Low Phonon Energy Research Articles

Phonon softening in Lu(Pt1−xPdx)2In close to a zero-temperature structural instability

The structural transition in Lu(Pt${}_{1\ensuremath{-}x}$Pd${}_{x}$)${}_{2}$In attracts special interest since it can be tuned to zero temperature with the appearance of a superconducting dome around structural quantum criticality. The authors combined inelastic neutron and x-ray scattering measurements to study this structural transition. They determined the low-energy phonon dispersions and detected the relevant phonon branch becoming soft. While theoretical calculations broadly agree with the measured dispersion, a clear non-mean-field behavior is demonstrated at this phase transition, which might be connected with quantum criticality and the superconducting dome.

Review of condensed matter laser cooling using electric-dipole-allowed transitions

The cooling of solids via laser-excited anti-Stokes luminescence has reached the cryogenic regime and is now being considered for several applications that benefit from vibration-free cryogenic refrigeration. 4f-4f transitions in rare-earth-doped crystals with low phonon energies currently offer the best laser-cooling performance. These transitions, however, are electric-dipole (ED) forbidden and thus have low cross sections, limiting the minimum achievable temperatures in practical materials to the 50–100 K regime. In contrast, ED-allowed transitions may offer greater cross sections and potentially enable lower minimum temperatures or more compact devices. The search for laser cooling with ED-allowed transitions has been an active field of research over the past decades with a wide variety of materials having been studied. This paper reviews the fundamentals of solid-state laser cooling, provides a preliminary theoretical assessment showing that laser-cooling with ED-allowed transitions is in principle possible, and reviews the experimental results reported to date. While cooling with ED-allowed transitions has not yet been realized in the solid state, the existing experimental body of work along with the theoretical considerations presented here suggest that further research on this topic may be fruitful.

2–3 μm mid-infrared luminescence of Ho3+/Yb3+ co-doped chloride-modified fluorotellurite glass

In this paper, Ho3+/Yb3+ co-doped chloride-modified fluorotellurite glasses with 2–3 μm mid-infrared luminescence are prepared. By measuring and investigating the transmission spectra and emission spectra, the prepared glasses show a high transmittance (91 %) and low maximum phonon energy (813 cm−1). Based on the measured absorption spectra, the Judd-Ofelt parameters and radiation characteristics were calculated in depth. In addition, with the assistance of phonons, the energy transfer between Ho3+/Yb3+ ions further increases the mid-infrared fluorescence intensity. The calculated emission cross-section at 2.0 μm and 2.85 μm reach 16.47 × 10-21 cm2 and 7.8 × 10-21 cm2, respectively. It is worth mentioning that the quantum efficiencies of Ho3+: 5I7→5I8 and 5I6→5I7 reach 51.47 % and 84.14 % respectively. The results having also in mind good thermal stability (ΔT = 102℃) and refractive index (n = 1.645) indicate that this glass has a promising application for the study of fiber lasers in the mid-infrared band.

Prediction of phonon-mediated superconductivity and topological surface states in NbRu[formula omitted

Based on the first-principles calculations, we obtain the superconducting transition temperature (Tc) of 21.17 K for NbRu3C. For the superconductivity of NbRu3C, the strong electron–phonon coupling mainly comes from the coupling between Ru 4d orbitals and low-energy phonon modes which include triply degenerate T1u and doubly degenerate E. Moreover, NbRu3C is found to have a topological insulating phase, confirmed by the calculated nontrivial surface states and spin textures. The presence of superconductivity and topological surface states in NbRu3C suggests that NbRu3C may be a candidate for the topological superconductor.

Downconversion mechanism in Er3+/Yb3+ codoped fluorotellurite glasses to enhance the efficiency of c-Si PV cells

Fluorotellurite glass codoped with erbium and ytterbium was elaborated through the melting-quenching route. Through the DSC and Raman measurements, the investigated glass samples presented high thermal stability and low phonon energy, respectively. By using the laser excitation at 488 nm, the downconversion mechanism in the Er3+/Yb3+ system was investigated and the energy transfers implicated in the emissions of erbium and ytterbium ions were discussed based on the different cross-relaxation processes. The addition of ytterbium resulted in an effective energy transfer from the excited 4S3/2/2H11/2 state of Er3+ ions to the emitting 2F5/2 state of Yb3+ ion, which considerably enhanced NIR emissions at 1000 nm and reduced experimental lifetimes of 4S3/2/2H11/2 states. Through luminescence decay analysis, the maximum values of quantum efficiency (ηQE) and energy transfer efficiency (ηET) were estimated as 61.2% and 161.2%, respectively. This result indicates that the proposed glasses can convert visible photons into NIR emissions at 1000 nm suitable to improve the spectral response of crystalline silicon PV cells.

