Phonon Linewidths Research Articles

DynaPhoPy: A code for extracting phonon quasiparticles from molecular dynamics simulations

We have developed a computational code, DynaPhoPy, that allows us to extract the microscopic anharmonic phonon properties from molecular dynamics (MD) simulations using the normal-mode-decomposition technique as presented by Sun et al. (2014). Using this code we calculated the quasiparticle phonon frequencies and linewidths of crystalline silicon at different temperatures using both of first-principles and the Tersoff empirical potential approaches. In this work we show the dependence of these properties on the temperature using both approaches and compare them with reported experimental data obtained by Raman spectroscopy (Balkanski et al., 1983; Tsu and Hernandez, 1982). Program summaryProgram Title: DynaPhoPyProgram Files doi:http://dx.doi.org/10.17632/v493dkxk8r.1Licensing provisions: MIT LicenseProgramming language: Python and CExternal routines/libraries: phonopy, numpy, matplotlib, scipy and h5py python modules. Optional: FFTW and CudaNature of problem: Increasing temperature, a crystal potential starts to deviate from the harmonic regime and anharmonicity is getting to be evident (M. T. Dove, Introduction to lattice dynamics, Vol. 4, Cambridge university press, 1993). To treat anharmonicity, perturbation approach often describes successfully phenomena such as phonon lifetime and lattice thermal conductivity. However it fails when the system contains large atomic displacements.Solution method: Extracting the phonon quasiparticles from molecular dynamics (MD) simulations using the normal-mode-decomposition technique.Restrictions: Quantum effects of lattice dynamics are not considered.

Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

The discovery of an enhanced superconducting transition temperature $T_c$ in monolayers of FeSe grown on several oxide substrates has opened a new route to high-$T_c$ superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon (e-ph) interaction across the interface that peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge instabilities at long-wavelengths. However, we further find that the inclusion of Coulombic screening significantly reduces the phonon broadening. Our results show that one might not expect anomalously broad phonon linewidths in the FeSe interface systems, despite the fact that the e-ph interaction has a strong peak in the forward scattering (small $q$) direction.

Comment on “Electron-phonon coupling in two-dimensional silicene and germanene”

In their work, Yan et al. [Phys. Rev. B 88, 121403 (2013)] employing density functional perturbation theory (DFPT) calculations, demonstrate that silicene and germanene show weaker Kohn anomalies in the $\Gamma$-$E_g$ and $K$-$A_1$ phonon modes, compared to graphene. Furthermore, they compute the electron phonon (e-ph) coupling matrix elements using the frozen phonon approach and found that in silicene the average e-ph coupling matrix-element square over the Fermi surface, $\langle g_{{\bf q}\nu}^2\rangle_{F}$, is about 50 % of those in graphene, but in germanene is weaker and nearly negligible. However, Yan et al. argues that the smaller Fermi velocity in silicene compensates the reduced $\langle g_{{\bf q}\nu}^2\rangle_{F}$, leading to phonon linewidths ($\gamma_{{\bf q}\nu}$) slightly larger than those in graphene. In this Comment, we show that the DFPT and the frozen phonon results of Yan et al. for silicene are inconsistent. Additionally, we have evaluated the e-ph coupling using direct DFPT calculations, analytical relations, and frozen phonon calculations, and we found systematically that $\langle g_{{\bf q}\nu}^2\rangle_{F}$ and $\gamma_{{\bf q}\nu}$ in silicene are one order of magnitude smaller than in graphene, in contrast to the conclusions of Yan et al.

Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe2

We investigate the interactions of photoexcited carriers with lattice vibrations in thin films of the layered transition metal dichalcogenide (TMDC) WSe_{2}. Employing femtosecond electron diffraction with monocrystalline samples and first-principles density functional theory calculations, we obtain a momentum-resolved picture of the energy transfer from excited electrons to phonons. The measured momentum-dependent phonon population dynamics are compared to first-principles calculations of the phonon linewidth and can be rationalized in terms of electronic phase-space arguments. The relaxation of excited states in the conduction band is dominated by intervalley scattering between Σ valleys and the emission of zone boundary phonons. Transiently, the momentum-dependent electron-phonon coupling leads to a nonthermal phonon distribution, which, on longer time scales, relaxes to a thermal distribution via electron-phonon and phonon-phonon collisions. Our results constitute a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of TMDCs.

