Phonon Coupling Parameter Research Articles

Superconductivity in Ternary Mg4Pd7As6

AbstractThe synthesis and characterization of a new compound Mg4Pd7As6, which is found to be a superconductor with Tc = 5.45 K is reported. Powder X‐ray diffraction confirms the U4Re7Si6 structure (space group Im‐3m, no. 229) with the lattice parameter a = 8.2572(1) Å. Magnetization, specific heat, and electrical resistivity measurements indicate that it is a moderate‐coupling (λ = 0.72) type‐II superconductor. The electronic and phonon structures are calculated, highlighting the importance of antibonding Pd–As interactions in determining the properties of this material. The calculated electron–phonon coupling parameter λ = 0.76 agrees very well with the experimental finding, which confirms the conventional pairing mechanism in Mg4Pd7As6.

Ab Initio Investigation of the Relativistic Effect in the Physical Properties of Intermetallic Superconductor TlBi2${\rm TlBi}_2$ with AlB2${\rm AlB}_2$‐Type Hexagonal Layer Structure

AbstractIn this work, the role of spin‐orbit coupling (SOC) in the physical properties of is reported. These electronic calculations reveal that the heavy elements Tl and Bi promote strong spin‐orbit coupling, lifting some of the degeneracies at the high‐symmetry points. The presence of SOC causes significant decreases in both elastic constants and elastic moduli, which in turn improve the compatibility of the calculated Debye temperature ( = 80 K) with the recent experimental data of 83 K. Furthermore, our quasi‐static harmonic approximation calculation, like this elastic constant calculation, confirms that taking SOC into account improves agreement with the experiment by decreasing value of Debye temperature from 95 to 85 K. Activating the SOC causes significant modifications in the phonon spectrum and the density of phonon states, as well as in the Eliashberg spectral function. As a result of these modifications, the electron–phonon coupling parameter () increases from 1.178 to 1.259, by . The calculated values of both with and without SOC imply that can be treated as phonon‐mediated superconductor with strong coupling. The SOC‐induced increase in brings the superconducting transition temperature of from 6.076 to 6.211 K, which is almost equal to the recent experimental value of 6.2 K.

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.

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.

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.

A complete study on the temperature dependences of zero-field splitting b4 0 for Eu2+ and Gd3+ ions in CdF2 crystal

ABSTRACT This paper gives for the first time a complete equation to analyze the temperature dependence (or thermal variation) of spin-Hamiltonian parameter from low to high temperatures for transition metal and rare-earth ions in crystals. This equation includes not only the dynamic effect (only considered in previous papers) caused by electron–phonon interaction but also the static effect owing to lattice thermal expansion. Based on the complete equation, the thermal variations of zero-field splitting b4 0 from low to high temperatures for cubic Eu2+ and Gd3+ centers in CdF2 crystal are studied. The contributions due to static effect are estimated from the pressure dependence of zero-field splitting b4 0. The static parameters B (charactering the static effect) and the electron–phonon coupling parameters K (charactering the dynamic effect) for both systems are determined and the coefficients (B + K) in the complete equation are obtained. The results are discussed.

Lattice dynamics and spin–phonon coupling in the noncollinear antiferromagnetic antiperovskite Mn[formula omitted]NiN

Antiferromagnetic antiperovskites, of the form Mn3BN (B= Ni, Cu, Zn, Sn, Ir, and Pt), have shown an outstanding behavior in which, giant negative thermal expansion and chiral magnetic structures are intertwined. As such, aiming to shed light on the magnetostructural behavior, related to the magnetic ordering and crystalline structure of this type of materials, we studied theoretically, based-on first-principles calculations, the lattice dynamics and spin–phonon coupling in the Manganese-based antiperovskite Mn3NiN, as a prototype in the Mn3BN family. We found a strong spin–lattice coupling by means of the understanding of the phonon-dispersion curves obtained when including the chiral noncollinear magnetic structures. To do so, we highlight the importance of the exchange–correlation scheme selected and its influence on the structural, electronic, magnetic, and vibrational degrees of freedom. Finally, we found two unstable vibrational modes at the M- and R-points when the magnetic ordering is switched to ferromagnetic from the chiral Γ4g and Γ5g. These octahedral-associated modes with spin–phonon coupling parameters, λ, as large as 10.74 cm−1 and −2.53 cm−1 for the R5−, and M2+, respectively. The latter coupling is observed as a key signature of the strong spin–lattice interaction.

