Volkmar Schultze

High-T/sub c/ superconducting quantum interference filters for sensitive magnetometers

We present several kinds of Superconducting Quantum Interference Filters (SQIFs) which are all realized with high-T/sub c/ superconductors. All SQIFs use the same configuration of 30 loops of different size. The properties of these SQIF types - serial arrays, parallel arrays, and a combination of both - are discussed concerning their usefulness for magnetometry. These properties are the formation of the desired single voltage peak, its peak voltage and full width at half maximum, and the magnetic field noise. Concerning all parameters an improvement can be achieved with SQIFs of all types compared to a single SQUID.

Microfabricated atomic vapor cell arrays for magnetic field measurements

We describe a method for charging atomic vapor cells with cesium and buffer gas. By this, it is possible to adjust the buffer gas pressure in the cells with good accuracy. Furthermore, we present a new design of microfabricated vapor cell arrays, which combine silicon wafer based microfabrication and ultrasonic machining to achieve the arrays of thermally separated cells with 50 mm(3) volume. With cells fabricated in the outlined way, intrinsic magnetic field sensitivities down to 300 fT∕Hz(1∕2) are reached.

Characteristics and performance of an intensity-modulated optically pumped magnetometer in comparison to the classical M x magnetometer

We compare the performance of two methods for the synchronization of the atomic spins in optically pumped magnetometers: intensity modulation of the pump light and the classical M(x) method using B(1) field modulation. Both techniques use the same set-up and measure the resulting features of the light after passing a micro-fabricated Cs cell. The intensity-modulated pumping shows several advantages: better noise-limited magnetic field sensitivity, misalignment between pumping and spin synchronization is excluded, and magnetometer arrays without any cross-talk can be easily set up.

Investigation of multiple SQUID arrangements in single layer high T/sub c/ magnetometers

Single layer high T/sub c/ dc SQUID sensors on bicrystal substrates were investigated. Special attention was paid to achieve a large flux-to-voltage transfer function in order to ensure stable operation of the SQUID electronics and to get low flux noise even in disturbed environment. Josephson junctions on 30/spl deg/ bicrystal were tested as well as sub-/spl mu/m junctions an 24/spl deg/ bicrystals. The steep edges achieved by sub-/spl mu/m patterning increased the resistance of the superconducting structures against large external fields. The design for large flux-to-voltage transfer functions focused on multiple de SQUID arrangements. For magnetometers with a directly coupled pickup-loop three junction SQUIDs show no advantage compared to standard two junction SQUIDs, but the series connection of two ordinary dc SQUIDs does. Although for magnetometers with inductively coupled pickup-loop twenty SQUIDs in series were used the effect on the flux-to-voltage transfer coefficient was poor, due to differences in the critical current and inductance of individual SQUIDs. The sensitivity of the whole magnetometer achieved with inductively coupled single layer pickup-loops is much smaller than with directly coupled ones. For the latter a pickup-loop formed as a conventional square washer shows better results compared to a slotted square.

Bicrystal submicrometer Josephson junctions and dc SQUIDs

Submicrometer Josephson junctions and dc SQUIDs with such junctions have been prepared on symmetrical 24/spl deg/ bicrystal substrates and were electrically characterized. The submicrometer structures are patterned using e-beam lithography and a C/Ti/e-beam resist mask system. Junctions with dimensions down to about 350 nm show no degradation of their superconducting properties at 77 K, when patterned at low temperatures. Series connections of directly coupled SQUIDs with large (110-160 pH) coupling-inductance, prepared with 0.5 /spl mu/m wide junctions show flux-to-voltage transfer function values up to 100 /spl mu/V//spl Phi//sub 0/. For these SQUIDs, typical white noise levels of 10 /spl mu//spl Phi//sub 0///spl radic/Hz are measured at 77 K.

A LTS-SQUID System for Archaeological Prospection and Its Practical Test in Peru

We present a new geomagnetic field measurement system for the detection of archaeological signatures in the soil. The system provides a unique fast mapping of large areas with high magnetic field gradient resolution as well as lateral precision. The data acquired by the device are geographically referenced and suitable for embedding in a geographic information system (GIS).

