Abstract
The S = 1/2 Heisenberg antiferromagnet on the two-dimensional pyramid lattice is studied by the numerical-diagonalization method. This lattice is obtained by the combination of the Lieb lattice and the square lattice. It is known that when interaction on the square lattice is increased from the ferrimagnetic limit of strong interaction on the Lieb lattice, this system shows gradual decrease and disappearance of spontaneous magnetization in the ground state. The present study treats the region near the case of the square-lattice antiferromagnet accompanied by isolated spins by numerical-diagonalization calculations of finite-size clusters with the maximum size of 39 sites. Our numerical results suggest the existence of a new phase with small but nonzero spontaneous magnetization between two zero-spontaneous-magnetization phases.
Highlights
Ferrimagnetism is a fundamental magnetic phenomenon which shows ferromagnetic nature and antiferromagnetic nature at the same time
Some of them show intermediate states with spontaneous magnetization whose magnitude is smaller than that determined by the MLM theorem
Let us observe our numerical results for the M-dependence of the lowest-energy level; the observation makes us understand how to determine the spontaneous magnetization for a given N
Summary
Ferrimagnetism is a fundamental magnetic phenomenon which shows ferromagnetic nature and antiferromagnetic nature at the same time. It is widely known that the ferrimagnetism is mathematically understood by Marshall-Lieb-Mattis (MLM) theorem.[1,2] The MLM theorem holds only when systems do not include magnetic frustration. It is a nontrivial problem how the ferrimagnetism is suppressed and disappears quantum-mechanically by such a frustrating situation. From such a viewpoint, there are several reports[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17] which studied several models on lattices of various types. To understand the nontrivial behavior of the new spontaneous-magnetization
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