Phonon Coupling Research Articles

Effect of magnetic phase coexistence on spin-phonon coupling and magnetoelectric effect in polycrystalline Sm0.5Y0.5Fe0.58Mn0.42O3

The various physical properties with their intercorrelations in polycrystalline samples of Sm0.5Y0.5Fe0.58Mn0.42O3 [SYFM (58–42)] were investigated. The dc magnetization measurements revealed a weak ferromagnetic (WFM) transition at TÑ361 K that is followed by an incomplete spin reorientation (SR) transition at TSR1∼ 348 K. A first order magnetic transition (FOMT) around TSR2 ∼ 292 K completes the spin reorientation transition and the material enters into a nearly collinear antiferromagnetic (AFM) state. Robust magnetodielectric (MD) is found to be present in material at room temperature. True ferroelectric transition with TFE = 108 K and having a value of saturation polarization (∼0.06 μC/cm2 at 15 K) have been found in the specimen. This spin phonon coupling (SPC) stabilizes the ferroelectric state and responsible for magnetoelectric coupling (ME). We argue that wave vector (q) dependence of the spin-pair correlation affects the electronic and magnetodielectric properties in a similar way.

An Organic–Inorganic Tin Halide Perovskite with Over 2000‐Hour Emission Stability

AbstractOrganic–inorganic tin (Sn)‐based halide perovskites have attracted special attention due to their outstanding photophysical properties and eco‐friendly nature. However, the instability of Sn‐based perovskites is still a pressing issue that needs to be addressed. Here, a new lead‐free hybrid Sn(IV) bromide hybrid (4‐APEA)2SnBr6 (4‐APEA: 2‐(4‐aminophenyl)ethylammonium) is synthesized with remarkable stability. This compound has strong electron–phonon coupling, resulting in excitons self‐trapping and broadband yellow emission with a photoluminescence peak at 566 nm. The bare (4‐APEA)2SnBr6 shows impressive emission stability in the air; maintains 90% of photoluminescence quantum yield under ultraviolet light irradiation over 2000 h (T90 > 2000 h), representing the most stable halide hybrids. The excellent stability is profit from the stable valence state and few defect state densities. The findings show the promising prospect of environment‐friendly organic–inorganic halide perovskites and their usage in optoelectronic applications.

Evidence of Griffith phase in quantum critical region of Dy2Ti1.8Mn0.2O7

We herein present the spin freezing dynamics of doped polycrystalline compound Dy2Ti1.8Mn0.2O7 using ac and dc magnetic measurements. A new relaxation peak (T*) appears in ac susceptibility measurement with the fractional inclusion of Mn at the Ti site in Dy2Ti2O7, indicating the presence of a short-range magnetically ordered state in the paramagnetic state. The evidence of the ‘Griffiths phase’-like state has been observed in the quantum critical region of Dy2Ti1.8Mn0.2O7. Arrhenius fit of freezing temperature (T*) with frequency for Dy2Ti1.8Mn0.2O7 shows that the spin relaxation at T* is thermally induced. Low-temperature structural change in lattice parameters and crystal field phonon coupling has been studied using synchrotron X-ray diffraction. Debye-Grüneisen's temperature-dependent lattice-volume analysis shows the ‘crystal field’-phonon coupling emergence at a much higher temperature (70 K) in Dy2Ti1.8Mn0.2O7 in contrast to 30 K in Dy2Ti2O7. These findings make Dy2Ti1.8Mn0.2O7 a suitable system to explore the application of the spin ice phenomenon at a workable temperature.

Stimulated emission cross sections and temperature‐dependent spectral shifts of neutral nitrogen‐vacancy centers in diamonds

AbstractThe neutral nitrogen‐vacancy (NV0) defects in diamond are photostable color centers, suitable for a wide range of applications in science and engineering. However, the photophysical properties of the centers have not yet been fully characterized. This work measured the stimulated emission cross sections of NV0 in a single‐crystal diamond by two‐photon excitation of its matrix at 344 nm. From the measured photoluminescence spectrum and the fluorescence lifetime of 20 ± 1 ns, we determined a peak stimulated emission cross section of 1.43 ± 0.07 × 10−17 cm2 at 650 nm for the NV0 centers. In addition, we have also examined the thermal shifts of the zero‐phonon line of NV0 centers in nanoscale diamonds (~100 nm in diameter) over the temperature range of 30–120°C. A temperature measurement sensitivity of 0.2 K·Hz−1/2 was achieved, which is about two‐fold better than that of NV−, despite that the fluorescence intensity of NV0 is about six‐fold lower than that of NV− in the same nanoparticles. The result is attributed to the smaller electron–phonon coupling strength of the neutral center, compared with its negatively charged counterpart.

