Hopfield Parameters Research Articles

Sulfur doped FeSe: A first-principles study of its electronic and superconducting properties

The self-consistent first-principles calculations, based on density functional theory (DFT) approach and the Green’s-function-based Korringa-Kohn-Rostoker Atomic Sphere Approximation (KKR-ASA) method within coherent potential approximation (CPA), are performed to investigate the electronic and superconducting properties of FeSe1-xSx (x = 0.0, 0.05) alloys. The spin-unpolarized calculations show a significant effect on the electronic structure for the substitution of S in FeSe. The outcome for electronic and superconducting properties have been examined in terms of changes in the density of states, band structure, Fermi surface, bare Sommerfeld constant, Hopfield parameters, and the superconducting transition temperature (Tc) of FeSe and FeSe0.95S0.05 alloys, respectively. All calculated results in terms of Tc compare well with the available experimental data. It hopes that the studied properties of these alloys would give the possibility to understand the new physics of iron-based superconductors experimentally.

Pressure-induced high-temperature superconductivity in hypothetical H3X (X=As, Se, Br, Sb, Te and I) in the H3S structure with [formula omitted] symmetry

The discovery of high critical temperature Tc superconductivity in highly compressed H3S has opened up the question of searching for strong electron–phonon coupling in the hydrides outside the transition metal series. The specific objective of this work is to explore the possibility of discovering a material that exceeds the superconducting transition temperature of H3S. Our study includes the materials H3X (X=As, Se, Br, Sb, Te, and I), is limited to the Im3¯m crystal structure. The procedure we adopt involves performing linearized augmented plane wave (LAPW) calculations for many different volumes to compute the electronic densities of states and their pressure variation. This is combined with Quantum-ESPRESSO (QE) calculations from which we obtain the phonon frequencies and the electron–phonon coupling constant λ, and followed by applying the multiple scattering-based theory of Gaspari and Gyorffy (GG) to obtain the Hopfield parameters and the McMillan–Allen–Dynes theory. It should be stressed that the GG approach decouples the electronic contribution to λ from the corresponding phonon contribution, and provides additional insights for the understanding of superconductivity in these materials. Based on our analysis, the hydrogen is the main contributor to the Tc in these materials as it makes up 75∼80 % of the total λ. Our calculations for H3Se and H3Br give a Tc∼100 K. For the other materials in our study we find that H3As is unstable and H3Sb, H3Te and H3I have small values of the McMillan–Hopfield parameters which makes it unlikely to give high Tc. However, according to both of our rigid band model and virtual crystal calculations, we predict a Tc∼150 K for H3Br with a small amount of hydrogen doping. Our basic conclusion is that the materials studied here could not reach very high Tc because the Hopfield parameters, which are the strongest contributor to high Tc, are not large enough.

Stability and high-temperature superconductivity in hydrogenated chlorine

Recent computational studies confirmed by experiment have shown the occurrence of superconductivity in the compound $$\hbox {H}_3$$ S at a temperature, $$T_\mathrm{c}$$ , near 200 K when the pressure is close to 200 GPa. It has become clear that classic electron–phonon coupling quantitatively explains the high-temperature superconductivity in this material. In this work, we investigate the possibility of discovering high-temperature superconductivity in the material $$\hbox {H}_3$$ Cl. By performing electronic structure calculations and an extensive search over many crystal structures, we conclude that $$\hbox {H}_3$$ Cl is metastable in the same $$Im\bar{3}m$$ structure found for $$\hbox {H}_3$$ S at high pressures. We then determine the angular momentum components of the density of states, the scattering phase shifts at the Fermi level, and using the Gaspari–Gyorffy formula, we calculate the electron–ion matrix element and the Hopfield parameter. Our calculated Hopfield parameters and an application of the McMillan–Allen–Dynes theory lead to a coupling constant $$\lambda =1.07$$ , and critical temperature $$T_\mathrm{c}=109$$ K not as high as in $$\hbox {H}_3$$ S.

