Large Electron-phonon Coupling Constant Research Articles

Few-Hydrogen High-Tc Superconductivity in (Be4)2H Nanosuperlattice with Promising Ductility under Ambient Pressure.

The multi-hydrogen lanthanum hydride LaH10 is well recognized as having the highest critical temperature (Tc) of 250-260 K under unrealistically ultrahigh pressures of about 170-200 GPa. Here, we propose a novel idea for designing a new ambient-pressure high-Tc superconductor by inserting a hexagonal H-monolayer into two close-packed Be monolayers to form a new and stable few-hydrogen metal-bonded layered beryllium hydride (Be4)2H nanosuperlattice, with better ductility than multi-hydrogen, cuprate, and iron-based superconductors, completely contrary to the conventional design strategy for multi-hydrogen covalent high-Tc superconductors with poor ductility at several hundred GPa. We find that (Be4)2H is a phonon-mediated Eliashberg superconductor with a large electron-phonon coupling constant of 1.41 and a high Tc of 84-72 K with Coulomb repulsion pseudopotential μ* = 0.07-0.13. Importantly, (Be4)2H is the only new high-Tc superconductor and fills the gap in the absence of ambient-pressure superconductors around the liquid-nitrogen temperature with good ductility, which is highly beneficial for practical applications.

Evidence for Band Renormalizations in Strong-Coupling Superconducting Alkali-Fulleride Films.

There has been a long-standing debate about the mechanism of the unusual superconductivity in alkali-intercalated fullerides. In this Letter, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of superconducting K_{3}C_{60} thin films. We observe a dispersive energy band crossing the Fermi level with the occupied bandwidth of about 130meV. The measured band structure shows prominent quasiparticle kinks and a replica band involving the Jahn-Teller active phonon modes, which reflects strong electron-phonon coupling in the system. The electron-phonon coupling constant is estimated to be about 1.2, which dominates the quasiparticle mass renormalization. Moreover, we observe an isotropic nodeless superconducting gap beyond the mean-field estimation (2Δ/k_{B}T_{c}≈5). Both the large electron-phonon coupling constant and large reduced superconducting gap suggest a strong-coupling superconductivity in K_{3}C_{60}, while the electronic correlation effect is suggested by the observation of a waterfall-like band dispersion and the small bandwidth compared with the effective Coulomb interaction. Our results not only directly visualize the crucial band structure but also provide important insights into the mechanism of the unusual superconductivity of fulleride compounds.

Untangling the Fundamental Electronic Origins of Non‐Local Electron–Phonon Coupling in Organic Semiconductors

AbstractOrganic semiconductors with distinct molecular properties and large carrier mobilities are constantly developed in attempt to produce highly‐efficient electronic materials. Recently, designer molecules with unique structural modifications have been expressly developed to suppress molecular motions in the solid state that arise from low‐energy phonon modes, which uniquely limit carrier mobilities through electron–phonon coupling. However, such low‐frequency vibrational dynamics often involve complex molecular dynamics, making comprehension of the underlying electronic origins of electron–phonon coupling difficult. In this study, first a mode‐resolved picture of electron–phonon coupling in a series of materials that are specifically designed to suppress detrimental vibrational effects, is generated. From this foundation, a method is developed based on the crystalline orbital Hamiltonian population (COHP) analyses to resolve the origins—down to the single atomic‐orbital scale—of surprisingly large electron–phonon coupling constants of particular vibrations, explicitly detailing the manner in which the intermolecular wavefunction overlap is perturbed. Overall, this approach provides a comprehensive explanation into the unexpected effects of less‐commonly studied molecular vibrations, revealing new aspects of molecular design that should be considered for creating improved organic semiconducting materials.

