Superconducting Ceramics IV

1. Unbalanced effect

The previous cases (mentioned in the third of superconducting ceramics) are all carried out in an equilibrium state or an approximate equilibrium state. The distribution of particle velocity and momentum is determined by temperature. For non-equilibrium states, they have nothing to do with temperature, but with the distribution of particles according to momentum.

There are several methods for obtaining the unbalanced state. Optical excitation: It is to irradiate superconducting samples with light (including magnetic field and laser, etc.); neutron beam excitation, unbalanced electron excitation and electric field breakdown. When a superconductor is subjected to an electromagnetic field whose frequency greatly exceeds its energy gap, in addition to causing the formation of quasi-particles, additional quasi-particles will also be produced. This superconducting state is very sensitive to the value and form of the distribution function of the process quasi-particles, so that it has a series of essential characteristics. For example, when a superconductor undergoes a non-equilibrium state, the transition from a superconducting state to a normally-conducting state is not a jump, but a gentle one. Under the particle anti-distribution bar, the appearance of the superconducting state is obtained when two electrons repel each other to form a combined state. However, under equilibrium conditions, electrons form electron pairs by interacting with phonons to generate attraction. In addition, under the condition of particle anti-distribution, the superconducting energy gap will be greater than the maximum value of phonon energy. There is a non-attenuated current in the system, which is completely paramagnetic, and the sign of the current is opposite to the sign of the current under the balancer. Therefore, in this state, the superconductor will not exhibit the Meissner effect, but the magnetic field can enter the sample and other peculiar properties. It is precisely because of this peculiar performance that leads to the result of theoretical analysis that makes it possible for Tc to reach 1000K. Even ignoring the anti-distribution of particles, it is possible to obtain a high Tc temperature for a general non-equilibrium state.

There are many superconductivity theories, not to introduce them one by one. Up to now, it has not been able to make a satisfactory explanation for superconductivity and so on.

2. The nature and testing of superconductors

There are many properties of superconductors, as shown in the table.

Electrical properties of superconducting state

property

Features

Fully conductive

The DC resistance is zero, but the AC resistance is not zero. Carriers are pairs of superconducting electrons. To be exact, the DC resistance is infinitely close to zero.

Resistivity

Approaching zero

Thermoelectromotive force

Approaching zero

Current can destroy the superconducting state

When the current density exceeds the critical value Jc, the superconductor changes from the superconducting state to the normally conducting state. The essence is the destruction of the superconducting state by the magnetic field generated by the current. This phenomenon is an important physical basis of superconductivity engineering.

Current trending effect

Superconducting current can only flow into a thin layer of deep surface along the superconducting surface

Optical properties of superconducting state

property

Features

General optical properties

No change

Reflectivity

No transformation occurs, photons with energy lower than the energy gap cannot be absorbed

Magnetic properties of superconducting state

property

Features

Fully diamagnetic

An external magnetic field cannot enter the superconductor under normal circumstances, but can only penetrate into the deep surface layer of λL

The magnetic field can destroy the superconducting state

When the magnetic field strength exceeds the critical value Hc, the superconductor changes from a superconducting state to a normal conducting state. This phenomenon is also an important physical basis of superconducting electronics (or superconducting microelectronics)

There is a mixed state

The state that exists between the two critical magnetic fields Hc1 and Hc2 of the second type of superconductor. It has the properties of complete conductivity, but does not have the properties of complete diamagnetism.

There is an intermediate state

The intermediate state is a state in which the superconducting state and the normal conducting state alternately exist in the superconductor. This state is sometimes called the intermediate state.

Thermal properties of superconducting state

property

Features

New phase transition effect

When the superconductor’s transition from a superconducting state to a normally conducting state (or vice versa) is accompanied by endothermic or exothermic heat

Latent heat

When H>0, latent heat occurs during the phase change, and when H=0, no latent heat occurs during the phase change

Specific heat

Abnormal specific heat, discontinuity when T=Tc, and mutation effect

Temperature destroys superconductivity

When the temperature exceeds the critical temperature Tc, the superconductor changes from a superconducting state to a normally conducting state, and vice versa. This is also an important physical basis for superconductivity

Thermal conductivity

With discontinuity in the magnetic field, the thermal conductivity of the general superconducting state will become lower

Other properties of superconductors

 

Features

Crystal structure

constant

Shape size

constant

elasticity

Want to change

Absorption of electron beam

constant

Energy gap

Determined by the Fermi gas, in the electron energy spectrum of the superconductor, there can be no gaps in the electron energy

Isotope effect

The critical temperature Tc of a superconductor is related to the isotope mass M of the superconductor, and Tc decreases as the temperature increases

Tunnel effect

It is divided into superconducting electron tunneling effect and normal conduction quasi-electron tunneling effect, which is an important physical effect on the basis of superconducting electrons.

 

Subscribe

Advanced Ceramics Academic Center Subscription

Input your email now to get latest academics update!

Get the industry analyse, technology & appcation sharing easily by CERADIR!

Get the industry analyse, technology & appcation sharing easily by CERADIR!