Strong Spin-Phonon Coupling Mediated by Single Ion Anisotropy in the All-In-All-Out Pyrochlore Magnet Cd_{2}Os_{2}O_{7}.

Spin-phonon coupling mediated by single ion anisotropy was investigated using optical spectroscopy and first-principles calculations in the all-in-all-out pyrochlore magnet Cd_{2}Os_{2}O_{7}. Clear anomalies were observed in both the phonon frequencies and linewidths at the magnetic ordering temperature. The renormalization of the phonon modes was exceptionally large, signifying the presence of an unconventional magnetoelastic term from large spin-orbit coupling. In addition, the relative phonon frequency shifts show a strong correlation with the modulation of noncubic crystal field by the corresponding lattice distortion. Our observation establishes a new type of spin-phonon coupling through single ion anisotropy, a second-order spin-orbit coupling term, in Cd_{2}Os_{2}O_{7}.

Raman Spectroscopic Observation of Gradual Polymorphic Transition and Phonon Modes in CuPc Nanorod

A Raman scattering investigation of a single α-CuPc nanorod is demonstrated here within the temperature range 300–770 K. From the typical thermal dispersion of Raman shift and phonon line width of in-plane B1g mode at 1526 cm–1, the metastable α polymorph is observed to undergo unambiguous phase transition to thermally stable β phase. The phase transition temperature (456 K) is observed to be significantly lower than that reported for bulk samples. Extensive computation of normal modes associated with both α- and β-CuPc reveal that the “fingerprint” domain across 1300–1600 cm–1 is particularly sensitive to this polymorphic transition where a silent B2g mode appears at 1303 cm–1 in the spectral vicinity of molecular edge vibration sensitive Ag–Bg Davydov doublet.

Understanding the evolution of anomalous anharmonicity in Bi2Te3−xSex

The anharmonic effect in thermoelectrics has been a central topic for decades in both condensed matter physics and material science. However, despite the long-believed strong and complex anharmonicity in the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3\ensuremath{-}x}{\mathrm{Se}}_{x}$ series, experimental verification of anharmonicity and its evolution with doping remains elusive. We fill this important gap with high-resolution, temperature-dependent Raman spectroscopy in high-quality single crystals of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$, and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ over the temperature range from 4 to 293 K. Klemens's model was employed to explain the renormalization of their phonon linewidths. The phonon energies of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ are analyzed in detail from three aspects: lattice expansion, cubic anharmonicity, and quartic anharmonicity. For the first time, we explain the evolution of anharmonicity in various phonon modes and across the series. In particular, we find that the interplay between cubic and quartic anharmonicity is governed by their distinct dependence on the phonon density of states, providing insights into anomalous anharmonicity designing of new thermoelectrics.

Effects of electronic relaxation processes on vibrational linewidths of adsorbates on surfaces: The case of CO/Cu(100)

We investigate nonadiabatic effects for the vibrational stretch mode of the CO molecule adsorbed on the top site of the Cu(100) surface. By studying the long-wavelength ($\mathbf{q}\ensuremath{\approx}0$) imaginary and real parts of the density functional theory based phonon self-energy due to the electron-phonon coupling ${\mathrm{\ensuremath{\Pi}}}_{\ensuremath{\lambda}}$ we obtain the phonon linewidth and the frequency renormalization of the CO stretch mode, respectively. To simulate electronic scattering processes that lead to further damping of the phonon modes we include a phenomenological damping in the phonon self-energy, as well as in the single-electron spectral function that enters ${\mathrm{\ensuremath{\Pi}}}_{\ensuremath{\lambda}}$, through the momentum distribution function. For the specific case of electron-impurity scattering we explicitly show how this process opens the indirect intraband channel and broadens the linewidth of the CO stretch mode. To emphasize the importance of accounting for electronic scattering processes we compare the phonon linewidths in the clean noninteracting limit (infinite electron lifetime) and when electronic scattering processes are phenomenologically included (finite electron lifetime) with available experimental data. We find that the agreement with experiments is improved in the latter case.

