Electron-phonon Coupling Parameter Research Articles

Transient electron excitation and nonthermal electron-phonon coupling in dielectrics irradiated by ultrashort laser pulses

We calculate nonequilibrium electron distributions created by the absorption of an intense, ultrashort laser pulse in a transparent model-dielectric using a Boltzmann-type collision approach. We compare electron energy and particle density, as well as electron-phonon energy transfer of a full nonequilibrium simulation with values of a simulation enforcing instant thermalization, testing the inherent assumption of temperature-based models. Finally, we calculate electron-phonon coupling parameters of nonequilibrium distributions and compare these with those of thermalized distributions of the same particle and energy density. We find that the strength as well as the relaxation of the electron-phonon coupling depends considerably on the excitation parameters.

Investigations of the thermal red-shift of R1-line and the electron-phonon coupling parameter for Cr3+-doped YAl3(BO3)4 crystal

A complete expression containing both the static factor caused by lattice thermal expansion and the vibrational factor owing to electron-phonon interaction is employed to compute the thermal red-shift of R1-line (the transition from 4A2 to one component of 2E) for YAl3(BO3)4: Cr3+ crystal. The results bring out that the static factor in sign is contrary to and in magnitude is nearly 22% of the vibrational factor. The true electron-phonon coupling parameter α’ raises about 28% in comparison with the apparent electron-phonon coupling parameter α estimated by considering only the vibrational factor. So, differing from the previous opinion, the static factor is not negligible for YAl3(BO3)4: Cr3+ crystal, and the rational analyses of the thermal shift of a spectral line and the electron-phonon coupling parameter should take both the static and vibrational factors into account.

Electron-phonon superconductivity in the filled skutterudites LaRu4P12, LaRu4As12 , and LaPt4Ge12

We have reported on the structural, elastic, electronic, lattice dynamical and electron-phonon interaction properties of $\text{La}{T}_{4}{X}_{12}$ ($T=\text{Ru}$, Pt and $X=\text{P}$, As, Ge) by using the generalized gradient approximation of the density functional theory and the plane-wave pseudopotential method. These lanthanum-filled skutterudites are found to be characterized with a flat band, resulting in a sharp peak in the electronic density of states, near the Fermi level. The lattice dynamical properties of these materials change considerably when the P atom is replaced by larger As or Ge atoms. The Migdal-Eliashberg approach is used to determine the Eliashberg spectral function for all the considered compounds. Using the calculated Eliashberg spectral function, the value of average electron-phonon coupling parameter is found to be 0.74 for ${\mathrm{LaRu}}_{4}{\mathrm{P}}_{12}$, 1.03 for ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$, and 1.08 for ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$. The superconducting critical temperature (${T}_{c}$) values for ${\mathrm{LaRu}}_{4}{\mathrm{P}}_{12}, {\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$, and ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ are estimated to be 6.95, 11.56, and 8.32 K, respectively, which compare well enough with their experimentally measured values of 7.2, 10.45, and 8.23 K.

Excitation energy transfer in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by spectral hole burning.

The cyanobacterium Acaryochloris marina developed two types of antenna complexes, which contain chlorophyll-d (Chl d) and phycocyanobilin (PCB) as light-harvesting pigment molecules, respectively. The latter membrane-extrinsic complexes are denoted as phycobiliproteins (PBPs). Spectral hole burning was employed to study excitation energy transfer and electron-phonon coupling in PBPs. The data reveal a rich spectral substructure with a total of four low-energy electronic states whose absorption bands peak at 633, 644, 654, and at about 673nm. The electronic states at ~633 and 644nm can be tentatively attributed to phycocyanin (PC) and allophycocyanin (APC), respectively. The remaining low-energy electronic states including the terminal emitter at 673nm may be associated with different isoforms of PC, APC, or the linker protein. Furthermore, the hole burning data reveal a large number of excited state vibrational frequencies, which are characteristic for the chromophore PCB. In summary, the results are in good agreement with the low-energy level structure of PBPs and electron-phonon coupling parameters reported by Gryliuk et al. (BBA 1837:1490-1499, 2014) based on difference fluorescence line-narrowing experiments.

