Electron-phonon Coupling Calculations Research Articles

Two-dimensional superconductivity in hexagonal layered Na2B2H structure at ambient pressure

Alkaline metal and alkaline earth metal borohydrides may provide ideal candidates for high-temperature superconductors. By replacing the metal element of the layered hexagonal structure Li2B2H in our earlier work, a novel energetically metastable hexagonal Na2B2H, designated as h-Na2B2H, was proposed at ambient condition. First-principles calculations suggest that h-Na2B2H may be prepared from Na3B20, Na, and H2 or from the pure elements by utilizing high-temperature and high-pressure technology. The h-Na2B2H exhibits the highest tensile stress of 24.2 GPa at a strain of 0.245 along the [1–10] direction among the four directions [100], [001], [110], and [1–10]. Electronic property calculations reveal that h-Na2B2H holds two-dimensional conductivity along B atom layers. Electron–phonon coupling calculation indicates h-Na2B2H has a Tc of 42.4 K with λ = 1.73. The superconductivity in h-Na2B2H primarily depends on the Na and B atomic vibrations. Thus, h-Na2B2H can be a potential superconducting material with 2D conductivity at atmosphere pressure.

Theoretical exploration of novel compounds in Sr(B,C)x (x = 7,8) at high pressure

Boron-carbon-based compounds have attracted considerable attention due to their exceptional properties and increasing applications in industry. In this study, we conducted a systematic structural prediction of the SrBxC7-x and SrBxC8-x systems under ambient and high pressures up to 20 GPa using an unbiased global structure search. Through the calculations of formation enthalpy and phonon dispersion curves, we identified three new compounds, namely SrB5C2, SrB6C and SrB5C3, as thermodynamically and dynamically stable under ambient pressure and high pressures up to 20 GPa. Calculations of mechanical properties reveal that these three compounds have significant mechanical strength, with estimated theoretical Vickers hardness values ranging from 23.2 to 39.8 GPa. Electronic structure calculations suggest that both SrB5C2 and SrB5C3 are hole conductors, with the bands crossing the Fermi level, and SrB6C is a semiconductor with an indirect bandgap of 2.4 eV. The electron-phonon coupling calculations using the Migdal-Eliashberg theory indicate that SrB5C2 exhibits superconductivity at 10.5 K under ambient pressure, while SrB5C3 shows superconductivity at a much lower temperature of 0.4 K. This finding adds to our knowledge of boron-carbide materials and provides valuable insights for the development of similar materials with good mechanical properties.

Tuning the electronic and vibrational properties of Sn[formula omitted]P[formula omitted]S[formula omitted]: Pressure induced metallization and superconductivity

The influence of external pressure on the properties of Sn2P2S6 has been systematically studied using density-functional theory calculations. It has been found that the ambient-pressure polymorph (space group P21/c) is most stable up to 100 GPa than the other two polymorphs reported in the literature. We have also found that the electronic band gap closes under compression, being metallization achieved at 35 GPa. In addition, electron–phonon coupling calculations show that the studied material is a superconductor at pressures of 35 GPa and higher. The critical temperature increases under compression. We will also present results on phonon-dispersion calculations, showing that Sn2P2S6 becomes dynamically unstable near 60 GPa. Finally results on the linear and volumetric compressibility are reported showing that Sn2P2S6 is highly compressible with a bulk modulus of 34(1) GPa.

Electron-phonon coupling from GW perturbation theory: Practical workflow combining BerkeleyGW, ABINIT, and EPW

We present a workflow of practical calculations of electron-phonon (e-ph) coupling with many-electron correlation effects included using the GW perturbation theory (GWPT). This workflow combines BerkeleyGW, ABINIT, and EPW software packages to enable accurate e-ph calculations at the GW self-energy level, going beyond standard calculations based on density functional theory (DFT) and density-functional perturbation theory (DFPT). This workflow begins with DFT and DFPT calculations (ABINIT) as starting point, followed by GW and GWPT calculations (BerkeleyGW) for the quasiparticle band structures and e-ph matrix elements on coarse electron k- and phonon q-grids, which are then interpolated to finer grids through Wannier interpolation (EPW) for computations of various e-ph coupling determined physical quantities such as the electron self-energies or solutions of anisotropic Eliashberg equations, among others. A gauge-recovering symmetry unfolding technique is developed to reduce the computational cost of GWPT (as well as DFPT) while fulfilling the gauge consistency requirement for Wannier interpolation.

