Double-beta decay nuclear matrix element using finite-amplitude method

The quasiparticle random-phase approximation (QRPA) can be used to evaluate the nuclear matrix element of the double-beta decay but the standard approach based on the matrix diagonalization is computationally demanding. We propose double contour integration technique of the finite-amplitude method, which is the linear response formalism of the QRPA for nuclear density functional theory, to compute the QRPA nuclear matrix elements efficiently. We will show numerical applications to double Gamow-Teller transition strengths and 2$\nu$ matrix elements.