Phonon Heat Capacity Research Articles

Extracting Thermodynamic Properties of Carbyne from Tip-Enhanced Raman Scattering Images.

The measurement of thermodynamic properties for nanosystems is essential to comprehend the inherent characteristics of nanomaterials. Traditional spectroscopy measurements, such as Raman or ultraviolet-visible spectroscopies, are limited to offering insights near the Γ point in the Brillouin zone and thus cannot precisely determine the system's thermodynamic properties, for example, heat capacity. Utilizing the intrinsic broad momentum distribution in highly confined plasmonic fields, here we take sp-hybridized carbyne as a proof-of-the-principle example to show that ultrahigh-resolution tip-enhanced Raman scattering (TERS) images have the ability to access all k-points in the phonon Brillouin zone of one-dimensional nanosystems, allowing the comprehensive determination of vibrational features and heat capacity for finite carbon chains. Comparing phonon dispersion spectra and heat capacities under different boundary conditions, i.e., linear carbon chains and cyclic carbon molecules, we find that the heat capacities of linear structures converge more rapidly than the counterparts of cyclic structures to the benchmark of ideal carbyne. We also study the effects of different terminal groups in linear structures as well as the aromaticity in cyclic structures on heat capacity. This study provides a practical method for characterizing the thermodynamic properties of nanosystems, demonstrating the potential applications of TERS imaging in nanomaterial science.

Examining graph neural networks for crystal structures: Limitations and opportunities for capturing periodicity.

Graph neural networks (GNNs) have recently been used to learn the representations of crystal structures through an end-to-end data-driven approach. However, a systematic top-down approach to evaluate and understand the limitations of GNNs in accurately capturing crystal structures has yet to be established. In this study, we introduce an approach using human-designed descriptors as a compendium of human knowledge to investigate the extent to which GNNs can comprehend crystal structures. Our findings reveal that current state-of-the-art GNNs fall short in accurately capturing the periodicity of crystal structures. We analyze this failure by exploring three aspects: local expressive power, long-range information processing, and readout function. To address these identified limitations, we propose a straightforward and general solution: the hybridization of descriptors with GNNs, which directly supplements the missing information to GNNs. The hybridization enhances the predictive accuracy of GNNs for specific material properties, most notably phonon internal energy and heat capacity, which heavily rely on the periodicity of materials.

Abnormal heat capacity of phonons in solid 4He

In the solid 4He at zero temperature, there are the acoustic waves, which manifest themselves in the form of phonon pairs. We show that the phonon system in solid 4He can undergo the Bardeen–Cooper–Schrieffer (BCS) condensation. In the BCS condensation state, the phonon system possesses a new kind of quasiparticle, the dressed phonon, whose velocity is a monotonically increasing function of temperature. The phonon system undergoes a first-order phase transition from the normal state to the BCS condensation state. We derive the low-temperature expression of the phonon heat capacity in the BCS condensation state, which considerably deviates from the Debye [Formula: see text] law in classic solids.

Structure redetermination, transport and thermal properties of the YNi3Al9 compound

Single crystals of completely ordered variant of the YNi3Al9 compound were grown by self-flux method with excess of aluminum. The crystal structure of the title compound was redetermined from single crystal X-ray diffraction data. The structure adopts ErNi3Al9 type, space group R32, parameters of the unit cell a ​= ​7.2838(2) Å, c ​= ​27.4004(8) Å. The growth of relatively large single crystals of the YNi3Al9 compound, having completely ordered structure, indicates possible existence of region on phase diagram, where the title compound is in equilibrium with liquid. Comparison with the results in the literature on the investigations of the same and related compounds shows trend towards formation of more ordered structures in flux grown samples and more disordered ones in arc melted samples.Physical properties of the title compound - electrical resistivity, magnetoresistance and heat capacity - were measured for the first time. It shows metallic-like behavior with very high values of magnetoresistance up to 420% at low temperatures without presence of magnetic elements. Therefore electronic structure calculations were carried out. The phonon heat capacity reveals major Debye and minor Einstein contributions at the intermediate temperatures, and total heat capacity approaches Dulong-Petit limit at high temperatures. The calculated Debye temperature from the whole temperature range θD ​= ​480(6) K is typical for aluminium-rich compounds. Corresponding Einstein temperature is θE ​= ​198(8) K. The estimations of electron-phonon coupling constant λ ​= ​0.092 show very weak coupling and absence of superconducting transition.

