Electron-phonon Coupling Constant Research Articles

Anharmonic and quantum effects in Pm3̄ AlM(M = Hf, Zr)H6 under high pressure: A first-principles study.

First-principles calculations were employed to investigate the impact of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of Pm3̄ AlM(M = Hf, Zr)H6 at pressures of 0.3-21.2GPa (AlHfH6) and 4.7-39.5GPa (AlZrH6) within the stochastic self-consistent harmonic approximation. A correction is predicted for the crystal lattice parameters, phonon spectra, and superconducting critical temperatures, previously estimated without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for lattice dynamics. The findings suggest that quantum ionic fluctuations have a significant impact on the crystal lattice parameters, phonon spectra, and superconducting critical temperatures. Based on our anharmonic phonon spectra, the structures will be dynamically stable at 0.3GPa for AlHfH6 and 6.2GPa for AlZrH6, ∼6 and 7GPa lower than pressures given by the harmonic approximation, respectively. Due to the anharmonic correction of their frequencies, the electron-phonon coupling constants (λ) are suppressed by 28% at 11GPa for AlHfH6 and 22% at 30GPa for AlZrH6, respectively. The decrease in λ causes Tc to be overestimated by ∼12K at 11GPa for AlHfH6 and 30GPa for AlZrH6. Even if the anharmonic and quantum effects are not as strong as those of Pm3̄n-AlH3, our results also indicate that metal hydrides with hydrogen atoms in interstitial sites are subject to anharmonic effects. Our results will inevitably stimulate future high-pressure experiments on synthesis, structural, and conductivity measurements.

Anharmonic and quantum effects in Pm3̄ AlM(M = Hf, Zr)H6 under high pressure: A first-principles study.

First-principles calculations were employed to investigate the impact of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of Pm3̄ AlM(M = Hf, Zr)H6 at pressures of 0.3-21.2GPa (AlHfH6) and 4.7-39.5GPa (AlZrH6) within the stochastic self-consistent harmonic approximation. A correction is predicted for the crystal lattice parameters, phonon spectra, and superconducting critical temperatures, previously estimated without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for lattice dynamics. The findings suggest that quantum ionic fluctuations have a significant impact on the crystal lattice parameters, phonon spectra, and superconducting critical temperatures. Based on our anharmonic phonon spectra, the structures will be dynamically stable at 0.3GPa for AlHfH6 and 6.2GPa for AlZrH6, ∼6 and 7GPa lower than pressures given by the harmonic approximation, respectively. Due to the anharmonic correction of their frequencies, the electron-phonon coupling constants (λ) are suppressed by 28% at 11GPa for AlHfH6 and 22% at 30GPa for AlZrH6, respectively. The decrease in λ causes Tc to be overestimated by ∼12K at 11GPa for AlHfH6 and 30GPa for AlZrH6. Even if the anharmonic and quantum effects are not as strong as those of Pm3̄n-AlH3, our results also indicate that metal hydrides with hydrogen atoms in interstitial sites are subject to anharmonic effects. Our results will inevitably stimulate future high-pressure experiments on synthesis, structural, and conductivity measurements.

Differentiating contribution of electron–phonon coupling to the thermal boundary conductance of metal–metal–dielectric systems

Electron–phonon coupling thermal resistance in metals is a key factor affecting the thermal boundary conductance (TBC) of metal–metal–dielectric systems. However, quantitatively differentiating the contribution of electron–phonon coupling to TBC is still a challenge, as various thermal resistances are coupled in a complicated manner at the metal–metal–dielectric interface. Herein, we propose a two-step strategy to study electron–phonon coupling. We first decouple the phonon–phonon thermal conductance (TBCp-p) between metallic interlayer and dielectric from the metal–metal–dielectric interface by experimentally characterizing the TBCp-p of a single metallic interlayer deposited dielectric with the transient thermoreflectance technique; Combining metal–metal–dielectric TBC measurement and a thermal circuit model with measured TBCp-p as input, the contribution of electron–phonon coupling to TBC of the metal–metal–dielectric system is differentiated quantitatively. For the Au–Ni–GaN system, the contribution of electron–phonon coupling thermal resistance in the Ni interlayer (Re−ph,Ni) is substantially higher at lower Ni interlayer thickness, reaching 35% at ∼1 nm Ni. The electron–phonon coupling constant of Ni (gNi) was fitted in the range of 6.4 × 1016–36 × 1016 W/m3K. The above results were also verified in the Au–Ni–SiC system. This study will promote a deeper understanding of the thermal transport in the metal–metal–dielectric system and provide an insightful indication for the manipulation of TBC in this system.

