Cooper instability of nonlocal spin polarons in the CuO<Subscript>2</Subscript> plane of high-temperature superconductors

The energy structure of nonlocal spin polarons has been obtained for the real structure of the CuO₂ plane of cuprate superconductors in the ensemble of such Fermi quasiparticles. A nonlocal spin polaron is formed due to the exchange interaction of the spin of an oxygen hole with the spins of the two nearest copper ions. The scattering amplitude of nonlocal spin polarons in the cooper channel calculated using the diagrammatic technique indicates that the spin and charge degrees of freedom are strongly correlated.     

Nature of low-energy excitations in La<Subscript>1.87</Subscript>Sr<Subscript>0.13</Subscript>CuO<Subscript>4</Subscript> superconducting cuprate

The spectra of the conductivity and dielectric constant of La_1.87Sr_0.13CuO₄ cuprate have been directly measured in the frequency range of 0.3 to 1.2 THz (10–40 cm⁻¹) and the temperature range of 5 to 300 K in the E | c polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of Cooper pairs, which appears because Josephson junctions between CuO planes are modulated due to in-plane magnetic and charge stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ≈15 cm⁻¹ at liquid helium temperatures.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

Low-temperature thermal-expansion anomaly in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6</Subscript>

Single-crystal samples of the Bi_{2 + x}Sr_{2 ? x ? y}Cu_{1 + y}O_{6 + δ} system revealed anomalous (negative) thermal expansion in the temperature range 10–20 K. Magnetic fields of 1–3 T were found to strongly affect the position and width of the anomaly region. A thermal-expansion singularity was detected at temperatures T≈30–50 K, which may be related to the formation of a pseudogap.     

Electronic structure of prototype AFe<Subscript>2</Subscript>As<Subscript>2</Subscript> and ReOFeAs high-temperature superconductors: A comparison

We have performed ab initio LDA calculations of the electronic structure of newly discovered prototype high-temperature superconductors AFe₂As₂ (A = Ba, Sr) and compared it with the previously calculated electronic spectra of ReOFeAs (Re = La, Ce, Pr, Nd, Sm). In all cases, we obtain almost identical densities of states in a rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two dimensional in this energy interval, leading to the same basic (minimal) model of the electronic spectra, determined mainly by Fe d orbitals of the FeAs layers. The other constituents, such as A ions or rare-earth Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe₂As₂ are also very similar to that of ReOFeAs. This makes the more simple AFe₂As₂ a generic system to study the high-temperature superconductivity in FeAs-layered compounds. The text was submitted by the authors in English.     

Magnetic excitations in antiferromagnetic Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

Magnetic excitations in the antiferromagnetic Bi₂CuO₄ (T _N =42K) are investigated on the basis of anisotropic exchange interaction between spins of Cu²⁺ ions. We calculate the dispersion curves and evaluate the intensity of the inelastic neutron scattering by spin wave excitations. Spin contraction at OK and the effect of spin wave interaction are studied.     

Nonstoichiometry and critical temperature of MgB<Subscript>2</Subscript>

The influence of nonstoichiometry of the new high-temperature superconductor MgB₂ on its critical temperature was studied by the direct magnetooptical observations of the penetration and trapping of magnetic flux. To preclude the possible influence of accidental factors, a special sample with transition from pure boron to the MgB₂ with an excess of Mg was synthesized. In a narrow region near the unreacted boron, the magnetic-field trapping and screening disappear at a temperature 1.5 K higher than in the dominant stoichiometric region of the sample.     

Anisotropy of g-factor of Cu<Subscript>2+</Subscript> ion and Anisotropic exchange interaction between Spins in antiferromagnetic Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

To study the anisotropy of g-factor of Cu²⁺ ion in the antiferromagnetic Bi₂CuO₄ having an interesting crystal structure, we calculate the energy splitting of the orbital state of Cu²⁺ ion due to the electric crystalline field arising from the surrounding O²⁺, Bi³⁺ and Cu²⁺ ions. On the basis of Pryce's spin Hamiltonian we evaluate g-tensor of each Cu²⁺ ion in the unit cell. We also discuss the anisotropic exchange interaction which is important as the origin of the anisotropy energy for Cu²⁺ ion with S=1/2. The results are discussed in connection with observations.     