Pressure-Induced Emission from All-Inorganic Two-Dimensional Vacancy-Ordered Lead-Free Metal Halide Perovskite Nanocrystals

Although seeking an effective strategy for further improving their optical properties is a great challenge, two-dimensional (2D) halide perovskites have attracted a significant amount of attention because of their performance. In this regard, the pressure-induced emission accompanied by a remarkable pressure-enhanced emission is achieved without a phase transition in 2D vacancy-ordered perovskite Cs3Bi2Cl9 nanocrystals (NCs). Note that the initial Cs3Bi2Cl9 NCs possess extremely strong electron-phonon coupling, leading to the easy annihilation of trapped excitons by the phonon. Upon compression, pressure could effectively suppress phonon-assisted nonradiative decay and give rise to an intriguing emission from "0" to "1". Both the weakened electron-phonon coupling and the relaxed halide octahedral distortion benefiting from the vacancy-ordered structure contributed to the subsequent enhanced emission. This work not only elucidates the underlying photophysical mechanism but also identifies pressure engineering as a robust means for improving their potential applications in environmentally friendly solid-state lighting at extremes.

Nuclear and magnetic spin structure of the antiferromagnetic triangular lattice compound LiCrTe2 investigated by mu ^+SR, neutron and X-ray diffraction

Two-dimensional (2D) triangular lattice antiferromagnets (2D-TLA) often manifest intriguing physical and technological properties, due to the strong interplay between lattice geometry and electronic properties. The recently synthesized 2-dimensional transition metal dichalcogenide LiCrTe_2, being a 2D-TLA, enriched the range of materials which can present such properties. In this work, muon spin rotation (mu ^+SR) and neutron powder diffraction (NPD) have been utilized to reveal the true magnetic nature and ground state of LiCrTe_2. From high-resolution NPD the magnetic spin order at base-temperature is not, as previously suggested, helical, but rather collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within the ab-plane and AFM coupling along the c-axis. The value if the ordered magnetic Cr moment is established as mu _{textrm{Cr}}= 2.36~mu _{textrm{B}}. From detailed mu ^+SR measurements we observe an AFM ordering temperature T_{textrm{N}}approx 125 K. This value is remarkably higher than the one previously reported by magnetic bulk measurements. From mu ^+SR we are able to extract the magnetic order parameter, whose critical exponent allows us to categorize LiCrTe_2 in the 3D Heisenberg AFM universality class. Finally, by combining our magnetic studies with high-resolution synchrotron X-ray diffraction (XRD), we find a clear coupling between the nuclear and magnetic spin lattices. This suggests the possibility for a strong magnon–phonon coupling, similar to what has been previously observed in the closely related compound LiCrO_2.

Modulating the antiferromagnetic metallic and insulating states by lattice distortion for lightly-doped La0.92Sr0.08MnO3 films

In order to modulate the magnetic and electronic properties of antiferromagnetic (AFM) materials, La0.92Sr0.08MnO3 (LSMO) films were deposited on SrTiO3 (STO), LaAlO3 (LAO) and MgO substrates, which present the AFM metallic, AFM insulating and ferromagnetic (FM) insulating states, respectively. The temperature-dependent Raman spectra demonstrate that the FM state of LSMO/MgO results from the strong lattice distortion induced by large lattice mismatch (δ = −7.2%), which breaks the AFM superexchange interactions of the magnetic ground state. Interestingly, the LSMO/STO (δ = 0.13%) shows two metal–insulator transitions (MIT) at 218 and 53 K, respectively. Meanwhile, the LSMO/LAO (δ = 3.1%) exhibits the MIT at 195 K, while the LSMO/MgO presents the insulating behavior. This is attributed to the strong electron–phonon coupling induced by lattice distortion and thus the localization of itinerant electrons. With the increasing lattice distortion from LSMO/STO to LSMO/MgO, the localization of itinerant electrons is enhanced. Therefore, the insulating behavior occurs for LSMO/MgO. The dramatic change from AFM metallic to FM insulating state for lightly-doped LSMO films has potential application in AFM spintronic devices.

