Strange-Metal Behavior of Highly Disordered 2D Electron System in Epitaxial Graphene Grown on 6H-SiC

The electron transport properties of highly disordered 2D electron system in an epitaxial graphene (EG) film grown on a Si-face semi-insulating 6H-SiC substrate have been investigated as functions of temperature and magnetic field. Although the EG film was grown in confinement-controlled manners, the film morphology was noticed to be uneven, mixture of uphill and downhill step structures accompanied with point defects (vacancies), in atomic force microscopy and scanning tunneling microscopy images. This irregular morphology can make the electrons in the EG film disordered and also face randomly-distributed scattering centers in their transport through the film. In magnetotransport measurements, the negative magnetoresistance was observed for a wide range of out-of-plane magnetic field up to 9 T with weak localization signature near zero magnetic field. In temperature dependence of resistivity, the Kondo-like feature was observed with the temperature of minimum resistivity at ~120 K with no magnetic field applied. The minimum resistivity temperature was found to decrease as the magnetic field increases. Most interestingly, a broad regime of T-linear resistivity above the minimum resistivity temperature to room temperature was found to exist, which is the distinct characteristic of strange metal. The disordered aspect of electrons and the potential local magnetic moments residing in point defects are likely to arouse the intriguing electron transport properties observed experimentally.