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Journal of Applied Physics

Aluminum single-electron transistors studied at 0.3 K in different transport regimes

B. Buonomo, Maria Gabriella Castellano, Roberto Leoni, F. Mattioli, G. Torrioli, P. Carelli, F. Chiarello, C. Cosmelli

Published Jun 1, 2001• 6 citations
Publisher

Abstract

We measured the transport properties and charge noise of superconducting single-electron transistors at the temperature of 0.3 K. The devices were fabricated with different ratios between charging and Josephson energies in order to explore their behavior in different experimental situations. We show that, in spite of the substantial thermal fluctuation, it is possible to extract from the data the values of the device parameters and information on the various tunneling mechanisms. Furthermore, we measured the device noise at different bias points, corresponding to regions with different dominant tunneling mechanisms, namely in the supercurrent, in the Josephson quasiparticle, and in the sequential tunneling regions.

DOI
10.1063/1.1361236

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We measured the transport properties and charge noise of superconducting single-electron transistors at the temperature of 0.3 K. The devices were fabricated with different ratios between charging and Josephson energies in order to explore their behavior in different experimental situations. We show that, in spite of the substantial thermal fluctuation, it is possible to extract from the data the values of the device parameters and information on the various tunneling mechanisms. Furthermore, we measured the device noise at different bias points, corresponding to regions with different dominant tunneling mechanisms, namely in the supercurrent, in the Josephson quasiparticle, and in the sequential tunneling regions.
We analyze charge tunneling statistics and current noise in a superconducting single-electron transistor in a regime where the Josephson-quasiparticle cycle is the dominant mechanism of transport. Due to the interplay between Coulomb blockade and Josephson coherence, the probability distribution for tunneling events strongly deviates from a Poissonian and displays a pronounced even-odd asymmetry in the number of transmitted charges. The interplay between charging and coherence is reflected also in the zero-frequency current noise which is significantly quenched when the quasiparticle tunneling rates are comparable to the coherent Cooper pair oscillation frequency. Furthermore, the finite frequency spectrum shows a strong enhancement near the resonant transition frequency for Josephson tunneling.
We analyze charge tunneling statistics and current noise in a superconducting single-electron transistor in a regime where the Josephson-quasiparticle cycle is the dominant mechanism of transport. Due to the interplay between Coulomb blockade and Josephson coherence, the probability distribution for tunneling events strongly deviates from a Poissonian and displays a pronounced even-odd asymmetry in the number of transmitted charges. The interplay between charging and coherence is reflected also in the zero-frequency current noise which is significantly quenched when the quasiparticle tunneling rates are comparable to the coherent Cooper pair oscillation frequency. Furthermore, the finite frequency spectrum shows a strong enhancement near the resonant transition frequency for Josephson tunneling.
We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In samples with relatively large charging energy Ec>EJ, where EJ is the Josephson coupling energy, two transport mechanisms were found to be dominant, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island [in the conducting state of the transistor around the points Qg≡CgVg=e mod (2e), where Cg is the gate capacitance] or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the conducting state of our transistors had been found to be subject to quasiparticle poisoning, high-frequency gate cycling (at f∼1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data.
In this thesis, I will present the experimental results of the dynamics of \nsuperconducting single electron transistors (sSETs), under the influence of \ntunable dissipation. The sSET, consisting of two dc SQUIDs in series and \nthe third gate electrode, is deposited onto a GaAs/AlGaAs heterostructure \nwhich contains a two dimensional electron gas plane 100nm beneath the \nsubstrate surface. The Josephson coupling energy, charging energy and \ndissipation related Hamiltonian can all be tuned in situ, while keeping \nothers unchanged. We measured the switching current statistics and the \ntransport properties, as a function of the dissipation and gate charge at \ndifferent temperatures. \n \nIf the sSET is in the classical regime where phase is a good quantum \nvariable, we found that the switching current and corresponding Josephson \nenergy decrease as dissipation increases. Our observation agrees qualitatively \nwith the theoretical calculation of a single Josephson junction \nwith dominant Josephson energy, in a frequency dependent dissipative \nenvironment where energy barrier decreases as dissipation increases in \nthermally activated escape regime. This dissipation dependence result can \nbe understood as the consequence of a reduced quantum fluctuations in \nthe charge numbers. \n \nWhereas in the charging regime, the switching current shows a 1e \nperiodicity with respect to gate charge, indicating a pronounced charging \neffect. At a specific gate charge number, quantum fluctuations of the phase \nvariable are compressed as dissipation increases, resulting in an enhanced \nswitching current and Josephson energy. This result matches the theory of a sSET capacitively coupled to a dissipative environment qualitatively. \nThe temperature dependence of the switching current histogram indicates \nthe existence of both quantum and classical thermal phase diffusion. \nMoreover, quantum charge fluctuations are minimized at the degeneracy \npoint, causing a sharp dip on the width of the switching current histogram. \n \nFor a sSET with comparable Josephson energy and charging energy, \nquantum fluctuations of both phase and charge variables are significant. \nThe influence of dissipation on the dynamics of the device is distinct in the \nclassical and charging regimes. Dissipation compresses quantum phase \nfluctuations in the charging regime, whereas reduces the quantum charge \nfluctuations in the classical regime. The transition between these two \nregimes is found to be determined by the tunnel resistance of the SQUID. \nThe competition between Josephson and charging energies, however, is \nnot the intrinsic parameter of this transition. Our results imply that a \ndetailed theoretical calculation of a sSET with comparable Josephson \ncoupling energy and charging energy under the influence of dissipation is \nneeded.
Charge states in mesoscopic Josephson devices are considered promising candidates for a solid-state implementation of quantum computing. We studied three superconducting single electron transistors (SSETs) fabricated with small Al/AlO x /Al tunnel junctions, which, with a different choice of the construction parameters, can realize both the quantum bit and the readout system for the qubit charge state. We varied the junction capacitance and barrier transparency, making the Josephson energy E J of the device smaller, comparable and larger than the charging energy E C : the first case is suitable to build the qubit, while the latter can be suitable for the qubit readout.
We investigate a superconducting single-electron transistor capacitively coupled to a nanomechanical oscillator and focus on the double Josephson quasiparticle resonance. The existence of two coherent Cooper-pair tunneling events is shown to lead to pronounced back action effects. Measuring the current and the shot noise provides a direct way of gaining information on the state of the oscillator. In addition to an analytical discussion of the linear-response regime, we discuss and compare results of higher-order approximation schemes and a fully numerical solution. We find that cooling of the mechanical resonator is possible and that there are driven and bistable oscillator states at low couplings. Finally, we also discuss the frequency dependence of the charge noise and the current noise of the superconducting single electron transistor.