The encapsulation of atomically-thin MgB2-based superconductors in two-dimensional material bilayers

Two-dimensional (2D) superconductors are a class of materials with unique properties that can potentially exhibit novel superconductivity at a reduced dimensionality, as well as pave the way for miniaturizing superconducting devices at the atomic limit. However, such materials are highly sensitive to interaction with their substrates and the environment, and therefore it is necessary to find potential methods for chemically isolating them. We address this problem by examining the possibility of isolating the medium-temperature 2D MgB2-based superconductors. We examine two possible structures: the monolayer stoichiometric MgB2 and the B-caged Mg3B8 structure. By solving the anisotropic Eliashberg equations, we predict that the latter possesses K upon application of a small lateral (3% tensile) strain, with an electron–phonon coupling of 0.88. To investigate protecting the superconductivity of these two layers, we apply density functional theory calculations to examine their encapsulation in a graphene bilayer or a hexagonal boron nitride bilayer. We find that both have high potential as encapsulation systems to protect the superconductivity of the 2D MgB2-based superconductors, in particular B-caged (B–Mg) n systems.

Smaller Stokes Shift Induced Highly Efficient Broadband Near Infrared Garnet Phosphor

AbstractBroadband near‐infrared (NIR) phosphor converted light emitting diodes (pc‐LEDs) are novel compact light sources that enable portable devices for substance detection. However, to achieve such efficient pc‐LEDs is still a challenge due to lack of efficient phosphors. In this paper, Stokes shift as an efficiency predictor is emphasized. In Cr3+ activated Ca2LuZr2Al3O12 (CLZA) garnet phosphor, Stokes shift decreases from 2990 to 2854 cm−1 upon substituting Ca‐Zr with Lu‐Al. As a result, electron–phonon coupling related nonradiative process is reduced. The external quantum efficiency is significantly enhanced to 47.4%, and the as fabricated NIR pc‐LED offers 33.7% @10 mA electro‐optical efficiency. The results can advance the development of efficient NIR phosphors.

Trajectory Propagation of Symmetrical Quasi-classical Dynamics with Meyer-Miller Mapping Hamiltonian Using Machine Learning

The long short-term memory recurrent neural network (LSTM-RNN) approach is applied to realize the trajectory-based nonadiabatic dynamics within the framework of the symmetrical quasi-classical dynamics method based on the Meyer-Miller mapping Hamiltonian (MM-SQC). After construction, the LSTM-RNN model allows us to propagate the entire trajectory evolutions of all involved degrees of freedoms (DOFs) from initial conditions. The proposed idea is proven to be reliable and accurate in the simulations of the dynamics of several site-exciton electron-phonon coupling models and three Tully's scattering models. It indicates that the LSTM-RNN model perfectly captures the dynamical information on the trajectory evolution in the MM-SQC dynamics. Our work proposes a novel machine learning approach in the simulation of trajectory-based nonadiabatic dynamic of complex systems with a large number of DOFs.

Insights into interfacial thermal conductance in Bi2Te3-based systems for thermoelectrics

The thermal resistance at the interface between thermoelectric materials and metallic electrodes plays an important role in the energy conversion efficiency of thermoelectric devices. This is especially prevalent in the Bi2Te3-based flexible thermoelectric generators for wearable electronics, where the temperature difference between the human skin and the ambient environment is small. In this work, we study the phonon interfacial thermal conductance (ITC) in Bi2Te3/Au and Bi2Te3/Cu, by using non-equilibrium molecular dynamics simulations. It is found that the ITC in Bi2Te3/Cu is 3-fold higher than that in Bi2Te3/Au. We explore the underlying mechanism and find that both the stronger interfacial van der Waals interaction and phonon coupling contribute to the higher ITC in Bi2Te3/Cu. We further show that the ITC of Bi2Te3/Au and Bi2Te3/Cu can be tuned effectively by mechanical strain. Remarkably, a 0.04 compressive strain leads to the increase in ITC of Bi2Te3/Au and Bi2Te3/Cu by 120% and 62%, respectively. Our work not only provides physical insights into the ITC in thermoelectric devices, but is also useful in designing the Bi2Te3-based thermoelectric generators with enhanced energy conversion efficiency.

Phonon-induced breakdown of Thouless pumping in the Rice-Mele-Holstein model

Adiabatic and periodic variation of the lattice parameters can make it possible to transport charge through a system even without net external electric or magnetic fields, known as Thouless charge pumping. The amount of charge pumped in a cycle is quantized and entirely determined by the system's topology, which is robust against perturbations such as disorder and interactions. However, coupling to the environment may play a vital role in topological transport in many-body systems. We study the topological Thouless pumping, where the charge carriers interact with local optical phonons. The semi-classical multi-trajectory Ehrenfest method is employed to treat the phonon trajectories classically and charge carriers quantum mechanically. We find a breakdown of the quantized charge transport in the presence of phonons. It happens for any finite electron-phonon coupling strength at the resonance condition when the pumping frequency matches the phonon frequency, and it takes finite phonon coupling strength away from the resonance. Moreover, there exist parameter regimes with non-quantized negative and positive charge transport. The modified effective pumping path due to electron-phonon coupling accurately explains the underlying physics. In the large coupling regime where the pumping disappears, the phonons are found to eliminate the staggering of the onsite potentials, which is necessary for the pumping protocol. Finally, we present a stability diagram of quantized pumping as a function of time period of pumping and phonon coupling strength.