Ellipsometry study of the infrared-active phonon modes in strained SrMnO3 thin films

Abstract We performed infrared and time-domain terahertz spectroscopic ellipsometry measurements of thin films of the perovskite antiferromagnetic insulator SrMnO3 that were grown by pulsed laser deposition (PLD) on LaAlO3, SrLaGaO4, and LSAT substrates which yield an epitaxial strain ranging from −0.3 to 1.7%. Taking these thin films as a representative example, we discuss the strategies for analyzing the ellipsometry spectra and extracting the information about the thin film dielectric response that can be equally applied to a variety of oxide based thin films and heterostructures. In particular, for the room temperature spectra we show that the three infrared-active phonon modes of the cubic perovskite structure of SrMnO3 undergo the expected softening with increasing tensile strain. For the SrMnO3 film on SrLaGaO4, we find that the low-energy (TO1) phonon mode reveals anomalous temperature dependence in the vicinity of the Néel temperature of about 170 K that signifies a strong spin-phonon coupling. For the SrMnO3 film on LSAT, we identify some irreversible changes of the infrared ellipsometry spectra that occur as the sample is heated to elevated temperature up to 560 K. These changes of the ellipsometry spectra have been attributed to a partial oxygen loss of the SrMnO3 thin film since they can be reverted with a post annealing treatment under high oxygen pressure.

Raman scattering of low energy HgCdTe phonons

In this article the experimental results for the Raman spectra of HgCdTe samples with mole fraction from 0.15 to 0.4 are presented. The results encompass the peaks of the low-energy phonon bands within the energy range of 10 cm−1–80 cm−1. The theoretical predictions for which the energy of these peaks should be around 24 cm−1 and 45 cm−1 are confirmed. For all measured samples, both discussed peaks occur, and their energy is similar in each measured sample.

Enhanced blue and broadband near-infrared emission of Pr3+/Yb3+ co-doped PbF2 single crystal

High quality Pr3+ single-doped PbF2 and Pr3+/Yb3+ co-doped PbF2 single crystals were grown by temperature gradient technique (TGT). By co-doping Yb3+ ion as Pr3+ ions sensitizer, the visible light emission can be regulated and the largest luminescence branching ratio can be switched from red-orange to blue-green light. Besides, when Pr3+ ions doping concentration is as low as possible, the blue light self-absorption effect can be weakened, which is beneficial to high-efficiency blue laser output. Moreover, the broad band emission around 1.3 μm from 1G4 energy level can be observed in the Pr3+/Yb3+:PbF2 crystal with the low maximum phonon energy for the first time. Absorption, emission spectra and luminescence mechanism are well analyzed by the energy transfer process and J-O theory. The laser upper energy level lifetime of Pr3+ ions has a great increase by Yb3+ ion effect. These advantageous spectroscopic characteristics show that the Pr3+/Yb3+:PbF2 crystal might be potential laser gain materials for blue-green and ∼1.3 μm near-IR laser operation.

Unusual thermodynamics of low-energy phonons in the Dirac semimetal Cd3As2

By studying the thermal conductivity, specific heat, elastic modulus, and thermal expansion as a function of temperature for Cd3As2, we have unveiled a couple of important thermodynamic features of the low-energy phonons strongly interacting with Dirac electrons. The existence of soft optical phonons, as inferred from the extremely low thermal conductivity, is unambiguously confirmed by low-temperature specific heat revealing significant deviation from Debye’s description. The estimated Debye temperature is small in the range of 100–200 K and varies significantly depending upon the measurement used in its experimental determination. The thermodynamic Grüneisen ratio γ reveals a remarkable reduction below about 100 K, an energy scale that is highly relevant to the Dirac states, towards negative values below about 10 K that are indicative of lattice instability.

