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Abstract: Magnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterization are presented. The SQUIDs consists of two Dayem bridges of 90nm x 250nm and loop area of 4, 1 and 0.55μm2. The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Current-voltage characteristics, critical current as a function of the external magnetic field and switching current distributions were performed at liquid helium temperature. A critical current modulation of about 20% and a current-magnetic flux transfer coefficient (responsivity) of 30μA/Φ0 have been obtained, resulting in a magnetic flux resolution better than 1 mΦ0. In order to show the effectiveness of sensor for nanomagnetism applications, we performed measurements with and without magnetic nanoparticles on the SQUID loop applying a magnetic field parallel to the SQUID plane. In this configuration the magnetic flux coupled to the SQUID is mainly due to the presence of magnetic nanoparticles. The magnetic nanoparticles can be easily detected and their response to magnetic field studied. Measurements has been performed on Fe3O4 nanoparticles prepared by thermal decomposition method with a nominal particle size of 8nm. Some examples of magnetization measurements were recorded at low temperature after Zero Field Cooling. [Copyright &y& Elsevier]

Abstract: The design of the reported current sensor is based on a suitable superconducting intermediary (matching) magnetic flux transformer magnetically coupled to a niobium based dc-SQUID (Superconducting Quantum Interference Device). The 60 square niobium turns (20μm width) signal coil is tightly coupled to the matching flux transformer consisting of a square single turn primary coil connected in series with a multiturn secondary coil. The obtained signal current to magnetic flux transfer factor (current sensitivity) is equal to 62 nA/Φ0 measured by using a current sensing noise thermometer technique. The sensor has been characterized in liquid helium by using a direct coupling low noise readout electronic and the flux locked loop configuration. Despite the circuit complexity, the sensor has exhibited a smooth and free resonance voltage-flux characteristic ensuring a stable working operation. Considering a SQUID magnetic flux noise √SΦ =1.8μΦ0/√Hz at T=4.2K, a current noise as low as 110 fA/√Hz has been obtained. Due to his high performance such sensor can be employed in all application requiring an extremely current sensitivity, like the readout of the gravitational wave detectors and the current sensing noise thermometry. [Copyright &y& Elsevier]

An experimental investigation of the critical current noise in underdamped niobium based Josephson devices [junctions and Superconducting Quantum Interference Device (SQUID)] by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching using the standard time of flight technique and analyze the data to extract the current noise. Measurements on Josephson junctions having an area ranging from (4 × 4) to (40 × 40) μm2 in the temperature range from 4.2 to 1.2 K are reported. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. These measurements provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits. [ABSTRACT FROM AUTHOR]

An integrated ultrahigh sensitive current amplifier based on a niobium dc superconducting quantum interference device (SQUID) has been developed. The sensor design is based on a multiturn signal coil coupled to a suitable SQUID magnetometer. The signal coil consists of 60 square niobium turns tightly coupled to a superconducting flux transformer of a SQUID magnetometer. The primary coil (pick-up coil) of the flux transformer has been suitably designed in order to accommodate the multiturn input coil. It has a side length of 10 mm and a width of 2.4 mm. In such a way we have obtained a signal current to magnetic flux transfer coefficient (current sensitivity) as low as 62 nA/[uppercase_phi_synonym]0. The sensor has been characterized in liquid helium by using a direct coupling low noise readout electronic and a standard modulated electronic in flux locked loop configuration for the noise measurements. Beside the circuit complexity, the sensor has exhibited a smooth and free resonance voltage-flux characteristic guaranteeing a reliable and a stable working operation. Considering a SQUID magnetic flux noise of S[uppercase_phi_synonym]1/2 = 1.8 μ[uppercase_phi_synonym]0/Hz1/2 at T = 4.2 K, a current noise as low as 110 fA/Hz1/2 is obtained. Such a value is about a factor two less than the noise of other SQUIDs of the same category. As an application, Nyquist noise measurements of integrated test resistors using the current sensing noise thermometer technique are reported. Due to its high performance such a sensor can be employed in all applications requiring an extremely current sensitivity like the readout of the gravitational wave detectors and the current sensing noise thermometry. [ABSTRACT FROM AUTHOR]