The effect of spin orbit interaction on the physical properties of LaTSi3 (T = Ir, Pd, and Rh): First-principles calculations

We have presented the structural, elastic, electronic, phononic, and electron-phonon interaction properties of the La-based noncentrosymmetric superconductors, such as LaIrSi3, LaRhSi3, and LaPdSi3, by using the generalized gradient approximation of the density functional theory. The calculated elastic constants reveal the mechanical stability of all the studied compounds in their noncentrosymmetric structure, while the lack of inversion symmetry gives rise to lift the degeneracy of their electronic bands, except in the Γ-Z and X-P directions. The calculated Eliashberg spectral function shows that all phonon branches of these materials couple considerably with electrons, and thus, all of them make contribution to the average electron-phonon coupling parameter λ. Using the calculated values of λ and the logarithmically averaged phonon frequency ωln, the superconducting critical temperature Tc values for LaIrSi3, LaRhSi3, and LaPdSi3 are estimated to be 0.89, 2.56, and 2.40 K, respectively, which accord very well with their corresponding experimental values of 0.77, 2.16, and 2.60 K.

Increasing spin polarization in Fe3O4 films by engineering antiphase boundary densities

We present a systematical study on the evolution of antiphase boundary (APB) densities in Fe3O4 films, which were prepared by pulsed laser deposition and post annealing at different temperatures. By measuring the electron-phonon coupling parameter and using Allen's formula, we evaluate the films' antiphase boundary densities, which show a decreasing tendency with increasing annealing temperature. Consequently, a 50% increase of spin polarization in Fe3O4 films is achieved, and a 110% increase of the magnetoresistance ratio was found in 900 °C annealed Fe3O4 films compared to the as-grown sample. This work could contribute to the effective manipulation of APB densities and spin polarization in Fe3O4 films, which is desirable for the application of spintronics devices based on Fe3O4 films.

Analysis of optical constants and temperature-dependent absorption edge of GaS0.75Se0.25 layered crystals

GaS0.75Se0.25 single crystals were optically characterized through transmission and reflection measurements in the wavelength range of 450–1000nm. Derivative spectrophotometry analyses on temperature-dependent transmittance spectra showed that band gap energies of the crystal increase from 2.39eV (T=300K) to 2.53eV (T=10K). Band gap at zero temperature, average phonon energy, electron phonon coupling parameter and rates of change of band gap energy with temperature were found from the temperature dependences of band gap energies under the light of different models reported in literature. Furthermore, the dispersion of room temperature refractive index was discussed in terms of single effective oscillator model. The refractive index dispersion parameters, namely oscillator and dispersion energies, zero-frequency refractive index, were determined as a result of analyses.

The influence of spin orbit interaction on phonons and superconductivity in the noncentrosymmetric superconductors LaPt3Si and LaPtSi3

We have performed ab initio study of structural, electronic, vibrational and electron-phonon interaction properties of LaPt3 Si and LaPtSi3 by employing the density functional theory, a linear-response formalism, and the plane-wave pseudopotential method. The electronic structure, phonon dispersion relations and the Eliashberg spectral function for these materials have been examined with and without the inclusion of spin-orbit interaction (SOI). Our electron-phonon interaction results reveal that the influence of spin-orbit interaction on phonons and superconductivity in these noncentrosymmetric superconductors is very small. Thus, we can conclude that a mixing of the spin-singlet and the spin-triplet components in these superconductors is weak and the spin-singlet Cooper pairs dominate. By integrating the Eliashberg spectral function α2F(ω), the average electron-phonon coupling parameter λ is obtained to be 0.470 for LaPt3 Si and 0.488 for LaPtSi3, indicating these to be weak-coupling BCS superconductors. Using an acceptable value of μ∗ = 0.13 for the effective Coulomb repulsion parameter, the superconducting critical temperature Tc is calculated to be 0.67 K for LaPt3 Si and 1.39 K for LaPtSi3, in good accordance with experimental values of 0.65 K and 1.52 K, respectively.

