Narrow Results

We consider a parity-preserving QED₃ model with spontaneous breaking of the gauge symmetry as a framework for the evaluation of the electron–electron interaction potential underlying high-T_c superconductivity. The fact that the resulting potential, -C_sK₀(Mr), is non-confining and "weak" (in the sense of Kato) strongly suggests the mechanism of pair-condensation. This potential, compatible with an s-wave order parameter, is then applied to the Schrödinger equation for the sake of numerical calculations, thereby enforcing the existence of bound states. The results worked out by means of our theoretical framework are checked by considering a number of phenomenological data extracted from different copper oxide superconductors. The agreement may motivate a deeper analysis of our model viewing an application to quasiplanar cuprate superconductors. The data analyzed here suggest an energy scale of 1–10 meV for the breaking of the U(1)-symmetry.

We have fabricated metal doped PrBa₂Cu₃O₇ (PBCO), i.e. PrBa₂(Cu_1-xM_x)₃O₇ (PBCMO) with M = Al, Fe, and Ni, and x = 0.05, 0.10, 0.15, and 0.20. X-ray data indicate no significant second phase for Ni-doped samples until 15% doping level is reached. No second phase was found in Al- and Fe-doped samples up to the 20% doping level. All the doped samples are in orthorhombic structure and their lattice parameters are very close to those of YBa₂Cu₃O_7-δ (YBCO). However, for Al or Fe doped samples the difference in lattice parameters a-b has decreased from that of PBCO. At 77 K the electrical resistivity for Al- or Fe-doped samples is several orders in magnitude higher than that of PBCO. Therefore these compounds may be better buffer-layer materials for YBCO superconducting electronic circuits and devices.

Spatial variation of low temperature tunneling spectrum on atomic scale was examined on the slightly underdoped Bi2212 sample (T_c~74K, p~0.12) by using a STM/STS technique. We observed a V-shaped gap with sharp coherence peaks, which is expected for d-wave superconductors, over the distance of ~18 nm and no other kind of gap. The spatial variation of gap size Δ₀ is small, ranging from 41 to 53 meV. These results indicate that the superconductivity takes place rather homogeneously in the slightly underdoped sample.

Superconducting onset temperatures T_c up to 23-44 K are reported for the new RuCa₂RCu₂O_8+δ (R=Pr, Nd, Sm, Eu, Gd) system with a Ru-1212 RuCa₂PrCu₂O_8+δ-type structure. Preliminary study indicates that the crystal structure is an orthorhombic distortion of the tetragonal RuSr₂GdCu₂O_8+δ-type structure ,which has space group P4/mmm, T_m~136 K, and T_c(max)~65 K. With similar RuO₆ octahedral and CuO₅ pyramidal units, superconductivity from the Cu-O bi-layers apparently coexists with the weak ferromagnetic (WFM) order with magnetic ordering temperature T_m~49 K. Low temperature anomalous in magnetic properties observed in both field-cooled (FC) and zero-field-cooled (ZFC) modes are closely related to the complex interplay and coexistence between weak ferromagnetic order and superconductivity as well as from structural distortion and sensitive sample oxygen concentration variation. No superconductivity or Ru-1212 structure can be detected for larger rare earth compounds R=La and Ce.

Applying two-time Green-function techniques to the Friedberg-T.D. Lee phenomenological Hamiltonian of a many-fermion system, it is shown that positive-energy resonant bosonic pairs associated with four-fermion excitations above the Fermi sea are energetically lower in a ground-state that is a mixture of two coexisting and dynamically interacting many-particle subsystems: a) unpaired fermions and b) composite bosons. It is argued that an interaction between free fermions and bosons excited above the Fermi sea in the mixture, namely, the continuous processes of pair-formation from, and disintegration into, two unpaired electrons, results in a substantially lowering the total system energy. The positive-energy composite bosons begin to appear incoherently below a depairing temperature T* as their coupling- and temperature-dependent number density gradually increases from zero. This leads quite naturally to the pseudogap phenomenon observed in high-T_c cuprates.

