Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F8: Electrons, Phonons, and Electron Phonon Scattering IFocus
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Sponsoring Units: DCOMP Chair: Anderson Janotti, University of Delaware Room: 267 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F8.00001: Anharmonic phonons and second-order phase-transitions by the stochastic self-consistent harmonic approximation Invited Speaker: Francesco Mauri Anharmonic effects can generally be treated within perturbation theory. Such an approach breaks down when the harmonic solution is dynamically unstable or when the anharmonic corrections of the phonon energies are larger than the harmonic frequencies themselves. This situation occurs near lattice-related second-order phase-transitions such as charge-density-wave (CDW) or ferroelectric instabilities or in H-containing materials, where the large zero-point motion of the protons results in a violation of the harmonic approximation. Interestingly, even in these cases, phonons can be observed, measured, and used to model transport properties. In order to treat such cases, we developed a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity in the nonperturbative regime and to obtain, from first-principles, the structural, thermodynamic and vibrational properties of strongly anharmonic systems [1]. I will present applications to the ferroelectric transitions in SnTe, to the CWD transitions in NbS$_2$ and NbSe$_2$ (in bulk and monolayer) and to the hydrogen-bond symmetrization transition in the superconducting hydrogen sulfide system [2], that exhibits the highest Tc reported for any superconductor so far. In all cases we are able to predict the transition temperature (pressure) and the evolution of phonons with temperature (pressure).\\ $\rm [1]$ I. Errea, M. Calandra, F. Mauri, Phys. Rev. Lett. 111, 177002 (2013) and Phys. Rev. B 89, 064302 (2014)\\ $\rm [2]$ I. Errea, M. Calandra, C. J. Pickard, J. R. Nelson, R. J. Needs, Y. Li, H. Liu, Y. Zhang, Y. Ma, F. Mauri, Nature 532, 81 (2016) [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F8.00002: Large Scale GW Calculations Including Electron-Phonon Interactions Ryan McAvoy, Marco Govoni, Giulia Galli Including the effect of electron-phonon coupling in first principle electronic structure calculations is crucial for the accurate prediction of band-gaps and temperature dependent carrier lifetimes. We present results for the electronic properties of condensed and molecular systems, including electron-phonon coupling, obtained by merging GW calculations [1] of eigenvalues and an efficient implementation of the Fan-Migdal-Debye-Waller self-energy. Our implementation does not require summation over virtual states and inversion of large dielectric matrices. Advantages of the algorithm presented here over standard techniques and its scalability will be discussed. [1] Govoni, Marco, and Giulia Galli. "Large scale GW calculations."~\textit{Journal of chemical theory and computation}~11, no. 6 (2015): 2680-2696. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F8.00003: Implementation of Electron-Phonon Coupling in the KKR Formalism and its Applications to Simple Metals Carsten Eberhard Mahr, Michael Czerner, Christian Franz, Christian Heiliger Electron-phonon coupling is one of the main incoherent inelastic scattering mechanisms in a wide variety of crystalline material systems at room temperature. Therefore, it is necessary to incorporate those effects in any realistic calculation of thermoelectric properties. We do so by extending our density functional theory (DFT) based Korringa-Kohn-Rostocker (KKR) Green's function formalism code. By approximating the Fr\"ohlich-type interaction with a self-energy $\Sigma_\text{eph} = -i\frac{\hbar}{2\tau}$ we are able to compute the dressed propagator $G$ by solving Dyson's equation $G = G_\text{ref} + G_\text{ref}\cdot\left(\Delta{}V+\Sigma_\text{eph}\right)\cdot{}G$, where $G_\text{ref}$ is the Green's Function of an arbitrary (though typically repulsive) reference system. The corresponding electron-phonon scattering time $\tau$ is extracted from electron linewidth calculations. We demonstrate the physical validity of the beforementioned calculational scheme for non-equilibrium properties by comparing evaluated temperature dependent resistivity characteristics of transport systems consisting of copper, aluminum and other simple metals to experiment. Further, technical details of the implementation in the KKR basis set are presented. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F8.00004: One-shot calculation of temperature-dependent optical spectra and phonon-induced band-gap renormalization Marios Zacharias, Feliciano Giustino Electron-phonon interactions are of fundamental importance in the study of the optical properties of solids at finite temperatures. Here we present a new first-principles computational technique based on the Williams-Lax theory for performing predictive calculations of the optical spectra, including quantum zero-point renormalization and indirect absorption [1]. The calculation of the Williams-Lax optical spectra is computationally challenging, as it involves the sampling over all possible nuclear quantum states. We develop an efficient computational strategy for performing "one-shot" finite-temperature calculations [2]. These require only a single optimal configuration of the atomic positions. We demonstrate our methodology for the case of Si, C, and GaAs, yielding absorption coefficients in good agreement with experiment. This work opens the way for systematic calculations of optical