Enhanced Electron Phonon Coupling Research Articles

Superconductivity in Nb5Ir3N: a nitrogen-filled electride

The hexagonal Mn5Si3-type compounds possess the capability to accommodate specific atoms in the interstices, thereby creating filled Mn5Si3-type structures. In Nb-based Mn5Si3-type system, interstitial atoms like carbon (C) or oxygen (O) have been identified to induce or enhance superconductivity. However, the compounds filled with nitrogen (N) are scarce, and the existence of a N-filled superconductor remains unknown. Here, we report the discovery of a novel ternary nitride superconductor, Nb5Ir3N, synthesized via incorporating N into the electride Nb5Ir3. The crystal structure of Nb5Ir3N conforms to the filled Mn5Si3-type, belonging to the P63/mcm space group (No. 193), with cell parameters a = b = 7.8398(2) Å and c = 5.1108(1) Å. Electrical resistivity and magnetic susceptibility demonstrate that Nb5Ir3N is a type-II superconductor with a T c of 8.7 K. The estimated lower and upper critical fields are 11.0 mT and 12.16 T, respectively. Moreover, specific heat measurements confirm the bulk superconductivity with enhanced electron–phonon coupling in Nb5Ir3N, as demonstrated by the normalized specific heat jump ΔC e/γT c ∼ 1.59. First-principles calculations emphasize the strong spin–orbit coupling in Nb5Ir3N.

Temperature-dependent excitonic emission characteristics of highly crystallized carbon nitride nanosheets

Highly-crystallized carbon nitride (HCCN) nanosheets exhibit significant potential for advancements in the field of photoelectric conversion. However, to fully exploit their potential, a thorough understanding of the fundamental excitonic photophysical processes is crucial. Here, the temperature-dependent excitonic photoluminescence (PL) of HCCN nanosheets and amorphous polymeric carbon nitride (PCN) is investigated using steady-state and time-resolved PL spectroscopy. The exciton binding energy of HCCN is determined to be 109.26 meV, lower than that of PCN (207.39 meV), which is attributed to the ordered stacking structure of HCCN with a weaker Coulomb interaction between electrons and holes. As the temperature increases, a noticeable reduction in PL lifetime is observed on both the HCCN and PCN, which is ascribed to the thermal activation of carrier trapping by the enhanced electron–phonon coupling effect. The thermal activation energy of HCCN is determined to be 102.9 meV, close to the value of PCN, due to their same band structures. Through wavelength-dependent PL dynamics analysis, we have identified the PL emission of HCCN as deriving from the transitions: σ*–LP, π*–π, and π*–LP, where the π*–LP transition dominants the emission because of the high excited state density of the LP state. These results demonstrate the impact of high-crystallinity on the excitonic emission of HCCN materials, thereby expanding their potential applications in the field of photoelectric conversion.

Optical properties of negatively charged germanium-vacancy centers in detonation nanodiamonds with an average single-digit nanometer particle size

Nanodiamonds that contain germanium-vacancy centers (GeV-NDs) exhibit significant potential for biomedical and quantum science applications. GeV-NDs with an average particle size of 9 nm were recently fabricated through a detonation process that enables the practical-scale production of detonation NDs (DNDs). However, the optical properties of the GeV centers in the DNDs have not been studied thoroughly. In particular, the luminescence spectrum of these GeV-DNDs had an unassigned peak at 1.98 eV. Here, we investigate the optical properties of GeV-DNDs under various conditions. Although the GeV-DNDs exhibit a zero-phonon line (ZPL) with similar excitation energy dependence and photostability to their bulk counterparts, the ZPL linewidth is broader. The 1.98 eV-peak is attributed to a composite phonon sideband peak. The unique properties of the GeV centers in these small DNDs are explained by enhanced electron–phonon coupling.

