superconductivity

superconductivity

The property that the resistance of some substances drops to zero at a certain temperature. 19 1 1 year, Dutch physicist H. Kamelin-Agnes found that when the temperature dropped to about 4.2K, Mercury suddenly entered a new state, and its resistance was too small to be measured. He called this new state of mercury superconductivity. It was later discovered that many other metals also had superconductivity. A substance that shows superconductivity below a certain temperature is called a superconductor.

Main characteristics The main characteristics of superconductors are as follows:

① When the superconductor enters the superconducting state, its resistivity is actually equal to zero. The temperature from the normal state of non-zero resistance to superconducting state is called superconducting transition temperature or superconducting critical temperature, which is expressed by Tc.

② The external magnetic field can destroy the superconducting state. Superconductivity can only be maintained when the applied magnetic field is less than a certain value Hc, otherwise the superconducting state will change to the normal state, and Hc is called the critical magnetic field strength. The relationship between Hc and temperature is HC ≈ H0 [1-(t/TC) 2], and H0 is the critical magnetic field strength at t = 0K.

③ When the current intensity in the superconductor exceeds a certain value Ic, the superconductor becomes a normal conductor, and Ic is called critical current.

(4) No matter whether there is an external magnetic field or not at the beginning, only T < TC, after the superconductor becomes superconducting, the magnetic induction intensity in the body is always zero, that is, the superconductor can repel all the external magnetic lines and has complete diamagnetism. This phenomenon was first discovered by W. meissner and R. Oxenfield in 1933, which is called Messner effect. When a small permanent magnet falls near the surface of the superconductor, because the magnetic field lines of the permanent magnet cannot enter the superconductor, repulsive force is generated between the permanent magnet and the superconductor, and the permanent magnet is suspended on the superconductor.

The first and second types of superconductors are divided into the first type (also called Pippard superconductor or soft superconductor) and the second type (also called London superconductor or hard superconductor). Among the discovered superconducting elements, only vanadium, niobium and technetium belong to the second kind of superconductors, and other elements are the first kind of superconductors, but most superconducting alloys belong to the second kind of superconductors. There is only one critical magnetic field Hc in the first superconductor. When the external magnetic field H < HC, it is completely diamagnetic and the internal magnetic induction intensity is zero. Superconductors of the second kind have two critical magnetic fields, which are respectively represented by HC 1 (lower critical magnetic field) and HC2 (upper critical magnetic field). When the external magnetic field H < HC 1, it has complete diamagnetism and the magnetic induction intensity in the body is zero everywhere. The external magnetic field satisfies HC 1

Theoretical research The macroscopic theoretical research on superconductors began with the work of W·H· Kassem, A·J· Lager and C·J· Gott. They analyzed and discussed the phase transition between superconducting state and normal state with thermodynamic theory, and reached an important conclusion that the entropy of superconducting state is always lower than that of normal state, which means that superconducting state is more orderly than normal state.

Two-fluid model Gott and H.B.G casimir proposed a two-fluid model of 1934 superconducting state based on the above results. They think that the superconducting state is more orderly than the normal state, which is caused by some ordered change of * * * chemical electrons (see energy band theory), and assume that: ① when the superconductor is in superconducting state, * * chemical electrons can be divided into normal electrons and superconducting electrons, which constitute normal fluid and superconducting electrons respectively. ② The properties of normal fluid are the same as those of free electron gas in ordinary metal, and the entropy is not equal to zero, so it is in an excited state. Normal electrons will produce resistance due to the scattering of lattice vibration. Superconducting electronic fluid has zero contribution to entropy because of its order, and is in the ground state with the lowest energy. Superconducting electrons will not be scattered by lattice and will not produce resistance. ③ The order degree of superconducting states can be expressed by the order parameter ω (t) = Ns (t)/n, where n is the total number of electroNs and ns is the number of superconducting electrons. When t > TC, there is no superconducting electron, ω = 0; Superconducting electrons begin to appear when τ < TC, and with the decrease of temperature T, more normal electrons are converted into superconducting electrons. When t = 0K, all electrons become superconducting electrons, ω = 1. Many experimental phenomena related to superconductivity can be explained according to the above two-fluid model.

