Strong Electron-phonon Coupling Research Articles

Superconductivity in TlBi2 with a large Kadowaki-Woods ratio

In this article, the superconducting and normal state properties of TlBi$_2$ with the AlB$_2$-type structure were studied by the resistivity, magnetization and specific heat measurements. It was found that bulk superconductivity with $T_{C}$ = 6.2 K emerges in TlBi$_2$, which is a phonon-mediated $s$-wave superconductor with a strong electron-phonon coupling ($\lambda$$_{ep}$ = 1.38) and a large superconducting gap ($\Delta_{0}$/$k_{B}T_{C}$ = 2.25). We found that the $\rho$($T$) exhibits an unusual $T$-linear dependence above 50 K, and can be well described by the Fermi-liquid theory below 20 K. Interestingly, its Kadowaki-Woods ratio $A/\gamma^{2}$ [9.2$\times$10$^{-5}$ $\mu\Omega$ cm(mol K$^{2}$/mJ)$^{2}$] is unexpectedly one order of magnitude larger than that obtained in many heavy Fermi compounds, although the electron correlation is not so strong.

Small-polaron conductivity in perovskite ferroelectric BaTiO3 films

In ABO3 perovskite oxide ferroelectrics, electrical conductivity ranges from insulator- to superconductor-type and is virtually critical for all applications of these materials. Compared to bulk ceramics and crystals, ferroelectric thin films can enable advanced control of the conductivity. Here, small-polaron hopping conductivity was evidenced and examined in various pulsed-laser-deposited films of ferroelectric BaTiO3 and reference films of SrTiO3. For this, AC conductivity was studied in a broad range of temperatures and frequencies for films sandwiched between the bottom and top electrodes. In the BaTiO3 films, with increasing temperature, a significant increase in activation energy for small-polaron hopping was found and ascribed to strong electron–phonon coupling and complex lattice oscillations therein. Plain relations of the activation energy to microstructure, composition, or phase transitions were lacking, which corroborated the critical role of phonons. Additionally, a phonon-less transport was detected. It was anticipated that owing to strong electron–phonon coupling, rich phonon ensembles, and coexistence of phonon-stimulated and phonon-less processes, the small-polaron conductivity can heavily vary in ferroelectric films that necessitates further studies.

High-Throughput Screening of Strong Electron-Phonon Couplings in Ternary Metal Diborides.

We perform a high-throughput screening on phonon-mediated superconductivity in a ternary metal diboride structure with alkali, alkaline earth, and transition metals. We find 17 ground states and 78 low-energy metastable phases. From fast calculations of zone-center electron-phonon coupling, 43 compounds are revealed to show electron-phonon coupling strength higher than that of MgB2. An anticorrelation between the energetic stability and electron-phonon coupling strength is identified. We suggest two phases, i.e., Li3ZrB8 and Ca3YB8, to be synthesized, which show reasonable energetic stability and superconducting critical temperature.

Bypassing backmapping: Coarse-grained electronic property distributions using heteroscedastic Gaussian processes.

We employ deep kernel learning electronic coarse-graining (DKL-ECG) with approximate Gaussian processes as a flexible and scalable framework for learning heteroscedastic electronic property distributions as a smooth function of coarse-grained (CG) configuration. The appropriateness of the Gaussian prior on predictive CG property distributions is justified as a function of CG model resolution by examining the statistics of target distributions. The certainties of predictive CG distributions are shown to be limited by CG model resolution with DKL-ECG predictive noise converging to the intrinsic physical noise induced by the CG mapping operator for multiple chemistries. Further analysis of the resolution dependence of learned CG property distributions allows for the identification of CG mapping operators that capture CG degrees of freedom with strong electron-phonon coupling. We further demonstrate the ability to construct the exact quantum chemical valence electronic density of states (EDOS), including behavior in the tails of the EDOS, from an entirely CG model by combining iterative Boltzmann inversion and DKL-ECG. DKL-ECG provides a means of learning CG distributions of all-atom properties that are traditionally "lost" in CG model development, introducing a promising methodological alternative to backmapping algorithms commonly employed to recover all-atom property distributions from CG simulations.

