Thermoelectric effect in high mobility single layer epitaxial graphene (original) (raw)

2011, Applied Physics Letters

The thermoelectric response of high mobility single layer epitaxial graphene on silicon carbide substrates as a function of temperature and magnetic field have been investigated. For the temperature dependence of the thermopower, a strong deviation from the Mott relation has been observed even when the carrier density is high, which reflects the importance of the screening effect. In the quantum Hall regime, the amplitude of the thermopower peaks is lower than a quantum value predicted by theories, despite the high mobility of the sample. A systematic reduction of the amplitude with decreasing temperature suggests that the suppression of the thermopower is intrinsic to Dirac electrons in graphene. PACS numbers: 73.63.Bd, 72.15.Jf, 73.43.Fj Graphene, a single layer of graphite, has a unique band structure, in which electrons are described by the relativistic Dirac equation. Extensive electrical transport studies have been performed to understand Dirac electrons in the material. Compared with electrical transport, the thermoelectric properties provide complementary information to the electronic structure and the detail of electron scattering, but investigations have been started only recently . The newly discovered topological insulators, whose electrons in surface states are also Dirac electrons, are extraordinary thermoelectric materials. The possibility of further improving the performance in its nanostructures by exploiting the Dirac nature of electrons also calls for studies on the thermoelectric response of Dirac electrons . The thermoelectric effect of Dirac electrons has been experimentally investigated in exfoliated graphene on SiO 2 [6-8]. It was found that the Mott relation, which is used to describe the thermoelectric effect in conventional 2-dimensional electron gases, is basically obeyed, but not in the vicinity of the charge neutrality point. In the quantum Hall regime, though theories predict a quantized value for the thermopower , experiments saw a smaller value . In those studies, the mobilities of the samples are low, in the order of a few thousand cm 2 /V·s. Thus, questions have been raised about whether the quantization of the thermopower is intrinsic to graphene and can be realized in high mobility samples . Generally, high mobility is crucial for studying the intrinsic properties of Dirac electrons. Therefore, achieving high mobility in graphene samples has been a main effort in many experiments. A few milestone experiments are indeed consequences of improved or new techniques for obtaining high mobilities, e.g. recent success in greatly improving the mobility of graphene by suspending and in situ annealing it directly led to observation of the fractional quantum Hall effect .