Phonon Coupling Research Articles

A ten-fold coordinated high-pressure structure in hafnium dihydrogen with increasing superconducting transition temperature induced by enhancive pressure

High pressure is an effective method to induce structural and electronic changes, creating novel high-pressure structures with excellent physical and chemical properties. Herein, we investigate the structural phase transition of hafnium dihydrogen (HfH2) in a pressure range of 0 GPa–500 GPa through the first-principles calculations and the crystal structure analysis by particle swarm optimization (CALYPSO) code. The high-pressure phase transition sequence of HfH2 is I4/mmm → Cmma → P-3m1 and the two phase transition pressure points are 220.21 GPa and 359.18 GPa, respectively. A newly trigonal P-3m1 structure with 10-fold coordination first appears as an energy superior structure under high pressure. These three structures are all metallic with the internal ionic bonding of Hf and H atoms. Moreover, the superconducting transition temperature (T c) values of Cmma at 300 GPa and P-3m1 at 500 GPa are 3.439 K and 19.737 K, respectively. Interestingly, the superconducting transition temperature of the P-3m1 structure presents an upward trend with the pressure rising, which can be attributed to the increase of electron–phonon coupling caused by the enhanced Hf-d electronic density of states at Fermi level under high pressure.

Enhancing thermal transport across diamond/graphene heterostructure interface

The thermal properties of two-dimensional materials and their heterostructure are critical for efficient heat dissipation in nano-devices. A good example is graphene which exhibits excellent in-plane thermal transport properties. However, the substantial interfacial thermal resistance between graphene and the substrate greatly hinders its practical application. Diamond is a good choice as a substrate to reduce out-of-plane phonon scattering when graphene is contacted with the substrate because of their high structural similarity. Based on non-equilibrium molecular dynamics simulations, the effects of graphene layer count and the temperature on the thermal conductance of diamond/graphene heterostructure are investigated. The results show that the interfacial thermal conductance of diamond/single-layer graphene heterostructure is at least double that of diamond/multi-layer graphene heterostructure. Moreover, high temperature is also conducive to thermal transport for diamond/graphene heterostructure. Due to the anisotropy of graphene, the in-plane and out-of-plane phonon density of state were analyzed. The trend of overlap energy of out-of-plane phonon density of state is consistent with that of the interfacial thermal conductance, which suggests that out-of-plane phonon has a greater effect on heat transport at the interface. The increasing temperature excites more high-frequency phonons, and thus, promotes the phonon coupling of diamond and graphene. This well explains the increases in interfacial thermal conductance at a higher temperature.

Strong Electron–Phonon Coupling Induced Self‐Trapped Excitons in Double Halide Perovskites

Double halide perovskites exhibit impressive potential for the self‐trapped exciton (STEs) luminescence. However, the detailed mechanism of the physical nature during the formation process of STEs in double perovskites is still ambiguous. Herein, theoretical research on a series of double halide perovskites Cs2B1B2Cl6 (B1 = Na+, K+; B2 = Al3+, Ga3+, In3+) regarding their electronic structures, exciton characteristics, electron–phonon coupling performances, and geometrical configuration is conducted. These materials have flat valence band edges and thus possess localized heavy holes. They also show high exciton binding energies, and their short exciton Bohr radius indicates that the spatial size of their excitons is comparable to the dimension of their single lattice. Based on the Fröhlich coupling constant and Feynman polaron radius, the stronger electron–phonon coupling strength in Ga‐series double halide perovskites is revealed. In particular, Cs2NaGaCl6 shows a high and effective Huang–Rhys factor of 36.21. The phonon characteristics and vibration modes of Cs2NaGaCl6 are further analyzed, and the Jahn–Teller distortion of the metal–halogen octahedron induced by hole‐trapping after excitation is responsible for the existence of STEs. This study strengthens the physical understanding of STEs and provides effective guidance for the design of advanced solid‐state phosphors.

Spin–Phonon Interactions and Anharmonic Lattice Dynamics in Fe3GeTe2

AbstractRaman scattering is performed on Fe3GeTe2 (FGT) at temperatures from 8 to 300 K and under pressures from the ambient pressure to 9.43 GPa. Temperature‐dependent and pressure‐dependent Raman spectra are reported. The results reveal respective anomalous softening and moderate stiffening of the two Raman active modes as a result of the increase of pressure. The anomalous softening suggests anharmonic phonon dynamics and strong spin–phonon coupling. Pressure‐dependent density functional theory and phonon calculations are conducted and used to study the magnetic properties of FGT and assign the observed Raman modes, and . The calculations proved the strong spin–phonon coupling for the mode. In addition, a synergistic interplay of pressure‐induced reduction of spin exchange interactions and spin–orbit coupling effect accounts for the softening of the mode as pressure increases.

