Large Electron-phonon Coupling Research Articles

Phonon collapse and anharmonic melting of the 3D charge-density wave in kagome metals

The charge-density wave (CDW) mechanism and resulting structure of the AV3Sb5 family of kagome metals has posed a puzzling challenge since their discovery four years ago. In fact, the lack of consensus on the origin and structure of the CDW hinders the understanding of the emerging phenomena. Here, by employing a non-perturbative treatment of anharmonicity from first-principles calculations, we reveal that the charge-density transition in CsV3Sb5 is driven by the large electron-phonon coupling of the material and that the melting of the CDW state is attributed to ionic entropy and lattice anharmonicity. The calculated transition temperature is in very good agreement with experiments, implying that soft mode physics are at the core of the charge-density wave transition. Contrary to the standard assumption associated with a pure kagome lattice, the CDW is essentially three-dimensional as it is triggered by an unstable phonon at the L point. The absence of involvement of phonons at the M point enables us to constrain the resulting symmetries to six possible space groups. The unusually large electron-phonon linewidth of the soft mode explains why inelastic scattering experiments did not observe any softened phonon. We foresee that large anharmonic effects are ubiquitous and could be fundamental to understand the observed phenomena also in other kagome families.

Superconductivity in Monolayer Carbon Allotropes with High Thermal Stability.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

Quantification of the strong, phonon-induced Urbach tails in β-Ga2O3 and their implications on electrical breakdown

In ultrawide bandgap (UWBG) nitride and oxide semiconductors, increased bandgap (Eg) correlates with greater ionicity and strong electron–phonon coupling. This limits mobility through phonon scattering, localizes carriers via polarons and self-trapping, broadens optical transitions via dynamic disorder, and modifies the breakdown field. Herein, we use polarized optical transmission spectroscopy from 77 to 633 K to investigate the Urbach energy (Eu) for many orientations of Fe- and Sn-doped β-Ga2O3 bulk crystals. We find Eu values ranging from 60 to 140 meV at 293 K and that static (structural defects plus zero-point phonons) disorder contributes more to Eu than dynamic (finite temperature phonon-induced) disorder. This is evidenced by lack of systematic Eu anisotropy, and Eu correlating more with x-ray diffraction rocking-curve broadening than with Sn-doping. The lowest measured Eu are ∼10× larger than for traditional semiconductors, pointing out that band tail effects need to be carefully considered in these materials for high field electronics. We demonstrate that, because optical transmission through thick samples is sensitive to sub-gap absorption, the commonly used Tauc extraction of a bandgap from transmission through Ga2O3 >1–3 μm thick is subject to errors. Combining our Eu(T) from Fe-doped samples with Eg(T) from ellipsometry, we extract a measure of an effective electron–phonon coupling that increases in weighted second order deformation potential with temperature and a larger value for E||b than E||c. The large electron–phonon coupling in β-Ga2O3 suggests that theories of electrical breakdown for traditional semiconductors need expansion to account not just for lower scattering time but also for impact ionization thresholds fluctuating in both time and space.

Probing the origin of abnormally strong electron-phonon interaction in phonon transport of semiconductor C3B monolayer

Recently, the two-dimensional semiconductor C3B monolayer has attracted much attention owing to its excellent physical, optical, and electronic properties. In this work, the origin of electron-phonon interaction (EPI) on the thermal properties of C3B by n-type and p-type doping is systematically investigated via first-principles calculations to provide fundamental knowledge for the thermal management the C3B-based electronic devices. The carrier concentrations for the largest reduction of the lattice thermal conductivity (κ) appear at 4 × 1014 cm−2 for n-type and 9 × 1014 cm−2 for p-type, which is closely related to the electron density of states (DOS). The boron (B) atoms break the structural symmetry and induce mass disorder scattering, which renders C3B more prone to the influence of EPI. Moreover, the electronic band structure in the C3B monolayer exhibits multivalley characteristics, which leads to an intervalley scattering. It is worth noting that the anisotropy in the C3B monolayer can be significantly enhanced by EPI. Additionally, an abnormal phenomenon of strong electron-phonon scattering but low electron-phonon coupling strength is found in C3B monolayer, which indicates that large electron-phonon coupling strength is sufficient but not necessary for strong electron-phonon scattering.

