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(a) Experimental setup used to perform the measurements and designed for carrier frequencies of 2–4 GHz. (b) Photograph of the PCB with the two stubs terminated by a varactor diode and the QPC, respectively. Two capacitors and one inductor placed after the diode act as on-chip bias tee. (c) Absolute value of the reflection coefficient as a function of the carrier frequency f for a QPC conductance of .
Photographs of samples with the same design as (a) sample 1, (b)samples 2 and 3, (c) sample 4, and (d) samples 5 and 6. The white circles are conventional Ohmic contacts and the darkest grey areas are the mesa arms where the 2DEG remains after etching. In (c) and (d), the lightest grey areas are the coplanar waveguides where the thick dotted lines indicate additional Ohmic contacts. In the inset of (c), an AFM scan of the central mesa is shown and the fine red dotted line marks the AMF lithography oxide lines.
(a) The dc conductance of the QPC as a function of gate voltage . (b) The absolute value of the reflection coefficient depending on the gate voltage for different carrier frequencies and different voltages applied to the diode. (c) as a function of the conductance obtained by combining the data of (a) and (b). (d) The derivative of as a function of g. The dashed yellow line indicates the g value for which is the figure of merit for the sample.
for the 6 investigated samples. On-chip extensions of the coplanar waveguides (full squares/circles) increase the performance of the samples significantly. The distance from the Ohmic contact to the QPC (long: red squares, short: black circles) only plays a role for the samples without on-chip CPWG (empty squares/circles).
Characteristics of samples 1 to 6 including distance from the rf Ohmic contact to the QPC (long (≥785 μm) or short (≤115 μm)), on-chip coplanar waveguides (existent/inexistent) and wedge (W, samples 1-4a, 5, 6a) or ribbon (R, samples 4b, 6b) bonds to connect the samples to the PCB.
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