scholarly journals Strong coupling to multiple phonon modes in high-temperature superconductors

2007 ◽  
Vol 75 (21) ◽  
Author(s):  
Guo-meng Zhao
Keyword(s):  
High Temperature ◽  
Strong Coupling ◽  
High Temperature Superconductors ◽  
Phonon Modes

scholarly journals Electronic Structure and Thermodynamic Properties of High Temperature Superconductors

2021 ◽  
Author(s):  
◽  
James Glenn Storey
Keyword(s):  
High Temperature ◽  
Transition Temperature ◽  
Isotope Effect ◽  
High Temperature Superconductivity ◽  
Hole Concentration ◽  
Ac Susceptibility ◽  
High Temperature Superconductors ◽  
Phonon Modes ◽  
Bulk Properties ◽  
Doping Dependence

<p>The generic doping dependence of the thermodynamic, electrodynamic and transport properties of high-temperature superconductors remains a puzzle despite many years of study. We are still awaiting a rigorous scientific theory that explains the resistance-free flow of electric current in these novel materials. In conventional superconductors, observations of the predicted dependence of the superconducting transition temperature on isotopic mass played a key role in identifying a phononic pairing mechanism. In order to elucidate the role of phonons in the high-Tc superconductors, the oxygen isotope effect in the separate components of the penetration depth tensor of the high-temperature superconductor YBa2Cu4O8 was determined from AC susceptibility measurements, performed on biaxially-aligned powders set in epoxy. The results, extracted after assuming values for the upper cut-off radii in the particle size distributions, show that the isotope effect in the bc-plane is negligible compared to those of the ab- and ac-planes. This suggests that the electrons prefer to couple to phonon modes in which the motion of the atoms is perpendicular to the plane of transport. The electronic entropy, superfluid density, Raman response, spin susceptibility and thermoelectric power were calculated from energy-momentum dispersions determined by angle-resolved photoemission spectroscopy (ARPES). An excellent match with experimental data was obtained. This is a highly significant result because it provides the first comprehensive link between these bulk properties and the ARPES measurements which are dominated by the outermost CuO2 layer. Thus, in most respects surface effects do not appear to seriously modify or obscure the band structure which governs bulk properties. The calculations reveal the presence of a van Hove singularity (vHs) at the Fermi level (EF ) in the heavily overdoped regime to be a universal feature of the cuprates. The evolution of these properties with temperature and doping can be fully explained by the retreat of EF from the vHs and the opening of a normal state pseudogap as doping is decreased. Consequently, the pairing potential amplitude is found to be a strongly decreasing function of hole concentration, similar to the doping dependence of the exchange interaction, J. The pairing interaction is possibly a universal function of the EF â EvHs with the maximum in the transition temperature (Tc) governed by the exact magnitude of the density of states on the flanks of the vHs. These are key new discoveries which may provide a route forward to solving the puzzle of high-temperature superconductivity.</p>

scholarly journals An Analysis of Strong Coupling in the High Tc Superconductors

2014 ◽  
Vol 2014 ◽  
pp. 1-7
Author(s):  
J. H. Turner
Keyword(s):  
High Temperature ◽  
Strong Coupling ◽  
High Temperature Superconductors ◽  
High Tc Superconductors ◽  
High Tc ◽  
Cubic Symmetry

A new McMillan formula is assembled for the High Temperature Superconductors that evidences strong coupling in these materials. The McMillan-Hopfield parameter is calculated in each case, in addition to the Bergmann and Rainer enhancement frequency. The cubic symmetry of the CuO layers and the lattice cohesion and oxygen presence are suggested reasons for the effect.

scholarly journals Electronic Structure and Thermodynamic Properties of High Temperature Superconductors

2021 ◽  
Author(s):  
◽  
James Glenn Storey
Keyword(s):  
High Temperature ◽  
Transition Temperature ◽  
Isotope Effect ◽  
High Temperature Superconductivity ◽  
Hole Concentration ◽  
Ac Susceptibility ◽  
High Temperature Superconductors ◽  
Phonon Modes ◽  
Bulk Properties ◽  
Doping Dependence

<p>The generic doping dependence of the thermodynamic, electrodynamic and transport properties of high-temperature superconductors remains a puzzle despite many years of study. We are still awaiting a rigorous scientific theory that explains the resistance-free flow of electric current in these novel materials. In conventional superconductors, observations of the predicted dependence of the superconducting transition temperature on isotopic mass played a key role in identifying a phononic pairing mechanism. In order to elucidate the role of phonons in the high-Tc superconductors, the oxygen isotope effect in the separate components of the penetration depth tensor of the high-temperature superconductor YBa2Cu4O8 was determined from AC susceptibility measurements, performed on biaxially-aligned powders set in epoxy. The results, extracted after assuming values for the upper cut-off radii in the particle size distributions, show that the isotope effect in the bc-plane is negligible compared to those of the ab- and ac-planes. This suggests that the electrons prefer to couple to phonon modes in which the motion of the atoms is perpendicular to the plane of transport. The electronic entropy, superfluid density, Raman response, spin susceptibility and thermoelectric power were calculated from energy-momentum dispersions determined by angle-resolved photoemission spectroscopy (ARPES). An excellent match with experimental data was obtained. This is a highly significant result because it provides the first comprehensive link between these bulk properties and the ARPES measurements which are dominated by the outermost CuO2 layer. Thus, in most respects surface effects do not appear to seriously modify or obscure the band structure which governs bulk properties. The calculations reveal the presence of a van Hove singularity (vHs) at the Fermi level (EF ) in the heavily overdoped regime to be a universal feature of the cuprates. The evolution of these properties with temperature and doping can be fully explained by the retreat of EF from the vHs and the opening of a normal state pseudogap as doping is decreased. Consequently, the pairing potential amplitude is found to be a strongly decreasing function of hole concentration, similar to the doping dependence of the exchange interaction, J. The pairing interaction is possibly a universal function of the EF â EvHs with the maximum in the transition temperature (Tc) governed by the exact magnitude of the density of states on the flanks of the vHs. These are key new discoveries which may provide a route forward to solving the puzzle of high-temperature superconductivity.</p>

scholarly journals ARE THE NEW HIGH TEMPERATURE SUPERCONDUCTORS STRONG COUPLING SYSTEMS?

1987 ◽  
Vol 01 (07n08) ◽  
pp. 327-334 ◽  
Author(s):  
M. WEINSTEIN
Keyword(s):  
High Temperature ◽  
Strong Coupling ◽  
Qualitative Agreement ◽  
High Temperature Superconductors ◽  
Variational Calculation ◽  
Experimental Results ◽  
Bcs Theory ◽  
Collective States

Under certain circumstances the excitations of a superconductor will not be those predicted by BCS theory, but rather electron bag states. A variational calculation is used to establish this regime in which such collective states form. The fact that such bags may bind more than one electron implies the existence of more than one peak in the plots of dI/dV versus VP obtained in Giaever tunneling experiments. The estimate of the number and ratios of voltages associated with such peaks is in qualitative agreement with recent experimental results.

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