... Theoretical department of ISSP RAS . ... Pis'ma v ZhETF, 69 , 543, 1999; V.E.Bisti , Pis'ma v ZhETF, 73 , 25, 2001 ). (ii) Collective intrasubband charge density excitations are investigated in double GaAs/AlGaAs quantum wells in external parallel magnetic field. ... Goldstone mode relaxation in a quantum Hall ferromagnet . ... The considered system is a Quantum Hall Ferromagnet possessing "classical" spin S 0 ~N j /2 (N j being the number of magnetic flux quanta, S 0 || ... Spin biexciton. ...