Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

54 J.M. Elzerman et al.
V L (V)
-1.1
-1.0
a b
00
-0.9
0.0 -0.2 -0.4 -0.6
V PR (V)
-1.02
-1.00
-0.98
-0.96
12
22
01
11
21
00
10
-0.15 -0.20 -0.25 -0.30
V PR (V)
Fig. 17. Using the QPC to measure the charge configuration of a double quantum
dot in the few-electron regime. (a) dIQP C/dVL (in grayscale) versusVL and
VPR,withVSD2 = 100 µV andVSD1 = VDOT = 0. A small modulation (0.3 mV
at 17.77 Hz) is applied to VL, and the resulting modulation in IQP C is measured
with a lock-in amplifier to give dIQP C/dVL directly. The label “00” indicates the
region where the double dot is completely empty. In the bottom left corner the dark
lines are poorly visible. Here the tunnel rates to the reservoirs are quite large, leading
to smearing of the steps in the QPC current, and therefore to smaller dips in
dIQP C/dVL. (b) Zoom-in of Fig. 17a, showing the “honeycomb” diagram for the first
few electrons in the double dot. The black labels indicate the charge configuration,
with “21” meaning 2 electrons in the left dot and 1 on the right
change in the number of electrons on the left dot, whereas almost-vertical lines
indicate a change in the electron number on the right. In the upper left region
the “horizontal” lines are not present, even though the QPC can still detect
changes in the charge, as demonstrated by the presence of the “vertical” lines.
We conclude that in this region the left dot contains zero electrons. Similarly, a
disappearance of the “vertical” lines occurs in the lower right region, showing
that here the right dot is empty. In the upper right region, the absence of
lines shows that here the double dot is completely empty.
We are now able to identify the exact charge configuration of the double
dot in every honeycomb cell, by simply counting the number of “horizontal”
and “vertical” lines that separate it from the 00 region. In Fig. 17b the first
few honeycomb cells are labelled according to their charge configuration, with
e.g. the label “21” meaning 2 electrons in the left dot and 1 on the right.
Besides the dark lines, also short bright lines are visible, signifying a peak
in dIQP C/dVL. These bright lines correspond to an electron being transferred
from one dot to the other, with the total electron number remaining the same.
(The fact that some charge transitions result in a dip in dIQP C/dVL and others
in a peak, derives from the fact that we use the QPC on the right and apply