Magnetic and dielectric properties of one-dimensional array of S = 1/2 linear trimer system Na2Cu3Ge4O12

Magnetic susceptibility χ, specific heat C, capacitance Cp, and 23Na-NMR measurements have been carried out on polycrystalline samples of quantum spin linear trimer system Na2Cu3Ge4O12, which has the one-dimensional array of Cu3O8 trimers formed of edge-sharing three CuO4 square planes. The exchange interactions between the Cu2+ (S = 1/2) spins have been determined by analyzing χ-T and C-T curves. By employing the isolated S = 1/2 Heisenberg trimer model above 70 K, the nearest-neighbor exchange couplings J1 and the second-neighbor one J2 in trimer have been evaluated to J1/kB = 30 ± 20 K (antiferromagnetic) and J2/kB = 340 ± 20 K. At low temperature region, two spins of the edge in the Cu3O8 trimers form a nonmagnetic singlet by strong antiferromagnetic interaction J2, and the spin left in the center of the Cu3O8 trimer forms one-dimensional chains by the exchange interaction J3 between the trimers. By employing the S = 1/2 uniform Heisenberg chain model below 70 K, we have evaluated to J3/kB = 18 ± 1 K. The mechanism of multiferroic behavior at Tc = 2 K is discussed.