Strong electron–phonon coupling of Cr3+ ion provides an opportunity for superior sensitivity cryogenic sensing

Nowadays, optical temperature measurement is becoming a reliable and advanced technology. Most of optical temperature sensing materials have been reported in the first biological temperature region or even higher. However, cryogenic optical sensing materials seem to be ignored. Here, Cr3+-doped SrGa12O19 is reported, exhibiting a obvious sharp peak at 696 nm (2E→4A2) and near-infrared emission at 750 nm (4T2→4A2) at 77 K. As the temperature increases, the transition of 2E→4A2 is suppressed and the emission of 4T2 is broadened due to the increasing electron–phonon coupling effect. This feature provides an opportunity to obtain the obvious discrepancy of fluorescence intensity, and further realize the superior cryogenic sensing performance based fluorescence intensity ratio (FIR) method with the maximum absolute sensitivity (Sa) of 0.0085 K−1 and relative sensitivity (Sr) of 2.62 %·K−1 at 77 K. The value of Sr is 138.6 % higher than that of CaHfO3:Cr3+ system and the temperature uncertainty (δT) is one order of magnitude lower than the most popular ruby Al2O3:Cr3+, showing superior cryogenic sensing ability. This work not only exhibits a novel cryogenic sensing material but also provides a train of thought for designing cryogenic thermometer.

Observation of superconductivity and its enhancement at the charge density wave critical point in LaAgSb2

We discover superconductivity (SC) in LaAgSb$_2$ at ambient pressure and its close correlation with a charge density wave (CDW) under pressure. The superconducting transition temperature ($T_c$) exhibits a sharp peak at the CDW critical pressure of 3.2 GPa. We demonstrate that the carriers inhabiting the Sb-square net is crucial not only in the formation of CDW but also in SC for their relatively strong electron-phonon coupling (EPC). Furthermore, theoretical EPC strength in pristine LaAgSb$_2$ cannot explain the observed peak with $T_c\sim 1$ K, which indicates that an additional mechanism reinforces SC only around the CDW critical pressure.

Nematic electron and phonon dynamics in SnS crystals

Tin sulfide (SnS) is one of the promising materials for the applications of optoelectronics and photovoltaics. This study determines the nematic dynamics of photoexcited electrons and phonons in SnS single crystals using polarization-dependent pump–probe spectroscopy at various temperatures. As well as the fast (0.21–1.38 ps) and slow (>5 ps) relaxation processes, a 36–41 GHz coherent acoustic phonon with a sound velocity of 4883 m/s that is generated by the thermoelastic effect is also observed in the transient reflectivity change (ΔR/R) spectra. Electrons and coherent acoustic phonons show significant in-plane anisotropy from 330 to 430 K due to strong electron–phonon coupling. However, this in-plane anisotropy weakens dramatically in the low-temperature (<330 K) and high-temperature (>430 K) phases. These results add to the knowledge about the anisotropy of electrons and coherent acoustic phonons that give SnS applications in photovoltaic or optoelectronic devices.

Effect of hydrogen content on superconductivity in La–H compounds

Finding room-temperature superconductors is one of the most important subjects in scientific search. Due to its hydrogen clathrate structures, LaH10 has become one of the widely known room-temperature superconductors under pressure. So exploring the role of hydrogen content would help understanding the mechanism of superconductivity in the hydrides. Here, we explore the evolution of hydrogen content in lanthanum hydrides based on the first-principles calculations. We found new lanthanum hydrides at pressures, such as Cmcm-LaH. The electronic properties and electron–phonon coupling have been studied and found that Fm3¯m-LaH obtained an unexpected low Tc of ∼ 0.001 K. The superconductive mechanism has been studied in LaHx (x = 0, 1, 2, 3, 8, 10), where LaH0 means elemental lanthanum. We found that low content of hydrogen depresses the electron–phonon interaction in La–H compounds before H–H bonds formed. But with the increases of H content, the strong electron–phonon coupling of H atoms would help the compounds obtain high Tc. This result would explain the lanthanum hydrides with lower H content obtain weaker electron–phonon coupling and could not became potential high-temperature superconductors.

