  In metals near a quantum critical point, the electrical resistance is thought
to be determined by the lifetime of the carriers of current, rather than the
scattering from defects. The observation of $T$-linear resistivity suggests
that the lifetime only depends on temperature, implying the vanishing of an
intrinsic energy scale and the presence of a quantum critical point. Our data
suggest that this concept extends to the magnetic field dependence of the
resistivity in the unconventional superconductor
BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ near its quantum critical point. We find that
the lifetime depends on magnetic field in the same way as it depends on
temperature, scaled by the ratio of two fundamental constants $\mu_B/k_B$.
These measurements imply that high magnetic fields probe the same quantum
dynamics that give rise to the $T$-linear resistivity, revealing a novel kind
of magnetoresistance that does not depend on details of the Fermi surface, but
rather on the balance of thermal and magnetic energy scales. This opens new
opportunities for the investigation of transport near a quantum critical point
by using magnetic fields to couple selectively to charge, spin and spatial
anisotropies.
