High Temperature Superconductivity
1911 H. Kamerlingh Onnes discovered the first VSuperconductor
The superconducting phenomenon was discovered by Dutch physicist Onnes in 1911. He cooled the mercury below 4K, where he found that the resistance of the mercury suddenly disappeared. The superconducting phenomenon was observed for the first time. Seventy-five years later, the first high-temperature superconductor was discovered by Swiss scientist Muller and German scientist Bednorz during the study of the oxide conductive ceramic material lanthanum-strontium-copper-oxygen. They won the 1987 Nobel Prize in Physics for this important discovery. Subsequently, Chinese and American scientists discovered iridium-barium-copper-oxygen superconductors above the liquid nitrogen temperature (77.3K), which further promoted the research boom of high-temperature superconductivity. Scientists have now prepared a series of nearly one hundred kinds of high-temperature superconductors. The transition temperature of superconductors has reached over 160K, and applications in some areas have begun to emerge.
The BCS theory based on the interaction between electrons and lattices has predicted that the superconducting phase transition temperature will not exceed 40K, so the high temperature superconducting phenomenon cannot be explained in the existing theoretical framework. The mechanism of generating high-temperature superconducting oxides is one of the most challenging topics in condensed matter physics at present. The famous physicist and Nobel Prize winner Anderson once compared the research of high-temperature superconductivity to “the unfinished Babel Tower of science”, implying the problems in this field and their roles in science. The breakthrough in this regard depends on the understanding of quantum correlation phenomena and universal laws. This breakthrough not only has significance for the research of superconductivity, but also has important significance for the development of condensed matter physics. It is expected that the key factors determining the critical transition temperature of superconductivity will be discovered through this research, the phase transition temperature will be increased, and the room temperature superconductor will be found.
Two Energy Gaps
In the underdoped high-Tc cuprates, there exist two kinds of energy gap scales: the superconducting gap energy scale which is proportional to Tc, and the pseudogap energy scale which drops almost linearly with doping. This difference between the superconducting pairing energy scale and the pseudogap energy scale was first pointed out in the paper
· C. Panagopoulos and T. Xiang, “ Relationship between the superconducting energy gap and the superconducting transition temperature in high temperature superconductors”, Physical Review Letters 81 (1998) 2336-2339.
based on a systematical analysis of low temperature penetration depth, ARPES and STM measurement data. This important observation has now been extensively verified by the Raman scattering, ARPES, STM and quite many other experiments.
This figure shows how the gap amplitude (or gap slope) at the gap nodes varies with Tc. It indicates that the superconducting gap is proportional to Tc, in contrast to the pseudogap energy which shows an opposite trend in the underdoped regime.
Two-band Theory of Electron Doped Cuprates