1. Complete conductivity
Complete conductivity, also known as the zero resistance effect, refers to the phenomenon in which the resistance suddenly disappears when the temperature drops below a certain temperature.
Full conductivity applies to direct current, and superconductors experience alternating current losses in the presence of alternating currents or alternating magnetic fields, and the higher the frequency, the greater the losses. AC loss is an important problem that needs to be solved in the practical application of superconductors. On the macroscopic level, the AC loss is caused by the difference between the induced electric field and the induced current density generated inside the superconducting material; on the microscopic level, the AC loss is caused by the quantized magnetic flux line sticking. caused by stagnant motion. AC loss is an important parameter to characterize the performance of superconducting materials. If the AC loss can be reduced, the cooling cost of the superconducting device can be reduced and the operation stability can be improved.
2. Completely diamagnetic
Complete diamagnetism is also known as the Meissner effect. "Diamagnetism" refers to the phenomenon that the magnetic field lines cannot pass through the superconductor and the magnetic field inside the superconductor is zero when the magnetic field strength is lower than the critical value. "Complete" refers to reducing the temperature to reach the superconducting state , the order of applying the magnetic field can be reversed. The reason for complete diamagnetism is that a lossless diamagnetic superconducting current can be generated on the surface of the superconductor, and the magnetic field generated by this current cancels the magnetic field inside the superconductor.
It is well known that superconductors have zero resistance, but superconductors are not equivalent to ideal conductors. Starting from the electromagnetic theory, the following conclusions can be deduced: if an ideal conductor is first cooled to a low temperature and then placed in a magnetic field, the internal magnetic field of the ideal conductor is zero; but if an ideal conductor is first placed in a magnetic field and then cooled to a low temperature, the ideal The magnetic field inside the conductor is not zero. For superconductors, the two operations of reducing the temperature to a superconducting state and applying a magnetic field, no matter what the order is, the internal magnetic field of the superconductor is always zero. This is the core of complete diamagnetism and the key to distinguishing superconductors from ideal conductors.
3. Flux Quantization
Flux quantization, also known as the Josephson effect, refers to the phenomenon that when the insulating layer between two superconductors is as thin as atomic size, electron pairs can pass through the insulating layer to generate a tunnel current, that is, in the superconductor-insulator-superconductor structure, superconductors can be generated. Conductive current.
The Josephson effect is divided into the DC Josephson effect and the AC Josephson effect. The DC Josephson effect means that pairs of electrons can form a superconducting current through an insulating layer. The AC Josephson effect means that when the applied DC voltage reaches a certain level, in addition to the DC superconducting current, there is also an AC current. When the superconductor is placed in a magnetic field, the magnetic field penetrates into the insulating layer, and the maximum superconducting current of the superconducting junction follows. The magnitude of the magnetic field changes regularly.
4. Zero resistance
When the superconducting material is in the superconducting state, the resistance is zero and can transmit electrical energy without loss. If a magnetic field is used to induce an induced current in the superconducting loop, this current can be maintained without attenuation. This "persistent current" has been observed many times in experiments. Superconductivity is one of the great inventions of the 20th century. Scientists have found that when a substance is very cold, such as lead below 7.20K (-265.95 degrees Celsius), the resistance becomes zero.
5. Isotope effect
The critical temperature Tc of a superconductor is related to its isotopic mass M. The larger the M, the lower the Tc, which is called the isotope effect. For example, the mercury isotope with an atomic weight of 199.55 has a Tc of 4.18 Kelvin, while the mercury isotope with an atomic weight of 203.4 has a Tc of 4.146 Kelvin.
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