Average Electron-phonon Coupling Parameter Research Articles

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.

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.

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.

First-principles calculations of physical properties and superconductivity of orthorhombic ScRuSi and ZrRhSi

We have performed ab initio pseudopotential calculations of the structural, electronic, elastic, mechanical, vibrational, electron-phonon interaction, and superconducting properties of ScRuSi and ZrRhSi. An assessment of their elastic and mechanical properties reveals that ScRuSi is a ductile compound, while ZrRhSi is a brittle compound. For both compounds electronic energy bands in the vicinity of the Fermi level originate from the $d$ states of transition-metal atoms. The Eliashberg spectral function reveals that low-frequency phonon modes due to coupled motions of transition-metal atoms are more involved in the process of scattering of electrons than the high-frequency modes due to the vibrations of Si atoms. The average electron-phonon coupling parameter is calculated to be 0.54 for ScRuSi and 0.84 for ZrRhSi from the integration of the Eliashberg spectral function. It is found that the relatively weaker electron-phonon interaction in ScRuSi makes its superconducting transition temperature value $({T}_{c}=3.23 \mathrm{K})$ considerably lower than that of ZrRhSi $({T}_{c}=10.45 \mathrm{K})$.

Probing the electron–phonon interaction in superconductivity for KSn2 using the Migdal-Eliashberg theory and linear-response theory

ABSTRACTWe have studied the structural and electronic properties of potassium distannide KSn crystallizing in the MgZn-type (C14) Laves structure by employing the generalised gradient approximation of the density functional theory and the plane wave ab initio pseudopotential method. It is found that the bonding character in this system is a mixture of covalent, ionic and metallic bonds. The stress–strain method calculations confirm the mechanical stability, and the phonon dispersion results with real frequencies reveal the dynamical stability of the structure. The electron–phonon interaction has been investigated by using the linear response approach and the Migdal-Eliashbergh approach. The integration of the Eliashbergh spectral function gives the value of average electron-phonon coupling parameter to be of the weak strength 0.637. The superconducting temperature is obtained to be 2.7 K, which is in good accordance with its measured value of 3.2 K.

Theoretical investigation of the superconductivity mechanism of BaIr2As2

Based on first-principles pseudopotential plane-wave method within the generalised gradient approximation, I have studied the structural, electronic, vibrational and BCS superconducting properties of BaIr2As2. The calculated electronic structure and density of states suggest that the bonding is a combination of covalent, ionic and metallic in nature and that the Fermi level falls in one of the peaks in the electronic density of states. The electron–phonon interaction calculations suggest that the mechanism for superconductivity is heavily governed by interactions of electrons with acoustic phonon modes and low-frequency optical phonon modes. By integrating the Eliashberg spectral function, the value of average electron–phonon coupling parameter is found to be 0.64 and the superconducting critical temperature is calculated to be 2.337 K, in excellent agreement with the experimentally reported value of 2.450 K.

Theoretical investigation of electron-phonon interaction in the orthorhombic phase of Mo2C

We have studied the structural, electronic, elastic, mechanical, vibrational and electron-phonon interaction properties of Mo2C crystallizing in the simple orthorhombic ζ-Fe2N-type crystal structure by using the generalized gradient approximation of the density functional theory and the plane wave ab initio pseudopotential method. A critical assessment of the calculated electronic structure and density of states reveals that the bonding in this material is a combination of covalent, ionic and metallic in nature. The calculated values of the second order elastic constants signal its mechanical stability. An examination of the calculated Eliashberg spectral function reveals that Mo-related phonon modes couple strongly to electrons due to the significant presence of Mo d electrons at the Fermi energy. From the integration of this spectral function, the value of average electron-phonon coupling parameter is determined to be of the intermediate strength 0.709. Finally, the value of the superconducting critical temperature is calculated to be 7.37 K, in excellent accordance with its measured value of 7.30 K.

Theoretical investigation of superconductivity in the non-centrosymmetric SrPtGe3 and CaPtSi3 compounds

First-principles calculations are made to investigate structural, elastic, electronic, vibrational and electron–phonon interaction properties of non-centrosymmetric superconductors SrPtGe3 and CaPtSi3. Electronic structure calculation results show that the density of states at the Fermi level for both compounds mainly comes from the p electrons of Ge (or Si), which considerably hybridise with Pt d states. The calculated Eliashberg spectral function with and without spin–orbit interaction reveals that while all phonon branches are involved in the process of scattering of electrons near the Fermi level, the average electron–phonon coupling parameter values of 0.47 for SrPtGe3 and 0.49 for CaPtSi3 indicate that these compounds are phonon-mediated BCS-type superconductors with a weak strength. By using a reasonable value of = 0.11 for the effective Coulomb repulsion parameter, the superconducting transition temperatures for SrPtGe3 and CaPtSi3 are found to be 1.3 K and 2.4 K, respectively, which compare very well with the corresponding experimental values of 1.0 K and 2.3 K.

