Weak Electron-phonon Coupling Research Articles

High- Tc superconductivity in CaKFe4As4 in absence of nematic fluctuations

We employ polarization-resolved Raman spectroscopy to study multi-band stoichiometric superconductor CaKFe$_4$As$_4$. The B$_{2g}$ symmetry Raman response shows no signatures of Pomeranchuk-like electronic nematic fluctuations which is observed for many other Fe-based superconductors. In the superconducting state, we identify three pair-breaking peaks at 13.8, 16.9 and 21 meV and full spectral weight suppression at low energies. The pair-breaking peak energies in Raman response are about 20% lower than twice the gap energies as measured by single-particle spectroscopy, implying a sub-dominant $d$-wave symmetry interaction. We analyze the superconductivity induced phonon self-energy effects and give an estimation of weak electron-phonon coupling constant $\lambda^\Gamma$=0.0015.

Orbital-Engineering-Based Screening of π-Conjugated d8 Transition-Metal Coordination Polymers for High-Performance n-Type Thermoelectric Applications.

Extraordinary progress has been achieved in polymer-based thermoelectric materials in recent years. New emerging π-conjugated transition-metal coordination polymers are one of the best n-type polymer-based thermoelectric materials. However, the microscopic descriptions on geometric structures, orbital characteristics, and most importantly, thermoelectric properties remain elusive, which has seriously hampered the experimentalists to draw a straightforward design strategy for new n-type polymer-based thermoelectric materials. Herein, we assess the n-type thermoelectric properties of 20 π-conjugated d8 metal center coordination polymers and rationalize their thermoelectric properties in terms of molecular geometry, orbital nature, and electron-phonon coupling based on first-principles calculations. An explicit screening rule for high-performance n-type π-conjugated transition-metal coordination polymeric thermoelectric materials was found, i.e., smaller metal center d orbital component ratio in the conduction band minimum, weaker electron-phonon coupling, higher intrinsic mobility, and thereby higher thermoelectric power factor can be achieved. Guided by this rule, poly(Pd-C2S4) and poly(Ni-C2Se4) show very high power factors. We built a map of high-performance π-conjugated transition-metal coordination polymers for n-type thermoelectric applications, which will help to accelerate the screening and design of innovative n-type thermoelectric polymers.

Electron-Phonon Systems on a Universal Quantum Computer.

We present an algorithm that extends existing quantum algorithms for simulating fermion systems in quantum chemistry and condensed matter physics to include bosons in general and phonons in particular. We introduce a qubit representation for the low-energy subspace of phonons which allows an efficient simulation of the evolution operator of the electron-phonon systems. As a consequence of the Nyquist-Shannon sampling theorem, the phonons are represented with exponential accuracy on a discretized Hilbert space with a size that increases linearly with the cutoff of the maximum phonon number. The additional number of qubits required by the presence of phonons scales linearly with the size of the system. The additional circuit depth is constant for systems with finite-range electron-phonon and phonon-phonon interactions and linear for long-range electron-phonon interactions. Our algorithm for a Holstein polaron problem was implemented on an Atos quantum learning machine quantum simulator employing the quantum phase estimation method. The energy and the phonon number distribution of the polaron state agree with exact diagonalization results for weak, intermediate, and strong electron-phonon coupling regimes.

New Insights into the Role of Weak Electron⁻Phonon Coupling in Nanostructured ZnO Thin Films.

High-quality crystalline nanostructured ZnO thin films were grown on sapphire substrates by reactive sputtering. As-grown and post-annealed films (in air) with various grain sizes (2 to 29 nm) were investigated by scanning electron microscopy, X-ray diffraction, and Raman scattering. The electron–phonon coupling (EPC) strength, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminished by reducing the ZnO grain size, which mainly relates to the Fröhlich interactions. Our finding suggests that in the spatially quantum-confined system the low polar nature leads to weak EPC. The outcome of this study is important for the development of nanoscale high-performance optoelectronic devices.