Raman evidence for nonadiabatic effects in optical phonon self-energies of transition metals

We report a Raman study of the effect of temperature on the self-energies of optical phonons in a number of transition metals with hexagonal-close-packed structure. Anisotropic softening of phonon energies and narrowing of phonon linewidths with increasing temperature are observed. These effects are reproduced in the calculations of phonon spectral functions based on \textit{ab initio} electronic structures and with carrier scattering by phonons taken into account. The combined observations and results of simulations indicate a relation between observed anomalies and the renormalization of the electron spectrum due to electron-phonon interaction. It is emphasized that the temperature dependence of the phonon energies resembles anharmonic behavior but is actually an electron-induced effect.

Theoretical study of two-dimensional boron silicide from first-principles

Using first-principles calculations and evolutionary algorithm, we predict two honeycomb (P1- and C2-type) structures of BSi with two-atom thickness. Phonon spectrum calculations show that both structures are thermodynamically stable. A weak covalent bonding interaction between B atoms along [001] direction plays a very important role in structure stability of P1- and C2-type BSi. P1-type BSi is semi-metallic, while C2-type exhibits a semiconductor band structure and has a good absorption of photons in the range of 1.5–2.5eV that is very promising for photoelectric applications. An e22 piezoelectric coefficient of 1.65×10−10Cm−1 is predicted for C2-type BSi, which makes it a potential candidate for piezoelectric applications at nano-scale. Moreover, three-phonon interactions areadoptedtoevaluate phonon lifetimes, phonon linewidths and lattice thermal conductivity. Although both P1- and C2-type BSi have a similar honeycomb structure and same elements, the maximum lattice thermal conductivity of P1-type BSi is around 6 times higher than that of C2-type BSi. Additionally, P1-type BSi exhibits remarkably higher phonon lifetimes than C2-type BSi.

Role of defects, resonances, anharmonicities and electron–phonon scattering processes on thermal conductivity of YBa2Cu3O7−δ

In this work, thermal conductivity of high temperature superconductors (HTS) has been analyzed on the basis of modified Callaway model. In the new formulation, the relaxation times of various contributing processes have been observed in newer perspectives of electron and phonon line widths. To obtain line widths, the quantum dynamics of electron and phonon is carried out by using double time thermodynamic Green’s functions method via a general Hamiltonian. The outcome of this heuristic approach is utilized to successfully explain the spectacular behavior of thermal conductivity of HTS, and particularly in the vicinity of transition temperature.

Low phonon conductivity of layered BiCuOS, BiCuOSe, and BiCuOTe from first principles

Combining the effect of layer mixing, mass mismatch, and intrinsic defects, we have investigated the origin of very low phonon conductivity ${k}_{p}$ in thermoelectric (TE) $\text{BiCuO}Q$ $(Q$: S, Se, Te) compounds. Based on the first-principles anharmonic, lattice dynamics calculations, we use the single-mode relaxation time approximation of the linearized phonon Boltzmann equation, which shows good agreement with experiments. Here, we found that the most important parameter for low ${k}_{p}$ is the interlayer interaction between the BiO and $\text{Cu}Q$ layers. By analyzing the phonon linewidth distribution, which indicates the phonon scattering rate, we propose that the interlayer interactions play a critical role on suppressing ${k}_{p}$, i.e., the heterolayered crystal controls these interlayer interactions, achieving low ${k}_{p}$ and optimal TE properties.