Ab initio investigation of electron-phonon interaction in LaSn3 and CaSn3

ABSTRACTWe present results for the structural, electronic, vibrational, and electron-phonon coupling properties of LaSn3 and CaSn3 adopting the simple cubic AuCu3-type structure obtained using the the generalised gradient approximation of the density functional theory and plane wave ab initio pseudopotential method. Our electronic results show that both materials display metallic character with several bands, which have mainly Sn 5p character, crossing the Fermi level. The calculated phonon spectrum of LaSn3 accords very well with reported experimental measurements. The weights of the peaks in the Eliashberg spectral function of both compounds are enhanced with the use of experimental lattice constant in our electron-phonon calculation, increasing the value of average electron phonon coupling parameter from 0.876 to 0.937 for LaSn3 (by 7%) and from 0.642 to 0.725 for CaSn3 (by 13%). The use of experimental lattice constant also improves the agreement between theoretical and experimental values of the superconducting temperature for both compounds.

Band structure, electron-phonon interaction and superconductivity of yttrium hypocarbide

Band parameters and superconductivity of yttrium hypocarbide (Y2C) have been investigated. The computations are performed using first-principles pseudopotential method within a generalized gradient approximation. The equilibrium lattice parameters have been determined and compared with experiment. Moreover, the material of interest is found to be stiffer for strains along the a-axis than those along the c-axis. A band-structure analysis of Y2C implied that the latter has a metallic character.The examination of Eliashberg Spectral Function indicates that Y-related phonon modes as well as C-related phonon modes are considerably involved in the progress of scattering of electrons. By integrating this function, the value of the average electron-phonon coupling parameter (λ) is found to be 0.362 suggesting thus that Y2C is a weak coupling Bardeen-Copper-Schrieffer superconductor. The use of a reasonable value for the effective Coulomb repulsion parameter (μ* = 0.10) yielded a superconducting critical temperature Tc of 0.59 K which is comparable with a previous theoretical value of 0.33 K. Upon compression (at pressure of 10 GPa) λ and Tc are increased to be 0.366 and 0.89 K, respectively, showing thus the pressure effect on the superconductivity in Y2C.The spin-polarization calculations showed that the difference in the total energy between the magnetic and non-magnetic Y2C is weak.

Electron-phonon interaction in the binary superconductor lutetium carbide LuC2 via first-principles calculations

Structural, electronic, electron-phonon coupling and superconducting properties of the intermetallic compound LuC2 are investigated by means of ab initio pseudopotential plane wave method within the generalized gradient approximation. The calculated equilibrium lattice parameters yielded a very good accord with experiment. There is no imaginary phonon frequency in the whole Brillouin zone supporting thus the dynamical stability in the material of interest. The average electron-phonon coupling parameter is found to be 0.59 indicating thus a weak-coupling BCS superconductor. Using a reasonable value of μ* = 0.12 for the effective Coulomb repulsion parameter, the superconducting critical temperature Tc is found to be 3.324 which is in excellent agreement with the experimental value of 3.33 K. The effect of the spin-orbit coupling on the superconducting properties of the material of interest has been examined and found to be weak.

The effect of martensitic phase transition from cubic to tetragonal on the physical properties of V3Si superconductor

We report results of ab-initio studies for the structural, electronic, lattice dynamical and electron-phonon interaction properties of cubic and tetragonal phases of V3Si by using the plane-wave pseudopotential method, the density-functional theory, and a linear-response technique. Our total energy results propose that the martensitic transition from cubic to a tetragonal variant of the A15 structure takes place with a very little change in the unit cell volume and total energy. Thus, the electronic and lattice dynamical properties of both phases look like similar to each other. Our electron-phonon interaction calculations reveal that the phonon properties of V lattice considerably enter into electron-phonon interaction calculations due to noteworthy presence of transition metal d electrons at the Fermi level. Using the calculated value of 1.15 for the average electron-phonon coupling parameter of both phases, the superconducting critical temperature and the electronic specific heat coefficient are evaluated to be 17.3 K and 54.5 mJ/(molK2), respectively, in gratifying agreement with their measured values of 17 K and 53 mJ/(molK2).

Electron–phonon interaction and superconductivity in the borocarbide superconductor

We have made an ab initio investigation of electron–phonon interaction and superconductivity in the borocarbide super-conductor adopting the body-centred tetragonal -type of layered crystal structure. The calculated electronic structure and density of states suggest that the bonding is a combination of covalent, ionic and metallic in nature and that the Fermi level falls in one of the peaks in the electronic density of states. Our electron–phonon interaction calculations suggest that the mechanism for superconductivity is heavily governed by interactions of electrons with acoustic phonon modes and low-frequency optical phonon modes, which strongly modulate tetrahedral bond angles. By integrating the Eliashberg spectral function, the value of average electron–phonon coupling parameter is found to be 0.93 and the superconducting critical temperature is calculated to be 16.28 K, in excellent agreement with the experimentally reported value of 16.0 K.

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.

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.

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.

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.

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.