Triple quantum dot charging rectifier
Applied Physics Letters
Three tunnel-coupled quantum dots in the Coulomb blockade regime act as a molecular rectifier. We have realized this device in a GaAs/ Al 0.3 Ga 0.7 As heterostructure containing a two-dimensional electron gas using lithographically patterned gates. The current through two tunnel-coupled dots in series is recorded versus applied voltage. Ratchet behavior is created by a third dot, with one lead, tunnel-coupled to one of the two dots. An electron enters the third dot where it is trapped,
... is trapped, producing a jamming effect where no other electron may enter the device. The current-voltage characteristics show rectification and negative resistance arising from charging of the third dot. Electron transport of coupled quantum dots in the Coulomb blockade regime, of both serial and parallel configurations, 1-3 has been studied extensively. While single quantum dots can be considered as "artificial atoms," 4 coupled quantum dots can be considered as "artificial molecules." Interactions between dots, caused by interdot tunneling or capacitive coupling, give rise to changes in the conductance spectra and current-voltage characteristics of the system. Recently, it has been shown that Coulomb interaction between dots can give rise to ratchet effects. 5 The Coulomb blockade formalism, 6 used to describe single-electron charging and transport through a multiple quantum dot system, can be used to predict these new ratchet effects. Work with quantum dots displaying ratchet behavior has been explored both experimentally and theoretically. 7,8 In this letter, we present an experimental realization of a triple quantum dot rectifier, or Coulomb blockade charging ratchet, as proposed in Ref. 5. Here, three quantum dots are arranged in an asymmetric configuration such that a rectifying effect, characteristic of ratchet behavior, 9 is observed. Figure 1 (a) shows a scanning electron micrograph of the triple quantum dot charging ratchet. Fifteen independently tunable Cr: Au gates are used to define three coupled quantum dots in a GaAs/ Al 0.3 Ga 0.7 As heterostructure containing a two-dimensional electron gas (2DEG) located 57 nm below the surface. At 4 K, the 2DEG sheet carrier density and mobility are n s = 4.5ϫ 10 11 cm −2 and = 400 000 cm 2 V −1 s −1 . The three dots are arranged in a ring structure, with tunneling possible between dots 1 and 2 and dots 1 and 3. Tunneling is forbidden between dots 2 and 3. A finite-bias Coulomb blockade measurement of dot 2 is shown in Fig. 2 , from which we can deduce the total dot capacitance to be C 2 ϳ 310 aF. All measurements were done in a Helium-3 system at the base temperature 380 mK and measured electron temperature 440 mK. The ratchet mechanism observed in this triple quantum dot is a bias-dependent current rectification, 5 similar to the effect seen with a hybrid molecular electronic device. 10 As is typical for a ratchet, breaking of symmetry under inversion must be present, and is introduced here by placing an infinite barrier between dots 2 and 3. Because no tunneling takes place between dots 2 and 3, dot 3 is a quantum box that is tunnel-coupled with dot 1. Dot 2 and dot 3 are now only capacitively coupled. From the circuit model of the triple quantum dot shown in Fig. 1(b) , we are able to plot the stability diagram of the system, 11 as is routinely done for double quantum dots. 2,12 The stability diagram displays a "quadruple point," analogous to the triple point of a double dot system, where four states are degenerate. These four states correspond to no excess electrons in the system or one excess electron on one of the three dots. Therefore, the triple dot can be tuned such that zero or only one excess electron is a) Electronic mail: firstname.lastname@example.org. edu FIG. 1. (a) Scanning electron micrograph of the triple quantum dot. The light areas are Cr: Au gates used to define the quantum dots. The locations of the dots are highlighted by circles. This geometry allows for tunneling between dots 1 and 2, and between dots 1 and 3. Dots 2 and 3 are capacitively coupled, but no electrons may tunnel between these dots. (b) Circuit diagram of a triple quantum dot, with source-drain voltage V SD = V L − V R . Split boxes represent tunnel junctions with capacitance C L . Each quantum dot ͑i =1,2,3͒, with total capacitance C i , has its own independent capacitively coupled side gate, with gate voltage V Gi and capacitance C Gi . Crosscapacitances are neglected. Interdot and dot-lead tunnel junctions are modeled as a large resistor and capacitor in parallel. An asymmetry is introduced by not permitting any transfer of charge between dot 2 and dot 3 : C 23 is a pure capacitor.