Significant Electron-phonon Coupling Research Articles

Significant electron-phonon coupling in nanographene confined in single-walled carbon nanotubes due to the large amplitude of radial breathinglike vibrations

Significant electron-phonon coupling in nanographene confined in single-walled carbon nanotubes due to the large amplitude of radial breathinglike vibrations

Two-Dimensional Moiré Polaronic Electron Crystals.

Two-dimensional moiré materials have emerged as the most versatile platform for realizing quantum phases of electrons. Here, we explore the stability origins of correlated states in WSe_{2}/WS_{2} moiré superlattices. We find that ultrafast electronic excitation leads to partial melting of the Mott states on timescales 5 times longer than predictions from the charge hopping integrals and that the melting rates are thermally activated, with activation energies of 18±3 and 13±2 meV for the one- and two-hole Mott states, respectively, suggesting significant electron-phonon coupling. A density functional theory calculation of the one-hole Mott state confirms polaron formation and yields a hole-polaron binding energy of 16meV. These findings reveal a close interplay of electron-electron and electron-phonon interactions in stabilizing the polaronic Mott insulators at transition metal dichalcogenide moiré interfaces.

Imaging Ultrafast Dynamics of Pressure-Driven Phase Transitions in Black Phosphorus and Anomalous Coherent Phonon Softening.

Applying high pressure to effectively modulate the electronic and lattice structures of materials could unravel various physical properties associated with phase transitions. In this work, high-pressure-compatible femtosecond pump-probe microscopy was constructed to study the pressure-dependent ultrafast dynamics in black phosphorus (BP) thin films. We observed pressure-driven evolution of the electronic topological transition and three structural phases as the pressure reached ∼22 GPa, which could be clearly differentiated in the transient absorption images containing spatially resolved ultrafast carrier and coherent phonon dynamics. Surprisingly, an anomalous coherent acoustic phonon mode with pressure softening behavior was observed within the range of ∼3-8 GPa, showing distinct laser power and time dependences. Density functional theory calculations show that this mode, identified as the shear mode along the armchair orientation, gains significant electron-phonon coupling strength from out-of-plane compression that leads to decreased phonon frequency. Our results provide insights into the structure evolution of BP with pressure and hold potential for applications in microelectromechanical devices.

Broadband Emission Origin in Metal Halide Perovskites: Are Self-Trapped Excitons or Ions?

It has always been a goal to realize high efficiency and broadband emission in single-component materials. The appearance of metal halide perovskites makes it possible. Their soft lattice characteristics and significant electron-phonon coupling synergistically generate self-trapped excitons (STEs), contributing to a broadband emission with a large Stokes shift. Meanwhile, their structural/compositional diversity provides suitable active sites and coordination environments for doping of ns2 ions, allowing 3 Pn ( n =0,1,2) →1 S0 transitions toward broadband emission. The ns2 ions emission is phenomenologically similar to that of STE emission, hindering in-depth understanding of their emission origin, and leading to failure to meet the design requirements for practical applications. In this scenario, herein, the fundamentals and development of such two emission mechanisms are summarized to establish a clear and comprehensive understanding of the broadband emission phenomenon, which may pave the way to an ideal customization of broadband-emission metal halide perovskites.

Photoluminescence spectroscopy of excitonic emission in CsPbCl3 perovskite single crystals

We analyzed low-temperature photoluminescence (PL) and the band edge excitonic emission in bulk halide perovskite CsPbCl3 single crystals. Strong near band edge excitonic emission is observed at 420 nm and a series of excitonic states was observed. Temperature-dependent PL measurements on cesium lead chloride over a wide temperature range from 13 K to 295 K revealed significant electron phonon coupling with a Huang-Rhys parameter ranging from 1.34 to 3.38. A primary experimental finding is that the near band edge PL emission shows existence of multiple sharp features at low temperatures that are attributable to free excitons, bound excitons, and their phonon replicas. The PL also shows evidence of band tailing effects due to disorder. We investigated the recombination mechanism of photogenerated charge carriers by means of time-resolved PL (TR-PL) spectroscopy at low temperatures and various excitation powers and observed competing mono- and bimolecular recombination processes. A dynamic model is proposed to describe the TR-PL. We observed both long-lived and short-lived recombination where the short-lived time decay is attributed to free exciton recombination, whereas the long-lived decay to bound exciton recombination involving traps due to intrinsic disorder.

