Electron-phonon Coupling Parameter Research Articles

Electron, phonon, and superconducting properties of Mg2CuH6 under pressure

Recent theoretical prediction of Mg2IrH6 is a significant advance in achieving high-temperature superconductivity under atmospheric pressure, Mg2IrH6 is the hydride superconductor with the highest superconducting transition temperature (Tc ∼ 160 K) under ambient pressure so far. In general, Tc is usually related to the degree of hydrogen enrichment. As a representative of the hydrogen-poor structures, MgHCu3 was also predicted to have a Tc of 43 K under atmospheric pressure. In this work, we try to replace the Ir atom in Mg2IrH6 with the Cu atom to improve the superconductivity of the system. The phonon dispersion curves and the ab initio molecular dynamics (AIMD) simulation indicate that the novel ternary Mg2CuH6 hydride is dynamically and thermodynamically stable. But, regrettably, the Tc and electron-phonon coupling (EPC) parameters λ of Mg2CuH6 are calculated to be 12.5 K and 0.55 at 25 GPa, which is far inferior to the superconductivity of Mg2IrH6. Compared with Fm3¯m Mg2CuH6, Fm3¯m Mg2IrH6 produces obvious phonon softening in the G point near 30 meV, which significantly enhances the EPC and increases the Tc of the system. Moreover, Cu substitution for Ir results in a sharp decrease in the contribution of the electronic states for Mg and H to the total density of states at the Fermi level. The huge difference in Tc between Mg2CuH6 and Mg2IrH6 and their causes may provide insights into the design of high-temperature atmospheric superconductors and help to understand the superconducting mechanisms of related systems.

Electron phonon coupling and superconductivity in α-MoB2 as a function of pressure

We have studied the lattice dynamics, electron-phonon coupling, and superconducting properties of α-MoB2, as a function of applied pressure, within the framework of density functional perturbation theory using a mixed-basis pseudopotential method. We found that phonon modes located along the A−H, H−L, and L−A high-symmetry paths exhibit large phonon linewidths and contribute significantly to the electron-phonon coupling constant. Although linewidths are particularly large for the highest-frequency optical phonon modes (dominated by B vibrations), their contribution to the electron-phonon coupling constant is marginal. The latter is largely controlled by the acoustic low-frequency modes of predominantly Mo character. It was observed that at a pressure of 90 GPa, where α-MoB2 forms, the phonon-mediated pairing falls into the strong-coupling regime, and the estimate for the superconducting critical temperature T c agrees well with experimental observations. When further increasing the applied pressure, a reduction of T c is predicted, which correlates with a hardening of the acoustic low-frequency phonon modes and a decrease of the electron-phonon coupling parameter.

DFT approach into the physical properties of MTe3 (M = Hf, Zr) superconductors: A comprehensive study

In this article, we investigated the structural, electronic, mechanical, optical, and superconducting state properties of the trichalcogenides, MTe3(M = Hf, Zr) compounds using the density functional theory. Electronic energy dispersion curves demonstrate that the title compounds are metallic in nature, with a significant contribution from the Te atom. The technologically important mechanical properties (stiffness constant, elastic moduli, brittle/ductile behavior, Poisson’s ratio, elastic anisotropy, machinability index, and hardness) are thoroughly examined and addressed. The value of Pugh’s ratio indicates the ductility (brittleness) of ZrTe3 (HfTe3). The Vickers hardness value is 0.86 and 0.54 GPa for MTe3 (M = Hf, Zr), respectively, which confirms their softness. The value of lattice thermal conductivity (in W m−1 K−1) for HfTe3 (3.64) and ZrTe3 (2.36) is low due to significant phonon scattering as confirmed by the Grüneisen parameter study. The optical constants were computed, which confirmed the strong optical anisotropy of MTe3 (M = Hf, Zr). For ZrTe3, with the electric field polarization along the [100] direction, the highest reflectivity (51.36%) is obtained compared to HfTe3 (45.21%). This shows promise for application as a radiative heat reflector of these two compounds. The superconducting state properties, such as London penetration depth, coherence length, Ginzburg–Landau parameter, and electron–phonon coupling parameters are estimated and discussed. The value of electron–phonon coupling parameters suggests that both compounds are moderately coupled superconductors.

