Low-energy thermal excitations in the V-O system

The thermal excitations of the atoms of the V-O solid solution have been investigated by the slow-neutron scattering method for low-energy transfers. The features of the low-frequency excitations of a light impurity in the metal lattice for energy transfers ? ≈ 1.3 and 2.4 meV have been revealed. It is assumed that the observed low-energy excitations appear in V-O due to strong local distortions around an impurity atom, which are responsible for the formation of the effective potential of a complex shape with the almost flat bottom. Analysis of the interaction of oxygen with the nearest matrix atoms shows that the low-energy excitations of oxygen atoms can be attributed to the splitting of the ground state in the complex potential. The level scheme of the split state can be obtained in the representation of decelerated circular quantum rotation about the axis passing through the two nearest metal atoms or the motion in a two-dimensional potential well. The results indicate that rotational low-energy excitations can exist in (metal-interstitial impurity) solid solutions.     

Exchange effect on the electronic specific heat

By improving the evaluation of the contribution from the first order exchange graphs reported recently by Tsai and Isihara, the electronic specific heat of an electron gas is evaluated to ordere ². It is shown that the specific heat contains a logarithmic term.     

Exchange effect on the electronic specific heat

The evaluation of the contribution to the specific heat of an electron gas from the first order exchange graphs is reconsidered. The logarithmic temperature dependence found by Kojima and Isihara does not seem to exist.     

Low-energy excitations in the three-dimensional random-field Ising model

General conservation equations are derived for 2D dense granular flows from the Euler equation within the Boussinesq approximation. In steady flows, the 2D fields of granular temperature, vorticity and stream function are shown to be encoded in two scalar functions only. We checked such prediction on steady surface flows in a rotating drum simulated through the Non-Smooth Contact Dynamics method even though granular flows are dissipative and therefore not necessarily compatible with Euler equation. Finally, we briefly discuss some possible ways to predict theoretically these two functions using statistical mechanics.     

Hartree Fock energy and electronic specific heat

The Hartree-Fock energy and electronic specific heat are calculated numerically. The results differ from those derived with Sommerfield expansions.     

On the electronic specific heat of transition metals

A simple framework to calculate the electronic specific heat enhancement is presented and applied to the case of the electron-electron interaction in transition metals. A substantial contribution has been found in iron and niobium.     

Field-enhanced electronic specific heat of carbon nanotubes

The tight-binding model including curvature effects is used to study the effect of transverse electric field on the low-temperature electronic specific heat (C_v) for armchair and zigzag carbon nanotubes (ACNTs and ZCNTs). Electric field could effectively modulate energy dispersions of CNTs and cause a shift of electronic states toward the Fermi energy. As field strength reaches to a critical value (F_c), it induces special structures in the density of states near the Fermi energy and thus the giant specific heat. At F_cs, C_v has a value comparable to that of the phonon specific heat and reveals strongly non-linear dependence on temperature. The critical field strength and giant specific heat are closely related to nanotube's geometry. Moreover, under F_cs, the extra longitudinal magnetic flux could cause a re-enhancement in C_v for ZCNTs, whereas C_v is always diminished for ACNTs.     

Superconductivity and electronic specific heat in V-Mo system

The parameters of superconductivity and specific heat are determined from the low temperature specific heat measurements of V_(1−x)Mo_x (0 ⩽ x ⩽ 1) solid solutions. The coefficient of electronic specific heat γ decreases with increasing Mo concentration and shows a minimum around 80 at. %Mo. Debye temperature θ_D varies slightly over the whole composition range. The superconducting transition temperature T_c also decreases with increasing Mo concentration. The variation of T_c is explained by the variation of γ and discussed in terms of the band structure.     

