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arXiv:cond-mat/0110093 · Physical review. B, Condensed matter

Charge transport processes in a superconducting single-electron transistor coupled to a microstrip transmission line

Wei Dong Lu, Kevin D. Maranowski, A. J. Rimberg

Published Jan 9, 2002• 13 citations
arXivHTMLPDF

Abstract

We have investigated charge transport processes in a superconducting single-electron transistor (S-SET) fabricated in close proximity to a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The macroscopic bonding pads of the S-SET along with the 2DEG form a microstrip transmission line. We observe a variety of current-carrying cycles in the S-SET which we attribute to simultaneous tunneling of Cooper pairs and emission of photons into the microstrip. We find good agreement between these experimental results and simulations including both photon emission and photon-assisted tunneling due to the electromagnetic environment.

DOI
10.1103/physrevb.65.060501

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We have investigated charge transport processes in a superconducting single-electron transistor (S-SET) fabricated in close proximity to a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The macroscopic bonding pads of the S-SET along with the 2DEG form a microstrip transmission line. We observe a variety of current-carrying cycles in the S-SET which we attribute to simultaneous tunneling of Cooper pairs and emission of photons into the microstrip. We find good agreement between these experimental results and simulations including both photon emission and photon-assisted tunneling due to the electromagnetic environment.
We have developed a novel system consisting of a superconducting single-electron transistor (S-SET) coupled to a two-dimensional electron gas (2DEG), for which the dissipation can be tuned in the immediate vicinity of the S-SET. To analyze our results, we have developed a model of the environment for S-SET/2DEG systems that includes electromagnetic fluctuations coupled both through the S-SET leads and capacitively to the S-SET central island. We analyze this model, treating the leads as finite transmission lines, to find the probability function P(E) for exchanging energy E with the environment. We also allow for the possibility of low-frequency fluctuations of the S-SET offset charge. We compare our calculations with measurements of SET conductance versus 2DEG conductance and find good agreement for temperatures 100 mK, while unexplained discrepancies emerge for lower temperatures. By including the effects of charge averaging we are also able to predict the shape and evolution of IV curves as the 2DEG in the vicinity of the S-SET is changed.
A novel model system is developed to study the effects of the environment on transport properties of a superconducting single-electron transistor (S-SET). The impedance of a two-dimensional electron gas (2DEG) 50 nm below the sample surface serving as the environment can be tuned in situ. A quantum dot is readily formed in the 2DEG. Josephson tunneling processes in the SET are suppressed and quasiparticle tunneling processes are enhanced as the 2DEG is confined. Important energetic parameters of the SET such as the charging energy EsbcE\\sb{c}Esbc and superconducting energy gap triangle\\triangletriangle remain almost unchanged in this process.
We have developed a novel system consisting of a superconducting single-electron transistor (S-SET) coupled to a two-dimensional electron gas (2DEG), for which the dissipation can be tuned in the immediate vicinity of the S-SET. Within linear response, the S-SET conductance varies nonmonotonically with increasing 2DEG impedance. We find good agreement between our experimental results and a model incorporating electromagnetic fluctuations in both the S-SET leads and the 2DEG, as well as low-frequency switching of the S-SET offset charge.
This thesis will be divided into two parts. In the first part, theory and results of a novel system in which a superconducting single-electron transistor (S-SET) coupled to a two-dimensional electron gas (2DEG) serving as a tunable electromagnetic environment for the S-SET will be discussed, including effects of dissipation, resonant tunneling with photon emission, and photon-assisted tunneling. In the second part, we discuss the techniques for which the SET is incorporated in an RF resonant circuit, resulting in an ultra high charge sensitivity and bandwidth. After the 2DEG is confined into a quantum dot, random telegraph signals (RTS) caused by individual electrons tunneling on and off the dot have been observed. In the equilibrium configuration, the occupational probabilities of the charge states of the dot can be directly measured from the RTS and were found to follow a Fermi distribution. In the non-equilibrium configuration, the RTS correctly detected the onset of the current through the dot.
We study charge states of a superconducting single-electron transistor (SSET) fabricated on a heterostructure substrate that can be switched from insulating to conductive. We probe the charge states by microwave irradiation and subsequent observation of photon-assisted Josephson quasiparticle current, which allow us to obtain the energy dispersion relation in the quasicharge space. When the substrate is insulating, the charge states show an energy-dispersion curve with a gap, manifesting that coherent superposition of the two charge states is realized. When the substrate becomes conductive, its two-dimensional electrons capacitively couple to the SSET and modify the degree of dissipation to its electromagnetic environment. The dispersion relation then becomes linear, and its slope agrees with the value estimated from the charging energy including the capacitance of the two-dimensional electrons. This indicates that quantum coherence between the two charge states is lost due to the coupling with the environment.