Simultaneous Action of Intra- and Interband Pair Channels in Multiband Superconductivity

Interband superconductivity channels with pairs formed from the same (a) and different bands (b) are compared in common action. A simple mean-field multiband model is inspected. There are three order parameters Δ _a , Δ _{a d} and \({\Delta }_{b} \left ({\Delta }_{b1}^{2}={\Delta }_{b2}^{2} \right )\). Complicated quasiparticle energies induced by interactions of strengths W _a and W _b follow. The calculated operator averages lead to a coupled nonlinear system for the gap-type parameters. Illustrative calculations of them vs temperature have been made. Overlapping dispersive bands intersected by the chemical potential have been used. A novel result is that at a fixed parameter set the system of basic equations has two independent solutions. The free energy is of a complicated structure under the action of both channels. There are stable and metastable states. The W _a and W _b channels compete in simultaneous functioning. The phenomenon of the effective logout of one of the channels can be traced. Starting by the temperature where Δ _i reached zero, the solutions induced by W _j behaving as W _i were zero and define T _{c i} . The pairs Δ _a and Δ _{a d} have the same vanishing temperature. The general results are very sensible to |W _b | inclusive to critial behaviour. Reduced |W _b | stimulates the formation of closed “bubbles” built up by Δ _a , Δ _b and Δ _{a d} belonging to parallel solutions. The corresponding metastable state vanishes when the bubble closes.     

Interband Model for the Superconductivity of AxC60

A mechanism for A_xC₆₀, where superconductivity is induced by the repulsive interaction between the valence and the conduction band, has been developed. The T_c depends on the Fermi level. The isotope shift of T_c is caused by the small interband electron-phonon contribution into the total coupling.     

Interband Model Scenario of Cuprate Superconductivity

A multiband approach for hole- and electron-doped cuprate superconductors is developed. The model electron spectrum includes nodal and antinodal defect (polaronic) subbands created by doping. Bare gaps between itinerant and defect subbands close with extended doping. The overlap conditions determine phase diagram special points. There are analogies for both types of doping despite the activation of different sublattices. Illustrative calculations have given self-consistent results reproducing qualitatively doping dependences of T _c , superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, etc. Interband pairing scenario is suggested to be an essential aspect of cuprate superconductivity. This work was supported by the Estonian Science Foundation grant No. 7296.     

Relaxation of Superconducting Fluctuations in a Two-Component Scenario with Intra- and Interband Pairings

We study the relaxation of superconducting fluctuations in a two-component (two-band) model where the interband pair-transfer interaction and intraband effective attractions of electron-phonon nature are taken into account. The corresponding free energy and the system of Landau–Khalatnikov equations are derived. One can distinguish two different time scales in the damping of the fluctuations of superconducting order parameters in a two-band scenario. The formation of these relaxation channels is caused by the interband interaction leading to the redistribution of damping processes of the initially independent bands. The relevant relaxation times characterize the kinetics of critical and noncritical fluctuations which appear as the certain linear combinations of the deviations from the equilibrium band superconducting orders. The peculiarities of the temperature behavior of the relaxation times in dependence on intra- and interband couplings are established.     

Pairing Mechanism and Superconductivity of Rb3C60 Fullerides

The nature of electron pairing mechanism and the superconducting transition temperature (T _c ) of alkali metal (Rb) doped fullerenes is studied within the framework of strong coupling theory. Chemical substitution of alkali metal in the parent compound introduces free electrons in the lowest unoccupied molecular orbital and for Rb₃C₆₀, the band is filled up to the Fermi level. The intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of Rb atoms are investigated using nearest-neighbor interactions. The free electrons in lowest molecular orbital are coupled with intermolecular phonons. The renormalized Coulomb repulsive parameter * and the attractive coupling strength are obtained for the intermolecular phonon frequency _er. T _c is then estimated as 8.6 K, which is lower as compared with the published data of 30 K. The electrons also couple with the intramolecular phonons and introducing them in ad hoc way, T _c enhances to 34 K. Analytical results on T _c allow one to visualise the relative interplay between the strength of inter- and intramolecular phonons. To illustrate the usefulness of the above approach the carbon isotope effect exponent and the energy gap ratio are estimated which are consistent with the experimental data. The present analysis reveals that both low frequency intermolecular and high frequency intramolecular phonons participate in the pairing mechanism, T _c mainly arises from high frequency intramolecular phonons.     

