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arXiv:1508.03825 · Applied Physics Letters

Wideband superconducting nanotube electrometer

Pasi Häkkinen, Aurélien Fay, Dmitry S. Golubev, Pasi Lähteenmäki, Pertti Juhani Hakonen

Published Jul 6, 2015• 0 citations
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Abstract

We have investigated the microwave response of nanotube Josephson junctions at 600–900 MHz at microwave powers corresponding to currents from 0 to 2 × IC in the junction. Compared with theoretical modeling, the response of the junctions corresponds well to the lumped element model of resistively and capacitively shunted junction. We demonstrate the operation of these superconducting FETs as charge detectors at high frequencies without any matching circuits. Gate-voltage-induced charge QG modifies the critical current IC, which changes the effective impedance of the junction under microwave irradiation. This change, dependent on the transfer characteristics dIC/dQG, modifies the reflected signal and it can be used for wide band electrometry. We measure a sensitivity of 3.1×10−5 e/Hz from a sample which has a maximum switching current of 2.6 nA.

DOI
10.1063/1.4926400

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We have investigated the microwave response of nanotube Josephson junctions at 600–900 MHz at microwave powers corresponding to currents from 0 to 2 × IC in the junction. Compared with theoretical modeling, the response of the junctions corresponds well to the lumped element model of resistively and capacitively shunted junction. We demonstrate the operation of these superconducting FETs as charge detectors at high frequencies without any matching circuits. Gate-voltage-induced charge QG modifies the critical current IC, which changes the effective impedance of the junction under microwave irradiation. This change, dependent on the transfer characteristics dIC/dQG, modifies the reflected signal and it can be used for wide band electrometry. We measure a sensitivity of 3.1×10−5 e/Hz from a sample which has a maximum switching current of 2.6 nA.
The microwave response of the dc quasiparticle tunneling current in Josephson tunnel junctions, where the Josephson current is suppressed by an external magnetic field, has been studied quantitatively in order to clarify its characteristics as a probe for the measurement of the junction capacitance. Extensive experiments for both small and long junctions are carried out for distinguishing between microwave behaviors of lumped and distributed constant junctions. It is shown that the observed voltage dependence of the dc quasiparticle tunneling current modified by an applied rf field is in good agreement with a theoretical result which takes into account the influence of the microwave circuit connected to the junction. The comparison between theory and experiment gives the magnitude of the internal rf field in the junction. Together with the applied rf field, this internal rf field leads to the junction rf impedance which is dominated by the junction capacitance in our experimental condition. In the case of lumped junctions, this experimental rf impedance is in reasonable agreement with the theoretical one with the junction capacitance estimated from the Fiske step of the distributed junction fabricated on the same substrate; the obtained ratio of the experimental impedance to the theoretical one is approximately 0.6–1.7. In the case of distributed junctions, however, experimental values of their characteristic impedances are approximately 0.2–0.3 of theoretical values calculated by assuming the one-dimensional junction model and taking account of the standing-wave effect in the junction.
We have studied the responses of high temperature superconducting (HTS) Josephson junctions to monochromatic and polychromatic electromagnetic irradiations from 50 GHz to 4.25 THz. The highest operating frequency and the maximum value of normalized response are shown to be proportional to the I/sub C/R/sub N/ product of the junction, where I/sub C/ and R/sub N/ are the critical current and normal resistance of the junction, respectively. Using a junction with I/sub C/R/sub N/ of 330 /spl mu/V, direct detections of monochromatic irradiations from 76 GHz to 3.1 THz have been obtained, indicating that fast detectors can be realized to cover around 50 : 1 frequency band by operating only one junction at one temperature. Also reported are the superposition of polychromatic signals, as well as some possible applications of the system.
We have measured the characteristics of a novel SQUID magnetometer pumped at 9 GHz. The device comprises a niobium point-contact shunting a low impedance superconducting microwave transmission line which isolates the junction at high frequencies from a toroidal input transformer. The junction is matched to a 50 Ω coaxial transmission line and the SQUID is operated in a reflection mode. For low values of the weak link critical current we observe a modulation of the phase of the reflected microwave signal periodic in the applied flux. The modulation persists at high microwave power levels indicating that the junction response is maintained to at least 200 GHz. For large values of critical current, amplitude modulation is observed. We have obtained a differential sideband power of 230 pW/(\Phi_{0})^{2}which is close to the value predicted by a simple model. In flux-locked loop operation we have obtained an energy sensitivity referred to the input terminals of 7 × 10-31J/Hz, which is superior to that so far reported for any other SQUID.
Abstract⎯We have fabricated the Bloch transistor shunted on-chip by a small-sized Cr resistor with Rs ≈ 1 kΩ. The Bloch transistor normally consists of two small Josephson junctions connected in series, which in our case have been replaced by two superconducting interferometer loops, each with two junctions in parallel. A capacitively coupled gate is supplied to control the induced charge of the small intermediate electrode (island) of the transistor. The measured I-V curves show no hysteresis and correspond to the operation of a effective Josephson junction at the high-damping and strong-noise limits. The critical current of the system was found to be close to its nominal value, that is in accordance with the electromagnetic environment theory. The I-V curves were modulated by the gate with a period of e and a maximum swing of about 2 µV. Such rather moderate modulation results from the Josephson-tocharging energies ratio, EJ/EC ≈ 9, in our sample being far from its optimum value ( ≈ 0.3÷1). I.
We have studied the microwave response of a single Cooper-pair transistor (CPT) coupled to a lumped-element microwave resonator. The resonance frequency of this circuit, fr{f}_{r}fr​, was measured as a function of the charge ng{n}_{g}ng​ induced on the CPT island by the gate electrode and the phase difference across the CPT, ${\ensuremath{\varphi}}_{B}$, which was controlled by the magnetic flux in the superconducting loop containing the CPT. The observed ${f}_{r}({n}_{g},{\ensuremath{\varphi}}_{B})$ dependencies reflect the variations of the CPT Josephson inductance with ng{n}_{g}ng​ and ${\ensuremath{\varphi}}_{B}$ as well as the CPT excitation when the microwaves induce transitions between different quantum states of the CPT. The results are in excellent agreement with our simulations based on the numerical diagonalization of the circuit Hamiltonian. This agreement over the whole range of ng{n}_{g}ng​ and ${\ensuremath{\varphi}}_{B}$ is unexpected, because the relevant energies vary widely, from 0.1 to 3 K. The sensitivity of the CPT as an electrometer is peaked when the CPT excitation level approaches that of the microwave resonator.
Two-dimensional arrays of Josephson junctions have been shown to operate in a state in which the microwave oscillations of the junctions are phase-coherent. However, the performance of an array is strongly dependent on both the details of the load-coupling circuit and the characteristic parameters of the junctions. We have measured two-dimensional arrays of resistively shunted Josephson tunnel junctions which are capacitively coupled to a nearby detector junction. Changing the critical current of the junctions in our arrays, while keeping the junction parasitics and array coupling roughly the same, results in an unexpected change in device operation. We have also investigated variations of the coupling circuit. Numerical simulations of the detection circuit enable us to estimate the power output from the array and to optimize the parameters of the system.