Phonon density-of-states in the new superconductor MgB 2

We provide here experimental data on the phonon density-of-states of MgB₂ obtained by the inelastic neutron scattering technique. The measurements were performed for the natural boron-based magnesium diboride with use of a time-of-flight neutron spectrometer. Several phonon bands were observed in the phonon spectrum at energies of about 33, 55, 82 and 99 meV. We show that the cut-off energy of the density-of-states occurs at around 105 meV which is much higher than expected so far from heat-capacity data and partially explains the high T _c value observed for MgB₂. The characteristic phonon energies are indicative of an intermediate coupling regime in this compound. We conclude that a much needed neutron experiment aimed at the study of the isotopic effect in the phonon density-of-states of MgB₂ is conceivable. Received 19 March 2001     

Vibronic spectrum of the U2 isoelectronic center in Si: In

The photoluminescence spectrum of Si:In measured at 2 and 4 K using samples from several suppliers has been found to be preparation sensitive. In particular, intensity variations allow us to distinguish a sharp no-phonon line at 1.118 eV, variously referred to as U₂ of P, and its associated vibronic spectrum from the In(NP) lines and their phonon replicas. Whereas the intensity of the latter did not show preparation sensitivity, the former has been observed to change by three orders of magnitude. The U₂ vibronics form a broad-structured spectrum containing density-of-states features. The appearance of phonons other than those conserving crystal momentum demonstrates the exciton is bound to a low symmetry site. In addition, the spectrum includes a peak at 1.109 eV, called R, and a shoulder at 1.107 eV which involves too small an energy loss to be density-of-states related, and these features are most likely modes of the U₂ impurity complex. This complex has been tentatively identified as an isoelectronic center composed of an indium-phosphorus nearest-neighbor substitutional pair.     

First-principles lattice dynamics,thermodynamics, and elasticity of Cr2O3

We present the calculation of the lattice dynamics of chromia (Cr₂O₃), a typical Mott–Hubbard insulator, employing the first-principles density functional theory plus U approach. We first report the phonon dispersions at the theoretical equilibrium volume. Then the phonon density-of-states is calculated as a function of volume. Finally, the atomic volume, heat capacity, linear thermal expansion coefficient, bulk modulus, Grüneisen constant, and elastic constants are calculated as functions of temperature.     

Temperature variation of phonon frequency distribution in the fast ion conductor lead fluoride

A weighted phonon frequency distribution has been measured in PbF₂ at temperatures 10, 302, 660 and 910 K, using a neutron scattering technique. At 10 K good agreement is found between the measured distribution and the phonon density-of-states calculated from the low temperature dispersion relation of PbF₂. At the higher temperatures, near the ionic conductivity transition temperature, T_c ～ 700 K, the optic modes are observed to broaden into a high energy tail consistent with strong anharmonicity or extensive disorder. A low energy peak arising from transverse acoustic modes remains well defined even at temperatures above T_c.     

High-Pressure Studies of Magnetism and Lattice Dynamics by Nuclear Resonant Scattering of Synchrotron Radiation

A review is given on recent high-pressure experiments using nuclear resonant scattering of synchrotron radiation. We shortly introduce the methodological aspects connected with high-pressure experiments applying nuclear forward scattering and nuclear inelastic scattering of synchrotron radiation. Selected examples for the study of magnetism are given for the Laves phases LuFe₂ and ScFe₂, where we studied the variation of the magnetic ordering temperature and of the Fe band moment as a function of pressure. An actual example for the study of lattice dynamics is a recent investigation of the phonon density-of-states in metallic iron with special emphasis on hcp ε-Fe, where the pressure-induced texture is used, in conjunction with theoretical calculations, to extract density of phonon states as seen parallel and perpendicular to the hexagonal c-axis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Infrared and Raman study of Si2Te3

Results of ir and Raman investigations on trigonal layer-structured Si₂Te₃ single crystals are reported. The ir reflection spectrum withEc exhibits seven reststrahl-like bands, whereas the corresponding spectrum withEc shows only one very small bump. Values for ₀ and are given. The Raman spectra are very rich in structure and can only be interpreted qualitatively as being a mixture of single phonon lines and one- and two-phonon density-of-states contributions. One-phonon density-of-states effects are disorder-induced owing to the statistical occurrence of the Si atoms within the regular hexagonal Te sublattice.     

Vibrational properties of single-wall BC3 nanotubes

The phonon dispersions of single-wall BC₃ nanotubes with any chirality are calculated within a symmetry-based force constant model of the lattice dynamics. Based on the non-symmorphic symmetry group of the BC₃ tubes, the symmetries and number of the Raman- and infrared-active modes at Γ point of the one-dimensional Brillouin zone are given. The neighbor atom-atomic interaction force constants are recalculated by fitting them to the experimental phonon energy-dispersion curves of honeycomb BC₃ sheet. The frequencies of the optically active modes are presented as the function of diameters and chiralities for BC₃ tubes. The obtained phonon density-of-states spectra, phonon dispersion relations, and vibrational patterns of the zone-center phonons are presented and discussed in detail. The calculated frequencies of infrared-active modes are compared with the experimental values reported in the literature. The results provide comprehensive information about the vibrational properties of the BC₃ tubes and shed light on the interpretation of Raman scattering and infrared spectroscopies.     

