Abstract The reliable fabrication of high-sensitivity superconducting quantum interference devices (SQUIDs) with high critical transition temperature (high- T c ) is crucial for many practical applications such as weak magnetic sensing. In this work, we report the first realization of the YBa 2 Cu 3 O 7− δ (YBCO) radio frequency SQUIDs (rf SQUIDs) based on focused helium ion beam (FHIB) technique. A series of YBCO rf SQUIDs with high consistency were fabricated using a 30 keV FHIB with different helium ion (He + ) dose. The dose-dependent characteristics of the fabricated devices were systematically investigated at a temperature of 77 K. The experimental results verify that the variation in He + dose can effectively modulate the junction parameters and thereby the device noise performance. A non-monotonic white noise trend with increasing He + dose was observed, which could be primarily attributed to the transition of operational working regime of the fabricated rf SQUIDs. Based upon the dose optimization, a flux noise of 58 μ Φ 0 / H z 1 / 2 , corresponding to a field sensitivity of 185 f T / H z 1 / 2 , was achieved with our current device configuration. Approaches to further optimize the overall device performance are also discussed. Our work demonstrates FHIB-based high- T c SQUIDs with high junction parameter consistency a promising candidate for the implementation of multi-channel weak magnetic field detection systems and might hopefully serve as a platform for validating the theoretical models of high- T c SQUIDs.
Abstract The reliable fabrication of high-sensitivity superconducting quantum interference devices (SQUIDs) with high critical transition temperature (high- T c ) is crucial for many practical applications such as weak magnetic sensing. In this work, we report the first realization of the YBa 2 Cu 3 O 7− δ (YBCO) radio frequency SQUIDs (rf SQUIDs) based on focused helium ion beam (FHIB) technique. A series of YBCO rf SQUIDs with high consistency were fabricated using a 30 keV FHIB with different helium ion (He + ) dose. The dose-dependent characteristics of the fabricated devices were systematically investigated at a temperature of 77 K. The experimental results verify that the variation in He + dose can effectively modulate the junction parameters and thereby the device noise performance. A non-monotonic white noise trend with increasing He + dose was observed, which could be primarily attributed to the transition of operational working regime of the fabricated rf SQUIDs. Based upon the dose optimization, a flux noise of 58 μ Φ 0 / H z 1 / 2 , corresponding to a field sensitivity of 185 f T / H z 1 / 2 , was achieved with our current device configuration. Approaches to further optimize the overall device performance are also discussed. Our work demonstrates FHIB-based high- T c SQUIDs with high junction parameter consistency a promising candidate for the implementation of multi-channel weak magnetic field detection systems and might hopefully serve as a platform for validating the theoretical models of high- T c SQUIDs.
YBa2Cu3O7−δSingle Flux Quantum Flip Flop Directly Written With a Focused Helium Ion Beam
Superconducting logic circuits based on low transition temperature (Tc) are attractive due to their intrinsic low power consumption and high operating frequency. They have a wide variety of applications (particularly in future computing), but are difficult to use because they require cooling with liquid helium to operate. The current high Tclogic circuit based on YBa2Cu3O7-δ(YBCO) already has the ability to operate at a temperature above 4 K. However, this material is difficult to integrate into large-scale circuits by the traditional YBCO junction fabrication process. The limits of fabrication methods are the bottleneck stifling this material's potential for integration into logic circuits. As such, in this paper we aim to advance the state of the art in junction fabrication technology. Here we report the demonstration of a high Tcsuperconducting single flux quantum (SFQ) circuit made by a focused helium ion beam that is easily fabricated and can readily be incorporated into a large-scale integration with nano-scale junctions for future applications. An inductively coupled readout superconducting quantum interference device (SQUID) is employed to reduce the circuit complexity. The circuit parameters and performance are discussed.
In this work, we demonstrate the ability to fabricate superconducting quantum interference devices (SQUIDs) by directly writing Josephson junctions into the plane of YBa2Cu3O7−δ thin films with a focused helium ion beam. This technique allows for the control of the Josephson barrier transport properties through the single parameter, ion dose. SQUIDs written with a dose of 4 × 1016 ions/cm2 had metallic barrier junctions that exhibited nearly ideal electrical transport characteristics at 50 K and a flux noise of 20 μΦ0/Hz at 10 Hz. At higher irradiation doses, the SQUIDs had insulating barrier Josephson junctions with a quasi particle energy gap edge at 20 meV.