Laser-excitation of electrons and nonequilibrium energy transfer to phonons in copper

After the irradiation of a copper sample with an ultrashort laser pulse, electrons do not follow a Fermi distribution anymore but instead are in a nonequilibrium state. In contrast, the lattice cannot be excited directly by the laser pulse, due to the frequency mismatch. The energy increase in the phononic system only happens due to electron–phonon scattering. We investigate the initial electron dynamics using full Boltzmann-type collision integrals, including material-dependent characteristics by implementing a realistic density of states. We show results on the absorbed energy, details of the electronic nonequilibrium and the resulting electron–phonon coupling parameter in dependence on the photon energy. Our results show a counteracting dependence on the photon energy, which, on the one hand, enables the d-band electrons to absorb high-energy photons and on the other hand, increases the probability of multi-photon absorption.

Effect of asymmetric spin–orbit coupling on the electronic structure properties of noncentrosymmetric superconductor

We report a theoretical investigation of the electronic structure properties of weakly correlated electronic noncentrosymmetric superconductor ( K). The first-principle calculations have been performed using the full potential linearised-muffin-thin-orbital method and including both scalar relativistic and fully relativistic (FR) treatments. We detect distinct differences between the data obtained from the two applied approaches. In particular, the bands along the , and separate and Fermi surfaces become highly anisotropic in FR calculations. The observed differences are ascribed to a strong effect of the asymmetric spin–orbit coupling with a splitting energy of order of –100 meV. Furthermore, we unveil that the valence bands originating from the Fe-3d orbitals essentially compose peak-structured profile in density of states (DOS) with a sharp peak situated at 0.65 eV below . The latter feature reflects the presence of a Van Hove type singularity. The main contribution to DOS at the Fermi level , however, comes equally from the Th-6d and Fe-3d electronic orbitals. The calculated value = 16.7 eV/f.u results in theoretical Sommerfeld coefficient = 39.31 mJ/mol K and electron–phonon coupling parameter = 0.34. The theoretical electronic structure properties of are discussed in comparison with those of other noncentrosymmetric superconductors.

Is sodium a superconductor under high pressure?

Superconductivity has been predicted or measured for most alkali metals under high pressure, but the computed critical temperature (Tc) of sodium (Na) at the face-centered cubic (fcc) phase is vanishingly low. Here we report a thorough, first-principles investigation of superconductivity in Na under pressures up to 260 GPa, where the metal-to-insulator transition occurs. Linear-response calculations and density functional perturbation theory were employed to evaluate phonon distributions and the electron-phonon coupling for bcc, fcc, cI16, and tI19 Na. Our results indicate that the maximum electron-phonon coupling parameter, λ, is 0.5 for the cI16 phase, corresponding to a theoretical peak in the critical temperature at Tc≈1.2 K. When pressure decreases or increases from 130 GPa, Tc drops quickly. This is mainly due to the lack of p-d hybridization in Na even at 260 GPa. Since current methods based on the Eliashberg and McMillian formalisms tend to overestimate the Tc (especially the peak values) of alkali metals, we conclude that under high pressure-before the metal-to-insulator transition at 260 GPa-superconductivity in Na is very weak, if it is measurable at all.

Active Thermal Extraction and Temperature Sensing of Near-field Thermal Radiation.

Recently, we proposed an active thermal extraction (ATX) scheme that enables thermally populated surface phonon polaritons to escape into the far-field. The concept is based on a fluorescence upconversion process that also occurs in laser cooling of solids (LCS). Here, we present a generalized analysis of our scheme using the theoretical framework for LCS. We show that both LCS and ATX can be described with the same mathematical formalism by replacing the electron-phonon coupling parameter in LCS with the electron-photon coupling parameter in ATX. Using this framework, we compare the ideal efficiency and power extracted for the two schemes and examine the parasitic loss mechanisms. This work advances the application of ATX to manipulate near-field thermal radiation for applications such as temperature sensing and active radiative cooling.

