In the 1980s, scientists discovered that copper oxides could achieve high-temperature superconductivity. Last year, iron-based materials became the second member of the superconductor family. Its discovery has reignited physicists’ dreams of high-temperature superconductivity. Perhaps it will also help solve the long-standing puzzle of the mechanism behind high-temperature superconductivity that has troubled the academic community.
Written by Graham P. Collins
Translated by Hu Wanzheng
Reviewed by Wang Nan
Superconductivity
Conventional superconductors can only conduct current without energy loss at low temperatures near absolute zero. The copper oxide superconductors discovered in the 1980s completely broke the long-standing limit on superconducting transition temperatures. However, applying copper oxide superconductors to industry remains a highly challenging task.
The unique status of copper oxide superconductors among superconducting materials was maintained until 2008, when physicists discovered that iron-based superconductors could also enter a superconducting state at temperatures far above absolute zero.
Research into iron-based superconductors may help scientists finally understand the superconducting mechanism of copper oxides and even provide clues for finding room-temperature superconductors.
In 2006, a research group led by Hideo Hosono at the Tokyo Institute of Technology was conducting a project. Initially, the researchers were not looking for superconducting materials but hoped to synthesize a new type of transparent semiconductor for use in flat-panel displays. However, when the researchers tested the physical properties of their newly synthesized substance—a new compound containing lanthanum, oxygen, iron, and phosphorus—they found that the material had zero resistance when conducting current below 4K (about -269^\circC); that is, it superconducted.
Although 4K is far below the current laboratory record for the highest superconducting transition temperature of 138K, let alone the ultimate goal of room temperature (about 300K), for researchers, discovering a new superconductor is like a driver getting a new race car. The driver wants to know how fast the car can go; physicists want to know if a superconductor with a higher transition temperature can be found in this new superconducting family. Because cooling systems are complex to build, occupy large areas, and cost a fortune, the industrial application of superconductors is greatly limited. Therefore, every bit of increase in the superconducting transition temperature helps improve the deficiencies of existing systems, making new projects more technically and economically feasible. If the liquid helium cooling systems required for traditional low-temperature superconductors could be discarded, the troubles of high costs and complex equipment would vanish. Engineers could then apply cables capable of carrying large currents without loss, as well as small powerful magnets, to magnetic resonance imaging (MRI), maglev trains, particle accelerators, and other scientific concepts.
The research group began to experiment with doping the newly discovered compound—replacing a small amount of existing elements in the compound with other elements—hoping to increase the superconducting transition temperature. They replaced part of the oxygen atoms with fluorine, raising the superconducting temperature to 7K. Subsequently, the researchers completely replaced phosphorus with arsenic, raising the superconducting temperature to 26K. This major discovery was reported in late February 2008, immediately attracting the attention of physicists worldwide and triggering a research boom in iron-based superconductors. By late March of the same year, several Chinese research groups synthesized superconductors with transition temperatures exceeding 40K. A month later, the maximum superconducting transition temperature had reached 56K.
Despite the rapid progress in iron-based superconductor research, it is still not enough to challenge the record for the highest superconducting temperature set 20 years ago by copper oxide (cuprate) superconductors. Nevertheless, physicists find it difficult to contain their excitement. They believe that the potential for the maximum transition temperature in this system is still great; since copper oxides are very brittle and require complex technical processes to make long wires for cables or magnets, iron-based materials might be easier to apply in industry.
It is very rare for a superconductor to contain iron. Iron atoms are strongly magnetic, and magnetism usually suppresses superconductivity. In fact, the definition of a superconductor, besides zero resistance, includes another property: perfect diamagnetism (the Meissner effect), where the magnetic field is expelled from the interior of the superconductor. When the magnetic field is strong enough to enter the superconductor, superconductivity is destroyed. Why the superconductivity of iron-based superconductors is not destroyed by the magnetism of the internal iron atoms remains an unsolved mystery.
Perhaps the most attractive aspect of iron-based superconductors is that they provide a new member to the high-temperature superconductor family; copper oxides are no longer alone. Researchers have been troubled by copper oxides for over 20 years and have yet to find a theory that explains all their properties, especially why the transition temperature is so high. Now, researchers may be able to compare copper oxides and iron-based materials—these two types of high-temperature superconductors—to find key clues and finally solve the mystery of high-temperature superconductivity.
The greatest similarity between iron-based materials and copper oxides is their layered structure, but whether this structure is the key factor for high-temperature superconductivity remains to be proven.
