Relationship between Magnetic Anisotropy Below Pseudogap Temperature and Short-Range Antiferromagnetic Order in High-Temperature Cuprate Superconductor

The central issue in high-temperature cuprate superconductors is the pseudogap state appearing below the pseudogap temperature $T^*$, which is well above the superconducting transition temperature. In this study, we theoretically investigate the rapid increase of the magnetic anisotropy below the pseudogap temperature detected by the recent torque-magnetometry measurements on YBa$_2$Cu$_3$O$_y$ [Y. Sato et al., Nat. Phys., 13, 1074 (2017)]. Applying the spin Green's function formalism including the Dzyaloshinskii--Moriya interaction arising from the buckling of the CuO$_2$ plane, we obtain results that are in good agreement with the experiment and find a scaling relationship. Our analysis suggests that the characteristic temperature associated with the magnetic anisotropy, which coincides with $T^*$, is not a phase transition temperature but a crossover temperature associated with the short-range antiferromagnetic order.Comment: 4 pages, 2 figures; added the formula relating the pseudogap temperature and the antiferromagnetic correlation lengt

Time-reversal symmetry-breaking flux state in an organic Dirac fermion system

We investigate symmetry breaking in the Dirac fermion phase of the organic compound $\alpha$-(BEDT-TTF)$_2$I$_3$ under pressure, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene. The exchange interaction resulting from inter-molecule Coulomb repulsion leads to broken time-reversal symmetry and particle-hole symmetry while preserving translational symmetry. The system breaks time-reversal symmetry by creating fluxes in the unit cell. This symmetry-broken state exhibits a large Nernst signal as well as thermopower. We compute the Nernst signal and thermopower, demonstrating their consistency with experimental results.Comment: 6 pages, 3 figure

Chirality effect on superconductivity

International Workshop on Dirac Electrons in Solids 2015: 14–15 January 2015, Tokyo, JapanWe consider electron systems characterized by chirality, such as Dirac fermion systems and iron-based superconductors. We investigate the chirality effect on superconducting states in such a system. We show that chirality effect leads to a nodal structure in the superconducting gap function. The node creation mechanism depends on the wave vector q of the pairing interaction and vorticity that characterizes chirality of electrons. The node creation effect due to chirality is significant for the case of Dirac fermions with q = (π, 0) and for the case of iron-based superconductors with q = (π, π)