High-pressure effect on the superconductivity of NaAlGe and pressure-induced structural phase transition

The structural, electronic, phonon, and electron–phonon properties of NaAlGe have been investigated under high pressure by means of density-functional theory simulations. The calculated electronic structure and density of states reveal that the electronic states near the Fermi level, which are responsible for electrical conductivity, are mostly composed of Ge 4p states. The largest contribution to the average electron–phonon coupling parameter is from Ge-related vibrations, and this parameter is calculated to be λ∼ 0.609. The superconducting critical temperature at ambient pressure is calculated to be Tc = 2.31 K. This temperature agrees with the previously experimentally obtained value of 1.80 K. The electronic density of states at the Fermi energy, λ, and Tc decrease as pressure is increased, and superconductivity is suppressed at 12 GPa. We also report the phonon dispersion at different pressures showing that NaAlGe develops a dynamical instability at 13 GPa favoring a structural phase transition Based on enthalpy calculations the crystal structure of the high-pressure phase has been proposed to be orthorhombic and described by space group Pnma. This phase does not exhibit superconductivity. The pressure dependence of unit-cell parameters and Raman- and infrared-active phonons of the low-pressure and high-pressure phase is also reported.

ThIrGe superconductor: First-principles insights into the optoelectronic, mechanical stability, thermophysical, and superconducting properties

The structural, electronic, optical, elastic, vibrational, Vickers hardness, and thermophysical properties of the ThIrGe superconductor are given. The characteristics of this technologically important superconductor are computed using the CASTEP formulation, based on density functional theory. The computed physical properties of ThIrGe are compared to available data and demonstrate good agreement. The band structure of this compound exhibits metal-like properties as the valence and conduction bands overlap at the Fermi level. The reflectance spectrum indicates that this material has potential as an effective solar reflector. Under Born stability conditions, the compound is mechanically stable. Different anisotropic parameters provide information about anisotropic properties in various crystallographic directions. Vickers hardness demonstrates that ThIrGe is soft, and Pugh's, Poisson's, and Kleinman's parameters show that it is ductile. The anisotropic acoustic modes are also reported. Using present elastic constants data, the theoretical Debye temperature of ThIrGe is 235.85 K. The electron–phonon coupling parameter is used to calculate superconducting properties. The findings could provide a useful foundation for scientists and engineers.

Role of spin–orbit coupling on the physical properties of APb[formula omitted] (A = Na, Ca, Y, and Th) superconductors

Structural, electronic, elastic, mechanical phonon and electron–phonon interaction properties of APb3 (A = Na, Ca, Y, and Th) superconductors have been investigated by using the density functional theory and the spin–orbit coupling (SOC) effects. It is found that the inclusion of SOC leads to significant changes in the electronic band structure of all the investigated compounds while the changes in the phonon properties essentially depend on the type of A atoms. While the phonon dispersion curves of YPb3 and ThPb3 are only weakly influenced, the inclusion of SOC produces significant amount of softening in the phonon dispersion curves for NaPb3 and CaPb3. Similarly, while the inclusion of SOC only weakly affects the electron–phonon interaction in YPb3 and ThPb3, it increases the electron–phonon coupling parameters in NaPb3 and CaPb3 by 27% and 33%, respectively. From the calculated elastic constants and related mechanical properties, all the superconductors studied are found to be ductile, which is favorable for their technological applications.

Analyses of the R1-line thermal shifts for Bi2Al4O9: Cr3+ and Bi2Ga4O9: Cr3+ crystals with a complete equation

ABSTRACT The thermal red-shifts of R1-line for Cr3+-doped Bi2Al4O9 and Bi2Ga4O9 crystals are analyzed with a complete equation containing not only the dynamic factor (used previously) due to electron–phonon interaction, but also the static factor caused by lattice thermal expansion. From the analysis, the static parameters A (charactering the static factor), the more reasonable electron–phonon coupling parameters(charactering the dynamic factor) and hence the more accurate quadratic coupling constants w for the two crystals are gained. It is found that the static factors are about 17% and 9% of the dynamic factors for Bi2Al4O9: Cr3+ and Bi2Ga4O9: Cr3+ crystals, respectively. So, in the accurate and reasonable analysis of R1-line thermal shifts in the two crystals (or other spectral line thermal shifts in crystals), the static factor needs to be considered and the complete equation including both factors should be adopted.

