In this thesis, I will present the experimental results of the dynamics of
superconducting single electron transistors (sSETs), under the influence of
tunable dissipation. The sSET, consisting of two dc SQUIDs in series and
the third gate electrode, is deposited onto a GaAs/AlGaAs heterostructure
which contains a two dimensional electron gas plane 100nm beneath the
substrate surface. The Josephson coupling energy, charging energy and
dissipation related Hamiltonian can all be tuned in situ, while keeping
others unchanged. We measured the switching current statistics and the
transport properties, as a function of the dissipation and gate charge at
different temperatures.
If the sSET is in the classical regime where phase is a good quantum
variable, we found that the switching current and corresponding Josephson
energy decrease as dissipation increases. Our observation agrees qualitatively
with the theoretical calculation of a single Josephson junction
with dominant Josephson energy, in a frequency dependent dissipative
environment where energy barrier decreases as dissipation increases in
thermally activated escape regime. This dissipation dependence result can
be understood as the consequence of a reduced quantum fluctuations in
the charge numbers.
Whereas in the charging regime, the switching current shows a 1e
periodicity with respect to gate charge, indicating a pronounced charging
effect. At a specific gate charge number, quantum fluctuations of the phase
variable are compressed as dissipation increases, resulting in an enhanced
switching current and Josephson energy. This result matches the theory of a sSET capacitively coupled to a dissipative environment qualitatively.
The temperature dependence of the switching current histogram indicates
the existence of both quantum and classical thermal phase diffusion.
Moreover, quantum charge fluctuations are minimized at the degeneracy
point, causing a sharp dip on the width of the switching current histogram.
For a sSET with comparable Josephson energy and charging energy,
quantum fluctuations of both phase and charge variables are significant.
The influence of dissipation on the dynamics of the device is distinct in the
classical and charging regimes. Dissipation compresses quantum phase
fluctuations in the charging regime, whereas reduces the quantum charge
fluctuations in the classical regime. The transition between these two
regimes is found to be determined by the tunnel resistance of the SQUID.
The competition between Josephson and charging energies, however, is
not the intrinsic parameter of this transition. Our results imply that a
detailed theoretical calculation of a sSET with comparable Josephson
coupling energy and charging energy under the influence of dissipation is
needed.
| Identifer | oai:union.ndltd.org:WATERLOO/oai:uwspace.uwaterloo.ca:10012/6925 |
| Date | January 2012 |
| Creators | Meng, Shuchao |
| Source Sets | University of Waterloo Electronic Theses Repository |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
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