The energy associated with bosonic and fermionic pairs of topological spin defects in doped antiferromagnetic quantum spin-
1∕2 square lattice is estimated within a resonating valence bond scenario, as described by a $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}\text{\ensuremath{-}}J$-like model Hamiltonian, plus a ${t}_{\ensuremath{\perp}}$, responsible of a three-dimensional screening of the electrostatic repulsion within the bosonic pairs. For parameters appropriate for monolayered high-
Tc superconductors, both fermionic and bosonic pairs show ${x}^{2}\ensuremath{-}{y}^{2}$ symmetry. We find a critical value of doping such that the energy of the bosonic pairs goes below twice the energy of two fermionic pairs at their Fermi level. This finding could be related to the onset of high-
Tc superconductivity.
We have numerically evaluated the current-current correlations for three-band models of the CuO planes in high-T(c) superconductors at hole doping x = 1/8. The results show no evidence for the orbital current patterns proposed by Varma. If such patterns exist, the associated energy is estimated to be smaller than 5 meV per link even if [formula: see text]. Assuming that the three-band models are adequate, quantum critical fluctuations of such patterns hence cannot be responsible for phenomena occurring at significantly higher energies, such as superconductivity or the anomalous properties of the strange metal phase.
Astrophysical photons with a sub-millimetre wavelength are of great value to research star and galaxy formation. Superconductors have a small energy gap with which these sub-millimetre photons can be measured. Superconductors can absorb photons so that quasiparticles are created, which will in turn change the complex conductivity: incoming photons change the electrical response of the superconductor. This principle can be used to realise a detector. This is done with the Microwave Kinetic Inductance Detectors (MKIDs) that consist of a superconducting film embedded in a resonant circuit, so that photons falling in will change the resonant frequency. This can be measured with a readout signal. To enhance the sensitivity of the MKID, we want to increase the responsivity of an MKID as much as possible.A previous study has proposed a figure of merit regarding the material properties of a superconductor to achieve a high responsivity. However, the readout frequency used is not included in this figure. The goal of this work is to extend the figure of merit so that the readout frequency of the MKID can be optimised for a higher responsivity.