EPR Theories for Selection Rules to Observe the Spin Gap
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Applied Magnetic Resonance
REVIEW
EPR Theories for Selection Rules to Observe the Spin Gap Tôru Sakai1 Received: 25 August 2020 / Revised: 21 October 2020 / Accepted: 28 October 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract The direct electron paramagnetic resonance (EPR) absorption between the singlet ground state and the triplet excited states of spin gap systems is investigated. Such an absorption, which is forbidden by the conservation of the total spin quantum number in isotropic Hamiltonians, is allowed by some anisotropies; the staggered g-tensor and the Dzyaloshinskii–Moriya interaction. Selection rules in the presence of these interactions were presented in our previous works. We review the selection rules of the EPR transition of the spin gap with some numerical demonstrations and discuss some applications.
1 Introduction The electron paramagnetic resonance (EPR) is one of most useful experimental techniques to investigate elementary excitations of condensed matters. Recently EPR has been applied for the gapped spin systems, which attract a lot of current interest in the field of the strongly correlated electron and quantum spin systems. Since the Haldane gap [1, 2] was directly detected in the high-field magnetization measurements [3, 4], on the quasi-one-dimensional S = 1 antiferromagnet Ni(C2 H8 N2 )2 NO2 (ClO4 ) , abbreviated NENP, the behavior of low-lying excitations under external field has been extensively studied with various methods [5–7]. The experimental investigation of the EPR transition among the first excited triplet states yielded the estimation of the amplitude of the anisotropies of NENP [8]. Some lowtemperature EPR measurements [9, 10] successfully detected the direct absorption between the singlet ground state and the first excited state in NENP, although it should be forbidden because of the spin conservation law. To explain the mechanism of the direct EPR transition corresponding the Haldane gap, in the previous works [11–13], we introduced the effective staggered field due to the alternation in the inclination of the g-tensor of the Ni2+ ion in NENP. The staggered g-tensor had been revealed by the NMR measurement [14]. The staggered g-tensor mechanism * Tôru Sakai [email protected] 1
Graduate School of Material Science, University of Hyogo and QST SPring-8, Hyogo, Japan
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was also consistent with the specific heat measurement [15]. Thus, at present, we believe that the origin of the direct EPR transition of the Haldane gap in NENP is the staggered g-tensor. In this case, the EPR intensity should strongly depend on the external field. Later the direct singlet-triplet EPR transition was also observed [16] for the inorganic compound CuGeO3 in the spin Peierls phase [17]. Since the observed EPR intensity had been almost independent of the external field, the mechanism based on the Dzyaloshinskii–Moriya (DM) interaction [18] was proposed as a possible origin of the direct transition [19]. However, the mechani
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