### Abstract

The S = 1/2 ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg quantum spin chain is investigated by the quantum Monte Carlo method. The randomness is only present on the strong bonds taking values of ±2J randomly, and the weak bonds are uniformly antiferromagnetic with J. The model corresponds to the quasi one-dimensional compound, (CH_{3})_{2}CHNH_{3}Cu(Cl_{x}, Br_{1-x})_{3}, whose ground state interpolates the Haldane state of the F-AF alternating spin chain at x = 1 and the singlet-dimer state of the AF-AF alternating spin chain at x = 0. The nonequilibrium relaxation function of the staggered magnetic susceptibility exhibits algebraic divergence in the intermediate region of 0.44 < x < 0.87, which explains the experimental observation of the magnetic phase transition. The results suggest that the antiferromagnetic order becomes critical by the randomness of the bond distributions even in the presence of the rich ferromagnetic bonds.

Original language | English |
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Pages (from-to) | 353-356 |

Number of pages | 4 |

Journal | Progress of Theoretical Physics |

Issue number | 145 SUPPL. |

Publication status | Published - 2002 Dec 1 |

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### ASJC Scopus subject areas

- Physics and Astronomy (miscellaneous)

### Cite this

*Progress of Theoretical Physics*, (145 SUPPL.), 353-356.