The reduced intrinsic noise of high-Tc squids with slots and holes should make these devices a competitive technology for geophysical studies employing magnetotellurics -- the use of naturally occurring electromagnetic radiation to study the earth's crust. Magnetotellurics in combination with conventional seismic imaging enables geophysicists to locate potential earthquake hazards or petroleum and gas reservoirs despite the presence of salt, lava flows, or thick carbonate rock.

As for external magnetic noise, although it can be sufficiently reduced with the proper shielding, such shielding is expensive and its use is not always practical, i.e., during a medical operation in a hospital.

A team consisting again of Clarke, Dantsker and Tanaka, plus Oliver Froehlich and Konstantin Kouznetsov, may have found a solution. They have developed what they call an "asymmetric gradiometer." Consisting of a high-Tc superconducting flux transformer superimposed on a superconducting magnetometer, the new gradiometer helps its users detect and measure a magnetic field from a local source, such as the human heart or brain, amid a background of external magnetic noise.

"Detecting a magnetic signal amid background noise (without shielding) is like trying to have a conversation in a room full of loud people," says Dantsker, "but with a gradiometer, we become deaf to all the chatter and only hear what our source signal has to say."

The flux transformer component of this new gradiometer is a closed loop of YBCO film deposited on a 4-inch wafer -- the deposition was done by collaborators at Conductus, Inc. The closed YBCO loop is configured to have a small input loop at one end and a much larger pickup loop at the other. The distance between the two is called the baseline. This flux transformer is positioned with the input loop above and close to the magnetometer which is placed close to the source of the magnetic field being measured.

"Background magnetic noise will be nearly uniform over the baseline," says Dantsker. "This will generate a superconducting current in the flux transformer that precisely cancels the magnetic field that the background noise produces in the magnetometer."

Since the magnetometer and the input coil of the flux transformer are closer to the source of the magnetic field being measured than the flux transformer's pickup loop, the source signal is not uniform over the baseline and is clearly detected.

Dantsker says the new gradiometer is equally suited to single and multichannel magnetometers made from either high or low-temperature superconductors. Because of its simplicity and demonstrated performance, and its potential for low-cost manufacturing, its inventors think this gradiometer may well replace other gradiometers now on the market. It should be especially useful for helping physicians identify certain problem areas in the heart and brain.

Another major technological breakthrough achieved by Clarke's group has been the development of a high-Tc squid microscope for room temperature samples. The new microscope was constructed by a team that included, in addition to Clarke and Dantsker, Tom Lee, Yann Chemla, and Tim Shaw.

A high-Tc squid microscope bears no resemblance to what most people think when they hear the word "microscope." Inside a fiberglass vacuum chamber, a high-Tc squid is mounted on a sapphire rod that is clamped to a can of liquid nitrogen. Separating the squid from the world outside the vacuum chamber is a window of silicon nitride that is three-microns thick. This window is attached to a positioning mechanism which allows the gap between it and the squid to be adjusted. During operations, the entire microscope is placed inside shielding to keep out external magnetic noise.

The advantage of a high-Tc squid microscope over existing low-Tc squid microscopes is the ability to study samples that must be maintained at room temperature. Samples can be placed on the outside surface of the silicon nitride window or scanned across it to produce a magnetic image. In either mode, the sample is at all times outside the vacuum chamber. This arrangement is possible only with liquid nitrogen as the coolant, hence the need for a high-Tc squid.