Theory of interface and anharmonic phonon interactions in nanocomposite materials

© Published under licence by IOP Publishing Ltd.We present a theory of phonon scattering rates resulting from mass smudging across interfaces and from anharmonicity in nanocomposite materials. We discuss the derivation of the anharmonic scattering term and present some recently published results indicating that if certain models of interface mass-mixing are adopted, the cross-planar thermal conductivity of an ultra-thin superlattice system will decrease dramatically with the number of bilayers n until n 4, above which value it begins to increase with n.Engineering and Physical Sciences Research Council (EPSRC

Origin of superconductivity in layered centrosymmetric LaNiGa2

The following article appeared in Applied Physics Letters , Vol. 104, 022603 (2014) and may be found at: http://dx.doi.org/10.1063/1.4862329We have examined the origin of superconductivity in layered centrosymmetric LaNiGa2 by employing a linear response approach based on the density functional perturbation theory. Our results indicate that this material is a conventional electron-phonon superconductor with intermediate level of coupling strength, with the electron-phonon coupling parameter of 0.70, and the superconducting critical temperature of 1.90 K in excellent accordance with experimental value of 1.97 K. The largest contribution to the electron-phonon coupling comes from the La d and Ga p electrons near the Fermi energy and the B3g phonon branch resulting from vibrations of these atoms along the Γ-Z symmetry line in the Brillouin zone. © 2014 AIP Publishing LLC

Phonon anomalies and superconductivity in the Heusler compound YPd 2Sn

The following article appeared in Journal of Applied Physics, Vol. 116, 013907 (2014) and may be found at: http://dx.doi.org/10.1063/1.4887355We have studied the structural and electronic properties of YPd 2Sn in the Heusler structure using a generalized gradient approximation of the density functional theory and the ab initio pseudopotential method. The electronic results indicate that the density of states at the Fermi level is primarily derived from Pd d states, which hybridize with Y d and Sn p states. Using our structural and electronic results, phonons and electron-phonon interactions have been studied by employing a linear response approach based on the density functional theory. Phonon anomalies have been observed for transverse acoustic branches along the [110] direction. This anomalous dispersion is merely a consequence of the strong coupling. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter is found to be λ=0.99. Using this value, the superconducting critical temperature is calculated to be 4.12K, in good accordance with the recent experimental value of 4.7K. © 2014 AIP Publishing LLC

Evolution of thermal properties from graphene to graphite

PublishedJournal ArticleWe report on the evolution of thermal properties from graphene to graphite as a function of layer thickness and temperature. The onset of the inter-layer compressional elastic constant C33 and the shear elastic constant C44 results in a large difference between the magnitudes and temperature dependencies of the specific heat and in-plane lattice thermal conductivity of bi-layer graphene (BLG) and single-layer graphene. The changes between BLG and few-layer graphene (FLG) decrease with increase in the number of layers. The cross-plane lattice thermal conductivity increases almost linearly with the number of layers in ultra-thin FLG. © 2014 AIP Publishing LLC.Taibah University, Madinah, Saudi Arabi

Tuning phonon properties to enhance the thermoelectric figure of merit

This paper has been accepted for publication in Electronic, Photonic, Plasmonic, Phononic and Magnetic Properties of Nanomaterials; Mahi R. Singh. AIP Conf. Proc. 1590, 1 (2014); http://dx.doi.org/10.1063/1.4870187. Conference date: 12–16 August 2013. Location: London, Canada. The paper is available via http://dx.doi.org/10.1063/1.4870203We will discuss a theory of the thermoelectric properties of semiconductor alloys and superlattices, with an emphasis on the role of phonons. After summarising our previous calculations of the lattice thermal conductivity and the thermoelectric figure of merit ZT of an n-type SiGe alloy system, we will present some recent results for an ultra-thin SiGe superlattice and discuss how they differ from the alloy results

