Research Results – Sensorics and Spectrometry
Sensorics and Spectrometry: High-Tc Quantum Interferometer Sensors for Magnetic Microscopy of Microstructures and Electric Circuits
The focus of our recent work has been the development of a scanning magnetic microscope based on high-Tc superconducting quantum interferometers (SQUIDs) with a ferromagnetic antenna. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid nitrogen-cooled SQUID-sensor and back again. The flux guide passes through the pick-up loop of the high-Tc SQUID providing an improved coupling of the object’s magnetic flux to the SQUID. The device measures the z-component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in x-y direction by a motorized computer controlled scanning stage. A lateral resolution of better than 10 µm was achieved. Fields of wires carrying currents of less than 1 µA could be detected with a time constant of 1 sec. An increase in the flux noise of the SQUID due to the presence of the soft magnetic foil was not essential.
Sensorics and Spectrometry: High-Tc Quantum Interferometer Sensors for Geomagnetic Surveys
The most often used high-Tc flip-chip magnetometers have a pick-up loop size of about 8 mm x 8 mm, a magnetic field sensitivity 1 nT/F0 and magnetic field resolution down to about 15 fT/sqrt(Hz) at 77 K. These magnetometers are able to operate outside magnetic shielding, which is essential for geomagnetic surveys. Due to their relatively small size they can be integrated into a compact mobile vector magnetometer or tensor systems. Cooling with liquid nitrogen has significant advantages compared to the low-Tc systems; the system’s signal-to-noise ratio and slew rates are significantly lower than those of room temperature systems. Another advantage is that the supply of liquid helium is a serious problem, especially in remote areas, whereas liquid nitrogen can be readily obtained simply from the air.
Sensorics and Spectrometry: High-Tc Quantum Interferometer Sensors for Biomagnetic Measurements
The expected increase in the price of helium is set to seriously impact on operation costs associated with magnetoencephalography (MEG) which currently relies on low-Tc (LTc) SQUIDs that are cooled exclusively by liquid helium. High-Tc (HTc) SQUIDs are cooled by cheap, easy-to-handle and readily available liquid nitrogen, and can be placed much nearer to magnetic sources than LTc SQUIDs. The problem lies in producing HTc SQUIDs with a magnetic field resolution better than 10 fT/sqrt(Hz) at 77 K, required for their application in MEG systems. We have developed 16-mm HTc SQUID magnetometers possessing a magnetic field resolution » 4 fT/sqrt(Hz) at 77 K and, together with INM-4, have demonstrated MEG measurements with HTc SQUIDs compared to those obtained using a commercial LTc MEG system. The low intrinsic noise and closer positioning of the HTc SQUIDs to the cortex contributed to the high signal-to-noise ratio of the MEG data obtained using the HTc system. These results open up new ways to upgrade MEG systems using HTc SQUIDs, which would make these systems independent of helium, more user-friendly and would save on a large part of the operating costs leading, in turn, to the widespread utilization of MEG systems.
Sensorics and Spectrometry: High-Tc Quantum Interferometer Sensors for Non-destructive Evaluation
We have developed an HTc DC SQUID gradiometric measurement system for routine applications in non-destructive evaluations. Low values of the white flux noise and white field gradient noise of about 15 fT/cm.sqrt(Hz) were measured for gradiometers operating at 77 K. Low-frequency noise was suppressed using an ac-bias technique, even in a magnetically unshielded environment. First order planar DC SQUID flip-chip gradiometers were used in the measurement system to ensure stable operation under electromagnetically noisy conditions. A synchronous filter was employed to remove all harmonic components of parasitic line-frequency interference. This improved the resolution of the system in a typical laboratory or industrial environment, showing strong magnetic gradients of the line-frequency signal due to nearby metallic constructions. Test scans of contaminations with magnetic microparticles were performed.
Sensorics and Spectrometry: Liquid Identification using Hilbert Spectrometry
The fast and reliable identification of liquids is of great importance in the fields of security, biology and in the beverage industry. An unambiguous identification of liquids could be made by taking electromagnetic measurements of their dielectric functions in the frequency range of their main dispersions, but this frequency range, ranging from a few GHz to a few THz, has not yet been covered by any conventional spectroscopy.
We have developed a concept of liquid identification, based on our new Hilbert spectroscopy and high-Tc Josephson junctions, which can operate at the intermediate range from microwaves to terahertz frequencies. The first proof-of-principle measurements of reflection spectra from various bottled liquids in the range 10- 300 GHz have already been demonstrated with a total scanning time of 1 second.
Sensorics and Spectrometry: Terahertz Losses, 1/f Noise and Electrical Transport in -tilt YBa2Cu3O7-x Bicrystal Junctions with High IcRn Product
Our studies focus on electrical transport and 1/f noise of -tilt bicrystal YBa2Cu3O7-x junctions before and after annealing in oxygen. Annealing in atomic oxygen resulted in a two-fold decrease of the resistance Rn, more than three-fold decrease of the spectral density of the 1/f noise, eight-fold decrease of the THz losses and the same IcRn values. The resistance and critical current 1/f fluctuations in -tilt junctions were found to be completely antiphase-correlated and their normalised intensities were equal, i.e. quasiparticles and Cooper pairs tunnel directly through the same parts of the barrier. An agreement of the I-V curves with those of the RSJ model, with an accuracy of better than 1% and IcRn-values up to 3 mV, was found for the annealed junctions at temperatures of up to 50 K, thus validating their use for THz Hilbert and admittance spectroscopies.
Sensorics and Spectrometry: Thz JOSEPHSON Spectroscopy of Optical Phonons in -tilt YBa2Cu3O7-x Bicrystal Junctions
Terahertz losses in -tilt YBa2Cu3O7-x bicrystal junctions at low temperatures were studied using admittance Josephson spectroscopy. The absorption of Josephson radiation by optical phonon modes in YBa2Cu3O7-x was found to be reflected in the I-V curve of the -tilt junctions. The most prominent feature in the dI/dV vs. V curves lies where the voltage V =~ 9.5 mV, which gives the corresponding Josephson frequency of 152 cm-1 in a good agreement with the frequency of the strongest IR active optical phonon mode in YBa2Cu3O7-x, polarised along the c-axis. Assignment of the observed localised features in the dI/dV vs. V curves is discussed according to available lattice dynamic calculations and experimental data for a dynamic conduction σ (f) of YBa2Cu3O7-x.