Phonon Coupling Strength Research Articles

Phonon-assisted excitonic absorption in diamond

The phonon-assisted optical transitions of excitons in intrinsic diamond have been investigated theoretically and experimentally. Excellent agreement is indicated between absorption spectra measured and calculated based on the phonon coupling strengths from first-principles calculations using just a single adjustable parameter. Temperature shift and broadening are incorporated via a pseudo-Voigt function analysis developed for the derivative of the experimental absorption spectra. The absorption coefficients of excitons in discrete and continuum states are separated and contributions from different phonon modes are discussed, which enable theoretical derivation of the exceptionally high phonon-assisted radiative rate of excitons in diamond among indirect-gap semiconductors. This study expands the applicability of the second-order perturbation theory involving electron-phonon interactions in materials important for future photonics and power electronics.

Unvailing the spin–phonon coupling in nanocrystalline BiFeO3 by resonant two-phonon Raman active modes

We report on temperature dependence of two-phonon Raman spectra in BiFeO3 nanocrystals, above and below the Néel temperature TN using a resonant laser excitation line (λ=532 nm). Two-phonon modes exhibited anomalous frequency hardening and deviation from the anharmonic decay below TN. Such behavior strongly supported the existence of spin-two-phonon interaction, because these modes are known to be very sensitive to the antiferromagnetic ordering. Within the mean-field theory for the nearest-neighbor interaction, the linear relationship between spin–spin correlation function and observed two-phonon frequency shift below TN was obtained. This approach enabled to quantify the spin–phonon interaction by spin–phonon coupling strength for both two-phonon modes and justified the application of mean-field approach. Magnetic measurements revealed the coexistence of antiferromagnetic and weak ferromagnetic phases below TN, which were found non competitive, additionally supporting the mean-field approach from which we deduced that the two-phonon modes in BiFeO3 are correlated with antiferromagnetic ordering below TN.

Photoluminescence and excited states dynamics of Tm2+-doped CsCa(Cl/Br)3 and CsCa(Br/I)3 perovskites

In this study, we systematically vary the Cl/Br and Br/I ratios in CsCaX3:Tm2+ (X = Cl, Br, I) and hereby gradually shift the positions of the Tm2+ 4f125d1-levels as relative to the two 4f13 levels. At low temperatures up to five distinct Tm2+ 4f125d1→4f13 emissions and the 4f13→4f13 emission can be observed. As the temperature increases, most of the 4f125d1→4f13 emissions undergo quenching via multi-phonon relaxation (MPR) and at room temperature only the lowest energy 4f125d1→4f13 and the 4f13→4f13 emission remains. For all compositions a 4f13→4f13 risetime phenomenon is then observed whose duration matches the 4f125d1→4f13 decay time. It shows the feeding of the 4f13 state after 4f125d1 excitation. Surprisingly, the feeding time becomes longer from Cl→Br→I, while the related 4f125d1-4f13 energy gap becomes smaller. The temperature dependence of the 4f125d1→4f13 and 4f13→4f13 emission intensity shows a anticorrelation as earlier observed in other systems and confirms that the feeding process is thermally stimulated. However, the thermally stimulated activation energies that control the feeding process, increase from Cl→Br→I despite our observation that the 4f125d1-4f13 energy gap becomes smaller. An analysis reveals that the unexpected behaviour in risetime and activation energy, as a function of composition, cannot be explained by 4f125d1→4f13 feeding via interband crossing, but more likely via MPR where the electron–phonon coupling strength decreases from Cl→Br→I. No strong relation was found between composition and the quantum efficiency (QE) of the 4f13→4f13 emission, due to the presence of fluctuations that are likely caused by intrinsic differences in sample quality. Nevertheless, a 4f13→4f13 QE of up to 70% has been observed and the materials can therefore be used in luminescence solar concentrators.