Macro-electromagnetic theory of superconductors 1935, F. London and H. London put forward a macro-electromagnetic theory of superconductors based on the two-fluid model by using Maxwell's electromagnetic theory, which successfully explained the zero resistance phenomenon and Messner effect of superconductors. According to London's theory, the magnetic field can penetrate the surface of superconductor, and the magnetic induction intensity decays exponentially with the depth X: B(x)∝e-x/λ, and the attenuation constant λ is called the penetration depth. When the linearity of the superconductor is less than the penetration depth, the magnetic induction intensity in the body is not equal to zero, so only when the linearity of the superconductor is much greater than the penetration depth can the superconductor be regarded as completely diamagnetic. The actual measurement confirms the existence of the theoretical predicted penetration depth, but the theoretical value is inconsistent with the experiment. 1953a.b. Pippard revised London's theory. London's theory does not consider the correlation between superconducting electrons. Pippard thinks that superconducting electrons are interrelated in a certain spatial range, and introduces the concept of coherence length to describe the correlation distance of superconducting electrons (that is, the spatial range of superconducting electron wave function). Pippard obtained the penetration depth consistent with the experiment.

Ganci Fort-Landau Theory 1950, V.L. Ganci Fort and L.D. Landau put forward a superconducting phenomenological theory based on the second-order phase transition theory, which is called Ganci Fort-Landau Theory (GL Theory for short). The mutual transformation between superconducting state and normal state is a second-order phase transition (no volume change, no latent heat of phase transition). In 1937, Landau put forward the second-order phase transition theory, holding that the difference between two phases lies in the degree of order, and introduced the order parameter η to describe two phases with different degrees of order. η = 0 is completely disordered, and η = 1 is completely ordered. GL theory applies the second-order phase transition theory to the phase transition process between normal state and superconducting state. Its uniqueness lies in introducing an effective wave function ψ as a complex order parameter, where ψ | 2 represents the number density of superconducting electrons. The Ganci-Landau equation about ψ is established by applying thermodynamic theory. According to GL theory, many conclusions consistent with experiments can be obtained, such as the relationship between critical magnetic field, coherence length, penetration depth and temperature. GL theory also gives the criteria for distinguishing the first type superconductor from the second type superconductor. According to GL theory, Abri Kausov discussed in detail the basic characteristics of the second kind of superconductors. L.P. Gokov derived the GL equation from the microscopic theory of superconductors. Today, GL theory and the later work of Abri Kosov and Gokov are called GLAG theory.

BCS theory J. Bading, L.-N. Cooper and J. R. schrieffer established the microscopic theory of superconducting state in 1957, which is called BCS theory for short. Based on Fermi liquid, the microscopic theory of superconducting state is applied to electrons? A theory based on the premise that phonon interaction is very weak. It believes that the reason of superconductivity is that the electrons near Fermi surface exchange phonons to generate gravity. Because of this attraction, electrons near Fermi's surface are paired into two combinations, which are called Cooper pairs. BCS theory can derive equations similar to fritz london equation, Pippard equation and Gunzburg-Landau equation, which can explain a large number of superconducting phenomena and experimental facts. For some superconductors, such as mercury and lead, some phenomena cannot be explained by it, but the superconducting strong coupling theory developed on the basis of BCS theory can be explained.

The application of superconductivity has important application value. For example, a sensitive superconducting thermometer can be made by using the law that resistivity changes rapidly with temperature near the critical temperature. Using the unimpeded effect of superconducting state, powerful current can be transmitted to manufacture superconducting magnets, superconducting accelerators, superconducting motors and so on. Using the magnetic levitation effect of superconductors, frictionless bearings and suspended trains can be manufactured. Various superconducting devices made of Josephson effect have been widely used in the measurement of basic constants, voltage and magnetic field, microwave and infrared detection and electronics. The appearance of superconducting materials with high critical temperature will greatly expand the application prospect of superconductivity.