Electric & Thermoelectric transport phenomena in bilayer Graphene under High Magnetic & Electric Fields

• Main Authors: Wen-Sen Lu, 呂文森
• Other Authors: Ching-Ray Chang
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/09332725439126092337

碩士 === 國立臺灣大學 === 物理研究所 === 99 === Electric and thermoelectric properties on bilayer graphene (BLG) under either high magnetic field or perpendicular electric field are investigated experimentally. For BLG under high magnetic field, the transverse thermoelectric conductivity αxy is determined from four transport coefficients which are measured experimentally. αxy(Vbg) attains a peak value of αxy,peak whenever the chemical potential μ, which can be tuned by Vbg, lies in the center of a Landau level. The results show that the temperature dependence of αxy,peak is dictated by the disorder width WL. For kBT/WL ≤ 0.2, peak value αxy,peak is basically linear in temperature, which gives a slope αxy,peak /T=0.019±0.03 nA/K2 and is independent of the magnetic field, temperature, and Landau-level index. At kBT/WL ≥ 0.5, αxy,peak saturates to a value close to the predicted universal value of 4(ln2) kBe/h from the theory of Girvin and Jonson. In addition to the universality of αxy,peak, we also find two anomalies around charge neutrality point (CNP) which cannot be explained by the generalized Mott relation. We attribute the failure of Mott relation to the proposed phase of counter propagating edge channels with opposite spin which presents when spin degeneracy is lifted, as indicated from the observed double peaks feature in the longitudinal conductivity σxx near CNP in our device. Considering the long spin diffusion length in graphene system, the proposed spin polarized counter propagating edge channels in the spin degeneracy lifted BLG around CNP is very interesting for spintronics application. For the BLG under perpendicular electric field, a full electric-field tuning of thermoelectric power (TEP) in a BLG device is demonstrated experimentally with dual-gated geometry. Under a strong electric field bias along the out of sample plane direction, a band-gap is opened in BLG. Resistance and TEP signal are then measured at different biased fields readily tuned by the dual-gated geometry for temperature ranging from 15K to 300K. The TEP exhibits an enhancement as biased field increases, and grows larger at lower temperature. At 15K and a biased field of 0.7V/nm, the TEP reaches a value of 48μV/K and is four-fold increased compared to the unbiased BLG. We suggest that with the introduction of material with high dielectric constant serves as the dielectrics, the enhancement could be enhanced further by increase the biased field further. This enhanced TEP at low temperature makes dual gated BLG device an idea candidate for the thermoelectric application at low temperature such as the power source for spacecrafts. We also find evidences indicating the band structure deformation when larger biased field applied, suggesting redistribution of states near the CNP. Further experiments could be made to confirm this scenario.