Anharmonic Phonon Research Articles

Non-Perturbative Determination of Isotope-induced Anomalous Vibrational Physics.

In general, vibrational physics has been well described by quantum perturbation theory to provide footprint characteristics for common crystals. However, despite weak phonon anharmonicity, the recently discovered cubic crystals have shown anomalous vibrational dynamics with elusive fundamental origin. Here, we developed a non-perturbative ab initio approach, in together with spectroscopy and high-pressure experiments, to successfully determine the exact dynamic evolutions of the vibrational physics for the first time. We found that the local fluctuation and coupling isotopes significantly dictate the vibrational spectra, through the Brillouin zone folding that has been previously ignored in literature. By decomposing vibrational spectra into individual isotope eigenvectors, we observed both positive and negative contributions to Raman intensity from constitutional atoms (10B, 11B, 75As or 31P). Importantly, our non-perturbative theory predicts that a novel vibrational resonance appears at high hydrostatic pressure due to broken translational symmetry, which was indeed verified by experimental measurement under a pressure up to 31.5 GPa. Our study develops fundamental understandings for the anomalous lattice physics under the failure of quantum perturbation theory and provides a new approach in exploring novel transport phenomena for materials of extreme properties.

Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi4O4SeCl2.

Crystalline Bi4O4SeCl2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κL), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κL in crystalline and pellet Bi4O4SeCl2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi4O4SeCl2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κL underscores an intrinsic approach for designing extremely low κL materials.

Anharmonic phonon scattering study in wide bandgap semiconductor β -Ga2O3 by Raman spectroscopy

The low thermal conductivity of β-Ga2O3 single crystal imposes limitations on its device performance under high-temperature and high-power conditions. Phonons are widely recognized to play the main role in heat transport, while the anharmonic scattering among these quasi-particles significantly diminishes thermal conductivity. Therefore, investigating the anharmonic phonon scattering within β-Ga2O3 single crystal holds paramount significance. In this study, we mainly employed temperature-dependent Raman spectroscopy and first-principles calculation to give a quantitative analysis on the anharmonic phonon scattering. The mode Grüneisen parameters and temperature-dependent thermal expansion coefficient were studied. Strong anharmonic phonon scattering was also confirmed, which was mainly caused by cubic scattering, but quartic scattering is also nonnegligible and causes nonlinear linewidth broadening. Moreover, high wavenumber peaks are more likely to undergo asymmetric phonon scattering by comparing the symmetric and asymmetric scattering models. In all, this study provides a profound understanding of the anharmonic phonon scattering properties within the β-Ga2O3 single crystal. It enriches our understanding of quasi-particle interaction dynamics and offers theoretical pathways for expanding the applications of β-Ga2O3 single crystal and optimizing its device performance.

Substrate-induced bonding asymmetry leading to strong phonon anharmonicity and largely reduced thermal conductivity of monolayer XS2 (X = Mo and W) from first-principles calculations

Thermoelectric materials have properties such as direct conversion of waste heat into electricity and environmental friendliness, which make them promising for renewable energy utilization, however, the lower conversion efficiency limits the large-scale application of thermoelectric materials. Monolayer transition metal dichalcogenides (TMDs) are candidates for thermoelectric materials due to their high power factor, but their higher thermal conductivity (TC) limits the further improvement of the figure of merit (ZT). Here, we employ first-principles calculations and solve the Boltzmann transport equation (BTE) to compare the lattice TC of free-standing monolayer XS2 (X = Mo and W) and monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates. We find that the lattice TC of monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates are sharply reduced. Especially supported on the ZnO (0001) substrate, the lattice TC of XS2 reduce by 65 %–70 %, which is expected to improve the thermoelectric properties of monolayer XS2. In addition, we report the underlying cause of this significant reduction in TC. Firstly, the substrate drives softening of the acoustic branch, which leads to a decrease of phonon group velocity in the low-frequency and an enhancement of the three-phonon scattering. Then, the asymmetric bonding of XS2 is induced by the substrates, which enhance phonon anharmonicity and reduce the phonon lifetime. Our results demonstrate that the lattice TC of monolayer XS2 can be tuned by adjusting the substrate, which has certain reference values for the thermoelectric design of XS2.

