Non-metallic Crystals Research Articles

Propagation of acoustic phonon solitons in nonmetallic crystals

We consider the theory of the propagation of acoustic-phonon solitons in nonmetallic crystals. For a soliton with a high strain amplitude, the width is small and so the soliton contains Fourier components of high frequency. These components are attenuated as a result of spontaneous anharmonic decay processes and, in addition, in some crystals there will be scattering due to the variation in mass of the different isotopes. At nonzero temperature there will be additional attenuation due to anharmonic interactions with thermal phonons. These attenuation mechanisms result in a steady decrease in the amplitude and velocity of a soliton, and we calculated attenuation constants for several nonmetallic crystals. We derive expressions for the rate of decay of the soliton amplitude and test these results by comparison with the results of numerical simulations.

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi2Se3 topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi2Se3 crystals are obtained. On the other hand, the Bi2Se3 topological insulator can also be induced to become a bulk superconductor, with Tc∼3.8K, by copper intercalation in the van der Waals gaps between the Bi2Se3 layers. Likewise, an as-grown crystal of metallic Bi2Te3 can be turned into a non-metallic crystal by slight variation in the Te content. The Bi2Te3 topological insulator shows small amounts of superconductivity with Tc∼5.5K when reacted with Pd to form materials of the type PdzBi2Te3.

Nanophotonic crystals with chiral elements by a hot embossing process in SU-8

We report a novel nanoprocess to fabricate 2D metallic and non-metallic nanophotonic crystals on SU-8 using hot embossing combining with a UV-curing process. The fabricated photonic crystals were characterized by optical diffraction experiments. Theoretical simulation of the light diffraction through such kind of artificial crystals agrees with the measured optical property very well, indicating that the developed fabrication technique is able to produce large area nanosize photonic crystals at economic costs. The success of this work not only provides scientists with a good opportunity to pioneer novel photonic physics in metallic and non-metallic chiral as well as photonic crystal systems, but also bridges the gap between laboratory and industrial application for mass production of photonic nanostuctures.

‘Comment on ‘A novel experimental method: Electrochemical detection of phase transition in ferroelectric single crystals’, Chem. Phys. Lett. 384 (2004) 262 by K. Gatner and R. Jakubas’

The electrochemical method presented by us has been used for the first time to study the phase transition in ionic, nonmetallic ferroelectric single crystals. Our experimental results were obtained for the crystals grown in the Faculty of Chemistry, University of Wrocław.

Comparison of localization procedures for applications in crystal embedding

With the aim of future applications in quantum mechanical embedding in extended systems such as crystals, we suggest a simple and computationally efficient method which enables construction of a set of nonorthogonal highly localized one-electron orbitals for periodic nonmetallic crystals which reflect their chemical nature. The orbitals are also used to build up the Hartree-Fock (HF) electron density of the entire crystals. The simplicity of the method stems from the fact that it does not require usage and/or modification of periodic electronic structure codes, and is instead based on the HF calculation of a sequence of finite clusters with subsequent application of a localization procedure to transform the HF canonical molecular orbitals. Two extreme cases of chemical bonding, ionic (MgO crystal) and covalent (Si crystal), are considered for which a number of known localization schemes are applied and compared. With some modifications our method can also be applied to nonperiodic nonmetallic systems as well.

Linear Dependence of the Phonon Thermal Resistance of Nonmetallic Crystals on the Isobaric Thermal Strain

This paper reports on the results of investigations into the thermal resistance W and thermal expansion coefficient β for single-crystal samples of Si, SiO2, Al2O3, and NaCl. The available experimental data on the thermal resistance and thermal expansion coefficient for materials with different types of interatomic bonding and different Landau criteria for convection are analyzed. It is demonstrated that the reduced phonon thermal resistance is equal to the isobaric thermal strain at any temperature.

High Thermal Conductivity Silicon Nitride Ceramic

Since the confirmation that nonmetallic single crystals with a diamond-like structure, such as SiC, BP, and AlN, have high intrinsic thermal conductivities of over 300 W m−1 K−1,1,2 a great deal of effort has been focused on the development of nonoxide polycrystalline ceramics with high thermal conductivity.

