Contributions Of Phonon Modes Research Articles

A consistent comparison of lattice thermal conductivities and phonon properties of single layer and bilayer graphene systems

Using nonequilibrium molecular dynamics (NEMD) based direct method and spectral energy density (SED) method, we calculate the size-dependent thermal conductivities (TCs) of single layer graphene (SLG), AB-stacked bilayer graphene (AB-BLG), and 21.78° twisted BLG (tBLG) in a robust and consistent manner. Our NEMD analysis reveals discrepancies in high TC reported for graphene systems in some of the earlier studies. Similarly, some of the previous SED based studies were done with unreliable SED Φ′ approach. We conduct size-dependent analysis of the graphene systems by the SED method for the first time and report that bulk TCs for SLG and tBLG systems are nearly the same when calculated by either the direct or the SED method. Contrary to studies that claim that phonon group velocities of AB-BLG and tBLG samples do not change, we find that although average group velocities in SLG and AB-BLG are almost the same, they are around 30% higher when compared to tBLG samples with different twist angles. On the other hand, average phonon lifetimes are almost similar for AB-BLG and 21.78° tBLG samples but around 43% lower than the average phonon lifetime of SLG. Together these trends suggest the reason behind the decreasing order of TCs across three systems. We also systematically study the basic phonon mode contributions to TCs and their properties and find that the high-symmetry modes contribute the most in all three systems.

Heat capacity of nanocrystalline Yb2O3

A study of the structural, Raman scattering and thermodynamic properties of a nanocrystalline ceramic rare earth oxide Yb 2 O 3 and its bulk counterpart is reported. The nanosized sample was obtained by mechanical milling of bulk Yb 2 O 3 (99.998%, high purity powder). The Rietveld analysis of the X-ray diffraction data indicates the presence of nanoparticles with a mean grain size of 12 ± 1 nm after 75 h of milling time. The crystallographic structure within nanoparticles is cubic Ia-3 and the lattice parameter a = 10.455 ± 0.002 Å. The nanocrystalline structure is confirmed by the evaluation of transmission electron microscopy images, showing a size distribution with a mean size D TEM = 8 ± 2 nm. Measurements of the specific heat (2 K–300 K) reveal an excess contribution respect to the unmilled (bulk) compound in the high temperature region above 70 K. At lower temperatures the results are consistent with a drastic change of the antiferromagnetic contribution (ordered below T N = 2.2 K) as a result of the magnetic disorder arising from the size reduction process. The specific heat above T N for the bulk and nanocrystalline samples are explained by the interplay among the phonon contribution, crystalline field and the presence of anharmonic effects. In the nanocrystalline state, broadening and shifts of the contribution of phonon modes to the Raman spectra, and a further reinforcement of the anharmonic contribution are found.

Twisting of 2D Kagomé Sheets in Layered Intermetallics.

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal, and superconducting properties. Here, we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ∼4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measurements, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ∼300 to 100 K, no evidence for a phase transition was found in physical property measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modeling of the displacive disorder in the crystal structure shows the presence of a second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits interlayer twisting, but with a different pattern, consistent with the change in electron count due to the removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.

Thermodynamics Properties and Optical Conductivity of Bipolaron in Graphene Nanoribbon Under Laser Irradiation

In this work, we are studying thermodynamics properties and optical absorption of bipolaron in graphene under a laser field using the variational method. We obtain the ground and first excited states of the bipolaron which strongly depend on laser parameter and graphene characteristics. It is seen that the optical absorption of a bipolaron in graphene depends strongly on the laser parameter. We observed that laser reduces the absorption of photons by laser. The latter also reinforces the electron–electron bound state in graphene leading to its strong energy storage capabilities. Our results show that the electron–electron interaction and laser parameters are important to control the disorder of the system. Due to its intensity and frequency, laser contribution is highest on the entropy, internal energy and specific heat, while the contribution of phonon modes is minimum on these thermodynamics properties suggesting the importance of laser on graphene structures. The results obtained are efficient because bipolaron is well formed and stable in graphene. This investigation may enlighten the understanding of the charge transport mechanism in graphene nanomaterials.