Soft-phonon anharmonicity, floppy modes, and Na diffusion in Na3FY (Y=S,Se,Te) : Ab initio and machine-learned molecular dynamics simulations

A class of Na-based antiperovskites ${\mathrm{Na}}_{3}XY$ $(X=\mathrm{F},\mathrm{H};Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ was recently reported with a remarkably high ionic conductivity \ensuremath{\sim}0.1 mS/cm near room temperature. Herein, we report comprehensive atomic dynamics investigations using ab initio and large-scale machine-learned molecular dynamics (MLMD) simulation on these sodium superionic conductors. Previous studies identified the role of soft phonons involving rotations of ${\mathrm{FNa}}_{6}$ octahedral units in Na diffusion. In contrast, our MLMD simulations show that the Na diffusion pathways are essentially uncorrelated and do not involve the collective dynamics of octahedral rotations or reorientations. Moreover, while the soft phonons do not involve any $Y$ atomic dynamics, the diffusion pathways are sterically hindered unless facilitated by $Y$ displacements and Na vacancies. However, we do find that the anharmonic low-energy phonon modes of wave vectors along the $M\text{\ensuremath{-}}R$ line at the Brillouin zone (BZ) boundary are important precursors of Na diffusion. These modes involve vibrations perpendicular to the F-Na ionic bond, along $\ensuremath{\langle}100\ensuremath{\rangle}$ directions, which provide the initial pathways for the Na diffusion. We have calculated the branch-resolved phonon spectral energy density (SED) in the entire BZ using the MLMD simulations. The SED results as a function of temperature reveal the large anharmonicity of the soft phonon modes, which leads to floppy dynamics of Na atoms. The volume of cubic-${\mathrm{Na}}_{3}\mathrm{FS}$ is the smallest among the three antiperovskites and has the largest mean-phonon energy. Our calculated Na diffusion coefficient at 700 K in ${\mathrm{Na}}_{3}\mathrm{FS}$ of $\ensuremath{\sim}0.63\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ is significantly larger than $\ensuremath{\sim}0.16\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ in ${\mathrm{Na}}_{3}\mathrm{FTe}$, which clearly shows contrast with the hypothesis of a softer lattice leading to faster diffusion. The estimated diffusion barrier energy for ${\mathrm{Na}}_{3}\mathrm{FS}$ with 2% Na vacancy is \ensuremath{\sim}0.44 eV, which is in fair agreement with the reported value of 0.44 eV estimated from the total conductivity measurement with Na-vacant iodine-doped ${\mathrm{Na}}_{3}\mathrm{FS}$. Similarly, for ${\mathrm{Na}}_{3}\mathrm{FSe}$ and ${\mathrm{Na}}_{3}\mathrm{FTe}$, the barrier energies have been estimated to be 0.50 and 0.55 eV, respectively. Our large-scale MLMD-based theoretical study provides a comprehensive understanding of the role of soft phonons, host dynamics, and vacancies in Na diffusion in ${\mathrm{Na}}_{3}\mathrm{F}Y$ $(Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ and other materials of this class, which will be helpful in designing materials for application in solid-state batteries.

Study of visible, NIR, and MIR spectroscopic properties of Er3+‐doped tellurite glasses and glass–ceramics

AbstractIn this paper, the structural, thermal, optical, and spectroscopic properties of Er3+‐doped tellurite glasses with the composition 68.25TeO2–19.5ZnO–9.75X–2.5Er2O3 (in mol%) with X = BaO, Na2O, and Bi2O3 are reported. The glasses were prepared using the standard melt quenching method. The investigated glasses exhibit low phonon energy (∼745 cm−1) and low glass transition temperature varying between 300 and 350°C depending on the glass composition. The Raman spectra show a regular tellurite structure with variations in the number of bridging and non‐bridging oxygens depending on the glass composition, the Na2O and Bi2O3‐containing glasses having the most and the least polymerized network, respectively. A thermal treatment of the glasses leads to the formation of crystals, the composition of which depends on the glass composition, as revealed by X‐ray diffraction analysis and confirmed using scanning electron microscope‐energy‐dispersive spectroscopy. The precipitation of Er‐containing crystals in the Na2O and BaO‐containing glasses leads to an increase in the intensity of the upconversion emissions. Although the Er3+ ions remain in the amorphous part of the Bi2O3‐containing glass after heat treatment, it is the precipitation of Bi3.2Te0.8O6.4 crystals in this glass, which is thought to decrease the distance between the Er3+ ions leading to an increase in the intensity of the upconversion and mid‐infrared emissions.