First-principles calculation of electron-phonon coupling in doped KTaO3.

Background: Motivated by the recent experimental discovery of strongly surface-plane-dependent superconductivity at surfaces of KTaO 3 single crystals, we calculate the electron-phonon coupling strength, λ, of doped KTaO 3 along the reciprocal-space high-symmetry directions. Methods:Using the Wannier-function approach implemented in the EPW package, we calculate λ across the experimentally covered doping range and compare its mode-resolved distribution along the [001], [110] and [111] reciprocal-space directions. Results: We find that the electron-phonon coupling is strongest in the optical modes around the Γ point, with some distribution to higher k values in the [001] direction. The electron-phonon coupling strength as a function of doping has a dome-like shape in all three directions and its integrated total is largest in the [001] direction and smallest in the [111] direction, in contrast to the experimentally measured trends in critical temperatures. Conclusions: This disagreement points to a non-BCS character of the superconductivity. Instead, the strong localization of λ in the soft optical modes around Γ suggests an importance of ferroelectric soft-mode fluctuations, which is supported by our findings that the mode-resolved λ values are strongly enhanced in polar structures. The inclusion of spin-orbit coupling has negligible influence on our calculated mode-resolved λ values.

First-principles study on novel LiB2 phases and superconductivity at ambient pressure

By using the particle swarm optimization (PSO) algorithm implemented in the CALYPSO code, two novel metastable LiB2 structures, namely, P42/mmc-LiB2 and I41/amd-LiB2, were proposed at ambient pressure. Compared with the Cm-LiB2 and AlB2-type P6/mmm-LiB2, the four predicted LiB2 structures are potentially hard materials as indicated by the empirical model formula at atmospheric pressure. The electronic band structure and density of state calculations showed that the conductivities of P42/mmc-, I41/amd-, and P6/mmm-LiB2 were strong, whereas that of Cm-LiB2 was weak. Electron–phonon coupling calculations indicated that the Tc values of P42/mmc-, I41/amd-, and P6/mmm-LiB2 structures were approximately 20.1 K, 12.6 K, and 31.4 K at ambient pressure, respectively. The present results may serve as a reference in the experimental and theoretical study of LiB2 compounds.

Superconductivity determined by the S-H framework in CH4-inserted S-H framework hydrides under high pressures.

Recently, a debate is raising the concern of possible carbonaceous sulfur hydrides with room-temperature superconductivity around 270GPa. In order to systematically investigate the structural information and relevant natures of C-S-H superconductors, we performed an extremely extensive structure search and first-principles calculations under high pressures. As a result, the metastable stoichiometries of CSH7, C2SH14, CS2H10, and CS2H11 were unveiled under high pressure, which can be viewed as CH4 units inserted into the S-H framework. Given the super-high superconductivity of Im3̄m-SH3, we performed electron-phonon coupling calculations of these compounds,the metastable of R3m-CSH7, Cm-CSH7, Cm-CS2H10, P3m1-CS2H10, Cm-CS2H11, and Fmm2-CS2H11 are predicted to become good phonon-mediated superconductors that could reach Tc of 130, 120, 72, 74, 92, and 70K at 270GPa, respectively. Furthermore, we identified that high Tc is associated with the large contribution of the S-H framework to the electron density of states near the Fermi level. Our results highlight the importance of the S-H framework in superconductivity and verify that the suppression of density of states of these carbonaceous sulfur hydrides by CH4 units results in Tc lower than that of Im3̄m-SH3, which could act as a useful guidance in the design and optimization of high-Tc superconductors in these and related systems.

Phonon-mediated superconductivity in the Sb square-net compound LaCuSb2

We investigated the electronic structure and superconducting properties of single-crystalline LaCuSb$_2$. The resistivity, magnetization, and specific heat measurements showed that LaCuSb$_2$ is a bulk superconductor. The observed Shubnikov-de Haas oscillation and magnetic field dependence of the Hall resistivity can be reasonably understood assuming a slightly hole-doped Fermi surface. Electron-phonon coupling calculation clarified the difference from the isostructural compound LaAgSb$_2$ indicating that (i) low-frequency vibration modes related to the interstitial layer sandwiched between the Sb-square nets significantly contribute to the superconductivity and (ii) carriers with sizable electron--phonon coupling distribute isotropically on the Fermi surface. These are assumed to be the origin of higher superconducting transition temperature compared with LaAgSb$_2$. We conclude that the superconducting properties of LaCuSb$_2$ can be understood within the framework of conventional phonon-mediated mechanism.

