Site-selective NMR for odd-frequency Cooper pairs around vortex in chiral p-wave superconductors

In order to identify the pairing symmetry with chirality, we study site-selective NMR in chiral p-wave superconductors. We calculate local nuclear relaxation rate 1/T_1 in the vortex lattice state by Eilenberger theory, including the applied magnetic field dependence. We find that 1/T_1 in the NMR resonance line shape is different between two chiral states p_{pm}(=p_x{pm}ip_y), depending on whether the chirality is parallel or anti-parallel to the vorticity. Anomalous suppression of 1/T_1 occurs around the vortex core in the chiral p_- wave due to the negative coherence term coming from the odd-frequency s-wave Cooper pair induced around the vortex with Majorana state.Comment: 6 pages, 4 figure

Spin-polarized local density of states in the vortex state of helical p -wave superconductors

Properties of the vortex state in helical p-wave superconductor are studied by the quasiclassical Eilenberger theory. We confirm the instability of the helical p-wave state at high fields and that the spin-polarized local density of states M(E,r) appears even when Knight shift does not change. This is because the vorticity couples to the chirality of up-spin pair or down-spin pair of the helical state. In order to identify the helical p-wave state at low fields, we investigate the structure of the zero-energy M(E = 0,r) in the vortex states, and also the energy spectra of M(E,r)

Local NMR relaxation rates T-1(-1) and T-2(-1) depending on the d-vector symmetry in the vortex state of chiral and helical p-wave superconductors

Local NMR relaxation rates in the vortex state of chiral and helical p-wave superconductors are investigated by the quasiclassical Eilenberger theory. We calculate the spatial and resonance frequency dependences of the local NMR spin-lattice relaxation rate T-1(-1) and spin-spin relaxation rate T-2(-1). Depending on the relation between the NMR relaxation direction and the d-vector symmetry, the local T-1(-1) and T-2(-1) in the vortex core region show different behaviors. When the NMR relaxation direction is parallel to the d-vector component, the local NMR relaxation rate is anomalously suppressed by the negative coherence effect due to the spin dependence of the odd- frequency s-wave spin-triplet Cooper pairs. The difference between the local T-1(-1) and T-2(-1) in the site-selective NMR measurement is expected to be a method to examine the d-vector symmetry of candidate materials for spin-triplet superconductors

Coherence effect in a two-band superconductor: Application to iron pnictides

From a theoretical point of view, we propose an experimental method to determine the pairing symmetry of iron pnictides. We focus on two kinds of pairing symmetries, $s_{+-}$ and $s_{++}$, which are strong candidates for the pairing symmetry of iron pnictides. For each of these two symmetries, we calculate both the density and spin response functions by using the two-band BCS model within the one-loop approximation. As a result, a clear difference is found between the $s_{+-}$- and $s_{++}$-wave states in the temperature dependence of the response functions at nesting vector $\bf{Q}$, which connects the hole and electron Fermi surfaces. We point out that this difference comes from the coherence effect in the two-band superconductor. We suggest that the pairing symmetry could be clarified by observing the temperature dependence of both the density and spin structure factors at the nesting vector $\bf{Q}$ in neutron scattering measurements.Comment: 15 pages, 7 figures, 1 tabl

Orbital Order, Structural Transition and Superconductivity in Iron Pnictides

We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below TQ which induces the tetragonal-orthorhombic structural transition at Ts = TQ, together with the stripe-type antiferromagnetic order below TN. Near the phase transitions, the system shows the s++ wave superconductivity due to the orbital fluctuation for a large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation for a small g case with TQ < TN. The former case is consistent with the phase diagram of doped iron pnictides with Ts > TN.Comment: 5 pages, 5 figures, minor changes, published in J. Phys. Soc. Jp

Simple Real-Space Picture of Nodeless and Nodal s-wave Gap Functions in Iron Pnictide Superconductors

We propose a simple way to parameterize the gap function in iron pnictides. The key idea is to use orbital representation, not band representation, and to assume real-space short-range pairing. Our parameterization reproduces fairly well the structure of gap function obtained in microscopic calculation. At the same time the present parameterization is simple enough to obtain an intuitive picture and to develop a phenomenological theory. We also discuss simplification of the treatment of the superconducting state.Comment: 4 page

Pair breaking of multigap superconductivity under parallel magnetic fields in the electric-field-induced surface metallic state

The roles of paramagnetic and diamagnetic pair-breaking effects in superconductivity in the electric-field-induced surface metallic state are studied using the Bogoliubov–de Gennes equation when magnetic fields are applied parallel to the surface. The multigap states of the subbands are related to the depth dependence and the magnetic field dependence of the superconductivity. In the Fermi-energy density of states and the spin density, subband contributions successively appear from higher-level subbands with increasing magnetic fields. The characteristic magnetic field dependence may be a key feature to identify the multigap structure of the surface superconductivity

Superconductivity in the Three-Fold Charge-Ordered Metal of the Triangular-Lattice Extended Hubbard Model

The quarter-filling extended Hubbard model on the triangular lattice is studied to explore pairing instability in the three-fold charge-ordered (CO) metal. We derive a second-order strong-coupling effective Hamiltonian of doped carriers into the three-fold CO insulator at electron density of $n=2/3$, and then study the $f$- and $d_{xy}$-wave superconductivities down to $n=1/2$ by using the BCS mean-field approximation. It is found that the triplet $f$-wave pairing is more stable than the $d_{xy}$-wave one. We also point out that this coexisting state of the charge ordering and superconductivity is possible to have critical temperature $T_c \sim 0.01 t$.Comment: 4 pages, 7 figure