I am going through the paper Surface code with decoherence: An analysis of three superconducting architectures and I have a doubt about how the authors get what they refer to as the combined channel of amplitude and phase damping. In the section IIA, the authors discuss amplitude damping and dephasing channels with their Kraus operators in order to describe decoherence first, and afterwards they combine their effects. They state that the combined channel is represented by 3 Krauss operators depending on the amplitude damping and dephasing parameters.
I am wondering how such Kraus operators are obtained from the Kraus operators describing the individual channels, as the authors do not explain nor give any reference to such statement. My initial approach has been to consider a sequential combination of the channels, but my results are not successful (probably because the simultaneous action of the channels is not equivalent to their sequential actions on the quantum information).