The impact of Rashba spin-orbit coupling in charge-ordered systems

We study the impact of the Rashba spin–orbit coupling (RSOC) on the stability of charge-density wave (CDW) in systems with large electron-phonon coupling (EPC). Here, the EPC is considered in the framework of the Holstein model at the half-filled square lattice. We obtain the phase diagram of the Rashba–Holstein model using the Hartree–Fock mean-field theory, and identify the boundaries of the CDW and Rashba metal phases. We notice that the RSOC disfavors the CDW phase, driving the system to a correlated Rashba metal. Also, we employ a cluster perturbation theory (CPT) approach to investigate the phase diagram beyond the Hartree–Fock approximation. The quantum correlations captured by CPT indicate that the RSOC is even more detrimental to CDW than previously anticipated. That is, the Rashba metal region is observed to be expanded in comparison to the mean-field case. Additionally, we investigate pairing correlations, and the results further strengthen the identification of critical points.

Donor-acceptor pairs in wide-bandgap semiconductors for quantum technology applications

We propose a quantum science platform utilizing the dipole-dipole coupling between donor-acceptor pairs (DAPs) in wide bandgap semiconductors to realize optically controllable, long-range interactions between defects in the solid state. We carry out calculations based on density functional theory (DFT) to investigate the electronic structure and interactions of DAPs formed by various substitutional point-defects in diamond and silicon carbide (SiC). We determine the most stable charge states and evaluate zero phonon lines using constrained DFT and compare our results with those of simple donor-acceptor pair (DAP) models. We show that polarization differences between ground and excited states lead to unusually large electric dipole moments for several DAPs in diamond and SiC. We predict photoluminescence spectra for selected substitutional atoms and show that while B-N pairs in diamond are challenging to control due to their large electron-phonon coupling, DAPs in SiC, especially Al-N pairs, are suitable candidates to realize long-range optically controllable interactions.

An exponential-potential description for the guest–host interactions in rattler-containing cage materials

Anomalous low-energy excitations (ALEs) associated with the vibrations of atoms accommodated in a space with large freedom of motion are frequently observed in cage materials. Such ALEs have been of great interest for their important roles in large electron–phonon coupling targeting high-TC superconductivity as well as phonon–phonon scattering giving rise to low thermal conductivity to achieve high thermoelectric performance. In our previous work, we showed that van der Waals-like repulsive interactions between the atoms forming the host cage and the atoms accommodated inside the cage (rattler) play a crucial role in giving small force constants and consequently nearly dispersionless low-energy phonons. In the present work, we show that the exponential law derived from the revised Morse potential model that we used previously could provide a universal trend for the ALEs of three typical cage materials: clathrates, skutterudites, and pyrochlores. This not only confirms the effectiveness of the potential model but also provides a clear description for different types of rattler vibrational modes based on the deduced parameters from the potential model. In addition to the free space in which a rattler can move and the mass of the rattler, which we considered in our previous work, structural anisotropy, electronegativity of the component cage atoms, and the coordination number of the rattler are shown to be important factors for description of an ALE.

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.

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.

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.

Anomalous lattice thermal conductivity driven by all-scale electron-phonon scattering in bulk semiconductors

Electron-phonon coupling (EPC) has been broadly considered to govern the charge transport in solids, whereas its effect on thermal conductivity, especially for semiconductors at ambient temperatures, is a widely debated issue in condensed-matter physics. Employing state-of-the-art first-principles calculations to quantify all the possible scattering factors, we show the dominant role of EPC in phonon transport of $n$-doped GaSb, which yields an unprecedented reduction of lattice thermal conductivity and triggers anomalous temperature-independent behavior. The significant EPC impact hinges on the joint effect of multiple conduction pockets, large EPC strength, and bunched heat-carrying acoustic branches, provoking strong electron-phonon scattering to surpass the intrinsic phonon-phonon scattering for all-scale phonons, in contrast to the case in silicon that only long-wave phonons can be scattered by carriers even at high doping level. This work advances the understating of phonon transport in doped semiconductors and stimulates potential heat management application of anomalous thermal conductivity.