First-principles study on phonon transport properties of MoTe2 and WTe2 monolayers in different phases

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDC) monolayer is a key method for manipulating the lattice thermal conductivity. In this paper, we investigate the phonon thermal transport properties of MoTe2 and WTe2 monolayers with H and T′ phases (H-TMDC and T′-TMDC) via first-principles calculations. H-MoTe2 and H-WTe2 monolayers have isotropic lattice thermal conductivities, whereas the lattice thermal conductivities of T′-MoTe2 and T′-WTe2 monolayers are highly anisotropic. Meanwhile, in comparison with the H phase, the maximal lattice thermal conductivity in T′ phase has a reduction of 83.85% and 77.92% at 300 K for MoTe2 and WTe2 monolayers, respectively. Furthermore, we observe that phonon heat capacities and phonon lifetimes of T′-TMDC monolayers are lower than those of H-TMDC monolayers, while the phonon group velocities of T′-TMDC monolayers are larger. Hence, the competitive relationship among these parameters leads to a lower lattice thermal conductivity of the T′-TMDC monolayer. In addition, lower critical phonon MFPs (mean free path) of T′-TMDC monolayers prove that the lattice thermal conductivity has a weaker size dependence. These findings demonstrate that MoTe2 and WTe2 monolayers with different phases have a great potential in 2D nanoelectronic devices.

Phonon transport mechanism of HfO2 ultrathin film with temperature-correction full-band Monte Carlo simulation

Hafnium dioxide (HfO2) has been widely used in microelectronics nowadays and commonly withstands extremely high temperatures, so the investigation of its thermodynamic properties is particularly essential. This paper develops a temperature-correction full-band Monte Carlo (TFMC) method to investigate the HfO2 ultrathin film. The phonon dynamics parameters of HfO2 are calculated based on the first-principles method. TFMC can better describe the phonon cumulative distribution function in different temperatures by modifying the phonon relaxation time and heat capacity. The thermal conductivity of HfO2 ultrathin film is calculated based on the above method and is in good agreement with the literature. It is observed that the optical phonons in HfO2 ultrathin film are prominent in the phonon heat transport, which is quite different from the mechanism in common semiconductor materials. Combined with the full-band diffuse mismatch model, the Si-based HfO2 ultrathin film is studied. It is found that the existence of the interface with substrates improves the thermodynamic properties of the ultrathin film, which provides a reference for the selection of substrate materials.

Phonon heat capacity and self-heating normal domains in NbTiN nanostrips

Self-heating normal domains in thin superconducting NbTiN nanostrips with the granular structure were characterized via steady-state hysteretic current–voltage characteristics measured at different substrate temperatures. The temperature dependence and the magnitude of the current, which sustains a domain in equilibrium at different voltages, can only be explained with a phonon heat capacity noticeably less than expected for 3D Debye phonons. This reduced heat capacity coincides with the value obtained earlier from magnetoconductance and photoresponse studies of the same films. The rate of heat flow from electrons at a temperature to phonons in the substrate at a temperature is proportional to with the exponent p ≈ 3, which differs from the exponents for heat flows mediated by the electron–phonon interaction or by escaping of 3D Debye phonons via the film/substrate interface. We attribute both findings to the effect of grains on the phonon spectrum of thin NbTiN films. Our findings are significant for understanding the thermal transport in superconducting devices exploiting thin granular films.