Highly correlated structural, local structural, Raman spectroscopic and magnetic properties of Mn-substituted Cu2V2O7

Low-dimensional quantum spin ½ system Cu2V2O7 has been investigated in the framework of Mn-substitution at the Cu site, which is really un-investigated. The studied compounds Cu2 − x Mn x V2O7 (x = 0, 0.05, 0.1 and 0.15) have been synthesized and characterized structurally, spectroscopically, local structurally and magnetically via x-ray diffraction, Raman, x-ray absorption and temperature, field dependent magnetization measurements respectively. Although Cu2V2O7 can be found in α, β and γ-phase, however all of the studied compounds are found in single orthorhombic α-phase which has crucial magneto-electric application potential. Temperature dependent Raman spectra indicated anharmonic phonon–phonon scattering but there is no spin–phonon coupling for VO4 vibrational modes. The local structure probed via x-ray absorption near edge structure and extended x-ray absorption fine structure spectroscopy at 15 K, 300 K indicates Cu2+, V5+ and mixed valent Mn2+ and Mn3+ ionic states and justified local structure for the probed ions. Magnetic measurements indicate long-range antiferromagnetic ordering with doping independent Neel temperature (32.5 K). Further observations are strong magnetic hysteresis at 5 K (due to canted spin structure), zero field exchange-bias and their noteworthy enhancement upon Mn-substitution. Interesting correlation between structural parameters and magnetic exchanges has been developed.

Graphene on SiO2/Si and Al2O3 under thermal annealing and electric current: Competition of dopant desorption and conformation to substrate

In this work, we study phonon and electronic properties of graphene on SiO2/Si and Al2O3 by simultaneous Raman and electrical measurements in the temperature range from room temperature to 550 °C, or at voltages from 20 to −20 V. The dependencies of G and 2D peak parameters and electrical resistance on temperature and voltage made it possible to observe in situ a competition between the p-type adsorbate removal from graphene surface and substrate-induced doping due to graphene-substrate conformality increase, both stimulated by either ambient or Joule heating. The analyzed parameters were dominated by the conformality increase, with the hole density increasing significantly and unidirectionally, while resistance and I-V curves fluctuated due to the competition. Having calculated Raman peak shift temperature coefficients, resistance temperature coefficients, total variations of carrier density, resistance and strain, we show that Al2O3 substrate can be used to reduce the desorption barrier, the overall doping, the impact on graphene resistance and on phonon anharmonicity – however, it should be used with regard to the possibility of introducing strain or stronger doping after longer treatments. The conformality effects should be taken into account when performing annealing, as well as graphene applications for sensors or strong electric currents.

Equation of state predictions for ScF3 and CaZrF6 with neural network-driven molecular dynamics.

In silico property prediction based on density functional theory (DFT) is increasingly performed for crystalline materials. Whether quantitative agreement with experiment can be achieved with current methods is often an unresolved question, and may require detailed examination of physical effects such as electron correlation, reciprocal space sampling, phonon anharmonicity, and nuclear quantum effects (NQE), among others. In this work, we attempt first-principles equation of state prediction for the crystalline materials ScF3 and CaZrF6, which are known to exhibit negative thermal expansion (NTE) over a broad temperature range. We develop neural network (NN) potentials for both ScF3 and CaZrF6 trained to extensive DFT data, and conduct direct molecular dynamics prediction of the equation(s) of state over a broad temperature/pressure range. The NN potentials serve as surrogates of the DFT Hamiltonian with enhanced computational efficiency allowing for simulations with larger supercells and inclusion of NQE utilizing path integral approaches. The conclusion of the study is mixed: while some equation of state behavior is predicted in semiquantitative agreement with experiment, the pressure-induced softening phenomenon observed for ScF3 is not captured in our simulations. We show that NQE have a moderate effect on NTE at low temperature but does not significantly contribute to equation of state predictions at increasing temperature. Overall, while the NN potentials are valuable for property prediction of these NTE (and related) materials, we infer that a higher level of electron correlation, beyond the generalized gradient approximation density functional employed here, is necessary for achieving quantitative agreement with experiment.

Intrinsically Low Thermal Conductivity in a Novel Cu-S Modified ZrS2 Compound with Asymmetric Bonding.