Anharmonic effects in the thermoelectric properties of PbTe

The following article appeared in Journal of Applied Physics, Vol. 116, 043702 (2014), and may be found at: http://dx.doi.org/10.1063/1.4891201In this work, we investigate the crystal anharmonic effects in the thermoelectric properties of n-type PbTe. The lattice thermal transport coefficient is computed by employing an isotropic continuum model for the dispersion relation for acoustic as well as optical phonon branches, an isotropic continuum model for crystal anharmonicity, and the single-mode relaxation time scheme. The electronic components of the transport coefficients in a wide temperature range are calculated using the isotropic-nearly-free- electron model, interaction of electrons with deformation potential of acoustic phonons, and the effect of the band non-parabolicity. It is found that the transverse optical branches play a major role in determining the phonon conductivity and the thermoelectric figure of merit of this material. © 2014 AIP Publishing LLC

Theoretical investigation of superconductivity in SrPd2Ge2, SrPd2As2, and CaPd2As2

This is the final version of the article. Available from American Physical Society via the DOI in this record.© 2016 American Physical Society. Ab initio pseudopotential calculations have been performed to investigate the structural, electronic, and vibrational properties of SrPd2Ge2, SrPd2As2, and CaPd2As2 crystallizing in the ThCr2Si2-type body-centered tetragonal structure. Our electronic results show that the density of states at the Fermi level is mainly dominated by the strong hybridization of Pd d states and Ge (or As) p states. The linear response method and the Migdal-Eliashberg approach have been used to calculate the Eliashberg spectral function for all these compounds. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter (λ) is found to be 0.74 for SrPd2Ge2, 0.66 for SrPd2As2, and 0.72 for CaPd2As2. Using the calculated values of λ and the logarithmically averaged phonon frequency ωln the superconducting critical temperature (Tc) values for SrPd2Ge2, SrPd2As2, and CaPd2As2 are found to be 3.20, 2.05, and 2.48 K, respectively, which are in acceptable agreement with the corresponding experimental values. The relative differences in the Tc values between the Ge and As compounds have been explained in terms of some key physical parameters.This work was supported by the Scientific and Technical Research Council of Turkey (TUB¨ ˙ITAK) (Project No. MFAG- 114F192). Some of the calculations for this project were carried out using the computing facilities on the Intel Nehalem (i7) cluster (ceres) in the School of Physics, University of Exeter, United Kingdo

Ab initio investigation of BCS-type superconductivity in LuNi2 B2 C -type superconductors

Journal ArticleAb initio pseudopotential calculations have been made to calculate the structural, electronic, vibrational, and superconducting properties of the borocarbide superconductors RM2B2C(R=Lu,La,Y;M=Ni,Pd,Pt). Using the electronic and vibrational spectra, the average electron-phonon coupling parameter is calculated to be 0.78, 0.83, 0.96, and 1.48 for LaPt2B2C, LuNi2B2C, YPt2B2C, and YPd2B2C, respectively. From a detailed spectral analysis throughout the irreducible Brillouin zone (IBZ), we found that the highest contribution to the electron-phonon coupling parameter for these materials comes from the acoustic and low-frequency optical modes characterized with anomalous dispersion, instead of the A1g optical-phonon mode claimed in previous studies. Using the calculated electron-phonon coupling parameter values, the superconducting critical temperatures for LaPt2B2C, LuNi2B2C, YPt2B2C, and YPd2B2C are found to be 10.40, 15.94, 11.30, and 20.60 K, respectively, which are in good accordance with their experimental values of 10.50, 16.10, 10.00, and 21.00 K.This work was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK) (Project No. MFAG-114F192). Some of the calculations for this project were carried out using the computing facilities on the Intel Nehalem (i7) cluster (ceres) in the School of Physics, University of Exeter, United Kingdom

Coherent phonon modes of crystalline and amorphous Ge2Sb2Te5 thin films: A fingerprint of structure and bonding

Copyright © 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics, Volume 117, article 025306, and may be found at http://dx.doi.org/10.1063/1.4905617Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge2Sb2Te5 (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T2-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe4 tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe6 octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3-4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb2Te3 or GeTe4 tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.Engineering and Physical Sciences Research Council (EPSRC