Enhanced electron–phonon coupling by delocalizing phonon states for desirable interlayer transfer of excited charges in MoSSe/WS2 heterobilayer

Regulating the interlayer transfer of excited charges is challenging but crucial for high-efficiency photoelectric conversion devices based on semiconductor heterojunction. In this work, the interlayer transfer and recombination of excited charges are investigated for the heterobilayers based on monolayer MoSSe and WS2 by combining density functional theory calculation with nonadiabatic molecular dynamics simulation. Our results reveal that the heterobilayers possess type-II band alignments and the interface engineering from S–Se to S–S stacking configuration reverses the spatial distribution of valence and conduction bands from MoSSe and WS2 to WS2 and MoSSe layers, respectively, which produces interlayer transfer of excited charges in opposite direction. The interface engineering also causes the delocalization of out-of-plane phonon states from the WS2 layer to both WS2 and MoSSe layers. This delocalized character in S–S configuration facilitates the simultaneous coupling of out-of-plane phonon states with the localized donor and acceptor electronic states, accelerates the motion of interface atoms, and reduces the band energy differences, which synergistically promote interlayer transfer of excited charges. As a result, the interlayer transfer of excited charges in S–S configuration is faster than that in S–Se configuration. Our investigation demonstrates that delocalizing phonon states through interface engineering can regulate electron–phonon coupling and interlayer transfer of excited charges.

Direct Observation of Enhanced Electron-Phonon Coupling in Copper Nanoparticles in the Warm-Dense Matter Regime.

Warm dense matter (WDM) represents a highly excited state that lies at the intersection of solids, plasmas, and liquids and that cannot be described by equilibrium theories. The transient nature of this state when created in a laboratory, as well as the difficulties in probing the strongly coupled interactions between the electrons and the ions, make it challenging to develop a complete understanding of matter in this regime. In this work, by exciting isolated ∼8 nm copper nanoparticles with a femtosecond laser below the ablation threshold, we create uniformly excited WDM. Using photoelectron spectroscopy, we measure the instantaneous electron temperature and extract the electron-ion coupling of the nanoparticle as it undergoes a solid-to-WDM phase transition. By comparing with state-of-the-art theories, we confirm that the superheated nanoparticles lie at the boundary between hot solids and plasmas, with associated strong electron-ion coupling. This is evidenced both by a fast energy loss of electrons to ions, and a strong modulation of the electron temperature induced by strong acoustic breathing modes that change the nanoparticle volume. This work demonstrates a new route for experimental exploration of the exotic properties of WDM.

Probing Enhanced Electron-Phonon Coupling in Graphene by Infrared Resonance Raman Spectroscopy.

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D^{'} peaks with respect to that measured in graphite. Comparing with fully abinitio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

Electron–Phonon Superconductivity in Boron‐Based Chalcogenide (X= S, Se) Monolayers

AbstractElectron–phonon mediated superconductivity is deeply investigated in two boron based monolayer materials, namely, , a metal exhibiting the ability to superconduct, and a new metal, , presenting perfect kinetic stability. Calculations based on density functional perturbation theory combined with the maximally localized Wannier function also reveal that both materials exhibit anisotropic planar hexagonal structure like graphene. The key parameters involved in the superconductor behavior are all calculated. The electronic density in the Fermi surface is given to provide the environment for enhanced electron–phonon coupling. The longitudinal and transverse vibration modes of optical phonons mainly contribute to the electron–phonon coupling strength. Furthermore, the binding energy between the bosonic Cooper pair superfluid is quantified and determined. The critical temperature for the two materials is 20 and 10.5 K, respectively. The results obtained show the potential use of such materials for superconducting applications.

Laser-induced Fano asymmetry, electron-phonon coupling, and phase transition in lanthanide sesquioxide (Ln2O3; Ln = Eu, Gd, Dy) nanoparticles: A Raman spectroscopic investigation

Laser power-dependent Raman spectroscopy is deployed to probe Fano interference in asymmetrically broadened Tg modes and the associated line shift in three technologically sound, meticulously characterized rare-earth sesquioxide systems. Group theoretical analysis is accompanied to introspect the Raman-active optic modes in cubic, monoclinic, and trigonal phases and identify the laser heating-induced local phase transitions. With increasing laser intensity, a regular redshift and larger negative asymmetry in the Raman peaks are detected, which is attributed to moderations in Fano scattering by enhanced electron–phonon coupling amid the focussed photoexcited electron plasma and is illustrated using a Feynman diagram. A quantitative study is thereby performed to unveil the intrinsic nature of discrete-continuum Fano resonance in the nanoparticles of interest emphasizing the high sensitivity of Raman spectra to the excitation strength that perturbs the generic vibrational features at the Brillouin zone center by influencing the interference conditions, force constant, and length of the associated bonds compelled by tensile stress. A rising trend of the charge–phonon coupling constant (λ) with laser power validates a stronger particle–quasiparticle coupling, whereas a shorter anharmonic phonon lifetime (τanh) indicates faster interactions. Using Allen's formalism, the charge density of states [N(εF)] at the Fermi level per spin and molecule is calculated, which pertains to a negative regression dependence in the λN(εF)−τanh dynamics.