Muon spin rotation (μ⁺SR) measurements conducted on crystalline YBa₂Cu₃O₇ are consistent with s-wave pairing, not d-wave, suggesting that the superconducting hole condensate resides in the BaO layers, not in the cuprate-planes. The specific heat and thermal conductivity data are explained by the superconducting BaO layers alone, unlike the failed interpretation based on CuO₂-plane superconductivity. The layer charges of the CuO₂ planes are almost -2 |e|, indicating that those planes are primarily carriers of electrons, not holes. The cuprate-planes are not the dominant hole-carriers of high-T_C superconductivity, as demonstrated by doped YBa₂RuO₆, which has no such CuO₂ lanes, yet superconducts at ~ 93 K. Moreover the trio of related compounds, YSr₂RuO₆ (doped with Cu on Ru sites), undoped GdSr₂Cu₂RuO₈, and undoped Gd_2-zCe_zSr₂Cu₂RuO₁₀ all start superconducting near 49 K in their SrO layers, not in the cuprate planes of the two compounds that have such planes, because those planes are either antiferromagnetic or weakly ferromagnetic and so do not superconduct. In PrBa₂Cu₃O₇, a Pr-on-Ba-site (Pr_Ba) defect kills the superconductivity, but Pr-on-Pr-site (Pr_Pr) does not. Both defects are approximately equidistant from the intervening cuprate plane, suggesting that the cuprate plane does not carry significant superconductivity. In GdBa₂Cu₃O₇, Gd-on-a-Gd-site (Gd_Gd) does not break Cooper pairs, but Gd-on-a-Ba-site (Gd_Ba) does, indicating that the superconductivity is in the BaO layers, and not in the cuprate-planes. In HgBa₂Ca_n-1Cu_nO_2n+2, the BaO layers, not the cuprate-planes, gain positive charge as T_C, pressure, and the number of layers n increase. The reason that theories based on holes in the cuprate-planes have done so poorly is that those planes were incorrectly identified as the source of high-temperature superconductivity on the basis of a single datum by Cava et al., that was first contradicted by Jorgensen et al., and then endorsed by Jorgensen alone on the basis of the Cava datum, not his own. This error is the main reason why cuprate-plane superconductivity, which probably does not exist, has been so widely accepted.

An analytic expression for the contribution σ_B(λ, T) to the conductivity from charged bosonic Cooper pairs (CPs) is derived via two-time Green function techniques as a function of the BCS interelectron interaction model parameter λ and temperature T. Within the framework of a binary boson-fermion gas mixture model, it is shown that a self-consistent description of the resistivity data observed in high-temperature superconductors is possible only by assuming the presence of a finite gap between the energy spectra of free fermions and bosonic CPs.

Bulk superconducting samples of type Tl₂Ba₂Ca_n-1Cu_n-xZn_xO₈ (where n = 2, 3 and x = 0, 0.1, 0.25, 0.50) were prepared by standard ceramic method. The gross structural characteristics/phase identification of the as synthesized samples were carried out by powder X-ray diffraction technique. The microstructural characteristics of these samples were explored by scanning electron microscopic and transmission electron microscopic techniques. The chemical compositions of these samples were determined by energy dispersive analysis of X-rays technique. The powder X-ray diffraction patterns indicate that the lattice parameter "c" vary and "a" showed no regular variation but the gross structure of Tl-2212 and Tl-2223 remain tetragonal with Zn substitution upto 0.50. The transition temperature (T_c) measured by standard four-probe method decreases as x increases from 0 to 0.50. The transport critical density (J_c) values were also measured by standard four-probe method as potential difference of 1 μV/cm appears across the sample by increasing current. The electron microscopic explorations exhibit various structural features (stacking faults and uniform distribution of ZnO nanoparticles) which improve the physical properties (e.g., transport critical current density "J_c") of these superconductors. The observed enhancement in transport critical current densities (J_c) of at least one order of magnitude (from 10² to 10³A/cm²) is due to the uniform distribution of ZnO nanoparticles which act as flux pinning centres.

