High-field NMR study on the topologically nontrivial 1/3 magnetization plateau state in doped Na2Cu3Ge4-xSix O12

The spin-1/2 trimer system Na2Cu3Ge4O12 is proposed to host topologically nontrivial 1/3 magnetization (M) plateau state. In this paper, we report a high-field nuclear magnetic resonance (NMR) study on the low-energy spin excitations of the 1/3-M plateau state in Na2Cu3Ge4O12 and Si-doped samples. First, the spin excitation gap supporting the high-field Haldane phase is indicated by the thermally activated behavior of spin-lattice relaxation rates in the 1/3-M plateau state. Second, by substituting Ge4+ with Si4+, the critical field for the 1/3-M plateau significantly decreases, e.g., μ0Hc=17 T in the x=1.0 sample, which results from the suppressed intertrimer coupling J2. The gapped spin excitation is observed above 17 T whose size shows the temperature-dependent behavior for μ0H≥25.72 T. These observations support the topological nontrivial characteristic of the 1/3-M plateau state.