Exploring the AgSb1−xBixI4 phase diagram: Thermochromism in layered CdCl2-type semiconductors

AgSbI4 and several AgSb1–xBixI4 compositions have been synthesized via solid state reactions. Both end members and all intermediate compositions crystallize with the CdCl2 structure type (space group ​= ​R3¯m) and a random distribution of the Ag+ and Sb3+/Bi3+ cations. Hypothetical cation ordered structures have been generated and their stabilities evaluated with density functional theory. This analysis suggests that while charge balanced layers are strongly favored, the long-range interactions that favor ordered stacking of layers are too weak to produce three-dimensional cation ordering. The colors of these materials change noticeably when heated, signaling a large thermochromic effect. For example, the band gap of AgSb0·25Bi0·75I4 changes from 1.68 ​eV ​at room temperature to 1.55 ​eV ​at 150 ​°C. This thermochromic shift in band gap is larger than comparable diamond-like semiconductors, such as CdTe. VASP calculations and molecular dynamics simulations indicate the large thermochromic effect is primarily caused by significant electron-phonon coupling.

Phonon-mediated superconductivity in doped monolayer materials

Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying electron-phonon coupling and superconductivity in doped monolayer graphene and hexagonal boron nitride based on intrinsic phonon modes. Solving the graphene gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models, the various system parameters can be tuned at will. Consistent with results in the literature, we find slight gap modulations along the Fermi surface, and the high energy phonon modes are shown to be the most significant for the superconductivity instability. The Coulomb interaction plays a major role in suppressing superconductivity at realistic dopings. Motivated by the direct onset of a large density of states at the Fermi surface for small charge dopings in hexagonal boron nitride, we also calculate the dimensionless electron-phonon coupling strength there, but the comparatively large density of states cannot immediately be capitalized on, and the charge doping necessary to obtain significant electron-phonon coupling is similar to the value in graphene.

2D vibrational properties of epitaxial silicene on Ag(111)

The two-dimensional silicon allotrope, silicene, could spur the development of new and original concepts in Si-based nanotechnology. Up to now silicene can only be epitaxially synthesized on a supporting substrate such as Ag(111). Even though the structural and electronic properties of these epitaxial silicene layers have been intensively studied, very little is known about its vibrational characteristics. Here, we present a detailed study of epitaxial silicene on Ag(111) using in situ Raman spectroscopy, which is one of the most extensively employed experimental techniques to characterize 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous. The vibrational fingerprint of epitaxial silicene, in contrast to all previous interpretations, is characterized by three distinct phonon modes with A and E symmetries. Both, energies and symmetries of theses modes are confirmed by ab initio theory calculations. The temperature dependent spectral evolution of these modes demonstrates unique thermal properties of epitaxial silicene and a significant electron–phonon coupling. These results unambiguously support the purely two-dimensional character of epitaxial silicene up to about 300 °C, whereupon a 2D-to-3D phase transition takes place. The detailed fingerprint of epitaxial silicene will allow us to identify it in different environments or to study its modifications.