Strong electron-phonon coupling superconductivity in compressed α−MoB2 induced by double Van Hove singularities

A recent experiment of ${\mathrm{MgB}}_{2}$-type structure $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{MoB}}_{2}$ has realized $\ensuremath{\sim}32$ K superconductivity (SC) at 90 GPa, exhibiting the highest superconducting transition temperature (${T}_{\mathrm{c}}$) among transition-metal diborides. Although the SC was characterized by the electron-phonon coupling (EPC), the microscopic mechanism of how the large EPC constant and high ${T}_{\mathrm{c}}$ are attained is unclear. Here, based on first-principles calculations, we found that in contrast to ${\mathrm{MgB}}_{2}$, B atoms contribute most to electronic states near Fermi level ($E{}_{\mathrm{F}}$), Mo ${d}_{{z}^{2}}$ orbital is more dominant component in ${\mathrm{MoB}}_{2}$ and provides two impressive peaks in density of states near ${E}_{\mathrm{F}}$ associated with emergent double Van Hove singularities (VHS). The EPC analysis reveals that the electronic sates around double VHS could strongly interact with the softened acoustic modes of Mo out-of-plane vibration, giving rise to a large single gap with the ${T}_{\mathrm{c}}$ up to $\ensuremath{\sim}37$ K, which distinctly differs from the superconducting feature of ${\mathrm{MgB}}_{2}$. Furthermore, by electron doping into ${\mathrm{MoB}}_{2}$, the VHS is tuned to be aligned with the ${E}_{\mathrm{F}}$ and ${T}_{\mathrm{c}}$ can be increased to $\ensuremath{\sim}43$ K. Our findings not only elucidate the microscopic mechanism of observed high ${T}_{\mathrm{c}}$ in ${\mathrm{MoB}}_{2}$, but also demonstrate that ${\mathrm{MoB}}_{2}$ provides an ideal platform to explore the role of the VHS in emergent strong EPC SC.

Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride.

The discovery of superconductivity at 200 kelvin in the hydrogen sulfide system at high pressures1 demonstrated the potential of hydrogen-rich materials as high-temperature superconductors. Recent theoretical predictions of rare-earth hydrides with hydrogen cages2,3 and the subsequent synthesis of LaH10 with a superconducting critical temperature (Tc) of 250 kelvin4,5 have placed these materials on the verge of achieving the long-standing goal of room-temperature superconductivity. Electrical and X-ray diffraction measurements have revealed a weakly pressure-dependent Tc for LaH10 between 137 and 218 gigapascals in a structure that has a face-centred cubic arrangement of lanthanum atoms5. Here we show that quantum atomic fluctuations stabilize a highly symmetrical [Formula: see text] crystal structure over this pressure range. The structure is consistent with experimental findings and has a very large electron-phonon coupling constant of 3.5. Although ab initio classical calculations predict that this [Formula: see text] structure undergoes distortion at pressures below 230 gigapascals2,3, yielding a complex energy landscape, the inclusion of quantum effects suggests that it is the true ground-state structure. The agreement between the calculated and experimental Tc values further indicates that this phase is responsible for the superconductivity observed at 250 kelvin. The relevance of quantum fluctuations calls into question many of the crystal structure predictions that have been made for hydrides within a classical approach and that currently guide the experimental quest for room-temperature superconductivity6-8. Furthermore, we find that quantum effects are crucial for the stabilization of solids with high electron-phonon coupling constants that could otherwise be destabilized by the large electron-phonon interaction9, thus reducing the pressures required for their synthesis.

Ultrafast dynamics of non-equilibrium electrons and strain generation under femtosecond laser irradiation of Nickel

We present a theoretical study of the ultrafast electron dynamics in transition metals of large electron–phonon coupling constant using ultrashort pulsed laser beams. The significant influence of the dynamics of produced nonthermal electrons to electron thermalisation and electron–phonon interaction is thoroughly investigated for various values of the pulse duration (i.e., from 10 fs to 2.3 ps). The model correlates the role of nonthermal electrons, relaxation processes and induced stress–strain fields. Simulations are presented by choosing Nickel (Ni) as a test material to compute electron–phonon relaxation time due to its large electron–phonon coupling constant. We demonstrate that the consideration of the aforementioned factors leads to significant changes compared to the results the traditional two-temperature model provides. The proposed model predicts a substantially (~ 33%) smaller damage threshold and a large increase of the stress (~ 20%, at early times) which first underlines the role of the nonthermal electron interactions and second enhances its importance with respect to the precise determination of laser specifications in material micromachining techniques.

