The possibility to achieve the room temperatures superconductivity has been argued for decades in the superconductivity research field. Because the real mechanism of superconductivity has never been revealed, so the estimates about the upper bound on the superconducting transition temperature are all empirical. Based on the superconductivity mechanism proposed in this paper, clearly, the room temperatures superconductivity must lie in the materials in which the three criteria for superconductivity have to be optimally satisfied. For the time being, we cannot predict what the upper bound of the superconducting transition temperature should be, but we assert that it is definitely higher than the room temperatures. We believe that the dream to achieve the room temperatures superconductivity will come true in the near future.
The physical mechanism of superconductivity is proposed on the basis of carrier-induced dynamic strain effect. By this new model, superconducting state consists of the dynamic bound state of superconducting electrons, which is formed by the high-energy nonbonding electrons through dynamic interaction with their surrounding lattice to trap themselves into the three – dimensional potential wells lying in energy at above the Fermi level of the material. The binding energy of superconducting electrons dominates the superconducting transition temperature in the corresponding material. Under an electric field, superconducting electrons move coherently with lattice distortion wave and periodically exchange their excitation energy with chain lattice, that is, the superconducting electrons transfer periodically between their dynamic bound state and conducting state. Thus, the intrinsic feature of superconductivity is to generate an oscillating current under a dc voltage. The coherence lengths in cuprates must have the value equal to an even number times the lattice constant. A superconducting material must simultaneously satisfy three criteria required by superconductivity. Almost all of the puzzling behavior of the cuprates can be uniquely understood under this new model. We demonstrate that the factor 2 in Josephson current equation, in fact, is resulting from 2V, the voltage drops across the two superconductor sections on both sides of a junction, not from the Cooper pair, and the magnetic flux is quantized in units of h/e, postulated by London, not in units of h/2e. The central features of superconductivity, such as Josephson effect, the tunneling mechanism in multijunction systems, and the origin of the superconducting tunneling phenomena, are all physically reconsidered under this superconductivity model.
A superconducting material must simultaneously satisfy the following three necessary conditions required by superconductivity.
First, the material must possess the high-energy nonbonding electrons with certain concentrations requested by coherence lengths. Following this criterion, it is not surprising that most of alkaline metal, the covalent and closed-shell compounds, and the excellent conductors, copper, silver and gold do not show superconductivity at normal condition.
Second, the material must have the three-dimensional potential wells lying in energy at above the Fermi level of the material, and the dynamic bound state of superconducting electrons in potential wells of a given superconducting chain must have the same binding energy and symmetry. According to the types of potential wells in which the superconducting electrons trap themselves to form superconducting dynamic bound state, the superconductors as a whole can be divided into two groups. One of them is called as usual as the conventional superconductors in which the potential well are formed by the microstructures of materials, such as crystal grains, clusters, nanocrystals, superlattice, and the charge inversion layer in metal surfaces. We propose that the type 1 superconductors are most likely achieved by the last kind of potential wells above. The common feature for this sort of superconductors is that the volume of the potential wells for trapping superconducting electrons varies with the techniques using to synthesize the superconductors, so that the superconducting transition temperature in conventional superconductors usually shows strongly sample-dependent and irreproducible. Since the potential wells in conventional superconductors generally have relatively large confined volume and low potential height, so the conventional superconductors normally have relatively low transition temperature, but magnesium diboride is an exception. Another group is referred to as the high-Tc superconductors in which the potential wells for trapping superconducting electrons are formed by the lattice structure of material only, such as CuO6 octahedrons and CuO5 pyramids potential wells for cuprates, BiO6 octahedron for BaKBiO3 compounds, C60 in A3C60 fullerides and FeAs4 tetrahedrons in LaOFeAs compounds. The small and fixed volume of potential wells makes the high-Tc superconductors usually have relatively high and fixed transition temperature.
Finally, in order to enable the normal state of the material being metallic, the band structure of the superconducting material must have a widely dispersive antibonding band, which crosses the Fermi level and runs over the height of potential wells. The symmetry of the antibonding band into which the superconducting electrons trap themselves to form a dynamic bound state dominates the types of the superconducting distortion waves. The typical example for superconductivity derived from this criterion perhaps belongs to transition metals and their compounds. Matthias was the first to propose that the transition temperature in transition metals depends upon the number of valence electrons per atom, Ne, and two values Ne = 5e/a for V, Nb, and Ne = 7e/a for Tc and Re are favorable to have high value of Tc.62 The similar phenomenon was also found in transition metal compounds. It has been confirmed that the density of electronic states for both bcc and hcp transition metals are all resulted from a number of the narrow density peaks derived from the d – orbitals bonding states overlapping with a broad low density of states arisen from the s – electron antibonding band. Based on the rigid band model, the Fermi levels for the transition metal with Ne = 1 to 4 all fall in the region where the density of states is dominated by the d – electron bonding states. The potential wells formed by the grain boundaries, which normally have a potential height less than 0.1 eV, should also overlap with bonding states of the d – orbitals. In this case, the dynamic bound state cannot be formed in the potential wells, thus it is not surprising that the superconductivity cannot be found in these transition metals. However, for V and Nb, which have five valence electrons, Ne = 5e/a, the Fermi level shifts toward the high energies at where the density of states is mainly resulted from the s electron-antibonding band. In this circumstance, the energy levels at the top of potential wells formed by grain boundaries are derived from the s electron-antibonding band, and so the superconducting state can be achieved and has a s-symmetry wave. The similar process is repeated for the transition metal Tc and Re with Ne = 7 e/a.
On the basis of the mechanism of superconductivity proposed above, the key point to achieve superconductivity is that the superconducting electron must periodically exchange its excitation energy with chain lattice. That is, the excitation energy of the superconducting electrons must be reversibly transferred between superconducting electrons and chain lattice. It is well known that the interaction between electrons and atomic magnetic moments is irreversible, which, thus, in any case cannot become the driving force of superconductivity. However, it can be seen from this new model that superconductivity and atomic magnetic moments in principle are not intrinsically exclusive each other. As long as there exists the same magnetic moment in every potential well in a given superconducting chain, as in the case of the ferromagnetic materials LaOFeAs, and the three necessary conditions required for superconductivity are satisfied, the superconducting state can be formed and the superconducting process will persist without dissipating energy. Since the electromagnetic interaction energy for superconducting electrons with atom magnetic moment maintains the same in every potential well, thus the binding energy of superconducting electrons in potential wells cannot be affected by the atom magnetic moment, and so the scattering centers for superconducting electrons cannot be introduced. But this condition essentially cannot be achieved for conventional superconductors, so the atomic magnetic moments are generally detrimental to superconductivity.
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