Time-Resolved Raman Scattering in Exfoliated and CVD Graphene Crystals

Phonon lifetimes are fundamental physical parameters playing a significant role in phonon transport and thermal conductivity of two-dimensional materials. We employed the time-resolved incoherent anti-Stokes Raman scattering (TRIARS) technique to capture the G phonon dynamics and to extract the G phonon lifetimes in supported exfoliated and polycrystalline chemical vapor deposited (CVD) graphene samples of various thickness. We have found that substrate reduces considerably the G phonon lifetime of exfoliated monolayer graphene while the corresponding lifetimes for bilayer and trilayer converge gradually close to that of graphite. The lifetimes of CVD monolayers are systematically lower than exfoliated ones mainly due to the presence of various types of structural defects. Besides, the electron–phonon coupling strength of graphite is determined to be 10.6 cm–1, in excellent agreement with theoretical predictions for graphene, justifying that the lifetimes of suspended graphene and graphite are similar.

Possible high-T C superconductivity at 50 GPa in sodium hydride with clathrate structure

Ambient-pressure room-temperature superconductivity is one ultimate goal of science, for it will bring worldwide revolutionary changes in all kinds of technology. Several room temperature and near room temperature hydride superconductors at ultra high pressure (≳100 GPa) have been predicted theoretically. In particular, the hydrogen sulfide (H3S) with T C ≃ 203 K at 200 GPa has soon been confirmed experimentally, establishing a milestone toward room temperature superconductivity. However, high-T C superconductors at lower pressure (≲100 GPa) have not been reported before. In this work, we present high-T C superconductivity of 180 K at a relatively low pressure of 50 GPa in sodium hydride clathrate structure NaH6. The T C can be raised up to 206 K at 100 GPa, similar to the T C of H3S but at a much lower pressure. At 200 GPa, it reaches the highest T C of 210 K, slightly higher than that of H3S. The strong electron–phonon coupling strength given by the T 2g phonon mode at Γ point plays the key role in superconductivity. Our work demonstrates theoretically that hydrides could stabilize at a relatively low pressure and host high-T C superconductivity.

Controlling Voronoi partitions on femtosecond-laser-superheated metal surfaces

Ultrafast and high intensity laser pulses can drive materials into highly non-equilibrium conditions. At a sufficiently high intensity, femtosecond laser pulses can turn a solid material into a superheated solid or a superheated liquid state. For metals with low electron–phonon coupling strength, e.g., gold (Au), a superheated liquid state can lead to the formation of a family of unique surface structures, the so-called Voronoi partitions (VPs). Although chaotic in nature, we demonstrate here a control of VP formation through varying the laser fluence and pulse number, resulting in a rich variety of VPs. To understand the formation of complex surface geometries under laser irradiation, we utilizes a numerical approach that integrates the finite difference time domain of electromagnetic field method with the two-temperature model. By studying the spatio-temporal distribution of electromagnetic and temperature fields and by analyzing the geometric properties of the VPs, we provide a framework for understanding the emerging surface patterns. With VPs, novel structures are generated with promising potential applications, namely, Gecko-skin-like nano-spikes and nano-cauliflower-like surface structures.

Relaxation dynamics of hot electrons in the transition metals Au, Ag, Cu, Pt, Pd, and Ni studied by ultrafast luminescence spectroscopy

The ultrafast relaxation dynamics of photoexcited electrons in six transition metals, Au, Ag, Cu, Pt, Pd, and Ni, were investigated using femtosecond luminescence spectroscopy in the infrared region between 0.4 and 1.05 eV. The behaviors of the time-resolved spectra are significantly different between group 11 noble metals (Au, Ag, and Cu) and group 10 transition metals (Pt, Pd, and Ni), which are neighbors in the periodic table of elements. In the latter group, the instantaneous luminescence intensities are one order of magnitude lower and the lifetimes (around 200 fs) are far shorter than those of the group 11 metals (typically 700 fs). The time-resolved spectra, decay profiles, and excitation power dependence were analyzed using a phenomenological model that considers both the nonthermal and thermal electrons. It was found that the nonthermal component is remarkably small in the group 10 transition metals. These systematic differences between group 10 and 11 metals are ascribed to the differences in their electron band structures and/or electron–phonon coupling strengths.