Adsorption properties of black phosphorene for actinides U, Th, and Pu: A first‐principles study

AbstractDensity functional theory (DFT) calculations were carried out in this work to systematically investigate the adsorption properties of monolayer black phosphorene (BP) for actinide uranium (U), thorium (Th), and plutonium (Pu) atoms. More specifically, the linear response method was used and the HubbardUvalues of 2.97 and 2.61 eV were fitted describing the strong lattice point Coulomb interactions of the U and Pu 5forbitals, respectively. From the DFT + Ucalculations, it was demonstrated that the U, Th, and Pu atoms can be favorably adsorbed on the hollow sites of the BP surface, with an adsorption energy of 3.48, 4.93, and 0.98 eV, respectively. By analyzing the electronic structure, charge transfer, and highest occupied molecular orbital, it was revealed that the U and Th adatoms can induce the generation of impurity states within the band gap of BP and stabilize their adsorption by strongp‐dcoupling with the phosphorus atoms. In addition, from the electron–phonon coupling calculations, it was revealed that the temperature for the stable adsorption of the U and Th adatoms can reach as high as 500 and 673 K. By using the adsorption rate equation, the critical temperature for the adsorption–desorption transition of U and Pu adatoms was estimated to be 632 and 185 K, respectively, while the Th adatom did not tend to desorb. Our results clearly indicate that BP is an ideal adsorption and separation material for actinides with promising potential applications, such as uranium extraction from seawater.

Topological and superconducting properties in bilayer kagome metals YT6Sn6 ( T =V, Nb, Ta)

Kagome materials exhibit fascinating properties and hold important research significance. Inspired by the extensive studies on the $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ family of materials, here, we investigate the topological and superconducting properties of the bilayer kagome metal materials $\mathrm{Y}{T}_{6}{\mathrm{Sn}}_{6}$ ($T$=V, Nb, Ta) using first-principles calculations. These kagome materials exhibit the ${\mathrm{MgFe}}_{6}{\mathrm{Ge}}_{6}$-prototype structure and have no magnetism. The calculated results for formation energy and the phonon dispersion spectrum demonstrate their stability. Based on the calculation of topologically invariant and surface states, they can be categorized as ${\mathbb{Z}}_{2}$ topological metals. The Van Hove singularity and Dirac points are also observed near the Fermi level. According to electron-phonon coupling (EPC) calculations, $\mathrm{Y}{T}_{6}{\mathrm{Sn}}_{6}$ systems are all predicted to be weak superconductors. Using the Allen-Dynes modified McMillan formula, the superconducting critical temperatures ${T}_{c}$ of ${\mathrm{YV}}_{6}{\mathrm{Sn}}_{6}, {\mathrm{YNb}}_{6}{\mathrm{Sn}}_{6}$, and ${\mathrm{YTa}}_{6}{\mathrm{Sn}}_{6}$ are predicted to be 0.65, 1.17, and 0.89 K, respectively. The EPC is mainly contributed by the vibrations of V atoms and partially by the out-of-plane vibrational modes of Sn atoms. The coexistence of nontrivial topological properties and superconducting properties in these bilayer kagome systems is helpful to study the relationship between different physical properties and to design new topological superconductors.

DFTBephy: A DFTB-based approach for electron–phonon coupling calculations

The calculation of the electron–phonon coupling from first principles is computationally very challenging and remains mostly out of reach for systems with a large number of atoms. Semi-empirical methods, like density functional tight binding (DFTB), provide a framework for obtaining quantitative results at moderate computational costs. Herein, we present a new method based on the DFTB approach for computing electron–phonon couplings and relaxation times. It interfaces with phonopy for vibrational modes and dftb+ to calculate transport properties. We derive the electron–phonon coupling within a non-orthogonal tight-binding framework and apply them to graphene as a test case.

Cr_2AlN and the search for the highest temperature superconductor in the M_2AX family

We have developed a high-throughput computational method to predict the superconducting transition temperature in stable hexagonal M_2AX phases, and applied it to all the known possible choices for M (M: Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Lu, Hf and Ta). We combine this with the best candidates for A (A: Al, Cu, Ge and Sn ) and X (X: C and N) from our previous work, and predict T_c for 60 M_2AX-phase materials, 53 of which have never been studied before. From all of these, we identify Cr_2AlN as the best candidate for the highest T_c, and confirm its high T_c with more detailed density functional theory electron-phonon coupling calculations. Our detailed calculations predict T_c = 14.8 K for Cr_2AlN, which is significantly higher than any T_c value known or predicted for any material in the M_2AX family to date.

