1PO1-30 From BCS to Bose-Einstein condensation in a non-Fermi superconductor

Ionel Tifrea, Pascu C. Moca, and Mircea Crisan, Dept. of Theoretical Physics, "Babes-Bolyai" University, 3400 Cluj, Romania

Presenting Author: I. Tifrea

We studied the possibility of a crossover from the BCS theory to a Bose-Einstein condensation (BEC) for a non-Fermi system using the field theoretical method. We use as a description for the non-Fermi system the model proposed by Anderson, based on an anomalous Green function of the form G(k w )=g(a ) w _c-a [i\w Î (k)]^1-a . We showed that this crossover is possible as a function of the interaction strength: the weak coupling limit can be described by the BCS theory and the strong coupling limit by the Bose-Einstein condensation of a preformed pairs gas. Using numerical methods we showed that the crossover is smooth, varying continously as a function of the interaction constant. A more interesting result is the fact that the same crossover can be discussed in terms of the non-Fermi Anderson constant a .

1PO1-31 Pseudogapped Normal and Superconducting States in Doped High-T_c Oxides: A Novel Fermi (Precursor) –Bose (Superfluid) -Liquid Approach

S. Dzhumanov, Institute of Nuclear Physics, 702132, Tashkent, Uzbekistan

Presenting Author: S. Dzhumanov

About ten years ago (see [1]), we have motivated that the pseudogapped normal and superconducting (SC) states in the oxide high-T_c superconductors (HTSC) are characterized by the formation of normal BCS-like k-space pairs both above and below T_c and by the superfluid (SF) (i.e. attractive) pair and single particle condensation of these pairs below the temperature T_c.

There are now many key experimental facts which are in favour of such two -stage Fermi(precursor)-Bose(SF)-liquid (FBL) scenario of superconductivity. The important features of the complicated pseudogapped electronic states in doped HTSC still need special theoretical and experimental investigations . Here using the well-developed continuum theory of real (r)-space (bi)polarons [2] and the novel two-stage FBL theory of superconductivity [1], we show the existence possibilities of two so-called carrier -confinement (or high-energy) and BCS-like (or low-energy) pseudogaps opening above T_c and one true SC gap which evolves below T_c within the both low -and high-energy pseudogaps.

In some overdoped HTSC only carrier -confinement pseudogap can be opened above T_c. The BCS-like pseudogap opens simultaneously both in the charge and spin channel [1] in underdoped and optimally doped HTSC. We have demonstrated that the electronic structure of HTSC evolves as a function of temperature T and doping x from ordinary gapless metal to pseudogapped insulator phase with two novel pseudogapped metallic, SC and mixed metallic and insulator stripe phases between them.

Generic and relevant T-x phase diagrams for LSCO and BSCCO compounds with eight electronic states are presented.

[1] S. Dzhumanov, Physica C 235-240, 2269 (1994); Int. J. Mod. Phys. B12, 2151 (1998).

[2] S. Dzhumanov, A.A. Baratov , S. Abboudy, Phys. Rev. B54, 13121 (1996-II).

1PO1-32 Spin wave superconductivity in antiferromagnetic Kondo lattices

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

Presenting Author: F. Lopez-Aguilar

In this communication, we analyze the spectrum of the Kondo lattices whose states are based on charged modes coupled to magnetic fluctuations of the spin liquid. For a given value of the initial Kondo coupling parameter an insulator-metallic phase transition can occur. The metallic phase has the characteristic that the strongly correlated coupled states form two narrow bands (one of particles and another of holes) which cross the Fermi level. We analyze these two bands and determine some thermodynamical and electrical properties such as the specific heat and the optical conductivity. This analysis allows us to conclude that in these antiferromagnetic Kondo lattice systems, a huge increase of the conductivity for zero frequency can appear in the case that the specific heat of the system is larger than a critical value. The critical value depends on the concomitant action of the renormalized Kondo effects and the induced RKKY correlations between the components of the spin field. This electrical behavior is a phase previous to the appearance of a completely coherent state whose features are in agreement with the experimental data observed in compounds such as UPt(3) and others. Several routes for the pair formation between these strongly correlated states are analyzed.

1PO1-33 Time evolution of superconducting transitions in hole-overdoped Bi₂Sr_2-xLa_xCuO_6+y Crystals: an indication of phase separation in overdoped superconductors

W.L. Yang 1, H.H. Wen 2, Y.M. Ni 2, and Z.X. Zhao 2. 1 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Science and Center for Condensed Matter Physics, Beijing 100080. 2 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Science and Center for Condensed Matter Physics, Beijing 100080.