Superconductivity in breathing kagome-structured C14 Laves phase XOs2(X = Zr, Hf)

Recently, the emergence of superconductivity in kagome metals has generated significant interest due to its interaction with flat bands and topological electronic states, which exhibit a range of unusual quantum characteristics. This study thoroughly investigates largely unexplored breathing kagome structure C14 Laves phase compounds XOs2 (X = Zr, Hf) by XRD, electrical transport, magnetization, and specific heat measurements. Our analyses confirm the presence of the MgZn2-type structure in ZrOs2 and HfOs2 compounds, exhibiting type-II superconductivity with critical temperature ( TC ) values of 2.90(3) K and 2.69(6) K, respectively. Furthermore, specific heat measurements and electron–phonon coupling constants for both compounds indicate weakly coupled fully gapped superconductivity.

Superconductivity in monolayer Janus titanium-sulfur hydride (TiSH) at ambient pressure

Janus two dimensional (2D) materials are new and novel materials. As they break out-of-plane symmetry, they possess several fascinating properties which can be applied in catalytic reactions and opto-electronics. Recent synthesis of MoSH and the prediction of phonon-mediated superconductivity have opened a new way to investigate the properties of hydrogenated Janus materials (Novoselov et al 2004 Science 306 666–9; Mehta et al 2023 Solid State Commun. 375 115347; Naik et al 2023 Comput. Theor. Chem. 1228 114278). In this work, we performed the density functional theory calculations to demonstrate that titanium sulfur hydride (TiSH) is dynamically stable and becomes phonon-mediated superconductor with the superconducting critical temperature, Tc = 9.24 K with the corresponding value of electron–phonon coupling constant, λ = 0.71, in the weak interaction limits, under ambient conditions. Eliashberg spectral function α2F(ω) was well converged for dense grid of q1 × q2 × q3 = 12 × 12 × 1 and nk1 × nk2 × nk3 = 140 × 140 × 1. The effect of smearing broadening was also considered for determining well converged value of Tc and λ. Figure (b) shows that after smearing broadening of 0.02 Ry, λ shows convergent values, and subsequent changes are as low as less that 5% of the peak value. Overall, our findings predicted a new member in the 2D Janus hydride family with possible applications in 2D nanomaterials and superconducting devices applications.

Anomalous superconductivity in Li/F modified two-dimensional molybdenene

Anomalous superconductivity in Li/F modified two-dimensional molybdenene

High-throughput discovery of metal oxides with high thermoelectric performance via interpretable feature engineering on small data

In this work, we have proposed a data-driven screening framework combining the interpretable machine learning with high-throughput calculations to identify a series of metal oxides that exhibit both high-temperature tolerance and high power factors. Aiming at the problem of weak generalization ability of small data with power factors at high temperatures, we employ symbolic regression for feature creation which enhances the robustness of the model while preserving the physical meaning of features. 33 candidate metal oxides are finally targeted for high-temperature thermoelectric applications from a pool of 48,694 compounds in the Materials Project database. The Boltzmann transport theory is utilized to perform electrical transport properties calculations at 1,000 K. The relaxation time is approximated by employing constant electron-phonon coupling based on the deformation potential theory. Considering band degeneracy, the electron group velocity is obtained using the momentum matrix element method, yielding 28 materials with power factors greater than 50 μWcm−1K−2. The high-throughput framework we proposed is instrumental in the selection of metal oxides for high-temperature thermoelectric applications. Furthermore, our data-driven analysis and transport calculation suggest that metal oxides rich in elements such as cerium (Ce), tin (Sn), and lead (Pb) tend to exhibit high power factors at high temperatures.

