arXiv subjects

Tian De Cao

Publications and source records attributed to Tian De Cao.

At least 19 recordsLinked to original sources

arXiv2013

What is the highest Tc for phonon mediated superconductivity?

We suggest that the high-temperature superconductivity is attributed to the director-roles of van Hove singularity between electron-electron interaction and electron-phonon interaction. Difference between the critical temperature and pairing temperature is presented, and the Fermi arc, d-wave symmetry and poor conductor etc are discussed. Particularly, non-s-wave symmetry is predicted to have the highest Tc for superconductors.

cond-mat.supr-con↗

arXiv2012

Ten questions and answers about superconductivity

This work answers the basic questions of superconductivity in a question-and-answer format. We extend a basic hypothesis to various superconductors. This hypothesis is that superconductivity requires that the pairing gap locates around the Fermi level. On the basis of this hypothesis our calculations give the so-called three factor theory with which some key problems of the high temperature superconductivity are explained.

physics.gen-ph↗

arXiv2012

A new boson-fermion model of superconductivity

It is shown that the superconducting energy gap necessarily lead to the disappearance of some quasi-electrons, thus we suggest a new boson-fermion Hamiltonian to describe superconductivity. The new supercurrent equations are derived with this Hamiltonian. Some new results can be found besides the zero resistance effect, the Meissner effect and the magnetic flux quantum can be explained.

physics.gen-ph↗

arXiv2012

Competition between superconductivity and spin density wave

The Hubbard model has been investigated widely by many authors, while this work may be new in two aspects. One, we focus on the possible effects of the positions of the gaps associated with the pairing and the spin density wave. Two, we suggest that the models with different parameters are appropriate for different materials (or a material in different doped regions). This will lead to some new insights into the high temperature superconductors. It is shown that the SDW can appear at some temperature region when the on-site Coulomb interaction is larger, while the SC requires a decreased U at a lower temperature. This can qualitatively explain the relationship between superconducting and pseudogap states of Cu-based superconductors in underdoped and optimally doped regions. The superinsulator is also discussed.

physics.gen-ph↗

arXiv2012

Reverse circling supercurrents along a superconducting ring

The reason why high temperature superconductivity has been being debated is that many basic ideas in literatures are wrong. This work shows that the magnetic flux quantum in a superconducting ring have been inaccurately explained in fact, thus we suggest a reinterpretation of the magnetic flux quantum in a superconducting ring on the basis of the translations of pairs. We also predict that the internal and external surface of a superconducting tube have the reverse circling supercurrents. This means that a more thick tube could trap a larger amount of flux. Both the magnetic flux quantum and the reverse circling supercurrents could not be found with the London equation.

physics.gen-ph↗

arXiv2011

Possible temperature control DC switch effect between two superconductors

The lifetime of an electron pair could not be unlimited long, on the basis of this, we suggest a model. The model means that the movements of charge carriers in a superconductor should have three forms: the single-electron movement, the single-pair movement, and the revolving around the mass center of two electrons in a pair. Thus the current in a superconductor has three possible parts. Similarly, there should be three possible effects in a SIS junction: the tunneling of single electron, the tunneling of single pair, and the pair-forming following the pair-breaking. This paper will discuss these problems and present a possible temperature control DC switch effect between two superconductors.

physics.gen-ph↗

arXiv2011

Crossover from a pseudogap state to a superconducting state

On the basis of our calculation we deduce that the particular electronic structure of cuprate superconductors confines Cooper pairs to be firstly formed in the antinodal region which is far from the Fermi surface, and these pairs are incoherent and result in the pseudogap state. With the change of doping or temperature, some pairs are formed in the nodal region which locates the Fermi surface, and these pairs are coherent and lead to superconductivity. Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface. It is also shown that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure, and non-s wave symmetry gap favors the high-temperature superconductivity. Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.

physics.gen-ph↗

arXiv2010

The free states-related Fermi pocket of cuprate superconductors

To stress the effect of the pairing position deviating from the Fermi level, we must investigate the pairs in the wave vector space, and then we use the dynamic equation to study some correlation functions. This article shows that the Fermi pocket is related to the effect of free electron states on the ARPES experiment. This also leads us to understand why the Fermi arc appears in Bi2212 while the Fermi pocket appears in Bi2201 with the valence bandwidth and the work function known for them.

physics.gen-ph↗

arXiv2010

Theory of angle-resolved photoemission experiments on a two-band model

Considering the electron states inside and outside the solid, we derive a formula of photoemission intensity. A general theoretical way to determine electronic structures of solids from ARPES experiments is outlined. It is shown that the spectral function inside the solids cannot be measured directly by ARPES, the effects of free electron states on the electronic structure observed by ARPES measurements must be considered, and the results from ARPES experiments cannot be understood until these results have been made consistent with a theoretical calculation.

cond-mat.supr-con↗

arXiv2009

Basic principle of superconductivity

The basic principle of superconductivity is suggested in this paper. There have been two vital wrong suggestions on the basic principle, one is the relation between superconductivity and the Bose-Einstein condensation (BEC), and another is the relation between superconductivity and pseudogap.

