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

44 J.M. Elzerman et al.
The resulting current coming from the drain reservoir is fed to a low-noise
IV convertor. In this work we use two types, depending on the desired bandwidth.
The first one is designed for low-frequency measurements. It has a
bandwidth of about 1 kHz, and a noise floor of ∼5 fA/Hz 1/2 . The feedback
resistance can be set to 10 MΩ, 100 MΩ or 1 GΩ, with an input resistance
that is a factor 10 3 or 10 4 smaller (for the “low noise” or “low input resistance”
setting, respectively). The faster IV convertor has a bandwidth of
about 150 kHz, and a current noise of ∼1 pA/Hz 1/2 at 100 kHz. The feedback
resistance is 10 MΩ, corresponding to an input resistance of 1.3 kΩ. More
characteristics are given in Sect. 4.
The signal from the IV convertor is then sent to an isolation amplifier,
to provide optical isolation and possibly gain. Again we can choose a lowfrequency
version (up to ∼1 kHz) or a high-frequency one (up to ∼300 kHz).
The voltage from the isolation amplifier is finally measured by a digital multimeter
(Keithley 2700) and sent to the computer via GPIB interface. Alternatively,
we can use a lock-in amplifier (Stanford EG&G 5210) if the signal to be
measured is periodic, or an ADwin Gold module for very fast measurements
(up to 2.2 × 10 6 14-bit samples per second).
Measurement Wires
To make contact to the sample, 2 × 12 twisted pairs of wires run from two
connector boxes at room temperature all the way down to the “cold finger”
at base temperature. The diameter and material of these wires is chosen to
minimize the heat load on the mixing chamber. From room temperature to 1
Kelvin, 2 × 9 pairs consist of manganine wires (100 µm diameter), and 2 × 3
pairs of copper wires (90 µm diameter). The copper wires can be used if a large
current has to be applied. From 1 Kelvin to the mixing chamber, superconducting
“Niomax” wires (50 µm diameter) are used. From the mixing chamber
to the bottom of the cold finger, where thermal conductivity is no longer a
constraint, we have standard copper wires. At base temperature, one wire of
each twisted pair is connected to “cold ground” (i.e. the cold finger), which is
electrically connected to clean ground via the metal parts of the fridge.
All wires are thermally anchored to the fridge, by carefully wrapping them
around copper posts, at several temperature stages (4 K, 1 K, ∼100 mK and
∼10 mK). At room temperature, the resistance of the wires is about 250 Ω or
150 Ω for the manganine or copper wires, respectively. At low temperature it
is about 50 Ω. The wires have various parasitic capacitances to their twisted
partner and to ground, as indicated in Fig. 11 and Fig. 12.
Filtering
The wires connect the device to the measurement electronics at room temperature,
so they have to be carefully filtered to avoid that the electrons in