Expert opinion: High density of superconducting current in the magnetic state of FeSe
At the end of December, our leading scientist, Dr. Sci. Professor, Head of the Department of Low Temperature and Superconducting Physics, Moscow State University Lomonosov and project manager "Metal oxide and polymer composite thermoelectrics" in NITU "MISiS" A.N. Vasilyeva in one of the most authoritative world scientific journals NATURE MATERIALS Impact Factor - 36.5 article published Strong interplay between stripe spin fluctuations, nematicity and superconductivity in FeSe
Alexander Nikolaevich Vasilyev, both to us and to the press service of the Moscow State University, provided a press release on this experiment, in which he spoke in a popular science format about Superconductivity and nematism in iron selenide . In the future, this work will help to create new superconductors, modify existing ones and, possibly, will be able to make them work in indoor conditions in the distant future, and this will allow to create superconducting computers on their basis. This press release was distributed by almost all electronic media. But in the same period, A.N. Vasilyeva published another article in the Nature Publishing Group's SCIENTIFIC REPORTS magazine, Impact Factor - 5.5 FeSe , which reported an increase in the density of the superconducting current in the magnetic state of FeSe. Since this article was left almost without attention in our Russian-language segment, and for Alexander Nikolaevich and the physical community it is of great importance, we asked him to write for us an exclusive expert opinion on this article!
All applications of superconducting materials suggest the preservation of a state with zero resistance at high temperatures and high current densities. The discovery of high-temperature copper-based superconductors aroused great interest not only due to unusual aspects of superconductivity, but also because the high temperature of the superconducting transition promised new revolutionary applications at temperatures above the boiling point of liquid nitrogen (~ 77 K). The key point in the realization of the technological potential of superconductors is the critical current density at which the magnetic flux lines (or vortices) begin to move and energy dissipation occurs. For decades, methods for increasing the critical current density have relied on the creation of artificial defects. Due to the fact that vortices have a normal core, they are able to hook (pinn) to defects where superconductivity is suppressed.
Another approach to increasing the critical current density is based on the inherent characteristics of materials. In particular, it was suggested that the coexistence of magnetism and superconductivity may be useful for fixing the eddies. Some of the high-temperature superconductors, such as La2-xSrxCuO4 and Ba (Fe1-xCox) 2As2, are of most interest in this regard, since the superconductivity in them is realized in the vicinity of an antiferromagnetically ordered state. Superconductivity in these materials, however, requires chemical substitutions, which is inevitably accompanied by the appearance of defects or structural disorder. This, in turn, leads to the intersection of positive and negative aspects of external and internal pinning. In addition, it is still not clear whether the magnetic order coexists with superconductivity at the microscopic or macroscopic levels. Thus, in order to clarify the effect of intrinsic pinning on the critical current density, it was necessary to conduct a study of the material, which is superconducting in a stoichiometric state and allows control of the superconducting and magnetic state by external influences.
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The binary high-temperature superconductor FeSe is the main candidate for testing the effects of intrinsic pinning on the critical current density, since the critical temperature of this material ~ 10 K can be increased to 37 K by applying pressure. The emergence of magnetism at a pressure of ~ 0.8 GPa is of interest both in terms of fundamental physics and practical applications. Especially important is the observation of the critical temperature of the superconducting state in monolayers of FeSe exceeding 100 K. In this paper, the evolution of the critical current density with increasing temperature of the superconducting transition in an FeSe single crystal under the action of quasi-hydrostatic pressure is reported.
The current-voltage characteristics, as well as the temperature dependences of the electrical resistance, show dramatic changes when a critical pressure of ~ 0.8 GPa is reached, over which the appearance of an antiferromagnetic state coexisting with superconductivity has been proved by methods of muon spectroscopy. In this state, the amplitude of the critical current increases dramatically. The fact that the application of pressure does not lead to additional disorder indicates that the intrinsic characteristics of the substance (antiferromagnetic ordering) affect the vortex pinning mechanisms. Two features on the current-voltage characteristics determine the behavior of a superconductor. The first of them, the critical depinning current Jc, was determined by the criterion 1 μV, when the vortices break away from the pinning centers and begin to move. The second, the free magnetic flux current Jf, corresponds to the regime when the vortices do not notice pinning centers and move freely. Diagrams of critical current densities, Jc and Jf, are shown in the figure. The borders of the existence of various crystallographic modifications of FeSe are also shown here, and the region of existence of the antiferromagnetic state is determined.
Thus, we investigated the correlations between the temperature of the superconducting transition and the critical current density in the high-temperature FeSe superconductor. The critical current increases sharply in the region of the coexistence of magnetic and superconducting order parameters. Fluctuations of the coherence length of a superconductor, due to the inhomogeneous superconducting state, may be important for additional pinning of the vortices. In combination with well-known methods of creating artificial defects, own pinning mechanisms can be used to increase the critical characteristics of superconductors. This, in turn, approximates the time of practical use of high-temperature superconductivity.
