Abstract:
High temperature superconductivity is a remarkable phenomenon in which certain materials
conduct electricity with zero electrical resistance and expel an internal magnetic field at
temperatures higher than 30 K. Bednorz and Muller discovered this phenomenon in copper oxide
compounds. Following this finding, researchers continued looking for new high-temperature
superconducting families, which is still going on. Superconductivity and magnetism were
formerly assumed to be mutually exclusive until the discovery of several rare earth ternary
compounds that demonstrate their coexistence. Layered superconductivity and magnetism
coexist in several of the recently found iron-based materials.
By establishing a compressive model and generic equations, the major goal of this research work
is to examine the nature of high-temperature superconductivity in a two-band model for Na and
Ni doped BaFe2As2 iron-based superconductors. By developing a model Hamiltonian for the
system and using quantum field theory Green's function formalism and phenomenological
Ginzburg-Landau (GL) free energy density functional theory in a two-band model, we obtained
the expression for the superconducting order parameters, superconducting transition temperature,
density of states, electronic specific heat, entropy, condensation energy, the possible coexistence
of superconductivity and spin density wave (SDW), the upper critical magnetic field (HC2),
Ginzburg-Landau coherence length ( ) and Ginzburg-Landau penetration depth ( ).
Besides, the effects of electron and hole intra-band superconducting pairing interactions, as well
as the inter-band pair interaction between the two bands in the two-band model in the Bardeen,
Cooper, and Schrieffer (BCS) mean field approximation for Na and Ni doped BaFe2As2
superconductor, have been studied in this dissertation.
Phase diagrams of superconducting order parameter ( ) versus temperature, density of state
versus temperature, electronic specific heat versus temperature, entropy versus temperature, and
condensation energy versus temperature have been plotted for all bands using the obtained
expressions. The effect of all intra-inter band pairings on the total superconducting order
parameters at zero temperature is 17 meV, and demonstrates the contribution
of combined intra-inter band pairing in the two-band model on strengthening the pairing
mechanism of high-temperature superconductors.
Parameterization of High Temperature Superconductivity
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We have also found a mathematical expression for the relationship between the superconducting
transition temperature (TC) and the inter-band pairing potential (Ueh), as well as the relationship
between density of states and excitation energy and temperature, and plotted their phase
diagrams. In addition, we are curious to observe whether the transition between the two states is
first order or the two pure phases are separated by an intermediate phase with superconductivity
and SDW orders coexisting. In the same way, phase diagrams of TC versus in the electron
and hole intra-bands, as well as in the inter-band, are plotted using the experimental and the
theoretical values of the parameters in the generated equations. The phase diagram of spin
density wave transition temperature (TSDW) versus spin density wave order parameter ) is
depicted using the obtained equations for the spin density wave transition temperature (TSDW)
dependency on the spin density wave order parameter ( ). The possibility of the coexistence
of superconductivity and spin density wave (SDW) in the two-band model of Ba1-xNaxFe2As2 is
demonstrated by merging the two-phase diagrams.
Finally, we have investigated the upper critical magnetic field (HC2), Ginzburg-Landau
coherence length ( ), and Ginzburg-Landau penetration depth ( ) in a two-band model of Na
and Ni doped BaFe2As2 iron-based superconductors. The phase diagrams for the temperature
dependence of upper critical magnetic fields parallel and perpendicular to the crystallographic c axis versus temperatures for our considered system are displayed in the same way, using the
experimental values in the obtained equations. The temperature dependence of GL coherence
lengths, penetration depths, and the characteristics parameters in the ab-plane, and along the c axis versus temperature are plotted using the obtained expressions. We showed that the upper
critical magnetic field along the ab-plane is greater than along the crystallographic c-axis, and
thus our iron-based superconductor has higher upper critical magnetic field values.
In general, we presented a framework for understanding some of the universal features of high temperature iron-based superconductors in the current work. The conclusions of this study are in
a broad agreement with prior experimental findings.
Parameterization of High Temperature Superconductivity
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Keywords: High-temperature superconductivity, iron-based superconductors, Ba1-xNaxFe2As2,
Ba(Fe1-xNix)2As2, two-band model, Green‟s function, Ginzburg-Landau theory, order
parameters( ), density of states(DS), electronic specific heat (Ces), entropy(S), condensation
energy(EC), spin density wave, coexistence of superconductivity and spin density wave, upper
critical magnetic field(HC2), GL coherence length(ӠGL), GL penetration depth(λGL).