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Written by | Ising
Condensed matter exhibits superconductivity, no matter where, it is magical for those who have learned a little physics. The properties of zero resistance and strong antimagnetic are great, but the physics that can pair each other and depend on each other from now on is of course even better! Therefore, physicals say that "Cooper's pair and its condensation" is the core of superconductivity, and many people should not object. Among them, Cooper pairs, according to the BCS theory, it is said that lattice phonons connect a pair of electrons with opposite spins in series, just like connecting two electrons at the left and right ends of a spring.
is more physical, saying that two mutually repulsive charges ( fermion ) form a quasi-particle called a boson, and then a large number of such bosons condense on the same energy level "in a consistent step" (bose condensed) . From a simplified and ideal perspective, there is no scattering between bosons, such as there is no scattering between phonons under simple harmony approximation. Since there is no scattering, there is no resistance. These condensed "electron pairs" form a dissipative superfluid, carrying the charges, and a river of spring water flows eastward, forming superconductivity! Superconducting antimagnetics can also be understood from the perspective of electromagnetics : one-to-one pairs of Coopers' electrons have the opposite magnetic moment, just like two reverse-moving electrons on the orbit outside the nucleus of . When the orbital magnetic moment is reversed, the external magnetic field will definitely show antimagnetic properties. Such images can be made with various expressions, and Figure 1 is one of them.
Figure 1. Some basic science popularization images in BCS theory.
(upper part) Electric-phonon coupled to form a real-space image of a pair of Cooper pairs. (Lower left) Boson (Cooper pair) condenses into one energy level, and has no repulsion or scattering between each other, so it is social, that is, likes lively; fermions (electrons) can only fill to different energy levels, and they can not be close to each other, and they repel scattering between each other, so it is not social, that is, they like loneliness. (bottom right) One-to-one Cooper pair dances.
http://www.supraconductivite.fr/en/index.php?p=supra-explication-cooper, https://www.nasa.gov/vision/earth/technologies/12feb_fermi.html
The problem is that among the four basic forces of the universe, the main body that determines the solid physical level of is electromagnetic force. When the distance is about the lattice constant scale (~ 0.1 nm), the Coulomb potential (~ 1.0 eV) between a pair of charges is much greater than the average lattice phonon energy (~ 0.01 eV). This means that the Cooper pair, implicated in the phonon spring, cannot be too close to each other in real space, just like the moon and the sky, interpreting the taste of "It is better to miss each other than meeting each other". Being far apart, Cooper's energy gap for superconducting condensation is not high, and the superconducting condensation energy gap (ability to resist external interference) is probably not very large, and the superconducting transition temperature is not much higher than that (Macmillan limit?) , and its ability to withstand external fields is not much stronger than that (the critical magnetic field of magnetic flux is not high) . Ising is a superconducting layman and has always understood superconducting physics in this way. Expert readers, please don’t mind ^_^!
Therefore, for many years, superconductors have "unwilling to accept BCS theory and accept their superconducting research career under their shade. This situation only changed after high-temperature superconducting (unconventional superconducting) appeared in the 1980s. At least many extremely smart and insightful physicists have begun to propose some pairing mechanisms such as non-phonon springs, trying to pull a pair of electrons into it. Here, the layman will ask: Can you not revolve around "Cooper's versus"? Is there any superconductivity that does not require electron pairing and condensation? This question seems to be too layman and civil science, so I will not give it aside for the time being.How to connect a pair of electrons without using phonons? How to connect two electrons with another mechanism instead? How to make a pair of electrons in real space closer and Coopers stronger in condensation? Such a mechanism, if any, should have higher superflow temperature, stronger stability and antimagnetic properties? Ising understands that this is probably the knot in the heart of a high-temperature superconductor: there are thousands of knots in the heart, and the belt will never stop until it becomes wider!