Magnetotransport measurements on Bi2Te3 nanowires electrodeposited in etched ion-track membranes

We report on the magneto-transport properties of individual Bi2Te3 nanowires with diameters ranging from 25 nm to 120 nm fabricated by electrodeposition in etched ion-track membranes. The metallic behavior of all nanowires is typical for highly-degenerate semiconductors and the weak temperature dependence of the resistivity suggests a weak electron-phonon coupling, as expected for topological surface states with quantum confinement. The diameter dependence of the resistivity further confirms the quasi-ballistic nature of charge carriers, in agreement with the reduction of the number of surface conductance channels in disordered quantum wires with a stronger quantum confinement. Although the bulk conductance is not negligible, both the transport length of bulk carriers and their electrostatic screening length remain shorter than the nanowire diameter, probably leading to a small contribution to the resistivity change. Quantum transport measurements give further evidence of finite-size effects, as shown by the weak anti-localization correction to the conductance, below 15K, and reveal the coexistence of surface Aharonov-Bohm oscillations and bulk universal conductance fluctuations, below 5K.

Fast thermal relaxation in cavity-coupled graphene bolometers with a Johnson noise read-out

High sensitivity, fast response time and strong light absorption are the most important metrics for infrared sensing and imaging. The trade-off between these characteristics remains the primary challenge in bolometry. Graphene with its unique combination of a record small electronic heat capacity and a weak electron-phonon coupling has emerged as a sensitive bolometric medium that allows for high intrinsic bandwidths1-3. Moreover, the material's light absorption can be enhanced to near unity by integration into photonic structures. Here, we introduce an integrated hot-electron bolometer based on Johnson noise readout of electrons in ultra-clean hexagonal-boron-nitride-encapsulated graphene, which is critically coupled to incident radiation through a photonic nanocavity with Q = 900. The device operates at telecom wavelengths and shows an enhanced bolometric response at charge neutrality. At 5 K, we obtain a noise equivalent power of about 10 pW Hz-1/2, a record fast thermal relaxation time, <35 ps, and an improved light absorption. However the device can operate even above 300 K with reduced sensitivity. We work out the performance mechanisms and limits of the graphene bolometer and give important insights towards the potential development of practical applications.

Temperature dependent Cr3+ photoluminescence in garnets of the type X3Sc2Ga3O12 (X = Lu, Y, Gd, La)

Broad band near-infrared (NIR) light sources have great potential, e.g., in optical communication and non-invasive medical imaging. Here we report the optical properties of X3Sc2Ga3O12 (X = Lu, Y, Gd, La) garnets doped with Cr3+ showing efficient 4T2 → 4A2 broad band NIR emission between 600 and 1000 nm. The chromium-doped garnets were investigated in view of their capability for blue to NIR conversion in high power phosphor converted LEDs (pcLEDs). Typical high power (In,Ga)N LEDs reach junction temperatures up to 450 K. Hence, the photoluminescence (PL) quenching temperature is a crucial parameter for on-chip phosphors. The Cr3+-doped garnets reveal very high quenching temperatures for the broad band NIR emission, ~ 700 K for Lu3Sc2Ga3O12, Y3Sc2Ga3O12, Gd3Sc2Ga3O12, and 450 K for La3Sc2Ga3O12. The high quenching temperatures are in line with a small Stokes shift (~ 2400 cm−1) and weak electron-phonon coupling. To gain further insight in the luminescence properties, low temperature photoluminescence measurements were done. The optical properties of Cr3+ ions in garnets were analyzed in terms of the crystal field strength, Racah parameters, and phonon coupling parameters. The suitability for application in pcLEDs for high power broad band NIR sources was confirmed by emission measurements of ceramic pellets upon blue LED excitation.

Towards sensitive terahertz detection via thermoelectric manipulation using graphene transistors

Graphene has been highly sought after as a potential candidate for hot-electron terahertz (THz) detection benefiting from its strong photon absorption, fast carrier relaxation, and weak electron-phonon coupling. Nevertheless, to date, graphene-based thermoelectric THz photodetection is hindered by low responsivity owing to relatively low photoelectric efficiency. In this work, we provide a straightforward strategy for enhanced THz detection based on antenna-coupled CVD graphene transistors with the introduction of symmetric paired fingers. This design enables switchable photodetection modes by controlling the interaction between the THz field and free hot carriers in the graphene-channel through different contacting configurations. Hence a novel “bias-field effect” can be activated, which leads to a drastic enhancement in THz detection ability with maximum responsivity of up to 280 V/W at 0.12 THz relative to the antenna area and a Johnson-noise limited minimum noise-equivalent power (NEP) of 100 pW/Hz0.5 at room temperature. The mechanism responsible for the enhancement in the photoelectric gain is attributed to thermophotovoltaic instead of plasma self-mixing effects. Our results offer a promising alternative route toward scalable, wafer-level production of high-performance graphene detectors.

