The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in quasi-one-dimensional systems with warped Fermi surfaces is examined in strong parallel magnetic fields. It is shown that the state is extremely stable for field directions around nontrivial optimum directions, at which the upper critical field exhibits cusps, and that the stabilization is due to a Fermi-surface effect analogous to the nesting effect for the spin density wave and charge density wave. Interestingly, the behavior with cusps is analogous to that in a square lattice system in which the hole density is controlled. For the organic superconductor (TMTSF) 2 ClO 4 , when the hopping parameters obtained by previous authors based on X-ray crystallography results are assumed, the optimum directions are in quadrants consistent with the previous experimental observations. Furthermore, near this set of parameters, we also find sets of hopping parameters that more precisely reproduce the observed optimum in-plane field directions. These results are consistent with the hypothesis that the FFLO state is realized in the organic superconductor.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is systematically examined in a generic model of quasi-onedimensional (Q1D) type-II superconductors that has six hopping integrals of electrons as model parameters. For a magnetic field parallel to the conductive layers, the upper critical field H c2 is strongly enhanced by the FFLO state at low temperatures and sensitively depends on the angle ϕ between the in-plane magnetic field and the highly conductive chain (the crystal a-axis). As a result, H c2 exhibits sharp peaks at the optimal angles ϕ = ±ϕ 0 . Since the optimal angle ϕ 0 strongly depends on the structure of the Fermi surface, we examine their correlation, searching for an intuitive method to find ϕ 0 from the shape of the Fermi surface. For this purpose, we define quantities that quantify the warp of each sheet (k x > 0 or k x < 0) of the Q1D open Fermi surface and the shear distortion between the two sheets. We estimate the optimal angles for numbers of the parameter sets chosen systematically from a large area of the parameter space. It is found that in most cases, the optimal direction of the in-plane magnetic field tends to be roughly parallel to the a-axis. This result, together with the fact that the orbital pair-breaking effect is weakest for ϕ = 0, implies that the FFLO state is most stabilized for a small ϕ. However, when the warp is small while the shear distortion is moderate, the FFLO state can be maximally stabilized for any in-plane magnetic-field direction except for the directions between the b-and b ′ -axes, where the b ′ -axis is perpendicular to the a-axis. The phase diagrams of the optimal angle and the upper critical field at zero temperature are also presented. A jump of the optimal angle ϕ 0 when the pressure varies is predicted.

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We numerically calculate the magnetization by applying the magnetic field (B) perpendicular to two-dimensional corbino disk system without electron spin. We obtain that the period of the Aharonov-Bohm (AB) oscillations of magnetization [M (B)] exhibit φ 0 , φ 0 /2 and almost φ 0 /3 as a function of φ, depending on numbers of electrons (N ), where φ 0 = hc e is a unit flux and φ is the magnetic flux through the hollow of the disk. The oscillations of M (B) are classified into five patterns at 1 ≤ N ≤ 27 investigated in this study. These are expected to be observed in the two-dimensional semiconductor corbino disk with the hollow radius of about 10 nm and small electron numbers at the high magnetic field.