Imprinting Ferromagnetism and Superconductivity in Single Atomic Layers of Molecular Superlattices

Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer-space-confined chemical design (ICCD) is reported for the synthesis of dilute single-atom-doped TaS₂ molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS₂ layers. The intercalation of 2H-TaS₂ crystal with bulky organic ammonium molecule expands its van der Waals gap for single-atom doping via co-intercalated cobalt ions, resulting in the formation of quasi-monolayer Co-doped TaS₂ superlattices. Isolated Co atoms are decorated in the basal plane of the TaS₂ via substituting the Ta atom or anchoring at a hollow site, wherein the orbital-selected p–d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal-size TaS₂ molecular superlattices.     

Pairing, Stripes, Lattice Distortions, and Superconductivity in Cuprate Oxides

We propose a model for a spatially modulated collective state of superconducting cuprates in which the electronic properties vary locally in space. In this model, the regions of higher hole density (called stripes) are described as Luttinger liquids and the regions of lower density (antiferromagnetic ladders) as an interacting bosonic gas of hole pairs. We show that the transition to the superconducting state is topologic and driven by decay processes among these elementary excitations in the presence of vibrations.     

Pairing, Magnetic Spin Fluctuations, and Superconductivity Near a Quantum Critical Point

The properties of a wide variety of intermetallic compounds exhibiting magnetic localized spin and superconducting fluctuations near a quantum critical point (QCP) are reviewed. They show highly anomalous critical indices (anomalously small). Laws of corresponding are observed in these materials and a theory is presented which gives a fully quantitative explanation of these laws. The theory employs a gauge transformation which rotates the electron spin quantization axis into the direction of the instantaneous staggered localized spin direction , where is the localized spin array wave vector. Many properties of these materials are worked out on the basis of this theory. The technological promise of these substances is truly immense, including energy generation, storage and transmission, MRI magnets, industrial and scientific magnets, maglev, cellular communications, -wave electronics, etc.     

Superconductivity of the Induced Pairing Model in the Presence of Diagonal Disorder

We analyze the influence of randomness of local pair (LP) site energies on the superconducting properties of a mixture of coexisting local pairs and itinerant electrons coupled via charge exchange mechanism. It has been found that the diagonal disorder effects depend in a crucial way on the total particle concentration n and the LP level position ₀. Depending on the parameters, the system can exhibit various types of superconducting behaviors, including the LP-like, intermediate (MIXED), and the BCS-like.     

Generation and Detection of Pairing Bosons--A New Method to Determine the Mechanism of Superconductivity in the Oxides

We propose a new method to determine the coupling boson in the cuprate superconductors based on the generation and detection of the appropriate boson analogous to the experiments performed on the generation and detection of phonons by conventional BCS superconductors. The appropriate choice of substrate and junction detector will allow the unambiguous determination of the boson that is formed by the recombination of energy gap edge quasiparticles produced by excitations into the normal state of the cuprates.     

The Order Parameters for Pairing and Phase Coherence in Cuprates; the Magnetic Origin of the Coherent Gap. The MCS Model of High-T c Superconductivity

The phase diagram of order parameters for pairing and phase coherence in hole-doped cuprates is discussed. By examining carefully some recent inelastic neutron scattering data obtained on hole-doped cuprates and heavy fermion compound UPd₂Al₃ in which the superconductivity is mediated by spin fluctuations, we conclude that the coherent gap in hole-doped cuprates has most likely the magnetic origin and scales with T _c, on average, as 2_c/k _B T _c = 5.4. We discuss a model of the superconductivity in hole-doped cuprates and the symmetries of two order parameters.     