Role of vibrational properties and electron–phonon coupling on thermal transport across metal-dielectric interfaces with ultrathin metallic interlayers

We systematically analyze the influence of 5 nm thick metal interlayers inserted at the interface of several sets of different metal-dielectric systems to determine the parameters that most influence interface transport. Our results show that despite the similar Debye temperatures of Al2O3 and AlN substrates, the thermal boundary conductance measured for the Au/Al2O3 system with Ni and Cr interlayers is ∼2× and >3× higher than the corresponding Au/AlN system, respectively. We also show that for crystalline SiO2 (quartz) and Al2O3 substrates having highly dissimilar Debye temperature, the measured thermal boundary conductance between Al/Al2O3 and Al/SiO2 are similar in the presence of Ni and Cr interlayers. We suggest that comparing the maximum phonon frequency of the acoustic branches is a better parameter than the Debye temperature to predict the change in the thermal boundary conductance. We show that the electron–phonon coupling of the metallic interlayers also alters the heat transport pathways in a metal-dielectric system in a nontrivial way. Typically, interlayers with large electron–phonon coupling strength can increase the thermal boundary conductance by dragging electrons and phonons into equilibrium quickly. However, our results show that a Ta interlayer, having a high electron–phonon coupling, shows a low thermal boundary conductance due to the poor phonon frequency overlap with the top Al layer. Our experimental work can be interpreted in the context of diffuse mismatch theory and can guide the selection of materials for thermal interface engineering.

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.

In-plane optical phonon modes of current-carrying graphene

In this work, we study the in-plane optical phonon modes of current-carrying single-layer graphene whose coupling to the $\pi$ electron gas is strong. Such modes are expected to undergo a frequency shift compared to the non-current-carrying state due to the non-equilibrium occupation of the Dirac cone electronic eigen-states with the flowing $\pi$ electron gas. Large electron-phonon coupling (EPC) can be identified by an abrupt change in the slope of the phonon mode dispersion known as the Kohn anomaly, which mainly occurs for (i) the in-plane longitudinal/transverse optical (LO/TO) modes at the Brillouin zone (BZ) center ($\Gamma$ point), and (ii) the TO modes at the BZ corners ($K$ points). We show that the breaking of the rotational symmetry by the DC current results in different frequency shifts to the $\Gamma$-TO and $\Gamma$-LO modes. More specifically, the DC current breaks the TO-LO mode degeneracy at the $\Gamma$ point which ideally would be manifested as the splitting of the Raman G peak.

Heat flow dynamics in planar multilayer antennas excited by far-infrared picosecond laser pulses

Thin metallic buffer layers are commonly used between semiconductor substrates and noble metal films or nanostructures. We propose an extended two-temperature heat transfer model that can be used to investigate the heat flow in multilayer antennas and includes metallic layers, a dielectric substrate, and their respective interfaces. It is demonstrated how the heat flow dynamics in a layered material, excited by short laser pulses, can be controlled by the thermal properties of the buffer layer. A large buffer layer electron-phonon coupling was found to induce a non-intuitive reverse internal heat flow in the layered material and transient energy retention on time scales of a few tens of picoseconds, while the buffer layer-substrate Kapitza conductance was found to be the limiting parameter for the heat dissipation on nanosecond time scales.

First-principles study of the Kohn anomaly in TaTe4

The tetrachalcogenide TaTe4 is known as an excellent example of a charge-density wave (CDW) system that has a commensurately modulated structure at room temperature. Using density function perturbation theory, we find that the unmodulated phase of TaTe4 has a giant Kohn anomaly at room temperature, which manifests itself as softened phonon modes at the CDW vector (1/2a*,1/2b*,1/3c*). Interestingly, after the application of 8 GPa hydrostatic pressure, this CDW instability can be effectively suppressed and disappears at room temperature. By studying the topology of the Fermi surface and the phonon linewidth, we show that the Kohn anomaly in TaTe4 is driven by a large electron–phonon coupling coefficient at the CDW vector and not by Fermi surface nesting.

Exploring Exciton and Polaron Dominated Photophysical Phenomena in Ruddlesden-Popper Phases of Ban+1ZrnS3n+1 (n = 1-3) from Many Body Perturbation Theory.

Ruddlesden-Popper (RP) phases of Ban+1ZrnS3n+1 are an evolving class of chalcogenide perovskites in the field of optoelectronics, especially in solar cells. However, detailed studies regarding its optical, excitonic, polaronic, and transport properties are hitherto unknown. Here, we have explored the excitonic and polaronic effect using several first-principles based methodologies under the framework of Many Body Perturbation Theory. Unlike its bulk counterpart, the optical and excitonic anisotropy are observed in Ban+1ZrnS3n+1 (n = 1-3) RP phases. As per the Wannier-Mott approach, the ionic contribution to the dielectric constant is important, but it gets decreased on increasing n in Ban+1ZrnS3n+1. The exciton binding energy is found to be dependent on the presence of large electron-phonon coupling. We further observed maximum charge carrier mobility in the Ba2ZrS4 phase. As per our analysis, the optical phonon modes are observed to dominate the acoustic phonon modes, leading to a decrease in polaron mobility on increasing n in Ban+1ZrnS3n+1 (n = 1-3).

