Electron-phonon Coupling Calculations Research Articles

Computational Design of Novel Hydrogen-Rich YS-H Compounds.

The recent successful findings of H3S and LaH10 compressed above 150 GPa with a record high Tc (above 200 K) have shifted the focus on hydrogen-rich materials for high superconductivity at high pressure. Moreover, some studies also report that transition-metal ternary hydrides could be synthesized at a relatively low pressure (∼10 GPa). Therefore, it is highly desirable to investigate the crystal structures of ternary hydrides compounds at high pressure since they have been long considered as promising superconductors and hydrogen-storage materials with a high Tc, and can be possibly synthesized at low pressure as well. In this work, combining state-of-the-art crystal structure prediction and first-principles calculations, we have performed extensive simulations on the crystal structures of YSHn (n = 1–10) compounds from ambient pressure to 200 GPa. We uncovered three thermodynamically stable compounds with stoichiometries of YSH, YSH2, and YSH5, which became energetically stable at ambient pressure, 143, and 87 GPa, respectively. Remarkably, it is found that YSH contains monoatomic H atoms, while YSH2 and YSH5 contain a mixture of atomlike and molecular hydrogen units. Upon compression, YSH, YSH2, and YSH5 undergo a transition from a semiconductor to a metallic phase at pressures of 168, 143, and 232 GPa, respectively. Unfortunately, electron–phonon coupling calculations reveal that these compounds possess a weak superconductivity with a relatively low Tc (below 1 K), which mainly stem from the low value of density of states occupation at the Fermi level (EF). These results highlight that the crystal structures play a critical role in determining the high-temperature superconductivity.

Electronic structure and superconductivity in hexagonal Li3B2 and Li2B2H phases under pressure

The relatively low-pressure layered hexagonal Li3B2 (h-Li3B2) phase under 20 GPa was identified on the basis of ab initio calculations. Inspired by the high-temperature superconductivity of metallic hydrogen, this study obtained a novel hexagonal Li2B2H (h-Li2B2H) structure by replacing Li2 with H. Contrast to h-Li3B2, h-Li2B2H may be directly synthesized by α-LiB and H2 under pressure. The two phases exhibited electronic features similar to those in MgB2 and were expected to be good superconductors. Electron–phonon coupling calculations illustrated that the layered h-Li3B2 structure is superconducting at 2 K with λ = 0.39 under 20 GPa. Moreover, h-Li2B2H was predicted to possess a large electron–phonon coupling parameter λ of approximately 0.81 and a resulting superconducting critical temperature about 28 K at 20 GPa. The introduced H in the h-Li2B2H structure had strong vibrational energy due to its light atomic mass and could enhance the energy of σ and π bonds by bonding with Li. Thus, the introduced H in the h-Li2B2H structure is important due to its enhanced electron–phonon coupling and superconducting property.

Topological and superconducting properties in YD3 ( D=In , Sn, Tl, Pb)

Using first-principles calculations, we explore the electron properties of ${\text{Y}D}_{3}$ ($D=\text{In}$, Sn, Tl, Pb) and predict that ${\mathrm{YIn}}_{3}$, ${\mathrm{YTl}}_{3}$, and ${\mathrm{YPb}}_{3}$ are topological superconductor candidates. In the presence of spin-orbit coupling effect, continuous band gaps for ${\mathrm{YIn}}_{3}$, ${\mathrm{YSn}}_{3}$, and ${\mathrm{YPb}}_{3}$ are opened between their highest occupied bands ($N$) and the lowest unoccupied bands ($N+1$), where the different ${Z}_{2}$ invariants are obtained. Differently and specially, there are type-II Dirac points (DPs) at the high-symmetry lines in ${\mathrm{YTl}}_{3}$, indicating one possibility of topological Dirac semimetal. Furthermore, the nontrivial Rashba-like topological surface states are achieved at the $\overline{\mathrm{X}}$ point on the (001) surface for ${\mathrm{YIn}}_{3}$, ${\mathrm{YTl}}_{3}$, and ${\mathrm{YPb}}_{3}$, as well as the Fermi arcs in ${\mathrm{YTl}}_{3}$ connecting the DPs. In addition to the topological properties, our electron-phonon coupling calculations indicate clearly that these four intermetallics are all phonon-mediated superconductors. The calculated superconducting transition temperatures of ${T}_{c}=0.96$, 6.34, 2.17, and 4.37 K respectively for ${\mathrm{YIn}}_{3}$, ${\mathrm{YSn}}_{3}$, ${\mathrm{YPb}}_{3}$, and ${\mathrm{YTl}}_{3}$ agree well with experiments.