Strong magnetoelectric and spin phonon coupling in SmFeO3/PMN-PT composite

We have investigated spin phonon coupling in the strain coupled magnetoelectric SmFeO3/0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT) composite in the temperature range of 300–650 K by Raman spectroscopy and magnetic measurements. The SmFeO3/PMN-PT composite shows sharp rise in magnetic moment across ferroelectric transition temperature (Tc) of PMN-PT. Around this transition temperature (Tc of PMN-PT), the temperature evolution of Raman spectra of the composite also shows anomalies in the phonon frequencies and line width corresponding to the SmFeO3 phase which indicate structural modifications in the SmFeO3 phase around Tc of PMN-PT. The observed structural, magnetic, and phonon anomalies of SmFeO3 around Tc of PMN-PT in SmFeO3/PMN-PT are attributed to spin-phonon coupling providing evidence of strong strain mediated magnetoelectric effects.

Probing spin–phonon coupling in magnetoelectric CaCu3Ti4O12–NiFe2O4 composite nanostructures

We investigated temperature‐dependent magnetic and phonon anomalies in epitaxial self‐assembled composite CaCu3Ti4O12–NiFe2O4 films where NiFe2O4 rods embedded in CaCu3Ti4O12 matrix. The temperature evolution of the Raman spectra for pure NiFe2O4 shows usual anharmonic behaviour in the temperature range of 80–300 K, while composite films show significant phonon anomalies in octahedral (O) site (Fe–O) phonon modes around 100 K, where sudden change in magnetic order is observed. The anomalies in A1g (O site) phonon mode position and linewidth of NiFe2O4 indicate lattice deformations around 100 K. It is suggested that spin strongly couples with phonons in magnetoelectric CaCu3Ti4O12–NiFe2O4 nanostructures. Copyright © 2016 John Wiley & Sons, Ltd.

On possibility of superconductivity in SnSb: A first principle study

The electronic, phonon structure and superconducting properties of tin antimonide (SnSb) in rock-salt (RS) structure are calculated using first-principles density functional theory. The electronic band structure and density of states show metallic behavior. The phonon frequencies are positive throughout the Brillouin zone in rock-salt structure indicating its stability in that phase. Superconductivity of SnSb in RS phase is discussed in detail by calculating phonon linewidths, Eliashberg spectral function, electron-phonon coupling constant and superconducting transition temperature. SnSb is found to have a slightly lower TC (3.1K), as compared to SnAs.

Magneto-elastic coupling in a potential ferromagnetic 2D atomic crystal

Cr2Ge2Te6 has been of interest for decades, as it is one of only a few naturally forming ferromagnetic semiconductors. Recently, this material has been revisited due to its potential as a two-dimensional semiconducting ferromagnet and a substrate to induce anomalous quantum Hall states in topological insulators. However, many relevant properties of Cr2Ge2Te6 still remain poorly understood, especially the spin-phonon coupling crucial to spintronic, multiferrioc, thermal conductivity, magnetic proximity and the establishment of long range order on the nanoscale. We explore the interplay between the lattice and magnetism through high resolution micro-Raman scattering measurements over the temperature range from 10 to 325 K. Strong spin-phonon coupling effects are confirmed from multiple aspects: two low energy modes splits in the ferromagnetic phase, magnetic quasielastic scattering in the paramagnetic phase, the phonon energies of three modes show clear upturn below TC, and the phonon linewidths change dramatically below TC as well. Our results provide the first demonstration of spin-phonon coupling in a potential two-dimensional atomic crystal.

Debye-Waller Factor in an Isotopically Disordered Semiconductor Crystal

The diagonal and non-diagonal parts for the Debye–Waller factor have been established using equation of motion technique of quantum dynamics and the Dyson equation approach. The double time temperature dependent phonon Green function has been taken to find the phonon linewidth and phonon shift. Renormalized mode frequency has been investigated in terms of electron–phonon coupling constant and temperature. The effect of electron–phonon interaction on the Debye–Waller factor has been studied in low temperature limit in low impurity concentration in semiconductor crystals.