Alexander Nikolaevich Vasilyev, both to us and to the press service of the Moscow State University, provided a press release on this experiment, in which he spoke in a popular science format about Superconductivity and nematism in iron selenide . In the future, this work will help to create new superconductors, modify existing ones and, possibly, will be able to make them work in indoor conditions in the distant future, and this will allow to create superconducting computers on their basis. This press release was distributed by almost all electronic media. But in the same period, A.N. Vasilyeva published another article in the Nature Publishing Group's SCIENTIFIC REPORTS magazine, Impact Factor - 5.5 FeSe , which reported an increase in the density of the superconducting current in the magnetic state of FeSe. Since this article was left almost without attention in our Russian-language segment, and for Alexander Nikolaevich and the physical community it is of great importance, we asked him to write for us an exclusive expert opinion on this article!
Critical current densities, Jc and Jf, in the high-temperature FeSe superconductor
All applications of superconducting materials suggest the preservation of a state with zero resistance at high temperatures and high current densities. The discovery of high-temperature copper-based superconductors aroused great interest not only due to unusual aspects of superconductivity, but also because the high temperature of the superconducting transition promised new revolutionary applications at temperatures above the boiling point of liquid nitrogen (~ 77 K). The key point in the realization of the technological potential of superconductors is the critical current density at which the magnetic flux lines (or vortices) begin to move and energy dissipation occurs. For decades, methods for increasing the critical current density have relied on the creation of artificial defects. Due to the fact that vortices have a normal core, they are able to hook (pinn) to defects where superconductivity is suppressed.
Another approach to increasing the critical current density is based on the inherent characteristics of materials. In particular, it was suggested that the coexistence of magnetism and superconductivity may be useful for fixing the eddies. Some of the high-temperature superconductors, such as La2-xSrxCuO4 and Ba (Fe1-xCox) 2As2, are of most interest in this regard, since the superconductivity in them is realized in the vicinity of an antiferromagnetically ordered state. Superconductivity in these materials, however, requires chemical substitutions, which is inevitably accompanied by the appearance of defects or structural disorder. This, in turn, leads to the intersection of positive and negative aspects of external and internal pinning. In addition, it is still not clear whether the magnetic order coexists with superconductivity at the microscopic or macroscopic levels. Thus, in order to clarify the effect of intrinsic pinning on the critical current density, it was necessary to conduct a study of the material, which is superconducting in a stoichiometric state and allows control of the superconducting and magnetic state by external influences.
')
The binary high-temperature superconductor FeSe is the main candidate for testing the effects of intrinsic pinning on the critical current density, since the critical temperature of this material ~ 10 K can be increased to 37 K by applying pressure. The emergence of magnetism at a pressure of ~ 0.8 GPa is of interest both in terms of fundamental physics and practical applications. Especially important is the observation of the critical temperature of the superconducting state in monolayers of FeSe exceeding 100 K. In this paper, the evolution of the critical current density with increasing temperature of the superconducting transition in an FeSe single crystal under the action of quasi-hydrostatic pressure is reported.
The current-voltage characteristics, as well as the temperature dependences of the electrical resistance, show dramatic changes when a critical pressure of ~ 0.8 GPa is reached, over which the appearance of an antiferromagnetic state coexisting with superconductivity has been proved by methods of muon spectroscopy. In this state, the amplitude of the critical current increases dramatically. The fact that the application of pressure does not lead to additional disorder indicates that the intrinsic characteristics of the substance (antiferromagnetic ordering) affect the vortex pinning mechanisms. Two features on the current-voltage characteristics determine the behavior of a superconductor. The first of them, the critical depinning current Jc, was determined by the criterion 1 μV, when the vortices break away from the pinning centers and begin to move. The second, the free magnetic flux current Jf, corresponds to the regime when the vortices do not notice pinning centers and move freely. Diagrams of critical current densities, Jc and Jf, are shown in the figure. The borders of the existence of various crystallographic modifications of FeSe are also shown here, and the region of existence of the antiferromagnetic state is determined.
Thus, we investigated the correlations between the temperature of the superconducting transition and the critical current density in the high-temperature FeSe superconductor. The critical current increases sharply in the region of the coexistence of magnetic and superconducting order parameters. Fluctuations of the coherence length of a superconductor, due to the inhomogeneous superconducting state, may be important for additional pinning of the vortices. In combination with well-known methods of creating artificial defects, own pinning mechanisms can be used to increase the critical characteristics of superconductors. This, in turn, approximates the time of practical use of high-temperature superconductivity.
Source: https://habr.com/ru/post/367713/
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