A new ternary molybdenum phospho-silicide Mo3Si2P: Phase equilibria, structural, electronic, transport and thermal properties

Here we report the successful preparation and systematical characterization of a novel molybdenum compound Mo3Si2P. As the first genuine ternary molybdenum phospho-silicide, it augments the Mo–Si–P family and may be served as an innovative archetype to develop exotic electronic states. The phase-equilibrium relation of Mo–Si–P system was studied, which defines the phase-stability region of Mo3Si2P, and a phase diagram that makes reviews on the abundant material and electronic physics in the system was established. Mo3Si2P crystallizes in a non-centrosymmetric orthorhombic structure (space group: Ama2) that appears as alternately stacking flat (Mo1)3P3 sheets and buckled (Mo2)3Si3 double-layers along the a-axis direction. The Bloch-Grüneisen-Mott model is employed to describe the T-dependent multiband metallic-conduction which is principally governed by electrons at high temperatures. The thermal-conductivity components of charge carrier and lattice vibration were extracted from κtotal by following the extended Wiedemann-Franz law. Poor thermoelectric performance is evidenced by very low thermoelectric power factor and figure-of-merit that are ascribed to small Seebeck coefficient and high thermal conductivity. Sommerfeld coefficient (γ) and Wilson ratio (RW) demonstrate that the electron correlation is extremely weak. First-principles calculations predict significant contribution from Mo-4 dx2−y2 orbital electrons to the Fermi surface of Mo3Si2P which shows different band fillings from Mo3Ge2P where dz2 electrons take the maximal occupation. Electron-phonon coupling parameter λep was calculated as 0.74, which reveals a medium interaction strength.

Phase stability and physical properties of lanthanum dicarbide under pressure

ABSTRACT First-principles calculations of electronic parameters, mechanical and dynamical stabilities on superconducting properties of the body-centered tetragonal intermetallic lanthanum dicarbide LaC2 have been reported at zero and high pressures. The investigated properties have been established through both full-potential-linearized augmented plane wave (FP-LAPW) and plane-wave pseudopotential (PW-PP) approaches, in conjunction with the generalized gradient approximation (GGA). Our investigations validate that the LaC2 crystal has a mechanical and dynamical stability under normal conditions. The generalised mechanical stability criteria applied to compressed LaC2, suggest that the compound remains stable for pressures below 18.30 GPa. The evaluated superconducting critical temperature Tc and the electron–phonon coupling parameter λ at zero pressure are found to be in excellent concordance with the experimental data. Furthermore, at elevated pressures, Tc and λ are predicted to decrease and the normalised specific heat jump remains unchanged.

Impact of spin–orbit coupling on physical properties and superconductivity in noncentrosymmetric superconductors Ru[formula omitted]B[formula omitted] and Re[formula omitted]B[formula omitted

We have made a first-principles investigation of the effect of spin–orbit coupling (SOC) the physical properties and superconductivity of noncentrosymmetric hexagonal superconductors T7B3 (T = Ru and Re). In general, the influence of SOC on the electronic, elastic and mechanical properties is slightly larger for Re7B3 due to larger nuclear charge of Re as compared to that of Ru since the strength of this coupling varies as Z4 (Z is atomic number). For both compounds, the transition metal-related phonon modes form more than 90% of the electron–phonon coupling parameter. SOC is slightly more effective on the Eliashberg spectral function of Re7B3 than that of Ru7B3, increasing the electron–phonon coupling parameter for the former by around 3.0% and decreasing it by less than 1.0% for the latter. As a consequence, SOC increases the superconducting transition temperature of Re7B3 from 2.20 to 2.51 K and decreases that of Ru7B3 4.34 to 3.79 K.

