The thermoelectric power of a material, typically governed by its band structure and carrier density, can be varied by chemical doping that is often restricted by solubility of the dopant. Materials showing large thermoelectric power are useful for many industrial applications, such as the heat-to-electricity conversion and the thermoelectric cooling device. Here we show a full electric-field tuning of thermoelectric power in a dual-gated bilayer graphene device resulting from the opening of a band gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at a low temperature, which may open up a new possibility in low temperature thermoelectric application using graphene-based device.

BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magneto-transport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov-de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov-de Haas oscillation were observed giving extremal Fermi surface areas of AS = 9.1×10 12 cm −2 and AL = 3.47×10 14 cm −2 with corresponding cyclotron masses m * S = 0.0353 me and m * L = 0.178 me, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I refined or calculated positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin splitting effect in BiTeI. PACS numbers:BiTeI emerges as an intriguing material that shows a large Rashba effect [1-3] and a possible topological phase transition under pressure [4]. Its crystal structure comprises alternating layers of bismuth (Bi), tellurium (Te) and iodine (I) each with trigonal planar lattice as illustrated in Fig. 1(a). It was proposed [5] to constitute a semi-ionic structure along the stacking direction, where (BiTe) + layer is positively charged and (Bi-I) layer is ionic. Angle-resolved photo-emission spectroscopy experiments (ARPES) [1,6] have revealed evidence for the giant Rashba spin splitting, and its bulk nature was further confirmed by bulk-sensitive optical spectroscopy [7] and soft x-ray ARPES [8]. When comparing to band structure calculation, the Te and I coordinations turn out to be crucial parameters that can result in dramatic difference in the calculated band property. There are three different Te and I coordinations reported in the literature: coordination A with Te(2/3,1/3,0.6928) and I(1/3,2/3,0.2510) from the refinement analysis of X-ray experiment [5], as well as coordination B with Te(2/3,1/3,0.7111) and I(1/3,2/3,0.2609) [9], and coordination C with Te(2/3,1/3,0.7482) and I(1/3,2/3,0.3076) [10], from two different theoretical structural determinations using the same band structure method. Regardless of the small variation, only coordination C with a shortest Bi-Te bond length (d Bi−Te = 3.05Å) gives rise to a giant Rashba spin-splitting in the bulk band with a Rashba parameter α R ∼ = 5.4 eVÅ according to our calculations, which may infer a close connection between d Bi−Te and its Rashba effect.In this paper, we show magneto-transport measurement results on high quality Cu x BiTeI single crystals with copper (Cu) doping x up to 0.2. Comparing to earlier works on Shubnikov-de Haas (SdH) oscillations [11,12], the SdH oscillation in our crystals exhibits two distinct frequencies derived from a large Fermi surface (LFS) and a small F...

We performed electric and thermoelectric transport measurements of bilayer graphene in a magnetic field up to 15 Tesla. The transverse thermoelectric conductivity α xy , determined from four transport coefficients, attains a peak value of α xy,peak whenever chemical potential lies in the center of a Landau level. The temperature dependence of α xy,peak is dictated by the disorder width W L . For k B T/W L ≤0.2, α xy,peak is nominally linear in temperature, which gives α xy,peak /T = 0.19 ± 0.03nA/K 2 independent of the magnetic field, temperature and Landau Level index. At k B T/W L ≥0.5, α xy,peak saturates to a value close to the predicted universal value of 4 × (ln2)k B e/h according to the theory of Girvin and Jonson. We remark that an anomaly is found in α xy near the charge neutral point, similar to that in single-layer graphene.

The quest for materials showing large thermoelectric power has long been one of the important subjects in material science and technology. Such materials have great potential for thermoelectric cooling and also high figure of merit ZT thermoelectric applications. We have fabricated bilayer graphene devices with ionic-liquid gating in order to tune its band gap via application of a perpendicular electric field on a bilayer graphene. By keeping the Fermi level at charge neutral point during the cool-down, we found that the charge puddles effect can be greatly reduced and thus largely improve the transport properties at low T in graphene-based devices using ionic liquid gating. At (Vig, Vbg) = (−1 V, +23 V), a band gap of about 36.6 ± 3 meV forms, and a nearly 40% enhancement of thermoelectric power at T = 120 K is clearly observed. Our works demonstrate the feasibility of band gap tuning in a bilayer graphene using ionic liquid gating. We also remark on the significant influence of the charge puddles effect in ionic-liquid-based devices.

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