Superconductivity at 23 K in MgLi compound at ultrahigh pressure

The recent observation of superconductivity at ambient temperature and pressure conditions in an Au-Ag based nanostructure has attracted a great attention. It is critical task to explore the issue whether the pure metals or metal alloys are outstanding candidates for high-temperature superconductors. We have selected the simplest sample, magnesium-lithium (MgLi) compound with stoichiometric composition of 1:1, as the prototype to explore the structural, electronic, and superconducting properties and their evolution behaviors at high pressure conditions. Based on particle-swarm optimization algorithm combined with first-principles calculations, we have identified the transition sequence and established the corresponding phase boundaries. According to electron-phonon coupling calculations, the superconducting critical temperatures for all the predicted thermodynamically stable structures increase with the increasing pressure in their own stable pressure ranges and the resulting highest critical temperature reaches ∼23 K.

Potential high- Tc superconductivity in CaYH12 under pressure

The high-pressure phases and superconductivity of ${\mathrm{CaYH}}_{12}$ have been explored by using a particle swarm optimization structure prediction methodology in combination with first-principles calculations. Our results show that ${\mathrm{CaYH}}_{12}$ becomes stable with a cubic $Fd\overline{3}m$ structure above 170 GPa, where metal atoms form body-centered-cubic (bcc) lattices and hydrogens occupy all the tetrahedral interstices of these bcc lattices, completing sodalitelike cages. The electron-phonon coupling calculations indicate that the $Fd\overline{3}m$ structure is a potential high-temperature superconductor, with a calculated ${T}_{c}$ of 258 K at 200 GPa. Our current study provides a possibility for searching new high-${T}_{c}$ superconductors in ternary hydrides.

β–RhPb2 : A topological superconductor candidate

A topological superconductor candidate $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{RhPb}}_{2}$ is predicted by using first-principles electronic structure calculations. Our calculations show that there is a band inversion around the Fermi level at the $Z$ point of Brillouin zone. The calculated nonzero topological invariant ${\mathrm{Z}}_{2}$ indicates that $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{RhPb}}_{2}$ is a topological insulator defined on a curved Fermi level. Slab calculations further demonstrate that the gapless nontrivial topological surface states (TSS) are not overlapped by the bulk states and they cross the Fermi level. Phonon calculations confirm the dynamical stability of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{RhPb}}_{2}$, while electron-phonon coupling (EPC) calculations predict that the superconducting transition temperature $({T}_{c})$ of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{RhPb}}_{2}$ can reach 9.7 K. The coexistence of nontrivial topological band structure with the TSS crossing the Fermi level as well as the superconducting ${T}_{c}$ above the liquid-helium temperature suggest that the layered compound $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{RhPb}}_{2}$ is a topological superconductor, which deserves further experimental verification.