Relativistic effects on the optical response of InSb by time-dependent density-functional theory

In this paper we show how relativistic effects can be included in the time-dependent density-functional theory (DFT) for the optical response properties of nonmetallic crystals. The dominant scalar relativistic effects have been included using the zeroth-order regular approximation (ZORA) in the ground-state DFT calculations, as well as in the time-dependent response calculations. We show that this theory can also be applied to indium antimonide in the zinc-blende structure, notwithstanding the fact that it turns into a semimetal when scalar relativistic effects are included. Results are given for the band structure, the static dielectric constant ε∞ and the dielectric function ε(ω), for the various levels on which relativity can be included, i.e., nonrelativistic, only in the ground-state, or also in the response calculation. Comparisons of our calculated results are made with experiment and other theoretical investigations. With the inclusion of scalar relativistic effects, the band structure of InSb becomes semimetallic within the local density approximation and we find a deviation of 5% from experiment for the static dielectric constant. Also the dielectric function is improved and the spectra are in good agreement with experiment, although the spectral features are shifted to somewhat lower energies compared to experiment.

Application of time-dependent density-functional theory to the dielectric function of various nonmetallic crystals

The dielectric function of a range of nonmetallic crystals of various lattice types is studied by means of a real-space and full-potential time-dependent density-functional method within the adiabatic local-density approximation. Results for the dielectric constant e ‘ ~at optical frequencies! are given for crystals in the sodium chloride, the fluoride, the wurtzite, the diamond, and the zinc-blende lattice structure. The frequency-dependent dielectric function e(v) for the crystals in the diamond and zinc-blende lattice structure are also presented. We compare our calculated results with experimental data and other theoretical investigations. Our results for the dielectric constants e ‘ and the dielectric functions e(v) are in good agreement with the experimental values. The accuracy of the results is comparable to the one which is commonly found for time-dependent densityfunctional theory calculations on molecular systems. On average we find a deviation of 4‐5 % from experiment for the group IV and III-V compounds in the wurtzite, zinc-blende and diamond lattice structure, 8‐9 % for the II-VI and I-VII compounds in the zinc-blende and sodium chloride lattice structure, and up to 14% deviation for the fluoride lattice structure. The spectral features of the dielectric functions e(v) appear in the

Efficient real-space approach to time-dependent density functional theory for the dielectric response of nonmetallic crystals

Time-dependent density functional theory has been used to calculate the static and frequency-dependent dielectric function ε(ω) of nonmetallic crystals. We show that a real-space description becomes feasible for crystals by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field as perturbation in a periodic structure calculation. The induced density and microscopic potential can be obtained self-consistently for fixed macroscopic field by using linear response theory in which Coulomb interactions and exchange-correlation effects are included. We use an iterative scheme, in which density and potential are updated in every cycle. The explicit evaluation of Kohn–Sham response kernels is avoided and their singular behavior as function of the frequency is treated analytically. Coulomb integrals are evaluated efficiently using auxiliary fitfunctions and we apply a screening technique for the lattice sums. The dielectric function can then be obtained from the induced current. We obtained ε(ω) for C, Si, and GaAs within the adiabatic local density approximation in good agreement with experiment. In particular in the low-frequency range no adjustment of the local density approximation (LDA) band gap seems to be necessary.

Determination of Unit Cell Parameter and One-Electron Model Potential of KCl by Using Soft X-Ray Absorption Spectra

A multiparameter scheme of determining geometric characteristics and constructing empirical muffin-tin potential for nonmetallic crystals by using XANES data is proposed. It is based on the S-matrix pole equation for many-centre systems. Input data are energies and halfwidths of spectral maxima of the one-electron origin. The scheme is tested by determining the lattice parameter of KCl.

High-order optical-harmonic generation in films of nonmetallic crystals

High-order optical-harmonic generation in nonmetallic films interacting with pulses of laser light is examined. The wave functions of the current carriers in a crystal in an external electromagnetic field are chosen in the form of Volkov-Keldysh solutions. An explicit expression for the intensity of the sth harmonic, which depends on the crystal parameters, is derived. A plateau and a cutoff effect, similar to those in the case of harmonic generation on an isolated atom, have been detected. Finally, numerical estimates are made for GaAs films excited by pulses of radiation from a carbon dioxide laser.

