Photoconductive Response of a Quasi-One Dimensional Channel

Much of the physics of quasi-one dimensional (ID) systems has been studied in so-called “ballistic channels” or “quantum point-contacts” defined by a negative voltage applied to two gates a fraction of a micron wide and a similar distance apart on top of a GaAs--(Ga,Al)As heterojunction containing a two dimensional electron gas (2DEG) (van Houten et al, 1990): the channel length is much shorter than the scattering lengths in the 2DEG, and so transport through it is ballistic. At B = 0 the resistance is quantized to a value h/2ie 2, where i is the number of occupied ID subbands (van Wees et al., 1988; Wharam et al., 1988). Much of the interest has focussed on the relationship between the gate voltage (Vg) and the electrostatic potential between the gates, which determines the ID subband spacing in the channel. In transport studies, the potential is determined indirectly by fitting theoretical predictions of the magnetic depopulation of ID subbands to the experimental data (Wharam et al, 1989). An alternative approach is to use a multiple quantum wire system, where an array of ~ 10,000 electron channels ~1 mm long is fabricated (see e.g., Alsmeier et al., 1989; Hansen et al., 1987). Although the large area off such samples means that the transmission of far-infrared (FIR) radiation can be used to detect transitions between ID states, it also leads to poorly defined transport properties, due toj averaging effects (Thornton et al, 1986; Gao et al, 1990). In addition, the transmission spectra of the multiple quantum wire systems are complicated by possible presence of coupling between the quantum wires (plasmons) (Batke et al, 1985; Hansen et al., 1986), and non-local effects such as depolarization shifts (Hansen et al, 1986).