Phonon Behavior Research Articles

Phonon anharmonicity in single-crystalline SnSe

Understanding the temperature-dependent behavior of phonons that affects the thermal transport properties is crucial for developing efficient thermoelectric materials. Previously, SnSe was reported to exhibit a record high thermoelectric performance above 900 K, which was debated in subsequent reports. Nonetheless, there is a growing consensus that anharmonicity is one of the contributing factors which yields ultralow thermal conductivity in SnSe. While there is experimental and theoretical evidence for anharmonicity in SnSe, its effect on the entire phonon band structure, which is responsible for the thermal transport, is investigated here. We performed a combined temperature-dependent polarized Raman spectroscopic and heat-capacity study on fully dense single-crystalline SnSe, which revealed that the anharmonicity is driven by soft optical modes in the $b\text{\ensuremath{-}}c$ plane. These modes exhibited strong high-temperature broadening, indicating ultrashort phonon lifetimes and high scattering rates. Analysis of the Raman peak frequencies and linewidths revealed phonon decay to be dominated by a three-phonon scattering process. Consistent with this observation, the analysis of our temperature-dependent heat-capacity data also revealed the presence of strong anharmonicity in SnSe. The anharmonic coefficients ${\ensuremath{\alpha}}_{R}$ and ${\ensuremath{\alpha}}_{C}$ calculated from the Raman and heat-capacity measurements, respectively, are in excellent agreement with each other. This study provides a deeper understanding of the role of phonon-phonon scattering and anharmonicity leading to outstanding thermal transport properties of SnSe.

Phonon thermal conduction in a graphene–C3N heterobilayer using molecular dynamics simulations

Two-dimensional (2D) graphene (GRA) and polyaniline (C3N) monolayers are attracting growing research interest due to their excellent electrical and thermal properties. In this work, in-plane and out-of-plane phonon thermal conduction of GRA–C3N heterobilayer are systematically investigated by using classical molecular dynamics simulations. Effects of system size, temperature and interlayer coupling strength on the in-plane thermal conductivity (k) and out-of-plane interfacial thermal resistance (R) are evaluated. Firstly, a monotonic increasing trend of k with increasing system size is observed, while a negative correlation between thermal conductivity and temperature is revealed. The interlayer coupling strength is found to have a weak effect on the in-plane thermal conductivity of the heterobilayer. Secondly, at T = 300 K and χ = 1, the predicted R of GRA → C3N and C3N → GRA are 1.29 × 10−7 K m2 W−1 and 1.35 × 10−7 K m2 W−1, respectively, which indicates that there is no significant thermal rectification phenomenon. It can also be observed that R decreases monotonically with increasing temperature and coupling strength due to the enhanced Umklapp phonon scattering and the phonon transmission probability across the interface. Phonon density of states, phonon dispersions and participation ratios are evaluated to reveal the mechanism of heat conduction in the heterobilayer. This work contributes the valuable thermal information to modulate the phonon behaviors in 2D heterobilayer based nanoelectronics.

Precursor Phenomena of Barium Titanate Single Crystals Grown Using a Solid-State Single Crystal Growth Method Studied with Inelastic Brillouin Light Scattering and Birefringence Measurements.

The nature of precursor phenomena in the paraelectric phase of ferroelectrics is one of the main questions to be resolved from a fundamental point of view. Barium titanate (BaTiO3) is one of the most representative perovskite-structured ferroelectrics intensively studied until now. The pretransitional behavior of BaTiO3 single crystal grown using a solid-state crystal growth (SSCG) method was investigated for the first time and compared to previous results. There is no melting process in the SSCG method, thus the crystal grown using a SSCG method have inherent higher levels of impurity and defect concentrations, which is a good candidate for investigating the effect of crystal quality on the precursor phenomena. The acoustic, dielectric, and piezoelectric properties, as well as birefringence, of the SSCG-grown BaTiO3 were examined over a wide temperature range. Especially, the acoustic phonon behavior was investigated in terms of Brillouin spectroscopy, which is a complementary technique to Raman spectroscopy. The obtained precursor anomalies of the SSCG-grown BaTiO3 in the cubic phase were similar to those of other single crystals, in particular, of high-quality single crystal grown by top-seeded solution growth method. These results clearly indicate that the observed precursor phenomena are common and intrinsic effect irrespective of the crystal quality.