Evidence of a pseudogap driven by competing orders of multi-band origin in the ferromagnetic superconductor Sr0.5Ce0.5FBiS2

From temperature and magnetic field dependent point-contact spectroscopy on the ferromagnetic superconductor Sr0.5Ce0.5FBiS2 (bulk superconducting K) we observe (a) a pseudogap in the normal state that sustains to a remarkably high temperature of 40 K and (b) two-fold enhancement of Tc upto 5 K in the point-contact geometry. In addition, Andreev reflection spectroscopy reveals a superconducting gap of 6 meV for certain point-contacts suggesting that the mean field Tc of this system could be approximately 40 K, the onset temperature of pseudo-gap. Our results suggest that quantum fluctuations originating from other competing orders in Sr0.5Ce0.5FBiS2 forbid a global phase coherence at high temperatures thereby suppressing Tc. Apart from the known ordering to a ferromagnetic state, our first-principles calculations reveal nesting of a multi-band Fermi surface and a significant electron-phonon coupling that could result in charge density wave-like instabilities.

Emergence of an Out-of-Plane Optical Phonon (ZO) Kohn Anomaly in Quasifreestanding Epitaxial Graphene

In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at q=Γ (LO branch) and q=K (TO branch), reflecting a significant electron-phonon coupling (EPC) to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the Γ point in quasifreestanding graphene epitaxially grown on Pt(111). The Kohn anomaly for the LO phonon is observed at finite momentum q~2k_{F} from Γ, with a shape in excellent agreement with the theory and consistent with known values of the EPC and the Fermi level. More strikingly, we also observe a Kohn anomaly at the same momentum for the out-of-plane optical phonon (ZO) branch. This observation is the first direct evidence of the coupling of the ZO mode with Dirac electrons, which is forbidden for freestanding graphene but becomes allowed in the presence of a substrate. Moreover, we estimate the EPC to be even greater than that of the LO mode, making graphene on Pt(111) an optimal system to explore the effects of this new coupling in the electronic properties.

Correlation between phonon anomaly along [211] and the Fermi surface nesting features with associated electron-phonon interactions in Ni2FeGa: A first principles study

First principles calculation reaffirms the presence of phonon anomaly along [211] direction in Ni2FeGa shape memory alloy supporting the experimental findings of J. Q. Li et al. Fermi surface scans have been performed in both austenite and martensite phase to see the possible Fermi nesting features in this alloy. The magnitude of observed Fermi surface nesting vectors in (211) plane exactly match the phonon anomaly wavevectors along [211] direction. Electron-phonon calculation in the austenite phase shows that there is significant electron-phonon coupling in this alloy which might arise out of the lattice coupling between lower acoustic modes and higher optical modes combined with the observed strong Fermi nesting features in the system.

Deoxidation of graphene oxide nanosheets to extended graphenites by “unzipping” elimination

Low-temperature scanning tunneling microscopy on alkyl-surface-functionalized graphene oxide nanosheets reveals the formation of low-dimensional graphenite nanostructures with extended pi-conjugation at deoxidation temperatures above 150 degrees C. The elimination of these alkyl chains from the surface of the nanosheets does not occur uniformly, but in distinctive patterns that correspond to the formation of an underlying network of graphenite one-dimensional "tracks" and "dots." Atomic-resolution imaging of these graphenite regions reveals a defective honeycomb lattice characteristic of single-layer graphenes. These extended graphenite structures percolate the nanosheet even for moderate levels of deoxidation and regraphenization of the basal plane. The formation of extended conjugation indicates a regioselective rather than random elimination of the oxygen atoms and alkyl chains. The resultant network morphology allows bandlike transport of charge carriers across the sheets despite defects and disorder. The sub-meV apparent activation energies for the field-effect mobilities at low temperatures (70-30 K) for both electrons and holes rule out significant electron-phonon coupling. This suggests a remarkable potential for electronic applications of these solution-processable functionalized graphene oxide nanosheets.

