J. M. Hergenrother

Charge transport and photon-assisted tunneling in the NSN single-electron transistor

Abstract We present extensive experimental data concerning charge transport and photon-assisted tunneling in the NSN single-electron transistor. At subgap bias voltages and low temperatures, the current through this system arises from a combination of Andreev reflection and single-electron tunneling. The experimental data clearly show the Coulomb blockade of Andreev reflection, as well as the “trapping” of quasiparticles on the superconducting island. The voltage thresholds for Andreev reflection and single-electron tunneling as well as the dependence of the Andreev current on the gate charge Q 0 are in excellent agreement with theory. The magnitude of the Andreev current agrees well with theory in which the probability of Andreev reflection is greatly enhanced by phase-coherent multiple scattering. When the sample is exposed to small amounts of microwave radiation, electron transport is modified dramatically and several additional features appear in the experimental data. These new features are well described by including the effects of photon-assisted tunneling which greatly increases the rates of energetically unfavorable single-electron transitions. Finally, we discuss why the SET transistor with a superconducting island is such a sensitive microwave detector and present an estimate of its ultimate sensitivity.

Fabrication and characterization of single-electron tunneling transistors in the superconducting state

Electron-beam lithography was used to fabricate single electron charging effect devices with ultrasmall capacitance Al/Al/sub 2/O/sub 3//Al tunnel junctions. The single electron transistor is a three-terminal device composed of two series tunnel junctions and a gate electrode capacitively coupled to the island between them. Typical junctions are of area 60 nm*60 nm with a capacitance of 190 aF. The authors outline the fabrication procedures, discuss operational properties, and give sample handling considerations. These devices exhibit a highly nonlinear I-V characteristic which is modulated by the gate voltage, as expected for the Coulomb blockage. In the superconducting state, the superconducting gap in the quasiparticle density of states leads to transistor action above 1.3 K, a temperature easily reached with pumped liquid /sup 4/He refrigeration. The authors also discuss the observation of an intermittent intrinsic switching noise in the offset charge of the central island.<<ETX>>

Discrete energy levels and superconductivity in nanometer-scale Al particles

We have fabricated single-electron tunneling transistors in which the central island is an aluminum grain with radius in the range 2–10 nm. The corresponding spacing between electron-in-a-box levels is in the range ∼0.01 to ∼1 meV. Using tunneling spectroscopy at 50 mK, we have, for the first time, resolved these discrete levels in a metallic grain. By observing the Zeeman spin splitting in a magnetic field, we can distinguish grains with even vs. odd numbers of electrons. A superconducting energy gap can be seen if the grain is large enough so that the level spacing is smaller than the energy gap. This gap is reduced continuously to zero by a magnetic field of 3–4 Tesla. While the superconducting gap adds to the Coulomb gap in determining the threshold voltage for tunneling into a grain with an initially even number of electrons, it subtracts from the Coulomb gap for a grain with an initially odd number of electrons because the tunneling electron can pair with the odd electron, forming a lower-energy fully-paired state.

The single-electron transistor as an ultrasensitive microwave detector

We have measured single-electron tunneling transistors with superconducting islands and conclude that they may be used as ultrasensitive detectors of microwave radiation for frequencies /spl ges/80 GHz. These devices contain a small superconducting Al island that is weakly coupled to a bias circuit through two small-capacitance tunnel junctions and a capacitive gate. At low bias voltages and temperatures, a single quasiparticle may only be introduced to the island through photon-assisted tunneling. Once this occurs, the quasiparticle is trapped on the island for /spl sim/1 /spl mu/s because it takes a relatively long time for this specific quasiparticle to tunnel off. While it is trapped, charge is transported through the system two electrons at a time. Since the photon-assisted transition merely switches the detector current on, this device is not limited to one electron tunneled through the system per absorbed photon. Measurements indicate that at least 100 electrons can tunnel for every absorbed photon, which corresponds to a noise-equivalent power of 3/spl times/10/sup -20/ W//spl radic/Hz at 80 GHz if the current is measured with a commercial current amplifier.<<ETX>>

A CRYOGENIC AMPLIFIER FOR DIRECT MEASUREMENT OF HIGH-FREQUENCY SIGNALS FROM A SINGLE-ELECTRON TRANSISTOR

We have developed a cryogenic voltage amplifier using GaAs metal semiconductor field effect transistors and coupled it to a superconducting single-electron transistor (SET) inside a dilution refrigerator. With this amplifier, we could extend the maximum output frequency of the SET, normally less than a kilohertz, up to 1 MHz. By placing this amplifier off-chip, we could maintain the low SET temperature required for proper SET operation.

Single-electron tunneling transistors incorporating Cooper pair processes

A superconducting single-electron tunneling transistor composed of two ultrasmall capacitance Al/Al/sub 2/O/sub 3//Al tunnel junctions and a capacitively coupled gate electrode has been fabricated. Transistor operation is based on single electron and Cooper pair charging effects. This three-terminal device exhibits novel I-V characteristics not seen in either conventional superconducting tunnel junctions or normal metal Coulomb blockage devices. Current peaks appear above V approximately 2 Delta /e which originate from combined Cooper pair/quasi-particle tunneling processes. These peaks show e-periodic modulation with respect to the gate-induced charge. At lower voltages, the I-V curve shows features which are 2e-periodic. In a magnetic field, it is found that the 2e-periodicity changes into e-periodicity above a crossover line, T*(H). The data strongly suggest the existence of a free energy difference between states with an even versus an odd number of electrons on the metal island between the two junctions.<<ETX>>

Even-odd electron number effects in small superconducting islands

Abstract We present experiments in which the even-odd electron number dependence of a superconducting island is investigated by way of electron tunneling. The experiments are performed in a single-electron transistor configuration in which a superconducting Al island is in tunneling contact with either superconducting Al or normal Au bias electrodes. A gate electrode is used to modify the island charge. A salient 2 e periodic behavior appears in the tunneling current versus gate charge characteristic as a result of an even-odd free energy difference F o . The experimental 2 e to e periodicity crossover line T * (H) is in excellent agreement with theoretical predictions. With normal leads the current is attributed to an Andreev-like tunneling process which depends on the parity of the number of electrons on the island.