Polar optical phonon states and their degenerative behaviors of wurtziteZnO/MgZnOcoupling quantum dots

Analytical polar optical phonon states in a wurtzite ZnO -based cylindrical coupling quantum dots (CQDs) with arbitrary number of quantum dots (QDs) are deduced and analyzed. It is found that there are four types of polar mixing optical phonon modes, i.e., the z-IO/ρ-QC modes, the z-PR/ρ-IO modes, the z-QC/ρ-QC modes and the z-HS/ρ-IO modes coexisting in the ZnO -based CQDs. Within the framework of the macroscopic dielectric continuum model, the dispersive equations are derived by using the transferring matrix method. And the Fröhlich electron–phonon interaction Hamiltonians are obtained via a standard procedure of field quantization. The relationships between the present ZnO -based CQDs and the ZnO -based quantum wells (QWs) or the nanowires (NWs) are analyzed, and the general features of phonon modes in ZnO -based low-dimensional quantum structures are concluded and discussed. Under certain conditions, the present theoretical results in wurtzite ZnO -based CQDs can be naturally degenerate into those in wurtzite ZnO -based single or double QDs, wurtzite NWs and QWs and even into cubic quantum confined structures. This just embodies the intrinsic consistency of phonon mode theories in low-dimensional confined systems with different confined dimensions. Due to the ternary mixing effect of MgxZn1-xO crystal, the dielectric functions of MgxZn1-xO crystals are quite complicated, and the phonon modes in ZnO -based quantum structures have both the features of phonon modes in anisotropic wurtzite confined systems and isotropic rock-salt crystal quantum systems. The characteristics of electron–phonon coupling strength in ZnO -based quantum systems are summarized. Very strong polaronic effect could be prognosticated and anticipated in ZnO -based low-dimensional quantum structures because of their quite large electron–phonon coupling constants. The theoretical results and conclusions described in this paper also can be looked on as a summary of phonon states and their general features in ZnO -based quantum confined systems.

Carrier heating in disordered conjugated polymers in electric field

The electric field dependence of charge carrier transport and the effect of carrier heating in disordered conjugated polymers were investigated. A parameter-free multiscale methodology consisting of classical molecular dynamics simulation for the generation of the atomic structure, large system electronic structure and electron-phonon coupling constants calculations and the procedure for extracting the bulk polymer mobility, was used. The results suggested that the mobility of a fully disordered poly(3-hexylthiophene) (P3HT) polymer increases with electric field which is consistent with the experimental results on samples of regiorandom P3HT and different from the results on more ordered regioregular P3HT polymers, where the opposite trend is often observed at low electric fields. We calculated the electric field dependence of the effective carrier temperature and showed however that the effective temperature cannot be used to replace the joint effect of temperature and electric field, in contrast to previous theoretical results from phenomenological models. Such a difference was traced to originate from the use of simplified Miller-Abrahams hopping rates in phenomenological models in contrast to our considerations that explicitly take into account the electronic state wave functions and the interaction with all phonon modes.

Roles of inter- and intramolecular vibrations and band-hopping crossover in the charge transport in naphthalene crystal

We calculate the hole and electron mobilities in naphthalene crystal from 10 to 300 K within the framework of the Holstein-Peierls model coupled with first-principles density-functional-theory-projected tight-binding band structures. All the electron-phonon coupling constants, including both local and nonlocal parts for inter- and intramolecular vibrations, have been taken into considerations through density functional theory. The band-hopping crossover transition temperature for the electron transport in the c' axis is calculated to be around 23 K. We have identified a few high frequency intramolecular vibrations which are very important to the charge transport in naphthalene crystal due to their comparatively large electron-phonon coupling constants. However, their contributions to the temperature dependence of mobility are minor because of the small phonon occupations and small nonlocal coupling strengths. The low frequency intermolecular modes (longitudinal optical modes) are found to be the major contributions to the temperature dependent charge transfer properties in naphthalene crystal. Even though the calculated qualitative temperature dependence is in agreement with experiment, the predicted absolute mobility is about one to two orders of magnitude larger.

Strong vibronic interactions and possible electron pairing in the photoinduced excited electronic states in nanosized molecules

Electron–phonon interactions in the photoinduced excited electronic states are investigated, and compared with those in the monoanionic and monocationic electronic states in acenes. The total electron–phonon coupling constants for the monoanionic ( l LUMO), monocationic ( l HOMO), and photoinduced excited electronic states ( l B 1 u ( HOMO → LUMO ) ) in acenes are estimated. The l B 1 u ( HOMO → LUMO ) values are much larger than the l LUMO and l HOMO values in acenes. The complete phase patterns difference between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) in acenes, are the main reason why the C–C stretching modes around 1500 cm −1 afford much larger electron–phonon coupling constants in the photoinduced excited electronic states than in the monoanionic and monocationic electronic states in acenes, and the reason why the l B 1 u ( HOMO → LUMO ) values are much larger than the l LUMO and l HOMO values in acenes.