Enhanced exciton binding energy, Zeeman splitting and spin polarization in hybrid layered nanosheets comprised of (Cd, Mn)Se and nitrogen-doped graphene oxide: implication for semiconductor devices

The exciton properties of (Cd,Mn)Se-NrGO (nitrogen doped reduced graphene oxide) hybrid layered nanosheets have been studied in a magnetic field up to 10 T and compared to those of (Cd,Mn)Se nanosheets. The temperature dependent photoluminescence reveals the hybridization of inter-band exciton and intra-center Mn transition with enhancement of the binding energy of exciton-Mn hybridized state (80 meV with respect to 60 meV in (Cd,Mn)Se nanosheets) and increase of exciton—phonon coupling strength to 90 meV (with respect to 55 meV in (Cd,Mn)Se nanosheets). The circularly polarized magneto—photoluminescence at 2 K provides evidence for magnetic field induced exciton spin polarization and the realization of excitonic giant Zeeman splitting with g eff as high as 165.4 ± 10.3, much larger than in the case of (Cd,Mn)Se nanosheets (63.9 ± 6.6), promising for implementation in spin active semiconductor devices.

Probability density and oscillating period of magnetopolaron in parabolic quantum dot in the presence of Rashba effect and temperature* *Project supported by the National Natural Science Foundation of China (Grant No. 11975011).

We study the property of magnetopolaron in a parabolic quantum dot under the Rashba spin–orbit interaction (RSOI) by adopting an unitary transformation of Lee–Low–Pines type and the variational method of Pekar type with and without considering the temperature. The temporal spatial distribution of the probability density and the relationships of the oscillating period with the RSOI constant, confinement constant, electron–phonon coupling strength, phonon wave vector and temperature are discussed. The results show that the probability density of the magnetopolaron in the superposition of the ground and first excited state takes periodic oscillation (T 0/period) in the presence or absence of temperature. Because of the RSOI, the oscillating period is divided into different branches. Also, the results indicate that the oscillating period increases (decreases) when the RSOI constant, electron-phonon coupling strength and phonon wave vector (the confinement constant) increase in a proper temperature, and the temperature plays a significant role in determining the properties of the polaron.

Superconductivity and strong anharmonicity in novel Nb–S phases

In this work we explore the phase diagram of the binary Nb–S system from ambient pressures up to 250 GPa using ab initio evolutionary crystal structure prediction. We find several new stable compositions and phases, especially in the high-pressure regime, and investigate their electronic, vibrational, and superconducting properties. Our calculations show that all materials, besides the low-pressure phases of pure sulfur, are metals with low electron–phonon (ep) coupling strengths and critical superconducting temperatures below 15 K. Furthermore, we investigate the effects of phonon anharmonicity on lattice dynamics, ep interactions, and superconductivity for the novel high-pressure phase of Nb2S, demonstrating that the inclusion of anharmonicity stabilizes the lattice and enhances the ep interaction.

Experimental evidence of polarons in silver doped ZnO films

Pure and silver (0.27 at.%)-doped zinc oxide (Ag:ZnO) films were deposited on sapphire substrates by e-beam evaporation technique at 290 °C and post-annealed at 350 °C for 1 h under ambient air. The XRD analysis confirms the formation of wurtzite hexagonal structure. From the UV–Vis absorption spectra, the incorporation of Ag into the ZnO involves a slight decrease in the optical band gap as compared to that of undoped ZnO film. Infrared reflection spectral data of the Ag:ZnO films indicate hole polaron states placed in the range of 0.136–0.439 eV above the valence band maximum. According to the calculated electron–phonon coupling strength (1.04) and binding energy (− 76 meV), the polaron of large radius is defined. The polaron optical conductivity and carrier relaxation time are determined from the experimental spectra using Kramers–Kronig relations.

Exciton transfer in organic photovoltaic cells: A role of local and nonlocal electron-phonon interactions in a donor domain.

We theoretically investigate an exciton transfer process in a donor domain of organic photovoltaic cells focusing on the roles of local and nonlocal electron-phonon interactions. Our model consists of a three-level system described by the Holstein-Peierls Hamiltonian coupled to multiple heat baths for local and nonlocal molecular modes characterized by Brownian spectral distribution functions. We chose tetracene as a reference donor molecule, where the spectral distribution functions of the local and nonlocal modes are available. We then employ the reduced hierarchical equations of motion approach to simulate the dynamics of the system under the influence of the environment as a function of the electron-phonon coupling strength and temperature. We rigorously calculate the reduced density matrix elements to explain the time scale of dynamics under the influence of the dissipative local and nonlocal modes. The results indicate that the strong nonlocal electron-phonon interaction under high temperature conditions favors the exciton transfer process and enhances the efficiency of organic photovoltaic materials, while the lifetime of the exciton becomes shorter due to a low-frequency local mode.