Predictions of thorium super nitrides and superconductivity under pressure: Ab initio calculations

Thorium nitrides have been the topic of intense studies due to their prospective applications as advanced nuclear fuels. The phase diagram of the Th–N scheme, however, continues unknown at low temperatures and extremely high pressures. In this article, we examine the Th–N system's phase diagram up to 300 GPa from the first-principle approach using universal structure predictor: evolutionary Xtallography (USPEX) method. Apart from the experimentally observed phase (ThN, Th2N3, and Th3N4), there are several unique chemical stoichiometries, i.e., ThN3, ThN4, ThN6, ThN8, ThN10, and ThN12 are found to have stability fields on the Th–N phase diagram at pressure of 3.0, 32, 100, 42, 28, and 236 GPa along with previously predicted composition ThN2 at 3.5 GPa. The structural stability of the predicted compositions is further assessed by evaluating the elastic and dynamic stability. Out of all above mentioned compositions, ThN3 is possibly a metastable one at 0 GPa. Electronic structure calculations predict that all newly discovered compositions are metallic except ThN10, which is semi-metallic at high pressures. Further, we predict that ThN4 and ThN6 have high electron–phonon coupling constant of 1.874 and 0.894 with Tc around 21.22 and 25.02 K, respectively, at 100 GPa.

First-principles study of superconducting transition temperature of niobium and vanadium as a function of pressure

Ultra-high pressure has recently led to the observation of high Tc superconductors. The exploration of superconducting properties under high pressure conditions offers valuable insights into the fundamental mechanisms governing superconductivity and opens avenues for the synthesis of novel materials with enhanced superconducting properties. In this study, the superconducting transition temperature, Tc, for niobium and vanadium has been investigated to maximum pressures of 250.0 and 70.0 GPa, respectively. Density functional theory and density functional perturbation theory were used to undertake first-principles computations to investigate the variations of superconducting properties under pressure. For both the metals niobium (Nb) and vanadium (V), we observed that the density of states at the Fermi level decreased, while the overall phonon frequency generally increased with pressure. For Nb, we find that the electron–phonon coupling constant (λ) and the superconducting transition temperature (Tc) decrease steadily with pressure; however, for V, we find that λ and Tc decrease smoothly up to 26.0 GPa, and then gradually increase to 70.0 GPa pressure. The observed phonon spectrum nature for both Nb and V is in good agreement with the neutron scattering approach’s output.

Superconductivity in transition metal carbides, TaC and HfC: A density functional theory study with local, semilocal, and nonlocal exchange–correlation functionals

We studied electronic, dynamical, and superconducting properties of TaC and HfC using local, semilocal, and nonlocal exchange–correlation functionals in the density functional theory. It turns out that there is a slight shift of Fermi energy and the corresponding density of states with different functionals. However, the phonon frequencies of, especially, the optical modes for both TaC and HfC shift by moderately larger values. The most important differences are observed for the electron–phonon interactions where the electron–phonon coupling constant, λ≈0.9, calculated using the nonlocal correlation functionals, vdW-DF2 and vdW-DF3 for TaC, are in the closest agreement with the experimental results. Although the Kohn anomalies and Fermi surface nesting are critical for the electron–phonon coupling, a significant contribution to λ, for TaC, comes from the regions of the Brillouin zone where the matrix elements have a predominant role. Moreover, the optical modes give 20%–23% of λ for TaC and 50%–57% for HfC.

Electronic structure of BaBiO3 and electron–phonon coupling in K-doped superconducting bismuthate—A first-principles study

The electronic structure of BaBiO3 and Ba1−xKxBiO3, where x = 0.375, 0.4, and 0.425 is investigated using the density functional theory (DFT) in the generalized gradient approximation (GGA). The charge-density-wave (CDW) nature of the host compound is suppressed via potassium doping at Ba-site, resulting in the emergence of a metallic state. A strong hybridization between Bi-6s and O-A1g orbitals is observed in the electronic states near the Fermi level, particularly enhanced in the collapsed BiO6 octahedra of the undoped compound. The K-doped systems show stronger spσ hybridization than that of the host compound. The self-doped holes condensate in the antibonding combination of Bi-s and O-A1g orbitals of both the doped systems and in the collapsed octahedra of the undoped system. The exploration of lattice dynamics and electron–phonon coupling in Ba1−xKxBiO3 systems, elucidate that mainly the phonons corresponding to the oxygen bond stretching vibrations contribute most to the total electron–phonon coupling. The GGA-calculated electron–phonon coupling constants (λ) across varying K-concentrations inadequately account for experimental Tcs. The inclusion of long-range interactions through the HSE06 hybrid exchange–correlation functional substantially augments both λ and Tc, closely mirroring experimental observations and underscoring the crucial role of nonlocal correlations in these systems.

Physical properties of newly synthesized noncentrosymmetric TaIr2B2 and NbIr2B2 superconductors: an extensive comparison of GGA and LDA functional investigations.

