Spin-stretching modes in non-centrosymmetric magnets: spin-wave excitations in the multiferroic Ba2CoGe2O7

K. Penc, 2 J. Romhanyi, 2 T. Room, U. Nagel, A. Antal, T. Feher, A. Janossy, H. Engelkamp, H. Murakawa, 6 Y. Tokura, 6, 7, 8 D. Szaller, S. Bordacs, 5, 6 and I. Kezsmarki Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, P.O.B. 49, Hungary Department of Physics, Budapest University of Technology and Economics and Condensed Matter Research Group of the Hungarian Academy of Sciences, 1111 Budapest, Hungary National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University, 6525 ED Nijmegen, The Netherlands Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), Japan c/o The University of Tokyo, Tokyo 113-8656, Japan Quantum-Phase Electronics Center, Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan Cross-correlated materials group (CMRG) and correlation electron research group (CERG), RIKEN Advanced Science Institute, Wako 351-0198, Japan (Dated: February 20, 2012)