spectra at finite temperature. [1] M. Zacharias, C. E. Patrick, and F. Giustino, Phys. Rev. Lett. 115, 177401 (2015). [2] M. Zacharias and F. Giustino, Phys. Rev. B 94, 075125 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F8.00005: Using atomic orbitals to compute and interpolate the electron-phonon interaction Luis Agapito, Marco Bernardi The electron-phonon interaction (EPI) controls key dynamical properties in materials, including charge and spin transport, and ultrafast carrier dynamics. Accurately computing these properties involves obtaining the EPI on very fine Brillouin zone (BZ) grids; since direct ab initio computation is prohibitively expensive, schemes to interpolate the EPI are needed. We present a new method that uses atomic-orbitals (AOs) to compute and interpolate the EPI on fine BZ grids, with important advantages over the alternative Wannier function (WF) interpolation. Different from WFs, AOs are readily available from databases. Their use can be automated without the trial-and-error optimization required in WFs, making AOs suitable for high-throughput study of carrier dynamics in materials. We will discuss the formalism and numerical implementation of our approach, comparing computational cost and accuracy with WFs. The impact of the completeness of the AO set (i.e., single zeta, double zeta, etc.) on the accuracy will be analyzed. Applications to complex materials for which WFs are not ideally suited will be discussed, along with processes that are naturally mapped onto AO sets, such as polaron transport in conjugated molecules and correlated oxides. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F8.00006: First-principles calculation of indirect absorption in transparent conducting oxides Hartwin Peelaers, Emmanouil Kioupakis, Chris G. Van de Walle Transparent conducting oxides (TCOs) are a technological important class of materials that combine high electrical conductivity with transparency in the visible light spectrum. The most frequently used material is Sn-doped In$_2$O$_3$, also called ITO. The Sn doping provides the free carriers required for the conductivity. The transparency of the material can be attributed to the fact that direct absorption processes, either across-the-gap or by direct free-carrier absorption, require energies larger than that of visible light photons. But light absorption can still occur due to indirect free-carrier absorption, which is usually described by a phenomenological Drude model. To improve the fundamental understanding of the processes limiting the transparency in these materials, we use a fully first-principles methodology based on density functional theory [1]. We will discuss the importance of phonon- and defect-assisted absorption and compare the results for ITO with our earlier results for SnO$_2$ [1]. \\ \\~ [1] H. Peelaers, E. Kioupakis, and C.G. Van de Walle, Phys. Rev. B 92, 235201 (2015). [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F8.00007: First-principles carrier transport using the Boltzmann Transport Equation in EPW Samuel Ponc\'e, Elena Roxana Margine, Feliciano Giustino Understanding transport and carrier mobilities in semiconductors is crucial in electronics, optoelectronics, and energy applications. To predict mobilities in theoretical (not yet made) materials would be a remarkable achievement. Fully predictive first-principles calculations of mobilities have only been made possible recently with the advance of ab-initio tools to compute electron-phonon interactions with high accuracy. Here, we present a comprehensive framework to study electron and hole transport in metals and insulators within the full self-consistent linearized Boltzmann transport equation and its approximations. The theory is implemented into the free and open-source code EPW [1,2]. We study four representative semiconductors: Si, GaAs, GaN and bulk MoS$_2$ and discuss challenges associated with ultra dense sampling and polar electron-phonon interactions. \newline [1] S. Ponc\'e \textit{et al.}, Comput. Phys. Commun. \textbf{209}, 116 (2016) \newline [2] http://epw.org.uk [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F8.00008: DQMC study on Holstein model with momentum-dependent electron-phonon coupling Wei-ting Chiu, Richard Scalettar We use the determinant quantum Monte Carlo (DQMC) method to study the Holstein model with a momentum-dependent electron-phonon coupling $\lambda(\textbf{q})$. The correlation functions and their Fourier transformed structure factors are calculated. In addition, the single particle Green's function and spectral functions are obtained. These quantities are used to infer the nature of long-range charge order as well as the renormalization of the electron and phonon propagators. Study of this model is motivated by the suggestion that electron-phonon interaction with large "forward scattering" is relevant to the understanding of the physics of the thin layers of single-unit-cell iron selenide (FeSe) grown on a strontium titanate (SrTiO$_{3}$) substrate, possibly giving rise to the enhanced superconducting transition temperature and replica bands seen in ARPES experiments. