The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
D.g. Walmsley - One of the best experts on this subject based on the ideXlab platform.
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BCS Theory has to be overhauled: reassurance from numerical survival rate
Solid State Communications, 2016Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Abstract The BCS Theory has conceptual and numerical difficulties. We have previously overhauled it with a new scheme of phonon-mediated electron pairing that can be expressed analytically in terms of an empirical pairing survival rate factor, S ( q ) = 0 or 1/2, depending on phonon momentum, q . Now we evaluate S ( q ) numerically entirely from experimental data on normal state electrical resistivity and on superconducting tunnelling conductance. The empirical and numerical S ( q ) are reassuringly close in aluminium and lead and particularly so in two other cases, niobium and tantalum.
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Further test of new pairing scheme used in overhaul of BCS Theory
Physica C: Superconductivity and its Applications, 2014Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Abstract A new electron pairing scheme, rectifying a fundamental flaw of the BCS Theory, is tested extensively. It postulates that superconductivity arises solely from residual umklapp scattering when it is not in competition for the same destination electron states with normal scattering. It reconciles a long standing theoretical discrepancy in the strength of the electron–phonon interaction between the normal and superconductive states. The new scheme is exploited to calculate the superconductive electron–phonon spectral density, α 2 F ( ν ) , entirely on the basis of normal state electrical resistivity. This leads to first principles superconductive properties (zero temperature energy gap and tunnelling conductance) in seven metals which turn out to be highly accurate when compared with known data; in other cases experimental verification is invited. The transition temperatures involved vary over almost three orders of magnitude: from 9.5 K for niobium to 0.012 K for tungsten.
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New pairing scheme to overhaul BCS Theory
Solid State Communications, 2014Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Abstract In a superconductor pair occupancy probabilities are doubly defined with conflicting values when normal and umklapp scattering coexist with the same destination momentum. To resolve this issue a new pairing scheme is introduced to assert normal–umklapp frustration under such circumstances. Superconductivity then arises solely from residual umklapp scattering to destination momenta not reached by normal scattering. Consequent T c calculations from first principles for niobium, tantalum, lead and aluminum turn out to be accurate within a few percent. A new perspective is revealed to support Matthias׳ rule. New light is also shed relevant to the future study of metallic hydrogen.
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Coulomb repulsion and Tc in BCS Theory of superconductivity
Physical Review B, 2005Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Coulomb repulsion among the many electrons in a metal is in a balance, which can be toppled by even a weak electron-phonon attractive interaction. Therefore neglecting the Coulomb term from the BCS reduced Hamiltonian has little effect on Tc. This is shown by a field-theoretic argument, an analysis based on the Bogoliubov model potential and a direct numerical calculation. Detailed knowledge about electrons and phonons for various materials can be incorporated into the BCS Theory through a refined treatment of the self-consistent gap equation. Consequently the universal ratio 3.5 in the BCS Theory is replaced by a range of values varying from 3.51 for Ga to 4.76 for Hg. It is found that the phonon cutoff frequency is much lower than the Debye frequency. Extraordinarily high Tc could be expected if all phonons were involved in pairing electrons in a BCS superconductor.
X.h. Zheng - One of the best experts on this subject based on the ideXlab platform.
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BCS Theory has to be overhauled: reassurance from numerical survival rate
Solid State Communications, 2016Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Abstract The BCS Theory has conceptual and numerical difficulties. We have previously overhauled it with a new scheme of phonon-mediated electron pairing that can be expressed analytically in terms of an empirical pairing survival rate factor, S ( q ) = 0 or 1/2, depending on phonon momentum, q . Now we evaluate S ( q ) numerically entirely from experimental data on normal state electrical resistivity and on superconducting tunnelling conductance. The empirical and numerical S ( q ) are reassuringly close in aluminium and lead and particularly so in two other cases, niobium and tantalum.
-
Further test of new pairing scheme used in overhaul of BCS Theory
Physica C: Superconductivity and its Applications, 2014Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Abstract A new electron pairing scheme, rectifying a fundamental flaw of the BCS Theory, is tested extensively. It postulates that superconductivity arises solely from residual umklapp scattering when it is not in competition for the same destination electron states with normal scattering. It reconciles a long standing theoretical discrepancy in the strength of the electron–phonon interaction between the normal and superconductive states. The new scheme is exploited to calculate the superconductive electron–phonon spectral density, α 2 F ( ν ) , entirely on the basis of normal state electrical resistivity. This leads to first principles superconductive properties (zero temperature energy gap and tunnelling conductance) in seven metals which turn out to be highly accurate when compared with known data; in other cases experimental verification is invited. The transition temperatures involved vary over almost three orders of magnitude: from 9.5 K for niobium to 0.012 K for tungsten.
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New pairing scheme to overhaul BCS Theory
Solid State Communications, 2014Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Abstract In a superconductor pair occupancy probabilities are doubly defined with conflicting values when normal and umklapp scattering coexist with the same destination momentum. To resolve this issue a new pairing scheme is introduced to assert normal–umklapp frustration under such circumstances. Superconductivity then arises solely from residual umklapp scattering to destination momenta not reached by normal scattering. Consequent T c calculations from first principles for niobium, tantalum, lead and aluminum turn out to be accurate within a few percent. A new perspective is revealed to support Matthias׳ rule. New light is also shed relevant to the future study of metallic hydrogen.