Phonon behaviors and dielectric functions in Bi0.5Na0.5TiO3‐based ceramics by Raman scattering and optical ellipsometry

AbstractThe dielectric functions and lattice vibrations of (0.935‐x)Bi0.5Na0.5TiO3‐0.065BaTiO3‐xSrTiO3 (BNBSTx) ceramics were investigated by temperature‐dependent spectroscopic ellipsometry and Raman scattering. Based on the analysis of dielectric functions with the Tauc‐Lorentz dispersion model, it was found that the band gap and the temperature coefficient increased with the addition of SrTiO3. Moreover, the frequency, intensity and/or full width at half maximum of phonon modes related to Bi and Ti‐localized phonon modes exhibited local maximum/minimum around the depolarization temperature and permittivity‐maximum temperature, which indicated the ongoing structural transformation. In addition, the Sr introduction affected the TiO6 tilting and the off‐center displacement of Ti4+. These results seem to be significant for the enhanced response functions with the addition of SrTiO3 in BNBSTx ceramics.

Influence of Co Doping on the Structural, Dielectric and Raman Properties of Ba0.75Sr0.25Ti1−xCoxO3

We synthesized Ba1−xSrxTiO3 (BST) and Co doped Ba0.75Sr0.25CoxTi1−xO3 (BSCTx) compositions (x = 0.03, 0.05, 0.07) by the solid state reaction route and probed the effect of Co-doping on the structural, microstructural, dielectric and phonon behaviour. Room temperature x-ray diffraction patterns of sintered ceramics reveal perovskite type average cubic crystalline structure (space group Pm-3m) for all doped compositions, whereas, pure BST composition stabilizes in the tetragonal crystalline phase (space group P4mm). Absence of any impurity peaks in the doped samples reveals that Co-ions are entering into the lattice without any metal clustering. Quantitative full profile fitting using Reitveld refinement fits well in the cubic phase giving a small value of goodness of the fitting parameter (Rwp). The real part of the dielectric permittivity as well as dielectric loss (tan δ) decreases with Co-doping indicating that intrinsic defects get annealed with doping. \( (\varepsilon^{\prime}/\varepsilon^{\prime\prime} - T) \) plots clearly show the cubic to tetragonal phase transition in BST at around 50°C, whereas Co-doped samples show no such transition in the temperature range studied. Temperature dependent Raman spectra reveal the characteristic phonon modes associated with the tetragonal phase at room temperature, whereas, Co-doped samples exhibit only bands associated with local disorder in the cubic phase. The spectral features of tetragonal phase appear around − 10°C in the 3% Co-doped system indicating the decrease of transition temperature with Co-doping. The analysis of deconvoluted Raman parameters using two sub-lattice models clearly reveals the structural disorder due to doping with slightly distorted TiO6 octahedra getting almost perfect thus giving rise to a cubic phase in the doped system.

Acoustic phonons in nanowires probed by ultrafast pump-probe spectroscopy

Abstract The fascinating relationship between structure and property in nanowires has enabled a wealth of applications in photonics and electronics. The behavior of phonons in nanowires is also modified compared to their bulk counterparts. In this review, we provide an overview of the recent efforts to investigate the properties of acoustic phonons in nanowires using ultrafast optical methods. In particular, we focus on the calculation of the modified phonon dispersion in nanowires and how to address them optically. We then discuss experimental investigations in arrays of nanowires and a single nanowire. The analysis of phonon behavior reveals the possibility to perform advanced mechanical characterization and to vary the thermal properties of nanowires. The review concludes with a brief perspective on future research directions, from phonon-induced control over properties to three-dimensional (3D) acoustic nano-imaging.

Electronic, magnetic, thermoelectric and lattice dynamical properties of full heusler alloy Mn2RhSi: DFT study

The manganese based heusler compounds are important class of materials having remarkable ferrimagnetic and thermoelectric properties. Among them, Mn2RhSi which crystallizes in both Hg2CuTi and Cu2MnAl type structure have undergone detailed investigation for its electronic, magnetic and mechanical properties in Hg2CuTi type structure. However, no studies have been performed on Cu2MnAl type structure of Mn2RhSi. In this study we have performed a detailed comparative study on electronic, magnetic, mechanical, thermoelectric and lattice dynamical behavior of both types of Mn2RhSi structures. The higher value of thermal conductivity reduces the thermo electric figure of merit in the material and has large value of Seebeck coefficient making these alloys as a potential candidate for thermoelectric device application. Our results show high value of spin polarization ratio for Mn2RhSi which can increase the efficiency of spintronics devices which further have direct application in magnetic random access memory device. The heusler compound are least studied for their lattice dynamical behavior. In our work, we perform detail investigation on lattice dynamical behavior of both type structures of Mn2RhSi. Our calculations revealed the anomalous behavior of optical phonons in Cu2MnAl type structure which was attributed to the presence of peak right at the Fermi level in electronic density of states whereas phonon dispersion curves of Hg2CuTi type structure confirms its dynamical stability.