Prediction of phonon-mediated superconductivity in new Ti-based M_2AX phases

A high-throughput computational method is used to predict 39 new superconductors in the Ti-based M_2AX phases, and the best candidates are then studied in more detail using density functional theory electron–phonon coupling calculations. The detailed calculations agree with the simple predictions, and Ti_2AlX (X: B, C and N) materials are predicted to have higher values of T_c than any currently known hexagonal M_2AX phases. The electronic states at the Fermi level are dominated by the Ti 3d states. The choice of X (X: B, C and N) has a significant impact on the electronic density of states but not on the phonon characteristics. The electron–phonon coupling parameter for Ti_2AlX (X: B, C and N) was determined to be 0.685, 0.743 and 0.775 with a predicted T_c of 7.8 K, 10.8 K and 13.0 K, respectively.

Effect of Atomic-Temperature Dependence of the Electron-Phonon Coupling in Two-Temperature Model.

Ultrafast laser irradiation of metals can often be described theoretically with the two-temperature model. The energy exchange between the excited electronic system and the atomic one is governed by the electron–phonon coupling parameter. The electron–phonon coupling depends on both, the electronic and the atomic temperature. We analyze the effect of the dependence of the electron–phonon coupling parameter on the atomic temperature in ruthenium, gold, and palladium. It is shown that the dependence on the atomic temperature induces nonlinear behavior, in which a higher initial electronic temperature leads to faster electron–phonon equilibration. Analysis of the experimental measurements of the transient thermoreflectance of the laser-irradiated ruthenium thin film allows us to draw some, albeit indirect, conclusions about the limits of the applicability of the different coupling parametrizations.

Physical properties and superconductivity in the cubic A15-type Ta[formula omitted]Ge compound: A first principles study

In this theoretical study, we have targeted to present the characteristic features of the superconductivity in the cubic A15-type compound Ta3Ge by investigating its structural, electronic, elastic, mechanical, phonon, and electron–phonon interaction properties in detail. A critical examination of elastic moduli of Ta3Ge reveals that it is soft (ductile) due to significant metallic bonding. A critical surveying of electronic density of states reveals that Ge electronic states make negligible additive to the main valence band region of Ta3Ge while its transition metal d states dominate this region. An examination of our phonon results indicate that the low-frequency optical phonon branches and three acoustic phonon branches of Ta3Ge are mainly dominated by the vibrations of Ta atoms. Thereby, their d states and their vibrations become the main actors of the transition from the normal state to the superconducting state for Ta3Ge. From the integration of the Eliashberg spectral function of Ta3Ge, the values of average electron–phonon coupling parameter and logarithmically averaged phonon frequency are found to be 1.08 and 123.05 K, respectively, producing its superconducting temperature Tc = 8.3 K, being in good accordance with the experimental value of 8.0 K.

Ultrafast Dynamics of Colloidal Copper Nanorods: Intraband versus Interband Excitation

Colloidal copper nanorods (NRs) display transverse and longitudinal localized surface plasmon resonances. The longitudinal localized surface plasmon modes are tunable through the near‐infrared electromagnetic radiation energies with NR aspect ratios. Visible and near‐infrared transient optical response of the copper NRs is investigated under excitation conditions spanning intraband and interband excitation (0.79−3.50 eV). In both the visible and near‐infrared regions, the spectral response of the samples under intraband excitation (<2 eV) differs substantially from their response under interband excitation (>2 eV). However, the timescale of the electron−phonon coupling estimated from pump fluence‐dependent measurements (τep) is less sensitive to excitation conditions than reports for gold. τep shortens slightly from ≈616 fs with intraband excitation (at visible probe energies) to ≈565 fs with interband excitation. The observed dynamics correspond to an average sample electron−phonon coupling parameter varying across all conditions from 4.4 × 1016 to 6.4 × 1016 J m−3 K−1, which is similar to bulk copper. Furthermore, coherent acoustic phonons are observed for the longitudinal localized surface plasmon resonance with a range of oscillatory periods reflecting sample size dispersion.