Zintl Phase Compounds Mg3Sb2−xBix (x = 0, 1, and 2) Monolayers: Electronic, Phonon and Thermoelectric Properties From ab Initio Calculations

The Mg3Sb2−xBix family has emerged as the potential candidates for thermoelectric applications due to their ultra-low lattice thermal conductivity (κL) at room temperature (RT) and structural complexity. Here, using ab initio calculations of the electron-phonon averaged (EPA) approximation coupled with Boltzmann transport equation (BTE), we have studied electronic, phonon and thermoelectric properties of Mg3Sb2−xBix (x = 0, 1, and 2) monolayers. In violation of common mass-trend expectations, increasing Bi element content with heavier Zintl phase compounds yields an abnormal change in κL in two-dimensional Mg3Sb2−xBix crystals at RT (∼0.51, 1.86, and 0.25 W/mK for Mg3Sb2, Mg3SbBi, and Mg3Bi2). The κL trend was detailedly analyzed via the phonon heat capacity, group velocity and lifetime parameters. Based on quantitative electronic band structures, the electronic bonding through the crystal orbital Hamilton population (COHP) and electron local function analysis we reveal the underlying mechanism for the semiconductor-semimetallic transition of Mg3Sb2-−xBix compounds, and these electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) were calculated. We demonstrate that the highest dimensionless figure of merit ZT of Mg3Sb2−xBix compounds with increasing Bi content can reach ∼1.6, 0.2, and 0.6 at 700 K, respectively. Our results can indicate that replacing heavier anion element in Zintl phase Mg3Sb2−xBix materials go beyond common expectations (a heavier atom always lead to a lower κL from Slack’s theory), which provide a novel insight for regulating thermoelectric performance without restricting conventional heavy atomic mass approach.

First principle design of new thermoelectrics from TiNiSn based pentanary alloys based on 18 valence electron rule

In this study we have reported electronic structure, lattice dynamics, and thermoelectric (TE) transport properties of a new family of pentanary substituted TiNiSn systems using the 18 valence electron count (VEC) rule. We have modeled the pentanary substituted TiNiSn by supercell approach with the aliovalent substitution, inspite of the traditional isoelectronic substitution. We have performed the detailed analysis of electronic structure, lattice dynamics, and TE transport properties of selected systems from this family. From our calculated band structures and density of states we show that by preserving the 18 VEC through aliovalent substitutions at Ti site of TiNiSn semiconducting behavior can be achieved and hence one can tune the band structure and band gap to maximize the thermoelectric figure of merit (ZT) value. Two approaches have been used for calculating the lattice thermal conductivity (κL), one by fully solving the linearized phonon Boltzmann transport (LBTE) equation from first-principles anharmonic lattice dynamics calculations implemented in Phono3py code and other using Slack’s equation with calculated Debye temperature and Grüneisen parameter using the calculated elastic constant values. At high temperatures the calculated κL and ZT values from both these methods show very good agreement. The calculated κL values decreases from parent TiNiSn to pentanary substituted TiNiSn systems as expected due to fluctuation in atomic mass. The substitution of atoms with different mass creates more phonon scattering centers and hence lower the κL value. The calculated κL for Hf containing systems La0.25Hf0.5V0.25NiSn and non Hf containing system La0.25Zr0.5V0.25NiSn calculated from Phono3py (Slack’s equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550K, respectively and the corresponding ZT value are found to be 0.54 (0.4) and 0.77 (0.53). Among the considered systems, the calculated phonon spectra and heat capacity show that La0.25Hf0.5V0.25NiSn has more optical-acoustic band mixing which creates more phonon–phonon scattering and hence lower the κL value and maximizing the ZT. Based on the calculated results we conclude that one can design high efficiency thermoelectric materials by considering 18 VEC rule with aliovalent substitution.

Thermal Properties of 2D Dirac Materials MN4 (M = Be and Mg): A First-Principles Study.