Electron-phonon superconductivity in the ternary phosphidesBaM2P2(M=Ni,Rh,and Ir)

Ab initio plane-wave pseudopotential calculations of electronic and vibrational properties have been carried out for the ternary phosphides $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$ $(M=\text{Ni},\text{Rh}\text{ and Ir})$ with a ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. The calculated electronic results show the metallic character of $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$, and the plots of total and partial density of states of $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$ exhibit strong hybridization between the $d$ states of the $M$ atom and the $p$ states of the P atom below the Fermi energy. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges for these compounds are presented and discussed. The Eliashberg spectral function for these compounds has been calculated by using a linear response approach based on the density functional theory. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter $(\ensuremath{\lambda})$ is determined to be 0.61 for ${\mathrm{BaNi}}_{2}{\mathrm{P}}_{2}$, 0.55 for ${\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$, and 0.43 for ${\mathrm{BaRh}}_{2}{\mathrm{P}}_{2}$. Using the calculated values of $\ensuremath{\lambda}$ and the logarithmically averaged phonon frequency ${\ensuremath{\omega}}_{ln}$ the superconducting critical temperature $({T}_{c})$ values for ${\mathrm{BaNi}}_{2}{\mathrm{P}}_{2},{\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$, and ${\mathrm{BaRh}}_{2}{\mathrm{P}}_{2}$ are obtained to be 2.80, 1.97, and 0.70 K, respectively, which compare very well with their experimental values of 3.0, 2.1, and 1.0 K.

Electronic structure and electron-phonon coupling in TiH2.

Calculations using first principles methods and strong coupling theory are carried out to understand the electronic structure and superconductivity in cubic and tetragonal TiH2. A large electronic density of states at the Fermi level in the cubic phase arises from Ti-t2g states and leads to a structural instability towards tetragonal distortion at low temperatures. However, constraining the in-plane lattice constants diminishes the energy gain associated with the tetragonal distortion, allowing the cubic phase to be stable at low temperatures. Calculated phonon dispersions show decoupled acoustic and optic modes arising from Ti and H vibrations, respectively, and frequencies of optic modes to be rather high. The cubic phase has a large electron-phonon coupling parameter λ and critical temperature of several K. Contribution of the hydrogen sublattice to λ is found to be small in this material, which we understand from strong coupling theory to be due to the small H-s DOS at the Fermi level and high energy of hydrogen modes at the tetrahedral sites.

First-principles investigation of superconductivity in the body-centred tetragonal

We have investigated the ground state and electronic properties of in the structure using a generalised gradient approximation of the density functional theory and the ab initio pseudopotential method. We find that the calculated electronic properties of exhibit three-dimensional rather than two-dimensional characteristics in spite of the apparent two dimensionality in its atomic structure. An interesting feature of the phonon dispersion curves is the phonon anomaly in the lowest transverse acoustic branch as well as the longitudinal acoustic branch. We have shown that these phonon anomalies give rise to large electron-phonon interaction, as is evident from the calculated Eliashberg spectral function F(). By integrating this spectral function, the value of average electron-phonon coupling parameter is calculated to be 0.85, from which the superconducting critical temperature is estimated to be 3.74 K, in gratifying accord with its experimental value of 4.0 K.

The effect of spin orbit interaction for superconductivity in the noncentrosymmetric superconductor CaIrSi3

We have carried out an ab initio study of the electronic, vibrational and electron-phonon interaction properties of the body-centred tetragonal CaIrSi3 by employing the density functional theory, a linear-response formalism, and the plane-wave pseudopotential method. The electronic structure and phonon dispersion relations of this material have been analyzed with and without the inclusion of spin-orbit interaction (SOI). Our electron-phonon interaction results reveal that Si-related phonon modes are more involved in the process of scattering of electrons than the remaining phonon modes due to considerable existence of the Si 3p states near the Fermi level. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter is found to be 0.58 which compares very well with its experimental value of 0.56. Using the calculated value of λ, the superconducting critical temperature (Tc) for CaIrSi3 is found to be 3.20 K which is in good accordance with its experimental value of 3.55 K. Furthermore, we have shown that the effect of SOI on the values of λ and Tc is very small.