Bandgap Tunability of Transition Metal Dichalcogenide Atomic Layers.

The temperature-dependent bandgap of transition metal dichalcogenides (TMDCs, MX2; M = Mo or W; X = S, Se, or Te) is analyzed using the O'Donnell and Chen relation with parameters including the average acoustic phonon energy (〈ħω〉) and the electron-phonon coupling strength (s). Wider (narrower) tunability of the bandgap results from the larger (smaller) electron-phonon coupling strength for a constant acoustic phonon energy. A 1.5 eV bandgap change was observed for weak electron-phonon coupling (s = 2) as well as with the strong electron-phonon coupling (s = 30). However, the weak electron-phonon coupling leads to a linear decrease in the bandgap energy as a function of temperature above ~85 K while the strong coupling exhibits similar behavior after ~60 K. Narrower (wider) tunability of the bandgap results from the larger (smaller) acoustic phonon energy for a constant electron-phonon coupling strength. The slope of negative entropy of exciton formation is large (small) at lower (higher) temperature. The management of the electron-phonon interaction as well as the average acoustic phonon energy indicates the ability to control the bandgap.

Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites.

Metal-halide perovskite (MHP) solar cells exhibit long nonradiative lifetimes as a crucial feature enabling high efficiencies. Long nonradiative lifetimes occur if the transfer of electronic into vibrational energy is slow due to, e.g., a low trap density, weak electron-phonon coupling, or the requirement to release many phonons in the electronic transition. Here, we combine known material properties of MHPs with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI3 lead to a strong dependence of recombination rates on trap position, which we deduce from the underlying physical effects determining nonradiative transitions. This is important for nonradiative recombination in MHPs, as it implies that they are rather insensitive to defects that are not at midgap energy, which can lead to long lifetimes. Therefore, the low phonon energies of MHPs are likely an important factor for their optoelectronic performance.

Ultra-broadband photodetectors based on epitaxial graphene quantum dots

AbstractGraphene is an ideal material for hot-electron bolometers due to its low heat capacity and weak electron-phonon coupling. Nanostructuring graphene with quantum-dot constrictions yields detectors of electromagnetic radiation with extraordinarily high intrinsic responsivity, higher than 1×109 V W−1 at 3 K. The sensing mechanism is bolometric in nature: the quantum confinement gap causes a strong dependence of the electrical resistance on the electron temperature. Here, we show that this quantum confinement gap does not impose a limitation on the photon energy for light detection and these quantum-dot bolometers work in a very broad spectral range, from terahertz through telecom to ultraviolet radiation, with responsivity independent of wavelength. We also measure the power dependence of the response. Although the responsivity decreases with increasing power, it stays higher than 1×108 V W−1 in a wide range of absorbed power, from 1 pW to 0.4 nW.

Tuning the thermoelectric performance of π–d conjugated nickel coordination polymers through metal–ligand frontier molecular orbital alignment

A good frontier molecular orbital alignment between the square planar metal-tetrasulfide fragment and the organic π-conjugated spacers results in a weak electron-phonon coupling, a high mobility and eventually a higher thermoelectric power factor.

Hot Carriers in CVD-Grown Graphene Device with a Top h-BN Layer

We investigate the energy relaxation of hot carriers in a CVD-grown graphene device with a top h-BN layer by driving the devices into the nonequilibrium regime. By using the magnetic field dependent conductance fluctuations of our graphene device as a self-thermometer, we can determine the effective carrier temperature Te at various driving currents I while keeping the lattice temperature TL fixed. Interestingly, it is found that Te is proportional to I, indicating little electron-phonon scattering in our device. Furthermore the average rate of energy loss per carrier Pe is proportional to (Te 2-TL 2), suggesting the heat diffusion rather than acoustic phonon processes in our system. The long energy relaxation times due to the weak electron-phonon coupling in CVD graphene capped with h-BN layer as well as in exfoliated multilayer graphene can find applications in hot carrier graphene-based devices.