Pressure-induced structural and electronic transitions in bismuth iodide

Using a machine-learning accelerated crystal structural search, and ab initio calculations together with high-pressure Raman measurements, we study the crystal and electronic structures of bismuth iodide (BiI) systematically up to 50 GPa. We find that the ambient $C2/m$ phase ($\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{4}{\mathrm{I}}_{4}$) transforms across a tetragonal $P{4}_{2}/mmc$ phase at 8.5 GPa and finally to a hexagonal $P{6}_{3}/mmc$ phase at 28.2 GPa. Our high-pressure Raman experiments identify a phase transition at around 8.6 GPa, and the experimental Raman modes evolution agrees with our calculations reasonably. Band-structures calculations suggest the BiI system undergoes a pressure-induced topological phase transition from a topological metal ($P{4}_{2}/mmc$ phase) to a trivial metal (the $P{6}_{3}/mmc$ phase). Our electron-phonon coupling calculations show both the $P{4}_{2}/mmc$ and $P{6}_{3}/mmc$ phases are superconductors and the estimated superconducting critical temperature agrees with previous measurements. Our study shows the previously reported pressure-induced superconductivity in BiI should originate from structure phase transitions.

Prediction of Li2B novel phases and superconductivity under varying pressures

Few experimental and theoretical superconductivity works have focused on Li–B systems under the Li-rich regime. Here, we utilized the recently developed particle-swarm optimization method for structural search to propose three novel Li2B phases. These phases include the layered sandwich structure hP6–Li2B (P-3m1, 164) and the two chain structures mS12–Li2B (C2/m, 12) and oC12–Li2B (Cmcm, 63). The structural stability of the three novel Li2B phases was confirmed on the basis of elastic constant and phonon dispersion calculations. Under ambient pressure, the three Li2B structures are notably more thermodynamically stable than the previously proposed Cmcm-Li2B structure and oC12–Li2B may be experimentally synthesized by α-LiB and Li. Electronic band structure and partial density of states analyses showed that our three proposed phases were metallic. The quasi-2D character in the band structure of hP6–Li2B was consistent with that of MgB2 and indicated that hP6–Li2B has potential superconductive properties. Our electron–phonon coupling calculations illustrated that mS12– and oC12–Li2B exhibited superconductivity at 0.03 K under 10 Gpa and 0.14 K under 0 Gpa, respectively. Moreover, we found that hP6–Li2B has low superconductivity with a predicted Tc of 0.09 K at 20 GPa, and its λ and Tc values increased within the pressure range of 20–50 GPa and then decreased under pressures exceeding 60 GPa. The maximum λ and Tc values under 60 GPa were 0.33 and 0.65 K, respectively.

Carbon network evolution from dimers to sheets in superconducting ytrrium dicarbide under pressure

Carbon-bearing compounds display intriguing structural diversity, due to variations in hybrid bonding of carbon. Here, first-principles calculations and unbiased structure searches on yttrium dicarbide at pressure reveal four new structures with varying carbon polymerisation, in addition to the experimentally observed high-temperature low-pressure I4/mmm dimer phase. At low pressures, a metallic C2/m phase (four-member single-chain carbide) is stable, which transforms into a Pnma phase (single-chain carbide) upon increasing pressure, with further transformation to an Immm structure (double-chain carbide) at 54 GPa and then to a P6/mmm phase (sheet carbide) at 267 GPa. Yttrium dicarbide is structurally diverse, with carbon bonded as dimers (at lowest pressure), four-member single chains, infinite single chains, double chains and eventually sheet structures on compression. Electron–phonon coupling calculations indicate that the high-pressure phases are superconducting. Our results aid the understanding and design of new superconductors and illuminate pressure-induced carbon polymerisation in carbides.

High-Pressure Evolution of Unexpected Chemical Bonding and Promising Superconducting Properties of YB6

As the prototype of MB6 (M = alkaline-earth and rare-earth metals) compounds, YB6 possesses the highest superconducting critical temperature in this family at the ambient pressure. Here, we performed a first principle exploration on the chemical bonding states and the corresponding electronic properties of YB6 at high pressure. Two phases with Cmcm and I4/mmm space groups are predicted using CALYPSO method, energetically more stable than the previously proposed structures. The B covalent network is eventually evolved from B6 octahedron in the cubic Pm-3m phase to B24 unit in I4/mmm phase. The calculated electron–phonon coupling parameters show that the contribution of B is significantly increased by the high-pressure effect in Cmcm and I4/mmm structure, contrary to that in Pm-3m structure. Further calculations of electron–phonon coupling indicate that the high-pressure phases are likely superconducting with the major contribution by boron phonon vibration.