Materials with low thermal conductivity have received significant attention across various research fields, including thermal insulation materials, thermal barrier coatings, and thermoelectric materials. Exploring novel materials with intrinsically low thermal conductivity and investigating their phonon transport properties, chemical bonding, and atomic coordination are crucial. In this study, a novel ternary sulfide is successfully discovered, Cu2 ZrS3 , which is achieved by introducing copper ions into both the interlayer and intralayer of ZrS2 . The resulting structure encompasses various coordination forms within each layer, such as [CuS4 ], [ZrS6 ], and [CuS3 ], leading to pronounced phonon anharmonicity induced by the asymmetric bonding of tri-coordinated Cu atoms within the [ZrS6 ] layer. As a result, Cu2 ZrS3 exhibits intrinsically low lattice thermal conductivity (κL ) of about 0.83 W m-1 K-1 at 300 K and 0.35 W m-1 K-1 at 683 K, which are in the exceptionally low level among sulfides. In comparison to the conventional approach of inserting guests between layers, the substitution of atoms within layers provides a novel and effective strategy for designing low κL materials in transition metal dichalcogenides (TMDCs).

Lattice Dynamics and Thermal Transport in Semiconductors with Anti-Bonding Valence Bands.

Achieving high thermoelectric performance requires efficient manipulation of thermal conductivity and a fundamental understanding of the microscopic mechanisms of phonon transport in crystalline solids. One of the major challenges in thermal transport is achieving ultralow lattice thermal conductivity. In this study, we use the anti-bonding character of the highest-occupied valence band as an efficient descriptor for discovering new materials with an ultralow thermal conductivity. We first examined the relationship between anti-bonding valence bands (ABVBs) and low lattice thermal conductivity in model systems PbTe and CsPbBr3. Then, we conducted a high-throughput search in the Materials Project database and identified over 600 experimentally stable binary semiconductors with an anti-bonding character in their valence bands. From our candidate list, we conducted a comprehensive analysis of the chemical bonds and the thermal transport in the XS family, where X = K, Rb, and Cs are alkaline metals. These materials all exhibit ultralow thermal conductivities less than 1 W/(m K) at room temperature despite simple structures. We attributed the ultralow thermal conductivity to the weakened bonds and increased phonon anharmonicity due to their ABVBs. Our results provide chemical intuitions to understand lattice dynamics in crystals and open up a convenient venue toward searching for materials with an intrinsically low lattice thermal conductivity.

Investigation of Temperature-Dependent Phonon Anharmonicity and Thermal Transport in SnS Single Crystals.

Tin sulfide has outstanding thermoelectric properties in the b-axis direction of crystallography as a IV-VI group layered compound, which arouses great attention. In this study, temperature-dependent Raman spectroscopy (TDRS) is used to quantify the phonon anharmonicity in SnS crystals from 77 to 475 K, where the three-phonon process dominates in this temperature region. Moreover, integration of the four-phonon process and lattice thermal expansion will better describe the temperature-dependent Raman experimental phenomenon. The good agreement between the calculated and experimental lattice thermal conductivity confirms the three-phonon scattering process is the dominant scattering mechanism at this temperature range. Further, combining the atomic thermal displacement and charge density through density functional theory calculation, the inherently low thermal conductivity of SnS is because of strong lattice anharmonicity, which is brought by the presence of asymmetric chemical bonding resulting from the Sn 5s2 lone pair electrons. These results provide key insights for studying thermal properties of other low-dimensional materials.

The filled tetrahedral compound NaZnP with strong quartic anharmonicity and good thermoelectric properties

Based on first-principles calculations, self-consistent phonon (SCP) theory, and the Boltzmann transport equation, we investigate the thermal transport and thermoelectric properties of the Nowotny-Juza filled tetrahedral compound NaZnP. The effects of anharmonic phonon renormalization and four-phonon scattering on the heat transport of NaZnP are also considered. The weak binding and rattling-like vibrational behavior of Na atoms lead to strong lattice anharmonicity, and the results show that NaZnP has low lattice thermal conductivity. Furthermore, the coexistence of high dispersion and high degeneracy of valence band maxima leads to high power factor under p-type doping. The combination of low lattice thermal conductivity and good electrical transport properties results in high thermoelectric performance. This indicates that NaZnP is a promising p-type high-temperature thermoelectric material.

