1PO1-40 On the non-phonon superconductivity of the flat niobium boron compounds

Rogdai O. Zaitsev 1 and Nadia V. Terekhina 2. 1 Russian Scientific Center, "Kurchatov Institute", 123182, Kurchatov Sq. 1, Moscow, Russia. 2 State Research Center "Niiteplopribor", 129085, Prospect Mira 95, Moscow, Russia.

Presenting Author: R.O. Zaitsev

In the compounds NbB₂ the 2s-states of boron are filled, while the 5s-niobium states are empty therefore the average number of electrons in the 2p-boron shell (n_p) are connected with the number of electron on the 4d-niobium shell (n_d) by means of the electroneutrality condition: 2n_p+n_d=7.

We use the simple model in which the single hopping transition t between the nearest flat Nb- and B- sublattices is taken into account. In line with this assumption we employ the Hubbard-Emery Hamiltonian. The equations of state connect the energetical difference r=e _p- e _d and the chemical potential m = -(e _p+e _d)/2 with the mean numbers of filling n_p, n_d.

The strong correlation of the electron states at U_p,d=µ leads to d-d- and p-p- scattering. The superconducting state can arise, if there is singularity in the two-particle vertex part G , which for the zero total spin and momentum is calculated. In the ladder approximation the appearance of this singularity can be written as the condition of solvability of the system, of the homogeneous equation.

The corresponding scattering amplitudes G ^0a ,b ;l ,n (p)® are determined as the coefficients at the product of the operators C l C n after the calculation of double permutative relations of the Hubbard X- operators {C a ,[C m ,H]}. Where H is the Hamiltonian expressed trough the Hubbard X-operators. Eventually the solvability conditions can be written as the BSC-relation T_c » t*exp(-1/gr ), where r is the density of states on the Fermi surface and g- is the function from e _p, e _d, n_p, n_d and the scattering amplitudes g _p, g d.

If one takes g=0 then it gives the condition for appearance of superconductivity. Together with the equation of state this condition defines the superconductive domains in the n_p, n_d- variables.

One can observe that the electroneutrality line 2n_p+n_d = 7 belongimg to superconductive Nb B₂ intersects the superconductive domain whereas the corresponding line 2n_p+n_d = 6 for non-superconductive TiB₂ are outside of the superconductive regions.

1PO1-41 Raman Scattering in Locally Anharmonic Model with Strong Electron Correlation

Ihor V. Stasyuk and Taras S. Mysakovych, Institute for Condensed Matter Physics, 1 Svientsitskii Str., UA-79011 Lviv, Ukraine

Presenting Author: I.V. Stasyuk

Anharmonic phonon contributions to Raman light scattering are investigated for the systems with locally anharmonic structure elements in presence of the strong electron-phonon interaction and the short-range Hubbard-type electron correlation (HTSC of YBaCuO-type are the objects of such type).

The investigation is carried out on the basis of the pseudospin-electron model. The microscopic approach based on the operator expansion of the polarizability operator in powers of the electron transfer constant is used to calculate the Raman scattering tensor. The effective interaction between pseudospins via conducting electrons is taken into account in the framework of generalized random phase approximation.

The contributions to the scattering tensor, which are of the zero order with respect to the electron transfer and describe the pseudospin (anharmonic phonon) scattering, are considered. It is established that the Raman scattering line consists of a coherent (delta-peak like) and incoherent (in the form of a band of the finite width) parts. This is the reflection of the collective dynamics of the pseudospin wave type as well as of the reconstruction of the electron spectrum at the reorientation of pseudospins. The form of lines and their width depend on the electron concentration and temperature; the asymmetry of lines is their characteristic feature. Comparison of the obtained results with the line profiles in the case of the antiferromagnetic magnon scattering is performed.