Scanning tunneling spectroscopy of subsurface Ag and Ge impurities in copper

We investigate single Ge and Ag impurities buried below a Cu(100) surface using low temperature scanning tunneling microscopy. The interference patterns in the local density of states are surface scattering signatures of the bulk impurities, which result from 3D Friedel oscillations and the electron focusing effect. Comparing the isoelectronic d scatterer Ag and the sp scatterer Ge allows to distinguish contributions from impurity scattering and the host. Energy-independent effective scattering phase shifts are extracted using a plane wave tight-binding model and reveal similar values for both species. A comparison with ab initio calculations suggests incoherent sp scattering processes at the Ge impurity. As both scatterers are spectrally homogeneous, scanning tunneling spectroscopy of the interference patterns yields real-space signatures of the bulk electronic structure. We find a kink around zero bias for both species that we assign to a renormalization of the band structure due to many-body effects, which can be described with a Debye self-energy and a surprisingly high electron–phonon coupling parameter λ. We propose that this might originate from bulk propagation in the vicinity of the surface.

Ultrafast lattice dynamics and electron-phonon coupling in platinum extracted with a global fitting approach for time-resolved polycrystalline diffraction data.

Quantitative knowledge of electron–phonon coupling is important for many applications as well as for the fundamental understanding of nonequilibrium relaxation processes. Time-resolved diffraction provides direct access to this knowledge through its sensitivity to laser-induced lattice dynamics. Here, we present an approach for analyzing time-resolved polycrystalline diffraction data. A two-step routine is used to minimize the number of time-dependent fit parameters. The lattice dynamics are extracted by finding the best fit to the full transient diffraction pattern rather than by analyzing transient changes of individual Debye–Scherrer rings. We apply this approach to platinum, an important component of novel photocatalytic and spintronic applications, for which a large variation of literature values exists for the electron–phonon coupling parameter . Based on the extracted evolution of the atomic mean squared displacement and using a two-temperature model, we obtain (statistical error). We find that at least up to an absorbed energy density of 124 J/cm3, is not fluence-dependent. Our results for the lattice dynamics of platinum provide insights into electron–phonon coupling and phonon thermalization and constitute a basis for quantitative descriptions of platinum-based heterostructures in nonequilibrium conditions.

Electron–phonon interaction and superconductivity in hexagonal ternary carbides Nb2 AC (A: Al, S, Ge, As and Sn)

The superconducting transition temperatures T c of hexagonal Nb2 AC (A: Al, S, Ge, As and Sn) are investigated using density functional perturbation theory to model the electron–phonon interaction. A critical assessment of the calculated electronic structure and density of states revealed that the electronic states near to the Fermi level are mostly composed of the Nb 4d states, which are responsible for the electrical conductivity. The theoretical T c data from electron–phonon calculations are in excellent agreement with the Fröhlich model, and this model was used as a computationally efficient screening method to identify promising Nb–C M 2 AX phase materials. For Nb2 AC (A: Zn, Cd, Al, Ga, In, Tl, Si, Pb and P), the model indicated that Nb2AlC should have the highest T c of this set, a little lower than Nb2GeC and comparable to Nb2SC and Nb2SnC. Superconductivity in Nb2AlC has not been studied experimentally, but this result was confirmed by full electron–phonon calculations, which also revealed that the mechanism for superconductivity is the interactions of Nb 4d-state electrons with low-frequency phonons (in particular, acoustic phonon and low-frequency optical phonons dominated by Nb and the A element). The average electron–phonon coupling parameter was found to be λ ∼ 0.646, 0.739, 0.685, 0.440 and 0.614 for Nb2 AC (A: Al, S, Ge, As and Sn), respectively, with a corresponding superconducting critical temperature T c ∼ 6.7 K, 7.7 K, 9.8 K, 2.1 K and 6.3 K, respectively.