Phonon backscatter, trapping, and misalignment effects on microscale thermal conductance below the Casimir limit

At nanometer to micron length scales, there exists a strong competition between intrinsic and extrinsic scattering mechanisms that can curtail the free flight of phonons and ultimately affect the thermal transport. Despite significant progress in showing the ability to reach behaviors significantly below the Casimir limit, little appears to be understood about the competition between these scattering sources. In this investigation, we propose a simple one-parameter geometry that simultaneously modulates backscattering and trapping effects to enable directed study of these different means of controlling phonons. The geometry is a simple sequence of chambers offset from one another by a defined distance. We use the geometry to study the effects of phonon backscatter, trapping, and corner-turning on the thermal conductance in Si nanowires (NWs). We employ a full Brillouin zone Boltzmann Transport Equation (BTE) method to determine spatially-varying phonon densities in the geometry. Significantly greater impact is seen due to backscatter than any other means of arresting phonon flow. By creating a geometry that maximizes backscatter, a roughly 8-fold reduction in thermal conductance below the Casimir limit can be achieved at room temperature which is a factor of four smaller than the nearest reported value in the literature. The geometry is also useful for systematic investigation of other means of controlling phonons and affecting thermal transport; particularly, we investigate diffuse versus specular boundary scattering and the induced misalignment between the phonon flow and thermal flux due to the shape of the geometry. These effects combine to offer new insights into fundamental phonon behaviors and possible routes to phonon control.

Design of Highly Efficient Thermoelectric Materials: Tailoring Reciprocal-Space Properties by Real-Space Modification.

Although restricted by the poor performance at present, thermoelectric materials for power-generation devices and solid-state Peltier coolers still possess unlimited vitality, thus capturing considerable attention. Understanding and manipulating the electrical and thermal transport mechanisms in thermoelectrics play significant roles in tailoring the properties of various thermoelectric materials. The transport behavior of electrons and phonons are closely related to the chemical composition and structure, which are defined in real space. Meanwhile, transport properties are also contingent on the band structure and phonon spectrum, both of which are represented in the reciprocal-space first Brillouin zone. Real space and reciprocal space are bridged by the Fourier transform, and the combination of real-space and reciprocal-space properties will provide more possibilities for regulating transport characteristics. Herein, a compendious discussion of the internal connection between real space and reciprocal space, and the underlying physics and chemistry is presented. Then, how the relationship between real and reciprocal space provides additional insights to govern electrical and thermal transport parameters is elaborated upon, thereby enabling the discovery and optimization of thermoelectric materials. In conclusion, specific challenges and feasible directions are discussed.

Wavevector and polarization resolved analysis of phonon scattering from embedded nanoparticles

Embedded nanocrystals are capable of dramatically reducing the thermal conductivity of alloy semiconductors through increased phonon scattering, but many aspects of thermal wavelength phonon interactions with embedded nanoparticles remain understudied. Here, the wavevector- and polarization-resolved capabilities of the Frequency Domain Perfectly Matched Layer (FDPML) computational technique are exploited to study several fundamentally important phonon scattering problems across the entire Brillouin zone. We compare the atomistic predictions of FDPML against continuum mechanics approaches for spherically embedded particles. For long to mid-wavelength phonons, reasonable agreement is found with continuum theories that consider the Mie regime accurately, while commonly used “patching” theories which empirically connect the Rayleigh scattering to the geometric limit are shown to have poor agreement with more rigorous approaches. Next, the scattering cross section of optical phonons from nanoparticles is explored for the first time. We show that the scattering behavior of optical phonons is fundamentally different than their acoustic counterparts in that long wavelength optical phonons exhibit a scattering cross section nearly independent of wavelength, which we interpret as being due to zone folding of short wavelength acoustic modes from the geometric scattering regime into long wavelength optical modes. Finally, we study the scattering cross section of nanoparticles exhibiting atomic interdiffusion at the matrix-particle interface, where we find that interdiffusion both suppresses Mie oscillations and substantially increases the scattering cross section at short wavelength, compared to a solid nanoparticle with the same number of impurity atoms.