Abstract The Josephson junction (JJ) is the corner stone of superconducting electronics and quantum information processing. While the technology for fabricating low T c JJ is mature and delivers quantum circuits able to reach the “quantum supremacy”, the fabrication of reproducible and low-noise high- T c JJ is still a challenge to be taken up. Here we report on noise properties at RF frequencies of recently introduced high- T c Josephson nano-junctions fabricated by mean of a Helium ion beam focused at sub-nanometer scale on a YBa 2 Cu 3 O 7 thin film. We show that their current-voltage characteristics follow the standard Resistively-Shunted-Junction (RSJ) circuit model, and that their characteristic frequency f c = (2 e / h ) I c R n reaches ~300 GHz at low temperature. Using the “detector response” method, we evidence that the Josephson oscillation linewidth is only limited by the thermal noise in the RSJ model for temperature ranging from T ~ 20 K to 75 K. At lower temperature and for the highest He irradiation dose, the shot noise contribution must also be taken into account when approaching the tunneling regime. We conclude that these Josephson nano-junctions present the lowest noise level possible, which makes them very promising for future applications in the microwave and terahertz regimes.
YBa2Cu3O7 nano-constriction Josephson junctions and SQUIDs fabricated by focused helium-ion-beam irradiation
By focused 30keV He ion beam irradiation, epitaxially grown YBa2Cu3O7 (YBCO) thin films can be driven from the superconducting to the insulating state with increasing irradiation dose. A properly chosen dose suppresses superconductivity down to 4K, while crystallinity is still preserved. With this approach we create areas of normal-conducting YBCO that can be used to define resistively shunted constriction-type Josephson junctions (cJJs) on the nanometer scale. We also demonstrate that the fabricated cJJs can be incorporated in direct current superconducting quantum interference devices and can be used as detector junctions in THz antennas.
YBa2Cu3O7 24∘ (30∘) bicrystal grain boundary junctions (GBJs), shunted with 60\,nm (20\,nm) thick Au, were fabricated by focused ion beam milling with widths 80nm≤w≤7.8μm. At 4.2\,K we find critical current densities jc in the 105A/cm2 range %\dkc{\#1} (without a clear dependence on w) and an increase in resistance times junction area ρ with an approximate scaling ρ∝w1/2. For the narrowest GBJs jcρ≈100μV, which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with w≈100\,nm wide GBJs we find an rms magnetic flux noise spectral density of SΦ1/2≈4μΦ0/Hz1/2 in the white noise limit. We also derive an expression for the spin sensitivity Sμ1/2, which depends on SΦ1/2, on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the not optimized SQUIDs presented here, we estimate Sμ1/2=390μB/Hz, which could be further improved by at least an order of magnitude.
We measured the amplitude–frequency characteristics of radio frequency superconducting quantum interference devices (rf SQUIDs) over a temperature range between 65 and 79 K. Using the expressions derived from the recently developed rf SQUID theory, valid also at large thermal fluctuations, we determined from these data the basic parameters of high-transition-temperature superconductor (HTS) rf SQUIDs. These parameters were: (a) the high-frequency coupling coefficient between the rf SQUID and the tank circuit resonator, k, (b) the SQUIDs hysteretic parameter, β, (c) the critical current of the Josephson junction, Ic, (d) its normal resistance, Rn, and (e) its noise parameter, Γ. We found a good agreement with the values of β(Ic) and Rn determined directly after destructively opening the SQUID loop. In accordance with the theoretical predictions, our experimental results show that at large thermal fluctuation levels (T≅77 K), rf SQUIDs with large loop inductance operate in nonhysteretic mode up to β values exceeding 3. Furthermore, we have shown that the optimal energy sensitivity is attained in the nonhysteretic mode at a value of β distinctly higher than 1. A quantitative comparison of white noise predicted by the theory with that obtained from the experiment showed a reasonable agreement. We also discussed the contribution of the phase information to the SQUID’s signal and noise at optimum operation conditions, when a mixer was used as a signal detector.