The transport characteristics of graphene devices with low n‐ or p‐type carrier density (∼1010–1011 cm‐2), fabricated using a new process that results in minimal organic surface residues, are reported. The p‐type molecular doping responsible for the low carrier densities is initiated by aqua regia. The resulting devices exhibit highly developed ν = 2 quantized Hall resistance plateaus at magnetic field strengths of less than 4 T.

We study the thermoelectric properties of a Kramer's pair of helical edge states of the quantum spin Hall effect coupled to a nanomagnet with a component of the magnetization perpendicular to the direction of the spin-orbit interaction of the host. We show that the transmission function of this structure has the desired qualities for optimal thermoelectric performance in the quantum coherent regime. For a single magnetic domain there is a power generation close to the optimal bound. In a configuration with two magnetic domains with different orientations, pronounced peaks in the transmission functions and resonances lead to a high figure of merit. We provide estimates for the fabrication of this device with HgTe quantum-well topological insulators. Introduction.Thermoelectricity in the quantum regime is attracting high interest for some years now [1,2]. Systems hosting edge states, like the quantum Hall and quantum spin Hall are paradigmatic realizations of quantum coherent transport. Several theoretical and experimental results on heat transport and thermoelectricity in these systems have been recently reported .Unlike the quantum Hall state, which is generated by a strong magnetic field, the quantum spin Hall (QSH) state taking place in two-dimensional (2D) topological insulators (TI), preserves time-reversal invariance. Therefore, the edge states appear in helical Kramer's pairs [32][33][34][35][36][37] with opposite spin orientations determined by the spin orbit of the TI. Several heat engines and refrigerators have been recently proposed, taking advantage of the fundamental chiral nature of the quantum Hall edge states, which manifests itself in multiple-terminal structures [19] and in quantum interference [20,21]. Recently, the property of charge fractionalization was also pointed out as a mechanism to enhance thermoelectricity [23]. All these setups rely on the existence of quantum point contacts and quantum dots in the structure, tunnel-coupled to the edge states, which are generated by recourse to constrictions. The fabrication of these elements is nowadays normal in the context of the quantum Hall effect [38][39][40]. However, their realization in the context of the QSH effect remains an experimental challenge so far [41], although they are widely investigated theoretically [42][43][44][45][46][47][48][49][50][51].In the quantum coherent regime the electronic transport properties take place without inelastic scattering and are fully characterized by a transmission function. Particle-hole symmetry breaking is a necessary condition for steady-state heat to work conversion. Having transmission functions rapidly changing in energy within the relevant transport window, is the key to achieve optimal thermoelectricity [2,[52][53][54][55]. The optimal performance is usually quantified by the figure of merit ZT , with the Carnot limit achieved for ZT → ∞. This ideal limit can be realized for transmission functions containing deltafunction like peaks [52]. In this sense, structures with resonant levels like quant...

A well-controlled technique for high-temperature epitaxial growth on 6H-SiC(0001) substrates is shown to allow the development of monolayer graphene that exhibits promise for precise metrological applications. Face-to-face and face-to-graphite annealing in a graphite-lined furnace at 1200 • C-2000 • C with a 101-kPa Ar background gas lowers the rates of SiC decomposition and Si sublimation/diffusion and thus provides a means to control the rate of graphene layer development. We studied a wide range of growth temperatures and times and describe the resulting sample surface morphology changes and graphene layer structures. The experimental results are compared to a kinetic model based on two diffusion processes: Si vapor diffusion in the Ar-filled gap and atomic diffusion through graphitic surface layers.

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We measure the thermoelectric response of Corbino structures in the quantum Hall effect regime and compare it with a theoretical analysis. The measured thermoelectric voltages are qualitatively and quantitatively simulated based upon the independent measurement of the conductivity, indicating that they originate predominantly from the electron diffusion. In contrast to earlier Hall-bar experiments, electronphonon interaction does not lead to a phonon-drag contribution. This implies a description of the Onsager coefficients on the basis of a single transmission function, from which both thermovoltage and conductivity can be predicted with a single fitting parameter. Furthermore, it lets us predict a figure of merit for the efficiency of thermoelectric cooling, which becomes very large for partially filled Landau levels and high magnetic fields.