N. B. Ivanov

Competition between plaquette and dimer phases in Heisenberg chains

Abstract We consider a class of one-dimensional Heisenberg spin models (plaquette chains) related to the recently found 1 5 - depleted square-lattice Heisenberg system CaV4O9. A number of exact and exact-numerical results concerning the properties of the competing dimer and resonating plaquette phases are presented.We consider a class of one-dimensional Heisenberg spin models (plaquette chains) related to the recently found 1/5-depleted square-lattice Heisenberg system

On the Violation of Marshall-Peierls Sign Rule in the Frustrated J1-J2 Heisenberg Antiferromagnet

CaV_4O_9

SPIN-WAVE SERIES FOR QUANTUM ONE-DIMENSIONAL FERRIMAGNETS

. A number of exact and exact-numerical results concerning the properties of the competing dimer and resonating plaquette phases are presented.

Diamond chains with multiple-spin exchange interactions

We present a number of arguments in favour of the suggestion that the Marshall-Peierls sign rule survives the frustration in the square-lattice Heisenberg antiferromagnet with frustrating next-nearest-neighbour (diagonal) bonds (J1-J2 model) for relatively large values of the parameter J2/J1. Both the spin-wave analysis and the exact-diagonalization data concerning the weight of Marshall states support the above suggestion.

Frustrated quantum Heisenberg ferrimagnetic chains

Second-order spin-wave expansions are used to compute the ground-state energy and sublattice magnetizations of the quantum one-dimensional Heisenberg ferrimagnet with nearest-neighbor antiferromagnetic interactions and two types of alternating sublattice spins

Collective excitations in ferrimagnetic Heisenberg ladders

{S}_{1}g{S}_{2}.

Marshall-Peierls phase rule and Lieb-Mattis level ordering in non-bipartite low-dimensional antiferromagnets

It is found that in the extreme quantum cases

Alternating-spin S = 3/2 and σ = 1/2 Heisenberg chain with isotropic three-body exchange interactions

{(S}_{1}{,S}_{2})

Analysis of the ground-state wave function of theJ1-J2 quantum Heisenberg antiferromagnet

=(1,1/2), (3/2,1), and (3/2,1/2), the estimates for the ground-state energy and sublattice magnetizations differ less than 0.03% for the energy and 0.2% for the sublattice magnetizations from the recently published density-matrix renormalization-group numerical calculations. The reported results strongly suggest that the quantum Heisenberg ferrimagnetic chains give another example of a low-dimensional quantum spin system where the spin-wave approach demonstrates a surprising efficiency.

Finite size and temperature effects in the J 1 − J 2 model on a strip

We study the phase diagram of a symmetric spin-1/2 Heisenberg diamond chain with additional cyclic four-spin exchange interactions. The presented analysis supplemented by numerical exact-diagonalization results for finite periodic clusters implies a rich phase diagram containing, apart from standard magnetic and spin-liquid phases, two different tetramer-dimer phases as well as an exotic fourfold degenerate dimerized phase. The characteristics of the established spin phases as well as the nature of quantum phase transitions are discussed as well.