Emergence of intrinsic superconductivity in monolayer W2N3

Two-dimensional (2D) materials possessing intrinsic superconductivity with high transition temperature and unconventional pairing are highly desired, but their realizations are few and far between. Recently, ${\mathrm{W}}_{2}{\mathrm{N}}_{3}$ nanosheets down to three layers were successfully prepared [Yu et al., Adv. Mater. 30, 1805655 (2018)]. By performing solid ab initio calculations based on the anisotropic Migdal-Eliashberg theory, we predict that monolayer ${\mathrm{W}}_{2}{\mathrm{N}}_{3}$ is an unexplored intrinsic (without the assistance of external gating, strain, or a special substrate) 2D superconductor with large electron-phonon coupling (EPC) and high critical temperature ${T}_{c}=38\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ accompanied with a single and broad superconducting gap \ensuremath{\sim}7.5 meV. The ratio between the gap and the critical temperature is much larger than the value derived from Bardeen-Cooper-Schrieffer (BCS) theory, further confirming the strong coupling feature of monolayer ${\mathrm{W}}_{2}{\mathrm{N}}_{3}$. The extremely strong EPC originates from the large deformation potential of low-frequency acoustic phonons rather than Fermi-surface nesting. Due to the partially filled $d$ orbitals, electron-electron correlation leads to remarkable enhancement of EPC based on frozen phonon analysis. Here, ${T}_{c}$ can be further enhanced via hydrogen passivation based on the McMillian-Allen-Dynes formula. In addition, the symmetry-restricted spin-orbit coupling (SOC) brings forth exotic type-I Ising pairing whose in-plane upper critical field is far beyond the Pauli paramagnetic limit. Our predictions provide a fascinating and highly feasible platform for realizing high-temperature 2D superconductivity and studying the interplay between electron-phonon coupling, electron-electron correlation, and SOCs.

Doping controlled Fano resonance in bilayer 1T'-ReS2: Raman experiments and first-principles theoretical analysis.

In the bilayer ReS2 channel of a field-effect transistor (FET), we demonstrate using Raman spectroscopy that electron doping (n) results in softening of frequency and broadening of linewidth for the in-plane vibrational modes, leaving the out-of-plane vibrational modes unaffected. The largest change is observed for the in-plane Raman mode at ∼151 cm-1, which also shows doping induced Fano resonance with the Fano parameter 1/q = -0.17 at a doping concentration of ∼3.7 × 1013 cm-2. A quantitative understanding of our results is provided by first-principles density functional theory (DFT), showing that the electron-phonon coupling (EPC) of in-plane modes is stronger than that of out-of-plane modes, and its variation with doping is independent of the layer stacking. The origin of large EPC is traced to 1T to 1T' structural phase transition of ReS2 involving in-plane displacement of atoms whose instability is driven by the nested Fermi surface of the 1T structure. Results are compared with those of the isostructural trilayer ReSe2.

Temperature-dependent lattice dynamics in iridium

The characterization of simple elemental systems is key to benchmarking first-principles modeling of electronic and vibrational behaviors of materials. A large body of literature has been built for most elemental systems in the periodic table. However, surprisingly little neutron work has been performed to probe the vibrational properties of iridium, likely due to its large neutron absorption cross section. Nonetheless, iridium is of significant scientific and technological interest due to large relativistic electron effects and electron-phonon coupling, particularly in strongly correlated iridate compounds. Here, we report temperature-dependent inelastic neutron scattering measurements of the vibrational properties of iridium, from which we extract key thermodynamic properties. To overcome the challenge of the large neutron absorption of iridium, we developed a simple postprocessing correction procedure. The measured densities of phonon states compare well with quasiharmonic density functional theory calculations, although the obtained experimental phonon Gr\uneisen parameters are much larger than expected, reaching as high as $\ensuremath{\gamma}=4.5$, indicating substantial anharmonicity.