In this communication we report the interplay of the normal electron-phonon (EP) interaction, dynamic Jahn–Teller (DJT) distortion and superconductivity in high temperature superconductors in presence of a static lattice strain. This model consists of a degenerate two orbital band separated by Jahn–Teller (JT) energy modified by the DJT interaction in the conduction band. The superconductivity is assumed to be s-wave type present in the same band. The interaction Hamiltonian is solved by Green's function method and a modified BCS gap equation is obtained with a modified conduction band energy and modified BCS order parameter with α = 1, 2 designating the two orbitals. This interplay displays some new interesting results which are different from the effect of the static lattice strain on superconducting (SC) order parameter. The interplay is studied by varying the normal EP coupling, the DJT coupling, the SC coupling, the phonon vibration frequency, the phonon wave vector and other model parameters of the system.

Small lattice bipolarons driven by external harmonic fields are considered in the adiabatic approximation. Resonant excitation of ions modulates the trapping potential and promotes hole transfer between neighboring atomic layers. It leads to a dramatic decrease of the apparent bipolaron mass compared to the undriven case. This effect offers an explanation for dynamic stabilization of superconductivity at high temperatures recently observed in layered cuprates.

The damping of spin waves in high-T${}_{\mathrm{\text{c}}}$ superconductors is investigated in this paper. We use the spin polaron formulation in the finite temperature (Matsubara) Green’s function method in a representation, where holes are described as spinless fermions (holons) and spins as normal bosons characterized by the hard-core bosonic operators in accordance with the Holstein–Primakoff transformation. The interaction of holes with spin waves is then described by a Hamiltonian, which resembles the conventional polaron problem and came to be known as the spin polaron Hamiltonian. The rate of the damping of spin waves is then obtained from the self-energy term of the spin wave Green’s function at finite temperature. In the limit of zero temperature and low frequency, the spin wave damping was subsequently determined. We evaluated the same quantity by analytic continuation to get the zero temperature result.

The mobility of holes in the spin polaron theory is discussed in this paper using a representation where holes are described as spinless fermions and spins as normal bosons. The hard-core bosonic operator is introduced through the Holstein–Primakoff transformation. Mathematically, the theory is implemented in the finite temperature (Matsubara) Green’s function method. The expressions for the zeroth-order term of the hole mobility is determined explicitly for hole occupation factor taking the form of Fermi–Dirac distribution and the classical Maxwell–Boltzmann distribution function. These are proportional to the relaxation time and the square of the renormalization factor. In the Ising limit, we showed that the mobility is zero and the holes are localized. The calculation of the hole mobility is generalized by considering the vertex corrections, which included the ladder diagrams. One of the vertex functions in the hole mobility can be evaluated using the Ward identity for hole-spin wave weak interaction. We also derived an expression for the hole mobility with vertex corrections in the low-temperature limit and vanishing self-energy effects. Our calculation is made up to second-order correction in the case where the hole occupation factor follows the Fermi–Dirac distribution.

We develop a phenomenological model for high-T_c superconductors. Some main features are emerging in copper oxides: characteristic quasi two-dimensional Cu-O planes, strong correlation of antiferromagnetism, existence of a vortex lattice structure, and observation of a small coherence length and a large penetration depth. These features indicate that the superconductive pair is reasonably constrained to a small volume in real space and may be conceived of as a string-carrying vortex, and therefore can be well simulated by the dual phenomenological local boson fields and Φ. The various mean-field ground states of the system are discussed. The field equations of motion are originally solved to get approximate analytical soliton solutions. The effective Hamiltonian is formulated by a variational method for finite temperatures. The model parameter behaviour described by the relationship of the variational parameters is investigated. We discuss the critical temperature T_c, the specific heat c_V and its jump Δc at T_c, and the critical magnetic fields H_c1 and H_c2. These results are in agreement with experimental observations, especially the critical behaviours and the zero temperature values. The model also allows interpretation of the variation of T_c with oxygen vacancy x and that with doping fraction δ in Cu-O planes, as well as the dependence of γ (defined as the specific heat coefficient of the T-linear term) on δ.

