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Magnetic field asymmetry of nonlinear transport in carbon nanotubes

We demonstrate that nonlinear transport through a two-terminal nanoscale sample is not symmetric in magnetic field B. More specifically, we have measured the lowest order B-asymmetric terms in single-walled carbon nanotubes. Theoretically, the size of these terms can be used to infer both the strength of electron-electron interactions and the handedness of the nanotube. Consistent with theory, we find that at high temperatures the B-linear term is small and has a constant sign independent of Fermi energy, while at low temperatures it develops mesoscopic fluctuations. We also find significant magnetoresistance of nanotubes in the metallic regime which is unexplained.