scholarly journals High Temperature Superconductors Through the Van Hove Singularity

2021 ◽  
Author(s):  
◽  
Keryn Anne Williams
Keyword(s):  
High Temperature ◽  
Critical Temperature ◽  
High Temperature Superconductors ◽  
Bcs Theory ◽  
Magnetic Data ◽  
Metallic State ◽  
Solid State Physics ◽  
Temperature Dependent ◽  
Doped Semiconductors ◽  
Exotic Physics

<p>The antibonding VHS of the high temperature superconductor Bi-2212 appears in the extreme overdoped regime, a part of the cuprate phase diagram little studied to date. Observation of this VHS motivated taking a fresh look at the cuprates using fundamentals of electronics as the foundation for understanding the physics involved in the superconductivity of these materials. In the study of the high temperature superconductors it appears important questions have been overlooked, notably the possible contribution of the gapped state and whether these materials are better considered as doped semiconductors rather than as 'poor' metals. We also find the question of the contribution of oxygen, a substance with a strong magnetic signature, to data of the oxygen-doped cuprates has been neglected. Comparison with non-oxygen doping is supportive of the view the oxygen dopant contributes noticeably to magnetic data. Through magnetic susceptibility measurements the antibonding VHS location, predicted by use of Fermi liquid theory, is well confirmed in polycrystals of the lead-doped cuprate Bi-2212. It was found that the peak in the DOS at the VHS produces no corresponding local peak in the critical temperature versus doping. Instead, the VHS appears associated with the disappearance of the superconductivity, rather than with the maximum critical temperature. We find the metal-insulator transition plays an important role. There are two of these in the cuprates, a horizontal doping dependent one and a vertical temperature dependent one. They affect each other. Noting the consequences of doping an insulator until a metallic state is reached enables a connection to be made between doping and pressure. Three requirements are identified for superconductivity to occur: 1. screening 2. pairing 3. charge mobility Each requirement may be separately satisfied in a manner whereby each can vary differently as a function of the same variable. The superconductivity of the cuprates is found to arise out of an underlying non-metallic state. As such, BCS theory, being formulated to explain superconductivity arising from metallic conduction, cannot be directly applicable. However, although HTS materials are a rich repository of both novel and familiar solid state physics, evidence does not appear to support the notion that superconductivity in the cuprates is caused by "exotic" physics. We also find cause for optimism regarding the development of new or improved superconducting materials.</p>

scholarly journals High Temperature Superconductors Through the Van Hove Singularity

2021 ◽  
Author(s):  
◽  
Keryn Anne Williams
Keyword(s):  
High Temperature ◽  
Critical Temperature ◽  
High Temperature Superconductors ◽  
Bcs Theory ◽  
Magnetic Data ◽  
Metallic State ◽  
Solid State Physics ◽  
Temperature Dependent ◽  
Doped Semiconductors ◽  
Exotic Physics

<p>The antibonding VHS of the high temperature superconductor Bi-2212 appears in the extreme overdoped regime, a part of the cuprate phase diagram little studied to date. Observation of this VHS motivated taking a fresh look at the cuprates using fundamentals of electronics as the foundation for understanding the physics involved in the superconductivity of these materials. In the study of the high temperature superconductors it appears important questions have been overlooked, notably the possible contribution of the gapped state and whether these materials are better considered as doped semiconductors rather than as 'poor' metals. We also find the question of the contribution of oxygen, a substance with a strong magnetic signature, to data of the oxygen-doped cuprates has been neglected. Comparison with non-oxygen doping is supportive of the view the oxygen dopant contributes noticeably to magnetic data. Through magnetic susceptibility measurements the antibonding VHS location, predicted by use of Fermi liquid theory, is well confirmed in polycrystals of the lead-doped cuprate Bi-2212. It was found that the peak in the DOS at the VHS produces no corresponding local peak in the critical temperature versus doping. Instead, the VHS appears associated with the disappearance of the superconductivity, rather than with the maximum critical temperature. We find the metal-insulator transition plays an important role. There are two of these in the cuprates, a horizontal doping dependent one and a vertical temperature dependent one. They affect each other. Noting the consequences of doping an insulator until a metallic state is reached enables a connection to be made between doping and pressure. Three requirements are identified for superconductivity to occur: 1. screening 2. pairing 3. charge mobility Each requirement may be separately satisfied in a manner whereby each can vary differently as a function of the same variable. The superconductivity of the cuprates is found to arise out of an underlying non-metallic state. As such, BCS theory, being formulated to explain superconductivity arising from metallic conduction, cannot be directly applicable. However, although HTS materials are a rich repository of both novel and familiar solid state physics, evidence does not appear to support the notion that superconductivity in the cuprates is caused by "exotic" physics. We also find cause for optimism regarding the development of new or improved superconducting materials.</p>

