Electron-phonon Coupling Parameter Research Articles

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.

A stable C2/m phase of H2S at 150–175 GPa

High temperature superconductivity of H2S at high pressure is fascinating, and there is still controversy about crystalline structures and stoichiometry responsible for high Tc. Determining the crystal structures of H2S at high pressure is a tremendous challenge in experiments. We found a previously missing structure with C2/m space group symmetry by genetic algorithm and first-principles calculations, which is an intermediate structure between the and Cmca structures and is competitive with the and Cmca phases at 150–175 GPa. The main difference between the C2/m and structure is the S–H3 bond lengths in the two structures. The C2/m and Cmca structures consist of similar structural units (H4S6 rectangular units). In the C2/m structure the chains formed by the H4S6 rectangular units are parallel, while in the Cmca structure the H4S6 rectangular units are twisted. The possible structural transformations from to C2/m to Cmca structure were also discussed. By calculating the electronic structure, phonon spectrum and electron–phonon coupling parameters, the C2/m structure was predicted to have a superconducting transition temperature Tc of 53–74 K at 150 GPa.

Thermal shifts and electron-phonon coupling parameters for Cr3+-doped Lu3Al5O12 and Lu3Ga5O12 garnet crystals

A full expression containing the vibrational contribution due to electron-phonon interaction and the static contribution due to lattice thermal expansion is employed to study the thermal shifts of R-line (corresponding to 2E→4A2 transition) for Cr3+-doped Lu3Al5O12 (LuAG) and Lu3Ga5O12 (LuGG) garnet crystals. The results indicate that the vibrational contribution results in the thermal red-shift, whereas the static contribution results in the thermal blue-shift. The magnitude of static contribution is nearly 14% that of the vibrational contribution, which leads the obtained true electron-phonon coupling parameter α′ to increase about 16% in comparison with the apparent electron-phonon coupling parameter α obtained by a simple expression including only the vibrational contribution. So, for reasonably describing the thermal shift and giving the true electron-phonon coupling parameter for a spectral line in crystals, one should adopt the full expression. The electron-phonon coupling parameter α′ in LuAG: Cr3+ is smaller than that in LuGG: Cr3+. The outcomes are discussed.

Thermal Shifts and Electron-Phonon Coupling Parameters for the Three Luminescence Lines of 5 D J→7 F J in SrFCl:Sm2+ Crystal

Abstract The thermal blue shifts of three spectral lines E 1(5 D 0→7 F 0), E 2(5 D 0→7 F 1), and E 3(5 D 1→7 F 0) in SrFCl:Sm2+ crystal are investigated by a complete expression consisting of both the static contribution due to lattice thermal expansion and the vibrational contribution owing to electron-phonon interaction. The obtained true electron-phonon coupling parameters α′ in both sign and magnitude are considerably different from the apparent electron-phonon coupling parameters α acquired in the previous paper by considering only the vibrational contribution. It is suggested that differing from the simple expression including only the vibrational contribution (many authors thought that it cannot be used to explain the thermal blue shifts), the complete expression containing both the vibrational and static contributions is effective in the studies of thermal shift (whether red shift or blue shift) and true electron-phonon coupling parameter for a spectral line in crystals.

Role of spin-orbit coupling in the physical properties of LaX3 ( X=In , P, Bi) superconductors

We report a comprehensive and complementary study on structural, elastic, mechanical, electronic, phonon, and electron-phonon interaction properties of $\mathrm{La}{X}_{3}$ ($X$ = In, Pb, and Bi) using first-principles density functional calculations within the local density approximation with and without the spin-orbit coupling (SOC). The calculated lattice parameters for these intermetallic compounds with and without SOC are found to differ by less than $2%$ from their experimental values. The effect of SOC on the elastic, mechanical, electronic, phonon, and electron-phonon interaction properties is more profound for ${\mathrm{LaPb}}_{3}$ and ${\mathrm{LaBi}}_{3}$ containing heavier $X$ elements rather than ${\mathrm{LaIn}}_{3}$ containing lighter $X$ element. The inclusion of SOC considerably removes the degeneracies of some bands near the Fermi level and makes some phonon branches in ${\mathrm{LaPb}}_{3}$ and ${\mathrm{LaBi}}_{3}$ softer and increases the strength of dominant peaks in their Eliashberg spectral functions. Thus the SOC related enhancement of their electron-phonon coupling parameter values can be related to both a softening of their phonon dispersion curves and an increase in their electron-phonon coupling matrix elements. The superconducting transition temperature with SOC is computed to be 0.69 K for ${\mathrm{LaIn}}_{3}$, 4.23 K for ${\mathrm{LaPb}}_{3}$, and 6.87 K for ${\mathrm{LaBi}}_{3}$, which agree very well with the respective measured values of 0.70, 4.18, and 7.30 K.