Supercurrent decay measurements of nano-superconducting quantum interference devices (SQUIDs) based on niobium constrictions (Dayem bridges) are reported. Such measurements provide useful information for applications that employ the SQUID as a trigger where the sensor works on the zero voltage state. The nanodevice consists of a niobium thin film (30 nm thick) ring with a hole side length of 200 nm including two Dayem bridges of 120×200 nm2. The measurements of the switching current distribution from the zero voltage state and the related escape rate, as function of the bias current, have been performed by a low noise fly time technique. The experimental data have shown an intrinsic current fluctuation less than 0.2% of the critical current at liquid helium temperature, corresponding to an intrinsic sensor magnetic flux resolution of a few m[uppercase_phi_synonym]0. The theoretical predictions based on the thermal escape process theory in the moderately damping limit are in a reasonable agreement with the experimental data. [ABSTRACT FROM AUTHOR]

A miniaturized niobium based dc superconducting quantum interference device (SQUID) magnetometer for high magnetic field sensitivity applications has been developed. The sensing coil consists of an integrated square superconducting coil with a length of 3 mm, involving a device area much smaller with respect to the standard SQUID magnetometers with a comparable magnetic field sensitivity; so it allows increasing the spatial resolution keeping the magnetic field sensitivity unaltered. Furthermore, a small pickup coil minimizes its antenna gain, reducing the radio frequency interference. At T=4.2 K, the sensors have shown smooth and resonance free V-[uppercase_phi_synonym] characteristics and an intrinsic white magnetic field noise spectral density as low as 5.8 fT/Hz1/2, measured in flux locked loop configuration. The good agreement with the theoretical predictions guarantees the reliability and the controllability of the sensors. Due to their compactness and good characteristic parameters, such sensors are suitable for large multichannel systems used in biomagnetic imaging. [ABSTRACT FROM AUTHOR]

A fully integrated dc-SQUID (superconducting quantum interference device) magnetometer based on niobium technology including a new feedback coil design is presented. With respect to a standard SQUID design, the new feedback coil design was optimized in order to reduce the mutual inductance between neighbors and to increase the coupling with the pickup coil of the SQUID itself. In such a way, it is possible to reduce cross talk due to both the feedback coil and wires. Experimental results for the characterization of the device and the cross talk measurements are reported. The measurements have been performed in liquid helium using a low noise readout electronics specifically designed for large multichannel SQUID-based instruments. The experimental data show a substantial reduction of cross talk between neighboring sensors with respect to a traditional feedback coil. Furthermore, the new feedback coil system does not introduce any noise degradation. [ABSTRACT FROM AUTHOR]

We present niobium-based Josephson devices as prototypes for applications to quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer for a fine tuning of flux qubit and interquibit coupling. These devices allow a control of the off-diagonal Hamiltonian terms of flux qubit and of the flux transfer function of a superconducting transformer. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control and precision by a small transversal magnetic field parallel to the radio-frequency superconducting loop plane and to the flux transformer. [ABSTRACT FROM AUTHOR]

The attainment of high-quality Josephson junctions on electro-optical substrates constitutes a key step toward the fabrication of functional superconducting opto-electronics. We report on the fabrication of high-quality all-refractory niobium superconducting tunnel junctions deposited on X-cut LiNbO3 single crystals. A high quality factor Vm=0.7 ΔIgR2 mV (where ΔIg is the current jump at the sum gap voltage and R2 mV the static resistance at V=2 mV) up to 61 mV has been measured at T=4.2 K. The very low contribution of subgap leakage currents is demonstrated by the BCS-like quasiparticle current measured at V=0.1 mV down to T=1.9 K. [ABSTRACT FROM AUTHOR]

We present Nb/AlO[sub x]/Nb–Al/AlO[sub x]/Nb stacked Josephson junctions which we propose as three-terminal elements to control the quantum behavior of Josephson complex systems. A bias current can be independently injected in the “top” junction, providing a fine control of the critical current of the “bottom” junction. The reported characterization of the device refers to data in the classical limit including measurements of the switching dynamics between metastable states of the system at different temperatures and bias conditions. At low temperature, the effective dissipation of the bottom junction remains substantially unchanged when different injection currents flow in the top junction. These results are interesting to project new configurations of macroscopic quantum experiments. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]