Recently, a novel two-dimensional (2D) Dirac material BeN4 monolayer has been fabricated experimentally through high-pressure synthesis. In this work, we investigate the thermal properties of a new class of 2D materials with a chemical formula of MN4 (M = Be and Mg) using first-principles calculations. First, the cohesive energy and phonon dispersion curve confirm the dynamical stability of BeN4 and MgN4 monolayers. Besides, BeN4 and MgN4 monolayers have the anisotropic lattice thermal conductivities of 842.75 (615.97) W m–1 K–1 and 52.66 (21.76) W m–1 K–1 along the armchair (zigzag) direction, respectively. The main contribution of the lattice thermal conductivities of BeN4 and MgN4 monolayers are from the low frequency phonon branches. Moreover, the average phonon heat capacity, phonon group velocity, and phonon lifetime of BeN4 monolayer are 3.54 × 105 J K–1 m–3, 3.61 km s–1, and 13.64 ps, which are larger than those of MgN4 monolayer (3.42 × 105 J K–1 m–3, 3.27 km s–1, and 1.70 ps), indicating the larger lattice thermal conductivities of BeN4 monolayer. Furthermore, the mode weighted accumulative Grüneisen parameters (MWGPs) of BeN4 and MgN4 monolayers are 2.84 and 5.62, which proves that MgN4 monolayer has stronger phonon scattering. This investigation will enhance an understanding of thermal properties of MN4 monolayers and drive the applications of MN4 monolayers in nanoelectronic devices.

Magnetoconductance and photoresponse properties of disordered NbTiN films

We report on the study of phonon properties and electron-phonon coupling in thin NbTiN films, which are intensively exploited in superconducting devices. Studied NbTiN films with thicknesses less than 10~nm are disordered with respect to electron transport, the Ioffe-Regel parameter of $k_F l_e = 2.5-3.0$ ($k_F$ is the Fermi wave vector and $l_e$ is the electron mean free path), and inelastic electron-phonon interaction, the product $q_T l_e \ll 1$ ($q_T$ is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derived the inelastic electron-phonon scattering rate $1/\tau_{e-ph}$ and determined sound velocities and phonon heat capacities. In the temperature range from 12 to 20~K, the scattering rate varies with temperature as $1/\tau_{e-ph}\propto T^{3.45\pm0.05}$; its value extrapolated to 10~K amounts to approximately 16~ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we found a systematic reduction of the sound velocity in all these films by about 50\% as compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology.

The Phonon Heat Capacity of Small Dielectric Crystals at Low Temperatures

The Phonon Heat Capacity of Small Dielectric Crystals at Low Temperatures

Phonon transport properties of two dimensional group-III nitrides (BN, AlN, and GaN)

In this work, the lattice thermal conductivity of two-dimensional honeycomb structures of BN, AlN, and GaN studied using first-principles density functional theory and solving the linearized Boltzmann transport equation by relaxation time approximation. Investigation of the influence of the phonon modes on the thermal conductivity revealed that ZA, and LA + TA + ZO modes have the most and TO + LO have the least contribution to the thermal conductivity of BN whereas in AlN, the contribution of ZA decreases while the effect of TO + LO increases. In GaN, the TO + LO branches Play the major role on the thermal conductivity. The observed differences in the lattice thermal conductivity of these monolayers were explained by considering the variations of their frequency-dependent phonon lifetimes, group velocities, and heat capacities. In BN monolayer, the group velocity and phonon lifetime of the acoustic phonons found to be higher whereas the heat capacity of it's LO and TO branches was lower than the monolayers of AlN and GaN and therefore, the contribution of the acoustical phonon branches to thermal conductivity has become more important. However in GaN, the LO and TO phonons revealed the highest phonon lifetime and specific heat resulting the dominance of the contribution of these phonon modes to the total thermal conductivity.