physics.gen-ph↗

arXiv2009

Pseudogap associated with precursor pairing

This work argues that the off-diagonal long range order (ODLRO) is not necessary for showing superconductivity while the electron pairing around Fermi surface is sufficient for superconductivity. It is shown that there exists the pseudogap state associated with the electron pairing in real space and the high temperature superconductivity could be only found in the metallic region near the Mott metal insulator transition (MIT).

physics.gen-ph↗

arXiv2009

Competition between singlet and triplet superconductivity

The competition between singlet and triplet superconductivity is examined in consideration of correlations on an extended Hubbard model. It is shown that the triplet superconductivity may not be included in the common Hubbard model since the strong correlation favors the singlet superconductivity, and thus the triplet superconductivity should be induced by the electron-phonon interaction and the ferromagnetic exchange interaction. We also present a superconducting qualification with which magnetism is unbeneficial to superconductivity.

physics.gen-ph↗

arXiv2009

High temperature superconductivity in metallic region near Mott transition

The spin-singlet superconductivity without phonons is examined in consideration of correlations on an extended Hubbard model. It is shown that the superconductivity requires not only the total correlation should be strong enough but also the density of state around Fermi energy should be large enough, which shows that the high temperature superconductivity could only be found in the metallic region near the Mott metal insulator transition (MIT). Other properties of superconductors are also discussed on these conclusions.

physics.gen-ph↗

arXiv2009

Positive and inverse isotope effect on superconductivity

This article improves the BCS theory to include the inverse isotope effect on superconductivity. An affective model can be deduced from the model including electron-phonon interactions, and the phonon-induced attraction is simply and clearly explained on the electron Green function. The focus of this work is on how the positive or inverse isotope effect occurs in superconductors.

physics.gen-ph↗

arXiv2009

Multiferroics seen from theoretic derivation of a tight binding model

One presented some lattice models, while the theoretic derivation has not been found, and the importance of correlation effects has to be emphasized. On the basis of the non-relativistic Hamiltonian from the Dirac equation, we derive in detail a tight binding model. We find that both ferromagnetism and ferroelectricity are from the correlation effect between electrons, and magnetic and electric orders are strongly coupled due to the spin-orbit interaction.

physics.gen-ph↗

arXiv2009

High-Tc superconductivity originated from strong spin-charge correlation: indication from linear temperature dependence of resistivity

Both the highest- and the linear temperature dependence of the resistivity in wide temperature range appear at the optimally doped regions of Cu-based superconductors1,2,3,4,5, and the highest- of Fe-based superconductors6,7 are also associated with the linear temperature dependence of the resistivity in normal states near superconducting states. This means that the high temperature superconductivity and the linear temperature dependence of the resistivity should be dominated by the same mechanism. This letter on theoretic calculation clearly shows that strong spin-charge correlation dominated resistivity behaves the linear temperature dependence, thus high-temperature superconductivity should be induced by strong spin-charge correlation.

physics.gen-ph↗

arXiv2009

A new model and its physics

The high temperature superconductivity in cuprate materials1 has puzzled scientists over twenty years. We must find a new way to understand superconductivity. It is found the spin-charge correlation may dominate the superconductivity2, and we base our judgment upon the features of various superconductors. Thus we presented the idea that superconductivity could be described by correlations. To develop this idea into a quantitative theory, the first work is to give a model and show that various superconductivities can be included in this model. Moreover, superconductivity can originate from the spin-singlet pairing3 or from the spin-triplet pairing4. The spin-singlet pairing favors to appear at the border of antiferromagnetism5, while the spin-triplet pairing favors to appear at the border of ferromagnetism6. The coexistence between superconductivity and magnetism is also possible7,8. Therefore, the second work is to reveal the relation between superconductivity and magnetism.

physics.gen-ph↗

arXiv2008

Superconductors described with CSM.(a new paper, 2008-10-16)

The properties of the known superconductors can be explained with the correlations dominated superconducting mechanism (CSM). The correlations have the spin correlation, the charge correlation and the spin-charge correlation, and their strengths can be described by the related correlation lengths in their correlation functions. Our evaluation from many superconductors is that superconductivities occur if both the spin correlation and the charge correlation are stronger, and the calculation of a Hubbard model showed that the spin-charge correlation may govern superconductivities1. Afterwards, this mechanism has led a model which includes various superconductivities and magnetisms, and the relation between superconductivities and magnetisms can be understood on this model2 (these results have been shown by calculations). This mechanism is very practical, for example, to turn a material into a superconductor or increase the Tc of a superconductor, what we will do is to increase the spin-charge correlation. In this letter, we first describe the relations between the spin-charge correlation, the spin correlation and the charge correlation, take these relations as the basis of constructing a new phase diagram, and then classify the known superconductors into various sections in this phase diagram. This letter also gives a new explanation about the pressure effect on Tc, the isotope effect on Tc and the pairing symmetry with the CSM.

physics.gen-ph↗