Anomalous thermopower oscillations in graphene-nanowire vertical heterostructures

Abstract

Thermoelectric measurements have the potential to uncover the density of states (DOSs) of low-dimensional materials. Here, we present the anomalous thermoelectric behavior of monolayer graphene-nanowire (NW) heterostructures, showing large oscillations as a function of the doping concentration. Our devices consist of InAs NW and graphene vertical heterostructures, which are electrically isolated by thin (∼10 nm) hexagonal boron nitride (hBN) layers. In contrast to conventional thermoelectric measurements, where a heater is placed on one side of a sample, we use the InAs NW (diameter ∼50 nm) as a local heater placed in the middle of the graphene channel. We measure the thermoelectric voltage induced in graphene due to Joule heating in the NW as a function of temperature (1.5–50 K) and carrier concentration. The thermoelectric voltage in bilayer graphene (BLG)-NW heterostructures shows sign change around the Dirac point, as predicted by Mott’s formula. In contrast, the thermoelectric voltage measured across monolayer graphene (MLG)-NW heterostructures shows anomalous large-amplitude oscillations around the Dirac point, not seen in the Mott response derived from the electrical conductivity measured on the same device. The anomalous oscillations are a signature of the modified DOSs in MLG by the electrostatic potential of the NW, which is much weaker in the NW-BLG devices. Thermal calculations of the heterostructure stack show that the temperature gradient is dominant in the graphene region underneath the NW, and thus sensitive to the modified DOSs resulting in anomalous oscillations in the thermoelectric voltage. Furthermore, with the application of a magnetic field, we detect modifications in the DOSs due to the formation of Landau levels in both MLG and BLG.

Extrinsic thermoelectric response of coherent conductors

“…Currents in response to local heating have been recently measured in low dimensional systems via electronic injection [73][74][75][76], laser illumination [77][78][79], or nanoheaters [80]. Transport is there dominated by the diffusion of the electron-hole excitations across a potential barrier, what can be interpreted as a nanoscale version of current induced by Landauer blowtorches [81][82][83].…”

Scanning probe-induced thermoelectrics in a quantum point contact

“…4 Spatially-resolved images of the Seebeck effect were measured as well by engineering local heaters with scanning tunneling 5 or thermal 3,4 microscopes, focused laser 6 or electron 7 beams, or Joule-heated nanowires. 8 Contrary to conventional (longitudinal) thermoelectric measurements that are performed across two terminals, such experiments involve a third terminal (e.g. the tip of the microscope) and give access to non-local thermoelectric effects.…”

“…Three-terminal thermoelectrics has attracted growing interest for a decade, 9 leading recently to experimental implementations. [3][4][5][6]8,[10][11][12][13][14] The prototypical system in this context consists of a central scattering region attached to two (say left and right) electronic reservoirs, and to a third one with which only heat can be exchanged. The third terminal may be a bosonic reservoir, [14][15][16][17][18][19][20] or a reservoir of electrons [10][11][12]21 capacitively coupled to the scattering region.…”

“…50 The same behavior is known for the local thermopower S 0 LL = − ∆µ L /(e∆θ L )| I e L =0 of a QPC without tip 40 and can be understood 48 in the non-local configuration from the analysis of the different terms in Eq. (8).…”