A theoretical analysis of the magnetic properties of La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The magnetic properties of the La₂CuO₄ are analyzed by means of the paramagnetic solution of the Hubbard model within the composite operator method. The experimental findings of the inelastic neutron magnetic scattering [R. Coldea et al., Phys. Rev. Lett. 86, 5377 (2001)] for the spin spectrum, the spin-wave intensity and the behavior of the dispersion at the zone boundary are well described by our results although the difference in phase. The Hubbard model emerges has a minimal model capable to describe the anomalous magnetic behavior of such a strongly correlated material. Received 29 July 2002 / Received in final form 2 January 2003 Published online 14 March 2003     

Band structure of new layered superconductors BaRh<Subscript>2</Subscript>P<Subscript>2</Subscript> and BaIr<Subscript>2</Subscript>P<Subscript>2</Subscript>

The results of ab initio FLAPW-GGA computations of the band structure of two new layered low-temperature superconductors BaRh₂P₂ and BaIr₂P₂ (with a ThCr₂Si₂ tetragonal structure) are presented. As distinct from the family of the isostructural FeAs superconductors, they feature the complete replacement of the magnetic (Fe) metal by the nonmagnetic 4d (Rh) and 5d (Ir) metals. For BaRh₂P₂ and BaIr₂P₂, the energy bands, the distributions of the densities of electronic states, the Fermi surface topology, and the coefficients of the low-temperature electron specific heat and the molar Pauli paramagnetic susceptibility have been determined. An increase in T _C in the BaRh₂P₂ (1 K) → BaIr₂P₂ (2.1 K) transition can assumingly be attributed to the features of their phonon subsystem.     

Influence of two-particle excited states on the interatomic exchange interaction in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The effective spin Hamiltonian for undoped cuprates is constructed in the framework of the realistic multiband p-d model with the parameters calculated from first principles. The exchange interaction parameter is defined as the sum of the antiferromagnetic and ferromagnetic contributions, which are determined by the two-hole triplet terms. The ferromagnetic and antiferromagnetic contributions of the excited terms compensate each other to a large extent. It is shown that the antiferromagnetic contribution of the two-hole ground singlet ¹ A _1g to the exchange interaction is dominant. Original Russian Text. V.A. Gavrichkov, S.G. Ovchinnikov, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 6, pp. 1037–1041.     

Magnetic susceptibility of fine crystalline high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (T_c ≤ T ≤ 400 K) and specific heat C(T) near temperature T_c of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa₂Cu₃O_y superconductor, having y and T_c close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.     

Charge ordering,superconductivity, and stripes in doped La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The special features of the phase diagrams of La_2?xSr_xCuO₄ are considered in terms of the high-temperature superconductivity model according to which the mechanism responsible for the anomalous properties of these compounds is the interaction of electrons with diatomic negative U-centers. A microstructural model that assumes the coexistence of domains with different types of dopant ion ordering is suggested for La_2?xSr_xCuO₄. According to this model, the main characteristics of the experimental phase diagrams of La_2?xSr_xCuO₄ only reflect square lattice geometric relations and competition between different dopant ordering types. Close agreement between the calculated and experimental “superconducting” and “magnetic” phase diagrams is an important argument in favor of the suggested high-temperature superconductivity model.     

Synthesis and structural characterization of uranium-doped Ca<Subscript>2</Subscript>CuO<Subscript>3</Subscript>, a one-dimensional quantum antiferromagnet

The technological settings of a modified sol-gel method for the preparation of highly fine homogeneous powder of Ca₂CuO₃ doped with uranium 238 (x= 0.0–0.05) is presented. The analysis of structure, purity of phases and the justification for the role of uranium in the given compounds are provided, together with an almost complete classification of the observed optical phonons by means of Raman, IR measurements and ab initio calculations. A significant reduction in particle size was achieved by doping, and a strong correlation between resistivity and doping concentration was observed and explained using the phonon-assisted electron hopping conduction model. The persistence of a covalent insulation state in all compounds is an interesting feature of this doping.     