1PO1-42 Full-critical behaviour of the heat capacity around T_c versus T_c-inhomogeneities in Y-123

C. Carballeira, M.V. Ramallo, and F. Vidal, Departamento de Fisica da Materia Condensada, Facultade de Fisica, Universidade de Santiago, E-15706 Spain

Presenting Author: F. Vidal

It is now well established that the so-called bilayered-Gaussian-Ginzburg-Landau (N=2-GGL) scenario explains at a quantitative level the thermal fluctuations effects on different observables around T_c in the high temperature superconductor Y-123.[1] In the case of the heat capacity, this conclusion has been showed to apply in the reduced temperature regions bounded by e º (T-T_c)/ T_c³ 3x10^-2 below T_c and e ³ 1.5x10^-2 above T_c.[1] In the present work we show that the presence of quite small T_c-inhomogeneities could easily explain the differences observed inside the remaining e -window.[2]

[1] M.V. Ramallo, F. Vidal, Phys. Rev. B 59, 4475 (1999).

[2] F. Sharify et al., Physica C 161, 555 (1989).

1PO1-43 Interaction of Two Different Structural Blocks in Bi-System Superconductors

L.L. Cheng and H. Zhang, Materials Physics Laboratory, State Key Laboratory for Artificial Microstructure and Mesoscope Physics, Department of Physics, Peking University, Beijing 100871, China

Presenting Author: L.L. Cheng

The influence of the interaction between two structural blocks on superconductivity was studied by calculating the cohesive energy in 3 Bi-system superconducting phases [Bi₂Sr₂CuO_y (2201), Bi₂Sr₂CaCu₂O_y (2211), and Bi₂Sr₂Ca₂Cu₃O_y (2223)] and the 2223 phase with different oxygen deficiency. We developed a program to calculate the combinative energy between the Cu-O planes and remaining parts, and between the blocks in the superconductors. The result indicates that if we consider the cell as the two blocks combined together, a close relationship among the combinative energy between the two blocks, the value of T_c, and the number of the Cu-O planes in the Bi-system superconductors is established. In contrast, the combinative energy between the Cu-O planes and the remains got from the method of separating all the Cu-O planes from the cell and leaving some discrete remaining parts does not show any relationship with the value of T_c. This means that considering the cell as the two blocks is more reasonable. Calculation of the combinative energy between the two blocks and analysis of the correlation between the combinative energy and the value of T_c in the Bi-system suggest that the interaction between the two structural blocks, perovskite and rock salt blocks, has a close relationship with the superconductivity.

1PO1-44 Electronic Structure of the Antiferromagnetic Ground State of La₂CuO₄ beyond LDA+U Method

A. Perez-Navarro, J. Costa-Quintana, and F. Lopez-Aguilar, Grup d'Electromagnetisme, Departament de Fisica, Universitat Autonoma de Barcelona, E-08193, Bellaterra, Barcelona, Spain

Presenting Author: F. Lopez-Aguilar

Considering the multiband Hubbard Hamiltonian and a spin density wave mean field, the antiferromagnetic correlations of wave vector Q=(p /a, p /a) are included obtaining the energy spectrum and density of states of La₂CuO₄ antiferromagnetic insulator. Despite the insulator character of this compound, no previous theoretical calculations has obtained zero for the number of states at the Fermi level, but this number of states is highly reduced when is calculated in the antiferromagnetic ground state.

We determine the self-energy in this antiferromagnetic ground state using several infinite perturbative series in the random phase approximation. With this self-energy Dyson's equation is solved by diagonalizing the Green function in k -space. The density of states is calculated by considering the renormalization factor and the life-times of the quasiparticle states in each pole of the Green function for a given momentum, and we also obtain the Fermi surface.

The self-energy leads to the splitting of the input density of states arising from the local density formalism. The new results are compared with available experimental data as well as with other theoretical calculations. We analyze the evolution of the gap when is simulated the variation of the doping in Sr_xLa_2-xCuO₄ with the rigid band model, and reasonable agreement with photoemision data is obtained.

