ICT2014 Abstracts

   Thermoelectric effects in two-dimensional electron systems (2DEGs) can be used to probe basic electronic properties and transport mechanisms and may also find application in green electronics [1]. We have measured photovoltage signals from 2DEGs in HgTe quantum-well structures (t_QW=6.1 nm) due to heating of the electron gas by THz cyclotron resonance power absorption. The conventional Hall-bar sample (3.6×0.4 mm) having six ohmic side-contacts was illuminated by a linearly polarized THz beam (v =2.53 THz, output power: 20/50 mW) propagating along (or counter to) the external magnetic field, which was applied normal to the sample surface.  There has been a prediction that the Nernst effect should be observable in the 2DEG of samples of materials with large spin-orbit coupling [2]. We have observed power dependent photovoltage signals from several different pairs of contacts in the magnetic field region near cyclotron resonance. An electron temperature gradient (T1K/mm) is produced by absorption of the focused THz laser beam with Gaussian spot size of roughly 2-3 mm diameter. This approximately circularly symmetric temperature gradient in a rectangular Hall-bar geometry leads to complex behavior of the photovoltage signals from different pairs of contacts. The measured signals between Hall contacts where beams focuses is the largest one, and the signal decreases moving away from this pair of contacts. Similarly, the photovoltage between the longitudinal contacts near the focused beam is larger than the contacts on opposite side. Some combination of the Nernst effect, related to the off-diagonal components of the conductivity tensor, and the thermopower, related to the diagonal components, appear to contribute to the different observed signals. The signal from an individual pair of contacts depends strongly on incident laser power.

[1]  M. Jonson and S. M. Girvin, Phys. Rev. B 29, 1939 (1984).

[2] S. G. Cheng et al., Phys. Rev. B 78, 0450302 (2008).