Coupled spin and electron-phonon interaction at the Tl/Si(111) surface from relativistic first-principles calculations

We investigate the role played by the electron spin and the spin-orbit interaction in the exceptional electron-phonon coupling at the Tl/Si(111) surface. Our first-principles calculations demonstrate that the particular spin pattern of this system dominates the whole low-energy electron-phonon physics, which is remarkably explained by forbidden spin-spin scattering channels. In particular, we show that the strength of the electron-phonon coupling appears drastically weakened for surface states close to the $\overline{K}$ and $\overline{{K}^{\ensuremath{'}}}$ valleys, which is unambiguously attributed to the spin polarization through the associated modulation due to the spinor overlaps. However, close to the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point, the particular spin pattern in this area is less effective in damping the electron-phonon matrix elements, and the result is an exceptional strength of the electron-phonon coupling parameter $\ensuremath{\lambda}\ensuremath{\sim}1.4$. These results are rationalized by a simple model for the electron-phonon matrix elements including the spinor terms.

A study of optical properties of Sm3+ -doped M′ -type structure YTaO4 powders synthesized by the disodium tetraborate flux method

The Y1-xSmxTaO4 (x = 0.1, 0.5, 1.0, 2.0 and 5.0) compounds have been synthesized by conventional solid state method using disodium tetraborate as flux. The growth process of title compounds was studied using X-ray diffraction and electron microscopy (SEM and TEM). The powder diffraction pattern was indexed on a monoclinic cell, space group P2/a, lattice parameters: a = 5.302, b = 5.454 and c = 5.115 Å, Z = 2 and V = 146.97 Å3. The absorption (300 K) and fluorescence spectra (5 and 300 K) as well as the fluorescence dynamics of the Sm3+:YTaO4 are presented and analysed in detail. The room-temperature absorption spectra have been analysed using the Judd-Ofelt approach. The intensity parameters have been evaluated and subsequently used to calculate fundamental spectroscopic parameters: the spontaneous transition probabilities, the branching ration, emission cross-sections and natural lifetime for the 4G(4)5/2 emitting level. The luminescence spectra recorded between 5 and 300 K are composed of transitions from the 4G(4)5/2 level to the 6H5/2, 6H7/2, 6H9/2, 6H11/2, 6H13/2, 6H15/26F1/2, 6F3/2 and 6F5/2 levels. The maximum emission cross-section has been calculated to be 0.935 and 0.985 × 10−21 cm2 at 609.1 and 657.1 nm, respectively. The electron–phonon coupling parameter α¯ was found to be in the range 127–512 cm−1 for the 4G(4)5/2 → 6H5/2, 4G(4)5/2 → 6H7/2 and 4G(4)5/2 → 6H9/2 transitions.

High-pressure effects on isotropic superconductivity in the iron-free layered pnictide superconductor BaPd2As2

While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Here we report on complementary measurements of specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffraction measurements supplemented with theoretical calculations for BaPd2As2. Its superconducting properties are completely isotropic as demonstrated by the critical fields, which do not depend on the direction of the applied field. Under the application of high pressure, Tc is linearly suppressed, which is the typical behavior of classical phonon-mediated superconductors with some additional effect of a pressure-induced decrease in the electronic density of states and the electron-phonon coupling parameters. Structural changes in the layered BaPd2As2 have been studied by means of angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we observed pressure induced lattice distortions manifesting as the discontinuity and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk modulus is B0=40(6) GPa below 12 GPa and B0=142(3) GPa below 27.2 GPa.

Pressure effect on the superconducting and the normal state of β−Bi2Pd

The pressure effect up to 24.0 kbar on superconducting and normal-state properties of $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ single crystal (${T}_{\mathrm{c}}\ensuremath{\approx}4.98\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure) has been investigated by measurements of the electrical resistivity. In addition, we have performed the heat capacity measurements in the temperature range 0.7--300 K at ambient pressure. The recent calculations of electronic density of states, electron-phonon interaction spectral function, and phonon density of states of $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ [Zheng and Margine, Phys. Rev. B 95, 014512 (2017)], are used to fit the resistivity and the heat capacity data. In the superconducting state we have focused on the influence of pressure on the superconducting transition temperature ${T}_{\mathrm{c}}$ and upper critical field ${H}_{\mathrm{c}2}$ and a negative effect with $d{T}_{\mathrm{c}}/dp=--0.025\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{kbar}$ and $d{H}_{\mathrm{c}2}/dp=--8\phantom{\rule{0.16em}{0ex}}\mathrm{mT}/\mathrm{kbar}$ is found. A simplified Bloch-Gr\uneisen model was used to analyze the pressure effect on the temperature dependence of the normal-state resistivity. The obtained results point to a decrease of the electron-phonon coupling parameter \ensuremath{\lambda} and to a shift of phonon frequencies to higher values with pressure. Moreover, the temperature dependence of the normal-state resistivity follows a ${T}^{2}$ dependence above ${T}_{\mathrm{c}}$ up to about 25 K. Together with the enhanced value of Sommerfeld coefficient $\ensuremath{\gamma}=13.23\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ these results point to a certain role of the electron-electron interaction in the superconducting pairing mechanism in $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$.