Anomalously large electron–phonon coupling constant in layered nitride superconductors as revealed by high-pressure experiments

Pressure dependence of T c, lattice constant, and phonon frequency has been investigated for layered nitride superconductors, Li 0.5(THF) y HfNCl and ZrNCl 0.7. Analysis based on McMillan’s theory has revealed that the electron–phonon coupling constant λ is larger than 3, and that the relevant phonon is of low frequency around 100 cm −1. The anomalously large value of λ is in sharp contrast with previous theories and an experiment.

The effects of H–F and H–D substitutions on Jahn–Teller effects and charge transfer in the monocations of B, N-substituted acenes

The effects of H–D and H–F substitution on the electron–phonon interactions in the monocations of B, N-substituted acenes are studied. The B–N stretching modes around 1400 cm −1 afford large electron–phonon coupling constants in the monocations of B 3N 3F 6 ( 1f) and B 5N 5F 8 ( 2f). The total electron–phonon coupling constants for the monocations ( l HOMO) are 0.479 and 0.402 eV in 1f and 2f, respectively. The l HOMO values become much larger by H–F substitution than by H–D substitution in the monocations of B, N-substituted acenes. The reason for the calculational results are discussed in detail in view of the phase patterns of the frontier orbitals and the vibronic active modes. The effects of H–D and H–F substitution on the charge transfer in the positively charged B, N-substituted acenes are also discussed.

Jahn–Teller effects and charge transfer in the positively charged triphenylene and coronene

Vibronic interactions and Jahn–Teller effects in the monocations of triphenylene and coronene are discussed. The E′ mode of 1634 cm −1 and the E 2g mode of 1668 cm −1 afford large electron–phonon coupling constants in the monocations of triphenylene and coronene, respectively. The single charge transfer through the molecules in these positively charged species are discussed. The reorganization energies for elementary charge transfer are estimated to be 0.045 and 0.028 eV for triphenylene and coronene, respectively. The optimized structures of the monocations are discussed in terms of the frontier orbitals and the vibrational modes.

Electron–phonon coupling in negatively charged acene- and phenanthrene-edge-type hydrocarbon crystals

Vibronic interaction and its role in the occurrence of possible superconductivity in the monoanions of phenanthrene-edge-type aromatic hydrocarbons are studied. The vibrational frequencies and the vibronic coupling constants are computed and analyzed and the electron–phonon coupling constants are estimated. The results for phenanthrene-edge-type hydrocarbons are compared with those for acene-edge-type hydrocarbons. The lowest frequency mode and the C–C stretching modes of 1400–1600 cm−1 afford large electron–phonon coupling constants in the monoanions of acene- and phenanthrene-edge-type hydrocarbons. The total electron–phonon coupling constants decrease with an increase in the number of carbon atoms in both acene- and phenanthrene-edge-type hydrocarbons, but those for the monoanions of phenanthrene-edge-type hydrocarbons are larger than those for the monoanions of acene-edge-type hydrocarbons. Possible superconducting transition temperatures Tcs for the monoanions are estimated. The monoanions of phenanthrene-edge-type hydrocarbons would have higher Tcs than the monoanions of acene-edge- type hydrocarbons if phenanthrene-edge-type hydrocarbons exhibit superconductivity. These results suggest that molecular edge structures as well as molecular sizes have relevance to the strength of electron–phonon coupling and Tcs. The fragment molecular-orbital method (FMO) method successfully characterizes the distinct electronic structures of the two small polynuclear aromatic hydrocarbons (PAHs) with different type of edges such as anthracene and phenanthrene.

Vibronic interactions and superconductivity in acene anions and cations

Vibronic interaction and its role in the occurrence of superconductivity in the monoanions and cations of benzene and acenes are studied. The vibrational frequencies and the vibronic coupling constants for benzene and acenes are computed and analyzed and electron—phonon coupling constants in the monoanions and cations are evaluated. The C–C stretching E2g mode of 1656 cm−1 affords large electron—phonon coupling constants in the monoanion and cation of benzene. The C–C stretching Ag modes of 1400–1600 cm−1 and the lowest frequency Ag mode play an important role in the electron–phonon coupling in the monoanions of acenes, while the only C–C stretching Ag modes of 1400–1600 cm−1 afford large electron–phonon coupling constants in the monocations of acenes. We estimate possible superconducting transition temperatures Tcs for the monoanions and cations of benzene and acenes. The predicted Tcs for the monocations of benzene and acenes are much lower than those for the monoanions. Possible high-temperature superconductivity in the monoanion and cation of benzene is suggested.