Angle-tunable intersubband photoabsorption and enhanced photobleaching in twisted bilayer graphene

Van der Waals heterostructures obtained by artificially stacking two-dimensional crystals represent the frontier of material engineering, demonstrating properties superior to those of the starting materials. Fine control of the interlayer twist angle has opened new possibilities for tailoring the optoelectronic properties of these heterostructures. Twisted bilayer graphene with a strong interlayer coupling is a prototype of twisted heterostructure inheriting the intriguing electronic properties of graphene. Understanding the effects of the twist angle on its out-of-equilibrium optical properties is crucial for devising optoelectronic applications. With this aim, we here combine excitation-resolved hot photoluminescence with femtosecond transient absorption microscopy. The hot charge carrier distribution induced by photo-excitation results in peaked absorption bleaching and photo-induced absorption bands, both with pronounced twist angle dependence. Theoretical simulations of the electronic band structure and of the joint density of states enable to assign these bands to the blocking of interband transitions at the van Hove singularities and to photo-activated intersubband transitions. The tens of picoseconds relaxation dynamics of the observed bands is attributed to the angle-dependence of electron and phonon heat capacities of twisted bilayer graphene.

Heat capacity in doped graphene under magnetic fields: the role of spin splitting

We study the electronic heat capacity in doped graphene under magnetic fields. The partition function is calculated considering only the thermal excitations in the last occupied energy levels. Due to the large energy separation between the Landau levels (LLs) and the Zeeman splitting, at low temperatures the heat capacity is dominated by the spin excitations in the last occupied LL. Correspondingly the heat capacity oscillates with maximum amplitude at half filling of each LL. At higher temperatures the inter-LLs excitations dominate the heat capacity, with maximum amplitude at full filling factors. The oscillation amplitudes are compared with the phonon heat capacity Cp. It is shown that the spin induced heat capacity oscillations have a maximum amplitude approaching 3% of Cp, whereas for the inter-LLs excitations the maximum amplitude is only 0.1% of Cp. These amplitudes decrease in the presence of impurities, although the effect is appreciable if the LLs broadening is bigger than the excitation energies.

Effect of biaxial strain on thermal transport in WS2 monolayer from first principles calculations

Abstract Recently, tungsten disulfide (WS2) monolayer has emerged as a two-dimensional material due to its outstanding physical properties. In this work, the thermal transports in strained WS2 monolayer are studied systematically by combining the first principles calculations and the Boltzmann transport equation. It can be found that the lattice thermal conductivity (k) decreases from 262.78 to 190.66 W m−1 K−1 for biaxial tensile strain (from 0% to 8%) and from 262.78 to 217.40 W m−1 K−1 for biaxial compressive strain (from 0% to −4%), respectively. The softened transverse/longitudinal acoustic phonon mode and the stiffened flexural acoustic phonon mode in strained WS2 monolayer change the phonon group velocities. We attribute the reduction of k in tensilely strained WS2 monolayer to the declining phonon group velocity, heat capacity and phonon lifetime of acoustic phonon modes. The comprehensive competition among the increasing phonon group velocity, heat capacity and the decreasing phonon lifetime suppresses the lattice thermal conductivity of compressively strained WS2 monolayer. Furthermore, the increasing phase space proves that the enhancing anharmonic phonon scattering, which results in the suppression of the k in tensilely strained WS2 monolayer. Additionally, the critical phonon mean free path (MFP) under different biaxial strain levels is calculated to demonstrate the contribution of the phonons with different MFPs to the total k of WS2 monolayer. This work provides a phonon behavior analysis of the thermal conduction in WS2 monolayer and paves the path for potential applications of WS2-based devices.

Study on phonon spectra and heat capacities of CL-20/MTNP cocrystal and co-formers by density functional theory method.