Equivalence of wave function matching and Green’s functions methods for quantum transport: generalized Fisher–Lee relation

We present a proof of an exact equivalence of the two approaches that are most used in computing conductance in quantum electron and phonon transport: the wave function matching and Green’s functions methods. We can obtain all the quantities defined in one method starting from those obtained in the other. This completes and illuminates the work started by Ando (1991 Phys. Rev. B 44 8017) and continued later by Khomyakov et al (2005 Phys. Rev. B 72 035450). The aim is to allow for solving the transport problem with whichever approach fits most the system at hand. One major corollary of the proven equivalence is our derivation of a generalized Fisher–Lee formula for resolving the transmission function into individual phonon mode contributions. As an illustration, we applied our method to a simple model to highlight its accuracy and simplicity.

Thermal expansion and vibrational properties of α-Se and α-TeSe2 based on first-principles calculations

Based on the first-principles calculations, we investigated the thermal expansion and related vibrational properties of monolayer α-Se and binary compounds α-TeSe2. The effect of binary compounds on thermal expansion properties is explored theoretically. We find the two monolayers show totally positive thermal expansion from 0 to 500 K. The linear thermal expansion coefficients are close to each other below 300 K. However, they are 3.5 and 2.5 × 10−5 K−1 for α-Se and α-TeSe2 at 500 K, showing much larger thermal expansion of the former. It can be attributed to the weaker Te–Se bond in α-TeSe2. Moreover, the phonon spectra, mode Grüneisen parameters, macroscopic Grüneisen parameters, and the contributions of phonon modes to total thermal expansion at different temperature are investigated detailed, in order to explore the underlying physical mechanisms of thermal expansion.

Exciton–phonon properties in cylindrical quantum dot with parabolic confinement potential under electric field

Abstract The mixed confinement potential effect on exciton in cylindrical quantum dot (QD) is calculated in the presence of an electric field with and without confined longitudinal optical (LO) phonon mode contribution. The variational approach within the effective mass approximation is used to describe the exciton–phonon interaction with three variational parameters of the trial wave function. The ground state binding energy of exciton has been calculated numerically for the parabolic (axial direction) and square (lateral direction) finite confinement potentials under the electric field applied along the growth direction of the QD. The system both with and without LO-phonon contribution is investigated. The combined effects of applied electric field and parabolic confinement on the ground state binding energy, the polaronic correction and the Stark shift are examined in detail. The findings indicate that the electric field strongly reduces the binding energy in both ignoring and considering LO-phonon interaction especially for wider thickness of QD. We have shown that the contribution of LO-phonon leads to an increase in the binding energy and to decrease in the Stark shift energy. Furthermore, our numerical results illustrate a strong dependence of Stark shift on the QD height, electric field strength and LO-phonon contribution.

Electronic and lattice properties of noncentrosymmetric superconductors ThTSi (T=Co, Ir, Ni, and Pt)

The theoretical studies on the electronic and lattice properties of the series of non-centrosymmetric superconductors ThTSi, where T = Co, Ni, Ir, and Pt are presented. The electronic band structure and crystal parameters were optimized within the density functional theory. The spin-orbit coupling leads to the splitting of the electronic bands and Fermi surfaces, with the stronger effect observed for the compounds with the heavier atoms Ir and Pt. The possible mixing of the spin-singlet and spin-triplet pairing in the superconducting state is discussed. The phonon dispersion relations and phonon density of states were obtained using the direct method. The dispersion curves in ThCoSi and ThIrSi exhibit the low-energy modes along the S-N-S0 line with the tendency for softening and dynamic instability. Additionally, we calculate and analyse the contributions of phonon modes to lattice heat capacity.

