Symmetry and Heterogeneity in High Temperature Superconductors

At one atmosphere 29 elements are classified as superconductors; at high pressures there are to date an additional 23, many of these being drawn from the lighter elements. The number of superconductors for the elements in combination appears to be illimitable. Observed symmetries in these systems generally include orderings in both nuclear and electronic degrees of freedom. The fluctuations impelling order in the electron sub-system include those originating with the nuclear degrees of freedom but also with the electrons themselves, both itinerant and localized. For the elements in combination coherent multipole fluctuations in localized states may arise, and the relative contributions of such excitations to electron-pairing is then of some especial interest. When the elements are placed in combination the effects of external pressure may be replicated in part by an equivalent internal pressure, this resulting from a form of chemical pre-compression.

We describe particular nanoarchitectures (superlattices of superconducting wires and layers) where a mechanism to evade temperature decoherence effects in a quantum condensate is switched on by tuning the charge density. The superlattice structure determines the subbands and the corresponding Bloch wavefunctions of charge carriers at the Fermi level with different parity and different spatial locations. The disparity and negligible overlap between electron wave-functions in different subbands suppress the single electron interband impurity scattering rate and allow the multiband superconductivity in the clean limit. The quantum trick that bestows to the system the property to resist to the decoherence attacks of high temperature is the Feshbach shape resonance in the interband off-diagonal exchange-like pairing term i.e., in the quantum configuration interaction between pairing channels in different subbands. It occurs by tuning the chemical potential at a particular point near a Van Hove singularity (vHs) in the electronic energy spectrum, or a Lifshitz electronic topological transition (ETT), associated with the change of the dimensionality of the Fermi surface topology of one of the subbands.

A simple model to cover the two-component scenario of cuprate superconductivity is developed. Interband pairing interaction acts between itinerant and defect states created by doping. Two defect system subbands correspond to “hot” and “cold” regions of the momentum space. Superconductivity energetic characteristics vs doping are compared to experimental findings. Transformations of two pseudogaps into superconducting and normal state gaps can be traced. Doping concentrations where the band components begin to overlap determine essential borders on the phase diagram. Qualitative agreement with observations is present including the effect of photodoping.

We using our two-band model for the explanation of two coupled superconductivity gaps for MgB₂. To study the effect of the increasing of T_c in MgB₂ due to the enhanced interband pairing scattering. We have proposed two channel scenario of superconductivity: the conventional channel which is connected with BCS mechanism in different zone and the unconventional channel which describes the transfer or tunneling of Cooper pair between two bands.

This article summarizes the principal points of discussions at the Erice workshop on the relevance of electron-lattice coupling to high temperature superconductivity. While the majority in the field still believes that the phonons and lattice are irrelevant to the superconductivity in the cuprates, such a view is strongly challenged by recent experimental results, particularly in conjunction with the increasing evidence of intrinsic electronic inhomogeneity. It is likely that phonons are a vital component of the phenomenon through the unconventional electron-phonon coupling.

Theory of non-adiabatic electron-vibration interactions has been applied to the study of MgB₂ superconductivity. It has been shown that at the non-adiabatic conditions when the Born-Oppenheimer approximation is not valid, and electronic motion is dependent not only on the nuclear coordinates but also on the nuclear momenta, the fermionic ground state energy of some systems can be stabilised by electron-phonon interactions at broken translation symmetry and an energy gap in one-particle metalic spectrum can be opened. More over, the new arising state is geometricaly degenerate — i.e., there is an infinite number of different nuclear configurations with the same fermionic ground state enegy. The superconducting state transition can be characterised as a Non-Adiabatic Sudden Increase of the Cooperative Kinetic Effect at Lattice Energy Stabilization (NASICKELES). Comparing to the experiments, the model study of MgB2 yields very good results. Calculated T_c is 39.5 K, and density of states exhibits two-gap character in full agreement with the tunneling spectra. The peaks are at +/−4 meV that is connected to π band and at +/−7.6 meV for σ band

In the low doping range of x from 0.01 to 0.06 in La_2−xSr_xCuO₄, we observed two electron paramagnetic resonance (EPR) signals: a narrow and a broad one. The narrow line is ascribed to metallic regions in the material, and its intensity increases exponentially upon cooling below ∼ 150 K. The activation energy deduced Δ = 460(50) K is nearly the same as that found in the doped superconducting regime by Raman and neutron scattering. Obtained results provide evidence of the microscopic phase separation and two type of quasiparticles in lightly doped La_2−xSr_xCuO₄