Presenting Author: W.L. Yang

Hole-overdoped Bi₂Sr_2-xLa_xCuO_6+y single crystals with two coexistent superconducting transitions have been intensively studied by measuring the temperature dependence DC magnetic susceptibility (M-T curves). In a previous paper (PRL82,410), we have presented the different behaviors of these two transitions when applying magnetic field. Besides, in this work, we performed the measurements on the time dependence of the M-T curves. It is found that the transition with the higher T_c is excellently reproducible, giving a very stable phase; while the other transition exhibits a rather dynamic and random behavior, which rules out any chemical mechanism. In summary, the characteristics, such as the time evolution and the field dependent behavior, of these two phases seem quite different, indicating the origin of them can not be attributed to any chemical imperfection but to some intrinsic driving force, such as the electronic-driven phase separation.

1PO1-34 Theoretically Possible Three-Dimensional Cuprate Superconductors

R.E. Allen, T. Dumitrica, J.S. Graves, B. Torralva, and D.N. Naugle, Physics Dept., Texas A&M Univ., College Station TX 77843

Presenting Author: R.E. Allen

We suggest the possibility of specific three-dimensional cuprate superconductors, with CuO₃ rather than CuO₂ conducting units, together with appropriate metal atoms which make the structure chemically stable. One variety of such material would have a large atom (like Pb) in the center of the cubic cell, and another would have small atoms (like Li) in the faces. We have studied the electronic structure by performing band-structure calculations with a model which is accurate for the standard cuprates, and we have also examined the structural stability with simple analytical models. These calculations, together with chemical arguments, indicate that there is no reason why these materials should not be stable and have electronic properties that are similar to the standard layered cuprates with CuO₂ sheets. Since the effect of fluctuations should be smaller in three-dimensional superconductors, there is an interesting potential for still higher transition temperatures if these materials can be grown.

Finally, we will discuss Fermi surfaces, Fermi-surface instabilities, and the possibility of the same basic mechanism that is responsible for high-temperature superconductivity in the standard layered cuprates.

1PO1-35 Temperature Dependence of the Anisotropic Superconducting Gap in YBa₂Cu₃O₇

G.L. Zhao and D. Bagayoko, Department of Physics, Southern University and A & M College, Baton Rouge, LA 70813, USA

Presenting Author: G.L. Zhao

We present ab-initio calculations for the electronic structure and the temperature dependence of the anisotropic superconducting gap in YBa₂Cu₃O₇ (YBCO). The electronic structure was calculated using a self-consistent ab-initio LCAO method. A non-orthogonal tight-binding method was used to calculate the electron-phonon interaction. We solved the anisotropic Eliashberg gap equation numerically. The strong coupling of the high energy optical phonons around 60 - 73 meV, with the electrons at the Fermi surface, leads to a high T_c in YBCO. The calculated T_c is about 89 to 92 K for m * in the range of 0.1 to 0.0. The calculated superconducting gap clearly shows a strong anisotropy on the Fermi surface but without nodes. The highly anisotropic superconducting gap of YBCO is mainly attributed to the anisotropic electron structure of the material. We also found that the smaller gap (at T=0 K) at some k - points of the Fermi surface tend to reduce faster than the larger ones as the temperature increases. Our studies suggest that some metallized nitride compounds in hexagonal structures, with very high optical phonon energies, are possible superconductors with a T_c near room temperature.

1PO1-36 Location and Properties of the Superconducting Hole Condensate in Sr₂YRu_1-uCu_uO₆

Dale R. Harshman 1,2, H.A. Blackstead 2, J.D. Dow 3, M.K. Wu 4, D.Y. Chen 4, F.Z. Chien 5, D.B. Pulling 1, W.J. Kossler 6, A.J. Greer 7, C.E. Stronach 8, E. Koster 9, and B. Hitti 10. 1 Physics Department, University of Notre Dame, Notre Dame, IN 46556 USA. 2 Physikon Research, Inc. P.O. Box 2421 Blaine, WA 98231 USA. 3 Physics Department, Arizona State University, Tempe, AZ 85287-1504 USA. 4 Department of Physics and Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan. 5 Physics Department, Tamkang University, Tamsui, Taiwan. 6 Physics Department, College of William and Mary, Williamsburg, VA 23185 USA. 7 Physics Department, Gonzaga University, Spokane, WA 99258 USA. 8 Physics Department, Virginia State University, Petersburg, VA 23806 USA. 9 Physics Department, University of British Columbia, Vancouver, B.C. Canada V6T 1Z1. 10 TRIUMF, Vancouver, B.C. Canada V6T 2A3.

Presenting Author: D.R. Harshman

The Cu-doped Sr₂YRu_1-uCu_uO₆ compound is a double-layered perovskite with non-magnetic SrO layers "sandwiched" between magnetic YRu_1-uCu_uO₄ layers. Below T_N ~30 K, the Ru spins freeze out and order ferromagnetically in each YRu_1-uCu_uO₄ plane, with alternating polarization direction in adjacent magnetic layers (net antiferromagnetism). Consequently the SrO layers experience zero field below TN. Local fields as high as ~3 kG are detected in the YRu_1-uCu_uO₄ layers, with no evidence of superconductivity. Signals associated with the SrO layers clearly indicate the formation of a vortex lattice below T_c~T_N ~30 K, consistent with bulk, i.e., fully developed, type-II superconductivity. Possible pair-breaking scenarios are discussed. Data acquired using several spectroscopies indicate that the superconducting hole condensate, which carries the supercurrent, clearly resides in the SrO layers.