Pressure Evolution of Ultrafast Photocarrier Dynamics and Electron-Phonon Coupling in FeTe0.5Se0.5.

Understanding the coupling between electrons and phonons in iron chalcogenides FeTexSe1-x has remained a critical but arduous project in recent decades. The direct observation of the electron-phonon coupling effect through electron dynamics and vibrational properties has been lacking. Here, we report the first pressure-dependent ultrafast photocarrier dynamics and Raman scattering studies on an iron chalcogenide FeTe0.5Se0.5 to explore the interaction between electrons and phonons in this unconventional superconductor. The lifetime of the excited electrons evidently decreases as the pressure increases from 0 to 2.2 GPa, and then increases with further compression. The vibrational properties of the A1g phonon mode exhibit similar behavior, with a pronounced frequency reduction appearing at approximately 2.3 GPa. The dual evidence reveals the enhanced electron-phonon coupling strength with pressure in FeTe0.5Se0.5. Our results give an insight into the role of the electron-phonon coupling effect in iron-based superconductors.

Superconductivity in Layered van der Waals Hydrogenated Germanene at High Pressure.

The emergence of superconductivity in two-dimensional (2D) materials has attracted tremendous research efforts because the origins and mechanisms behind the unexpected and fascinating superconducting phenomena remain unclear. In particular, the superconductivity can survive in 2D systems even with weakened disorder and broken spatial inversion symmetry. Here, structural and superconducting transitions of 2D van der Waals (vdW) hydrogenated germanene (GeH) are observed under compression and decompression processes. GeH possesses a superconducting transition with a critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa, while superconductivity remains. An abnormal increase of Tc up to 6.11 K was observed during the decompression process, while the GeH remained in the 2D amorphous phase. A combination study of in situ high-pressure synchrotron X-ray diffraction, in situ high-pressure Raman spectroscopy, transition electron microscopy, and density functional theory simulations suggests that the superconductivity in 2D vdW GeH is attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure even in the form of an amorphous phase. The unique pressure-induced phase transition of GeH from 2D crystalline to 2D amorphous metal hydride provides a promising platform to study the mechanisms of amorphous hydride superconductivity.

Polaron mobility modulation by bandgap engineering in black phase α-FAPbI3

Lead halide hybrid perovskites (LHP) have emerged as one of the most promising photovoltaic materials for their remarkable solar energy conversion ability. The transportation of the photoinduced carriers in LHP could screen the defect recombination with the help of the large polaron formation. However, the physical insight of the relationship between the superior optical-electronic performance of perovskite and its polaron dynamics related to the electron-lattice strong coupling induced by the substitution engineering is still lack of investigation. Here, the bandgap modulated thin films of α-FAPbI3 with different element substitution is investigated by the time resolved Terahertz spectroscopy. We find the polaron recombination dynamics could be prolonged in LHP with a relatively smaller bandgap, even though the formation of polaron will not be affected apparently. Intuitively, the large polaron mobility in (FAPbI3)0.95(MAPbI3)0.05 thin film is ∼30% larger than that in (FAPbI3)0.85(MAPbBr3)0.15. The larger mobility in (FAPbI3)0.95(MAPbI3)0.05 could be assigned to the slowing down of the carrier scattering time. Therefore, the physical origin of the higher carrier mobility in the (FAPbI3)0.95(MAPbI3)0.05 should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution. In addition, (FAPbI3)0.95(MAPbI3)0.05 will lose fewer active carriers during the polaron cooling process than that in (FAPbI3)0.85(MAPbBr3)0.15, indicating lower thermal dissipation in (FAPbI3)0.95(MAPbI3)0.05. Our results suggest that besides the smaller bandgap, the higher polaron mobility improved by the substitution engineering in α-FAPbI3 can also be an important factor for the high PCE of the black phase α-FAPbI3 based solar cell devices.