Two dimensional superconductors in electrides

Two-dimensional (2D) superconductors are always a research hotspot in superconductivity, due to the huge importance of 2D superconductors in quantum phenomena transitions. And the natural metallic 2D electride with exceptional physical properties is one member of candidate materials for 2D superconductors in the Bardeen–Cooper–Schrieffer theory. The properties of electron–phonon induced superconductivity in 2D electrides, Y2C and MgONa, are investigated by using first-principles calculations. Because of the weak electron–phonon coupling between surface electronic states and phonons, critical temperature of the prototypical Y2C and MgONa are estimated to be 0.9 K and 3.4 K, respectively. It is also found that the electronic doping can improve superconductivity to Tc = 2.4 K and 4.5 K, resulting from the increase of density of states and the coupling between inner-layer electrons and phonons. In addition, a small tensile strain can enhance the critical temperature of MgONa to 6.15 K@n cm−2, due to the increase of density of states and phonon softening.

Cerium and Ytterbium Codoped Halide Perovskite Quantum Dots: A Novel and Efficient Downconverter for Improving the Performance of Silicon Solar Cells.

Quantum cutting can realize the emission of multiple near-infrared photons for each ultraviolet/visible photon absorbed, and has potential to significantly improve the photoelectric conversion efficiency (PCE) of solar cells. However, due to the lack of an ideal downconversion material, it has merely served as a principle in the laboratory until now. Here, the fabrication of a novel type of quantum cutting material, CsPbCl1.5 Br1.5 :Yb3+ , Ce3+ nanocrystals is presented. Benefiting from the larger absorption cross-section, weaker electron-phonon coupling, and higher inner luminescent quantum yield (146%), the doped perovskite nanocrystals are successfully explored as a downconverter of commercial silicon solar cells (SSCs). Noticeably, the PCE of the SSCs is improved from 18.1% to 21.5%, with a relative enhancement of 18.8%. This work exhibits a cheap, convenient, and effective way to enhance the PCE of SSCs, which may be commercially popularized in the future.

Coupling of quantum well states and phonons in thin multilayer Pb films on Si(111)

Density functional theory calculations for the electronic and phononic band structures of Pb/Si(111) thin films with a thickness of 4 and 5 monolayers (ML) are performed. We employ a Si(111)$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ unit cell to model the Pb films including the Si substrate, and identify quantum well (QW) states for film thicknesses between 3 and 6 ML. The calculations show that the quantum-confined state closest to the Si band gap acquires the character of a quantum well resonance with a major part of its wave function extending into the Si(111) substrate. This finding explains the unusually low dispersion of this state and its lacking sensitivity to phonons in the 5 ML Pb film. Moreover, several unoccupied QW states are identified in the calculations and are assigned to previously observed features in structurally simpler freestanding Pb films. The calculated phonon band structures of the Pb/Si(111)$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ films display stiff surface phonon modes in the 2.3--2.5 THz range. The electron-phonon coupling strength in the quantum-confined states is addressed by means of deformation-potential theory using the calculated atomic displacements of $\mathrm{\ensuremath{\Gamma}}$-point phonons. It is found that both the acoustic shear deformation potential as well as the optical deformation potentials of unoccupied QW states are sizable. Comparing the results for 4 and 5 ML Pb films, we conclude that the optical deformation potentials are generally larger for the 4 ML film. The occupied QW resonance in the 5 ML Pb film shows weak electron-phonon coupling, in qualitative agreement with the small experimentally observed lifetime broadening of this state. Our results form the basis for addressing the role of electron-phonon scattering for the lifetime of unoccupied QWs acting as intermediate states in two-photon photoemission from Pb/Si(111) films.

