Phonon Temperature Research Articles

Molecular Dynamics Studies on Thermal Transport Through Multiwalled Carbon Nanotubes.

The influence of the temperature and strength of the inter-wall interaction on the thermal conductivities of the (5,5) and (10, 10) double-walled carbon nanotubes (DWNTs) is studied by using molecular dynamics (MD) simulations with two different temperature control methods. One method is imposing heat baths (HBs) only on the outer wall, while the other is imposing HBs on both the two walls. The results show that the thermal conductivities of the DWNTs with the first method are about two-third of those with the second method. The relationship is the same even if the temperature and strength of the inter-wall interaction vary. Besides, the thermal conductivities of the DWNTs with the two different temperature control methods both slightly increase with the increasing energy parameters of Lennard-Jones (LJ) potential describing the inter-wall interaction and decrease with increasing temperature. Based on the analyses of the temperature profiles and phonon density of states (PDOS) spectra of the DWNTs with the two different temperature control methods, the results are well explained and the thermal transport mechanisms of multiwalled carbon nanotubes (MWNTs) under different conditions are explored.

Influence of the electron and phonon temperature and of the electric-charge density on the optimal efficiency of thermoelectric nanowires

In this paper we study the thermodynamic efficiency of thermoelectric generators in which the heat transport is driven by phonons and electrons. It is assumed that the phonon temperature and the electron temperature are different, and that the electric-charge density is nonuniform. The mean temperature is defined by observing that the internal energy of the system is the same either in the presence of two temperatures, or of one temperature. In steady states, we determine the influence of the gradients of the mean temperature and of the electric-charge density on the theoretical values of the thermoelectric efficiency. The physical conditions under which such efficiency is optimal are determined as well.

Phonon spectra and temperature variation of bulk properties of Cu, Ag, Au and Pt using Sutton–Chen and modified Sutton–Chen potentials

Three potentials of the Finnis–Sinclair type are studied with regard to their suitability for predicting bulk thermal and elastic properties of fcc metals Cu, Ag, Au and Pt over a wide temperature range. We start with a particular parametrization of the Finnis–Sinclair model known as the Sutton–Chen potential and a later version of the same, known as the quantum Sutton–Chen potential. The quasiharmonic lattice dynamics method is used to study the temperature variation of the thermodynamic properties. Both models are found to yield poor results for thermal expansion, which can be traced to rapid softening of transverse phonon frequencies with increasing lattice parameter. The form of the Sutton–Chen potential is modified here to seek improvement in the agreement between quasiharmonic calculations and experimental data. It is found that the modified potential better predicts bulk properties in nearly all cases studied. Significant improvement is seen over the Sutton–Chen potential, while lesser but still substantial improvement is observed over the Quantum-Sutton Chen potential.

Observation of vacancy-induced suppression of electronic cooling in defected graphene

Previous studies of electron-phonon interaction in impure graphene have found that disorder can give rise to an enhancement of electronic cooling at high temperatures. We investigate the effect of lattice vacancy in both mono- and bilayer graphene and observe an order of magnitude suppression of electronic cooling at low temperatures compared with clean graphene. The dependence of the coupling constant on the phonon temperature implies its link to the dynamics of disorder. Our study highlights the effect of disorder on electron-phonon interaction in graphene. In addition, the suppression of electronic cooling holds great promise for improving the performance of graphene-based bolometer and photodetector devices.

Ultrafast thermalisation dynamics in an Au film excited by a polarization-shaped femtosecond laser double-pulse

Abstract We theoretically investigated the ultrafast non-equilibrium thermalisation dynamics in an Au film irradiated by a polarization-shaped femtosecond laser double-pulse. A non-equilibrium thermal relaxation model is proposed to study the relevance of the polarization of the double-pulse energy that couples to the Au film target. The phonon temperature can be greatly increased by optimizing double-pulse polarization combinations. The results are explained by an enhanced laser energy coupling with the Au film due to a synergetic effect arising from pulse-to-pulse polarization relevance. The study provides a basis for understanding ultrafast thermalisation dynamics for optimizing laser micro- and nano-fabrications by tailoring the polarization state of temporally shaped femtosecond lasers.

