlighting effects of parameter alternation.

Findings

Multiple quantum phase transition points depending on parameters

Plateau-like steps in zero-temperature transverse magnetization

Weak singularities in ground-state quantities for certain parameters

Abstract

Using the Jordan-Wigner transformation and continued fractions we calculate rigorously the thermodynamic quantities for the spin-1/2 transverse Ising chain with periodically varying intersite interactions and/or on-site fields. We consider in detail the properties of the chains having a period of the transverse field modulation equal to 3. The regularly alternating transverse Ising chain exhibits several quantum phase transition points, where the number of transition points for a given period of alternation strongly depends on the specific set of the Hamiltonian parameters. The critical behavior in most cases is the same as for the uniform chain. However, for certain sets of the Hamiltonian parameters the critical behavior may be changed and weak singularities in the ground-state quantities appear. Due to the regular alternation of the Hamiltonian parameters the transverse Ising chain may exhibit plateau-like steps in the zero-temperature dependence of the transverse magnetization vs. transverse field and many-peak temperature profiles of the specific heat. We compare the ground-state properties of regularly alternating transverse Ising and transverse XX chains and of regularly alternating quantum and classical chains. Making use of the corresponding unitary transformations we extend the elaborated approach to the study of thermodynamics of regularly alternating spin-1/2 anisotropic XY chains without field. We use the exact expression for the ground-state energy of such a chain of period 2 to discuss how the exchange interaction anisotropy destroys the spin-Peierls dimerized phase.