For this reason, based on the symmetry of electronic structure, discussing the electronic pairing mechanism (s-wave, p-wave, d-wave, etc.) will be more rigorous and physical. However, we might as well look at the problem from a more understandable perspective. In addition to phonons, another widely-watched pairing medium is spin fluctuation (imitated with phonon fluctuation) . The general image is to say that in unconventional superconductors, as the static antiferromagnetic sequence in the superconductor parent is dispersed and suppressed, the fluctuations of spin become significant. These fluctuations, like scattered "waves", will "resonate" with the electron pairing state at certain specific energy and momentum (wave vector) , forming a collective spin excitation mode. This is the so-called neutron spin resonance mode seen in neutron scattering experiments, reaching the superconducting state (such as d-wave electron pairing) . Many unconventional superconductors have this resonance pattern, which seems to be evidence of spin fluctuations and fall pairing electrons.
Intuitively speaking, there is no longer a need for phonon media here. It is just to directly use spin fluctuations and falls to form a spin single state or even triplet state. This medium seems to have more advantages in improving superconducting temperature and service stability. While there is a lot of evidence to support these pairing patterns, there is also a lot of evidence that is less consistent with this physics. There are many different things and some no consensus among them, which has formed the magnificent scene of superconducting mechanism research today. If you look at the copper-based high-temperature superconducting phase diagram, you will know that there is a typical "time" inversion symmetry breakdown: as time passes, phase diagram becomes more and more complex! This is somewhat romantically related to Cooper's magnetic pairing mechanism, because magnetism is the symmetry of time inversion. Figure 2 shows the so-called three possible pairing mechanisms (upper) and the orbital resolution band structure diagram that has been proposed about a few years ago.
Figure 2. (Upper) Three mainstream mechanisms for superconducting electron pairing: Lattice --- Electrophonon pairing media in classic superconducting; Spin --- Spin fluctuation pairing media in copper-based superconducting; Orbital --- Electronic orbital pairing media in iron-based superconducting. (lower) Orbital-resolved band structure in FeAs (kz = 0)
(upper) In classical superconductors, which function at very low temperatures, vibrations of atoms in the crystal lattice of the material provide the necessary glue for the pairing. In cuprates, the original high-temperature superconductor compounds, magnetic interactions based on an electron’s spin generate the superconductive pairing. In the pnictide high-temperature superconductors, electronic orbitals as a third kind of pairing glue for electronic pairs. From T. Shimajima et al, Orbital-independent superconducting gaps in iron pnictides, Science 332, 564 (2011), https://www.science.org/doi/10.1126/science.1202150, https://phys.org/news/2011-06-superconductivity-side-unmasked.html.
(lower) Schematic representation of orbitally resolved band structure of FeSe at kz = 0. From A. Kostin et al, Imaging orbital-selective quasiparticles in the Hund’s metal state of FeSe, Nature Mater. 17, 869 (2018), https://www.nature.com/articles/s41563-018-0151-0.
Next, the progress of iron-based superconducting has further improved our understanding of unconventional superconducting. As the saying goes, "Three thousand weak water, take one scoop." It is very interesting to look at the superconducting properties of the single layer FeSe grown on SrTiO3(STO). The superconducting Tc it shows is much higher than the Tc of the bulk FeSe-based compound, suggesting some new signs of Cooper's formation mechanism.In fact, from the analysis of FeSe itself, its superconducting temperature significantly deviates from the prediction of BCS theory, showing many characteristics of unconventional superconductivity: if you look at the spin resonance mode of neutron scattering, it should be that the d-wave pairing dominates. But looking at the energy band structures near the Fermi surface such as nodeless gap, there should be strong s-wave pairing participation. I don’t know if it is for this reason. Many physicals believe that there may be strong phonon intervention provided by the STO substrate in single-layer FeSe, which makes the superconducting temperature of single-layer FeSe high! Very strong phonon intervention, as an additional entry point, seems to be back to the way BCS theory is the master of the family. But if this is the case, BCS will also face a huge challenge: unconventional superconducting physics has accumulated too much evidence, results and effects that BCS cannot finalize, such as the copper-based superconducting phase diagram, such as the iron-based d-wave pairing here!