Effect of covalent bonding on the superconducting critical temperature of the H-S-Se system

Hydrogen-rich materials have attracted great interest since the recent discovery of superconductivity at 203 K in highly compressed hydrogen sulfide. To probe the role of covalent bonding in determining the ${T}_{\mathrm{c}}$ of hydrogen-related superconductors, we systematically studied the crystal structure and superconductivity of ${\mathrm{H}}_{6}\mathrm{SSe}$, a hypothetical compound derived from ${\mathrm{H}}_{3}\mathrm{S}$ with half its S atoms replaced by group neighbor Se. First-principles structure searches identify three dynamically stable structures for ${\mathrm{H}}_{6}\mathrm{SSe}$ at 200 GPa. Interestingly, all three structures keep the main feature of the cubic $Im\overline{3}m$ structure of ${\mathrm{H}}_{3}\mathrm{S}$, but with different Se substitution positions. Electron-phonon coupling calculations reveal the superconductive potential of the three phases of ${\mathrm{H}}_{6}\mathrm{SSe}$, with ${T}_{\mathrm{c}}$ decreasing (from 195 to 115 K) upon the declining strength of the weakest covalent H-S or H-Se bonds in each structure, thereby highlighting the key role of covalent bonding in determining ${T}_{\mathrm{c}}$. For comparison, O-substituted ${\mathrm{H}}_{6}\mathrm{SO}$ was predicted to assume a semiconducting phase with entirely different structural features from ${\mathrm{H}}_{6}\mathrm{SSe}$. We attribute this difference to the much stronger electronegativity of O (3.44) compared with S (2.58) or Se (2.55).

Impact of the electron-phonon coupling factor and electron heat capacity on the thermal response of targets irradiated by femtosecond laser

This work investigates the impact of the electron temperature-dependent electron-phonon coupling factor and the electron heat capacity on the thermal evolution of a copper target irradiated by femtosecond laser pulses. These effects were studied using their recently developed two-dimensional two-temperature model. Two models were implemented to account for the electron-phonon coupling calculation. The first were the calculations of the electronic structure of the density of state have been done at high electron temperature. The excitation of the d-band electrons and their contribution to the coupling of the heat between electrons and phonons were considered. The second were the electron-electron collisions and electron-lattice collision rates calculations were performed. The effect of fluence on the thermalization time and on the ablated depth of the copper irradiated by femtosecond laser pulses was studied using these two models and compared with the results assuming constant value for the electron-phonon coupling factor. A comparison between the model of the electron heat capacity based on the density of state calculation and the well-known linear relationship between the electron heat capacity and the electron temperature was also studied.

Stability and superconductivity properties of metal substituted aluminum diborides (M0.5Al0.5B2)

The stability table and superconducting properties such as screened Coulomb potential, electron-phonon coupling and critical temperature of planar boron sheet included ternary crystalline compounds of MgB2-type M0.5Al0.5B2 (M = Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Y, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re, and Os) have been investigated by first-principles density functional theory calculations with spin-orbit coupling. It is shown that the M = Li, Mg, Ca, Sc, Ti, V, Y, Zr, Nb, Mo, Tc, Hf, Ta, and W-substituted compounds are thermodynamically stable at the ambient conditions. Among them, Nb0.5Al0.5B2 has the lowest cohesive energy while Ti0.5Al0.5B2 has the lowest formation enthalpy. Except for Be0.5Al0.5B2, all compounds are mechanically stable, and fourteen of them are dynamically stable also. So, a dozen of M0.5Al0.5B2 (M = Li, Mg, Ca, Sc, Ti, V, Zr, Nb, Mo, Tc, Hf, and Ta) compounds are satisfying all three stability conditions. The calculated electronic properties show that all structures have metallic character and interestingly the radii of the substituted atoms correlate with surface area of regular B6 hexagons and volumes of metal-boron pyramids. All of them are hard materials, and V0.5Al0.5B2 has the highest semi-empirical microhardness (27 GPa). According to the precise phonon dispersion and electron-phonon coupling calculations, Tc’s of the stable compounds are lower than that of MgB2 (39 K), and Tc0.5Al0.5B2 has higher Tc (∼7 K) than the others.