scholarly journals Torque magnetometry on single-crystal high-temperature superconductors near the critical temperature: A scaling approach

2000 ◽  
Vol 62 (1) ◽  
pp. 631-639 ◽  
Author(s):  
J. Hofer ◽  
T. Schneider ◽  
J. M. Singer ◽  
M. Willemin ◽  
H. Keller ◽  
...  
Keyword(s):  
High Temperature ◽  
Critical Temperature ◽  
Single Crystal ◽  
High Temperature Superconductors ◽  
Torque Magnetometry

scholarly journals Superconducting Hydrides Under Pressure

2020 ◽  
Vol 11 (1) ◽  
pp. 57-76 ◽  
Author(s):  
Chris J. Pickard ◽  
Ion Errea ◽  
Mikhail I. Eremets
Keyword(s):  
Hydrogen Sulfide ◽  
High Temperature ◽  
Critical Temperature ◽  
First Principles ◽  
High Temperature Superconductors ◽  
Microscopic Structure

The measurement of superconductivity at above 200 K in compressed samples of hydrogen sulfide and in lanthanum hydride at 250 K is reinvigorating the search for conventional high temperature superconductors. At the same time, it exposes a fascinating interplay between theory, computation, and experiment. Conventional superconductivity is well understood, and theoretical tools are available for accurate predictions of the superconducting critical temperature. These predictions depend on knowing the microscopic structure of the material under consideration, which can now be provided by computational first-principles structure predictions. The experiments at the megabar pressures required are extremely challenging, but, for some groups at least, permit the experimental exploration of materials space. We discuss the prospects for the search for new superconductors, ideally at lower pressures.

scholarly journals THE 3d-TO-4s-BY-2p HIGHWAY TO SUPERCONDUCTIVITY IN CUPRATES

2002 ◽  
Vol 16 (30) ◽  
pp. 4577-4585 ◽  
Author(s):  
TODOR M. MISHONOV ◽  
JOSEPH O. INDEKEU ◽  
EVGENI S. PENEV
Keyword(s):  
High Temperature ◽  
Critical Temperature ◽  
High Temperature Superconductors ◽  
Electron Exchange ◽  
Theoretical Physics ◽  
Pairing Interaction ◽  
Electron Pairing

High-temperature superconductors are nowadays found in great variety and hold technological promise. It is still an unsolved mystery that the critical temperature T c of the basic cuprates is so high. The answer might well be hidden in a conventional corner of theoretical physics, overlooked in the recent hunt for exotic explanations of new effects in these materials. A forgotten intra-atomic s–d two-electron exchange in the Cu atom is found to provide a strong (~ eV) electron pairing interaction. A Bardeen–Cooper–Schrieffer approach can explain the main experimental observations and predict the correct dx2-y2 symmetry of the gap.

