The functions of hearing officers and those officers participating in decisions shall be conducted in an impartial manner with due regard for the rights of all parties and the facts and the law, and consistent with the orderly and prompt dispatch of proceedings. These officers, except to the extent required for the disposition of ex parte matters authorized by law, may not engage in interviews with, or receive evidence or argument from, a party, directly or indirectly, except upon opportunity for all other parties to be present. Copies of all communications with these officers shall be served upon all parties.