The effect of the spin–orbit coupling on the physical properties of ScGa[formula omitted] and LuGa[formula omitted] superconductors

In this work, we have aimed to reveal the impacts of spin–orbit coupling (SOC) on the elastic, mechanical, electronic, phonon, and electron–phonon interaction properties of ScGa3 and LuGa3 by executing both scalar and full-relativistic ab initio pseudopotential calculations based on the density functional theory with its local density approximation. The effect of this coupling on the physical properties of LuGa3 is slightly more pronounced than those of ScGa3. This finding has a physical explanation because spin–orbit interaction in an atom is due to the interaction of electronic spin moment with the magnetic field formed by the traveling electron in the electric field of the nucleus. Since the nuclear charge enhances with the rise in atomic number of the elements, heavier atoms are characterized by larger SOC strength and the mass of Lu atom is considerably heavier than that of Sc atom. In particular, the inclusion of SOC changes the value of the electron–phonon coupling parameter for LuGa3 from 0.593 to 0.583 by less 2%, while the corresponding parameter of ScGa3 remains almost unchanged (from 0.536 to 0.537). The superconducting transition temperature is estimated to be 2.09 K for ScGa3, and 2.15 K for LuGa3 are almost equal to their experimental values of 2.1 and 2.2 K, respectively. Finally, we have estimated three superconducting parameters: the Ginzburg–Landau parameter (κ(0)), the London penetration depth (λL(0)) and the coherence length (ξ(0)). Their calculated values confirm the existence of type-I superconductivity in both superconductors.

Prediction of superconductivity in sandwich XB4 (X = Li, Be, Zn and Ga) films.

Borophenes and 2D boron allotropes are metallic and exhibit a BCS superconducting state, unlike graphene. In-plane stretching vibrational modes in bulk MgB2 boron layers induce phonon-mediated superconductivity. However, the effect of stretching vibrational phonon modes on transition temperature (Tc) still requires further investigations. Here, we use first-principles calculations combined with conventional BCS theory to explore the superconducting properties in a series of dynamically stable boron-based sandwich films that have not been realized experimentally. The sandwich films of XB4 (where X = Li, Be, Zn, Ga) are predicted to exhibit good phonon-mediated superconductivity with high Tc values of 25.1 K, 28.7 K, 38.7 K, and 36.2 K, respectively. The origin of the superconducting states is mainly caused by the high metallicity and strong electron-phonon coupling (EPC), which can be attributed to the presence of intercalated atoms within the borophene layers. It is further demonstrated in the XB4 compounds (where X = Li, Be, Zn, Ga) that the pronounced EPC is not solely attributable to the in-plane vibrations of B atoms, but it is also influenced significantly by the out-of-plane vibrations of B atoms. Sandwich (Li,Be,Zn,Ga)B4 films may be a great choice for nanoscale superconductors as the electron-phonon coupling parameter becomes greater than unity, thereby providing a powerful approach for investigating these systems with high critical temperatures.

Scanning the latent phases and superconductivity in the Nb-Pb system at high pressure.

So far, few literature studies have been reported on niobium-lead binary intermetallic compounds, which are expected to have very different properties compared to existing niobium-carbon binary compounds, due to the distinct electronic properties of lead when compared to other carbon-group elements. Herein, we carry out a global structure search for the Nb-Pb system based on the evolutionary algorithm and density functional theory. Based on the dynamical and mechanical stability analyses, we unveiled five new phases, P4/m-Nb9Pb, Cmcm-Nb3Pb, I4/mmm-Nb2Pb, Pmm2-Nb5Pb3, and I4/mmm-NbPb2, that are promising candidates for experimental synthesis. Moreover, the superconducting transitions of all Nb-Pb binary intermetallic compounds are performed with electron-phonon calculations. As Nb9Pb exhibited the maximum Tc in the Nb-Pb intermetallics, greater than 3.0 K at 20 GPa, the phonon band structures, partial phonon density of states (PHDOS), the corresponding Eliashberg spectral functions α2F(ω), and integral electron-phonon coupling (EPC) parameters λ as a function of frequency of Nb9Pb were also studied. This work filled the gap in the pressure-tuned Nb-Pb phase transitions from a systematic first principles study for the first time.