Enhancement of charge density wave correlations in a Holstein model with an anharmonic phonon potential

Enhancement of charge density wave correlations in a Holstein model with an anharmonic phonon potential

Thermoelectric response of Janus monolayer M2P2S3Se3 (M = Zn and Cd).

Thermoelectric transport properties of Janus monolayers M2P2S3Se3 (M = Zn and Cd) are investigated by the first-principles based transport theory. The Zn2P2S3Se3 and Cd2P2S3Se3 monolayers are indirect-gap semiconductors. The high polarizability of M-Se/S bonds in the MS3Se3 distorted octahedrons leads to anharmonic phonon behavior, which produces an intrinsic lattice thermal conductivity (κl) as low as 1.06 and 1.99W m-1 K-1 at 300K for Zn2P2S3Se3 and Cd2P2S3Se3 monolayers, respectively. The lower κl of the Zn2P2S3Se3 monolayer is mainly attributed to more pronounced flat modes of the phonon dispersion in a frequency range of 1-1.7THz caused by the softer Zn-Se/S bonds. The polar optical phonon scattering of carriers surprisingly plays a dominant role in carrier transport of both the monolayers, which greatly suppresses the electrical conductivity and thereby the power factor by about an order of magnitude. The predicted figure of merit (zT) increases monotonically with the temperature at the optimal carrier density, and at the operating temperature of 1200K, it reaches an optimal value of 0.86 at an optimal electron density of ∼1.5×1013 cm-2 for the n-type Zn2P2S3Se3 monolayer and 0.30 at an optimal electron density of ∼7×1012 cm-2 for the n-type Cd2P2S3Se3 monolayer.

Phonon Anharmonicity and Spin-Phonon Coupling in CrI3.

We report on the far-infrared, temperature-dependent optical properties of a CrI3 transition metal halide single crystal, a van der Waals ferromagnet (FM) with a Curie temperature of 61 K. In addition to the expected phonon modes determined by the crystalline symmetry, the optical reflectance and transmittance spectra of CrI3 single crystals show many other excitations as a function of temperature as a consequence of the combination of a strong lattice anharmonicity and spin-phonon coupling. This complex vibrational spectrum highlights the presence of entangled interactions among the different degrees of freedom in CrI3.

Reciprocal space temperature-dependent phonons method from ab-initio dynamics

We present a robust reciprocal-space implementation of the temperature-dependent effective potential method, our implementation can scale easily to large cell and long sampling time. It is interoperable with standard ab-initio molecular dynamics and with Langevin dynamics. We prove that both sampling methods can be efficient and accurate if a thermostat is used to control temperature and dynamics parameters are used to optimize the sampling efficiency. By way of example, we apply it to study anharmonic phonon renormalization in weakly and strongly anharmonic materials, reproducing the temperature effect on phonon frequencies, crossing of phase transition, and stabilization of high-temperature phases.

Temperature dependence Raman spectroscopy studies of CVD grown few layer MoS2 triangular domains

The effect of thermal perturbations on different Raman-active phonon modes was investigated using temperature-dependent Raman Spectroscopy of few layers MoS2 triangular domains (FMTD). The MoS2 triangular domains were prepared using the chemical vapour deposition (CVD) method and were characterised using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and micro Raman spectroscopy. The micro Raman spectra of few FMTD recorded at higher temperatures (above room temperature) and were analysed using a theoretical framework developed by taking hot phonon anharmonicity into account. Temperature-dependent responses for the E12g, A1g, and 2LA(M) modes soften as the temperature rises from 25 °C to 300 °C. The first-order temperature coefficients of MoS2 triangular domains, E12g, A1g, and 2LA(M) modes are −0.018 cm-1K−1, −0.015 cm-1K−1, and −0.017 cm-1K−1, respectively. The experimental results revealed that phonon–phonon kinematics played a dominant role in broadening and peak shifts in the Raman mode, confirming the anharmonic effect's prevalence. Three or four phonon decay processes were used to explain the boarding and redshift in the peak position of temperature-dependent Raman mode. Moreover, thermal management is so crucial to the performance of low-dimensional devices, our findings pave the way for improved design strategies in this field.