Excess of manganese in an iron-based superconductor: a first-principles study of its electronic, magnetic and superconducting properties

Alloying and doping have always been an effective way of improving the properties of materials—be it electronic, magnetic or superconducting. In this direction, we present a first-principles investigation of the electronic, magnetic and superconducting properties of the systems (where x = 0.0, 0.01, 0.02 and 0.03). The calculations were performed using the Korringa–Kohn–Rostoker (KKR) atomic sphere approximation (ASA). In order to treat the disorder, we employed the coherent-potential approximation (CPA) into the KKR method. We performed both unpolarized and spin-polarized calculations and found that these systems are close to being magnetic. The results have been analyzed in terms of the density of states (DOS), electronic band structures, Fermi surfaces and the superconducting transition temperature Tc. We find that there are no significant changes in the shape of the Fermi surfaces in unpolarized calculations with respect to pure FeSe, but in spin-polarized calculations, the excess of the transition-metal Mn atoms substantially modifies the Fermi surface compared with the parent FeSe and compounds. We have also estimated the value of electron–phonon mass enhancement factor λ which is the sum of the electron–phonon coupling constant as well as the electron–electron interaction. Using λ, we have calculated the superconducting transition temperature Tc for the Mn doped superconductor. The calculated Tc are in good agreement with the experimental results. Additionally, Hopfield parameters are also calculated from the charge self-consistent potential using the KKR–ASA–CPA method for understanding the trend of Tc and DOS at Fermi energy.

First-principles study of electronic and magnetic properties of Fe1−xTMxSe0.5Te0.5 alloys (TM ≡ Cr, Mn, Co, Ni)

We have studied the effects of transition metal (TM) substitution on Fe sub-lattice in Fe1−xTMxSe0.5Te0.5 (TM ≡ Cr, Mn, Co, Ni and x = 0, 0.01, 0.05, 0.10) in terms of changes in its electronic and magnetic properties. The calculations were performed with the Korringa-Kohn-Rostoker (KKR) Atomic Sphere Approximation (ASA) method within the coherent-potential approximation (CPA). Our spin-polarized calculations show these alloys to be magnetic in nature. We discuss the calculated density of states (DOS), band structures and Fermi surfaces (FS) in terms of the contributions of Fe3d64s2, Cr3d54s1, Mn3d54s2, Co3d74s2, Ni3d84s2 and Se4s24p4 outer most electrons. Our results indicate that the effects of doping on the electronic properties are significant with respect to parent FeSe and FeSe0.5Te0.5 superconductor. We have also estimated the value of electron-phonon mass enhancement factor λ (which is the sum of electron–phonon coupling constant and the electron–electron interaction present in the material) to calculate the superconducting transition temperature Tc for the TM-doped FeSe0.5Te0.5. Hopfield parameters are also calculated from the charge self-consistent potentials obtained from KKR-ASA CPA method.

Neural Network-Based IIR All-Pass Filter Design

This paper presents a neural network-based Lyapunov energy function for the weighted least-squares design of IIR all-pass filters. In the proposed method, the error reflecting the difference between the desired phase response and the phase of the designed IIR all-pass filter is formulated as a Lyapunov error criterion. Based on the neural network architecture and suitable Hopfield parameters, the optimal filter coefficients can be obtained when convergence is achieved. Furthermore, a weight updating function is proposed to achieve accurate approximation of the equiripple response. The simulation results indicate that the proposed technique can achieve high performance in a parallel manner.

Electronic structure, superconductivity and magnetism study of Cr 3GaN and Cr 3RhN

The intermetallic perovskite compounds Cr 3GaN and Cr 3RhN were theoretically investigated, using the Korringa–Kohn–Rostoker method, as possible candidates for MgCNi 3-like superconductors and being near the magnetic critical point. The large values of McMillan–Hopfield parameters indicate the possibility of superconductivity onset as estimated from electron–phonon coupling constant λ > 1 . Moreover, spin-polarised electronic structure calculations resulted in the non-ferromagnetic ground state in both compounds, in spite of the n ( E F ) values on Cr atoms close to the Stoner limit.

Search forSc3XB(X=In,Tl,Ga,Al)perovskites superconductors and proximity of weak ferromagnetism

A possibility for a new family of intermetallic perovskite superconductors Sc3XB, with X = Tl, In, Ga and Al, is presented as a result of KKR electronic structure and pseudopotential phonon calculations. The large values of computed McMillan--Hopfield parameters on scandium suggest appearance of superconductivity in Sc3XB compounds. On the other hand, the possibility of weak itinerant ferromagnetic behavior of Sc3X systems is indicated by the small magnetic moment on Sc atoms in two cases of X =~ l and In. Also the electronic structure and resulting superconducting parameters for more realistic case of boron--deficient systems Sc3XB_x are computed using KKR--CPA method, by replacing boron atom with a vacancy. The comparison of the calculated McMillan--Hopfield parameters of the Sc3XB series with corresponding values in MgCNi3 and YRh3B superconductors is given, finding the favorable trends for superconductivity.