Selection Rules and Probabilities of Exciton-Exciton Collisions

Selection rules are summarized for a mutual collision of two Wannier-Mott excitons which produces one free hole and one free electron. Calculation of probabilities per second of the collision: exciton + exciton → electron + hole in a perfect nonmetallic crystal for slow excitons in 1s and 2p states with isotropic effective mass of electron and hole has been performed assuming that the conduction band minimum and the valence band maximum are localized at the center of the Brillouin zone.

Sixty years of dislocations

It is well-known that with the appearance of three independent papers by Taylor, Polanyi and Orowan in the year 1934, the concept of crystal dislocations was born. Since then, dislocation theory has had many spectacular successes. It is quite appropriate therefore to be aware of the state of development of this exciting subject, sixty years after its discovery. A flavour for the vast subject of the applications of dislocation mechanisms to real materials is presented by choosing three examples, one each, drawn from metallurgy, physics and electronics. The topic of ‘Strength of metals and alloys’ is the first one, as this is also the author’s area of research. The phenomenon of solidification and crystal growth forms the next topic, especially in view of the seminal contributions made by A R Verma and his school from India. Dislocations play a useful role in the strengthening of solids, but how influential are they in affecting the performance of modern semiconductor devices? In the third example, the interesting and painstaking work done to settle this question is reviewed. Can we regard carbon fibre as thetransistor of dislocation theory? How shall we understand the long-established effects such as corrosion-fatigue, superplasticity and shape memory as well as the electrochemical and electro-mechanical properties of dislocations in semiconductor and non-metallic crystals? Answers to these questions belong to the realms of the future developments in dislocations. The talk is concluded with a discussion of these topics.

Implications of a metal‐bearing chemical boundary layer in D″ for mantle dynamics

At lower mantle conditions, the thermal conductivity of non‐metallic crystals is an order of magnitude lower than that of dense metallic minerals such as FeO, FeSi and Fe, which may be present in a chemical boundary layer at the base of the mantle. Because the core‐mantle boundary is nearly isothermal, variations in the thickness of a metal‐bearing layer induce lateral temperature variations of several hundred kelvin, which in turn affect the pattern of mantle convection. Upwellings should occur where the layer is thickest; the resulting stability of the metal + silicate layer with respect to the flow may also stabilize the pattern of mantle convection, and reduce its time‐dependence.

Thermal conductivity of doped YBCO single crystals

Thermal conductivity K of the three systems: oxygen deficient YBa 2Cu 3O x ( x=6.05−6.95), Y 1−zPr zBa 2Cu 3O x ( z=0−1, x=6.05−6.95), and YBa 2(Cu 1−yZn y) 3O 6.95 ( y=0.0 −0.05) in a-b plane have been systematically studied as function of temperature and magnetic field. The evolution of the thermal conductivity upon doping depends on the origin of metal-insulator transition. The unusual temperature dependence K(T) of non-metallic crystals suggests that some unconventional mechanism of phonon relaxation dominates in these crystals. Analysis of the experimental data confirms the phononic explanation of the K(T) for YBa 2Cu 3O 7.

Electronic structure of point defects in SiO 2, NaCl and diamond: cluster model with features of molecular cluster and cyclic models

The cluster model is proposed for calculating the structure and spectroscopic properties (by means of Hartree-Fock-Roothaan and configuration interaction methods) of neutral and charged point defects in any non-metallic crystals (both in the bulk and on the surface). The defectless quasicluster has the point symmetry of a crystal, is stoichiometric, uncharged and is characterized by closed electron shells. Semiempirical INDO calculations of the electronic structure and potential surface of a number of defects in SiO 2, NaCl and diamond were carried out with the use of the proposed, periodic supercell 3periodic defect) and molecular cluster models. The presented results of quasicluster and periodic supercell calculations of the perfect crystal electronic structure are almost identical. Application of the proposed model to non-crystalline solids is discussed.

The deep freeze

The development of specialized cryogenic systems which can access temperatures below 1 K is an exciting challenge for physicists and engineers. And as the number of scientists interested in the thermodynamics of producing ultra-low temperatures is steadily decreasing, many more are becoming interested in using refrigerators capable of reaching such temperatures as a tool to perform a wide range of experiments. Sometimes certain phenomena only occur at low temperature, such as the superfluid phases of 3He and 4He. In other cases, the physical property being studied becomes less complex and more amenable to theoretical interpretation, for example, in non-metallic crystals. The range of applications of these systems is surprisingly diverse and with most refrigerators being used in fundamental research it is inevitable that new requirements develop continuously. The designers of such products need to liaise with potential users and to combine physics and engineering to develop cost-effective and thermodynamically efficient systems. This brings the ultra-low-temperature regime within reach of specialists from other fields.