Dynamics of thermalization in Au-Ti double-layered film excited by a femtosecond laser pulse

We theoretically investigate the dynamics of thermalization in Au-Ti double-layered film irradiated by a femtosecond laser pulse. A nonequilibrium thermal relaxation model is proposed to study the energy deposition and transport processes during femtosecond laser pulse heating of double-layered film. The maximum phonon temperature on the Au layer can be greatly adjusted by optimizing the thickness of the Au layer. In addition, the effect of Au-layer thickness on the thermalization dynamics of the Au-Ti system is examined in detail. This study provides a new way to increase the resistance of mirrors to thermal damage in applications of high-power lasers.

Thermal conductivity of transition metal containing type-I clathrates

ABSTRACTConcerning a materials ability to convert heat to electrical energy, the electrical power factor S2/ρ as well as the thermal conductivity at elevated temperatures are of special interest. Since Flash experiments measure the thermal diffusivity and standard steady-state heat-flow experiments are inaccurate at elevated temperatures due to radiation errors inherent to this technique, direct and accurate thermal conductivity data on type-I clathrate single crystals at elevated temperatures are scarce in literature. Here we report 3ω thermal conductivity data on single crystalline Ba8Cu5.09Ge40.91 (BCG), La1.23Ba6.99Au5.91Si39.87, and Ce1.06Ba6.91Au5.56Si40.47 in the temperature range between 80 and 330 K, and specific heat data on BCG between 2 and 300 K. The comparison of our room temperature phonon thermal conductivity data (κph) to results on transition metal (TM) free type-I clathrates in terms of the guest free space (Rfree) suggests a stronger dependence of κph on Rfree for the clathrates containing TM elements.

High Temperature Nanoplasmonics: The Key Role of Nonlinear Effects

In this work we show the effect of high temperature on the plasmonic properties of metallic nanorods. Within this context, the dielectric function of metal has been modified to keep into account both the electron–electron and electron–phonon temperature dependent scattering mechanisms. In fact, the mentioned damping processes are very sensible to temperature variation. It is found that the damping modifications due to temperature change substantially modify both the near and far field optical response of metallic nanorods. Furthermore, the response alteration can be very different depending on the rod aspect ratio, suggesting the need of accurate investigations when metallic nanostructures are to be employed within high-temperature environments or under high intensity irradiation. In this regard, by taking into account different nanorods aspect ratio and incident powers, we provide detailed calculations showing the error committed in evaluating absorption, scattering efficiencies, and near-field enhanceme...

Phonon transport in aluminum and silicon film pair: laser short-pulse irradiation at aluminum film surface

Phonon transport in paired aluminum and silicon thin films is considered under laser short-pulse irradiation at the aluminum film surface. The Boltzmann equation is incorporated to formulate energy transport in the films. To include a volumetric source resembling laser irradiation in the aluminum film, the Boltzmann equation is modified. Thermal boundary resistance is located at the interface of the film pair. An equivalent equilibrium temperature is introduced to assess the thermal resistance of the film during the laser heating process. The phonon temperature obtained from solution of the Boltzmann equation is compared with the findings of the two-temperature model. It is found that phonon temperature obtained from the solution of the Boltzmann equation is lower than that corresponding to the two-temperature model, which is particularly true in the surface region of the aluminum film. Phonon temperature increases gradually while, early on, the electron temperature rises and decays sharply in the surface region of the aluminum film.