Rare earth borocarbides: Electronic structure calculations and electric field gradients

The electronic structure of $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Y},$ $\mathrm{L}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r}--\mathrm{T}\mathrm{m},\phantom{\rule{0ex}{0ex}}\mathrm{L}\mathrm{u})$ is systematically studied using density functional theory (DFT). The partially occupied $4f$ states are assumed to be localized for both the light and heavy rare earths $(R=\mathrm{Pr},$ $\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{m},\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{b},\phantom{\rule{0ex}{0ex}}\mathrm{D}\mathrm{y},\phantom{\rule{0ex}{0ex}}\mathrm{H}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m})$ and treated in the ``open core approximation.'' In the case of Gd (Lu) the $4f$ states are treated both as itinerant and as part of the atomiclike core states. The calculations of the electronic density of states (DOS) show that the Fermi energy ${E}_{f}$ is located in a pronounced peak for $R=\mathrm{Y},$ Dy, Ho, Er, Tm, and Lu. This peak starts to be broadened for $R=\mathrm{Tb},$ Gd, and Sm and finally disappears for $R=\mathrm{Pr},$ Nd. This reduction is large enough to explain the depression of superconductivity to below 3 K in the light rare-earth borocarbides. Additional calculations of the Hopfield parameters support this conclusion. The charge density distribution and general features of the bonding mechanism are discussed. The relations between the DOS in the vicinity of ${E}_{f}$ and the lattice parameters $a,c$ and the free internal structural parameter ${z}_{B}$ of boron are studied using the DFT total energy and force calculations. The total energy is very sensitive to the $c/a$ ratio and the optimum DFT values of $c/a$ and ${z}_{B}$ are close to those observed in the experiment. The electric field gradients (EFG) on the Gd- $({\mathrm{GdNi}}_{2}{\mathrm{B}}_{2}\mathrm{C})$ and B-site $({\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C})$ are calculated and agree with experimental data. We also point out that the physical origin of this relatively large EFG on the Gd site results from a strong cancellation between positive $6p\ensuremath{-}6p$ and negative $5p\ensuremath{-}5p$ contributions.

Electronic structure of disordered Nb - Mo alloys studied using the charge-self-consistent Korringa - Kohn - Rostoker coherent potential approximation

We present an all-electron fully charge-self-consistent Korringa - Kohn - Rostoker coherent potential approximation (KKR-CPA) study of the electronic structures of disordered bcc alloys over the entire composition range. Specific computations are reported for x = 0.0, 0.25, 0.50, 0.75, and 1.0. Extensive comparisons with the predictions of the Nb-based rigid-band model (RBM) and other theoretical results, as well as with the relevant experimental results insofar as they are available are made. The particular issues that we focus on concern the evolution of the Fermi surface (FS), and the changes in the density of states and the superconducting transition temperature of Nb with increasing Mo content. The N- and H-centred FS sheets of Nb are found to shrink essentially rigidly, but the -centred sheets evolve in a highly non-rigid-band manner. The x = 0.25 Mo alloy displays an especially large disorder-induced smearing of the -centred FS sheets. Direct experimental information concerning the FS is available only for the N-centred sheet via positron annihilation and in this regard our results are in accord with the measurements. Concerning superconductivity, we have computed the Hopfield parameters and for the Nb and Mo sites, and used the results to obtain for via the McMillan formula. We find that this simple scheme describes the observed composition dependence of in reasonably well.

Optimal environmental dispatching of electric power systems via an improved Hopfield neural network model

This paper examines the usefulness and specifics of the Hopfield neural network as applied to optimal economic/environmental dispatching of thermal generating units in an electric power system. A simulator is developed and criteria for selecting its parameters are discussed to obtain an optimal dispatch in the minimum amount of iterations. Particular emphasis is placed on the effects of variations in the selection of Hopfield parameters both with respect to the final result and the number of iterations required to reach a final solution.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>