Theoretical investigation of superconductivity in SrAuSi3 and SrAu2Si2

The structural and electronic properties of BaNiSn3-type SrAuSi3 and ThCr2Si2-type SrAu2Si2 have been investigated by using the planewave pseudopotential method and the density functional theory. The electronic structures and phonon dispersion relations of these two materials have been analyzed with and without the inclusion of spin–orbit interaction, and similarities and differences highlighted. By integrating the Eliashberg spectral function α2F(ω), the average electron–phonon coupling parameter is determined to be λ=0.47 for SrAuSi3 and 0.42 for SrAu2Si2. The largest contribution to the electron–phonon coupling for SrAuSi3 comes from the Si p electrons near the Fermi energy and Si-related vibrations. Using a reasonable value of μ*=0.12 for the effective Coulomb repulsion parameter, the superconducting critical temperature Tc for SrAuSi3 is found to be 1.47K which compares very well with its experimental value of 1.54K.

Electron-Phonon Coupling and Energy Flow in a Simple Metal beyond the Two-Temperature Approximation

The electron-phonon coupling and the corresponding energy exchange was investigated experimentally and by ab initio theory in non-equilibrium states of the free-electron metal aluminium. The temporal evolution of the atomic mean squared displacement in laser-excited thin free-standing films was monitored by femtosecond electron diffraction. The electron-phonon coupling strength was obtained for a range of electronic and lattice temperatures from density functional theory molecular dynamics (DFT-MD) simulations. The electron-phonon coupling parameter extracted from the experimental data in the framework of a two-temperature model (TTM) deviates significantly from the ab initio values. We introduce a non-thermal lattice model (NLM) for describing non-thermal phonon distributions as a sum of thermal distributions of the three phonon branches. The contributions of individual phonon branches to the electron-phonon coupling are considered independently and found to be dominated by longitudinal acoustic phonons. Using all material parameters from first-principle calculations besides the phonon-phonon coupling strength, the prediction of the energy transfer from electrons to phonons by the NLM is in excellent agreement with time-resolved diffraction data. Our results suggest that the TTM is insufficient for describing the microscopic energy flow even for simple metals like aluminium and that the determination of the electron-phonon coupling constant from time-resolved experiments by means of the TTM leads to incorrect values. In contrast, the NLM describing transient phonon populations by three parameters appears to be a sufficient model for quantitatively describing electron-lattice equilibration in aluminium. We discuss the general applicability of the NLM and provide a criterion for the suitability of the two-temperature approximation for other metals.

Theoretical investigation of superconductivity in ternary silicide NaAlSi with layered diamond-like structure

We have investigated the electronic structure, phonon modes and electron–phonon coupling to understand superconductivity in the ternary silicide NaAlSi with a layered diamond-like structure. Our electronic results, using the density functional theory within a generalized gradient approximation, indicate that the density of states at the Fermi level is mainly governed by Si p states. The largest contributions to the electron–phonon coupling parameter involve Si-related vibrations both in the x–y plane as well as along the z-axis in the x–z plane. Our results indicate that this material is an s-p electron superconductor with a medium level electron–phonon coupling parameter of 0.68. Using the Allen–Dynes modification of the McMillan formula we obtain the superconducting critical temperature of 6.98 K, in excellent agreement with experimentally determined value of 7 K.

Ab initio investigation of superconductivity in orthorhombic MgPtSi

We have performed an ab initio study of electronic, vibrational and superconducting properties of the orthorhombic MgPtSi by employing the density functional theory, a linear-response formalism, and the plane-wave pseudopotential method. Our electronic results suggest that the density of states at the Fermi level is primarily contributed by Pt 5d and Si 3p states with much smaller contribution from Mg electronic states. Phonon anomalies have been found for all three acoustic branches. Due to these phonon anomalies, the acoustic branches make large contributions to the average electron-phonon coupling parameter. From the Eliashberg spectral function, the value of average electron-phonon coupling parameter is found to 0.707. Using this value, the superconducting critical temperature is obtained to be 2.4 K, in excellent accordance with its experimental value of 2.5 K.