Reexamination of the microscopic couplings of the quasi one-dimensional antiferromagnet CuGeO_3
Abstract
Experimental data for the magnetic susceptibility and magnetostriction of CuGeO_3 are analyzed within a one-dimensional antiferromagnetic model with nearest ({J_1}) and next-nearest neighbour interactions ({J_2}). We show that the ratio of the exchange constants in the antiferromagnetic chains of CuGeO_3 amounts to
J
2
/
J
1
= 0.354(0.01), i.e. it is significantly larger than the critical value for the formation of a spontaneous gap in the magnetic excitation spectrum without lattice dimerization. The susceptibility data are reproduced by our numerical results over the temperature range from 20K to 950K to a high degree of accuracy for
J
1
=80.2(3.0)
and
J
2
=28.4(1.8)
. The pressure dependence of the exchange constants is estimated from magnetostriction data. Furthermore, the specific heat data are checked on consistency against the calculated entropy of the above model.