In recent years, noncentrosymmetric (NCS) structural compounds have received much attention from the scientific community in the exploration for the unconventional nature of superconductivity with exciting physical properties. This study uses the comprehensive generalized gradient approximation (GGA) and local density approximation (LDA) to gain insights into the physical properties of two recently synthesized Ir-based NCS superconductors, TaIr2B2 and NbIr2B2. The structural parameters, mechanical performance, electronic structure, Debye temperature, melting temperature, electronic specific heat, and electron-phonon coupling constant of TaIr2B2 and NbIr2B2 are explored and discussed in detail. Density functional theory (DFT) optimized structural parameters of both NCS phases agree well with experimental observation. Both GGA and LDA calculations show that the compounds are ductile, machinable, mechanically stable, and anisotropic in nature. The elastic moduli and hardness calculations reveal that TaIr2B2 is harder than NbIr2B2. The calculation of the melting temperature reveals that TaIr2B2 is more suitable for high temperature technology applications compared to NbIr2B2. Both GGA and LDA functionals reveal that the optical functions are very similar. Both compounds display a significant amount of reflectivity spectra over a wide range of photon energies. The GGA functional reveals a somewhat higher density of states value compared to that of LDA. The present calculated values of the electron-phonon coupling constant of both compounds are consistent with values previously reported from experimental studies.

Few‐Hydrogen Metal‐Bonded Perovskite Superconductor MgHCu3 with a Critical Temperature of 42 K under Atmospheric Pressure

AbstractSince the prediction of multi‐hydrogen high‐temperature superconductor by Ashcroft in 2004, many possible candidates have been proposed, e.g., LaH10 showing the highest superconducting transition temperature (Tc) around 250–260 K at 170‐200 GPa hitherto. However, this pressure is too large to be taken into practical use. To address this challenge, it proposes a few‐hydrogen metal‐bonded perovskite superconductor, MgHCu3, by combining a novel design idea with first‐principles calculations. Different from multi‐hydrogen hydrides, whose high Tc relies on extreme pressure, the metallic bond in few‐hydrogen superconductor MgHCu3 improves the structural stability and ductility at atmospheric pressure. Here, the small amount of hydrogen is found to be vital for Tc. After the incorporation of hydrogen, the electron–phonon coupling constant of MgHCu3 is increased to 0.83, which is larger than that of the well‐known MgB2. Moreover, the anisotropy of MgHCu3 also plays an important role in enhancing Tc. Based on the Migdal‐Eliashberg theory, it predicts that the phonon‐mediated metal‐bonded perovskite MgHCu3 is a superconductor with Tc of 42 K. The first predicted ternary metal‐bonded perovskite, MgHCu3, enriches the family of perovskite and will promote further investigation on few‐hydrogen superconductors under atmospheric pressure.

The Electron–Phonon Interaction at Vicinal Metal Surfaces Measured with Helium Atom Scattering

Recently, it was demonstrated that inelastic helium atom scattering from conducting surfaces provides a direct measurement of the surface electron–phonon coupling constant (mass enhancement factor λ) via the temperature or the incident wave vector dependence of the Debye–Waller exponent. Here, previous published as well as unpublished helium atom scattering diffraction data from the vicinal surfaces of copper (Cu(11α), with α = 3, 5, 7) and aluminum (Al(221) and Al(332)) were analyzed to determine λ. The results suggested an enhancement with respect to the corresponding data for the low-index surfaces (111) and (001) above the roughening transition temperature. The specific role of steps compared to that of terraces is briefly discussed.

Effects of anisotropy and disorder on the superconducting properties of niobium

We report results for the superconducting transition temperature and anisotropic energy gap for pure niobium based on Eliashberg’s equations and electron and phonon band structures computed from density functional theory. The electronic band structure is used to construct the Fermi surface and calculate the Fermi velocity at each point on the Fermi surface. The phonon bands are in excellent agreement with inelastic neutron scattering data. The corresponding phonon density of states and electron–phonon coupling define the electron–phonon spectral function, α2F(p, p′; ω), and the corresponding electron–phonon pairing interaction, which is the basis for computing the superconducting properties. The electron–phonon spectral function is in good agreement with existing tunneling spectroscopy data except for the spectral weight of the longitudinal phonon peak at ℏωLO = 23 meV. We obtain an electron–phonon coupling constant of λ = 1.057, renormalized Coulomb interaction μ⋆ = 0.218, and transition temperature Tc = 9.33 K. The corresponding strong-coupling gap at T = 0 is modestly enhanced, Δ0 = 1.55 meV, compared to the weak-coupling BCS value Δ0wc=1.78kBTc=1.43meV. The superconducting gap function exhibits substantial anisotropy on the Fermi surface. We analyze the distribution of gap anisotropy and compute the suppression of the superconducting transition temperature using a self-consistent T-matrix theory for quasiparticle-impurity scattering to describe niobium doped with non-magnetic impurities. We compare these results with experimental results on niobium SRF cavities doped with nitrogen impurities.

