  Negative differential conductance (NDC), characterized by the decreasing
current with increasing voltage, has attracted continuous attention for its
various novel applications. The NDC typically exists in a certain range of bias
voltages for a selected system and controlling the regions of NDC in curves of
current versus voltage (I-V) is experimentally challenging. Here, we
demonstrate an unusual magnetic-field-controlled NDC in graphene npn junction
resonators. The magnetic field not only can switch on and off the NDC, but also
can continuously tune the regions of the NDC in the I-V curves. In the graphene
npn junction resonators, magnetic fields generate sharp and pronounced
Landau-level peaks with the help of the Klein tunneling of massless Dirac
fermions. A tip of scanning tunneling microscope induces a relatively shift of
the Landau levels in graphene beneath the tip. Tunneling between the misaligned
Landau levels results in the magnetic-field-controlled NDC that may have
potential applications for future graphene-based technology.