Large carrier-capture rate of PbI antisite in CH3NH3PbI3 induced by heavy atoms and soft phonon modes

The $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ antisite was reported to be a possible deep-level recombination-center defect in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ solar cells with a concentration higher than ${10}^{15}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ under I-poor conditions. However, whether it is really an effective nonradiative recombination center and limits the photovoltaic efficiency depends also on its cross sections for capturing the electron or hole carriers, which is difficult to determine in both experiment and theory. Here we use a recently developed method to overcome the high computational cost of electron-phonon coupling calculation and implement it in the standard first-principles code quantum espresso so that we can calculate the carrier-capture cross sections effectively for the point defects in complicated semiconductors, such as $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$. The calculation showed that both the hole- and the electron-capture cross sections and capture rates of $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ are large relative to those of the point defects in conventional semiconductors, such as Si or GaP, which is attributed mainly to the heavy Pb-I atoms and the soft phonon modes in this Pb halide. Nonradiative recombination increases the thermal energy by exciting the phonons in the soft Pb-I lattice, whereas the contribution of the organic $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}$ group is negligible. Since $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ has a higher concentration under I-poor conditions, especially when the semiconductor is $p$-type, our results suggest that the minority-carrier lifetime and thus the photovoltaic efficiency should be very limited in $p$-type $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ and $p$-type doping should be avoided for fabricating high-efficiency $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ solar cells under I-poor conditions. Similar calculations can be used for studying the influence of various defects on the photovoltaic performance of other organic-inorganic hybrid and inorganic halide perovskites, shedding light on the design of high-efficiency solar cells.

Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure

A systematic structure search in the La-H and Y-H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH8 a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen-Cooper-Schrieffer (BCS) arguments indicate they might be high-Tc superconductors. In particular, the superconducting transition temperature Tc calculated for LaH10 is 274-286 K at 210 GPa. Similar calculations for the Y-H system predict stability of the sodalite-like fcc YH10 and a Tc above room temperature, reaching 305-326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

Pressure-Stabilized Superconductive Ionic Tantalum Hydrides.

High-pressure structures of tantalum hydrides were investigated over a wide pressure range of 0-300 GPa by utilizing evolutionary structure searches. TaH and TaH2 were found to be thermodynamically stable over this entire pressure range, whereas TaH3, TaH4, and TaH6 become thermodynamically stable at pressures greater than 50 GPa. The dense Pnma (TaH2), R3̅m (TaH4), and Fdd2 (TaH6) compounds possess metallic character with a strong ionic feature. For the highly hydrogen-rich phase of Fdd2 (TaH6), a calculation of electron-phonon coupling reveals the potential high-Tc superconductivity with an estimated value of 124.2-135.8 K.