Fluorine Substitution in High Temperature Superconductors

1999 ◽  
Vol 13 (09n10) ◽  
pp. 973-978 ◽  
Author(s):  
E. Bellingeri ◽  
G. Grasso ◽  
R. Gladyshevskii ◽  
E. Giannini ◽  
F. Marti ◽  
...  
Keyword(s):  
Crystal Structure ◽  
High Temperature ◽  
Critical Temperature ◽  
Low Temperature ◽  
High Temperature Superconductors ◽  
Irreversibility Field ◽  
Fluorine Substitution ◽  
Superconducting Phases

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. We obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200-400 °C) fluorination process. Fluorine substitutes completely the oxygen sites in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and anions were induced, especially in the Bi and partially in the Sr layer. F-doped Tl(1223) has been prepared in the same way as Bi(2223) and Bi(2212) (low temperature fluorination), but also starting from precursor containing fluorides of different elements. No significant differences in the crystal structure have been observed between the Tl-based samples with F inclusions and without. The critical temperature (116 K) remains unchanged but a significant increase of the irreversibility field at low temperature was found.

scholarly journals Unusual symmetries of the order parameter in cuprates

2020 ◽  
Vol 2 (4) ◽  
pp. 207-212
Author(s):  
Tran Van Luong ◽  
Nguyen Thi Ngoc Nu
Keyword(s):  
High Temperature ◽  
Order Parameter ◽  
Cuprate Superconductors ◽  
High Temperature Superconductivity ◽  
Spin Fluctuation ◽  
High Temperature Superconductors ◽  
Coulomb Repulsion ◽  
Bcs Theory ◽  
S Wave ◽  
Wave Symmetry

The BCS superconducting theory, introduced by J. Bardeen, L. Cooper and R. Schriffer in 1957, succeeded in describing and satis-factorily explaining the nature of superconductivity for low-temperature superconductors. However, the BCS theory cannot explain the properties of high-temperature superconductors, discovered by J. G. Bednorz and K. A. Müller in 1986. Although scientists have found a lot of new superconductors and their transition temperatures are constantly increasing, most high-temperature superconductors are found by experiment and so far no theory can fully explain their properties. Many previous studies have suggested that the order parameter in high-temperature copper-based superconductors (cuprate superconductors - cuprates) is in the form of d-wave symmetry, but recent results show that the order parameter has an extended s-wave symmetry (extended s wave). Studying the symmetric forms of order parameters in cuprate can contribute to understanding the nature of high-temperature superconductivity. In this article, the authors present an overview of the development of high-temperature supercon-ductors over the past 30 years and explains unusual symmetries of the order parameter in copper-based superconductors. The com-petition of three coupling mechanisms of electrons in cuprates (the mechanism of coupling through coulomb repulsion, electron-phonon mechanism and spin-fluctuation mechanism) affects the unusual symmetry of the order parameter. The solution of the self-consistency equation in simple cases has been found and the ability to move the phase within the superconducting state has been shown.

scholarly journals A Schematic Two Overlapping-Band Model for Superconducting Sulfur Hydrides: The Isotope Mass Exponent

2019 ◽  
Vol 2019 ◽  
pp. 1-7 ◽  
Author(s):  
R. M. Méndez-Moreno
Keyword(s):  
High Temperature ◽  
Critical Temperature ◽  
Fermi Surface ◽  
Coupling Parameter ◽  
High Temperature Superconductors ◽  
Band Model ◽  
Intermediate Coupling ◽  
Electronic Density ◽  
Knowing That ◽  
Multicomponent Model

The high value of the isotope shift in sulfur hydrides supports a phonon-mediated pairing scenario of superconductivity for these high-temperature superconductors which are consistent with the Bardeen–Cooper–Schrieffer (BCS) framework. Knowing that a large electronic density of states enhances the critical temperature (Tc), generalized Fermi surface topologies are used to increase it. A multicomponent model within the BCS framework is proposed in this work for sulfur hydride superconductors. This model is used to evaluate some properties of the H3S superconductor. Strong and intermediate coupling effects are taken into account with the effective McMillan approximation, and the isotope coefficient is evaluated as a function of the coupling parameter as well as other relevant parameters of the model.

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