The Pseudogaps of HTSC

The problem of the pseudogaps in oxide HTSC is analyzed by means of the off-diagonal propagators, in analogy with the method of Nambu in superconductivity. We conjecture that the small pseudogap is obtained as the Fock diagram and the large pseudogap as the total off-diagonal self-energy (which is the sum of the Hartree diagram and the Fock diagram). We obtain real gap for undoped materials, and pseudogaps (with states in the gap) for doped and conducting materials. We find that disorder is essential for the results of the doped materials.     

Double Correlations in HTSC

The Nambu–Gorkov generalized Hartree–Fock theory of superconductivity is further generalized to treat simultaneously two kinds of correlations: the correlations that lead to superconductivity, as well as the correlations that produce the pseudogap in the normal state. The treatment of these simultaneous double correlations is done by enlarging the dimension of the propagator and self-energy matrices, from two to four. The off-diagonal self-energies and the DOS are calculated. The results are compatible with tunneling and PES experiments.     

Preparationin situ and properties of thin films HTSC YBa2Cu3O x

We describe the technique ofin situ preparation of thin (d=0, 1-1μm) YBCO films by the RF magnetron sputtering of the ceramic target in the argon-oxygen atmosphere on the heated substrates, and report the preliminary results of investigation of the multilayer structure based on such films.     

Further Establishment of the Field Perturbation Theory of the Pseudogaps in HTSC

Here I establish the field perturbation theory of pseudogaps in HTSC. The proposed ground state suggests an internal particle-hole field, which is normal to nesting surfaces, and having twice the Fermi wave number. It is proved that the system violates momentum conservation by the wave vector of this internal field. This violation applies to the quasi-particle propagators, as well as to the interactions. Interaction vertices via the Pauli matrix-τ₁ are established. This, in turn, establishes the validity of the pseudogap Hartree self-energy.     

Sketching the Theory of Copper Oxide High Temperature Superconductivity

The role of the antibonding state in the electron correlations in copper oxide HTSC is analyzed. Then the t-J Hamiltonian is used to establish the formation of the charge stripes in underdoped oxides. It is proposed that these stripes make up the boundaries between the two degenerate antiferromagnetic (AFM) states, and that they are a key factor in switching between these states. We also provide a theoretical expression to the charge driven AFM magnons that have been observed by Neutron scattering experiments. Finally, the double correlation theory is applied to the stripe phase of holes to result in the superconductive gap and in the “pseudogap.”     

Universal Correlations of Tc Ion Mass Scaling with Distance

We present a short overview of the critical temperature correlations with different parameters for low and high T _c superconductors. The overview underlines the importance of crystalline structure in determining the critical temperature value. The structure of copper-based superconductors is studied in comparison with low-temperature superconductors and a new correlation of the transition temperature is found. Namely, the layered structure, more exactly a certain distribution of the mass of the ions along the c-axis is the key factor in determining the intrinsic critical temperature, parameter introduced by Kresin et al. [4].     

Intrinsic Nanoscale Disorder and the Physical Properties of HTSC

Recent STM studies revealed nanoscale electronic disorder on the crystal surface in many cuprates. In BSCCO, strong correlations between oxygen defect distributions on its surface and both the gap map and the coherence peak amplitude showed that the off-center distortions in the positions of oxygen atoms are responsible for most of the electronic disorder. How do these nanoscale inhomogeneities affect the bulk macroscopic physical properties (such as transport properties) of these compounds? What is the effect of a local oxygen disorder on these properties? Persistent circulating supercurrents, which are known to bypass regions of a reduced order parameter (macroscopic crystal defects), have been used to investigate superconducting properties. Our investigations identified universal (sample independent) features in these properties (such as Josephson effects, filamentary and percolative flow of the transport current, etc.) which can be attributed to the presence of a nanoscale inhomogeneity. Local oxygen redistribution, induced either by careful low temperature annealing or by room temperature aging, was found to modify substantially both the superconducting and the normal state properties.     

Evidence for Polaron Formation in High-Temperature Superconducting Cuprates: Experiment and Theory

We provide compelling support for the key role played by polaron formation to the physics of cuprate superconductors, which is evidenced above the pseudogap temperature T ^*, is the origin of the pseudogap phase itself and persists in the superconducting phase. Experimental and theoretical results are compared and show convincing agreement with each other.     

