Anomalous Self-energy Research Articles

Non-Fermi-liquid-like behaviors of the CuO 2 model of the high- Tc systems

We have studied the non-Fermi-liquid properties of the doped three-band CuO 2 model for the high- T c superconductors in the infinite U d slave-boson approach. The Green's functions of quasiparticles (d- and p-characters) as well as of the Cu d-electron for this model have been derived for the first time in the closed form by taking into account all the self-energies to order O(1/ N). We have examined the energy and temperature dependences of the imaginary parts of the self-energies and the spectral functions of Cu d- and oxygen p-electrons. It is found that charge fluctuations of quasiparticles due to constraint-field bosons give rise to the anomalous self-energy of quasiparticles which is consistent with experiment. Here the imaginary part of the d-electron self-energy exhibits a crossover from a quadratic (canonical Fermi liquid) to linear (non-canonical Fermi liquid) energy dependence above a small energy scale ω c≈0.01 eV for the hole doping (δ≈0.14), while the p-electron self-energy remains to be Fermi-liquid-like and does not contribute to the anomalous behavior. The origin of this characteristic energy scale ω c is discussed. The spectral functions of quasiparticles are asymmetric and are similar to the predictions of the marginal Fermi-liquid hypothesis.

Theory for dirty superconductors. II. McMillan solution and Tc degradation.

For a recent strong-coupling theory for dirty superconductors, a solution for ${T}_{c}$ of the McMillan type is obtained, which still contains complete disorder renormalizations of both anomalous and normal self-energies. All disorder effects leading to diffusion enhancement of vertices are calculated. The resulting ${T}_{c}$ formula gives good account of the observed ${T}_{c}$ degradation in high-${T}_{c}$ materials. A detailed discussion of the approximations used and of the relation to previous approaches is given.

Ordered pairing in liquid metallic hydrogen

We study two possible types of pairing involving the protons of a proposed low-temperature liquid phase of metallic hydrogen. Electron-proton pairing, which can result in an insulating phase, is investigated by using an approximate solution of an Eliashberg-type equation for the anomalous self-energy. A very low estimate of the transition temperature is obtained by including proton correlations in the effective interaction. For proton-proton pairing, we derive a new proton pair potential based on the Abrikosov wave function. This potential includes the electron-proton interaction to all orders and has a much larger well depth than is obtained with linear screening methods. This suggests the possibility of either a superfluid paired phase analogous to that in ${\mathrm{He}}^{3}$, or alternatively a phase with true molecular pairing.

Ferroelectric mode and molecular structure in the hydrogen-bonded ferroelectric arsenates and their deuterated isomorphs

Light-scattering measurements of the temperature dependence of the Raman-active modes of vibration in both the paraelectric and ferroelectric states of the hydrogen-bonded ferroelectrics K${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$, Rb${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$, and Cs${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$ and their deuterated isomorphs K${\mathrm{D}}_{2}$As${\mathrm{O}}_{4}$, Rb${\mathrm{D}}_{2}$As${\mathrm{O}}_{4}$, and Cs${\mathrm{D}}_{2}$As${\mathrm{O}}_{4}$ are reported. Through examination of these spectra we have identified the protonic or deuteronic, transverse-optic, As$\mathrm{O}_{4}^{}{}_{}{}^{3\ensuremath{-}}$ internal, and O-H-O or O-D-O valence vibrations in the Raman scattering and determined their symmetries and temperature dependence. Evidence for the collective protonic or deuteronic motions is observed in both the low-frequency ${B}_{2}$ and $E$ spectra in the paraelectric phase of each arsenate. The low-frequency ${B}_{2}$ spectra for the hydrogenated compounds are well fitted by a coupled-oscillator system representing the ferroelectric mode and a low-frequency phonon. The low-frequency ${B}_{2}$ spectra for the deuterated materials reveals the ferroelectric mode to be coupled to at least two low-frequency phonons, but, in a limited way, the spectra can be fitted to a coupled-oscillator system representing the ferroelectric mode and the lowest ${B}_{2}$ phonon. The results reveal the $\frac{T}{\ensuremath{\tau}}$ for the ferroelectric mode of all the compounds to decrease linearly with decreasing temperature; for real coupling the $\frac{T}{\ensuremath{\tau}}$ extrapolate to zero well below the transition temperature but for imaginary coupling the $\frac{T}{\ensuremath{\tau}}$ vanish much closer to the transition. In the limit of small damping, the results for real coupling are found to be in only partial agreement with the Kobayashi model, while the results for imaginary coupling are in accord with the Cowley-Coombs anharmonic theory of the transition and reveal the anomalous self-energy to be small compared to the conventional anharmonic self-energy of the soft mode.