Results of systematic Raman studies of phase separation induced on cuprates by doping, hydrostatic pressure, and atomic substitutions are presented. In YBa₂Cu₃O_x, oxygen doping induces a separation into microphases from the ordering of the chains, at relative amounts that vary with concentration. In the overdoped region, a similar coexistence of phases appears, which is due to the modification of the CuO₂ buckling. The excess doping of YBa₂Cu₃O_7−δ by Ca creates domains of pure Y123 and others where the Ca atoms are surrounded by yttrium as first neighbours. Hydrostatic pressure induces non-linear effects in almost all A_g-symmetry phonons of YBa₂Cu₃O_x, YBa₂Cu₄O₈, and the Bi₂Sr₂CaCu₂O₈ superconductors. A coexistence of phases seems to appear at a critical pressure where changes in the superconducting transition temperature have been observed.

Intrinsic heterogeneity, from nano- to meso- scales, of lattice/charge/spin variables, is common in complex oxides such as cuprates and manganites, that have strain-based ferroelastic transitions. We summarize our viewpoint that the central player is the power-law, anisotropic, strain-strain force that lies concealed in the apparently innocuous St. Venant compatibility constraint, namely the maintenance of lattice integrity, under strain texturing induced by charges and spins, that act as ‘local stresses’ and ‘local transition temperatures’

A modified Van der Waals scheme for cuprates to give the co-existence of multiple phase separations in cuprates is presented. The model includes the tendency of charge carriers to form anisotropic directional bonds at preferential volumes for the formation of different “liquid phases”. We obtain the variation of the pseudogap temperature T*(δ) (for phase separation between pseudogap matter and normal matter) with hole density (δ) in agreement with experiments. We discuss the thermodynamic parameters that control the variation of the phase diagram of different cuprate families.

We have investigated the transport critical current density J _c as a function of the angle θ between the crystallographic c-axis and the applied magnetic field in high quality YBa₂Cu₃O_7−δ thin film. Measurements were performed for various temperature and magnetic field values. Our results show that the critical current density maximum occurs when the applied magnetic field is parallel to the ab planes (θ = 90°). The angular dependence of the critical current density shows the existence of the intrinsic pinning between the CuO₂ layers for H parallel to the ab planes and the extrinsic pinning in the configuration where the magnetic field is parallel to the c-axis. We have analyzed our results in the framework of the intrinsic pinning model proposed by Tachiki and Takahachi

The symmetry classification of superconducting states is reviewed. Based on purely symmetry considerations, a simple proof is given for the enhancement of d_x ² _-y ² superconductivity at the surface of cuprate materials. A novel method to study mixed superconducting phases is introduced.

We consider three group experiments on interlayer tunneling on Bi-2212 mesa-type structures, fabricated by focused ion beam (FIB) technique from Bi-2212 single crystal whiskers, pointing out to the d-wave type of the order parameter (OP) in this compound. We specify the experiments on low temperature interlayer quasiparticle conductivity and magneto-conductivity on small Bi-2212 mesas and the experiments on Josephson flux-flow dissipation on long Bi-2212 mesas. All the results are shown to be consistent with a dwave Fermi-liquid model with a significant contribution of coherent interlayer tunneling.

It is shown that the electronic structure in layered cobalt oxides with hexagonal crystal structure is described as a Kagomé lattice hidden in the CoO₂ layer which consists of stacked triangular lattices of oxygen ions and of cobalt ones. The Kagomé lattice is derived because of the degeneracy of t _2g orbitals. We discuss that the electronic structure causes a variety of unique properties in the cobalt oxides such as superconductivity and ferromagnetism, which are in contrast to the high-T _c cuprates.

The trio of ruthenate compounds, doped Sr₂YRuO₆, GdSr₂Cu₂RuO₈, and Gd_2−zCe_zSr₂Cu₂RuO₁₀ all superconduct in their SrO layers, which is why they have almost the same ≈49 K onset temperatures for superconductivity. The sister compound Ba₂GdRuO₆, whether doped or not, does not superconduct, because the Gd breaks Cooper pairs. These fit in with the superconducting cuprates: the superconducting layers are SrO or BaO, not CuO₂.

It is demonstrated that in the cuprates a dynamical, itinerant antiferromagnetic (AF)SDW state (with SDW/CDW stripe structure and d-wave SDW-gap (pseudogap)) is an additional, underlying order for the s-wave Cooper pairing (due to electron-phonon interaction) to appear at higher temperatures. The possible treatment of the nature of AF ordering observed in resistive state in recent neutron scattering experiments on the cuprates is presented. The effects in SC/SDW heterostructures with subnanolayers are discussed.