Superconductivity of metals, the discovery of Heike Kamerling-Onnes

The first to come across the phenomenon of superconductivity Heike Kamerling Onnes — Dutch physicist and chemist. The year of discovery of the phenomenon was 1911. And already in 1913, the scientist will receive the Nobel Prize in Physics for his research.

Conducting a study of the electrical resistance of mercury at ultra-low temperatures, he wanted to determine to what level the resistance of a substance to an electric current could drop if it was cleaned of impurities, and to reduce as much as possible what can be called. » thermal noise «, that is, to lower the temperature of these substances. The results were unexpected and astounding. At temperatures below 4.15 K, the resistance of mercury suddenly disappeared completely!

Below is a graph of what Onnes observed.

In those days, science already knew at least that much current in metals is the flow of electrons, which are separated from their atoms and, like the charged gas, are carried away by the electric field.It is like wind when air moves from an area of ​​high pressure to an area of ​​low pressure. Only now, in the case of current, instead of air, there are free electrons, and the potential difference between the ends of the wire is analogous to the pressure difference for the air example.

In dielectrics, this is impossible, because the electrons are tightly bound to their atoms and it is very difficult to tear them from their places. And although in metals the electrons forming the current move relatively freely, they occasionally collide with obstacles in the form of vibrating atoms and a kind of friction occurs called electrical resistance.

But when at ultra-low temperature it starts to manifest itself superconductivity, the friction effect disappears for some reason, the resistance of the conductor drops to zero, which means that the electrons move completely freely, unimpeded. But how is this possible?

To find the answer to this question, physicists have spent decades researching. And even today, ordinary wires are called "normal" wires, while conductors in a state of zero resistance are called "superconductors".

It should be noted that although ordinary conductors decrease their resistance with decreasing temperature, copper, even at a temperature of several kelvins, does not become a superconductor, and mercury, lead and aluminum do, their resistance turns out to be at least a hundred trillion times lower than that of copper under the same conditions.

It is worth noting that Onnes did not make unsubstantiated claims that the resistance of mercury during the passage of current became exactly zero, and did not simply drop so much that it became impossible to measure it with instruments of the time.

He set up an experiment in which the current in a superconducting coil immersed in liquid helium continued to circulate throughout until the genie evaporated. The compass needle, which followed the magnetic field of the coil, did not deviate at all! In 1950, a more accurate experiment of this kind will last a year and a half, and the current will not decrease in any way, despite such a long period of time.

Initially, it is known that the electrical resistance of a metal depends significantly on temperature, you can build such a graph for copper.

The higher the temperature, the more the atoms vibrate. The more the atoms vibrate, the more significant an obstacle they become in the path of the electrons forming the current. If the temperature of the metal decreases, then its resistance will decrease and approach a certain residual resistance R0. And this residual resistance, as it turned out, depends on the composition and "perfection" of the sample.

The fact is that defects and impurities are found in any sample made of metal. This dependence interested Ones above all in 1911, initially he did not strive for superconductivity, but only wanted to achieve such a frequency of the conductor as possible to minimize its residual resistance.

In those years, mercury was easier to purify, so the researcher came across it by accident, despite the fact that platinum, gold and copper are better conductors than mercury at ordinary temperatures, it is just more difficult to purify them.

As the temperature decreases, the superconducting state occurs abruptly at a certain moment when the temperature reaches a certain critical level. This temperature is called critical, when the temperature drops even lower, the resistance drops sharply to zero.

The purer the sample, the sharper the drop, and in the purest samples this drop occurs in an interval of less than a hundredth of a degree, but the more polluted the sample, the longer the drop and reaches tens of degrees, this is especially noticeable in high temperature superconductors.

The critical temperature of the sample is measured in the middle of the sharp drop interval and is individual for each substance: for mercury 4.15K, for niobium, 9.2K, for aluminum, 1.18K, etc. Alloys are a separate story, their superconductivity was discovered later by Onnes: mercury with gold and mercury with tin were the first superconducting alloys he discovered.

As mentioned above, the scientist performed the cooling with liquid helium. By the way, Onnes obtained liquid helium according to his own method, developed in his own special laboratory, founded three years before the discovery of the phenomenon of superconductivity.

To understand a little about the physics of superconductivity, which occurs at a critical temperature of the sample so that the resistance drops to zero, it should be mentioned phase transition… The normal state, when the metal has normal electrical resistance, is the normal phase. Superconducting phase — this is the state when the metal has zero resistance. This phase transition occurs immediately after the critical temperature.

Why does the phase transition occur? In the initial "normal" state, the electrons are comfortable in their atoms, and when current flows through a wire in this state, the energy of the source is expended to force some electrons to leave their atoms and start moving along the electric field, even though encounter flickering obstacles in their paths.

When the wire is cooled to a temperature below the critical temperature and at the same time a current is established through it, it becomes more convenient for the electrons (energy favorable, energy cheap) to be in this current, and to return to the original "normal" state, it would be necessary in this case, to get extra energy from somewhere, but it doesn't come from anywhere. Therefore, the superconducting state is so stable that matter cannot leave it unless it is reheated.

See also:Meissner effect and its use