Power Load and Temperature Dependence of Cold-Electron Bolometer Optical Response at 350 GHz

Cold-electron bolometers (CEBs) integrated with twin-slot antennas have been designed and fabricated. Optical response was measured at bath temperatures of 0.06 to 3 K using blackbody radiation source at temperatures of 3 to 15 K. The responsivity of 0.3 * 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> V/W was measured at 2.7-K blackbody temperature that is close to the temperature of the cosmic microwave background. Optical measurements indicate quasi-optical coupling efficiency of up to 60% at low phonon temperature and low signal level. Estimations for bolometer responsivity were made for practical range of bath temperatures and blackbody radiation temperatures. The estimated ultimate dark responsivity at 100-mK bath temperature can approach S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</sub> = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> V/W and reduces down to 1.1 * 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> V/W at 300 mK for a device with absorber volume of 5 * 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-20</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

Modeling heat generation in high power density nanometer scale GaAs/InGaAs/AlGaAs PHEMT

A model of heat generation rate for two-dimensional semiconductor devices is established. Electron motion and electron–phonon scattering in nanometer scale GaAs/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) under high power density are simulated by Monte Carlo method. Parameters such as electric field, phonon temperature, and heat generation rate distribution are obtained by solving Boltzmann equation and phonon conservation equations. The heat generation rate distribution and the number of net optical-phonon emission in the presence of different applied voltages are presented. The electric field, electron density, phonon emission and absorption, and lattice temperature are investigated to elucidate the mechanism of heat generation in the devices.

Dynamical stability of Mo under high pressure and high temperature

Considering the phonon-phonon interactions, we obtain the high temperature phonons of Mo under high pressure. The dynamically stable regions of bcc and fcc Mo in the phase diagram are predicted. By comparing the anharmonic free energy, we determine the bcc-fcc boundary. The bcc Mo is the stable phase up to 700 GPa. Around 210 GPa, there is no bcc-fcc phase transition, which is different with the results from quasiharmonic approximation.

Influence of electron and phonon temperature on the efficiency of thermoelectric conversion

Abstract In the framework of Extended Irreversible Thermodynamics it is developed a two-temperature model (for electrons and phonons, respectively) of thermoelectric effects. The expression of the maximum efficiency in terms of these two temperatures is derived as well. It is proved that, for the electron temperature higher than the phonon temperature, the two-temperature model yields an efficiency which is higher with respect to that of the single-temperature model. Two possible experiments to estimate the electron temperature are suggested.

Overheating or overcooling of electrons in a metal because of the effect of an interface with an insulator

It has been shown that the temperatures of electrons and phonons are different at a heat flow through a metal-insulator interface. This effect leads to an additional contribution to the Kapitza thermal resistance because electrons transferring heat in the metal do not transfer it through the interface, but are rather involved in heat transfer only at a certain distance from it. Consequently, heat transfer near the interface is less efficient. The effect is independent of the insulator adjacent to the metal. An exact solution has been obtained in a linear approximation. The results explain the qualitative difference of predictions of previously accepted models from experimental data in the case of large transmission coefficients of phonons through the interface.

Role of phonon drag and carrier diffusion in thermoelectric power of polycrystalline La0.97Na0.03MnO3 manganites

We develop a theoretical model for quantitative analysis of temperature-dependent thermoelectric power of monovalent (Na) doped La0.97Na0.03MnO3 manganites. In the ferromagnetic regime, we have evaluated the phonon thermoelectric power by incorporating the scattering of phonons with impurities, grain boundaries, charge carriers and phonons. In doing so, we use the Mott expression to compute the carrier (hole) diffusion thermoelectric power (S c diff ) using Fermi energy as carrier (hole)-free parameter, and S c diff shows linear temperature dependence and phonon drag S c drag increases exponentially with temperature which is an artifact of various operating scattering mechanisms. It is also shown that for phonons the scattering and transport cross-sections are proportional to ω4 in the Rayleigh regime where ω is the frequency of the phonons. Numerical analysis of thermoelectric power from the present model shows similar results as those revealed from experiments.

Evidence of scaling in the high pressure phonon dispersion relations of some elemental solids.