2D Superlattices for Efficient Energy Storage and Conversion

2D genuine unilamellar nanosheets, that are, the elementary building blocks of their layered parent crystals, have gained increasing attention, owing to their unique physical and chemical properties, and 2D features. In parallel with the great efforts to isolate these atomic-thin crystals, a unique strategy to integrate them into 2D vertically stacked heterostuctures has enabled many functional applications. In particular, such 2D heterostructures have recently exhibited numerous exciting electrochemical performances for energy storage and conversion, especially the molecular-scale heteroassembled superlattices using diverse 2D unilamellar nanosheets as building blocks. Herein, the research progress in scalable synthesis of 2D superlattices with an emphasis on a facile solution-phase flocculation method is summarized. A particular focus is brought to the advantages of these 2D superlattices in applications of supercapacitors, rechargeable batteries, and water-splitting catalysis. The challenges and perspectives on this promising field are also outlined.     

Cuprate Interband Superconducting Density for Doping Driven Spectral Overlaps

We calculate the superconducting density for a model cuprate with gapped itinerant and defect subsystems brought into spectral overlap by progressive doping. The interband nature of the pairing prevents the manifestation of the normal state gaps in the superconducting density. These “extrinsic” gaps drive the pseudogap behavior until the overlap concentrations are reached. Further doping changes the nature of the quasiparticle minimal excitation energy and a pseudogap is transformed into the corresponding superconducting gap. The Uemura-type plot contains a sublinear segment at underdoping.     

Probing High-Temperature Superconductivity with Positive Muons

Muon-spin rotation (SR) is a unique tool to investigate internal magnetic fields in superconductors on a microscopic scale. In particular, the complex vortex structure in high-temperature superconductors can be explored with this technique. As an example, SR experiments on the vortex phase diagram of single-crystal Bi₂Sr₂CaCu₂O₈₊ are briefly described. A novel SR technique using low-energy muons allows the measurement of internal magnetic fields near the surface of a superconductor with a few nanometers' resolution. First results obtained with this technique on a thin YBa₂Cu₃O_7- film are presented.     

Superconductivity with Local, Short-Range Attraction

The approaches to short coherence length superconductors based on theconcepts of local electron pairing are discussed. The properties of systemswith intersite pairing, the nature of the BCS-Bose superconductivitycrossover as well as a two-component model of local pairs and fermions areanalyzed. The question of a pseudogap and a possible scenario of crossoversin high temperature superconductors are briefly examined.     

Binary Nanoparticle Superlattices for Plasmonically Modulating Upconversion Luminescence

Engineering a facile and controllable approach to modulate the spectral properties of lanthanide‐doped upconversion nanoparticles (UCNPs) is always an ongoing challenge. Herein, long‐range ordered, distinct two‐dimensional (2D) binary nanoparticle superlattices (BNSLs) composed of NaREF₄:Yb/Er (RE = Y and Gd) UCNPs and plasmonic metallic nanoparticles (Au NPs), including AB, AB₃, and AB₁₃ lattices, are fabricated via a slow evaporation‐driven self‐assembly to achieve plasmonic modulation of upconversion luminescence (UCL). Optical measurements reveal that typical red–green UCL from UCNPs can be effectively modulated into reddish output in BNSLs, with a drastically shortened lifetime. Notably, for AB₃‐ and AB₁₃‐type BNSLs with more proximal Au NPs around each UCNP, modified UCL with fine‐structured spectral lineshape is observed. These differences could be interpreted by the interplay of collective plasmon resonance introduced by 2D periodic Au arrays and spectrally selective energy transfer between UCNPs and Au. Thus, fabricating UCNP‐Au BNSLs with desired lattice parameters and NP configurations could be a promising way to tailor the UCL through controlled plasmonic modulation.