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.

Effect of Electron–Phonon Coupling on the Color Purity of Two-Dimensional Ruddlesden–Popper Hybrid Lead Iodide Perovskites

Two-dimensional (2D) lead halide perovskites have become a class of promising luminescent materials due to their excitonic nature and excellent environmental stability. However, it seems counterintuitive that the 2D perovskites possess extremely narrow emission, while at the same time they have been reported to reveal strong electron–phonon coupling, which generally leads to significant emission broadening. Here, the main factors affecting the electron–phonon coupling of 2D perovskites are investigated to interpret the observed light emission with full width at half maximum as small as 12.7 nm (∼58 meV) at room temperature (RT). We demonstrate that the electron–phonon coupling in 2D perovskites is structure-dependent, and coupling with the phonon mode in the organic layer rather than the inorganic one plays an important role in the broadening of emission. This scenario is evidenced by temperature-dependent photoluminescence spectroscopy and structural change of 2D perovskites with various bulky organic cations (L-series) and different numbers of inorganic layers (n-series). Our results suggest that the electron–phonon coupling in L-series 2D perovskites could be weak below 250 K, which leads to a small broadening, thus mainly contributing to high color purity at RT. While for the n-series, the increase of inorganic layer number induces a monotonic increase of the line width broadening in the low-temperature region, which reduces the color purity at RT.

Observation of photoinduced polarons in semimetal 1T-TiSe2

In this work, ultrafast carrier dynamics of mechanically exfoliated 1T-TiSe2 flakes from the high-quality single crystals with self-intercalated Ti atoms are investigated by femtosecond transient absorption spectroscopy. The observed coherent acoustic and optical phonon oscillations after ultrafast photoexcitation reveal the strong electron–phonon coupling in 1T-TiSe2. The ultrafast carrier dynamics probed in both visible and mid-infrared regions indicate that some photogenerated carriers localize near the intercalated Ti atoms and form small polarons rapidly within several picoseconds after photoexcitation due to the strong and short-range electron–phonon coupling. The formation of polarons leads to a reduction of carrier mobility and a long-time relaxation process of photoexcited carriers for several nanoseconds. The formation and dissociation rates of the photoinduced polarons are dependent on both the pump fluence and the thickness of TiSe2 sample. This work offers new insights into the photogenerated carrier dynamics of 1T-TiSe2, and emphasizes the effects of intercalated atoms on the electron and lattice dynamics after photoexcitation.

Analyses of the R1-line thermal shifts for Bi2Al4O9: Cr3+ and Bi2Ga4O9: Cr3+ crystals with a complete equation

ABSTRACT The thermal red-shifts of R1-line for Cr3+-doped Bi2Al4O9 and Bi2Ga4O9 crystals are analyzed with a complete equation containing not only the dynamic factor (used previously) due to electron–phonon interaction, but also the static factor caused by lattice thermal expansion. From the analysis, the static parameters A (charactering the static factor), the more reasonable electron–phonon coupling parameters(charactering the dynamic factor) and hence the more accurate quadratic coupling constants w for the two crystals are gained. It is found that the static factors are about 17% and 9% of the dynamic factors for Bi2Al4O9: Cr3+ and Bi2Ga4O9: Cr3+ crystals, respectively. So, in the accurate and reasonable analysis of R1-line thermal shifts in the two crystals (or other spectral line thermal shifts in crystals), the static factor needs to be considered and the complete equation including both factors should be adopted.

Synthetic magnetism for solitons in optomechanical array

Discrete optical and mechanical solitonic waves are widely used for photonic/phononic information processing. Here we propose a synthetic magnetism to generate and to control solitonic waves in 1D−optomechanical array. Each optomechanical cavity in the array couples to its neighbors through photon and phonon coupling. We create the synthetic magnetism by modulating the phonon hopping rate through both the modulation of frequency and phase between resonators at different sites. When the synthetic magnetism effect is not taken into account, the mechanical coupling play a crucial role of controlling and switching the waves from bright to dark solitons, and it even induces rogue wave-like a shape in the array. For enough mechanical coupling strength, the system enters into a strong coupling regime through splitting/crossing of solitonic waves, leading to multiple waves propagation in the array. Under the synthetic magnetism effect, the phase of the modulation enables a good control of the wave propagation, and it also switches soliton shape from bright to dark, and even induces rogue waves as well. Similarly to the mechanical coupling, the synthetic magnetism offers another flexible way to generate plethora of solitonic waves for specific purposes. This work opens new avenues for optomechanical platforms and sheets light on their potentiality of controlling and switching solitonic waves based on synthetic magnetism.

