Experimental evidence for Coulomb charging effects in an open quantum dot at zero magnetic field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. For the first time, at zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot G exceeds 2e 2 /h. We ascribe the observed conductance oscillations to evidence for Coulomb charging effects in an open dot. This is supported by the evolution of the oscillating features for G > 2e 2 /h as a function of both temperature and barrier transparency. The use of sub-micron surface gates fabricated over GaAs/AlGaAs heterostructures to electrostatically squeeze [1] the underlying two-dimensional electron gas (2DEG) into various shapes, has been proved a powerful tool to realize low-dimensional electron systems. By employing this technique, it is possible to define a quantum dot which confines electrons in an isolated region within a 2DEG. Consider a lateral quantum dot [2] weakly coupled to the source and drain contacts where the tunnel-ing conductance through the dot G is low, G ≪ 2e 2 /h. If the thermal smearing k B T and the chemical potentials in the leads are much smaller than the Coulomb charging energy e 2 /C which is required for adding an extra electron to the quantum dot, transport through the dot is inhibited. This is the Coulomb blockade (CB) of single electron tunneling [3]. It has been demonstrated [4] that transport through small quantum dots is determined by Coulomb charging effects [5] as well as zero-dimensional (0D) quantum confinement effects [6]. A versatile quantum dot may be defined by two pair of split-gates which introduce two quantum point contacts acting as the entrant and exit barriers to the dot, and two side-gates which are used to deplete electrons within the dot [7]. It has been reported that at zero magnetic field Coulomb charging effects only occur when the con-ductance of the two quantum point contacts, and of the quantum dot as a whole, falls below 2e 2 /h [7–9]. No oscillations were found when either or both of the quantum point contacts were set to above G > 2e 2 /h so that the dot was open to the 2DEG reservoirs [7–9]. Thus at present, it is widely accepted that at zero magnetic field, the conductance 2e 2 /h is the upper limit for which Coulomb charging effects can occur [10]. Nevertheless, previously we presented evidence …