The phonon spectra and heat capacities of 2, 4, 6, 8, 10, 12-hexanitrohexaazaisowurtzitane/1-methyl-3, 4, 5-trinitropyrazole (CL-20/MTNP) cocrystal and co-formers were calculated in the framework of DFT. By analyzing the phonon density of states (DOS), the energy flow directions and trigger bonds of cocrystal and co-formers have been obtained and the microscopic physical nature was revealed for thermal decomposition mechanism, detonation performance, and sensitivity. For CL-20/MTNP cocrystal, the phonon number of "doorway" modes and the characteristic vibrational frequencies Δωd are between those of its co-formers, which can provide the microscopic understanding for the ordering of impact sensitivity at experiment, ε-CL-20 > CL-20/MTNP > MTNP. In CL-20/MTNP cocrystal, more phonons and stronger phonon DOS peaks of CL-20 molecules than those of MTNP molecules mean cocrystal's detonation performance is mainly dominated by CL-20 molecules. The heat capacities obtained by the Debye model rise with elevated temperatures at 0-600K and the order is ε-CL-20 > CL-20/MTNP > MTNP. Graphical abstract The phonon spectra and heat capacities of CL-20/MTNP cocrystal and co-formers were calculated by density functional theory (DFT). In CL-20/MTNP cocrystal, the detonation performance and impact sensitivity are mainly dominated by CL-20 molecules. The broken bonds caused by energy transfer may undergo a multi-phonon pumping process.

Structural, mechanical, electronic, and thermodynamic properties of pure tungsten metal under different pressures: A first‐principles study

AbstractThe structural, mechanical, electronic, and thermodynamic properties of pure W metal under different pressures have been investigated using the first‐principles method. Our calculated structural parameters are in good agreement with experimental and previous theoretical results. The obtained elastic constants show that pure W metal is mechanically stable. Elastic properties such as the bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio (ν), Cauchy pressure (C′), and anisotropy coefficients (A) are calculated by the Voigt‐Reuss‐Hill method. The results show that the pressure can improve the strength of pure tungsten and has little effect on the ductility. In addition, the total density of states as a function of pressure is analyzed. Thermodynamic properties such as the Debye temperature, phonon dispersion spectrum, free energy, entropy, enthalpy, and heat capacity are also discussed.

The Role of Multiple Fluctuation Events in NbN and NbTiN Superconducting Nanostrip Single-Photon Detectors

We report on measurements of the switching current distributions on two-dimensional NbN superconducting nanostrip single-photon detectors (SNSPD), 5 nm thick and 80 nm wide, in an interval of temperatures from 6 K down to 0.3 K and compare the data with those obtained for similar NbTiN nanostrips. The standard deviations of the switching distributions show an extended region at high temperatures where multiple phase slip switching events occur. This is probably related to a decreasing critical current and an increasing electron and phonon heat capacities. In this temperature region, the width of the switching distribution, and therefore the dark count rate, is considerably reduced down to values below those observed at the lowest temperature. Finally, we also quantify the energy scale of the fluctuation phenomena. The proposed experimental approach may result in a powerful tool for the diagnostic of SNSPD operation mode.

Analysis of electron and phonon heat capacities of La2−xSrxCuO4 cuprate superconductor

Adopting the many body quantum dynamical Green's function (GF) theory via a versatile Hamiltonian that incorporates the contribution due to harmonic electron and phonon fields and their interactions, anharmonicities and point impurities; the expressions for phononic heat capacity (PHC) and electronic heat capacity (EHC) have been obtained using the method of density of states of phonons and electrons (PDOS and EDOS). The theory is applied to the high temperature superconductor (HTS) La2−xSrxCuO4 for successful numerical estimation. The expressions of PDOS and PHC can be separated into diagonal and non-diagonal components. The non-diagonal contribution emerges only in the impure crystals and becomes extinct in pure crystals. The presence of electron-phonon (ep) coupling constant in each term of EHC is an additional and unique feature of present formulation. The results are compared with the experimental data of heat capacity for La2−xSrxCuO4 superconductor and results are found in good agreement. The electronic contribution is quite small in the comparison to the phononic contribution to total heat capacity. The doping concentration and ep coupling constant dependence of the PHC and EHC independently are investigated for La2−xSrxCuO4 crystal and observed that orderly increases with doping concentration and ep coupling constant.