Thermal conductivity of bulk and porous ThO2: Atomistic and experimental study

Thorium dioxide (ThO2) is proposed to play a vital role in the world's future energy needs and is considered a better and safer alternative to the currently used nuclear fuel, uranium dioxide (UO2). Thermo-physical properties of ThO2 are superior to UO2, but the fundamental physics governing the heat transport in ThO2 is still ambiguous, and the available data for the thermal conductivity (k) of ThO2 was scattered. In this article, a systematic investigation regarding the lattice thermal conductivity (kL) of the bulk and porous ThO2 is carried out theoretically and validated with experiments. The phonon transport calculations were done using two different methods; ab-initio calculations combined with the Boltzmann transport equation (BTE) and the equilibrium molecular dynamics (EMD) simulations using Green Kubo (GK) approach. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space modes, Grüneisen parameter, and mean free path (MFP) distributions were analyzed to understand the underlying physics in the thermal transport of ThO2. The effect of porosity on the kL by measurements and molecular dynamics (MD) simulations was explored. The measurements were performed on specimens with different porosity, that were prepared by spark plasma sintering (SPS) using the laser flash (LFA) technique. The results obtained demonstrated that the kL values predicted by both the BTE and the EMD simulations were in excellent agreement with our experimental measurements. Moreover, the model to simulate the 95% theoretical density (TD) using MD simulations also captured the decrease in thermal conductivity with porosity and agreed well with the measured results for 95% TD dense sintered pellets.

Atomistic and experimental study on thermal conductivity of bulk and porous cerium dioxide

Cerium dioxide (CeO2) is a surrogate material for traditional nuclear fuels and an essential material for a wide variety of industrial applications both in its bulk and nanometer length scale. Despite this fact, the underlying physics of thermal conductivity (kL), a crucial design parameter in industrial applications, has not received enough attention. In this article, a systematic investigation of the phonon transport properties was performed using ab initio calculations unified with the Boltzmann transport equation. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space, mode Grüneisen parameter and mean free path (MFP) distributions were also conducted. To further augment theoretical predictions of the kL, measurements were made on specimens prepared by spark plasma sintering using the laser flash technique. Since the sample porosity plays a vital role in the value of measured kL, the effect of porosity on kL by molecular dynamics (MD) simulations were investigated. Finally, we also determined the nanostructuring effect on the thermal properties of CeO2. Since CeO2 films find application in various industries, the dependence of thickness on the in-plane and cross-plane kL for an infinite CeO2 thin film was also reported.

Phonon Mean Free Path in Few Layer Graphene, Hexagonal Boron Nitride, and Composite Bilayer h-BN/Graphene

In this study, ab-initio calculations were performed to obtain the phonon dispersions of seven different structures: single layer graphene, bilayer graphene, graphite, single layer h-BN, bilayer h-BN, bulk h-BN, and finally composite bilayer h-BN/graphene. Using these dispersions specific heat, group velocity, and single mode relaxation times of phonons were obtained to calculate their thermal conductivities, and mean free paths at room temperature. Calculated variables were used to understand the effects of additional layers to thermophysical properties, phonon mode contributions to thermal conductivity, and the limits for ballistic-diffusive heat transfer of all seven structures. Finally, based on the obtained data, thermal properties of the composite bilayer h-BN/graphene were investigated and compared to those of few layer graphene and h-BN structures.

Effects of electron–optical phonon interactions on the polaron energy in a wurtzite ZnO/MgxZn1−xO quantum well**Project supported by the National Natural Science Foundation of China (Grant Nos. 11264027 and 11364030), the Project of Prairie Excellent Specialist of Inner Mongolia, China, and the “Thousand, Hundred and Ten” Talent Training Project Foundation of Inner Mongolia Normal University, China (Grant No. RCPY-2-2012-K-039).