Structural, thermal and spectroscopic properties of samarium (Sm3+) doped tungsten zinc tellurite glass for application in orange light emitting devices

Using melt quenching method 1 mol% Sm2O3 doped WO3–ZnO–TeO2 glass has been prepared. The structural, thermal, optical and luminescence properties are studied by analyzing the XRD, FTIR, DTA, UV–Vis–NIR absorption, excitation, photoluminescence emission, decay profile, CIE1931 and CIELab diagrams. The DTA profile shows that the glass transition (Tg), crystallization (Tc) and melting temperature (Tm) occur at 384 °C, 559 °C and 810 °C, respectively. The FTIR spectra confirm that the glass has low phonon energy around 666 cm−1. Using absorption spectra the Judd-Ofelt intensity parameters and hence the radiative parameters are determined. The glass emits intense orange light at 600 nm due to 4G5/2 → 6H7/2 transition upon excitation at 403 nm, and the decay profile of the 4G5/2 level shows double-exponential nature. The values of radiative parameters, quantum efficiency (η) and CIE indicate that the glass sample is promising for orange light emitting photonics devices.

Local atomic structure distortions in the Dion-Jacobson ferroelectric CsBiNb2O7

Despite recent interest in Bi-containing layered-perovskite ferroelectrics due to high ${T}_{c}$, their rational design is constrained by a poor understanding of local atomic structures. Here, we use neutron total scattering and first-principles theory to examine the local atomic structure in Dion-Jacobson layered-perovskite ferroelectric $\mathrm{CsBi}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$. We show that the instantaneous short-range atomic structure can be described by a Debye-Einstein model that accommodates atomic motions due to acoustic and low-energy optic phonon modes. We determine structural distortions due to localized atomic dynamics with extremely high resolution. These are in addition to ground-state relative ionic displacements. The magnitude of dynamic variations in ionic positions points toward strong electron-phonon coupling in this material.

Phonon-assisted carrier cooling in h -BN/graphene van der Waals heterostructures

Being used in optoelectronic devices as ultra-thin conductor-insulator junctions, detailed investigations are needed about how exactly h-BN and graphene hybridize. Here, we present a comprehensive ab initio study of hot carrier dynamics governed by electron-phonon scattering at the h-BN/graphene interface, using graphite (bulk), monolayer and bilayer graphene as benchmark materials. In contrast to monolayer graphene, all multilayer structures possess low-energy optical phonon modes that facilitate carrier thermalization. We find that the h-BN/graphene interface represents an exception with comparatively weak coupling between low-energy optical phonons and electrons. As a consequence, the thermalization bottleneck effect, known from graphene, survives hybridization with h-BN but is substantially reduced in all other bilayer and multilayer cases considered. In addition, we show that the quantum confinement in bilayer graphene does not have a significant influence on the thermalization time compared to graphite and that bilayer graphene can hence serve as a minimal model for the bulk counterpart.

Systematic study on thermal conductivity of organic triangular lattice systems β′-X[Pd(dmit)2]2

Systematic variation of the low-temperature heat transport properties of the quasi-two-dimensional quantum spin liquid (QSL) candidate ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{EtMe}}_{3}\mathrm{Sb}[\mathrm{Pd}({\mathrm{dmit})}_{2}{]}_{2}$ and its analog compounds showing antiferromagnetic (AFM) ordering is discussed using thermal conductivity $\ensuremath{\kappa}$ measurements. All compounds with the QSL ground state show a monotonic decrease in $\ensuremath{\kappa}$ with decreasing temperature, which is a typical feature of glass or amorphous materials, caused by their low-energy phonon structures. In contrast, the AFM compounds exhibit a peak structure of $\ensuremath{\kappa}$ around 10 K, which is the characteristic temperature dependence of normal crystals with coherent phonons. Our results indicate that the anomalous glassy behavior in ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{EtMe}}_{3}\mathrm{Sb}[\mathrm{Pd}({\mathrm{dmit})}_{2}{]}_{2}$ is an intrinsic and universal feature of the organic QSL phase and is not caused by extrinsic factors, such as the quality of the samples and the microcracks occurring during the cooling process. We propose a scenario for the origin of the glassy behavior of QSL compounds in which the charge fluctuation within the Pd(${\mathrm{dmit})}_{2}$ dimers changes the phonon properties from crystalline to glasslike.

The study on BWGd:Nd glass for new laser amplifier: Properties, theoretical and practical investigations

The new laser-amplifier glass, Nd3+ doped tungsten gadolinium borate glass (BWGd:Nd), was investigated in this work. Glasses with different Nd2O3 concentration were prepared by melt-quenching technique. The measured density, refractive index and other calculated physical parameters leaded to the assumption that the degree of Nd–O covalency and the number of NBO increased with addition of Nd2O3 content. The enhancement of both factors possibly increased the strength of ligand field that slightly shifted and split some energy levels of Nd3+. The energy level splitting from 4F5/2 state was an intermediate level which accelerated the Nd3+ population on the 4F3/2 luminescence level by single-phonon relaxation. This electron-phonon coupling and low phonon energy of BWGd host glass was a key for strong Nd3+ luminescence of 4F3/2 → 4I11/2 transition (1074 nm) and high radiative parameters, especially the high quantum efficiency around 96%. Due to high value of Δλeff as well as σem but no long τexp value, this glass is suitable for laser amplifier. In practical laser-amplifier experiment, the gain amplification of developed glass was around 1.5 times under pumping power of 3.55 W. The BWGd:Nd glass performed high ability and may using as laser-amplifier material in the laser fusion and other applied laser system.