Raman scattering of Ni and Cr amorphous disilicides

The Raman spectra and reflectivity of r.f. sputtered amorphous semimetallic a-NiSi₂ and a-CrSi₂ thin films are reported. Substantial differences between the two amorphous disilicide Raman spectra are associated with changes in the phonon density-of-states as a result of variations in Si(p)-transition metal (d) interactions. The greater width of the bands in the Cr alloy suggests a less ordered local environment. Neither spectrum indicates distinct a-Si-like bands, demonstrating that significant short range order changes have occured.     

Effet d’ordre dans le supraconducteur V3Au: Effet électronique ou phononique?

Specific heat measurements from1 to 40 K give some insight into the phonon spectrum of the alloy V₃Au. The minute differences between the as-cast and annealed states indicate that the drastic change of the superconducting transition temperature upon ordering is not likely to be of phononic origin. Alternatively, this behaviour can be understood as a consequence of an electronic density-of-states change in the presence of a large Coulomb pseudopotential term. This assumption is supported by susceptibility measurements.     

First principles study on the structural,electronic and phonon properties of monolayer B2Si

Using first principles density functional theory, we predict a monolayer B₂Si structure with space group Pmm2 in the present work. This structure is confirmed to be dynamically stable. Based on the plane wave pseudopotential approach, the charge density, electron localization function, density of states, energy band, phonon property and thermal conductivity of Pmm2-B₂Si are systematically studied. It is interesting that the sp² hybridization and coordination bond of Si are found in Pmm2-B₂Si, which is the most important factor for its structural stability. The density of states and energy band analysis reveals that Pmm2-B₂Si is metallic because of the partial occupied Si 3pz and B 2pz states. Moreover, the acoustic-optical coupling is important for phonon transport in Pmm2-B₂Si, and the contribution of optical modes to the lattice thermal conductivity along the [100] and [010] directions is 13% and 12%, respectively. This study gives a fundamental understanding of the structural, electronic and phonon properties in Pmm2-B₂Si.     

Electron tunneling spectroscopy of the phonon spectrum of MgB<Subscript>2</Subscript>

The specific features of the phonon spectrum of the MgB₂ compound (T _c = 38 K) are investigated by tunneling spectroscopy. It is demonstrated that both the position and the energy width of the fundamental optical mode E _2g in the phonon spectrum are in good agreement with inelastic X-ray spectroscopy data but differ substantially from Raman spectroscopy results. Among possible factors responsible for this discrepancy, the anharmonic and nonadiabatic effects that are characteristic of the MgB₂ system are discussed.     

Raman spectroscopy study of the Fermi resonance between a local vibration and the two‐phonon spectrum in PbI2 crystal

The problem of complex Fermi resonance (FR) involving a local vibration is considered. It is shown for the first time that in the PbI₂ crystal, owing to the features of phonon bands, the following two effects superpose on each other: (1) the traditional FR between the fundamental Ω(A_1g) vibration and the two‐phonon states (TPS) arising from the 2Ω(E_u) vibrations, and (2) the FR between the impurity vibration and the same TPS band. It is shown theoretically that in the case of such complex FR the shape of the spectrum strongly depends on the impurity concentration, anharmonic constants and the phonon band structure. An analysis of experimental data and comparison with theory is made. Copyright © 2008 John Wiley & Sons, Ltd.     

Evolution of the 251 cm-1 infrared phonon mode with temperature in Ba0.6K0.4Fe2As2

The far-infrared optical reflectivity of an optimally doped Ba1-xKxFe2As2(x =0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm-1 . This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Ba0.6K0.4Fe2As2 compound.     

Soft phonon mode at the martensitic phase transformation of AuCuZn2

The lattice dynamical property of AuCuZn₂ has been investigated by means of inelastic neutron scattering technique in connection with its martensitic phase transformation. The temperature dependent soft phonon was observed transformation. The temperature dependent soft phonon was observed around $\text{q =}\text{2}\text{3}\text{[110]}$ of TA₂ phonon branch. We have also found a premartensitic metastable phase, in which new satellite reflections at (H± $\text{2}\text{3}$, K ? $\text{2}\text{3}$, 0) have been observed around fundamental reflections with H + K = 4n. The atomic displacements of the soft phonon mode correspond to the atomic arrangement of the premartensitic phase.     