Finite-temperature optical conductivity with density-matrix renormalization group methods for the Holstein polaron and bipolaron with dispersive phonons

A comprehensive picture of polaron and bipolaron physics is essential to understand the optical absorption spectrum in many materials with electron-phonon interactions. In particular, the finite-temperature properties are of interest since they play an important role in many experiments. Here, we combine the parallel two-site time-dependent variational principle algorithm (p2TDVP) with local basis optimization (LBO) and purification to calculate time-dependent current-current correlation functions. From this information, we extract the optical conductivity for the Holstein polaron and bipolaron with dispersive phonons at finite temperatures. For the polaron in the weak and intermediate electron-phonon coupling regimes, we analyze the influence of phonon dispersion relations on the spectra. For strong electron-phonon coupling, the known result of an asymmetric Gaussian is reproduced for a flat phonon band. For a finite phonon bandwidth, the center of the Gaussian is either shifted to larger or smaller frequencies, depending on the sign of the phonon hopping. We illustrate that this can be well understood by considering the Born-Oppenheimer surfaces. A similar behavior is seen for the bipolaron for strong coupling. For the bipolaron with weak and intermediate coupling strengths and a flat phonon band, we obtain two very different spectra. The latter also has a temperature-dependent resonance at a frequency below the phonon frequency.

Temperature induced giant shift of phonon energy in epitaxial boron nitride layers

The recent progress in the growth of large-area boron nitride epilayers opens up new possibilities for future applications. However, it remains largely unclear how weakly attached two-dimensional BN layers interact with their substrate and how their properties are influenced by defects. In this work, we investigate hBN layers grown by metal organic vapor phase epitaxy using Fourier-transform infrared spectroscopy in the temperature range of 160–540 K. Our measurements reveal strong differences in the character of layer-substrate interaction for as-grown and delaminated epitaxial layers. A much weaker interaction of as-grown layers is explained by wrinkles formation that reduces strain at the layer-substrate interface, which for layers transferred to other substrates occurs only in a limited temperature range. The most striking result is the observation of a giant increase in the E 1u phonon energy of up to ∼6 cm−1 in a narrow temperature range. We show that the amplitude and temperature range of the anomaly is strongly modified by UV light illumination. The observed giant effect is explained in terms of strain generation resulting from charge redistribution between shallow traps and different defects, which can be interpreted as a result of strong electron-phonon coupling in hBN. The observed narrow temperature range of the anomaly indicates that the effect may be further enhanced for example by electrostrictive effects, expected for sp2 boron nitride.

Energy gap in hydride superconductors due to spin-amplified phonons

Room temperature superconductivity has been realized in hydrides under ultrahigh pressure. A recent study has revealed that charge pairing in pressurized hydrides is mediated by spin-amplified phonons. Little research has been undertaken to reveal the energies involved in pressurized superconductivity. In this paper, we study the evolution of the energy gap under immense pressure in hydride superconductors. We adopt the non-standard BCS theory in which multiple bands with different symmetry cross the chemical potential and multiple gaps form in different spots of the k-space and all multiple Fermi surfaces are in the BCS limit. The energy Epen>ℏωD due to enhanced phonons produces multiple bandgaps. Applying the multiband BCS model in the presence of spin-amplified phonons reveals electron–phonon coupling beyond the Eliashberg–Migdal limit. The energy gap function Δ0 in the strong electron–phonon coupling limit shows dependence on ionic spin and Tc in which the effect of pressure and phonons on the gap is confined. Further, we show that pressure at Tc→0 is nonzero.

PPh3H]2[SbCl5]: A Zero-Dimensional Hybrid Metal Halide with a Supramolecular Framework and Stable Dual-Band Emission

Zero-dimensional (0D) organic–inorganic metal halides (OIMHs) have attracted tremendous attention due to their fascinating optoelectronic properties. Herein, we report an antimony(III)-based 0D OIMH of [PPh3H]2[SbCl5] comprising discrete [SbCl5]2– pyramids and protonated triphenylphosphine (PPh3H) cations with a low charge density. Abundant supramolecular interactions including C–H···Cl and rare P–H···Cl hydrogen bonds and C–H···Π and anion···Π interactions lead to a three-dimensional (3D) supramolecular framework structure of [PPh3H]2[SbCl5]. Photophysical characterizations indicate that [PPh3H]2[SbCl5] has singlet/triplet dual-band emission. The intersystem crossing between the single and triplet states is tunable upon changing the temperature. Analyses of the Huang–Rhys factor (S), exciton activation energy (Ea), and electron–phonon coupling energy (Γop) verify that the soft lattice and strong electron–phonon coupling result in the broad emission of [PPh3H]2[SbCl5], while the excited-state reorganization of the [SbCl5]2– pyramid leads to a large Stokes shift. DFT calculations help provide a deep understanding on the optical property of [PPh3H]2[SbCl5]. The title compound could remain stable for more than 1 year without the obvious decay of emission, which may be related to the rigid 3D supramolecular framework.