Archaeometric prospection with high-T/sub c/ SQUID gradiometers

Mapping of the Earths magnetic field or its gradient is a widely used method in archaeological prospection. The use of SQUIDs promises to be advantageous for archaeometry, since they combine a high field resolution with a large bandwidth. Compared to conventional Cs vapor sensors SQUIDs can be used for much faster magnetic mapping, allowing, for the first time, the investigation of huge archaeological features in a reasonable time period. We have investigated several SQUID systems for their usability in archaeometry by measuring a Neolithic double ring ditch enclosure. We have used two electronic High Temperature Superconductor SQUID (HTS SQUID) gradiometers with base lengths of about 60 cm and a Low Temperature Superconductor SQUID (LTS SQUID) gradiometer with a base length of 4 cm. Their intrinsic magnetic field resolution was 6 pT/m//spl radic/Hz for the HTS SQUID gradiometers and about 0.1 pT/m//spl radic/Hz for the LTS SQUID gradiometer. In contrast to Cs vapor gradiometers, which measure the gradient of the total magnetic field, SQUID gradiometers measure one component of the gradient tensor. Since measurements have to be performed whilst movement in the background of the Earths magnetic field, balancing is the limiting factor for the magnetic gradient field resolution of vector gradiometers.

SQUID gradiometers for archaeometry

The mapping of the Earths magnetic field or field gradient is a proven method in surface exploration and archaeometry. Caesium vapour magnetometers show the best magnetic field resolution of commercial devices, but their sampling frequency is limited to 10 Hz. Using SQUIDs it is possible to achieve the same or even better magnetic field resolution with a sampling frequency as high as 100 Hz or more. This allows significantly shorter acquisition times, which is essential for the mapping of large objects. In this paper we check the performance of our developed systems on a neolithic double-ring ditch enclosure near Weimar, Germany. We compare mappings of this area using an electronic caesium gradiometer, an electronic HTS SQUID gradiometer and an integrated planar LTS SQUID gradiometer. With all three systems the magnetic pattern of the ditch is visible; however, the electronic HTS gradiometer shows disturbances of the same order of magnitude as the gradient signal of the ditch, due to an insufficient common mode rejection whilst being moved. The planar LTS SQUID gradiometer shows superior performance. Its mapping shows a much better contrast and features that are not visible in the mapping of the caesium gradiometer.

HTS dc SQUID systems for geophysical prospection

We describe the use of high T/sub c/ flip-chip dc SQUID magnetometers in systems for several geophysical prospection methods. For use in the Transient Electro Magnetics (TEM) method, a one-channel SQUID magnetometer system (vertical field component) was adapted to rough conditions in the field. We report on measurements in South Africa over a geophysically interesting target in real production mode. Transients up to 200 ms could be recorded, which is about a factor of 100 more than with conventional coils. For investigation of the magnetic field gradient a two-channel system, built as a first-order electronic gradiometer with a baseline up to 1 m, was designed and manufactured. It can be used, e.g., for surface exploration or for archeometry. The system can be moved in the Earths magnetic field. We achieve a common mode rejection of 100. The reasons for the imbalance and the possibility of further improvement of the system are discussed.

Highly balanced single-layer high-temperature superconductor SQUID gradiometer freely movable within the Earth's magnetic field

We developed a gradiometer system based on a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID), which can be freely moved within the Earths magnetic field during measurement. The problem of circumferential shielding currents in the parallel gradiometer pick-up loop is solved by the use of an appropriately designed magnetometer SQUID integrated on the gradiometer chip. The magnetometers feedback coil of the flux-locked loop is laid out as a small Helmholtz coil pair, thus keeping the homogeneous magnetic field constant for both the magnetometer and the gradiometer. Therefore, the balance of the directly coupled gradiometer SQUID is enhanced from 100 up to 3800. The noise limited magnetic field gradient resolution of 45 pT m−1 Hz−1/2 is preserved down to frequencies of several Hz even after strong motion in the Earths magnetic field.