Thermal transport across metal silicide-silicon interfaces: First-principles calculations and Green's function transport simulations

In this work, we use a combination of first-principles calculations under the density functional theory framework and heat transport simulations using the atomistic Green's function (AGF) method to quantitatively predict the contribution of the different scattering mechanisms to the thermal interface conductance of epitaxial CoSi$_2$-Si interfaces. An important development in the present work is the direct computation of interfacial bonding from density functional perturbation theory (DFPT) and hence the avoidance of commonly used `mixing rules' to obtain the cross-interface force constants from bulk material force constants. Another important algorithmic development is the integration of the recursive Green's function (RGF) method with B\"{u}ttiker probe scattering that enables computationally efficient simulations of inelastic phonon scattering and its contribution to the thermal interface conductance. First-principles calculations of electron-phonon coupling reveal that cross-interface energy transfer between metal electrons and atomic vibrations in the semiconductor is mediated by delocalized acoustic phonon modes that extend on both sides of the interface, and phonon modes that are localized inside the semiconductor region of the interface exhibit negligible coupling with electrons in the metal. We also provide a direct comparison between simulation predictions and experimental measurements of thermal interface conductance of epitaxial CoSi$_2$-Si interfaces using the time-domain thermoreflectance technique. Importantly, the experimental results, performed across a wide temperature range, only agree well with predictions that include all transport processes: elastic and inelastic phonon scattering, electron-phonon coupling in the metal, and electron-phonon coupling across the interface.

Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First-Principles.

Environmental effects and intrinsic energy-loss processes lead to fluctuations in the operational temperature of solar cells, which can profoundly influence their power conversion efficiency. Here we determine from first-principles the effects of temperature on the band gap and band edges of the hybrid pervoskite CH3NH3PbI3 by accounting for electron-phonon coupling and thermal expansion. From 290 to 380 K, the computed band gap change of 40 meV coincides with the experimental change of 30-40 meV. The calculation of electron-phonon coupling in CH3NH3PbI3 is particularly intricate as the commonly used Allen-Heine-Cardona theory overestimates the band gap change with temperature, and excellent agreement with experiment is only obtained when including high-order terms in the electron-phonon interaction. We also find that spin-orbit coupling enhances the electron-phonon coupling strength but that the inclusion of nonlocal correlations using hybrid functionals has little effect. We reach similar conclusions in the metal-halide perovskite CsPbI3. Our results unambiguously confirm for the first time the importance of high-order terms in the electron-phonon coupling by direct comparison with experiment.

Versatile Titanium Silicide Monolayers with Prominent Ferromagnetic, Catalytic, and Superconducting Properties: Theoretical Prediction.

On the basis of global structure search and density functional theory calculations, we predict a new class of two-dimensional (2D) materials, titanium silicide (Ti2Si, TiSi2, and TiSi4) monolayers. They are proved to be energetically, dynamically, and thermally stable and own excellent mechanical properties. Among them, Ti2Si is a ferromagnetic metal with a magnetic moment of 1.37 μB/cell, while TiSi2 is an ideal catalyst for the hydrogen evolution reaction with a nearly zero free energy of hydrogen adsorption. More importantly, electron-phonon coupling calculations suggest that TiSi4 is a robust 2D phonon-mediated superconductor with a transition temperature of 5.8 K, and the transition temperature can be enhanced up to 11.7 K under a suitable external strain. The versatility makes titanium silicide monolayers promising candidates for spintronic materials, hydrogen evolution catalysts, and 2D superconductors.

Pressure-Stabilized Ir3– in a Superconducting Potassium Iridide

The first charge-separated iridide (Ir3–) in an extended solid was identified at elevated pressure when combined with potassium. According to an unbiased structure searching method that combines first-principles calculations with particle swarm optimization algorithms, K3Ir in the Cu3Ti-type structure shows a favorable formation enthalpy (ΔH) compared to the elements and is dynamically stable above 10 GPa. This novel semiconductor (Eg ≈ 1.6 eV) has sufficient orbital separation to allow complete charge transfer from K to Ir, and Bader charge analysis supports the formation of a formally anionic Ir3–. Further, electron doping of K3Ir through Pt substitution makes the system metallic, and electron–phonon coupling calculations indicate that K3(Ir0.875Pt0.125) falls in the strong-coupling regime, with a predicted superconducting transition temperature (Tc) of ∼27 K at 20 GPa. These results suggest that systems containing elements isoelectronic with classical BCS superconductors such as mercury might have an i...