Computational Method for Evaluating the Thermoelectric Power Factor for Organic Materials Modeled by the Holstein Model: A Time-Dependent Density Matrix Renormalization Group Formalism.

Organic/polymeric materials are of emerging importance for thermoelectric conversion. The soft nature of these materials implies strong electron-phonon coupling, often leading to carrier localization. This poses great challenges for the conventional Boltzmann transport description based on relaxation time approximation and band structure calculations. In this work, combining the Kubo formula with the finite-temperature time-dependent density matrix renormalization group (FT-TD-DMRG) in the grand canonical ensemble, we developed a nearly exact algorithm to calculate the thermoelectric power factor PF = α2 σ, where α is the Seebeck coefficient and σ is the electrical conductivity, and apply the algorithm to Holstein Hamiltonian with electron-phonon coupling to model organic materials. Our algorithm can provide a unified description covering the weak coupling limit described by the bandlike Boltzmann transport to the strong coupling hopping limit.

Polarons in spinless metals—a variational solution

We propose a simple variational solution for calculating one-particle spectral functions in lattice models of spinless metals with strong electron-phonon coupling. It is based on a generalization of the Momentum Average variational approximation for single polarons, combined with the assumption that the other fermions in the system are locked into an inert Fermi sea. We expect the method to be accurate for fermion addition spectral functions in metals with a small Fermi energy (nearly empty band), and for fermion removal spectral functions in metals with a large Fermi energy (nearly full band), provided that the characteristic phonon frequency is not too small. Both these regions are far from the region where the Migdal theorem holds, thus our results offer new insights into polaronic behavior in a largely unexplored part of the parameter space. Here, we show results for the Holstein coupling in one-dimension and present ways to gauge their accuracy, but ultimately this will need to be verified against numerical calculations. This variational method can be extended straightforwardly to higher dimensions and other forms of electron-phonon coupling.

Superconducting properties of doped blue phosphorene: effects of non-adiabatic approach

We study the effects of Kohn anomalies on the superconducting properties in electron- and hole-doped cases of monolayer blue phosphorene (BLP), considering both adiabatic and non-adiabatic phonon dispersions using first-principles calculations. We show that the topology of the Fermi surface is crucial for the formation of Kohn anomalies of doped BLP. By using the anisotropic Eliashberg formalism, we further carefully consider the temperature dependence of the non-adiabatic phonon dispersions. In cases of low hole densities, strong electron–phonon coupling (EPC) leads to a maximum critical temperature of K for superconductivity. In electron-doped regimes, on the other hand, a maximum superconducting critical temperature of K is reached at a doping level that includes a Lifshitz transition point. Furthermore, our results indicate that the most prominent component of EPC arises from the coupling between an in-plane (out-of-plane) deformation and in-plane (out-of-plane) electronic states of the electron (hole) type doping.

Proton Storage in Metallic H1.75MoO3 Nanobelts through the Grotthuss Mechanism.

The proton, as the cationic form of the lightest element-H, is regarded as most ideal charge carrier in "rocking chair" batteries. However, current research on proton batteries is still at its infancy, and they usually deliver low capacity and suffer from severe acidic corrosion. Herein, electrochemically activated metallic H1.75MoO3 nanobelts are developed as a stable electrode for proton storage. The electrochemically pre-intercalated protons not only bond directly with the terminal O3 site via strong O-H bonds but also interact with the oxygens within the adjacent layers through hydrogen bonding, forming a hydrogen-bonding network in H1.75MoO3 nanobelts and enabling a diffusion-free Grotthuss mechanism as a result of its ultralow activation energy of ∼0.02 eV. To the best of our knowledge, this is the first reported inorganic electrode exhibiting Grotthuss mechanism-based proton storage. Additionally, the proton intercalation into MoO3 with formation of H1.75MoO3 induces strong Jahn-Teller electron-phonon coupling, rendering a metallic state. As a consequence, the H1.75MoO3 shows an outstanding fast charging performance and maintains a capacity of 111 mAh/g at 2500 C, largely outperforming the state-of-art battery electrodes. More importantly, a symmetric proton ion full cell based on H1.75MoO3 was assembled and delivered an energy density of 14.7 Wh/kg at an ultrahigh power density of 12.7 kW/kg, which outperforms those of fast charging supercapacitors and lead-acid batteries.