Research on the thermal-shifts and electron-phonon coupling parameters for two spectral lines in Nd3+-doped Gd3Sc2Ga3O12 and La3Lu2Ga3O12 garnets

The thermal red-shift of spectral line E1 (R2 → Y3) and the thermal blue-shift of spectral line E2 (R1 → Z5) for Nd3+ in Gd3Sc2Ga3O12 (GSGG) and La3Lu2Ga3O12 (LLGG) garnet crystals are studied by using a full expression including both the dynamic factor due to electron-phonon interaction and the static factor due to lattice thermal expansion. The results indicate that the full expression enables explaining not only the thermal red-shift, but also the thermal blue-shift. For both the spectral lines E1 and E2, the dynamic factor leads to the red-shift because the true electron-phonon coupling parameters α′ are always negative, whereas the static factor results in the blue-shift due to the positive static parameter A. The observed thermal blue-shift of line E2 for GSGG: Nd3+ and LLGG: Nd3+ is owing to the static factor being larger than the dynamic one rather than to the positive apparent electron-phonon parameter α obtained by a simple expression containing only the dynamic factor. The true electron-phonon parameters α′ acquired by the full expression in sign (for line E2) and in magnitude (for both the lines E1 and E2) differ from the apparent parameters α obtained from the simple expression. So, the reasonable studies of thermal-shift and electron-phonon coupling parameters should employ the full expression.

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).

Electronic, phonon and superconducting properties of LaPtBi half-Heusler compound

In the framework of density functional theory based on plane wave pseudopotential method and linear response technique, we have studied the electronic, phonon and superconducting properties of LaPtBi half-Heusler compound. The electronic band structure and density of states show that it is gapless semiconductor which is consistent with previous results. The positive phonon frequencies confirm the stability of this compound in cubic MgAgAs phase. Superconductivity is studied in terms of Eliashberg spectral function (α2F(ω)), electron-phonon coupling constants (λ). The value of electron-phonon coupling parameter is found to be 0.41 and the superconducting transition temperature is calculated to be 0.76 K, in excellent agreement with the experimentally reported values.

Electronic transport properties of intermediately coupled superconductors: PdTe2 and Cu0.04PdTe2

We have investigated the electrical resistivity (1.8–480 K), Seebeck coefficient (2.5–300 K) and thermal conductivity (2.5–300 K) of PdTe2 and 4% Cu intercalated PdTe2 compounds. The electrical resistivity for the compounds shows a Bloch-Gruneisen–type linear temperature (T) dependence for , and Fermi liquid behavior for . Seebeck coefficient data exhibit a strong competition between Normal (N) and Umklapp (U) scattering processes at low T. The low-T, thermal conductivity of the compounds is strongly dominated by the electronic contribution, and exhibits a rare linear T-dependence below 10 K. However, high-T, shows the usual -dependence, dominated by the U-scattering process. The electron-phonon coupling parameters, estimated from the low-T, specific-heat data and first-principle electronic structure calculations suggest that PdTe2 and Cu0.04PdTe2 are intermediately coupled superconductors.

Strain-induced modulations of electronic structure and electron-phonon coupling in dense H3S.

A ∼200 K superconducting phase in dense hydrogen sulfide is an important milestone for the development of novel superconductors with high critical temperature. Here, we systematically studied the effect of uniaxial strain on the electronic and superconducting properties in dense H3S using density functional calculations. Our theoretical results show that inducing strain is an effective tool to control the electronic Fermi surface topology, logarithmic average frequency, and electron-phonon coupling parameter of dense H3S. Thus, uniaxial strain induces sensitive and considerable changes in superconducting critical temperature, which stem from the energy-level shifts and softening lattice vibrations.