Optical Phonon Behaviors of Photocharged Nanocrystals: Effects of Free Charge Carriers.

For semiconductor nanocrystals (NCs), the precise knowledge of phonons in the presence of free carriers is important for understanding their electronic and photonic properties in device applications. With Raman spectroscopy, this study investigates the effects of free charge carriers on optical phonon behaviors of NCs. The adoption of the photocharging method allows us to introduce free charge carriers into NCs without inducing other side effects. In the photocharged ZnO NCs, lower longitudinal optical (LO) phonon frequencies and weaker LO overtones relative to the fundamentals were found, which was explained by the screening and band-filling effects caused by the induced free carriers. The free carrier effects on optical phonon behaviors of NCs, usually neglected in previous studies, should be taken into consideration when discussing the electronic and photonic properties of NC-based devices.

Investigating Thermal Behavior of Surface Phonon in SiC by in-situ Vibrational Spectroscopy

Investigating Thermal Behavior of Surface Phonon in SiC by in-situ Vibrational Spectroscopy

Absence of superconductivity in iron polyhydrides at high pressures

Recently, C. M. P\'epin \textit{et al.} [Science \textbf{357}, 382 (2017)] reported the formation of several new iron polyhydrides FeH$_x$ at pressures in the megabar range, and spotted FeH$_5$, which forms above 130 GPa, as a potential high-\tc \ superconductor, because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A.~Majumdar \textit{et al.} [Phys. Rev. B \textbf{96}, 201107 (2017)] and A.~G.~Kvashnin \textit{et al.} [J. Phys. Chem. C \textbf{122}, 4731 (2018)] based on {\em ab initio} Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We conversely find, on the same theoretical-numerical basis, that neither FeH$_5$ nor its precursor, FeH$_3$, shows any conventional superconductivity and explain why this is the case. We also show that superconductivity may be attained by transition-metal polyhydrides in the FeH$_3$ structure type by adding more electrons to partially fill one of the Fe--H hybrid bands (as, e.g., in NiH$_3$). Critical temperatures, however, will remain low because the $d$--metal bonding, and not the metallic hydrogen, dominates the behavior of electrons and phonons involved in the superconducting pairing in these compounds.

A practical field guide to thermoelectrics: Fundamentals, synthesis, and characterization

The study of thermoelectric materials spans condensed matter physics, materials science and engineering, and solid-state chemistry. The diversity of the participants and the inherent complexity of the topic mean that it is difficult, if not impossible, for a researcher to be fluent in all aspects of the field. This review, which grew out of a one-week summer school for graduate students, aims to provide an introduction and practical guidance for selected conceptual, synthetic, and characterization approaches and to craft a common umbrella of language, theory, and experimental practice for those engaged in the field of thermoelectric materials. This review does not attempt to cover all major aspects of thermoelectric materials research or review state-of-the-art thermoelectric materials. Rather, the topics discussed herein reflect the expertise and experience of the authors. We begin by discussing a universal approach to modeling electronic transport using Landauer theory. The core sections of the review are focused on bulk inorganic materials and include a discussion of effective strategies for powder and single crystal synthesis, the use of national synchrotron sources to characterize crystalline materials, error analysis, and modeling of transport data using an effective mass model, and characterization of phonon behavior using inelastic neutron scattering and ultrasonic speed of sound measurements. The final core section discusses the challenges faced when synthesizing carbon-based samples and the measuring or interpretation of their transport properties. We conclude this review with a brief discussion of some of the grand challenges and opportunities that remain to be addressed in the study of thermoelectrics.

Comparison of Phonon Damping Behavior in Quantum Dots Capped with Organic and Inorganic Ligands.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment.