Low-energy excitations of Yb As in a magnetic field

We discuss the effects of an applied magnetic field on the low-energy excitations in the low temperature phase of Yb₄As₃. We show also why the magnetic interaction of the Yb³⁺ ions is nearly of an isotropic Heisenberg spin-1/2 type. A small anisotropy due to an intrachain dipolar interaction leads to the opening of a gap when a magnetic field is applied. The model agrees with available experimental data. Simple experiments are suggested in order to further test the present theory. Received 3 February 1999     

Valence,electronic specific heat and magnetic susceptibility of alpha-cerium

A sixfold degenerate Anderson Hamiltonian, which includes the Coulomb repulsion between localized and band states in the mean field approximation following Ramírez and Falicov, is treated in the limit of infinitely correlated 4? states and to second order in the mixing parameter. Assuming a line width Γ ≈ 0.02eV we obtain at low temperatures and 11 kbar, a valency of 3.76 and sizeable contributions from the 4? levels to the electronic specific heat and magnetic susceptibility of alpha Ce. Thus we show, that the highly correlated picture necessary to the explanation of the alpha—gamma transition by Ramírez and Falicov needs not to be abandoned in order to explain the anomalies in alpha Ce.     

Low-energy electronic spin excitations between filling factors ν=1 and studied by optically detected nuclear magnetic resonance

We report measurements of the spin relaxation time (T_1n) for nuclei in the potential well confining a high-mobility two-dimensional electron system at a single GaAs–GaAlAs heterojunction. At low temperatures nuclear spin relaxation is dominated by electron–nuclear spin scattering: we find that T_1n displays sharp maxima at incompressible states throughout the hierarchy of the fractional quantum Hall effect. This behaviour is consistent with the existence of low-energy spin excitations only where the electron system is compressible. Our measurements also provide evidence for a gap in the spin excitation spectrum at .     

Low-energy librational excitations in vitreous poly(methyl methacrylate)

The infrared (IR) and Raman spectra of vitreous poly(methyl methacrylate) (PMMA) are measured and investigated in the frequency range 10–150 cm^?1. A comparison of the results obtained from IR and Raman spectroscopic measurements permits the assignment of the low-frequency anomaly (boson peak) observed in the spectra to librational vibrations occurring in a segment of the main chain that is comparable in length to the statistical chain segment. It is demonstrated that coherent librational excitations are associated with the relaxation processes proceeding in the polymers.     

Temperature dependence of the electronic specific heat of superconductors with quantum defects

The electronic specific heat of metals with quantum defects in the superconducting state is examined. The role of the electron-polaron effect, as well as that of the level population factor of two-level states, is analyzed in an adiabatic approach. The cases of intermediate and strong coupling are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 413–418 (March 1998)     

On the computation of electronic excitations in solids

An approximation scheme is proposed for calculating electronic excitations in solids. It is described within a projection operator formalism of the Mori-Zwanzig type. The approximation consists in successively limiting the space of dynamical variables which are treated. The selection of the variables is done by making use of the local character of the correlation hole which is surrounding an electron. The method is distinct from a perturbation expansion and can be related to a variational ansatz via the Sauermann functional. The computation of the spectral density of the Green's function is reduced to the diagonalization of matrices. Their dimension depends on the required degree of accuracy of the calculations. The present method can be considered as an extension to excited states of a previously developed. Local Approach for ground state calculations.Dedicated to Prof. S. Methfessel on the occasion of his 60th birthday     

Susceptibility,electronic specific heat and magnetization of mixed-valent rare earth compounds

Applying our previous theory for mixed-valent materials, we report calculations of susceptibility, specific heat and magnetization of α-Ce, CePd₃, YbAl₃ and YbCuAl. The results are compared with experiment.     

Localized electronic excitations in nickel oxide

Valence-electron ionization and optical excitation energies of NiO have been calculated by the SCF?Xα cluster method and transition-state procedure. Ni 3d-like spin orbitals are localized above the O 2p band, leading to well defined peaks in the photoemission spectra. Ligand-to-metal charge-transfer excitations are responsible for strong optical absorption between 4 and 7eV.