Title: Ground state properties of the bond alternating spin-1/2 anisotropic Heisenberg chain
Author(s):

	S. Paul (Department of Physics, Scottish Church College, Urquhart Square, Kolkata 700006, India) ,
	A.K. Ghosh (Department of Physics, Jadavpur University, 188 Raja Subodh Chandra Mallik Road, Kolkata 700032, India)

Ground state properties, dispersion relations and scaling behaviour of spin gap of a bond alternating spin-1/2 anisotropic Heisenberg chain have been studied where the exchange interactions on alternate bonds are ferromagnetic (FM) and antiferromagnetic (AFM) in two separate cases. The resulting models separately represent nearest neighbour (NN) AFM-AFM and AFM-FM bond alternating chains. Ground state energy has been estimated analytically by using both bond operator and Jordan-Wigner representations and numerically by using exact diagonalization. Dispersion relations, spin gap and several ground state orders have been obtained. Dimer order and string orders are found to coexist in the ground state. Spin gap is found to develop as soon as the non-uniformity in alternating bond strength is introduced in the AFM-AFM chain which further remains non-zero for the AFM-FM chain. This spin gap along with the string orders attribute to the Haldane phase. The Haldane phase is found to exist in most of the anisotropic region similar to the isotropic point.

Key words: bond alternating, spin gap, bond operator, string orders, dimer order, scaling law
PACS: 75.10.Jm, 75.10.Pq, 75.50.Ee, 75.40.Mg, 75.30.Gw

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