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F8.00009: Electron-phonon scattering rates in GaAs and AlAs under hydrostatic pressure Nandan Tandon, L. R. Ram-Mohan, John D. Albrecht We present a first-principles plane wave pseudopotential study on the electron-phonon (el-ph) scattering rates in GaAs and AlAs under hydrostatic pressure. Effect of pressure on the band gap and phonon dispersion is well studied in the past, where the direct band gap of GaAs becomes larger than the indirect band gap at X making it an indirect bandgap semiconductor over $\sim 3$GPa. Under hydrostatic pressure AlAs remains an indirect bandgap semiconductor with the bandgap decreasing as pressure increases. Phonon dispersion under hydrostatic pressure results in softening of acoustic phonons and optical frequencies shift to higher values. We will discuss the resulting effect of pressure on the el-ph scattering rates. The hydrostatic pressure is varied between $0-15$GPa. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F8.00010: Ab initio Electron Mobility and Polar Phonon Scattering in GaAs Jin-Jian Zhou, Marco Bernardi In polar semiconductors and oxides, the long-range nature of the electron-phonon (\textit{e}-ph) interaction is a bottleneck to compute charge transport from first principles. Here, we develop an efficient ab initio scheme to compute and converge the \textit{e}-ph relaxation times (RTs) and electron mobility in polar materials. We apply our approach to GaAs, where using the Boltzmann equation with state-dependent RTs, we compute mobilities in excellent agreement with experiment at 250$-$500~K. The $e$-ph RTs and the phonon contributions to intra-valley and inter-valley $e$-ph scattering are also analyzed. Our work enables efficient ab initio computations of transport and carrier dynamics in polar materials. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F8.00011: First-principles studies of electron transport in Ga$_2$O$_3$ Youngho Kang, Karthik Krishnaswamy, Hartwin Peelaers, Chris G. Van de Walle Ga$_2$O$_3$ is a wide-gap semiconductor with a monoclinic crystal structure and a band gap of 4.8 eV. Its high carrier mobility and large band gap have attracted a lot of attention for use in high power electronics and transparent conductors. Despite its potential for adoption in these applications, an understanding of its carrier transport properties is still lacking. In this study we use first-principles calculations to analyze and compute the electron scattering rates in Ga$_2$O$_3$. Scattering due to ionized impurities and polar longitudinal-optical (LO) phonon is taken into account. We find that the electron mobility is nearly isotropic, despite the low-symmetry monoclinic structure of Ga$_2$O$_3$. At low carrier densities ($\sim$10$^{17}$ cm$^{-3}$), the mobility is limited by LO phonon scattering. Scattering by ionized impurities becomes increasingly important at higher carrier densities. This type of scattering is enhanced when compensating native point defects are present; in particular, gallium vacancies, which are triply negatively charged, can have a strong effect on mobility. These effects explain the downturn in mobility observed in experiments at high carrier densities. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F8.00012: Phonon Calculations Using the Real-Space Multigrid Method (RMG) Jiayong Zhang, Wenchang Lu, Emil Briggs, Yongqiang Cheng, A.J. Ramirez-Cuesta, Jerry Bernholc RMG, a DFT-based open-source package using the real-space multigrid method, has proven to work effectively on large scale systems with thousands of atoms. Our recent work has shown its practicability for high accuracy phonon calculations employing the frozen phonon method. In this method, a primary unit cell with a small lattice constant is enlarged to a supercell that is sufficiently large to obtain the force constants matrix by finite displacements of atoms in the supercell. An open-source package PhonoPy is used to determine the necessary displacements by taking symmetry into account. A python script coupling RMG and PhonoPy enables us to perform high-throughput calculations of phonon properties. We have applied this method to many systems, such as silicon, silica glass, ZIF-8, etc. Results from RMG are compared to the experimental spectra measured using the VISION inelastic neutron scattering spectrometer at the Spallation Neutron Source at ORNL, as well as results from other DFT codes. The computing resources were made available through the VirtuES (Virtual Experiments in Spectroscopy) project, funded by Laboratory Directed Research and Development program (LDRD project No. 7739) [Preview Abstract] |
Tuesday, March 14, 2017 2:03PM - 2:15PM |
F8.00013: Fr\"ohlich electron-phonon coupling from first-principles in anatase TiO$_2$ Carla Verdi, Feliciano Giustino The calculation of electron-phonon properties in polar materials from first principles remains largely unexplored despite their ubiquitous role in many technological applications, such as light-emitting devices and transparent electronics. In polar semiconductors and insulators the electrons can be strongly coupled to the macroscopic electric field induced by longitudinal optical (LO) phonons at long wavelength, leading to the so-called Fr\"ohlich interaction. In our work we develop a general formalism for calculating the Fr\"ohlich vertex from first principles, which can be used as a powerful tool in conjunction with \textit{ab initio} interpolation based on maximally localized Wannier functions [1]. We demonstrate our method by computing the electron lifetimes in anatase TiO$_2$ and we establish quantitatively the importance of the inclusion of the \textit{ab initio} Fr\"ohlich coupling. Finally, we address the problem of investigating the properties of polaronic quasiparticles, i.e. electrons dressed by a phonon cloud, that can easily form in polar materials and affect their charge transport properties.\\ [4pt] [1] C. Verdi and F. Giustino, Phys. Rev. Lett. \textbf{115}, 176401 (2015). [Preview Abstract] |
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