Designing ternary superconducting hydrides with A15-type structure at moderate pressures

Ever since hydrogen-based superconductors have been actively considered as candidates for room-temperature superconductors, binary hydrides have been almost completely explored theoretically. Now researchers are turning their interest to ternary hydrides. However, the enormous compositional and structural possibilities in the ternary hydride space limit the search efficiency. Here, using an advanced crystal structure search method combined with first-principles calculations, a series of ternary hydrides AXH6 (MgPH6, MgAsH6, CaSbH6, CaSnH6, NaSbH6 and ScSiH6) with the A15-type structure are predicted to be stable at around one million atmospheres. Among them, CaSnH6 presents the lowest thermodynamically stable pressure of 110 GPa. CaSbH6, CaSnH6 and ScSiH6 may be dynamically stable down to 41 (70), 71 (90) and 86 (70) GPa, respectively, without (with) anharmonic effects and temperature effects. Hydrogen atoms in these A15-type structures are found to form weak covalent bonds to X atoms. More interestingly, we find that ternary hydrides with the AXH6 stoichiometry may assume the A15-type structure under pressure when the sum of the chemical valences of the two non-hydrogen atoms is six or seven. Based on electron-phonon coupling (EPC) calculations, all these ternary hydrides are estimated to be superconductors. CaSbH6, CaSnH6 and ScSiH6 have superconducting transition temperatures (Tcs) of 65, 53 and 116 K at 45, 75 and 90 GPa, respectively, while MgPH6 has the highest Tc of 148 K at 135 GPa. Our study will provide guidance for designing high Tc ternary hydrides near ambient pressures.

A pressure-induced high-pressure metallic GeTe phase.

As an important phase-change material, GeTe has many high-pressure phases as well, but its phase transitions under pressure are still lack of clarity. It is challenging to identify high-pressure GeTe crystal structures owing to the phase coexistence in a wide pressure range and the reversibility of phase transitions. Hence, first-principles calculations are required to provide further information in addition to limited experimental characterizations. In this work, a new orthorhombic Cmca GeTe high-pressure phase has been predicted via the CALYPSO method as the most energetically favorable phase in the pressure range between ∼30 and ∼38.5GPa, which would update the GeTe high-pressure phase transition sequence. The crystal structure of the Cmca phase is composed of alternate stacking puckered layers of Ge six-membered rings and Te four-membered rings along the b direction. The high density of states near the Fermi level and delocalization of electrons from the two-dimensional electron localization function indicate a strong metallic property of the Cmca phase. Electron-phonon coupling calculations indicate that the Cmca phase is superconductive below ∼4.2K at 35GPa. The simulated x-ray diffraction pattern of the Cmca phase implies that this phase might coexist with the Pnma-boat phase under high pressure. These results offer further understanding on the high-pressure structural evolution and physical properties in GeTe and other IV-VI semiconductors.

First-principles calculations of electron-phonon coupling in NaSnSb: buckling and pressurization

In this paper, the electron-phonon coupling (EPC) properties of topological material NaSnSb has been explored, whose low phonon branch dominates the EPC constant λ. NaSnSb can remain the strength of its EPC with a slight compression of the distance of the Na and Sb atoms along the z-axis (z Na–Sb), while further compression or stretching would destroy the strength of its EPC. By pressing, in the pressure range of 2 GPa, the superconducting transition temperature (T c) of NaSnSb behaves similar to that of FeSe in the same pressure range, with a local minimum at 1 GPa and a maximum at 1.5 GPa. Both compressing z Na–Sb and pressure lead to phonon softening of NaSnSb. The phonon softening induced by compressing z Na–Sb is three-dimensional, whereas the phonon softening induced by compression is two-dimensional.

High-Throughput DFT-Based Discovery of Next Generation Two-Dimensional (2D) Superconductors.