Infrared spectroscopic study ofCaFe0.7Co0.3O3

Temperature-dependent infrared spectroscopy has been investigated for ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ which undergoes a ferromagnetic transition at ${T}_{C}\ensuremath{\approx}177\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. It is observed that the spectral weight is transferred from $\ensuremath{\sim}4800--14 000{\mathrm{cm}}^{\ensuremath{-}1}$ to $\ensuremath{\sim}0$--$4800{\mathrm{cm}}^{\ensuremath{-}1}$ as the temperature is lowered around ${T}_{C}$. Such a large-range spectral weight transfer is attributed to the Hund's interaction. The phonons in ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ show minor asymmetric line shapes, implying relatively weak electron-phonon coupling compared with the parent compound ${\mathrm{CaFeO}}_{3}$. The optical conductivity also reveals a broad peak structure in the range of $\ensuremath{\sim}700--1500{\mathrm{cm}}^{\ensuremath{-}1}$. Fit by the model of single-polaron absorption, the broad peak is interpreted by the excitation of polarons. From the fitting parameters of the polaron peak, we estimate the electron-phonon coupling constant $\ensuremath{\alpha}\ensuremath{\sim}0.4--0.5$, implying that ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ falls into the weak-coupling regime.

Nonequilibrium steady states and transient dynamics of conventional superconductors under phonon driving

We perform a systematic analysis of the influence of phonon driving on the superconducting Holstein model coupled to heat baths by studying both the transient dynamics and the nonequilibrium steady state (NESS) in the weak and strong electron-phonon coupling regimes. Our study is based on the nonequilibrium dynamical mean-field theory, and for the NESS we present a Floquet formulation adapted to electron-phonon systems. The analysis of the phonon propagator suggests that the effective attractive interaction can be strongly enhanced in a parametric resonant regime because of the Floquet side bands of phonons. While this may be expected to enhance the superconductivity (SC), our fully self-consistent calculations, which include the effects of heating and nonthermal distributions, show that the parametric phonon driving generically results in a suppression or complete melting of the SC order. In the strong coupling regime, the NESS always shows a suppression of the SC gap, the SC order parameter and the superfluid density as a result of the driving, and this tendency is most prominent at the parametric resonance. Using the real-time nonequilibrium DMFT formalism, we also study the dynamics towards the NESS, which shows that the heating effect dominates the transient dynamics, and SC is weakened by the external modulations, in particular at the parametric resonance. In the weak coupling regime, we find that the SC fluctuations above the transition temperature are generally weakened under the driving. The strongest suppression occurs again around the parametric resonances because of the efficient energy absorption.

Suppressing Electron-Phonon Coupling through Laser-Induced Phase Transition.

Using first-principle calculations, we introduced a strategy of laser-induced phase transition that suppress electron-phonon couplings in crystal lattice. We explained unusual irreversible phase transitions in previous experiments on MoTe2 and NaYF4 crystals. Laser irradiations produced local heats in 2H-MoTe2 and Hex NaYF4, driving atom reorganizations toward new lattices. The reorganization with effective electron-phonon couplings continues with spontaneously generated heats, whereas a 1T'-MoTe2 and a metastable cubic NaYF4 phases were kept because of suppressed vibrational relaxations. Long time laser treatments create phases with weak electron-phonon couplings. Such irreversible transitions guarantee complete conversions, opening a new door to selective material modifications.

Pump-Power Dependence of Coherent Acoustic Phonon Frequencies in Colloidal CdSe/CdS Core/Shell Nanoplatelets.

Femtosecond optical pump-probe spectroscopy resolves hitherto unobserved coherent acoustic phonons in colloidal CdSe/CdS core/shell nanoplatelets (NPLs). With increasing pump fluence, the frequency of the in-plane acoustic mode increases from 5.2 to 10.7 cm-1, whereas the frequency of the out-of-plane mode remains at ∼20 cm-1. Analysis of the oscillation phases suggests that the coherent acoustic phonon generation mechanism transitions from displacive excitation to subpicosecond Auger hole trapping with increasing pump fluence. The measurements yield Huang-Rhys parameters of ∼10-2 for both acoustic modes. The weak electron-phonon coupling strengths favor the application of NPLs in optoelectronics.