The complex nature of superconductivity in MgB2 as revealed by the reduced total isotope effect.

Magnesium diboride, MgB2, was recently observed to become superconducting at 39 K, which is the highest known transition temperature for a non-copper-oxide bulk material. Isotope-effect measurements, in which atoms are substituted by isotopes of different mass to systematically change the phonon frequencies, are one of the fundamental tests of the nature of the superconducting mechanism in a material. In a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor, where the mechanism is mediated by electron-phonon coupling, the total isotope-effect coefficient (in this case, the sum of both the Mg and B coefficients) should be about 0.5. The boron isotope effect was previously shown to be large and that was sufficient to establish that MgB2 is a conventional superconductor, but the Mg effect has not hitherto been measured. Here we report the determination of the Mg isotope effect, which is small but measurable. The total reduced isotope-effect coefficient is 0.32, which is much lower than the value expected for a typical BCS superconductor. The low value could be due to complex materials properties, and would seem to require both a large electron-phonon coupling constant and a value of mu* (the repulsive electron-electron interaction) larger than found for most simple metals.

Tuning the spectral and temporal response in PtAu core–shell nanoparticles

Ultrafast laser spectroscopy has been used to measure the electron–phonon coupling constant in PtAu core–shell bimetallic nanoparticles. A chemical reduction method was used to prepare Pt cores of 12.5 nm diameter and a γ-radiolytic deposition technique was then used to grow Au shells of variable thickness. The resulting nanoparticles have a spectrum that is characteristic of Au. It is found that the electron–phonon coupling time for these hybrid nanoparticles (τe–ph∼200 fs) is over a factor of 3 shorter than that for plain Au nanoparticles (τe–ph∼650 fs). The faster dynamical response is due to the large electron–phonon coupling constant for Pt, which provides efficient scattering of the excited electrons. Platinum dominates the temporal response, even for a 1:6 molar ratio of Pt to Au, because it has a much larger density of states near the Fermi level compared to Au.

Polaron cyclotron resonance observed for n-type ZnSe in high magnetic fields up to 180 T.

We have observed cyclotron resonance and impurity cyclotron resonance (1s-2${\mathit{p}}^{+}$ transition) in a wide far-infrared wavelength range between 9.27 and 70.5 \ensuremath{\mu}m under pulsed high magnetic fields up to 180 T. A very large resonant polaron effect reflecting a large electron-phonon coupling constant was observed in both resonances, as well as a large nonparabolicity. The experimental results of cyclotron resonance were found to be in good agreement with a theoretical calculation based on the Wigner-Brillouin second-order perturbation theory taking account of the nonparabolicity. The impurity cyclotron-resonance data can also be well explained by a variational calculation. The donor ionization energy was found to increase by a factor of 2.39 at 180 T due to the magnetic freeze-out effect.

Oxygen mobility in YBa2Cu3O7− δ: A Raman study

From Raman measurements on superconducting and non-superconducting YBa 2Cu 3O 7-δ single crystals, we show that close to room temperature there is an unaccounted for, intense, oxygen vibrational mode at 598 cm −1. This (zz) polarized mode appears to arise from some chain oxygen atoms occupying another site in the crystal. Using laser heating and a shutter, we find that the time it takes to occupy or empty this site is less than 0.1 sec. If a charge carrier in the Cu-O plane induces the occupation of this new site, then a large electron-phonon coupling constant could result.

Phonon spectra of amorphous superconductors

The techniques electron tunneling were applied to the study of the amorphous superconductors Ga, Bi(+Pb impurity), and Pb(+Bi impurity). The phonon spectra were computed using McMillan's inversion program. The characteristics of this kind of superconductor are the large Coulomb pseudopotentials and electron-phonon coupling constant. Calculated values of the T c for a disordered superconductor using the theory of Garland et al. were in good agreement with the measured values.