Enhanced electron–phonon coupling near an electronic quantum phase transition

I construct a simple model to demonstrate that when the many-electron quantum state of a material is near a quantum phase transition and the vibrational motion of a phonon explores the potential energy surface near the transition point, then an impenetrable barrier appears at the potential energy surface which restricts the phonon from crossing the transition point and abnormally increases the phonon frequency. The ensuing anomalous enhancement of the electron phonon coupling is general and independent of the specific nature of the electronic quantum phase transition. Understanding and modelling this strong electron–phonon coupling may potentially lead to the design of phonon superconductors with high critical temperatures by choosing their parameters appropriately near an electronic quantum phase transition.

Reversible Thermochromism in All‐Inorganic Lead‐Free Cs3Sb2I9 Perovskite Single Crystals

AbstractHere, lattice expansion modulated reversible thermochromism (TCM) in all‐inorganic lead‐free Cs3Sb2I9 perovskite single crystals (SCs) is demonstrated. Upon temperature change, Cs3Sb2I9 SC exhibits a reversible color change between red and dark‐brown. The corresponding optical band‐edge modulation and structural phase transition are monitored by temperature dependent UV–vis absorption spectroscopy and X‐ray diffraction study. This observed reversible TCM is attributed to the lattice expansion and strain relaxation at elevated temperatures. Additionally, Raman spectroscopy confirms enhanced electron–phonon coupling originating from stretched SbI terminal bonds at elevated temperature. This work validates that the all‐inorganic Cs3Sb2I9 perovskites have the potential in smart thermochromic windows and temperature sensors applications.

Temperature-dependent site selection of boron doping in chemically derived graphene

Site dependent light element doping in graphene can lead to exciting phenomenological prospects such as tunable bandgap, enhanced electron phonon coupling and anomalous transport properties for superconductivity, ferromagnetism and catalysis. However, they can lead to more additional defect sites and strain dependent effects as a residual fallout of the process and remains open to interpretations. In this work, we delineate a spectroscopic approach combined with ab-initio results to decipher these factors by using a prototypical in-situ boron doped reduced graphene oxide sample specimens and tuning them from low to moderate hole doping concentrations. The selectivity of doping configurations (BC3, BCO2 and BC2O) as well as their concentration is varied by regulating the annealing temperature (up to 1000 °C). We find a competitive relationship between the dopant and the residual surface oxygen atoms with gradual transformation of favorable doping configuration from out of plane to in-plane (substitutional) with increasing temperature. Furthermore, simultaneous induction of point defects and strain related effects in graphene lattice were also observed with increase in doping concentration. This led to anomalous bandgap crossover at high temperatures in boron doped graphene in comparison to the thermally reduced counterpart which could be important for electronic and transport applications.

Barriers to carriers: faults and recombination in non-stoichiometric perovskite scintillators

Tuning the efficiency and speed of charge carrier recombination in inorganic scintillators can potentially improve their performance in diverse applications. Recent work suggests that this maybe be achieved via a two-phase scintillator AB that naturally phase separates into A-rich and B-rich domains. In addition, a favorable electronic structure and band-edge alignment such that the charge carriers are confined or are thermodynamically driven to preferentially accumulate in one of the two domains, might lead to an improved radiative recombination rate. Here, we use density functional theory computations and ab initio molecular dynamics (AIMD), including non-adiabatic molecular dynamics (NAMD) simulations, to examine an alternative phase structure and its potential impact on recombination. Using a model perovskite SrTiO_3 system with one-, two- and three-dimensional Ruddlesden–Popper (RP) phases, we demonstrate that RP faults induce band structure changes in the material that can act as barriers to carrier transport. Our AIMD/NAMD simulations indicate competing effects of a lower mean free path (potentially enhancing the desired radiative recombination and overall scintillating efficiency) and faster non-radiative recombination (undesired) due to enhanced electron–phonon coupling in the faulted system. Full exploitation of such a rational design approach would require tuning of the effective scintillation efficiency by varying the perovskite chemistry using appropriate arrangements of RP faults in the bulk material. Finally, other effects, such as the tendency of point defects to segregate at the interface, that might affect the overall performance, are briefly discussed. We expect the basic results found here to apply to other nanostructured scintillators.Graphical

Temperature Dependent Excitonic Transition Energy and Enhanced Electron-Phonon Coupling in Layered Ternary SnS2-xSex Semiconductors with Fully Tunable Stoichiometry.