Simultaneous Double-Sided Deposition of Large Size HTSC Thin Films

An apparatus for simultaneous deposition of large size double-sided YBCO thin films by sputtering from single inverted cyclindrical target was constructed. Double-sided YBCO thin films on LaAlO₃ (100) substrate up to 30 mm in diameter were prepared to test the performance of the apparatus. The microwave surface resistance, R _s (77 K, 10 GHz, 0 T), of the YBCO thin films on both sides of the wafer ranged from 500 to 800 . Values of R _s (75 K, 145 GHz, 0 T) below 60 m were reached over the area of 20 mm × 15 mm on the wafer. The majority of the wafer area given in percent has R _s (75 K, 145 GHz, 0 T) values in the range of 21.5– 45.5 m. Lateral homogeneity of R _s values in the whole wafer was good enough to well meet the microwave application.     

Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3O x

Samples of the series Bi_1·9−x Pb _x Sb_0·1Sr₂Ca₂Cu₃O _y withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withT _c=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.     

High-Temperature Superconductivity: A Macroscopic View

Over the last 12 years, impressive progress has been made in characterizingthe high temperature superconducting phenomenon and in discovering trendsabout its occurrence. As a result, many models have been advanced to accountfor the observations. In this presentation, I would like to discuss a fewissues that may be important to the understanding of high temperaturesuperconductivity but have not yet been given the attention they deserve inthe development of theoretical models.     

D-Wave Superconductivity and Coulomb Correlations in the Two-Dimensional Hubbard Lattice

We considered anisotropic superconductivity within the two-dimensional Hubbard model extended by pairing correlations originating from the electron–phonon interaction. To discuss the onset of superconductivity close to the insulator–metal transition, we used the Hubbard I approximation to account for the formation of the insulating gap and see the role of Coulomb correlations for superconducting pairing. It has been shown that the Hubbard I approximation reflects effective pairing interactions genuine for correlated electron systems and leads to the stabilization of the superconductivity in the d-wave channel. One may expect the cooperation of phonon-free and phonon-induced mechanism in the formation of thed-wave superconducting state.     

High temperature superconductivity in orthorhombic LaBa2Cu3O7−δ

High temperature superconducting LaBa₂Cu₃O_7−δ has been prepared by ceramic and nitrate methods to understand the influence of preparation conditions on superconductivity. The characterization was carried out by X-ray diffraction and magnetic susceptibility measurements.T _c onset was observed at 88 K. Meissner effect has been observed above the liquid nitrogen temperature.     

Spin Fluctuation Effect on Electron-Electron Coupling in High Temperature Superconductors

The spin fluctuation effect on the electron-phonon coupling factor has been calculated. The system is considered to be two-dimensional and we consider two different models which describe the spin fluctuations. These models correspond to strongly correlated electron systems. The effect of disorder introduced by nonmagnetic impurities is investigated.     

Flow-Equation Method for a Superconductor with Magnetic Correlations

The flow equation method has been used to calculate the energy of single impurity in a superconductor for the Anderson model with U0. We showed that the energy of the impurity depends only on the _R ² (renormalized order parameter), which depends on the renormalized Hubbard repulsion U ^R. For a strong Hubbard repulsion U ^R = U and ^R = ^I the effect of the s–d interactions are nonrelevant, a result that is expected for this model.     

Effect of disordered magnetic correlations in a layered high-temperature superconductor

Using the Eliashberg equations for superconductivity in a layered high-temperature superconductor and taking into consideration the phonon-mediated electron-electron interaction and the effect of the disordered magnetic correlations, we calculated the expressions for the critical temperature. The phonon and the magnetic kernels have a logarithmic divergence in the lowfrequency limit and the critical temperature differs from the McMillan form. The effect of various parameters on the decrease of the critical temperature has been analyzed. The renormalization functionZ _ph () contains a logarithmic dependence on.     

The Breakdown of Migdal's Theorem and the Divergence of Electron Polarizations in HTSC

We assume that Fermi surface nesting is a common feature in all oxide high temperature superconductors (HTSC) of perovskite or layered-perovskite structure. We show that this feature results in the breakdown of the Migdal's theorem, and the singularity of some electronic polarizations. These singularities trigger various kinds of instabilities in these materials.     

A High Temperature Superconductor (HTSC) Hot Electron (HE) THz Heterodyne Thermal Sensor (HTS): Computational Analysis of Conversion Gain in

The solution of the two temperature (electron and phonon) heat transfer equations, for nonequilibrium elctron–phonon cooling processes happening between electrons, sensor lattice, and the substrate system, is used to express the conversion gain of the thermal sensor in the form of a mathematical analytic expression. The full expression exposes the relative importance of various material and geometry dependent parameters of the device. The detector is pumped by a local infrared laser of picosecond pulses. The external signal is fed to the device through a chopper of frequency close to THz. It is found that in the band of operation GHz–THz frequency (), the time of escape of phonons (t _mes) to the substrate basically controls the performance of the sensor.