High-throughput density functional theory (DFT) calculations allow for a systematic search for conventional superconductors. With the recent interest in two-dimensional (2D) superconductors, we used a high-throughput workflow to screen over 1000 2D materials in the JARVIS-DFT database and performed electron-phonon coupling calculations, using the McMillan-Allen-Dynes formula to calculate the superconducting transition temperature (Tc) for 165 of them. Of these 165 materials, we identify 34 dynamically stable structures with transition temperatures above 5 K, including materials such as W2N3, NbO2, ZrBrO, TiClO, NaSn2S4, Mg2B4C2, and the previously unreported Mg2B4N2 (Tc = 21.8 K). Finally, we performed experiments to determine the Tc of selected layered superconductors (2H-NbSe2, 2H-NbS2, ZrSiS, FeSe) and discuss the measured results within the context of our DFT results. We aim that the outcome of this workflow can guide future computational and experimental studies of new and emerging 2D superconductors by providing a roadmap of high-throughput DFT data.

Direct Measurement of Ballistic and Diffusive Electron Transport in Gold

We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron-phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron-phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.

Predicted superconductivity in one-dimensional A3Hf2B3-type electrides.

Inorganic electrides are considered potential superconductors due to the unique properties of their anionic electrons. However, most electrides require external high-pressure conditions to exhibit considerable superconducting transition temperatures (Tc). Therefore, searching for superconducting electrides under low or moderate external pressures is of significant research interest and importance. In this work, a series of A3Hf2B3-type compounds (A = Mg, Ca, Sr, Ba; B = Si, Ge, Sn, Pb) were constructed and systematically studied based on density functional theory calculations. According to the analysis of the electronic structures and phonon dispersion spectrums, stable one-dimensional electrides Ca3Hf2Ge3, Ca3Hf2Sn3, and Sr3Hf2Pb3, were screened out. Interestingly, the superconductivity of these electrides were predicted from electron phonon coupling calculations. It is highlighted that Sr3Hf2Pb3 showed the highest Tc, reaching 4.02 K, while the Tc values of Ca3Hf2Ge3 and Ca3Hf2Sn3 were 1.16 K and 1.04 K, respectively. Moreover, the Tc value of Ca3Hf2Ge3 can be increased to 1.96 K under 20 GPa due to the effect of phonon softening. This work enriches the types of superconducting electrides and has important guiding significance for the research on constructing electrides and related superconducting materials.

Electron-phonon coupling, bipolar effects, and thermoelectric performance of the CuSbS2 monolayer.

The thermoelectric performance of the CuSbS2 monolayer is determined using the relaxation times obtained from electron-phonon coupling calculations and the transport properties of phonons and electrons. Based on the fully relaxed structure, the lattice thermal conductivity and the electronic transport coefficients are evaluated by solving the Boltzmann transport equation for phonons and electrons under relaxation time approximation, respectively. The tendencies of the transport coefficients depending on the carrier concentrations and temperatures are studied to understand the thermoelectric performance. Based on the bipolar effect, the transport coefficients and intrinsic carrier concentrations, we determined the dimensionless figure of merit ZT in the 300-800 K range. The results demonstrate that the CuSbS2 monolayer should be an p-type semiconductor, and the maximum ZT of 1.36 is obtained, indicating that the monolayer is a good candidate for high-temperature thermoelectric devices. Substantial bipolar effects are observed, and the ones in the x-direction are stronger in comparison to those in the y-direction, which is responsible for the smaller ZT in the x-direction.

Theoretical Prediction of Superconductivity in Boron Kagome Monolayer: MB3 (M = Be, Ca, Sr) and the Hydrogenated CaB3

Using first-principles calculations, we predict a new type of two-dimensional (2D) boride MB3 (M = Be, Ca, Sr), constituted by boron kagome monolayer and the metal atoms adsorbed above the center of the boron hexagons. The band structures show that the three MB3 compounds are metallic, thus the possible phonon-mediated superconductivity is explored. Based on the Eliashberg equation, for BeB3, CaB3, and SrB3, the calculated electron–phonon coupling constants λ are 0.46, 1.09, and 1.33, and the corresponding superconducting transition temperatures T c are 3.2, 22.4, and 20.9 K, respectively. To explore superconductivity with higher transition temperature, hydrogenation and charge doping are further considered. The hydrogenated CaB3, i.e., HCaB3, is stable, with the enhanced λ of 1.39 and a higher T c of 39.3 K. Moreover, with further hole doping at the concentration of 5.8 × 1011 hole/cm2, the T c of HCaB3 can be further increased to 44.2 K, exceeding the McMillan limit. The predicted MB3 and HCaB3 provide new platforms for investigating 2D superconductivity in boron kagome lattice since superconductivity based on monolayer boron kagome lattice has not been studied before.