The superconductivity, in which the electrical resistance becomes zero at extremely low temperatures, was discovered in 1911 by Kamerlingh Onnes of Leiden University in the Netherlands, who first succeeded in liquefying helium (-269 ℃ (4.2K)). Since then, it has become clear that a large number of materials show superconductivity at extremely low temperatures. Then, in 1987, Chu et al. Of the University of Houston discovered an yttrium-based cuprate superconductor that becomes superconducting at liquid nitrogen temperature (-196 ℃(77K)). For convenience, materials that become superconducting at liquid nitrogen temperature are called high-temperature superconductors (HTS), and superconductors that require liquid helium cooling are called low-temperature superconductors (LTS). We develop and provide high-performance magnetic sensors that utilize the superconducting phenomenon. It is called a superconducting QUantum Interference Device (SQUID) and is a high-sensitivity magnetic sensor. Our HTS-SQUID, which uses yttrium-based cuprate superconductors, can be cooled with liquid nitrogen, which is inexpensive and easy to handle, so it can be actively used outdoors for resource exploration and non-destructive inspection.

In the superconducting state, the electrical resistance becomes completely zero, and current flows even when the voltage is zero. This current is called superconducting current. Electrons (e-) are flowing in the normal current that has electrical resistance and requires voltage, but a pair of electrons (2e-) called a Cooper pair, are flowing in the superconducting current. All Cooper pairs are in the same energy state and have the property (phase) of a highly coherent wave. When the superconductor is made into a ring shape (closed loop), the circular superconducting current is subject to the condition that the phase matches when it makes one round. This is because if the phases do not match, the current will disappear and it will not exist. For this reason, only discontinuous values are allowed. The magnetic flux corresponding to the circular current becomes a discontinuous value, which is called magnetic flux quantization (Fig. 1). Specifically, inside all superconductors, the magnetic flux is quantized to an integral multiple of the magnetic flux quantum Φ₀ (2.07x10^-15Wb).

　The SQUID magnetic sensor has a structure in which a ring-shaped superconductor is separated by two Josephson junctions (JJ) as shown in Fig.2. The Josephson junction has a structure in which two superconductors are weakly connected, and only a small amount of superconducting current (about several tens of μA) called Josephson current flows. Phase shifts are allowed at the Josephson junction, and the external magnetic flux (Φ_ex) can take continuous values within the SQUID ring. However, the phase difference of the superconducting current at the Josephson junction changes in response to the magnetic flux interlinking in the ring. If the phase difference is large, the Josephson current decreases, and the phase difference changes with the period of the magnetic flux quantum Φ₀.

When a constant bias current (I_b) slightly larger than the maximum Josephson current of SQUID is applied and the voltage (V_SQUID) generated in SQUID is measured with respect to an external magnetic field, the voltage-magnetic flux characteristics shown on the right can be obtained. When the external magnetic flux is zero, the phase difference is the smallest and the generated voltage is low. As the external magnetic flux increases, the phase difference increases, and the Josephson current that flows at zero voltage decreases, so the voltage applied to SQUID increases. When the phase difference becomes 180 ° or more, the Josephson current ratio starts to increase as the next cycle approaches, and the voltage applied to SQUID decreases. As a result, the voltage output changes periodically (sine wave) with Φ₀ as the period (Fig. 3). SQUID functions as a non-linear magnetic flux-voltage conversion device.

Since the SQUID has a non-linear output that changes periodically, the input signal (detected signal) cannot be extracted as it is. Therefore, using a control circuit called FLL (Flux Locked Loop) circuit, a magnetic field in the opposite direction is input to the SQUID so as to cancel the input signal. Specifically, as shown in the figure on the right, the operating point is the point where the output is 0V. This voltage is used to pass a current through the feedback coil so that a magnetic field that cancels the magnetic field is applied to SQUID. During the feedback operation, the relationship of external magnetic field equal to feedback magnetic field is established, so it is possible to obtain an output signal that is linear with respect to the input magnetic field by reading the current of the feedback coil. If sudden magnetic field changes occur, feedback will be delay and the operating point cannot be maintained. The maximum amount of change in the magnetic field per unit time that feedback control can follow is called the slew rate. The higher the slew rate, the higher the magnetic field resistance to disturbance, and stable operation is possible even under severe conditions. Our SQUID has the highest record of 10mT / s in HTS-SQUID.