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

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

Effect of pressure on superconductivity of LaBi3

The electronic properties, phonon properties, and conventional superconducting properties of the newly synthesized AuCu3-type intermetallic compound LaBi3 using first-principle density functional methods have been reported in this research article.Variations in conventional superconducting properties like transition temperature, electron–phonon coupling constant, Eliashberg spectral function and average logarithmic phonon frequency with applied hydrostatic pressure have been reported in this research article. At ambient pressure, the transition temperature and coupling constant was computed to be 7.2 K and 2.78 respectively. The calculated decrease in transition temperature with applied pressure as reported in this article agrees with the previously reported experimental results. Ambient pressure fermi surface was employed to explain the high electron–phonon coupling constant.

Effects of nonlocal correlation functionals on electron-phonon interactions in NbC

The first-principles approach is applied to study the electronic, vibrational, and superconducting properties of B1-NbC. We used a variety of density functional theory methods that involve local density approximation (PW), generalized gradient approximation; PBE, PBEsol, and GGA functionals corrected with nonlocal correlation functionals (rVV10, vdW-DF2, vdW-DF3). Although the electronic band structures from different methods show an overwhelming agreement, the phonon dispersion curves display significant differences at certain points. The optical phonons make an appreciable part of the electron-phonon coupling constant, λ (up to 23%). This contribution is found to be the largest for PW and the smallest for PBE whereas the nonlocal functionals give intermediate values. This behavior is in perfect analogy with the interaction energies for the three types of methods. The nonlocal functionals, rVV10 and vdW-DF3, fine tuned λ to obtain a value in close agreement with a recently reported experimental value of λ = 0.848 Yan et al. Despite the fact that variations in λ due to nonlocal functionals are not very large, the effects on the superconducting transition temperatures are significant. In addition, the Coulomb screening potential, μ * = 0.1525 gives superconducting transition temperature in the best agreement with the experiment.

Roles of Fe-ion irradiation on MgB2 thin films: Structural, superconducting, and optical properties

The effects of Fe-ion irradiation on the crystal structure and superconducting properties of MgB2 thin films were investigated. Pristine samples were prepared using hybrid physical-chemical vapor deposition (HPCVD), and ion irradiation was performed at three different doses of 5 × 1013, 1 × 1014, and 2 × 1014 ions/cm2. The measured temperature-dependent resistivity showed that as the irradiation dose increased from pristine to most irradiated, the superconducting critical temperature, Tc, significantly decreased from 38.33 to 3.02 K. The crystal structures of the films were investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements. The results showed that the higher the dose, the greater the change in crystal structure, such as the lattice constant and bond length. This suggests that the destruction of the crystal structure at higher doses leads to the degradation of superconductivity in the irradiated MgB2 thin films. Raman spectroscopy showed that the electron-phonon coupling constant decreased with increasing irradiation dose, which was directly related to the reduction of Tc in the samples. The optical conductivity indicates that the charge-carrier density of the σ-band plays an important role in the superconductivity of ion-irradiated MgB2.

G phonon linewidth and phonon-phonon interaction in p-type doped CVD graphene crystals

We employed conventional and time-resolved Raman spectroscopy to study the effects of electron-phonon and phonon-phonon interactions on the linewidth of the G phonon in both unintentionally and intentionally doped polycrystalline chemical vapor deposited (CVD) graphene samples. We directly measured the G phonon lifetime, observing a ≈14% decrease with doping up to EF = 270 meV. The anticipated reduction of G phonon linewidth due to a decrement in electron-phonon contribution deviates from first principles calculations. CVD samples exhibit a ≈30% decrease in the electron-phonon coupling constant, λΓ(EF = 0), compared to exfoliated graphene samples. Additionally, phonon-defect scattering makes a significant contribution to the G band linewidth in CVD graphene samples, owing to their lower crystal quality compared to exfoliated graphene.