The Physics of Phonons

Elements of crystal symmetry: Direct lattice Reciprocal lattice Brillouin zone Crystal structure Point groups Space groups Symmetry of the Brillouin zone Jones zone Surface Brillouin zone Matrix representations of point groups. Lattice dynamics in harmonic approximation - semiclassical treatment: Introduction Lattice dynamics of a linear chain Lattice dynamics of three-dimensional crystals - phenomenological models Density of normal modes Numerical calculation of g(w) Lattice heat capacity. Lattice dynamics in the harmonic approximation - ab initio treatment: Introduction The frozen-phonon approach The linear response approach The planar force constant method. Anharmonicity: Introduction Hamiltonian of a general three-dimensional crystal Effect of anharmonicity on phonon states Effects of the selection rules on three-phonon processes Hamiltonian of an anharmonic elastic continuum Evaluation of three-phonon scattering strengths The quasi-harmonic approximation and Grueneisen's constant. Theory of lattice thermal conductivity: Introduction Relaxation-time methods Gree-Kubo linear response theory Second sound and Poiseuille flow of phonons. Phonon scattering in solids: Boundary scattering Scattering by static imperfections Phonon scattering in alloys Anharmonic scattering Phonon-electron scattering in doped semiconductors Phonon scattering due to magnetic impurities in semiconductors Phonon scattering from tunnelling states of impurities Phonon-photon interaction. Analysis of phonon relaxation and thermal conductivity results: Anharmonic decay of phonons Lattice thermal conductivity of undoped semiconductors and insulators Non-metallic crystals with high thermal conductivity Thermal conductivity of complex crystals Low-temperature thermal conductivity of doped semiconductors. Phonons in low dimensional solids: Introduction Surface vibrational modes Attenuation of surface phonons Phonons in superlattices Thermal conductivity of superlattices. Phonons in impure and mixed crystals: Introduction Localised vibrational modes in semiconductors Experimental studies of long-wavelength optical phonons in mixed crystals Theoretical models for long-wavelength optical phonons in mixed crystals Phonon conductivity of mixed crystals. Phonons in quasi-crystalline and amorphous solids: Introduction Phonons in quasi-crystals Structure and vibrational excitations of amorphous solids Vibrational properties of amorphous solids Low-temperature properties of amorphous solids. Phonon spectroscopy: Introduction Heat pulse technique Superconducting tunnel junction technique Optical techniques Phonons from Landau levels in 2DEG Phonon focusing and imaging Frequency crossing phonon spectroscopy Phonon echoes. Phonons in liquid helium: Introduction Dispersion curve and elementary excitations Specific heat Interactions between the excitations Kapitza resistance Quantum evaporation. Appendices: Density functional formalism The pseudopotential method Evaluation of integrals in sections 6.4.1.4 Negative-definitenss of the phonon off-diagonal operator ^D*L. References. Index.

Accumulation and annihilation processes of cascades in metallic and nonmetallic crystals under irradiation with ions and/or electrons

In situ observation has been performed under dual-beam irradiation with heavy ions and fast electrons in the HVEM-accelerator facility at Kyushu University. The objectives of the present study are to understand the structure and the accumulation process of cascade damage and the synergistic effect of electronic excitation and/or free point defects on the behavior of cascade damage in metallic and nonmetallic crystals, such as Cu, graphite, SiC, Si, Ge, Ge-20at%Si, MgO, α-Al 2O 3, MgAl 2O 4 and others. The HVEM-accelerator facility has been confirmed to be extremely useful for observing the behavior of cascade damage: (1) Cascade damage is directly produced without the help from other collision cascades in Cu. (2) Most collisions themselves in Si, Ge and Ge-20at%Si induce invisible structural change through electron microscopy, but they are converted into visible ones by the impact from other collision cascades. (3) No visible cascade damage is formed in covalent crystals with light masses such as graphite and SiC and in ionic crystals such as MgO, α-Al 2O 3 and MgAl 2O 4. (4) Free interstitials suppress the evolution of visible cascade damage and assist the nucleation and the growth of interstitial loops around collision cascades in metallic and nonmetallic crystals.