First principles searches are carried out for the existence of an asymptotic scaling law for the zero temperature phonon dispersion relation of several elemental crystalline solids in the high pressure regime. The solids studied are Cu, Ni, Pd, Au, Al, and Ir in the face-centered-cubic (fcc) geometry and Fe, Re, and Os in the hexagonal-close-packed (hcp) geometry. At higher pressures, the dependence of the scale of frequency on pressure can be fitted well by a power law. Elements with a given crystalline geometry have values of the scaling exponent very close to each other (0.32 for fcc and 0.27 for hcp - with a scatter below five percent of the average).

Longitudinal spin current induced by a temperature gradient in a ferromagnetic insulator

Based on the solution of the stochastic Landau-Lifshitz-Gilbert equation discretized for a ferromagnetic chain subject to a uniform temperature gradient, we present a detailed numerical study of the spin dynamics with a focus particularly on finite-size effects. We calculate and analyze the net longitudinal spin current for various temperature gradients, chain lengths, and external static magnetic fields. In addition, we model an interface formed by a nonuniformly magnetized finite-size ferromagnetic insulator and a normal metal and inspect the effects of enhanced Gilbert damping on the formation of the space-dependent spin current within the chain. A particular aim of this study is the inspection of the spin Seebeck effect beyond the linear response regime. We find that within our model the microscopic mechanism of the spin Seebeck current is the magnon accumulation effect quantified in terms of the exchange spin torque. According to our results, this effect drives the spin Seebeck current even in the absence of a deviation between the magnon and phonon temperature profiles. Our theoretical findings are in line with the recently observed experimental results by M. Agrawal et al., Phys. Rev. Lett. 111, 107204 (2013).

Observation of the spin Peltier effect for magnetic insulators.

We report the observation of the spin Peltier effect (SPE) in the ferrimagnetic insulator yttrium iron garnet (YIG), i.e., a heat current generated by a spin current flowing through a platinum (Pt)|YIG interface. The effect can be explained by the spin transfer torque that transforms the spin current in the Pt into a magnon current in the YIG. Via magnon-phonon interactions the magnetic fluctuations modulate the phonon temperature that is detected by a thermopile close to the interface. By finite-element modeling we verify the reciprocity between the spin Peltier and spin Seebeck effect. The observed strong coupling between thermal magnons and phonons in YIG is attractive for nanoscale cooling techniques.

Thermal and optical properties of freestanding flat and stacked single-layer graphene in aqueous media

Graphene, a two-dimensional atomic layer of carbon atoms, represents a class of nanostructures whose physical properties are strongly dependent on their morphology as well as the environment in which they exist. Aqueous media is one of the most common environments that play an important role in influencing the performance of these materials. Here, we investigate the thermal and optical properties of suspended flat and stacked graphene ribbons that are typical structures in aqueous media. We demonstrate that stacked graphene structures thermalize much more rapidly than flat graphene and display unequilibrated electron and phonon temperatures upon laser excitation. The interface thermal conductivity between graphene and water of (7.2 + 1.4/−5.5) × 105 W m−2 K−1 is also obtained. We also show that graphene hot electron luminescence not only depends on Fermi energy, but also exhibit dramatic differences between flat and stacked regions. This indicates the morphology of a graphene structure may affect its optical and thermal properties.

Looking for footprint of bulk metallic glass in electronic and phonon heat capacities of Cu55Hf45−xTix alloys

We report on the heat capacity investigation of Cu55Hf45−xTix metallic glasses. The most appropriate procedure to estimate low temperature electronic and phonon contributions has been determined. Both contributions exhibit monotonous Ti concentration dependence, demonstrating that there is no relation of either the electron density of states at the Fermi level or the Debye temperature to the increased glass forming ability in the Ti concentration range x = 15–30. The thermodynamic parameters (e.g., reduced glass temperature) remain better indicators in assessing the best composition for bulk metallic glass formation.