Composition and dimension dependent static and dynamic stabilities of inorganic mixed halide antimony perovskites

We systematically show that alloys of low-dimensional mixed-halide antimony perovskites exhibit weaker vibrational anharmonicity and electron–phonon coupling strengths, which may enhance their photo-stabilities compared to 3D halide perovskites.

Unveiling the Excited-State Dynamics of Mn2+ in 0D Cs4PbCl6 Perovskite Nanocrystals.

Doping is an effective strategy for tailoring the optical properties of 0D Cs4PbX6 (X = Cl, Br, and I) perovskite nanocrystals (NCs) and expanding their applications. Herein, a unique approach is reported for the controlled synthesis of pure‐phase Mn2+‐doped Cs4PbCl6 perovskite NCs and the excited‐state dynamics of Mn2+ is unveiled through temperature‐dependent steady‐state and transient photoluminescence (PL) spectroscopy. Because of the spatially confined 0D structure of Cs4PbCl6 perovskite, the NCs exhibit drastically different PL properties of Mn2+ in comparison with their 3D CsPbCl3 analogues, including significantly improved PL quantum yield in solid form (25.8%), unusually long PL lifetime (26.2 ms), large exciton binding energy, strong electron–phonon coupling strength, and an anomalous temperature evolution of Mn2+‐PL decay from a dominant slow decay (in tens of ms scale) at 300 K to a fast decay (in 1 ms scale) at 10 K. These findings provide fundamental insights into the excited‐state dynamics of Mn2+ in 0D Cs4PbCl6 NCs, thus laying a foundation for future design of 0D perovskite NCs through metal ion doping toward versatile applications.

Suppressing Strong Exciton–Phonon Coupling in Blue Perovskite Nanoplatelet Solids by Binary Systems

AbstractQuasi‐two‐dimensional (2D) perovskites are promising candidates for light generation owing to their high radiative rates. However, strong exciton–phonon interactions caused by mechanical softening of the surface act as a bottleneck in improving their suitability for a wide range of lighting and display applications. Moreover, it is not easily available to tune the phonon interactions in bulk films. Here, we adopt bottom‐up fabricated blue emissive perovskite nanoplatelets (NPLs) as model systems to elucidate and as well as tune the phonon interactions via engineering of binary NPL solids. By optimizing component domains, the phonon coupling strength can be reduced by a factor of 2 driven by the delocalization of 2D excitons in out‐of‐plane orientations. It shows the picosecond energy transfer originated from the Förster resonance energy transfer (FRET) efficiently competes with the exciton–phonon interactions in the binary system.

Suppressing Strong Exciton-Phonon Coupling in Blue Perovskite Nanoplatelet Solids by Binary Systems.

Quasi-two-dimensional (2D) perovskites are promising candidates for light generation owing to their high radiative rates. However, strong exciton-phonon interactions caused by mechanical softening of the surface act as a bottleneck in improving their suitability for a wide range of lighting and display applications. Moreover, it is not easily available to tune the phonon interactions in bulk films. Here, we adopt bottom-up fabricated blue emissive perovskite nanoplatelets (NPLs) as model systems to elucidate and as well as tune the phonon interactions via engineering of binary NPL solids. By optimizing component domains, the phonon coupling strength can be reduced by a factor of 2 driven by the delocalization of 2D excitons in out-of-plane orientations. It shows the picosecond energy transfer originated from the Förster resonance energy transfer (FRET) efficiently competes with the exciton-phonon interactions in the binary system.

Temperature Dependence on State Energies and Transition Frequency of Polaron in a Quantum Well with Asymmetric Gaussian Potential: Strong Coupling Method

In the present work, the influence of temperature effect on polaron in a quantum well (QW) with asymmetric Gaussian potential (AGP). By employing strongly coupling method of Pekar variational (SCMPV), the ground state (GS), the first-excited state (FES) energies and transition frequency (TF) of the polaron are derived. The state-energies and the TF as relationship of the temperature T and the electron phonon coupling strength (EPCS) are obtained by using quantum statistics theory (QST). We have found that the state-energies and the TF increase when the temperature is increasing. The states-energies and the TF are decreased with raising the EPCS.