In this study, a series of SnS2-xSex (0 ≤ x ≤ 2) layered semiconductors were grown by the chemical–vapor transport method. The crystal structural and material phase of SnS2-xSex layered van der Waals crystals was characterized by X-ray diffraction measurements and Raman spectroscopy. The temperature dependence of the spectral features in the vicinity of the direct band edge excitonic transitions of the layered SnS2-xSex compounds was measured in the temperature range of 20–300 K using the piezoreflectance (PzR) technique. The near band-edge excitonic transition energies of SnS2-xSex were determined from a detailed line-shape fit of the PzR spectra. The PzR characterization has shown that the excitonic transitions were continuously tunable with the ratio of S and Se. The parameters that describe the temperature variation of the energies of the excitonic transitions are evaluated and discussed.

Multivalley enhanced electron–phonon coupling and potential Ising superconductivity in monolayer aW-antimonene

Two dimensional group 15 monoelemental materials have ignited intense and wide research interests in condensed matter. aW-antimonene has suitable band gap, ultrahigh carrier mobility and experimental confirmed outstanding environmental stability, which is competitive for future electronic and optoelectronic applications. In this paper, we investigated the electron–phonon coupling (EPC) and superconductivity of aW-antimonene by first principles calculations. At optimal doping concentration multiple valleys are occupied, resulting in a remarkable increasing of EPC. In addition, mechanical strains change the electronic structure and distribution of EPC. Biaxial tensile strain significantly reduces the EPC strength, while strain along x direction increases EPC. Finally, the spin texture of noncentrosymmetric aW-antimonene fits the requirement of type-I Ising superconductor, therefore, aW-antimonene is highly expected to be a novel candidate for realizing Ising superconductivity.

Electron–phonon coupling enhancement and displacive magnetostructural transition in SrCr2 As2 under magneto-Raman spectroscopy

A Raman spectroscopy study on high quality single crystals of SrCr2 As2 (SCA) in the temperature T range 4 K < T < 300 K and high applied magnetic fields up to H = 9 T is presented. The chromium B 1g phonon analysis reveals two anomalous shifts in the frequency, the first below T = 250 K at H = 0 T in the saturated AFM G-type order likely due to an enhanced electron–phonon coupling by the magnetic order, whereas the second anomaly occurs above H = 4 T at T = 4 K likely as a consequence of a magnetostructural displacive transition. Renormalization of the electronic Raman spectra in both studies reveals a decrease in the electronic density of states with decreasing T and increasing H, respectively, with consequent changes in the Fermi surface, which are intrinsically related to the observed anomalies.

Enhanced Electron–Phonon Coupling and Superconductivity in Two-dimensional BC2N via Lithium Deposition: a First-Principles Investigation

Two-dimensional (2D) superconductors are important both for the basic understanding of pairing mechanism and potential applications in nanosuperconducting quantum interference devices. In this work, we explore the phonon-mediated superconductivity of hole-doped and lithium-deposited monolayer BC2N by first-principles calculations. Our results reveal that the hole-doped planar BC2N cannot superconduct due to weak electron–phonon coupling (EPC) with λ = 0.07. When lithium is deposited on the monolayer, the EPC constant is enhanced significantly to 0.52 and the superconducting transition temperature Tc is determined to be 3.58 K. Because more phonons, in particular the out-of-plane modes, are triggered to participate in the EPC process, the Tc is higher than 1.4 K of Ca-deposited graphene, 1.3 K of Li-deposited stanene, and 3 K of doped WS2 and NbSe2. It is also found that tensile biaxial strain weakens the EPC and leads to a decreased superconducting transition temperature. Our findings will provide a new guideline for designing novel 2D superconductors and enrich the potential application of 2D BC2N.

Phonon softening and higher-order anharmonic effect in the superconducting topological insulator SrxBi2Se3

We report the anharmonic effect of the Raman-scattering spectrum in the low-carrier density superconductor SrxBi2Se3, which is dominated by the quartic term. Compared to the parent Bi2Se3, the superconducting SrxBi2Se3 crystals show obvious phonon softening in the three optical phonon modes , , and . Based on the simulations by the Fano function, we present compelling evidence of the enhanced electron–phonon coupling in SrxBi2Se3 for its smaller asymmetric parameter q. Moreover, an anomalous broadening and intensity antiresonance in the characteristic Raman peaks were observed as the temperature decreased to around 160 K. Thus, the superconducting topological material SrxBi2Se3 represents a counterintuitive example where the electron–phonon interaction is strengthened by the accumulation of electron carriers.