The effect of a small amount of Sb doping (5%) on the formation of the high temperature superconducting phase has been investigated on the Bi_1.5Pb_0.5Sr₂Ca₂Cu₃O_y compound. It is observed that at 860°C the T_c zero value depends on the sintering time. It first increases to a maximum value of 96 K and then decreases to a stable lowest value of 88 K. The samples have been characterized by X-ray diffraction (XRD), four probe electrical resistance and current density measurements. The XRD revealed the presence of two phases: an orthorhombic high-T_c 2223 phases with lattice constants a = 5.395 Å, b = 5.508 Å, c = 37.0422 Å and another tetragonal low-T_c 2212 phase with lattice constants a = b = 5.419 Å, c = 30.832 Å.

This article summarizes and extends the recent developments in the microscopic modeling of the magnetic excitations in cuprate two-leg ladder systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange terms plus an additional four-spin interaction which is about five times smaller. We give an overview over the relevant energies like the one-triplon dispersion, the energies of two-triplon bound states and the positions of multi-triplon continua and over relevant spectral properties like spectral weights and spectral densities in the parameter regime appropriate for cuprate systems. It is concluded that an almost complete understanding of the magnetic excitations in undoped cuprate ladders has been obtained as measured by inelastic neutron scattering, inelastic light (Raman) scattering and infrared absorption.

In this paper, we systematically analyze the properties of the bosonic t–J model simulated in optical superlattices. In particular, by using a slave-particle approach, we show the emergence of a strange topological Fermi liquid with Fermi surfaces from a purely bosonic system. We also discuss the possibility of observing these phenomena in ultracold atom experiments. The result may provide some crucial insights into the origin of high-T_c superconductivity.

We report here a microscopic theory of the temperature dependence of specific heat in high-T_c cuprate superconductors. The system is described by a model Hamiltonian consisting the antiferromagnetic spin fluctuations due to the impurity f-electrons as well as the conduction electrons, besides the superconducting interaction due to the itinerant electrons. The Hamiltonian is treated within a mean-field approach. The transverse spin fluctuation parameters and the superconducting gap are calculated by Zubarev's Green's function technique and solved self-consistently. The temperature dependent specific heat is calculated from the free energy in order to study the anomalies appearing at the spin fluctuation and superconducting transition temperatures. The evolutions of these anomalies are studied by varying the model parameters of the systems and results are discussed in reference to experimental observations.

High quality polycrystalline GdBa₂Cu₃O_7−x samples with oxygen content equal to 7.0 were synthesized. Sensitive magnetization measurements were performed at low temperatures and in vicinity of T_c. From these measurements the intergrain material volume was calculated. The existence of two first penetration fields was demonstrated.

In high-$T_c$ superconductors, electrons form pairs and electric transport becomes dissipation-less at low temperatures. The iron-based superconductors (FeSCs) have the highest superconducting (SC) transition temperature next to copper oxides. The gap structure and pairing mechanism for FeSCs are hotly discussed as a central issue since their discovery. A model Hamiltonian for the superconductivity in FeSCs is proposed by a tight-binding two-orbital model. The SC gap, conduction electron density of states, specific heat and energy band structure for the system are calculated. We have proposed here a $s^{\pm }$-wave pairing symmetry of the form $cos{k}_x\times cos{k}_y$ in the model in the mean-field approximation. The model is solved by Zubarev’s double-time Green’s function technique to find the self-consistent gap equation and is solved self-consistently.