Phonon transport across GaN-diamond interface: The nontrivial role of pre-interface vacancy-phonon scattering

Diamond substrate with superior thermal conductivity has been the most promising heat sink to solve the heat dissipation issues in GaN-based power electronics. The phonon transport behaviors across the GaN-diamond interface crucially determine the thermal performance and still remain largely unexplored especially considering the presence of pre-interface vacancies. Herein, the influence of localized high-concentration Ga/N atomic vacancies on phonon transport across the GaN-diamond interface is fully investigated using non-equilibrium molecular dynamics. The calculated interface thermal resistance (ITR) of the perfect GaN-diamond interface at room temperature is approximately 1.5 m2·K/GW, consistent with prediction of the diffusion mismatch model. After introducing the Ga/N vacancies at a concentration of 30%, the existence of vacancy-phonon scattering increases the interfacial thermal resistance (ITR) by up to 67%. Analyzing the phonon density of states and the spectral heat current, the heat flux contributed by GaN high-frequency phonons decreases after introducing vacancy defects. This is attributed that the vacancy-phonon scattering in GaN enhances anharmonic phonon scattering, and the phonon energy is redistributed among different phonon modes. Moreover, the collective vibration of propagation phonons is disrupted randomly by the vacancy-phonon scattering. This work aims to understand the influence of vacancy-phonon scattering on the phonon transport across GaN-diamond interface and provide helpful guidance to engineering such interface for better heat dissipation performance.

Role of high-order anharmonicity and off-diagonal terms in thermal conductivity: A case study of multiphase CsPbBr3

We investigate the influence of three- and four-phonon scattering, perturbative anharmonic phonon renormalization, and off-diagonal terms of coherent phonons on the thermal conductivity of ${\mathrm{CsPbBr}}_{3}$ phase change perovskite, by using advanced implementations and first-principles simulations. Our study spans a wide temperature range covering the entire structural spectrum. Notably, we demonstrate that the interactions between acoustic and optical phonons result in contrasting trends of phonon frequency shifts for the high-lying optical phonons in orthorhombic and cubic ${\mathrm{CsPbBr}}_{3}$ as temperature varies. Our findings highlight the significance of wavelike tunneling of coherent phonons in ultralow and glasslike thermal conductivity in halide perovskites.

Visualizing electron–phonon and anharmonic phonon–phonon coupling in the kagome ferrimagnet GdMn6Sn6

Kagome magnet RMn6Sn6 (R = Gd-Tm, Lu) with unusual lattice geometry and breaking of time-reversal symmetry is a promising platform to investigate the interaction of topology and magnetism. Since phonons play a vital role in the coupling between magnetism and topological fermions, a fundamental understanding of phonon dynamics is of great significance in this emerging research field. Here, we report a systematic investigation of ultrafast coherent phonon dynamics in GdMn6Sn6 crystals as a function of temperature and excitation fluence using time-resolved pump-probe spectroscopy. When the temperature decreases, the coherent phonon exhibits a hardening trend in frequency with a suppressed decay rate, which can be well-explained by the anharmonic scattering model. Unexpectedly, both the frequency and decay rate of coherent phonons are almost independent of excitation fluence, suggesting a weak electron–phonon scattering process in GdMn6Sn6.

Probing anharmonic phonons in WS2 van der Waals crystal by Raman spectroscopy and machine learning

Understanding the optothermal physics of quantum materials will enable the efficient design of next-generation photonic and superconducting circuits. Anharmonic phonon dynamics is central to strongly interacting optothermal physics. This is because the pressure of a gas of anharmonic phonons is temperature dependent. Phonon-phonon and electron-phonon quantum interactions contribute to the anharmonic phonon effect. Here we have studied the optothermal properties of physically exfoliated WS2 van der Waals crystal via temperature-dependent Raman spectroscopy and machine learning strategies. This fundamental investigation will lead to unveiling the dependence of temperature on in-plane and out-of-plane Raman shifts (Raman thermometry) of WS2 to study the thermal conductivity, hot carrier diffusion coefficient, and thermal expansion coefficient.