Shulou(Shulou.com)11/24 Report--

The room temperature and atmospheric pressure superconducting materials in South Korea have made an uproar recently, and in order to prove the authenticity of the incident, they filmed a video of the material levitating. However, some physicists say that it is not necessarily the Meissner effect that can cause suspension, and some diamagnetism can also cause suspension. So what is the Meissner effect and diamagnetism?

We all know that magnets can attract metals such as iron, cobalt and nickel because they are paramagnetic, that is, they have unpaired electrons inside their atoms, and the spin and orbital movements of these electrons produce tiny magnetic moments, making the atoms behave like small magnetic needles. When the magnetic field is applied, the magnetic moments of these atoms tend to be consistent with the direction of the external field, which makes the whole metal show magnetization in the same direction as the external field.

However, not all metals are paramagnetic. When some metals apply a magnetic field, they will show the magnetization phenomenon opposite to the external field, which is diamagnetism. The principle of diamagnetism is that when an external magnetic field is applied, the free electrons inside the metal will be affected by the Lorentz force, resulting in a vortex current, which will produce a magnetic field opposite to the external field. as a result, the whole metal shows magnetization in the opposite direction of the external field.

Although diamagnetism is opposite to paramagnetism, they all have a linear response, that is, there is a proportional relationship between the external magnetic field and the internal magnetic field of the metal. This proportional coefficient is the magnetic susceptibility of the metal. The magnetic susceptibility of paramagnetic metals is positive, while that of diamagnetic metals is negative, but they are all very small, generally in the order of magnitude from 10 ^-5 to 10 ^-6.

However, in 1933, two German physicists Meissner and Oksenfeld discovered a strange phenomenon. They found that when some metals were cooled below a certain critical temperature, they not only showed zero resistance (that is, superconductivity). And it shows complete diamagnetism. Complete diamagnetism means that under the external magnetic field below the critical temperature and below the critical intensity, there is no magnetic induction inside the superconductor, that is, the superconductor completely repels the external magnetic line of force. This effect is called the Meissner effect.

There is an essential difference between Meissner effect and ordinary diamagnetism. Ordinary diamagnetism can only produce internal reverse current and reverse magnetic field in the presence of external magnetic field, while the Meissner effect is not affected by the existence or sequence of external magnetic field. In other words, if a material is first placed in an external magnetic field and then cooled below the critical temperature, or cooled below the critical temperature, and then placed in an external magnetic field, the result is the same: there is no magnetic induction inside the superconductor, that is, the superconductor completely repels the external magnetic line of force.

The theoretical explanation of the Meissner effect is the London equation put forward by the London brothers in 1935. The London equation describes the relationship between the current density and the magnetic field in the superconductor, indicating that there is a shielding current on the surface of the superconductor, and the magnetic field produced by it cancels out the external magnetic field inside the superconductor. The London equation also introduces an important physical quantity, the London penetration depth, which represents the distance required for the external magnetic field to decay to zero inside the superconductor. However, the London equation is a phenomenological theory, which can only explain the phenomenon of the Meissner effect, but can not reveal its microscopic mechanism. Later, in 1957, BCS theory successfully explained the Meissner effect at the micro level.

The principle of the Meissner effect is that when the superconductor is below the critical temperature, its free electrons form a special state called the Cooper pair. A Cooper pair is a quasiparticle formed by two opposing spin electrons attracting each other. Their total spin is zero, so they are not affected by the external magnetic field. When a magnetic field is applied, the Cooper pair forms a current layer on the surface of the superconductor, which produces a magnetic field that is completely offset by the external field, so that there is no magnetic induction inside the superconductor. This is the microscopic mechanism of the Meissner effect.

There are many interesting phenomena and applications of Meissner effect. For example, if a superconductor is suspended over a magnet, it will maintain a fixed position and direction and will not rotate or fall. This is the phenomenon of superconducting suspension. Superconducting levitation can be used to manufacture high-speed trains, aircraft, generators and other equipment. Another example is that if a superconducting ring is placed in an external magnetic field and then cooled below the critical temperature, a constant current will be generated inside the superconducting ring, which will not attenuate or disappear, which is the superconducting ring current phenomenon. Superconducting ring current can be used to manufacture high-precision magnetic measuring instruments, quantum computers and other equipment.

In short, diamagnetism is a phenomenon that a substance produces the opposite magnetization under an external magnetic field, thus reducing its own magnetic induction intensity. The Meissner effect is a phenomenon that when the superconductor is below the critical temperature, it completely repels the external magnetic field, so that the internal magnetic field of the superconductor is zero.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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