Ambient-pressure high- Tc superconductivity in doped boron-nitrogen clathrates La(BN)5 and Y(BN)5

${\mathrm{LaH}}_{10}$ exhibits the near-room-temperature superconductivity, but the high fabrication pressure exceeding 200 GPa limits the possibility of its experimental research and application. To examine the feasibility of high-temperature superconductivity at ambient pressure in light-element compounds, we study the crystal structure, metallization, and electron-phonon interaction of $M{(\mathrm{BN})}_{5}$ ($M$ = La and Y) with ${\mathrm{LaH}}_{10}$-like clathrate configuration, based on the first-principles calculations. The result shows that adopting B and N to substitute H, the strong $s{p}^{3}$ hybridization between B and N greatly reduces the pressure to keep the dynamical stability. We predict that the strong electron-phonon coupling exists in $M{(\mathrm{BN})}_{5}$ and the ${T}_{c}$ values of $\mathrm{La}{(\mathrm{BN})}_{5}$ and $\mathrm{Y}{(\mathrm{BN})}_{5}$ reach 69 and 59 K at ambient pressure, respectively. As a result, the high-temperature superconductivity at ambient pressure in clathrate BN compounds has been proved theoretically, which provides a way to lower the work pressure of hydride superconductors.

Carrier Multiplication in Transition Metal Dichalcogenides Beyond Threshold Limit.

Carrier multiplication (CM), multiexciton generation by absorbing a single photon, enables disruptive improvements in photovoltaic conversion efficiency. However, energy conservation constrains the threshold energy to at least twice bandgap (2 ). Here, a below threshold limit CM in monolayer transition metal dichalcogenides (TMDCs) is reported. Surprisingly, CM is observed with excitation energy of only 1.75 due to lattice vibrations. Electron-phonon coupling (EPC) results in significant changes in electronic structures, which favors CM. Indeed, the strongest EPC in monolayer MoS2 leads to the most efficient CM among the studied TMDCs. For practical applications, chalcogen vacancies can further lower the threshold by introducing defect states within bandgap. In particular, for monolayer WS2 , CM occurs with excitation energy as low as 1.51 . The results identify TMDCs as attractive candidate materials for efficient optoelectronic devices with the advantages of high photoconductivity and efficientCM.

Recompilation-enhanced simulation of electron–phonon dynamics on IBM quantum computers

Simulating quantum systems is believed to be one of the first applications for which quantum computers may demonstrate a useful advantage. For many problems in physics, we are interested in studying the evolution of the electron–phonon Hamiltonian, for which efficient digital quantum computing schemes exist. Yet to date, no accurate simulation of this system has been produced on real quantum hardware. In this work, we consider the absolute resource cost for gate-based quantum simulation of small electron–phonon systems as dictated by the number of Trotter steps and bosonic energy levels necessary for the convergence of dynamics. We then apply these findings to perform experiments on IBM quantum hardware for both weak and strong electron–phonon coupling. Despite significant device noise, through the use of approximate circuit recompilation we obtain electron–phonon dynamics on current quantum computers comparable to exact diagonalisation. Our results represent a significant step in utilising near term quantum computers for simulation of quantum dynamics and highlight the novelty of approximate circuit recompilation as a tool for reducing noise.

Lead-Free Potassium Alloyed Cs2AgBiBr6: Tunable Optoelectronic Properties and Carrier-Lattice Interaction

The role of the carrier-lattice interaction and associated self-trapped excitons (STEs) in broadband white-light emission in perovskite halides is one of the key aspects when aiming to develop next generation semiconductors for lighting applications. Herein, we provide microscopic insight on optoelectronic properties and electron–phonon interactions in potassium (K+) alloyed Cs2AgBiBr6, while evaluating its potential as a single material tunable light emitter. While K+ partially occupying the Cs+ site leads to blue-light emission at low K+ ion concentrations, a sudden photoluminescence broadening and white-light emission occur at relatively higher alloying concentrations where K+ starts to occupy Ag+ sites. Simulation reveals an anomalous increase in carrier lattice coupling strength (via the Fröhlich mechanism) when K+ replaces Ag+. This along with the increased density of STEs (aided by strong electron–phonon coupling and more octahedral distortion) and induced free exciton (FE) emission is proposed to play a major role behind the observed photoluminescence (PL) broadening and white-light emission. This study should stimulate further research on single material based white-light emitters for next generation solid-state lighting applications.