Superconductivity in two-dimensional ferromagnetic MnB

Using the universal structure predictor algorithm, we proposed that two-dimensional MnB structures with p4mmm (α-MnB) and pmma (β-MnB) symmetries could be synthesized. This finding was verified by calculating the dynamical stability, molecular dynamics, and mechanical properties. The α-MnB had an in-plane stiffness Yx (=Yy) around 100 N/m while the β-Mn displayed an asymmetric mechanical stiffness of Yx = 186 N/m and Yy = 139 N/m. Both systems displayed a ferromagnetic ground state with metallic band structures. The calculated magnetic moments were 2.14 and 2.34 µB per Mn-B pair in the α-MnB and β-MnB. Furthermore, we investigated the potential superconductivity. In the α-MnB, we found the unique feature of Kohn anomaly at q~2kF in the diagonal direction of the Brillouin zone. The β-MnB phonon spectra showed a valley of degenerated localized softening vibration modes at the edge of the Brillouin zone. The ZA and LA phonon branches in this valley induced the largest contribution to electron-phonon coupling strength. The calculated total electron-phonon coupling parameters were 1.20 and 0.89 in α-MnB and β-MnB systems. Overall, we predict that the α-MnB and β-MnB systems can display 2D ferromagnetic superconducting states with the estimated critical temperatures of Tc ≈ 10−13 K.

Phonon transport in a curved aluminum thin film due to laser short pulse irradiation

Laser short-pulse heating of a curved aluminum thin film is investigated. The Boltzmann transport equation is incorporated to formulate the heating situation. A Gaussian laser intensity distribution is considered along the film arc and time exponentially decaying of pulse intensity is incorporated in the analysis. The governing equations of energy transport in the electron and lattice sub-systems are coupled through the electron-phonon coupling parameter. To quantify the phonon intensity distribution in the thin film, equivalent equilibrium temperature is introduced, which is associated with the average energy of all phonons around a local point when the phonon energies are redistributed adiabatically to an equilibrium state. It is found the numerical simulations that electron temperature follows similar trend to the spatial distribution of the laser pulse intensity at the film edge. Temporal variation of electron temperature does not follow the laser pulse intensity distribution. The rise of temperature in the electron sub-system is fast while it remains slow in the lattice sub-system.

A study on the thermal red-shift of R1-line for LiAl5O8: Cr3+ crystal from the static and vibrational effects

This paper reports a theoretical study on the thermal red-shift of R1-line (one component of 2E2→4A2 transition) for LiAl5O8: Cr3+ crystal by using a full expression including both the static effect derived from the lattice thermal expansion and the vibrational effect coming from the electron-phonon interaction. The results indicate that the vibrational and static effects result in the red-shift and blue-shift of R1-line, respectively, and the static effect in magnitude is about 19% of the vibrational effect. The true electron-phonon coupling parameter is nearly 1.23 times as large as the apparent one which was obtained by considering only the vibrational effect in the previous paper. So for explaining reasonably the thermal-shift of a spectral line in crystals and giving its true electron-phonon coupling parameter, one should take into account both the static and vibrational effects.

Effects of spin-orbit coupling on the electron-phonon superconductivity in the cubic Laves-phase compounds CaIr2 and CaRh2

We report our ab initio pseudopotential results for the structural, electronic, vibrational, and electron-phonon interaction properties of the cubic Laves-phase compounds ${\mathrm{CaIr}}_{2}$ and ${\mathrm{CaRh}}_{2}$. While the spin-orbit coupling (SOC) does not result in any appreciable changes in structural parameters, it lifts the degeneracies of some bands near the Fermi level, albeit with a much smaller amount for ${\mathrm{CaRh}}_{2}$. The effect of SOC on the vibrational properties of both materials is considerable. The SOC results in a slight decrease in the electronic density of states near the Fermi level $N({E}_{F})$ and makes low-frequency phonon branches harder, and the electron-phonon coupling parameter $\ensuremath{\lambda}$ is lowered from 1.43 to 1.05 for ${\mathrm{CaIr}}_{2}$ and from 1.17 to 0.96 for ${\mathrm{CaRh}}_{2}$. On the other hand, the logarithmically averaged phonon frequency ${\ensuremath{\omega}}_{ln}$ is enhanced from 79.60 to 100.97 K for ${\mathrm{CaIr}}_{2}$ and from 120.20 to 140.80 K for ${\mathrm{CaRh}}_{2}$ with the inclusion of SOC. Using the calculated values of $\ensuremath{\lambda}$ and ${\ensuremath{\omega}}_{ln}$, the superconducting critical temperature is determined to be 5.94 K (7.34 K without SOC) for ${\mathrm{CaIr}}_{2}$ and 6.97 K (9.08 K without SOC) for ${\mathrm{CaRh}}_{2}$. The superconducting critical-temperature values with SOC compare very well with corresponding experimental values of 5.80 and 6.40 K, indicating the importance of SOC for the physical properties of both materials.

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.