Kohn anomalies and nonadiabaticity in doped carbon nanotubes

The high-frequency Raman-active phonon modes of metallic single-walled carbon nanotubes (SWCNT) are thought to be characterized by Kohn anomalies (KAs) resulting from the combination of SWCNT intrinsic one-dimensional nature and a significant electron-phonon coupling (EPC). KAs are expected to be modified by the doping-induced tuning of the Fermi energy level ${ϵ}_{F}$, obtained through the intercalation of SWCNTs with alkali atoms or by the application of a gate potential. We present a density-functional theory (DFT) study of the phonon properties of a (9,9) metallic SWCNT as a function of electronic doping. For such study, we use, as in standard DFT calculations of vibrational properties, the Born-Oppenheimer (BO) approximation. We also develop an analytical model capable of reproducing and interpreting our DFT results. Both DFT calculations and this model predict, for increasing doping levels, a series of EPC-induced KAs in the vibrational mode parallel to the tube axis at the $\mathbf{\ensuremath{\Gamma}}$ point of the Brillouin zone, usually indicated in Raman spectroscopy as the ${G}^{\ensuremath{-}}$ peak. Such KAs would arise each time a new conduction band is populated. However, we show that they are an artifact of the BO approximation. The inclusion of nonadiabatic effects dramatically affects the results, predicting KAs at $\mathbf{\ensuremath{\Gamma}}$ only when ${ϵ}_{F}$ is close to a band crossing ${E}_{X}$. For each band crossing, a double KA occurs for ${ϵ}_{F}={E}_{X}\ifmmode\pm\else\textpm\fi{}\ensuremath{\hbar}\ensuremath{\omega}∕2$, where $\ensuremath{\hbar}\ensuremath{\omega}$ is the phonon energy. In particular, for a $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ metallic nanotube, we predict a KA to occur in the so-called ${G}^{\ensuremath{-}}$ peak at a doping level of about ${N}_{\mathrm{el}}∕C=\ifmmode\pm\else\textpm\fi{}0.0015$ atom $({ϵ}_{F}\ensuremath{\approx}\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and, possibly, close to the saturation doping level $({N}_{\mathrm{el}}∕C\ensuremath{\sim}0.125)$, where an interlayer band crosses the ${\ensuremath{\pi}}^{*}$ nanotube bands. Furthermore, we predict that the Raman linewidth of the ${G}^{\ensuremath{-}}$ peak significantly decreases for $\ensuremath{\mid}{ϵ}_{F}\ensuremath{\mid}\ensuremath{\geqslant}\ensuremath{\hbar}\ensuremath{\omega}∕2$. Thus, our results provide a tool to determine experimentally the doping level from the value of the KA-induced frequency shift and from the linewidth of the ${G}^{\ensuremath{-}}$ peak. Finally, we predict KAs to occur in phonons with finite momentum $\mathbf{q}$ not only in proximity of a band crossing but also each time a new band is populated. Such KAs should be observable in the double-resonant Raman peaks, such as the defect-activated $D$ peak, and the second-order peaks $2D$ and $2G$.

Jahn-Teller effect and possible superconductivity in [18]annulene

Abstract We discuss the Jahn-Teller effect and possible superconductivity in negatively charged [18]annulene, (CH) 18 , which has a highly symmetric D 6h geometry in the neutral state. Electron-phonon coupling constants in model one-dimensional crystals of (CH) 18 − and (CH) 18 3− are calculated and analyzed using the density functional B3LYP method. The lowest two frequency modes (116 cm −1 and 405 cm −1 ) causing distortions of the C-C framework of [18]annulene and the highest frequency mode (3201 cm −1 ) of C-H stretching appear to give significant electron-phonon coupling constants. Possible preconditions under which idealized one-dimensional crystals of (CH) 18 − and (CH) 18 3− can exhibit superconductivity at finite temperature are considered.

Electron–phonon coupling in the one-dimensional crystals of negatively charged [18]annulene

We discuss the Jahn–Teller effect and possible superconductivity in negatively charged [18]annulene, (CH)18, which is an interesting molecular system with a highly symmetric D6h geometry in the neutral state. Electron–phonon coupling constants in model one-dimensional crystals of (CH)18−, (CH)183−, (CD)18−, and (CD)183− are calculated and analyzed using the B3LYP density functional method of Becke and Lee, Yang, and Parr. The lowest two frequency modes (116 cm−1 and 405 cm−1) causing distortions of the C–C framework of [18]annulene and the highest frequency mode (3201 cm−1) of C–H stretching appear to give significant electron–phonon coupling constants. The computed coupling constants of the monoanion are larger than those of the trianion. Possible preconditions under which idealized one-dimensional crystals of (CH)18− and (CH)183− can exhibit superconductivity at finite temperature are considered.