On synthesis, stability and superconductivity of ThNb2H12under pressure: ab-inito calculations

Systematic study employing first-principle electronic structure method along with the evolutionary structure search algorithm as implemented in the universal structure predictor evolutionary Xtallography (USPEX) code has been carried out to look for the possibility of synthesising the ternary hydride (ThxNbyHz) from constituent's thorium, niobium and hydrogen. Our study suggests that the ternary hydride with formula ThNb2H12 will be globally stable above 12.5 GPa indicating the possibility of its formation from constituents Th, Nb, H and binary compounds of Th-H and Nb-H. Further, though the global stability of this compound is predicted to be beyond 12.5 GPa, it is metastable at 0 GPa. We have theoretically explored different formation routes to synthesize this compound. The zero-pressure monoclinic structure (Cm) of metastable ThNb2H12 undergoes a transformation to hexagonal structure (P6/mmm) at 3.5 GPa and this hexagonal structure (P6/mmm) transforms to other hexagonal type structure (P6/m) at 50 GPa. The high pressure structural sequence so predicted through static lattice calculations has been further substantiated by confirming the elastic and lattice dynamic stability of each structure in the pressure regime of their stability. Employing Allen–Dynes modified McMillan [P. B. Allen and R. C. Dynes, Phys. Rev. B 12, 905 (1975)] formula along with the fixed Coulomb pseudopotential of 0.1 and calculated electron-phonon coupling parameter, the superconductive temperature has also been calculated for this compound. Our analysis indicates that this material will behave as superconductor with transition temperature (Tc) ∼ 25 K at 15 GPa, which, will reduce to 4 K upon further compression to 100 GPa. Our analysis on lattice thermal conductivity of this compound in P6/mmm phase, at 15 GPa, shows a significantly large anisotropy in conductivity tensor, especially at lower temperatures.

Investigations of temperature dependence of the thermal shifts in Sm2+-doped SrFCl crystal

Temperature dependences of thermal shifts for all transitions in Sm2+-doped SrFCl crystal were investigated by using a full expression including both the static effect and the vibrational effect at temperature ranges 87 and 610 K. The Debye temperatures, the electron-phonon coupling parameters and the static parameters (A) for each electronic transitions are derived by fit the obtained analytical expression to experimental data. We have compared our results with experimental data, and our results were found to be satisfactory

Unified theoretical description of out-of-equilibrium electron intraband dynamics in gold induced by femtosecond laser pulses

The modeling of the intraband dynamics of conduction electrons in gold induced by a femtosecond laser pulse is addressed. Current approaches, based on the numerical resolution of the Boltzmann equation, are only able to describe electron excitation $\mathit{or}$ relaxation processes. In the present paper, within a single formalism, our kinetic model accounts quantitatively for both excitation and relaxation processes, i.e., photon absorption, thermal conductivity, and electron-phonon coupling coefficient. We suggest that such an approach can only be built by including Umklapp processes in the description of electron collisions. In addition to normal electronic collisional processes with phonons and other electrons, the present theoretical description further includes these mechanisms: (i) a conduction electron can be scattered in the second Brillouin zone after a collision, contributing significantly to the photon absorption, and (ii) the influence of the discrete and periodic nature of the lattice on electron collisions is considered, enabling in particular the coupling between electron and transverse phonons. A good agreement of predictions of this unified modeling with experimental observations for absorption and energy transfer from electrons to the lattice is obtained. We have investigated the influence of the Umklapp processes on the imaginary part of the dielectric function, the electron-phonon coupling parameter, and the transient shape of the electron energy distribution.

Luminescence properties and phase transformation of broadband NIR emitting A2(WO4)3:Cr3+ (A=Al3+, Sc3+) phosphors toward NIR spectroscopy applications