1PO1-46 Dynamical Susceptibilities in Strong Coupling Approach

Andrij M. Shvaika, Institute for Condensed Matter Physics Nat. Acad. Sci. Ukr., Lviv UA-79011, Ukraine

Presenting Author: A.M. Shvaika

A general scheme to calculate dynamical susceptibilities of strongly correlated electron systems within the dynamical mean field theory is developed. Approach is based on an expansion over electron hopping around the atomic limit (within the diagrammatic technique for site operators: projection and Hubbard ones) in infinite dimensions. As an example, the Falicov-Kimball and simplified pseudospin-electron models are considered for which an analytical expressions for dynamical susceptibilities are obtained. Different types of ionic and pseudospin orderings as well as phase separation are considered.

1PO1-47 Polaron signatures in the temperature dependences of the electric resistivities in single-layer high-T_c cuprates

M. Georgiev 1, I. Polyanski 1, M. Dimitrova-Ivanovich 1, and A.D. Gochev 2. 1 Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria. 2 Chemical Abstracts Service, Columbus, OH 43221.

Presenting Author: M. Georgiev

The temperature dependences r (T) of the electric resistivities r _c (axial) and r _ab (in plane) in normal state of single-layer high-T_c super-conductors, such as the La_1-xSr_xCuO₄ and Bi₂Sr_2-xLa_xCuO_y families, show common trends: as the temperature T is raised, the resistivity first drops steeply (dr /dT<0) before it starts rising (dr /dT>0) µ T above a 0/00semiconductor-to-metal crossover temperature. Ingenious experimental techniques involving external magnetic fields or chemical substitution to suppress pair formation have made it possible to extend the range down to very low temperatures. No satisfactory explanation of both resistivity branches has so far been achieved even though a few models have been sugested. In our turn, we plot T/r versus T at various doping levels x for r _c and r _ab of both superconducting families. In ionic theory, T/r is proportional to the average traversal time across a potential energy barrier as an ionic particle undergoes a drift motion in an external electric field. We find the overall temperature dependence of T/r in good agreement with the expected quantum-rate features of a diffusing ionic particle incorporating a zero-point rate at the lowest temperatures and an Arrhenius higher-temperature portion. These features give rise to the semiconductor branch below the semiconductor-to-metal crossover. As the former branch grows less effective with the temperature, the metallic branch becomes predominating, its µ T character deriving from the coupling of the migrating particle to the external electric field. Fits to the absolute resistivity data proved convincing for r _c, while those to r _ab exhibited doping-dependent misfit factors. The agreement suggests that traditional metal physics does not suffice, in so far as ionic features play an essential role in transport phenomena. We attribute our findings to the drift motion of strong-coupling polarons, either axial or in-plane, irrespective of the specific form of the polaron potentials. Options are discussed for the microscopic details of polaron formation along with eminent implications for the superconducting pairs and the single-particle excitations.

1PO1-48 Anderson's ``Theorem'' and Bogoliubov-de Gennes Equations for Surfaces and Impurities

K. Tanaka and F. Marsiglio, Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1

Presenting Author: K. Tanaka

In order to incorporate spatial inhomogeneity due to nonmagnetic impurities, Anderson proposed [1] a BCS-type theory in which single-particle states in such an inhomogeneous system are used. We examine Anderson's proposal, in comparison with the Bogoliubov-de Gennes equations, for the attractive Hubbard model on a system with surfaces and impurities.

[1] P.W. Anderson, J. Phys. Chem. Solids {11}, 26 (1959).

1PO1-49 Unitarity scattering limit in multi-band and other anisotropic superconductors

Oleg V. Dolgov, Max-Planck-Institute, 70569 Stuttgart, Germany, and Miodrag L. Kulic, Physikalisches Institut, Uni. Bayreuth, 95440 Bayreuth, Germany

Presenting Author: O.V. Dolgov

Physical properties of multi-band and anisotropic superconductors like the critical temperature, the density of states, the vortex pinning energy etc. depend sensitively on impurity scattering. These quantities are usually analyzed in the Born scattering limit. We investigate here changes in the above quantities coming from anisotropies in the impurity potential considering the unitarity scattering limit. We calculate above mentioned quantities as a function of the scattering amplitudes and show their qualitative change in the unitarity limit compared with the Born limit. The considered models are of importance for HTS materials and for heavy-fermion superconductors.