It should be pointed out that the above text has been repeated over and over again, one is because Ising is an amateur, and the other is that this is the process of superconducting research! The reason why I continue to go around in circles is because the superconductor noticed the mixed storage of d-wave and s-wave in FeSe. Here, some superconductors think of and pay attention to the electronic pairing mechanism of "orbital selection" (orbital-selective pairing) , to mediate in a central manner, neither offends d-wave nor overhead s-wave. For example, Professor Qimiao Si (Rice University) , a well-known Chinese superconducting scholar at (Sqimiao ) , is one of them. He collaborated with his collaborators Emilian M. Nica (now should work for University of British Columbia in Vancouver) and Professor Yu Rong (now works for the Department of Physics of Renmin University) to propose the so-called orbital-selective pairing state's sτ3 theory, published in 2017's "npj QM" [E. M. Nica et al, Orbital-selective pairing and superconductivity in iron selenides, npj Quant. Mater. 2, 24 (2017), https://www.nature.com/articles/s41535-017-0027-6]. This work has an impact among colleagues. Subsequently, Teacher Si and Nica published another article on in "npj QM" in 2021 [E. M. Nica & Q. Si, Multiorbital singlet pairing and d + d superconductivity, npj Quant. Mater. 6, 3, (2021), https://www.nature.com/articles/s41535-020-00304-3], significantly deepening and expanding this theory. Regarding the historical rhyme of these two tasks, in an interview with the famous science media "Science Daily" in early 2021, Teacher Si shared the interesting stories [https://www.sciencedaily.com/releases/2021/01/210125144522.html]. Interested readers can go to Yulan.
"orbital selection" pairing naturally means that electronic pairing may exhibit orbital anisotropy. Sensitively speaking, that is, the system "likes" electronic pairing of one direction or one track. For example, spin fluctuations may be dominated by the out-of-plane c-axis direction, or in-plane ab-axis direction. If this effect can be observed in the experiment, it will be an important support for the relevant "orbital selection" pairing physics, which is of great significance. In fact, in the past few years, there have been significant progress and many related work has been published. Ising here to provide evidence that the three examples published in 2022 (Figure 3 shows part of the results of two of the work) are all completed by Chinese scholars, and it also declares that "npj QM" has a timely response to this issue.
Figure 3. Some experimental evidence recently obtained from "track selection" pairing physics.
(upper part) Luo Huiqian Professor they drew anisotropy of spin fluctuations. (Lower) Professor Wang Jian and his superconducting energy gap anisotropy obtained by detecting monolayer FeAs.
https://Physics.aps.org/articles/v15/s45, https://pubs.acs.org/doi/10.1021/acs.nanolett.1c04863
(1) Li Shiliang and Luo Huiqian's team from Institute of Physics, Chinese Academy of Sciences [CaK(Fe0.96Ni0.04)4As4] The spin excitation in uses inelastic neutron scattering as a detection tool to establish the general optimal orientation of spin fluctuations in the superconducting state of this system. The research work was published in the recent Phys. Rev. Lett. [C. Liu et al, Preferred spin excitations in the bilayer iron-based superconductor CaK(Fe0.96Ni0.04)4As4 with spin-vortex crystal order, PRL 128, 137003 (2022)].
(2) Wang Jian's team from the Center for Quantum Materials of Peking University cooperated with relevant European and American teams to use in-situ scanning tunneling microscopy to study the orbital selectivity of superconducting states for the classic system of FeSe monolayer/STO substrate, revealing orbital selectivity, and confirming that this orbital selectivity still exists in the FeSe monolayer. Research is published in the recent Nano Lett. [C. F. Liu et al, Orbital-selective high-temperature Cooper pairing developed in the two-dimensional limit, Nano Lett. 22, 3245 (2022)].
