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[en] This year is the 10th anniversary of the discovery of the first high-transition temperature (Tc) superconducting compounds. Since then, these materials have taught us much new physics. Few issues have been s hotly debated as the controversy about flux lattice melting. Two recent papers by Kes and co-workers at the University of Leiden drive right to the heart of this issue. The superconducting state, in general, does not like magnetic fields. To avoid the presence of a magnetic field in its interior, a superconductor responds in interesting ways. There are two important critical fields for a superconductor that define its response. The first is the lower critical field Hcl; for fields less than this value, superconductors expel the applied field (the Meissner effect). The second is the upper critical field Hc2; for fields above this value, superconductivity is destroyed. The high-Tc superconductors are examples of what are called open-quotes strongly type IIclose quotes materials, those for which Hc2>>Hcl. All of the technologically important superconductors are of this type, and so their study is essential if applications are to be realized. This article goes on to discuss the research, background, and future of these. 4 refs., 1 fig
[en] A model for uniaxial media which computed an irreversible component that lies along the easy axis and a reversible component that is sensitive to off-axis fields has been developed. By assuming that the distribution of critical fields is Gaussian, then the major loop will be an error function. For small rotating fields, the magnetization will oscillate between two extreme values on the same side of the origin. For larger fields, the irreversible magnetization can change sign and the magnetization will rotate about the origin. We start with a demagnetized specimen, then apply a field that starts along the easy axis and then rotates about the origin. For the first 90°, the magnetization will follow a circular path. Beyond that point the irreversible magnetization will start to change, and the magnetization will start to spiral in towards the origin. Therefore, we will get asymmetrical loops which can be compared with experiment.
[en] The behavior of nuclear matter in an external static field acting in a region of finite dimensions is investigated. It is shown that, at some critical value V/sub c/ of the field intensity, nuclear matter is expelled from the region in which the field acts. The quantity V/sub c/ is estimated, and found to be almost an order of magnitude smaller than the Fermi energy. The use of a mechanical analogy reveals that the equilibrium equation for nuclear matter in an external field can have three types of solutions simultaneously, namely, the traditional bubble and soliton solutions and a new, hybrid solution, and allows the solution at a qualitative level of the problem of localized hadronic states
[en] Complete text of publication follows. If thermal fluctuations are ignored, the theory for isolated single-domain particles predicts a very simple hysteresis loop involving jumps between two stable magnetization curves. The associated first-order reversal curve (FORC) function for randomly oriented single-domain particles has some distinctive features that are observed in real samples: a negative region near the Hu axis and a sharp ridge (theoretically infinite) on the Hc axis. However, in real samples there is generally a symmetric spreading about the Hc axis that is not predicted by the theory. This could be due to particle interactions or thermal fluctuations. A new theory is developed for the effect of thermal interactions in systems of randomly oriented single-domain particles. As the field approaches the critical field for instability, the probability of a jump increases because the energy barrier between states is decreasing. The main hysteresis loop shrinks and the simple two-curve hysteresis of a given particle is replaced by an area in which a first-order reversal curve passes through every point. All jumps are replaced by continuous transitions, the ridge becomes finite, and the symmetry of the FORC function is broken.
[en] The dependence of the upper critical field on temperature for all temperatures is calculated in the framework of the extended saddle-point model. A discussion is given of the present experimental situation. copyright 1997 The American Physical Society
[en] The era of near-room-temperature superconductivity started after experimental discovery by Drozdov et al (2015 Nature 525 73) who found that compressed H3S exhibits superconducting transition at T c = 203 K. To date, the record near-room-temperature superconductivity stands with another hydrogen-rich highly compressed compound, LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001), which has critical temperature of In this paper, we analyse available upper critical field, B c2(T), data for LaH10 (Drozdov et al 2019 Nature 569 528) and report that this compound in all considered scenarios has the ratio of T c to the Fermi temperature, T F, 0.009 < T c/T F < 0.038, which is typical range for unconventional superconductors. In attempt to extend our finding, we examined experimental B c2(T) data for superconductors in the palladium-hydrogen system and surprisingly find that PdHx compounds have the ratio of 0.008 < T c/T F < 0.012. Taking in account that H3S has the ratio of 0.012 < T c/T F < 0.039 (Talantsev 2019 Modern Phys. Lett. B 33 1950195) we come to conclusion that in the Uemura plot all discovered to date hydrogen-rich superconductors, i.e. PdHx, H3S and LaH10, lie in same band as all unconventional superconductors, particularly heavy fermions, fullerenes, pnictides, and cuprates, and former should be classified as a new class of unconventional superconductors. (paper)