Magnetic Excitation Spectra from Spin-Vortices and Spin-Vortex-Induced Loop Currents in Hole-Doped Cuprates

Abstract

We argue that spin-vortices and loop currents induced by them (spin-vortex-induced loop currents) are crucial ingredients for superconductivity in hole-doped cuprates. The existence of the former is indicated by the hourglass-shaped magnetic excitation spectrum obtained by the inelastic neutron scattering experiments. The existence of the latter is indicated by the Kerr effect measurement and the recently observed hidden magnetic excitation spectrum. The existence of both spin-vortices and spin-vortex-induced currents points to the occurrence of the spin-vortex superconductivity in the cuprates. The suppression of superconductivity in the x = 1/8 stripe-ordered sample is also explained by this theory.KEYWORDS: cuprates, magnetic excitations, spin-vortices, loop currents, the x = 1/8 problem According to the BCS theory, superconductivity is due to the Cooper-pair formation. The BCS theory provides a way to calculate transition temperature T c and explains various phenomena that occurs as consequences of the Cooper-pair condensation.1) However, it has been repeatedly argued that it does not have the predicting power whether a particular compound is a superconductor or not. 2, 3)Recently, a theory of superconductivity, the spin-vortex superconductivity, is put forward as a theory that explains superconductivity in materials close to antiferromagnetic insulators. 4) In this theory, spin-vortices arise in the background antiferromagnetism, and stable loop currents are induced around the spin-vortices [spin-vortex-induced loop currents (SVILCs)]. [4][5][6][7] Since the existence of spin-vortices is indicated by the magnetic excitation spectrum obtained by neutron scattering experiments, [8][9][10] this theory may be relevant for superconductivity in hole-doped cuprate. It is also noteworthy that the Kerr effect measurement 11) and the recently observed hidden magnetic excitation spectrum 12) indicate the presence of loop currents.In the derivation of the spin-vortex superconductivity theory, one of the basis of the present understanding of superconductivity is challenged; the textbook derivation of the AC Josephson effect is wrong; 13) the mistake is claimed to originate from the fact that it does not include the flow-in and -out of the particles between a Josephson junction and leads connected to it. If the above claim is valid, the elucidation of the mechanism of the cuprate superconductivity will actually lead to the reconstruction of the theory of superconductivity from the very fundamental level.Since the above-mentioned AC Josephson problem is extremely important, let us first reproduce it here 4) as a part of the introduction of the present work; we will do it succinctly, using the energy diagram depicted in Fig. 1.The Josephson current is obtained by assuming the existence of a "macroscopic wave function " in superconductors;where ρ is the superfluid density and θ is a phase variable that is conjugate to ρ. In the current understanding, ρ is the number * E-mail address: koizumi@ims.tsukuba.ac.jp It is assumed ...