Xiangyan Kong

High intrinsic noise and absence of hysteresis in superconducting quantum interference devices with large Steward-McCumber parameter

We investigated niobium thin film superconducting quantum interference devices (SQUIDs) with large Steward-McCumber parameter (βc > 1). No hysteresis was observed in the current-voltage (I-V) characteristics of the SQUIDs, even for βc ≈ 17. We attribute the absence of hysteresis to an excess voltage noise of the junctions which increases the SQUID intrinsic noise δΦs. It can be represented by an effective noise temperature T* of the SQUID which is higher than the bath temperature T. We simulated SQUID I-V characteristics using the measured device parameters and confirmed the absence of hysteresis.

Study of weakly damped superconducting quantum interference devices operated in different bias modes in presence of external shunt resistance

We experimentally studied weakly damped superconducting quantum interference devices (SQUIDs) shunted by an external resistor Rs and operated in either current- or voltage-bias mode. The SQUID parameters, such as the flux-to-voltage transfer coefficient ∂V/∂Φ and the dynamic resistance Rd, are reduced due to Rs, while the SQUID intrinsic noise remains unchanged. The reduced parameters can be enhanced again by using voltage feedback circuitry. Furthermore, Rs can be used to damp the SQUID in order to avoid the appearance of hysteresis or oscillation in SQUID characteristics. SQUID shunted by small Rs is always operated in mixed-bias mode.

Magnetic microscopy based on high-Tc SQUIDs for room temperature samples

The SQUID microscope is the most suitable instrument for imaging magnetic fields above sample surfaces if one is mainly interested in field sensitivity. In this paper, both the magnetic moment sensitivity and spatial resolution of the SQUID microscope are analysed with a simple point moment model. The result shows that the ratio of SQUID sensor size to sensor–sample distance effectively influences the sensitivity and spatial resolution. In comparison with some experimental results of magnetic images for room temperature samples from our high-Tc SQUID microscope in an unshielded environment, a brief discussion for further improvement is presented.

A novel quadruple excitation in high-Tc SQUID-based non-destructive evaluation

A high-Tc SQUID-based non-destructive evaluation (NDE) system has been set up in our laboratory. The SQUID was made on a 24° bicystal SrTiO3 substrate. A novel quadruple excitation coil was proposed for the first time and applied in the artificial holes in the aluminium multilayer structure in a noisy unshielded environment. The experimental data shows that it has good balance and is very effective at detecting small hole defects.

Analysis of a dc SQUID readout scheme with voltage feedback circuit and low-noise preamplifier

We analyzed the dc SQUID with voltage feedback circuit (VFC) and a low-noise roomtemperature preamplifier to evaluate the feasibility of a low-noise SQUID direct-coupled readout scheme (DRS), possibly eliminating the need for a two-stage scheme employing a SQUID preamplifier. The passive VFC, connected in parallel to the SQUID, consists of a resistor Rs in series with an inductor Ls. This inductor is coupled to the SQUID by a mutual inductance Ms .T he purpose of the VFC is to increase the SQUID’s flux-to-voltage transfer coefficient ∂V/∂Φ ,t hus reducing the preamplifier noise contribution δΦpreamp. However, at the same time, VFC introduces the thermal noise of Rs, δΦR, which may not be negligible. Generally, the noise of the readout scheme, δΦreadout, may thus include both δΦpreamp and δΦR ,i .e.,δΦreadout = δΦpreamp + δΦR . To characterize the SQUID operation with VFC we introduced two dimensionless parameters, r=Rs/Rd and Δ=(Ms/Mdyn)�(Rs/Rd), where Rd and Mdyn=1/(∂i/∂Φ) are dynamic properties of the SQUID itself. For assumed intrinsic SQUID parameters, we then numerically analyzed the dependence of δΦreadout noise components on r and Δ to determine their suitable ranges and the minimum of δΦreadout. To verify our analysis, we experimentally characterized, in liquid helium, three niobium SQUIDs with VFC, having suitably chosen r and Δ. The measured SQUID system flux noise was on the order of 1 μΦ0/√Hz, comparable to the intrinsic noise of the SQUID itself. The deduced equivalent voltage noise was comparable to that of a SQUID preamplifier in the twostage readout. Simple single-stage ultra-low-noise SQUID DRS readout was thus demonstrated.