Figure 4. Teacher Peng Yingying and others' RIXS experiments and some results of the (Li,Fe)OHFeSe system. The three tasks of
have their own characteristics, and it seems to be quite complementary to Teacher Si’s theoretical work. Here, Ising is so "deliberately" packaged, which seems to constitute the "resonance" understanding of this important issue by Chinese quantum materials scholars (maybe they are not closely connected) ^_^. Based on selfishness and tendency to protect children in "npj QM", Ising tried to add a few more words to the work of Teacher Peng Yingying and others. Teachers Li Shiliang, Luo Huiqian and Wang Jian are all fellow teachers. I believe they can understand this and give them understanding. Similarly, Ising is ignorant of the experimental discoveries and theoretical discussions of "orbital selection pairing" physics and failed to mention many of the work in the past few years, and also expressed his apology.
(a) (Li,Fe)OHFeSe system has a layered structure, superconducting Tc ~ 40 K, which is not much different from single-layer FeSe. Moreover, there have been recent reports that the Majorana zero mode exists, showing that it is also a vector of the non-mediocre topology quantum state .
(b) The (Li,Fe)OH layer in this compound is intervened between the single layer FeSe by means of the intercalation technology commonly used in two-dimensional material , which significantly weakens the coupling between FeSe. Finally, this win-win structure that is both a block single crystal (favorable for performance measurement) and is similar to the single-layer FeAs in terms of physical properties (favorable for revealing the superconducting physics of single-layer FeAs) is a smart move.
(c) excludes the huge impact of STO substrates in the classical system of FeSe single layer/STO substrate, and this effect may be non-inherent and decisive. Therefore, the study of (Li,Fe)OHFeSe is the study that is closest to the intrinsic properties.
(d) RIXS technology itself is a good means to study spin excitation and fluctuation, and its excitation mode changes are relatively rich.
and above are destined to be the (Li,Fe)OHFeSe bulk single crystals are the preferred object for revealing single-layer FeSe superconducting physics. There is no doubt that the reason why Teacher Peng Yingying and the others got unique and distinctive results was originally expected, and Ising here is just a presentation of reading notes and experiences.
Endlessly: Ising is an amateur, if you understand it wrong, please forgive me. If you are interested, please go to Yu to read the original text.The original link information is as follows:
Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors
Qian Xiao, Wenliang Zhang, Teguh Citra Asmara, Dong Li, Qizhi Li, Shilong Zhang, Yi Tseng, Xiaoli Dong, Yao Wang, Cheng-Chien Chen, Thorsten Schmitt & Yingying Peng
npj Quantum Materials volume 7, Article number: 80 (2022)
https://www.nature.com/articles/s41535-022-00492-0
Seven-character · Xia Reading North Building
Human history takes several floors, the three lives of the screen wall will not be planned
Reading the poem of reincarnation, and after writing, the permanent orchid will be rewarded
Sitting in the pavilion window, walking in the garden quiet and distant
A light and sunset like a waterfall, Green maple Flow down to the autumn flow
Note:
(1) Editor Ising, serving as , School of Physics, Nanjing University, and part-time editor of "npj Quantum Materials".
(2) The title of the article "There are thousands of tracks, and Cooper only loves one side" is an exaggerated word, not a physically rigorous statement. Here is just expressing the electronic Cooper pairing with orbital (direction) selectivity. The so-called "electronics have thousands of orbits, and Cooper only loves one side" seems to be a new feature of the iron-based superconducting mechanism.
(3) The picture on the background is taken from The place where the green maple leaks inside , the North Building of Nanjing University (20210724). The original poem describes the scenery of hiding North Building in summer. Here we pay tribute to the long history and abundant precipitation of superconducting physics. The image of "If you want to gain something, you need to make great efforts to be able to do one or two" (20220531).
(4) The cover image shows the image of spin fluctuations promoting Cooper's pairing. Images from Stephen Julian, Pairing with Spin Fluctuations, Physics 5 (Feb. 06), 17 (2012), https://physics.aps.org/articles/v5/17.
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