On the Electronic Structure of the New "Intermetallics" LnNi2B2C

The electronic structure of the new series of intermetallic compounds LnNi[sub 2]B[sub 2]C (Ln = lanthanide element) is calculated and discussed in terms of local chemical bonding in order to elucidate how superconductivity may be possible for some of these materials. The Fermi level occurs in a region that allows for a second-order Jahn-Teller mixing and can contribute to significant electron-phonon coupling in these systems. Moreover, the trend in the a lattice parameter for different Ln cations shifts the position of the x[sup 2]-y[sup 2] band, which significantly changes its occupation and affects the superconducting transition temperature. We also compare the electronic structures of LnNi[sub 2]B[sub 2]C with those from the related compounds LnNiBC. 20 refs., 7 figs., 2 tabs.

Raman scattering in ferromagnetic TDAE-C 60 compared to TDAE-C 70

The Raman scattering at 1.16 eV laser excitation has been studied in ferromagnetic TDAE-C 60 and compared to that in paramagnetic TDAE-C 70. The shift of H g and A g modes in TDAE-C 60 indicates transfer of one electron to C 60 while the broadening of H g modes is quite similar to metallic K 3C 60 meaning significant electron-phonon coupling of this Jahn - Teller vibrations. In TDAE-C 70 the lines are narrow sugesting suppresed electron - phonon interaction and hence smaller polaronic effects. Below T c of magnetic ordering the two-magnon excitations are not seen clearly however the low energy part of spectra shows somewhat increased intensity. Upon air-exposure of TDAE-C 60 all H g and A g Raman modes disappear while a new band appears at 1848 cm −1.

Vibrational spectroscopy and comparative study of trans-polyacetylene, poly(p-phenylene) and poly(p-phenylene vinylene)

Theoretical vibrational spectra of trans-polyacetylene (PA), poly(p-phenylene) (PPP) and poly(p-phenylene vinylene) (PPV) are presented, based on a single-periodic-chain model and a harmonic potential with parameters transferred from trans-hexatriene, toluene, and p-divinyl benzene, respectively. The out-of-plane degrees of freedom and the infrared absorption intensities were included in the calculations. The calculations evince the absence of the in-plane C-H bending infrared absorption band of the trans-vinylene portion of trans-PA and PPV, which is a consequence of a delocalized π-electron system. The electron-phonon coupling in PPP and PPV is relatively weak - in contrast to trans-PA, where the frequencies of the principal Raman lines are significantly lowered. As a consequence, the PPV is an example of pristine polymer with well delocalized π-electron system, but without significant electron-phonon coupling.

Theoretical study of the vibrational spectra in poly(p-phenylene vinylene).

Theoretical vibrational spectra of poly(p-phenylene vinylene) (PPV) are presented, based on a single--periodic-chain model and a harmonic potential with parameters transferred from n-divinyl benzene. The out-of-plane degrees of freedom and the infrared-absorption intensities were included in the calculations. The calculations evince a missing of the in-plane C-H bending infrared-absorption band of the trans-vinylene portion of PPV, as in trans-polyacetylene, which is a consequence of delocalized \ensuremath{\pi}-electron system. However, the electron-phonon coupling in PPV is relatively weak [as in poly(p-phenylene)] as compared to trans-polyacetylene (where the frequencies of the principal Raman lines are significantly lowered). As a conequence, PPV is an example of pristine polymer with well-delocalized \ensuremath{\pi}-electron system, but without significant electron-phonon coupling.