Interplay of spin, phonon, and lattice degrees in a hole-doped double perovskite: Observation of spin–phonon coupling and magnetostriction effect

Unlike a typical spin–phonon coupling, an exhibition of unconventional spin–phonon coupling, which is mediated via magnetostriction effect, is reported in a hole-doped double perovskite Pr1.5Sr0.5CoMnO6. Various investigations including electronic and crystal structures, spin structure, transport property, lattice dynamics, and theoretical density of states analysis by density-functional theory (DFT) have been performed. A substantial increase in the mean oxidation states of Co ions and a concurrent abrupt decrease in the resistivity upon Sr doping is observed, thus altering its underlying transport mechanism. An insulating and ferromagnetic (FM) ground state is predicted by DFT calculations. The neutron diffraction data analysis reveals a complex crystal structure of Pr1.5Sr0.5CoMnO6, which consists of B-site disordered monoclinic (P21/n) and orthorhombic (Pnma) structures, highlighting the presence of an anti-site disorder in the system. The analysis also suggests an overall FM ordering of Co/Mn spins below 150 K for the monoclinic phase, whereas no such magnetic ordering is found for the orthorhombic phase. More interestingly, the neutron powder diffraction study perceives the presence of a strong magnetostriction effect in the system. Raman spectroscopy unravels the presence of a spin–phonon coupling, which is essentially mediated by the magnetostriction effect.

Electrical regulation of interfacial thermal conductance of graphene/MoS2 heterojunction: Uncovering the role of electrons

The van der Waals heterojunction of graphene and MoS2 has drawn extensive research in recent years, since it combines the advantages of both materials. The thermal transport across the interface is an important issue in practical application, which has been seldom experimentally studied for graphene/MoS2 heterojunction. Previous work only focused on the phonon coupling without considering the role of interfacial electronics, which caused a deviation between experimental and theoretical results. Herein, we employed photo-thermal Raman technique to characterize the interfacial thermal conductance of graphene/MoS2 heterojunction and revealed a value of 93.2 ± 10.2 MW/m2K on average. This value is much larger than theoretical predictions. Using source-drain bias (VD) and gate voltage in a field effect transistor device, we achieved effect regulation of interfacial electron transport and corresponding thermal conductance in a range of 55.1 ± 5.8 to 304.2 ± 32.1 MW/m2K. At VD with different signs, we uncovered a possible thermal rectification at the interface. We owed the change of interfacial thermal conductance to electron-assisted phonon-coupling, when electron's transfer between graphene and MoS2 may results in the phonon density redistribution.

Femtosecond laser melting NbMoTaW refractory high entropy alloy: A micro-scale thermodynamic simulation

Refractory high entropy alloys (RHEA) exhibit excellent material properties, such as high strength, high hardness, high melting point and high temperature oxidation resistance. Ultrafast laser fabricating RHEAs is potential widely used in extreme environments at high temperature. In the present study, a multiscale simulation of the femtosecond laser pulse melting NbMoTaW RHEA film was performed to observe the transient interaction process between femtosecond laser and RHEA at the atomic scale. The temperature dependent electron thermophysical parameters (electron heat capacity, electron thermal conductivity and effective electron–phonon coupling factor) were calculated from density functional theory. The two temperature model coupling molecular dynamics was used to reveal the temperature change, the density variation and the pressure evolution during the interaction of a single pulse laser with the NbMoTaW RHEA. The simulated ablation and melting depths varied differently with laser fluence parameters. The ablation depth increases with the laser fluence, while the non-removed melting depth varies little with laser energy beyond the ablation threshold. The research reveals the mechanisms in melting process at atomic-level, and the results are helpful to the fabrication of RHEA.