Band structure and optical transitions in the Hg3Se2Cl2 crystals

First-principles calculations of the band structure for Hg₃Se₂Cl₂ crystal were performed by means of density functional theory (DFT). The exchange and correlation potential was described within a framework of the local density approximation based on exchange-correlation energy optimization to calculate the total energy. DOS/PDOS and valence charge distribution were studied in details. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions. For device applications based on Hg₃Se₂Cl₂ crystal, understanding these fundamental issues is highly important and essential. It should be noted that optoelectronic applications of Hg₃Se₂Cl₂ are caused by coexistence of the large polarized Hg cations and a huge contribution of an harmonic phonon subsystem caused by anions.     

Multi-quantum photoconductivity in Cdln2S4

A study has been made of photoconductivity in Cdln₂S₄ induced by 1.17 eV photons from a Q-switched neodymium laser. Both three-photon absorption, and two-photon absorption involving the emission of a phonon, have been investigated. The experimental data are in satisfactory agreement with the theory of multi-quantum transitions which takes into account the peculiarities of crystal zone structure.The investigation of competing multi-quantum transitions is a new method which permits one to obtain information on the fundamental optical constants of solids.     

Phonon density-of-states for high-T c (Y,RE) Ba2Cu3O7 superconductors and non-superconducting reference systems

From inelastic neutron scattering experiments, we determined the generalized phonon density-of-states (PDOS) for the high-T _c superconductors MEBA₂Cu₃O₇ (ME=Y, Y_0.5Pr_0.5, Nd) and the non-superconducting references YBa₂Cu₃O₆ and PrBa₂Cu₃O₇. A detailed study of the Y-O₇ compound at temperatures between 300 K and 6 K gave no evidence of any pronounced soft-mode behaviour. Moderate changes only occur in the PDOS between Nd and Pr samples and Y and Y_0.5Pr_0.5 samples, respectively, but drastic differences show up in the Y-O₇ and Y-O₆ spectra. Thus, we could not find correlations betweenT _c -values and particular features in the PDOS although there is some evidence that electron-phonon coupling is strong in this class of materials.     

Tuning of phonon anharmonicity in pyrochlore titanates: temperature‐dependent Raman studies of Sm2Ti2‐xZrxO7 (x=0, 1/2, 3/4, and 2) and stuffed spin‐ice Ho2 + xTi2 − xO7 − x/2 (x=0, 1/3, and 2/3)

Anomalous temperature dependence of Raman phonon wavenumbers attributed to phonon–phonon anharmonic interactions has been studied in two different families of pyrochlore titanates. We bring out the role of the ionic size of titanium and the inherent vacancies of pyrochlore in these anomalies by studying the effect of replacement of Ti^{4 +} by Zr^{4 +} in Sm₂Ti₂O₇ and by stuffing Ho^{3 +} in place of Ti^{4 +} in Ho₂Ti₂O₇ with appropriate oxygen stoichiometry. Our results show that an increase in the concentration of the larger ion, i.e. Zr^{4 +} or Ho^{3 +}, reduces the phonon anomalies, thus implying a decrease in the phonon–phonon anharmonic interactions. In addition, we find signatures of coupling between a phonon and crystal field transition in Sm₂Ti₂O₇, manifested as an unusual increase in the phonon intensity with increasing temperature. Copyright © 2011 John Wiley & Sons, Ltd.     

Nb对C-15相V2Zr和V2(HfZr)系列声子性能的影响

本文用中子飞行时间方法对C-15相的超导物质V₂Zr,V₂Zr_0.95Nb_0.05及V₂Hf_0.5Zr_0.5,V₂Hf_0.5Zr_0.3Nb_0.2的热中子非弹性散射谱作了测量,发现Nb的添加对V₂Zr和V₂Hf_{0.5 关键词：}     

First-principles investigation of the thermodynamic properties of two-dimensional MoS2

Density-functional perturbation theory has been applied to investigate the thermodynamic properties of two-dimensional MoS₂ within the Perdew-Burke-Ernzerhof genealized gradient approximation (PBE-GGA). The Murnaghan’s isothermal equation of state has been derived for the monolayer, giving how the pressure and the total energy of the monolayer evolve versus the volume of the unit cell. The temperature-dependent behavior of some thermodynamic quantities, including the Helmholtz free energy, total energy, Debye temperature, mean square displacements, specific heat, entropy and number of microstates, have been determined as well as their functional forms within the quasi-harmonic approximation, by calculating the partial phonon density of states as a fundamental quantity. The value of 45.6 cm${}^{?1}$ has been found for the phonon band gap in the frequency range, separating the acoustic and the optical phonon bands in fair agreement with the literature. The results broadly support the view that the validity of the Debye T³-law for low-temperature specific heat of solids is violated for the case of two-dimensional MoS₂ and therefore, a T²-law is proposed, accurately describing the behavior of the isochoric specific heat from 0 to 60 K.