Spin dynamics in the carrier-doped S=1/2 triangular lattice of NaxCoO2.yH2O

We probed the local electronic properties of the mixed-valent Co+4-x triangular lattice in NaxCoO2.yH(2)O by 59Co NMR. We observed two distinct types of Co sites for x > or =1/2, but the valence seems averaged out for x approximately 1/3. Local spin fluctuations exhibit qualitatively the same trend down to approximately 100 K regardless of the carrier concentration x, and hence the nature of the electronic ground state. A canonical Fermi-liquid behavior emerges below approximately 100 K only for x approximately 1/3.     

Raman studies of lattice dynamics in 1T-TiSe2

Raman scattering measurements were carried out on 1T-TiSe₂ above and below the phase transition temperature. Below _c many new lines appear, which are the Γ-point phonon modes folded from the original zone boundary points L, M and A due to the formation of the 2a₀ × 2a₀ × 2c₀ superlattice. Among them the strong A_1g line at 119 cm^?1 and the E_g line at 78 cm^?1 at 11 K show softening, as the temperature approaches to T_c, but the modes become overdamped before the energies go to zero.     

On the lattice dynamics of AgGaS2

Summary The vibrational spectrum and one-phonon density of states of a chalcopyrite crystal AgGaS₂ are calculated in an extended Keating’s model with two-bond-stretching and one-bond-bending force constants. Three charges of ions and three force constants are determined by a least-square fitting to experimental frequencies of long-wave phonons taken from Raman-scattering experiments. The calculated constant-volume specific heat, Debye temperature and elastic constants, of AgGaS₂ are in agreement with the experimental data of other authors.     

Intercalate lattice dynamics in stage-1 rubidium graphite

Collective intercalate models have been observed in C₈Rb by neutron scattering. The partial phonon density of states for in-basal-plane Rb vibrations agrees remarkably well with a calculation treating the Rb-Rb interactions as unscreened Coulomb forces.     

The lattice dynamics of SrCl2

Phonon dispersion curves of Sr Cl₂ have been measured at room temperature in the [001], [110] and [111] directions using inelastic neutron scattering. The results are interpreted by means of a polarizable ion model.     

Specific heat and lattice dynamics of layered TiS2 crystal

The specific heatC of the layered 1T-CdI₂ type TiS₂ crystal has been measured over the temperature range 20–300 K using a thermal relaxation method. Based on the available information about lattice dynamics for acoustic and optical phonon modes in TiS₂, the dispersion curve and the density of states for each mode are evaluated using a simplified phonon Brillouin zone of a cylindrical form. The calculatedC-T curve is in good agreement with the observation.     

Helium transport along lattice channels in crystalline quartz

The problem of He atom movement through channels of the quartz crystalline lattice is investigated. Providing the diameters of the atom and of the channel are of similar size the atom interacts with neighbor constituents of the wall. The conservation of momentum law in local form applied to the ‘helium-constituent’ interaction allows reduction of the problem to a one-dimensional one, which is similar to the movement of a dislocation in the Frenkel-Kontorova (FK) model. Within the framework of this model the activation energy for ‘helium+neighbor constituents’ is expressed by the shear modulus for the channel-forming material and the He polarizability. A metastable helium atom in the triplet state (2 ³S₁) is able to penetrate through the channel. In contrast, helium atoms in the singlet states, both ground state (1 ¹S₀) and metastable (2 ¹S₀), cannot penetrate.     

Spin dynamics of the spin-1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2

We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagomé lattice antiferromagnet ZnCu3(OH)6Cl2. The susceptibility indicates a Curie-Weiss temperature of theta CW approximately = -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagomé lattice system.     

Atomic structure and lattice dynamics of strain-compensated Si1-x-yGexCy layers

The local atomic structure and lattice dynamics are studied for strain compensated Si_1-x-yGe_xC_y layers grown by molecular beam epitaxy on Si (001) substrates. The layers were characterized by transmission electron microscopy, x-ray diffraction, and Raman scattering and modeled using a valence-force field model. For a [Ge]/[C] ratio of approximately ten, the lattice constant in the growth direction is equal to that of the substrate, indicating an absence of macroscopic strain. Experimental and theoretical results are compatible with Vegard's rule. To handle the large bond length distortions near C atoms properly, the valence-force field model used includes anharmonic effects via bond-length-dependent interatomic force constants which were determined from ab initio density-functional calculations. The dependence of the Raman spectra on strain and composition of Si_1-x-yGe_xC_y layers can be explained by the model calculations.     

Spin liquid state in the S = 1/2 triangular lattice Ba3CuSb2O9

The synthesis and characterization of Ba3CuSb2O9, which has a layered array of Cu2+ spins in a triangular lattice, are reported. The magnetic susceptibility and neutron scattering experiments of this material show no magnetic ordering down to 0.2 K with a θ(CW) = -55 K. The magnetic specific heat reveals a T-linear dependence with a γ = 43.4 mJ K(-2) mol(-1) below 1.4 K. These observations suggest that Ba3CuSb2O9 is a new quantum spin liquid candidate with a S = 1/2 triangular lattice.     

Continuum limit of lattice dynamics

A scheme of construction of the time evolution in QFT in algebraic framework is presented and verified for a linear theory in the flat space-time. The tool used is the lattice approximation.     

Influence of radiation damage on lattice vibration parameters in crystalline and vitreous SiO2

The influence of radiation damage on a number of lattice vibration parameters in two SiO₂ modifications was investigated using IR reflectance spectroscopy. The radiation kinetics of changes in spectral characteristics of SiO₂ fundamental vibrations in crystalline and glassy states were determined. The reflectivity coefficient and the frequency of degenerate vibrations as functions of dose showed minima, the locations of which were governed by the type of sample. At high neutron irradiation doses (10²¹ cm⁻²), certain characteristics of the bands had the same values for both modified materials. Features of the radiation kinetics of sample dynamic parameters were determined. It was deduced that the specific character of the observed radiation-induced changes in spectral and dynamic parameters of vibrations near degenerate modes was due to both the accumulation of radiation damage and a change in the force field surrounding bridging bonds, which is related to a change in the SiO₂ structure. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 354–358, May–June, 2008.     

Crystal lattice dynamics of ZnGeP2 and AgGaS2 in hard ion model

A calculation is made of the long-wavelength phonon spectrum of ZnGeP₂ and AgGaS₂ in the hard ion model. The parameters of short-range forces and dynamic charges are found by comparing the theory with the experimental data on infrared absorption and Raman scattering of light. The elastic and piezoelectric constants are calculated. The available experimental data are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–10, May, 1980.     

High-temperature dynamics of the anisotropic spin-1/2 square lattice Heisenberg model studied by moment methods

The spin dynamics of the anisotropic spin-1/2 nearest-neighbour Heisenberg model (XYZ model) on a plane square lattice is studied at infinite temperature. Low-order coefficients of the short-time expansions for spin-spin correlation functions are calculated. The necessary commutator algebra may be performed by a computer. The series obtained for the spin correlation function are the longest ones available up to now. The series coefficients are used to construct rigorous upper and lower bounds to autocorrelation functions and near-neighbour correlation functions.