We investigated the properties of polarons in a wurtzite ZnO/MgxZn1−xO quantum well by adopting a modified Lee–Low–Pines variational method, giving the ground state energy, transition energy, and phonon contributions from various optical-phonon modes to the ground state energy as functions of the well width and Mg composition. In our calculations, we considered the effects of confined optical phonon modes, interface-optical phonon modes, and half-space phonon modes, as well as the anisotropy of the electron effective band mass, phonon frequency, and dielectric constant. Our numerical results indicate that the electron–optical phonon interactions importantly affect the polaronic energies in the ZnO/MgxZn1−xO quantum well. The electron–optical phonon interactions decrease the polaron energies. For quantum wells with narrower wells, the interface optical phonon and half-space phonon modes contribute more to the polaronic energies than the confined phonon modes. However, for wider quantum wells, the total contribution to the polaronic energy mainly comes from the confined modes. The contributions of the various phonon modes to the transition energy change differently with increasing well width. The contribution of the half-space phonons decreases slowly as the QW width increases, whereas the contributions of the confined and interface phonons reach a maximum at d ≈ 5.0 nm and then decrease slowly. However, the total contribution of phonon modes to the transition energy is negative and increases gradually with the QW width of d. As the composition x increases, the total contribution of phonons to the ground state energies increases slowly, but the total contributions of phonons to the transition energies decrease gradually. We analyze the physical reasons for these behaviors in detail.

First principles study of thermal conductivity cross-over in nanostructured zinc-chalcogenides

Systematic first principles studies of zinc-chalcogenides have been performed to understand their thermal transport behaviour. We have applied the Boltzmann transport equation in the relaxation time approximation to calculate the thermal conductivity of ZnS, ZnSe, and ZnTe. We find a thermal conductivity cross-over between ZnS and ZnSe at nanostructure sizes around 0.1–0.2 μm and explain this in terms of the different contributions of phonon modes in these materials. We study the effect of nanostructuring using both the diffusive boundary scattering and confined mean free path limit and discuss the variations in the results. Furthermore, we show the strong influence of isotope scattering on the thermal conductivity. The calculated thermal conductivity is found to be strongly dependent on the volume and we explain the observed differences between local density and generalized gradient approximation calculations. We compare further calculated thermal properties, such as the thermal expansion coefficient, to experiment to validate our approach.

Thermal conductivity of skutterudite CoSb3 from first principles: Substitution and nanoengineering effects.

CoSb3-based skutterudites are promising intermediate-temperature thermoelectric materials and fundamental understanding of the thermal transport in CoSb3 is crucial for further improving its performance. We herein calculate the lattice thermal conductivity κL of CoSb3 with first-principles methods and conduct a comprehensive analysis on phonon mode contribution, relaxation time and mean free path (MFP) distributions. The contribution of optical phonons is found to be significant (28% at 300 K) and important optical modes usually involve two or more pnicogen atoms moving synchronously. The MFP (~135 nm at 300 K) corresponding to 50% κL accumulation in CoSb3 is much larger than that predicted from the kinetic theory (~4 nm), providing an opportunity to reduce κL by nanoengineering. The effects of elemental substitution and nanoengineering on κL are therefore investigated. A 10% substitution of Sb by As results in 57% reduction of κL while the in-plane (cross-plane) κL of a 50-nm CoSb3 thin film is only 56% (33%) of the bulk κL at 300 K. The impurity scattering and boundary scattering mainly suppress phonons in different frequency regimes. By combining these two effects, κL can be reduced by more than 70% at 300 K, potentially leading to much improved ZT near room temperature.

Investigation of direct and indirect phonon-mediated bond excitation in α-RDX

We evaluate whether lattice or internal phonons dominate the thermal excitation of the N-N bonds in α-cyclotrimethylene trinitramine (α-RDX) by computing the fractional contributions of phonon modes to the excitation of all atomic interactions. We derive a method to compute these contributions, which we call mode energy fractions, from the phonon eigenvectors and a splitting of the dynamical matrix. This enables identification of phonon modes that most strongly excite the N-N bonds that play a key role in molecular decomposition of α-RDX. Correlating these fractions with the mode populations and contributions to the specific heat and thermal conductivity, we analyze how thermal energy is distributed by phonons following a passing shock. Contrary to the common explanation that thermal energy is transferred to the N-N bonds indirectly, by internal phonons, we find that lattice phonons dominate this thermal energy transfer, implying that energy flow follows a direct route. We also comment on implications of these results for non-shock decomposition of α-RDX.

