The longitudinal magnetoresistance (MR) is assumed to be hardly realized as the Lorentz force does not work on electrons when the magnetic field is parallel to the current. However, in some cases, longitudinal MR becomes large, which exceeds the transverse MR. To solve this problem, we have investigated the longitudinal MR considering multivalley contributions based on the classical MR theory. We have showed that the large longitudinal MR is caused by off-diagonal components of a mobility tensor. Our theoretical results agree with the experiments of large longitudinal MR in IV-VI semiconductors, especially in PbTe, for a wide range of temperatures, except for linear MR at low temperatures.
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