Incommensurate and Commensurate Density-Wave in the Two-Dimensional Lattice

We have investigated anisotropic density-wave phase with arbitrary ordering wave vector (Q) and estimated the value of Q that corresponds to the minimum of the free energy. We have found that for a wide range of model parameters the commensurate d-density-wave (DDW) is actually the most stable density-wave state. However, for moderate doping the commensurate DDW state is stable only at finite temperatures and disappears when the temperature is sufficiently low. These features may assist in clarification of the mechanism of the pseudogap.     

Specific heat of the high-Tc oxide superconductors

We review the specific heat measurements on La₂CuO₄, La_2–xM_xCuO₄ (M = Ca, Sr, and Ba), YBa₂Cu₃O₇, and the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O systems. Tables of properties derived from the data are presented. Results on RBa₂Cu₃O₇ (R=rare earth elements other than Y) are summarized, as are results on YBa₂(Cu_3–xM_x)O₇ (M=Zn, Cr, Fe, or Ni). The difficulties of analyzing the specific heat data, and specifically the separation of the contributions associated with magnetic impurities, are discussed. It is tentatively concluded that the data nearT _c are consistent with BCS theory, although they show evidence of fluctuation effects. It is also concluded that the low-temperature zero-field data on a majority of the high-T _c oxide superconductors provide evidence of anintrinsic term that is proportional toT, a result that is inconsistent with a gap in the electronic density of states.     

Frustrated Phase Separation in the Overdoped Regime of “123” High-Tc Superconductors

Recent inelastic neutron scattering measurements revealed the crystal-field (CF) spectra of the overdoped R_{1 – y} Ca_yBa₂Cu₃O₇ (R = Ho, Er; 0 < y < 0.25) high-T _c superconductors to consist of two spectral components associated with the optimally doped and the overdoped domains. Increase of the Ca concentration does not affect the local charge density of domains, but changes the spectral weight of the components. In the sense of this two-phase picture, which was established earlier for the underdoped region, there is a smooth crossover between the under- and overdoped parts of the phase diagram. The charge origin of the observed superposition effect is confirmed by the systematic analysis of the concentration dependences of the CF parameters for both Ho- and Er-based compounds. Therefore, the local charge inhomogeneity in the CuO₂ planes of high-T _c cuprates is a characteristic feature of the doping process, which depends neither on the way to introduce doping nor on the doping level.     

NMR Lineshape in Anisotropic Superconductors with Nonregular Vortex Lattice

The nuclear magnetic resonance line shapes within a primitive cell of the vortex lattice of the type II anisotropic superconductors in a case when a vortex is displaced on small distance a from a regular position in a primitive cell are constructed. The results of the numerical calculations show that displacement of the flux line lattice essentially changes the NMR lineshape. The derivative of the power of the absorption energy with respect to the magnetic field is calculated. It allows to obtain more detailed information about the real vortex lattice of a superconductor.     

Fermi Surface and Electronic Structure of Incommensurate Charge-Density Wave Systems

We study the single-particle spectra of a model that shows an incommensurate charge-density wave (CDW) instability arising from the competition between phase separation and long-range Coulomb interactions. Starting from an open Fermi surface, we find that the resulting CDW is oriented along the (1, 0) and (or) (0, 1) direction, which allows for a purely one-dimensional (1D) or a two-dimensional (2D) eggbox type charge modulation. In both cases, the van Hove singularities are substantially enhanced, and the spectral weight of Fermi surface states near the M points tends to be suppressed. Remarkably, a leading edge gap arises near these point, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless.     

Magnetic correlations and disorder in a two-dimensional fermi liquid: Possible important effects for high-Tc superconductors

The influence of magnetic correlations and disorder in a two-dimensional Fermi liquid is considered in connection with the occurrence of the superconducting state. The disorder effect and magnetic correlations work against the superconducting state but at the same time are competitive phenomena. We show that the critical temperature can decrease due to the magnetic correlations and disorder, but the effect of correlations is reduced by the disorder. The electronic mass enhancement has been calculated taking into consideration the magnetic correlation effects and disorder. Its dependence on the concentration impurities, in agreement with the experimental data, shows the importance of the localization effects due to disorder. These results are in agreement with the experimental data obtained for the high critical temperature superconductors.     

Polarons in the infrared spectra of high-Tc materials

Polaronic absorption features, induced by excess charges in both electron and hole doped cuprates, have been measured. The features here reported include additional phonon-like peaks in the far infrared and a broad overtone band centered at 1000 cm^–1 (d band). The peaks in the far infrared are attributed to local modes. Thed band is produced by overtone and combination bands of a few of these local modes and is well fitted in a polaron model. Most of these polaronic contributions survive in the metallic phase, where they are superimposed on the Drude term due to free carriers.     

Eu Atomic Motion in EuSr2Cu3O7?δ and EuBa2Cu3O7?δ: A Comparative M?ssbauer Study

We present a comparative Mössbauer study of the ¹⁵¹Eu motions in the nonsuperconducting and superconducting cuprates EuSr₂Cu₃O_7– (ESCO) and EuBa₂Cu₃O_7– (EBCO), respectively, with the aim of investigating atomic-motion anharmonicities in these materials. In this study, a comparison with the results of a similar analysis of powdered samples of EBCO having oriented grains is also performed. We show that the Eu ion in ESCO, as well as in the common EBCO cuprates, does not move in a parabolic potential, but in a potential with a flat bottom. Moreover, for ESCO the flat part is about 1.5 times larger than for EBCO.     

Room-Tc Superconductivity on Whispering Mode in Quasi-1D Composite of Superconducting Nanotubes: Is It Possible?

Combining Little's and Ginzburg's ideas with recent progress in nanotubes research, a novel type of material is advanced as a perspective high-T_c superconductor on a base of a close-packed lattice of quasi-1D superconducting nanotubes. Idea is offered that superconducting coaxial multilayer nanotubes of the correlation length in diameter is an ideal and natural trap for pinning of Abrikosov vortex. Nanotube should be layered superconductor, such as LuNiBC. Mechanism of superconductivity was proposed and substantiated quantitatively on a base of a whispering mode, which is shown to be responsible for a strong enhancement of electron–phonon interaction and for an increase of critical temperature. Nanocomposite built from such quasi-1D nanotubes when coinciding with vortex lattice provides ideal conditions for the pinning, resonance, distortion, ordering, and Little–Parks effects, the joint action of which is suggested to result in synergetic effect increasing the superconductivity. Such quasi-1D or 2D nanotubular crystal is proposed to synthesize by template approach using zeolite-like membrane.