Fine Structure Splitting of Phonon-Assisted Excitonic Transition in (PEA)2PbI4 Two-Dimensional Perovskites

Two-dimensional van der Waals materials exhibit particularly strong excitonic effects, which causes them to be an exceptionally interesting platform for the investigation of exciton physics. A notable example is the two-dimensional Ruddlesden-Popper perovskites, where quantum and dielectric confinement together with soft, polar, and low symmetry lattice create a unique background for electron and hole interaction. Here, with the use of polarization-resolved optical spectroscopy, we have demonstrated that the simultaneous presence of tightly bound excitons, together with strong exciton-phonon coupling, allows for observing the exciton fine structure splitting of the phonon-assisted transitions of two-dimensional perovskite (PEA)2PbI4, where PEA stands for phenylethylammonium. We demonstrate that the phonon-assisted sidebands characteristic for (PEA)2PbI4 are split and linearly polarized, mimicking the characteristics of the corresponding zero-phonon lines. Interestingly, the splitting of differently polarized phonon-assisted transitions can be different from that of the zero-phonon lines. We attribute this effect to the selective coupling of linearly polarized exciton states to non-degenerate phonon modes of different symmetries resulting from the low symmetry of (PEA)2PbI4 lattice.

Efficient Yellow Emission and Near-Unified Photoluminescence Quantum Yield of Sb3+ in a One-Dimensional Confinement Cadmium Chloride Lattice

Sb3+, with a 5s2 electron configuration, has been widely studied as a dopant for its efficient self-trapped exciton (STE) broadband emission. However, the efficient emission of Sb3+-doped one-dimensional (1D) structures and the energy/charge transfer between the host and dopant are rarely reported. In this paper, a series of Sb3+-doped 1D (TDMP)CdCl4 (TDMP = trans-2,5-dimethylpiperazine) powder crystals were prepared by the solvothermal method. The 1D quantum confinement effect and Jahn–Teller distortion in this organic–inorganic hybrid with a soft lattice generate strong electron–phonon coupling. In addition, we also found a fast energy transfer process between the host ([CdCl6]2–) and the dopant ([SbCl6]3–). The synergy of strong electron–phonon coupling and fast energy transfer enables 8.8% Sb3+:(TDMP)CdCl4 to exhibit efficient broadband yellow emission under UV excitation with a photoluminescence quantum yield (PLQY) of 99.69%. DFT calculations indicate that the emission comes from the triplet self-trapped excitons (STEs) emission of the [SbCl6]3– octahedron. This work provides a deeper understanding of the emission of Sb3+ in doped systems and provides a strategy to guide the design of future efficient luminescent materials.

Ab initio study of pressure-dependent phonon heat conduction in cubic boron nitride

Phonon transport in cubic boron nitride (c-BN) at elevated pressure in several tens GPa is potentially of interest for cooling high-power electronics. In this work, we study the pressure-dependent thermal conductivity (up to 20 GPa) of isotope-engineered c-BN by solving the Boltzmann transport equation with inputs only from ab initio calculations. Four boron isotope compositions are analyzed, including natural abundance, enriched 10B and 11B, and a roughly equal mixture of the two. In all cases, the thermal conductivity of c-BN increases linearly with pressure. Compared to isotope-enriched c-BN, the thermal conductivity of isotopically mixed c-BN is less sensitive to pressure variations since phonon-isotope scattering causes substantial thermal resistance yet is pressure independent. Based on mode-resolved analysis, the thermal conductivity enhancement is mainly attributed to the suppression of the aao absorption processes with transverse-acoustic phonons in the frequency range from 300 to 600 cm–1, due to the synergistic effect between decreased coupling strength and phonon hardening. In addition, suppression of the aaa processes is also responsible for a slight increase in thermal conductivity. Our results are further supported by the frequency- and mean-free-path-dependent spectra of the thermal conductivity. This work provided a fundamental interpretation of pressure-dependent phonon transport in c-BN from a microscopic perspective.

