Ising Chain in Regularly Alternating Transverse Field: Spin Correlation Functions

We consider the spin1 2 Ising chain in a regularly alternating transverse field to examine the effects of regular alternation on the quantum phase transition inherent in the quantum Ising chain. The number of quantum phase transition points strongly depends on the specific set of the Hamiltonian parameters but never exceeds 2p where p is the period of alternation. Calculating the spin correlation functions numerically (for long chains of up to 5400 sites) and determining the critical exponents we have demonstrated that two types of critical behavior are possible. In most cases the square-lattice Ising model universality class occurs, however, a weaker singularity may also take place.