71 research outputs found
Sensitivity and spatial resolution of square loop SQUID magnetometers
We calculate the flux threading the pick-up coil of a square SQUID
magnetometer in the presence of a current dipole source. The result reproduces
that of a circle coil magnetometer calculated by Wikswo with only small
differences. However it has a simpler form so that it is possible to derive
from it closed form expressions for the current dipole sensitivity and the
spatial resolution. The results are useful to assess the overall performance of
the device and to compare different designs
Investigation of current noise in underdamped Josephson devices by switching current measurements
AbstractExperimental measurements on critical current noise in underdamped niobium based Josephson devices 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 as a function of the time using the standard time of flight technique. In such a way it is possible to obtain the critical current fluctuations ΔIc=Ic(t)-<Ic(t)> and the relative standard deviations which corresponds to the root square of the current fluctuation power. Pointing at the white noise fluctuations (above few Hz) and taking into account the physical frequency of the device, it is possible to evaluate the power spectral density of the critical current. The analysis has involved high quality underdamped Josephson junctions having an area ranging from (4x4) μm2 to (40x40) μm2 in the temperature range from 4.2K to few tenth of mK. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits
NANO-SQUIDs based on niobium Dayem bridges for nanoscale applications
Abstract. We report on the design, the fabrication and the performance of an integrated
magnetic nano-sensor based on niobium dc-SQUID (Superconducting QUantum Interference
Device) for nanoscale applications is presented. The nano-sensors are based on nanometric
niobium constrictions (Dayem bridges) inserted in a square loop having a side length of 200
nm. Measurements of voltage-flux characteristic, flux to voltage transfer factor and noise
performances are reported. In small signal mode, the sensors have shown a magnetic flux noise
spectral density of 1.5 µΦ0/Hz1/2 corresponding to a spin sensitivity in unit of Bohr magneton
of 60 spin/Hz1/2. Supercurrent decay measurements of these devices are also reported. Such
measurements provide useful information for applications which employ the SQUID as a
trigger where the sensor works on the zero voltage state. 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 mΦ0. In view of the
nano-SQUID employments in the detection of small spin populations, the authors calculated
the spin sensitivity and the magnetic response relative to the single spin, as a function of its
position within the SQUID hole. The results show that the SQUID response depends strongly
on the spin position
Mutations in the SPAST gene causing hereditary spastic paraplegia arerelated to global topological alterations in brain functional networks
Aim: Our aim was to describe the rearrangements of the brain activity related to genetic mutations in the SPAST gene. Methods: Ten SPG4 patients and ten controls underwent a 5Â min resting state magnetoencephalography recording and neurological examination. A beamformer algorithm reconstructed the activity of 90 brain areas. The phase lag index was used to estimate synchrony between brain areas. The minimum spanning tree was used to estimate topological metrics such as the leaf fraction (a measure of network integration) and the degree divergence (a measure of the resilience of the network against pathological events). The betweenness centrality (a measure to estimate the centrality of the brain areas) was used to estimate the centrality of each brain area. Results: Our results showed topological rearrangements in the beta band. Specifically, the degree divergence was lower in patients as compared to controls and this parameter related to clinical disability. No differences appeared in leaf fraction nor in betweenness centrality. Conclusion: Mutations in the SPAST gene are related to a reorganization of the brain topology
Fine-tuning and optimization of superconducting quantum magnetic sensors by thermal annealing
In the present article, we present the experimental results concerning the fine-tuning and optimization of superconducting quantum interference device (SQUID) parameters by thermal annealing. This treatment allows for the modification of the parameters in order to meet a specific application or to adjust the device parameters to prevent the increase of magnetic field noise and work instability conditions due to a different critical current with respect to the design value. In particular, we report the sensor critical current, the voltage-flux (V-Phi) characteristics and the spectral density of the magnetic field of SQUID magnetometers for different annealing temperatures. The measurements demonstrate that it is possible to achieve a fine control of the most important device parameters. In particular, we show that thermal annealing allows for the reduction of SQUID noise by more than a factor of 5 and makes the device working operations very stable. These results are very useful in view of quantum technology applications related to superconducting quantum computing where the correct functioning of the quantum bit depends on the fine control of the superconducting quantum device parameters and selectable annealing is possible by using a suitable laser as a thermal source
Integrated SQUID sensors for low cross-talk multichannel systems
We present a fully integrated dc-SQUID magnetometer based on niobium technology including a new feedback coil design. In respect to a standard SQUID design, such a feedback-coil design was optimized in order to reduce the mutual inductance with the neighbours and to increase the coupling with the pick-up coil of the SQUID itself. In such a way, it is possible to reduce cross-talks due to both feedback coil and wires. Experimental results about the characterization of the device and the crosstalk measurements are reported. The measurements have been performed in liquid helium using a low noise readout electronics specifically designed for large multichannel SQUID based instrumentations. The experimental data have shown a substantial reduction of cross-talk between neighbouring sensors. © 2006 IOP Publishing Ltd
Vertical nano superconducting quantum interference device based on Josepshon tunnel nanojunctions for small spin cluster detection
The ultra high sensitivity exhibited by Superconducting Quantum Interference Device (SQUIDS) could be the key to explore new field of nanoscience such as the investigation of small cluster of elementary magnetic moments. In this paper, an ultra high sensitive niobium nanoSQUID based on submicron Josephson tunnel junction is presented, It has been fabricated in a vertical configuration by using a threedimensional focused ion beam sculpting technique. In such a configuration, the nanosensor loop (area of 0.25 mu m(2)) is perpendicular to the substrate plane allowing to drastically reduce the spurious effects of the external magnetic field employed to excite the nano-objects under investigation. Main device characteristics have been measured at T=4.2 K by using a low noise readout electronics. Due to high voltage responsivity, the nanosensor has exhibited a spectral density of the magnetic flux noise as low as 1.6 mu Phi(0)/Hz(1/2)
- …