1PO1-37 Eu_2-zCe_zSr₂Cu₂RuO₁₀ Superconducts in its SrO Layers, Not in its Cuprate-planes

Howard A. Blackstead 1, J.D. Dow 2, D.R. Harshman 1,3, I. Felner 4, D.B. Pulling 1, W.J. Kossler 5, A.J. Greer 6, C.E. Stronach 7, E. Koster 8, and B. Hitti 9. 1 Physics Department, University of Notre Dame, Notre Dame, IN 46556 USA. 2 Physics Department, Arizona State University, Tempe, AZ 85287-1504 USA. 3 Physikon Research, Inc. P.O. Box 2421 Blaine, WA 98231 USA. 4 Raccah Institute of Physics, The Hebrew University, Jerusalem, Israel 91904. 5 Physics Department, College of William and Mary, Williamsburg, VA 23185 USA. 6 Physics Department, Gonzaga University, Spokane, WA 99258 USA. 7 Physics Department, Virginia State University, Petersburg, VA 23806 USA. 8 Physics Department, University of British Columbia, Vancouver, B.C. Canada V6T 1Z1. 9 TRIUMF, Vancouver, B.C. Canada V6T 2A3.

Presenting Author: H.A. Blackstead

Eu_2-zCe_zSr₂Cu₂RuO₁₀ superconducts at and below T_c ~ 43K. The RuO₂ layers and the CuO₂ planes of the crystal structures are magnetic from low temperatures up to temperatures over twice the superconducting transition temperature. Hence the primary superconductivity must originate in the SrO layers, not in either the CuO₂ layers or the RuO₂ layers. In these materials, as also shown in PrBa₂Cu₃O₇ and in NdBa₂Cu₃O₇, the primary superconductivity does not occupy the cuprate-planes.

1PO1-38 Role of Attractive Interlayer and Interlayer Interactions in High Temperature Cuprate Superconductors

S.L. Kakani, Institute of Physics, 4G45, Hastri Nagar Extension, Bhilwara-311001, India

Presenting Author: S.L. Kakani

Considering attractive intra and inter layer pairing and excitronic type interlayer correlations with the following model Hamiltonian

H = S h e _kC_r⁺_,k,s C_r,k,s + US e _kC_r⁺_,k+q­

C_r⁺_,k’-q¯ C_r,k’¯ - tS C_r⁺_,k,q C_s,k,s

+ WS e _kC_r⁺_,k+q,s C⁺_s,k’-q,s C_s,k’,s C_r,k,s

where r & s are layer indices, where r = 1(2) then s=2(1), C_k^+s (C_ks ) are creation (annihilation) operators of the charge carriers in CuO₂ plane with wave Vector k and spin s , and following equation of motion method and Green's function technique we have obtained expressions for specific heat, density of states, Free energy, Critical field. We have also made study of isotope effect and phase transition.

Using the available experimental values of various parameters [2], we have made comparison for the system YBa₂Cu₃O₇ system. The agreement between theory and experiment is satisfactory [3,4].

[1] Ajay, R.S. Tripathi, Physica C274,73(1997)

[2] K.P. Sinha and S L Kakani, "High Temperature Superconductivity: Current Results and Novel Mechanisms", Nova Science Publishers, New York (1995).

[3] A. Kashyapa, R.K.Paliwal and S.L.Kakani, J. Low Temp. Physics. (submitted 1999).

1PO1-39 The two-band model and the anisotropic fluctuation conductivity in different thin films

Peet Konsin 1, Boris Isorkin 1, and Stamatis K. Patapis 2. 1 Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia. 2 Solid State Section, Dept. of Physics, University of Athens, Panepistimiopolis, Zografos, Athens, Greece.

Presenting Author: S.K. Patapis

The formulae for the temperature dependence of the paraconductivity in the vicinity of a superconducting phase transition have been derived using the linearized Ginzburg-Landau equations within the Kubo formalism for a two-band model. In general, two correlation lengths exist in the two-band model. However, only one of them is critical and determines the fluctuation conductivity. In zero applied magnetic field the fluctuation conductivity is measured in different thin films of ReBa₂Cu₃O_7-y where Re: Y, Gd, Eu, Tm and La. From the comparison of the theory with the experiment we obtained reasonable values and the slope of fluctuation conductivity, the superconducting transition temperature and the coherence length. In the two-band model which consists of a narrow and a broad band the doping dependence of the paraconductivity in YBa₂Cu₃O_7-y is calculated. The dependence of the correlation length on the carrier concentration in this film is derived.