Graphitic carbon nitride-based upconversion photocatalyst for hydrogen production and water purification

Upconversion luminescence (UCL) is mainly a nonlinear optical method that some engineered nanomaterials can attain and helps in the transformation of low energy phonons (near-infrared photons) into higher energy phonons (ultraviolet (UV) and visible light photons). Upconversion (UC) nanomaterials are a suitable candidate for preparing near-infrared (NIR) light-responsive photocatalytic systems by mixing with other photocatalysts. Many reports have been published on lanthanide-based UC materials (Ln3+ ions as dopants) and carbon quantum dots (CQDs) carrying UC property with their use in photocatalytic removal of pollutants and energy production. Besides these UC nanomaterials, graphitic carbon nitride (g-C3N4) as a potential photocatalyst (metal-free and ecofriendly) has gained attention owing to its unique and amazing possessions. But some limitations and inadequate utilization of visible light restrict its photocatalytic applicability. Therefore, to enhance or widen its light-harvesting property towards the NIR region, the integration of upconversion nanocrystals (UC NCs) into g-C3N4 is considered an effective approach. Thus, the present review is focused on the amalgamation of g-C3N4 with UC nanomaterials for full solar spectrum absorption in H2 production and pollutant degradation via NIR light absorption.

Effect of biphasic-phase on the mid-infrared emission properties of Pr3+ doped GeSe2-Ga2Se3 chalcogenide glass ceramics

Mid-infrared (MIR) laser operating at 2–5 μm has gained increasing interest because of the extensive applications in fields from biomedicine, sensing to nanomaterial science. Chalcogenide glass (ChG) is a kind of matrix that is promising for MIR laser due to its low phonon energy and wide infrared transmittance region. However, significant challenges in the enhancement of MIR optical properties of rare-earth ion-doped ChGs still exist. Here, enhanced MIR emission was realized in Pr3+-doped chalcogenide glass ceramics (ChGCs). By controlling the heat treatment temperature and duration, biphasic-phase nanocrystals of Ga2Se3 and GeSe2 were successively precipitated in the glass host. Compared to the as-prepared glass, the GCs exhibit an obvious increase in emission intensity by more than 5 times. The improvement of the MIR emission may be induced by the enhanced cross-relaxation between Pr3+ ions and the increased multiple scattering effects. Furthermore, the obtained ChGC enables large stimulated emission cross section 0.42 × 10−20 cm2 at 2.44 μm and 0.62 × 10−20 cm2 at 4.75 μm. The superior optical properties of the ChGC indicate that it is an attractive candidate for MIR laser.

Interplay of thermal diffuse scattering and correlated compositional disorder in KCl1-xBrx.

Single-crystal X-ray diffuse scattering measurements are reported of the compositional series KCl1-xBrx, a model system for the broader family of disordered rocksalts. Using a combination of Monte Carlo simulations and lattice dynamical calculations, we show that the observed diffuse scattering is well described in terms of (i) non-statistical anion distributions, (ii) local lattice relaxations accompanying Cl/Br substitution, and (iii) the contribution from low-energy phonons. It is found that a tendency for compositional domain formation broadens the thermal diffuse scattering by splitting and softening the acoustic phonon branches. This effect, which is strongest for intermediate compositions, is seen in both experiment and calculation alike. These results establish a link between local compositional order and unconventional lattice dynamics in this system, and reinforce emerging design principles of exploiting compositional fluctuations to tailor physical properties, such as thermal conductivity, that depend on phonon broadening.

Ultra-narrow room-temperature emission from single CsPbBr3 perovskite quantum dots

Semiconductor quantum dots have long been considered artificial atoms, but despite the overarching analogies in the strong energy-level quantization and the single-photon emission capability, their emission spectrum is far broader than typical atomic emission lines. Here, by using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr3 quantum dots, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35−65 meV (vs. initial values of 70–120 meV), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20−60 meV). Ultra-narrow emission at room-temperature is desired for conventional light-emitting devices and paramount for emerging quantum light sources.