Crystal dynamics and thermal properties of neptunium dioxide

We report an experimental and theoretical investigation of the lattice dynamics and thermal properties of the actinide dioxide NpO$_2$. The energy-wavevector dispersion relation for normal modes of vibration propagating along the $[001]$, $[110]$, and $[111]$ high-symmetry lines in NpO$_2$ at room temperature has been determined by measuring the coherent one-phonon scattering of X-rays from a $\sim$1.2 mg single-crystal specimen, the largest available single crystal for this compound. The results are compared against ab initio phonon dispersions computed within the first-principles density functional theory in the generalized gradient approximation plus Hubbard $U$ correlation (GGA+$U$) approach, taking into account third-order anharmonicity effects in the quasiharmonic approximation. Good agreement with the experiment is obtained for calculations with an on-site Coulomb parameter $U = 4$ eV and Hund's exchange $J= 0.6$ eV in line with previous electronic structure calculations. We further compute the thermal expansion, heat capacity, thermal conductivity, phonon linewidth, and thermal phonon softening, and compare with available experiments. The theoretical and measured heat capacities are in close agreement with another. About 27% of the calculated thermal conductivity is due to phonons with energy higher than 25 meV ($\sim$ 6 THz ), suggesting an important role of high-energy optical phonons in the heat transport. The simulated thermal expansion reproduces well the experimental data up to about 1000 K, indicating a failure of the quasiharmonic approximation above this limit.

Polarized and spatially resolved Raman scattering from composition-graded wurtzite InGaAs nanowires

We report Raman scattering from wurtzite single-crystalline InGaAs nanowires (NWs) to probe optical phonon behaviors associated with spatial grading in alloy composition along the NW length. Polarized Raman spectra revealed several optical phonons and their scattering symmetries: (i) InAs-like A1(LO) and A1(TO) phonons and (ii) GaAs-like A1(LO), A1(TO), and E2(high) phonons. In addition, strong anisotropic behavior was observed in the Raman tensor elements of the A1(TO) phonon mode. Interestingly, a spatial mapping of the GaAs-like A1(TO) phonon along the NW length direction showed a systematic increase in energy from the NW top (~255 cm−1) to the midpoint (~263 cm−1), indicating an increase in the Ga mole fraction from about 0.5 to about 0.8. Further toward the NW bottom, the GaAs-like A1(TO) phonon energy saturated to the peak value at about 264 cm−1. In the upper half of the NW, the phonon linewidths broadened significantly due to the spatial grading in In/Ga composition along the NW length. When the composition grading was negligible in the bottom half of the NW, the spectral widths were considerably narrowed. The GaAs-like E2(high) phonon showed similar variations in both energy and spectral width along the NW length.

Correlation between magnon and magnetic symmetries of hexagonal RMnO3 (R = Er, Ho, Lu)

The correlation between the magnon scattering and the magnetic symmetries of hexagonal RMnO3 (R = Er, Ho) thin films and LuMnO3 single crystal was studied through the 2D Correlation Spectroscopy (2D COS) and Perturbation-Correlation Moving Window 2D (PCMW2D) Correlation Spectroscopy which were performed on the temperature-dependent Raman spectra of RMnO3 (R = Er, Ho, Lu). From the Raman spectra, we observed much stronger intensity and more asymmetrical magnon peak in LuMnO3 single crystal than in ErMnO3 and HoMnO3 thin films. While the ratio between magnon and phonon's linewidth of LuMnO3 and HoMnO3 display an anomalous behavior, that ratio of ErMnO3 is almost stable. The result from PCMW2D also supports these results. In addition, our 2D COS analysis showed that there are more overlap peaks in broad four-spin flipping magnon peak in LuMnO3 than that in ErMnO3 and HoMnO3. The differences of hexagonal RMnO3 (R = Er, Ho, Lu) in magnon scattering are very similar to the actual differences of the magnetic symmetries of these compounds. Therefore, we suggest that the magnon scattering of hexagonal RMnO3 is strongly correlated with the magnetic symmetries of these materials.