Surface carrier dynamics and diffusion in inorganic metal-halide perovskite films

Carrier dynamics in metal-halide perovskites, in particular diffusion, has typically been investigated on micrometer-length scales and above. However, surfaces and interfaces modify the carrier dynamics, e.g., by interface band bending and recombination. To investigate the carrier dynamics and diffusion at the surface, we use time-resolved photoelectron spectroscopy which is sensitive to the photoexcited carriers in the topmost few nm of the material. We extract electron mobilities in well-ordered ${\mathrm{CsPbBr}}_{3}$(001) and ${\mathrm{CsSnBr}}_{3}$(001) films at room temperature of $31\ifmmode\pm\else\textpm\fi{}6$ and $13\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. The mobility in ${\mathrm{CsPbBr}}_{3}$(001) increases to $200\ifmmode\pm\else\textpm\fi{}8\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 90 K, indicating strong electron-phonon coupling for electrons at the conduction band minimum. Temperature-dependent photoemission experiments find different phonon coupling mechanisms for high-energetic electrons and holes at the valence band maximum and in the main valence bands.

Regulating Polaron Transport Regime via Heterojunction Engineering in Cove‐Type Graphene Nanoribbons

AbstractGraphene nanoribbons (GNRs) are emerging materials inheriting excellent properties from graphene while potentially exhibiting semiconducting behavior. All these features sparked numerous efforts to insert GNRs in nanoelectronics. As a result, synthesis routes with atomic resolution are now a reality. Recently, the rise of heterojunction (HJ) engineering pushed even further the prospects, allowing the blending of different GNRs as building blocks. However, much of the potential behind it remains untouched for some junctions. In this work, the consequences of forming a cove‐type GNR (CGNR) HJ by assembling specimens with borders of different zig‐zag/armchair ratios are explored. The nanoribbons are simulated using the extended two‐dimensional Su–Schrieffer–Heeger model with electron–phonon coupling. The findings show that manipulating the junction creates multiple routes for smooth monotonic gap tuning. Moreover, the changes in the hopping mechanism, mobility, and effective mass are reported leading to variations up to 10 000 cm2 V−1 s−1 and 0.425 me. This work reveals a pathway to expand the modularity of CGNRs through smooth control of the charge carrier's properties. Future applications can explore this feature to design devices with highly specific charge transport characteristics. The study also serves as theoretical background, potentially inspiring new tuning strategies in other GNRs.

Elastic Phonon Scattering Dominates Dephasing in Weakly Confined Cesium Lead Bromide Nanocrystals at Cryogenic Temperatures.

Cesium lead halide perovskite nanocrystals (PNCs) have emerged as a potential next-generation single quantum emitter (QE) material for quantum optics and quantum information science. Optical dephasing processes at cryogenic temperatures are critical to the quality of a QE, making a mechanistic understanding of coherence losses of fundamental interest. We use photon-correlation Fourier spectroscopy (PCFS) to obtain a lower bound to the optical coherence times of single PNCs as a function of temperature. We find that 20 nm CsPbBr3 PNCs emit nearly exclusively into a narrow zero-phonon line from 4 to 13 K. Remarkably, no spectral diffusion is observed at time scales of 10 μs to 5 ms. Our results suggest that exciton dephasing in this temperature range is dominated by elastic scattering from phonon modes with characteristic frequencies of 1-3 meV, while inelastic scattering is minimal due to weak exciton-phonon coupling.

Fluctuating fractionalized spins in quasi-two-dimensional magnetic V0.85PS3

Quantum spin liquid (QSL), a state characterized by exotic low energy fractionalized excitations and statistics, is still elusive experimentally and may be gauged via indirect experimental signatures. A remnant of the QSL phase may reflect in the spin dynamics as well as quanta of lattice vibrations, i.e., phonons, via the strong coupling of phonons with underlying fractionalized excitations. Inelastic light scattering (Raman) studies on ${\mathrm{V}}_{1--x}\mathrm{P}{\mathrm{S}}_{3}$ single crystals evidence the spin fractionalization into Majorana fermions deep into the paramagnetic phase reflected in the emergence of a low frequency quasielastic response, along with a broad magnetic continuum marked by a crossover temperature ${T}^{*}\ensuremath{\sim}200$ K from a pure paramagnetic state to a fractionalized spin regime qualitatively gauged via dynamic Raman susceptibility. We found further evidence of anomalies in the phonons' self-energy parameters, in particular, phonon line broadening and line asymmetry evolution at this crossover temperature, attributed to the decaying of phonons into itinerant Majorana fermions. This anomalous scattering response is thus indicative of fluctuating fractionalized spins suggesting a phase proximate to the quantum spin liquid state in this quasi-two-dimensional magnetic system.