Because these two classes of superconductors share many similarities, researchers hope to find clues by studying iron-based superconductors to explore the mechanism of copper oxides. The transition temperatures of both materials are much higher than all other known superconductors. They both have their own optimal doping concentrations; that is, when doped to a certain concentration, the transition temperature of the system reaches a maximum value. Below this temperature, the material enters a superconducting state, also known as the critical temperature (T_c). The transition temperatures of under-doped and over-doped samples are both lower than that of the optimally doped sample. As the doping concentration moves away from the optimal point, the transition temperature gradually drops to absolute zero. In other words, if the doping concentration of the sample is too low or too high, it will not superconduct.
Of course, the greatest similarity between these two materials lies in their structure. Both copper oxides and iron-based superconductors are composed of different atomic layers stacked alternately. The main feature of copper oxides is the copper-oxygen (CuO_2) layer. Correspondingly, iron-based compounds also have atomic layers composed of iron and pnictogens (elements in the nitrogen group of the periodic table, such as phosphorus, arsenic, antimony, etc.). The 26K superconductor discovered by Hideo Hosono’s group is composed of alternating lanthanum-oxygen (LaO) layers and iron-arsenic (FeAs) layers.
If the crystal structure of these two superconductors is compared to a sandwich, the copper-oxygen layers and iron-arsenic layers are the meat inside the sandwich. Physicists believe that superconductivity originates from this filling layer. The “bread slices” on both sides merely provide extra electrons to the filling layer or remove some electrons from it. When lanthanum-oxygen-iron-arsenic (LaOFeAs) is doped with fluorine, fluorine replaces some oxygen atoms. Since each fluorine atom has one more electron than the oxygen atom it replaces, these extra electrons transfer to the iron-arsenic layer, thereby changing its electrical properties.
Looking down perpendicular to the layered surface, the atoms in the iron-arsenic layer appear to be placed on a nano-scale chessboard; each iron atom occupies a black square, and each arsenic atom occupies a white square. The situation in the copper-oxygen layer is similar, with the difference being that only half of the black squares on the chessboard are occupied by copper atoms. Each copper-oxygen layer is basically flat, meaning all atoms are coplanar. In contrast, the arsenic atoms in the iron-arsenic layer are located diagonally above and below the iron atoms. Each iron atom is surrounded by four arsenic atoms, forming a tetrahedron with arsenic atoms at the vertices. Whether the similarities or the differences in the structural characteristics of the two materials are more important remains to be verified.
Copper oxide superconductors have a layered structure, a feature that makes them respond differently to superconducting currents flowing along the layers versus perpendicular to them. In copper oxide superconductors, the effect of a magnetic field on the superconducting current depends on the direction of the field. When the magnetic field is parallel to the copper-oxygen plane, the superconductor can withstand a very large magnetic field and still maintain its superconducting state. However, when the magnetic field is perpendicular to the copper-oxygen plane, a smaller magnetic field can destroy the superconductivity. This property is very important in practical applications because many superconductors are used to generate strong magnetic fields. This characteristic of copper oxides is also considered a potential clue that might explain the principle of high-temperature superconductivity.
Theorists value these clues highly. They have spent 20 years focusing on developing a theory to explain how superconductivity arises within a copper-oxygen layer. They believe that the two-dimensional nature of copper oxides is a key factor. From a theoretical perspective, this view is reasonable; many examples in mathematics and physics show that the unique properties or phenomena of a two-dimensional system no longer exist or become quite complex in three dimensions. In the specific case of copper oxide superconductors, a large number of experimental results show that the status of the copper-oxygen layer in the entire compound is very special.
Some of the earliest studies on iron-based superconductors suggested they also had two-dimensional characteristics. However, in late July 2008, a research group led by Professor Wang Nanlin of the Chinese Academy of Sciences, and a collaborative team of Paul C. Canfield’s group at Iowa State University and researchers at Los Alamos National Laboratory, independently discovered that iron-based superconductors have similar responses to different directions of strong magnetic fields. That is to say, the potassium-doped barium-iron-arsenic material they studied, which has a transition temperature of up to 38K, appears to be a superconductor with three-dimensional characteristics.
In the view of Jan Zaanen, a theoretical physicist at Leiden University in the Netherlands, if both copper oxides and iron-based superconductors contain the same “secret of high-temperature superconductivity,” then the experimental results of the above two research groups suggest that “two-dimensionality seems to be a distraction that led theoretical physicists in the wrong direction.”
Source: “Global Science”, Issue 9, 2009
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