The synthesis, structural, and luminescence properties have been carried out for Cr3+-activated Al2(WO4)3 (AWO) and Sc2(WO4)3 (SWO) phosphors for application in pc-NIR LED. Upon blue excitation, these compounds are capable of exhibiting broadband NIR emission stems primarily from 4T2→4A2 transition in the range of 670–1200 nm (maxima ∼808 nm, FWHM ∼140 nm) for AWO:Cr and of 700–1300 nm (maxima ∼870 nm, FWHM ∼164 nm) for SWO:Cr. The significant shift of NIR emission is attributed to the substitution of AlO6 with larger ScO6 octahedrons. To gain insight into the luminescence the crystal field strength, Racah parameters, nephelauxetic effect, and electron-phonon coupling have been analyzed based on spectroscopic results. The electron-phonon coupling parameter S for SWO:Cr was determined to be 11.5, twice as large as that for AWO:Cr, which is in accordance with its strong thermal quenching. The abrupt changes occurring at 275 K in temperature-dependent luminescence spectra and decay lifetime of AWO:Cr is associated with temperature-driven phase transformation from low-temperature monoclinic to high-temperature orthorhombic phase. Pressure induced amorphization of AWO:Cr at pressures higher than 25 kbar was confirmed by employing high pressure evolution of Raman spectra. A high-power NIR pc-LED, fabricated by coating AWO:0.04Cr on a commercial 470 nm LED chip, shows good performance with an output power of 17.1 mW driven by a current of 320 mA, revealing potential application of studied materials for NIR light source.

Relaxation of photoexcited hot carriers beyond multitemperature models: General theory description verified by experiments on Pb/Si(111)

The equilibration of electronic carriers in metals after excitation by an ultra-short laser pulse provides an important class of non-equilibrium phenomena in metals and allows measuring the effective electron-phonon coupling parameter. Since the observed decay of the electronic distribution is governed by the interplay of both electron-electron and electron-phonon scattering, the interpretation of experimental data must rely on models that ideally should be easy to handle, yet accurate. In this work, an extended rate-equation model is proposed that explicitly includes non-thermal electronic carriers while at the same time incorporating data from first-principles calculations of the electron-phonon coupling via Eliashberg-Migdal theory. The model is verified against experimental data for thin Pb films grown on Si(111). Improved agreement between theory and experiment at short times (<0.3ps) due to non-thermal electron contributions is found. Moreover, the rate equations allow for widely different coupling strength to different phonon subsystems. Consequently, an indirect, electron-mediated energy transfer between strongly and weakly coupled groups of phonons can be observed in the simulations that leads to a retarded equilibration of the subsystems only after several picoseconds.

Temperature-dependent band modification and energy dependence of the electron-phonon interaction in the topological surface state on Bi2Te3

We report the temperature-dependent band structure and energy dependence of the electron-phonon interaction of the topological surface state (TSS) on the three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ using angle-resolved photoemission spectroscopy. Despite the nonrigid-band-like band modification as the temperature increases, the degeneracy at the Dirac point (DP) was robust, indicating topological protection. The electron-phonon coupling parameter depends on the initial-state electron energy, varying from 0.13 (at the DP) to 0.02 (100 meV above the DP), in agreement with reported first-principles calculations. Our findings provide insight into the physical properties derived from TSSs at finite temperatures.

Multi-temperature modeling of femtosecond laser pulse on metallic nanoparticles accounting for the temperature dependences of the parameters

This review considers the fundamental dynamical processes of metal nanoparticles during and after the impact of a femtosecond laser pulse on a nanoparticle, including the absorption of photons. Understanding the sequence of events after photon absorption and their timescales is important for many applications of nanoparticles. Various processes are discussed, starting with optical absorption by electrons, proceeding through the relaxation of the electrons due to electron–electron scattering and electron–phonon coupling, and ending with the dissipation of the nanoparticle energy into the environment. The goal is to consider the timescales, values, and temperature dependences of the electron heat capacity and the electron–phonon coupling parameter that describe these processes and how these dependences affect the electron energy relaxation. Two- and four-temperature models for describing electron–phonon relaxation are discussed. Significant emphasis is paid to the proposed analytical approach to modeling processes during the action of a femtosecond laser pulse on a metal nanoparticle. These consider the temperature dependences of the electron heat capacity and the electron–phonon coupling factor of the metal. The entire process is divided into four stages: (1) the heating of the electron system by a pulse, (2) electron thermalization, (3) electron–phonon energy exchange and the equalization of the temperature of the electrons with the lattice, and (4) cooling of the nanoparticle. There is an appropriate analytical description of each stage. The four-temperature model can estimate the parameters of the laser and nanoparticles needed for applications of femtosecond laser pulses and nanoparticles.