  We present a theoretical study of the spin-valley blockade transport effect
in a double quantum dot defined in a straight carbon nanotube. We find that
intervalley scattering due to short-range impurities completely lifts the
spin-valley blockade and induces a large leakage current in a certain confined
range of the external magnetic field vector. This current hot spot emerges due
to different effective magnetic fields acting on the spin-valley qubit states
of the two quantum dots. Our predictions are compared to a recent measurement
[F. Pei et al., Nat. Nanotech. 7, 630 (2012)]. We discuss the implications for
blockade-based schemes for qubit initialization/readout, and motion sensing of
nanotube-based mechanical resonators.