Dependence of SQUID Intrinsic Flux Noise on Stewart–McCumber Parameter

It is known that three superconducting quantum interference device (SQUID) parameters, namely, the intrinsic flux noise δΦ_s, the flux-to-voltage transfer coefficient ∂V/∂Φ, and the dynamic resistance R_d, increase with increasing the Stewart- McCumber parameter β_c of Josephson junction. It is very difficult to obtain the low values of δΦ_s for a strongly damped SQUID due to its small ∂V/∂Φ. In this paper, we employ a two-stage scheme (TSS) to measure δΦ_s directly. Here, we use single chip readout electronics and a weakly damped SQUID with a β_c of flux noise of the readout-SQUID system is about 4.8 μΦ₀/√Hz 1.2, both acting as a readout-SQUID system (a preamplifier). The with a corner frequency of about 10 Hz. The front-end SQUID is coupled to the readout SQUID via the input coil with a mutual inductance of M_i. In our experiment, the flux gain for a readout SQUID, i.e., G_Φ = M_i × (∂I/∂Φ)_front-end is adjusted to be 20. Using the TSS, we measured δΦ_s with different β_c changing from 0.3 to 13.5 and found δΦ_s increasing from 1.0 to 5.0 μΦ₀/√Hz. We also discussed the relations between L_s, β_c, and δΦ_s quantitatively. Finally, we proposed designing a suitable β_c to match the preamplifier in a direct readout scheme to simplify a SQUID measurement system.

\beta_{c}

In a superconducting quantum interference device (SQUID) probe microscope, the existence of a needle with high permeability as a flux guide can increase the spatial resolution. Also, the flux guide makes it possible to measure the samples in air at room temperature. Using a finite element method (FEM), we analyzed the property of a magnetic shielding cylinder with high permeability Permalloy around the needle. Simulation results show that a cylindrical shield around the flux guide can increase the spatial resolution of the SQUID probe microscope. Also, a model of a small SQUID has been designed and the calculated result shows that the spatial resolution is higher than a SQUID with the flux guide and the shield around it.

of Josephson Junction

Noise matching between superconducting quantum interference device (SQUID) sensor and readout electronics has always been an important problem. Conventionally, in order to avoid the hysteresis, SQUIDs should be operated in the range of β_c <; 1, where β_c is the Stewart-McCumber parameter. Recently, Liu et al. extended the SQUID operating area to βc > 1 due to a large noise parameter Γ*. When β_c > 1, SQUID exhibits a large ∂V/∂Φ, which is beneficial in reducing the noise contribution of the readout electronics, δΦ_e. In this work, we compare the SQUID system noise δΦ_s measured with four different readout electronics. Here, the four readout electronics employed are as follows: 1) the flux modulation scheme (FMS); 2) the direct readout scheme (DRS), which uses six parallel connected bipolar transistor as its preamplifier (DRS-PCBT); 3) the DRS, which uses a single preamplifier AD797 (DRS-AD797); and 4) the two-stage scheme. The preamplifier voltage noise of DRS-PCBT and DRS-AD797 are about 0.4 nV/√Hz and 0.9 nV/√Hz, respectively. For a two-stage scheme, the equivalent flux noise reaches about 0.25 μΦ₀/√Hz, the measured SQUID intrinsic noise, δΦ_i, will dominate δΦ_s. In other readout schemes, however, δΦ_s depends on the value of β_c. Finally, we found that FMS is suitable to match SQUIDs with β_c <; 1. DRS-PCBT is a good choice for SQUIDs with 1 <; β_c <; 3 whereas DRS-AD797 suffices for SQUIDs with β_c > 3.

A magnetic shield around the flux guide in a SQUID probe microscope with finite element simulation

In a multichannel magnetocardiography system, the problem of crosstalk is serious due to mutual inductances between the pickup coils of adjacent channels. In this paper, a superconducting quantum interference device gradiometer module based on weakly damped Josephson junctions with large βc (≈5) is presented. An external feedback design is also adopted to suppress the crosstalk between different channels. With a special superconducting connection between input coil and pickup coil, an axial first-order gradiometer with baseline of 70 mm and diameter of 18 mm is fabricated. Using a direct readout circuit with equivalent flux noise of 1.8 μΦ0/√Hz, the flux noise of this gradiometer is measured to be 6.5 μΦ0/√Hz in a white noise range, corresponding to a field sensitivity of 0.5 fT/(cm · √Hz). The crosstalk rates in external feedback mode and internal feedback mode are both successfully simulated with MAXWELL software. The results by experimental measurements using these gradiometers are in coincidence with the simulated ones; both simulation and experimental results show a substantial suppression of crosstalk in the external feedback design.

Study on Noise Matching Between SQUID Sensor and Its Readout Electronics

For a practical superconducting quantum interference device (SQUID) based measurement system, the Tesla/volt coefficient must be accurately calibrated. In this paper, we propose a highly efficient method of calibrating a SQUID magnetometer system using three orthogonal Helmholtz coils. The Tesla/volt coefficient is regarded as the magnitude of a vector pointing to the normal direction of the pickup coil. By applying magnetic fields through a three-dimensional Helmholtz coil, the Tesla/volt coefficient can be directly calculated from magnetometer responses to the three orthogonally applied magnetic fields. Calibration with alternating current (AC) field is normally used for better signal-to-noise ratio in noisy urban environments and the results are compared with the direct current (DC) calibration to avoid possible effects due to eddy current. In our experiment, a calibration relative error of about 6.89 × 10−4 is obtained, and the error is mainly caused by the non-orthogonality of three axes of the Helmholtz coils. The method does not need precise alignment of the magnetometer inside the Helmholtz coil. It can be used for the multichannel magnetometer system calibration effectively and accurately.