High-Pressure, High-Temperature Behavior of Silicon Carbide: A Review

The high-pressure behavior of silicon carbide (SiC), a hard, semi-conducting material commonly known for its many polytypic structures and refractory nature, has increasingly become the subject of current research. Through work done both experimentally and computationally, many interesting aspects of high-pressure SiC have been measured and explored. Considerable work has been done to measure the effect of pressure on the vibrational and material properties of SiC. Additionally, the transition from the low-pressure zinc-blende B3 structure to the high-pressure rocksalt B1 structure has been measured by several groups in both the diamond-anvil cell and shock communities and predicted in numerous computational studies. Finally, high-temperature studies have explored the thermal equation of state and thermal expansion of SiC, as well as the high-pressure and high-temperature melting behavior. From high-pressure phase transitions, phonon behavior, and melting characteristics, our increased knowledge of SiC is improving our understanding of its industrial uses, as well as opening up its application to other fields such as the Earth sciences.

On the diversity in the thermal transport properties of graphene: A first-principles-benchmark study testing different exchange-correlation functionals

Due to the extraordinary mechanical, electronic, optical, and thermal transport properties, graphene had been one of the most fascinating and extensively studied two-dimensional materials in recent years. Benefiting from the extremely high thermal conductivity, the thermal transport in graphene has received exceptional attention in both experimental and theoretical studies. In this paper, based on first-principles calculations, we performed systematic study on the thermal transport properties of graphene using 10 different exchange-correlation (XC) functionals combined with ultrasoft or projector augmented wave (PAW) pseudopotentials. The thermal transport properties of graphene show distinct diversity for different XC functionals despite their overall good agreement with experimental measurements. It is found that the thermal conductivity of graphene could range from 1936 to 4376 W m−1 K−1 with different XC functionals employed. The reason for the diversity in the thermal transport properties of graphene is analyzed based on the insight into the mode level phonon behaviors. Our study executed a comprehensive investigation of the thermal transport properties of graphene with different XC functionals employed, which would shed light on future researches related to graphene and other novel materials.

Diamond family of colloidal supercrystals as phononic metamaterials

Colloidal crystals provide a versatile platform for designing phononic metamaterials with exciting applications for sound and heat management. New advances in the synthesis and self-assembly of anisotropic building blocks such as colloidal clusters have expanded the library of available micro- and nano-scale ordered multicomponent structures. Diamond-like supercrystals formed by such clusters and spherical particles are notable examples that include a rich family of crystal symmetries such as diamond, double diamond, zinc-blende, and MgCu2. This work investigates the design of phononic supercrystals by predicting and analyzing phonon transport properties. In addition to size variation and structural diversity, these supercrystals encapsulate different sub-lattice types within one structure. Computational models are used to calculate the effect of various parameters on the phononic spectrum of diamond-like supercrystals. The results show that structures with relatively small or large filling factors (f > 0.65 or f < 0.45) include smaller bandgaps compared to those with medium filling factors (0.65 > f > 0.45). The double diamond and zinc-blende structures render the largest bandgap size compared to the other supercrystals studied in this paper. Additionally, this article discusses the effect of incorporating various configurations of sub-lattices by selecting different material compositions for the building blocks. The results suggest that, for the same structure, there exist multiple phononic variants with drastically different band structures. This study provides a valuable insight for evaluating novel colloidal supercrystals for phononic applications and guides the future experimental work for the synthesis of colloidal structures with desired phononic behavior.

Thermal Engineering in Low‐Dimensional Quantum Devices: A Tutorial Review of Nonequilibrium Green's Function Methods

AbstractThermal engineering of quantum devices has attracted much attention since the discovery of quantized thermal conductance of phonons. Although easily submerged in numerous excitations in macrosystems, quantum behaviors of phonons manifest in nanoscale low‐dimensional systems even at room temperature. Especially in nanotransport devices, phonons move quasiballistically when the transport length is smaller than their bulk mean free paths. It has been shown that the phonon nonequilibrium Green's function method (NEGF) is effective for the investigation of nanoscale quantum transport of phonons. Here, two aspects of thermal engineering of quantum devices are discussed using NEGF methods. One covers transport properties of pure phonons; the other concerns caloritronic effects, which manipulate other degrees of freedom, such as charge, spin, and valley, via the temperature gradient. For each part, the basic theoretical formalisms are outlined first, then a survey of related investigations on models or realistic materials is presented. Particular attention is given to phonon topologies and the generalized phonon NEGF method. Finally, conclusions are offered and the study is summarized with an outlook.

Reduction of Thermal Conductivity in Nanowires by Combined Engineering of Crystal Phase and Isotope Disorder.

Nanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors 28SiH4, 29SiH4, and 30SiH4 with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to ∼40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed 28Si x30Si1- x nanowires with composition close to the highest mass disorder ( x ∼ 0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic devices.