Heat Transport without Heating?—An Ultrafast X‐Ray Perspective into a Metal Heterostructure

AbstractWhen the spatial dimensions of metallic heterostructures shrink below the mean free path of its conduction electrons, the transport of electrons and hence the transport of thermal energy by electrons continuously changes from diffusive to ballistic. Electron–phonon coupling sets the mean free path to the nanoscale and the time for equilibration of electron and lattice temperatures to the picosecond range. A particularly intriguing situation occurs in trilayer heterostructures combining metals with very different electron–phonon coupling strength: Heat energy deposited in few atomic layers of Pt is transported into a nanometric Ni film, which is heated more than the Cu film through which the heat is released. Femtosecond pump‐probe experiments with hard X‐ray pulses provide a layer‐specific probe of the heat energy. A purely diffusive two‐temperature model with increased thermal conductivity of hot electrons excellently reproduces the observed signals from all three layers. At the time when the Ni lattice is maximally heated, no significant heat has entered the Cu lattice. This phenomenon would be enhanced in thinner layers where ballistic transport dominates. In this context it is shown that purely diffusive transport can lead to a linear time‐to‐length dependence that must not be misinterpreted as ballistic transport.

On the Red Antenna States of Photosystem I Mutants from Cyanobacteria Synechocystis PCC 6803.

To identify the molecular composition of the low-energy states in cyanobacterial Photosystem I (PSI) of Synechocystis PCC6803, we focus on high-resolution (low-temperature) absorption, emission, resonant, and nonresonant hole-burned spectra obtained for wild-type (WT) PSI and three PSI mutants. In the Red_a mutant, the B33 chlorophyll (Chl) is added to the B31-B32 dimer; in Red_b, histidine 95 (His95) on PsaB (which coordinates Mg in the B7 Chl within the His95-B7-A31-A32-cluster) is replaced with glutamine (Gln), while in the Red_ab mutant, both mutations are made. We show that the C706 state (B31-B32) changes to the C710 state (B31-B32-B33) in both Red_a and Red_ab mutants, while the C707 state in WT Synechocystis (localized on the His95-B7-A31-A32 cluster) is modified to C716 in both Red_b and Red_ab. Excitation energy transfer from C706 to the C714 trap in the WT PSI and Red_b mutant is hampered as reflected by a weak emission at 712 nm. Large electron-phonon coupling strength (exposed via resonant hole-burned spectra) is consistent with a strong mixing of excited states with intermolecular charge transfer states leading to significantly red-shifted emission spectra. We conclude that excitation energy transfer in PSI is controlled by fine-tuning the electronic states of a small number of highly conserved red states. Finally, we show that mutations modify the protein potential energy landscape as revealed by different shapes and shifts of the blue- and red-shifted antiholes.

Competitive effect of dopant concentration and the size of the nanorods over the electron phonon coupling in Cd doped ZnO nanorod arrays

Earlier reports on resonance Raman scattering in ZnO suggests that the electron phonon coupling in ZnO usually increase with nanoparticle size or decline with the doping concentration. We investigated the effect of combination of the concentration of dopant and the variation of size of nanorods over the electron phonon interaction in Cd doped ZnO nanorod array fabricated by a simple, low temperature (90 °C) growth process. The resonant Raman measurement revealed minor decrease in the electron phonon coupling strength as defined by the intensity ratio (I2LO/I1LO) at low doping concentration (20 mol %) and increase at high Cd concentration (>20 mol %) confirming a strong dependence of electron phonon coupling strength on the concentration of dopant. The enhancement in the electron phonon coupling strength at high doping concentration was due to the increase in the diameter of the nanorod rather than the concentration of dopants explaining the competitive effect of dopant and size over the electron phonon interaction in a range of doping concentration. The Cd doping led to the decrease in the band gap and the increase (decrease) in the intensity of the excitonic emission (visible luminescence) as observed by the photoluminescence property. X-ray, Fourier transform infrared (FTIR) analysis confirmed low temperature doping of Cd at the tetrahedral coordination site of the ZnO lattice Although, the Cd concentration in the growth solution was systematically varied from 1 mol % to 80 mol %, the maximum Cd doping varied from 3 at. % (80 mol %) at the surface to 0.2 at % - 0.6 at. % in the bulk after surface cleaning by Ar ion etching.