Binding energies of bound polaron in a wurtzite nitride nanowire: Contributions of IO and QC phonon modes

AbstractBy employing the two‐parameter variational approach, the binding energies of bound polarons in a quasi‐one‐dimensional wurtzite nanowire (NW) are investigated. Two types of polar optical phonon modes, i.e., the interface optical (IO) and quasi‐confined (QC) phonon modes of wurtzite NWs are taken in account. The results reveal that the phonon contribution to binding energy of bound polaron in GaN NWs reaches ∼370 meV. The quite large contribution of phonon modes to the total binding energy is mainly ascribed to the very strong electron–phonon coupling constant in GaN materials. Moreover, the IO modes plays more important role to the binding energies of bound polaron as the NW radius is small, while the QC modes dominates as the NW radius is relatively large. The calculated results of impurity binding energy are quite consistent with the recent experimental measurement. The numerical results also show that the two‐parameter variational approach is necessary and suitable for the description of impurity states in wurtzite GaN NW structures.

Thermal transport in graphene and effects of vacancy defects

We have performed molecular dynamics simulations to investigate phonon transport in graphene at 300 K with the Green-Kubo method. We show that the thermal conductivity (TC) of pristine graphene is $2903\ifmmode\pm\else\textpm\fi{}93$ W/mK, and the out-of-plane phonon mode contribution is $1202\ifmmode\pm\else\textpm\fi{}32$ W/mK. We further investigate thermal transport in graphene with different vacancy defect concentrations, and the TC shows that it is possible to achieve a maximum of a thousandfold reduction with a high defect density.

INFLUENCE OF BOTH ELECTRIC AND MAGNETIC FIELDS ON THE POLARON IN A CYLINDRICAL QUANTUM DOT

The polaronic correction to the ground-state energy of the electron confined in a cylindrical quantum dot (QD) subject to electric and magnetic fields along the growth axis has been investigated. Using a combinative approach of perturbative theory and variational wavefunction, calculations are performed for an infinitely deep confinement potential outside the QD within the effective mass and adiabatic approximation. We have treated the system by taking into consideration the interaction of the electron with the confined longitudinal optical (LO) phonons as well as the side surface (SSO) and the top surface (TSO) optical phonons.1,2 The ground-state energy shift is obtained as a function of the cylindrical radius and the strength of electric and magnetic fields. The results show that the magnetic field heavily enhances the three types of phonon mode contribution to the correction of the electron ground-state energy while the electric field only improves the contribution of surface phonons (SSO and TSO) but decreases the contribution of LO phonons.

Charge transfer vibronic excitons in ferroelectric oxides: experiment and theory

Review of the charge transfer vibronic excitons (CTVE) study on the basis of optical response investigations in ferroelectric oxides is presented. CTVE is the correlated pair of electron and hole small polarons whose stability is controlled by a pair pseudo-Jahn–Teller effect. The study is focused on the CTVE-induced critical dependence near the ferroelectric phase transition. The critical effect in absolute diffraction efficiency (ADE) formation of the laser-induced transient gratings near ferroelectric phase transition in KTa 1− x Nb x O 3 (KTN) is considered as an example. Critical concentration dependence for slow as well as for instantaneous (fast) optical response was detected. The peak positions for these x – dependences were different. The CTVE-model can explain these experiments taking into account not only the soft TO phonon mode contribution but also that of soft tunnel quasilocal modes of clusters of the second component.

Theory of uniaxial stress in the cooperative jahn-teller distortion of DyVO 4

A Hamiltonian for the co-operative Jahn-Teller transition in DyVO 4 is derived with both optic mode and strain couplings. A molecular field theory for this Hamiltonian predicts a second order phase transition and a dependence of the ground state splitting on stress. Contributions of phonon modes and strain can be distinguished by a comparison of the elastic constants and the parameters derived from the stress dependence of the ground state splitting. A correction to these molecular field results is derived by accounting for fluctuations. The effects of a magnetic field are investigated in the molecular field approximation; it is found that if the field is applied in the basal plane simultaneously with the stress at low temperatures, different distortions can be stabilized. The form of the first order phase boundary for these distortions is derived and an analogy is drawn between this ‘pseudospin flop’ transition and antiferromagnetic spin flop.