Recent Advances in Chromium‐Doped Near‐Infrared Luminescent Materials: Fundamentals, Optimization Strategies, and Applications

AbstractDevelopment of chromium‐doped luminescent materials is pertinent to many emerging applications, ranging from agriculture, food industry to noninvasive health monitoring. The fundamental importance of chromium‐activated luminescent materials in the field of optics and biomedicine makes the rapid development of novel materials and relevant applications. Herein, the recent advances on the luminescence principle and photoluminescence (PL) optimization for Cr3+‐activated luminescent materials together with their potential applications are reviewed. The different types of most recently developed Cr3+‐doped luminescent materials and the design principles are systematically summarized. The associations between crystal structure and near‐infrared (NIR) PL properties, as well as performance‐evaluating parameters are introduced with the examples of known NIR emitting phosphors, which will be helpful to explore future NIR luminescent materials. Based on crystal field control, site engineering, and electron–phonon coupling, several efficient strategies for optimizing luminescence performances including bandwidth, thermal stability, and quantum efficiency of Cr3+‐doped NIR luminescent materials are proposed. Then, potential applications in the fields of food analysis, night vision, information encryption, and optical sensors are surveyed. Finally, the challenges of promising Cr3+‐doped luminescent materials are proposed.

Infrared optical absorption of Fröhlich polarons in metal halide perovskites

The formation of Fröhlich polarons in metal halide perovskites, arising from the charge carrier–longitudinal optical (LO) phonon coupling, has been proposed to explain their exceptional properties, but the effective identification of polarons in these materials is still a challenging task. Herein, we theoretically present the infrared optical absorption of Fröhlich polarons based on the Huang–Rhys model. We find that multiphonon overtones appear as the energy of the incident photons matches the multiple LO phonons, wherein the average phonon number of a polaron can be directly evaluated by the order of the strongest overtone. These multiphonon structures sensitively depend on the scale of electronic distribution in the ground state and the dimensionality of the perovskite materials, revealing the effective modulation of competing processes between polaron formation and carrier cooling. Moreover, the order of the strongest overtone shifts to higher ones with temperature, providing a potential proof that the carrier mobility is affected by LO phonon scattering. The present model not only suggests a direct way to verify Fröhlich polarons but also enriches our understanding of the properties of polarons in metal halide perovskites.

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.

Exploring the Ti2CO2–WSe2 Heterostructure as a Direct Z-Scheme Photocatalyst for Water Splitting: A Non-Adiabatic Study

One of the potential strategies to solve renewable energy shortage is to design low-cost efficient photocatalysts for water splitting to aid hydrogen and oxygen evolution reactions (HER and OER). Two-dimensional (2D) materials like MXenes and transition metal dichalcogenides (TMDs) have caught special attention owing to their unique optical, electrical, and mechanical properties, but come with issues like photocorrosion and poor charge separation. Bilayer vdW heterojunctions of suitable constitutive monolayers are coming up as a potential solution to these hazards, accredited to their tunable band gap and efficient charge separation. In this paper, we have investigated the possibility of Ti2CO2–WX2 (X = S, Se, Te) vdW heterostructures to perform as Z-scheme photocatalysts by employing first-principle density functional calculations, and thereby, the Ti2CO–WSe2 heterostructure has emerged as the most promising material for Z-scheme photocatalysis. Further, excited-state dynamics simulation reveals that the timescales of electron transfer and hole transfer are greater (1.13 and 1.22 ps, respectively) than the maximum time limit of the photogenerated electron–hole recombination (1.0 ps) owing to the weaker non-adiabatic coupling and electron–phonon coupling. This affirms the fact that photogenerated electrons and holes with greater redox ability are preserved to drive the photocatalytic pathway. In addition to that, the free energy calculations associated with the HER and OER processes entail that the processes occur spontaneously on the surface of the heterostructure without any co-catalyst. This establishes the Ti2CO–WSe2 heterostructure as a potential mediator-free direct Z-scheme photocatalyst for overall water splitting.

Dephasing Dynamics across Different Local Vibrational Modes and Crystalline Environments.

The perturbed free induction decay (PFID) observed in ultrafast infrared spectroscopy was used to unveil the rates at which different vibrational modes of the same atomic-scale defect can interact with their environment. The N_{3}VH^{0} defect in diamond provided a model system, allowing a comparison of stretch and bend vibrational modes within different crystal lattice environments. The observed bend mode (first overtone) exhibited dephasing times T_{2}=2.8(1) ps, while the fundamental stretch mode had surprisingly faster dynamics T_{2}<1.7 ps driven by its more direct perturbation of the crystal lattice, with increased phonon coupling. Further, at high defect concentrations the stretch mode's dephasing rate was enhanced. The ability to reliably measure T_{2} via PFID provides vital insights into how vibrational systems interact with their local environment.