Magneto-Optic Investigation of the Haldane Gap
Abstract
Haldane has predicted that a finite energy gap exists between a non-magnetic singlet ground state and the lowest lying excited state for integer-spin Heisenberg antiferromagnetic (HAFM) chain systems. Considerable number of theoretical and experimental work have been done to verify the Haldane's prediction. Optical linear birefringence measurements were performed on three nickel chain (spin-1 1D HAFM) systems. Ni(C _2H_8N _2)_2 rm NO_2ClO _4, (NENP); Ni(C_2H_8N _2)_2NO_2PF_6, (NENF) and Ni(C_3H_{10}N_2) _2NO_2ClO_4, (NINO). The distinct anomalies in the birefringence data at 275 K and at 235 K for NENP and NENF, respectively, indicate crystallographic structural phase transitions. Low temperature x-ray diffraction measurements show that the nickel chains for both NENP and NENF remains magnetically uniform. The temperature derivative of the linear magnetic birefringence (TDLMB) of all three compounds has been measured between 2 and 300 K. It was been found from the TDLMB measurements that for NENP the magneto-elastic strain is induced by the 1D short range order only along the chain axis. Using the proportionality between magnetic specific heat and the TDLMB the intrachain exchange energies of NENP, NENF and NINO have been determined as -23.3(3) K, -23.4(5) K and -24.5(2) K respectively. The low temperature data were fit to an exponential function of temperature to determine the Haldane gap; the obtained values of the Haldane gaps of NENP, NENF and NINO are 17.5(6) K, 15.6(5) K and 17.4(6) K respectively. The observed values of intrachain exchange energies for NENP, NENF and NINO and the Haldane gap for NENP are in excellent agreement with both theoretical predictions and previous experimental results. This work confirms the validity of the TDLMB technique for the determination of exchange energies. Using the magneto-optic technique two new Haldane compounds have been investigated in this dissertation.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1990
- Bibcode:
- 1990PhDT........54R
- Keywords:
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- ANTIFERROMAGNET;
- Physics: Condensed Matter; Physics: Optics