Temperature dependence of ESR intensity for the nanoscale molecular magnet

The electron spin resonance (ESR) of nanoscale molecular magnet V15 is studied. Since the Hamiltonian of V15 has a large Hilbert space and numerical calculations of the ESR signal evaluating the Kubo formula with exact diagonalization method is difficult, we implement the formula with the help of the random vector technique and the Chebyshev polynominal expansion, which we name the double Chebyshev expansion method. We calculate the temperature dependence of the ESR intensity of V15 and compare it with the data obtained in experiment. As another complementary approach, we also implement the Kubo formula with the subspace iteration method taking only important low-lying states into account. We study the ESR absorption curve below 100K by means of both methods. We find that side peaks appear due to the Dzyaloshinsky-Moriya interaction and these peaks grows as temperature decreases.