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Coulomb repulsion and Tc in BCS Theory of superconductivity
Physical Review B, 2005Co-Authors: X.h. Zheng, D.g. WalmsleyAbstract:Coulomb repulsion among the many electrons in a metal is in a balance, which can be toppled by even a weak electron-phonon attractive interaction. Therefore neglecting the Coulomb term from the BCS reduced Hamiltonian has little effect on Tc. This is shown by a field-theoretic argument, an analysis based on the Bogoliubov model potential and a direct numerical calculation. Detailed knowledge about electrons and phonons for various materials can be incorporated into the BCS Theory through a refined treatment of the self-consistent gap equation. Consequently the universal ratio 3.5 in the BCS Theory is replaced by a range of values varying from 3.51 for Ga to 4.76 for Hg. It is found that the phonon cutoff frequency is much lower than the Debye frequency. Extraordinarily high Tc could be expected if all phonons were involved in pairing electrons in a BCS superconductor.
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BCS Theory for Binary Systems with 2D Electrons
arXiv: Superconductivity, 2001Co-Authors: X.h. ZhengAbstract:MgB_2 is considered as a binary system with 2D electrons. The classic BCS Theory is applied to this system. The transition temperature is found to be relatively high, because 2D electrons are more capable of moving with the atoms, on top of other features of this system to enhance the electron-phonon interaction. This system may also shed light on the nature of superconductivity in cuprates.
Rafael Baquero - One of the best experts on this subject based on the ideXlab platform.
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On the possibility of Universal Ratios within a van Hove scenario of BCS Theory
AIP Conference Proceedings, 1996Co-Authors: D. Quesada, C. Trallero-giner, Rafael BaqueroAbstract:The central results of the BCS Theory are the Universal Ratios (UR) which do not depend on any physical parameters whatsoever. This constitutes a well defined weak coupling limit for conventional superconductivity (CS). Furthermore, this formalism assumes that there is a bosonic mechanism acting between electrons, but no explicit hypothesis about the atracting mechanism is necessary. The UR constitute an important point of reference and characterization for CS. Here a generalization of the BCS Theory for the van Hove scenario for high‐Tc superconductivity is performed.
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BCS-universal ratios within the Van Hove scenario
Physica C: Superconductivity, 1996Co-Authors: Rafael Baquero, D. Quesada, C. Trallero-ginerAbstract:Abstract The central result of BSC Theory is the Universal Ratios which do not depend on physical parameters of the particular superconductor under study. Several attempts have been made to introduce the Van Hove Scenario within BCS Theory. We present here a formulation that allow us to obtain Universal Ratios independent of any parameters and therefore a precise definition of “deviation from BCS Theory”. This concept is at the basis of several applications of BCS Theory in characterizing conventional superconductors and should be useful as a reference value for high- T c superconductivity new theories.
M. Vénéroni - One of the best experts on this subject based on the ideXlab platform.
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N-PARITY PROJECTED BCS Theory
The Nucleus, 2000Co-Authors: Roger Balian, H. Flocard, M. VénéroniAbstract:Odd-even effects are manifest in many properties of small superconducting systems such as nuclei and nanometric aluminium grains. The BCS equations provide a natural tool to investigate these effects at zero temperature. Here we give and discuss the equations which generalize the BCS formalism at finite temperatures while ensuring (contrary to standard finite temperature BCS) an exact conservation of the particle number parity. Special attention is given to the low temperature limits in both the nuclear physics and the condensed matter contexts.
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Variational extensions of BCS Theory
Physics Reports, 1999Co-Authors: Roger Balian, H. Flocard, M. VénéroniAbstract:Abstract A variational principle is devised which optimizes the characteristic function at thermodynamical equilibrium. The Bloch equation is used as a constraint to define the equilibrium state, and the trial quantities are an unnormalized density operator and a Lagrangian multiplier matrix which is akin to an observable. The conditions of stationarity yield for the latter a Bloch-like equation with an imaginary time running backwards. General conditions for the trial spaces are given that warrant the preservation of thermodynamic relations. The connection with the standard minimum principle for thermodynamic potentials is discussed. We apply our variational principle to the derivation of equations which are tailored for (i) the consistent evaluation of fluctuations and correlations and (ii) the restoration through projection of broken symmetries. When the trial spaces are chosen to be of the independent-quasi-particle type, we obtain an extension of the Hartree–Fock–Bogoliubov Theory which optimizes the characteristic function. The expansion of the latter in powers of its sources yields for the fluctuations and correlations compact formulae in which the RPA kernel emerges variationally. Variational expressions for thermodynamic quantities or characteristic functions are also obtained with projected trial states, whether an invariance symmetry is broken or not. In particular, the projection on even or odd particle number is worked out for a pairing Hamiltonian, which leads to new equations replacing the BCS ones. Qualitative differences between even and odd systems, depending on the temperature T, the level density and the strength of the pairing force, are investigated analytically and numerically. When the single-particle level spacing is small compared to the BCS gap Δ at zero temperature, pairing correlations are effective, for both even and odd projected cases, at all temperatures below the BCS critical temperature Tc. There exists a crossover temperature T× such that odd–even effects disappear for T such that T×
Peter Thalmeier - One of the best experts on this subject based on the ideXlab platform.
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BCS Theory for s+g-wave superconductivity in borocarbides Y(Lu)Ni2B2C
Physical Review B, 2003Co-Authors: Qingshan Yuan, Peter ThalmeierAbstract:The $s+g$ mixed gap function ${\ensuremath{\Delta}}_{\mathbf{k}}=\ensuremath{\Delta}[(1\ensuremath{-}x)\ensuremath{-}x{\mathrm{sin}}^{4}\ensuremath{\theta}\mathrm{cos}4\ensuremath{\varphi}]$ $(x:$ weight of the g-wave component) has been studied within BCS Theory. By suitable consideration of the pairing interaction, we have confirmed that the coexistence of s and g waves, as well as the state with equal s and g amplitudes (i.e., $x=1/2)$ may be stable. This provides the semiphenomenological Theory for the $s+g$-wave superconductivity with point nodes which has been observed experimentally in borocarbide ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ and possibly in ${\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$