Induction of superconductivity of a La<Subscript>2</Subscript>CuO<Subscript>4</Subscript> thin film chemically oxidized by NaClO

High-quality, c-axis-oriented La₂CuO₄ thin films have been fabricated by the pulsed laser ablation technique. Superconductivity has been successfully induced in the films after chemical oxidation using sodium hypochlorite solution as oxidizing agent. The structural properties, surface morphology, and electrical resistivity before and after oxidation are compared. In addition, the oxidation mechanism is discussed. PACS 68.55.Jk; 74.76.Bz; 82.40.-g     

On the nature of the KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> high-temperature transformation

For over two decades, the high-temperature phase transition (HTPT) at around T _p = 180 °C on KH₂PO₄ (KDP), which involves an ionic conductivity increase, constitutes a controversial subject; while most authors ratify a physical transformation (tetragonal → monoclinic phase transition), others defend the chemical transformation. A careful high-temperature phase behavior examination of this acid salt by means of modulated and conventional differential scanning calorimetry, thermogravimetric analysis, simultaneous thermogravimetric and differential scanning calorimetry, impedance spectroscopy, and temperature evolution of X-ray diffraction was performed to provide a possible solution to this long-standing issue. We found that the structural phase transition does not take place. Instead, a chemical transformation occurs at T _p. When KDP is heated through this temperature, the sample initially corresponding to a single phase (tetragonal) transforms to a sample composed of two solid phases: tetragonal KDP, located at its bulk, and monoclinic potassium metaphosphate (KPO₃), located at its surface. Most of the water produced evaporates, but a small portion of liquid water bonds to KPO₃. Because this is of polymeric nature, it takes the role of a host matrix that contains liquid water regions. Consequently, given that part of the water dissolves a portion of surface salt (providing protons), the surface sample system behaves in a similar manner to a polymer electrolyte membrane where the proton transport mechanism includes the vehicle type, using hydronium (H₃O⁺) as a charge carrier. On further heating, the bulk tetragonal KDP phase reduced to its total decomposition. The metastability of the high-temperature phase below T _p is also explained.     

Domain-wall mechanism of nucleation in a magnetic structural phase transition in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The structure of domain walls and new-phase nucleation are investigated in a four-sublattice antiferromagnet (AFM) of the La₂CuO₄ type placed in a magnetic field which initiates an AFM-weak-ferromagnet (WFM) magnetic structural phase transition. The critical fields for nucleus growth are found in the case of two types of domain walls present. The magnetization curve is calculated and a two-step mechanism is proposed for the AFM-WFM phase transition observed in La₂CuO₄.     

Anomalous thermal expansion in La<Subscript>2−x</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript> system

It was found that thermal expansion Δl/l was anomalous in a wide temperature interval for single-and polycrystals of the La_2?xSr_xCuO₄ system with various doping levels: the samples with small Sr content contracted with heating. This effect was observed up to x=0.2. Anisotropy of thermal expansion was revealed, for different crystal directions. The Δl/l anomaly was sensitive, to the oxygen content. These results are consistent with the local bosons model.     

HgO-added YB<Subscript>a2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>−δ superconductors

The HgO-added YB_a2Cu₃O_7−δ (YBCO) superconductor has been studied for its structural and superconducting properties. Polycrystalline YBCO samples were synthesized through solid-state reaction method by adding HgO in different concentrations without using oxygen annealing. All the samples showed a sharp superconducting transition temperature around 90 K. The X-ray diffraction patterns of all the samples revealed monophasic Y-123 nature. The structural studies were carried out by neutron scattering and Rietveld analysis. The neutron scattering revealed that Hg is not incorporated in the Y-123 system and has shown optimum oxygen concentration. The significant role played by the HgO is to provide oxygen ambient through its decomposition, thus changing the oxygen balance in favour of high Cu-valence state.     

Electronic structure of Ti-doped Sr<Subscript>4</Subscript>Sc<Subscript>2</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>O<Subscript>6</Subscript> as a possible parent phase for the new FeAs superconductors

First principle FLAPW-GGA calculations have been performed with the purpose to understand the effect of Ti-doping on the electronic properties for the newly discovered tetragonal iron arsenide-oxide Sr₄Sc₂Fe₂As₂O₆ (abbreviated as FeAs42226) as the possible parent phase for the new FeAs superconductors. Our results show that the insertion of Ti into Sc sublattice of this five-component iron arsenide-oxide phase leads to the resolute change of electronic structure of FeAs42226. Namely, the insulating oxygen-containing [Sr₄Sc₂O₆] blocks in Ti-doped FeAs42226 became conducting, and this differs essentially from the known picture for all others FeAs superconductors where the conducting [Fe₂As₂] blocks are alternated with insulating blocks. Moreover in sharp contrast with FeAs-based superconductors with Fe 3d bands near the Fermi level, for Ti-doped FeAs42226 in this region the Ti 3d states are dominated, whereas the Fe 3d states are suppressed.