Phonon coupling versus pure dephasing in the photon statistics of cooperative emitters

Realizing scalable quantum networks requires a meticulous level of understanding and mitigating the deleterious effects of decoherence. Many quantum device platforms feature multiple decoherence mechanisms, often with a dominant mechanism seemingly fully masking others. In this paper, we show how access to weaker dephasing mechanisms can nevertheless be obtained for optically active qubits by performing two-photon coincidence measurements. To this end we theoretically investigate the impact of different decoherence mechanisms on cooperatively emitting quantum dots. Focusing on the typically dominant deformation-potential coupling to longitudinal acoustic phonons and typically much less severe additional sources of pure dephasing, we employ a numerically exact method to show that these mechanisms lead to very different two-photon coincidence signals. Moreover, surprisingly, the impact of the strongly coupled phonon environment is weak and leads to long-lived coherences. We trace this back to the superohmic nature of the deformation-potential coupling causing interemitter coherences to converge to a nonzero value on a short timescale, whereas pure dephasing contributions cause a complete decay of coherence over longer times. Our approach provides a practical means of investigating decoherence processes on different timescales in solid-state emitters, and thus contributes to understanding and possibly eliminating their detrimental influences.

Dynamic Disorder Drives Exciton Dynamics in Diketopyrrolopyrrole–Thiophene-Containing Molecular Crystals

There is a growing interest in controllable molecular materials for potential nanophotonic and quantum information applications where excitons move beyond the incoherent transport regime. Thus, the ability to identify the key parameters that correlate with the efficiency of the transport of the excitation energy is highly desirable. In this work, we investigate the effects of the dynamic disorder on the transport of the exciton in molecular crystals of several mono- and dialkylated 1,4-diketo-3,6-dithiophenylpyrrolo[3-4-c]pyrrole derivatives. These systems exhibit great potential for photovoltaic applications due to their broad optical absorption and efficient charge transport. The exciton dynamics are studied using a model Hamiltonian, in which the thermal fluctuations of the excitonic coupling (nonlocal electron–phonon coupling), as well as the local exciton–phonon couplings, have been appropriately taken into account. The computed reorganization energies for the most feasible transport pathway (π–π stacking) for the excitons in UBEQUQ and UBEQOK molecular crystals are 0.366 and 0.357 eV, respectively. These values are comparable with the magnitude of the average excitonic coupling ⟨J⟩ (≈ 0.1 eV) for these two molecular crystals. In this instance, the local exciton–phonon coupling is not large enough to form a small exciton-polaron. In addition, substantial coherences are observed on a time scale of less than 100 fs, which indicates that the dynamic disorder is sufficient to overcome quantum dephasing and help drive exciton transport in this class of organic semiconductors. On the other hand, the diffusion of the excitons reduces significantly when the thermal fluctuations of the excitonic coupling are omitted. Thus, dynamic disorder plays a vital role in the transport of the exciton, and the ability to control this inherent property in molecular aggregates will provide valuable tools for the design and development of efficient organic semiconductors.

Red emission material from Eu3+ ion doped in oxy-fluoroborate glass system

The aim of the work is to prepare (51-x)B2O3 + 23CaF2 + 10Li2O + 10Na2O + 6BaO + xEu2O3 glasses with various concentration, analyse the optical/radiative properties and compare with other reported literature. In this work, the Judd-Ofelt analysis was employed to evaluate oscillator strength, JO parameters. Using JO parameters, radiative properties of the prepared samples were evaluated to understand their branching ratios, stimulated emission cross-section and radiative lifetime. The phonon coupling constant was evaluated using phonon sideband and compared with other literatures. The present glasses doped with Eu3+ ions find a potential use in orange red light emitting optical display device applications.

Modeling laser interactions with aluminum and tantalum targets using a hybrid atomistic-continuum model

A hybrid atomistic-continuum method can model the microstructure evolution of metals subjected to laser irradiation. This method combines classical molecular dynamics (MD) simulations with the two-temperature model (TTM) to account for the laser energy absorption and heat diffusion behavior. Accurate prediction of the temperature evolution in the combined MD-TTM method requires reliable accuracy in electron heat capacity, electron thermal conductivity, and electron–phonon coupling factor across the temperatures generated. This study uses the electronic density of states (DOS) obtained from first-principle calculations. The calculated electron temperature-dependent parameters are used in MD-TTM simulations to study the laser metal interactions in FCC and BCC metals and the phenomenon of laser shock loading and melting. This study uses FCC Al and BCC Ta as model systems to demonstrate this capability. When subjected to short pulsed laser shocks, the dynamic failure behavior predicted using temperature-dependent parameters is compared with the experimentally reported single-crystal and nanocrystalline Al and Ta systems. The MD-TTM simulations also investigate laser ablation and melting behavior of Ta to compare with the ablation threshold reported experimentally. This manuscript demonstrates that integrating the temperature-dependent parameters into MD-TTM simulations leads to the accurate modeling of the laser–metal interaction and allows the prediction of the kinetics of the solid–liquid interface.