A Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state was previously reported in the quasi-2D heavy fermion CeCoIn5 when a magnetic field was applied parallel to the ab-plane. Here, we conduct 115 In NMR studies of this material in a perpendicular field, and provide strong evidence for FFLO in this case as well. Although the topology of the phase transition lines in the H-T phase diagram is identical for both configurations, there are several remarkable differences between them. Compared to H ab, the FFLO region for H ⊥ ab shows a sizable decrease, and the critical field separating the FFLO and non-FFLO superconducting states almost ceases to have a temperature dependence. Moreover, directing H ⊥ ab results in a notable change in the quasiparticle excitation spectrum within the planar node associated with the FFLO transition.

The discovery of high-T c superconductivity in lightly doped antiferromagnets has provided much interest in low-dimensional quantum spin systems. In order to obtain insight into the physics of the high-T c superconductivity, the Heisenberg spin-ladder systems, which consist of two (or more) chains of magnetic coupled ions, have been investigated intensively, since novel quantum phenomena appear in spin-ladder system [1-6]. The spin degree of freedom in the single ladder is well described by the Heisenberg Hamiltonianwhere J and J 0 are exchange couplings along the legs and the rungs of the ladder, respectively. Recent theories predicted that the S 1 2 antiferromagnetic spin ladder forms a spin liquid state and has an energy gap in the spin excitation spectra (spin gap) for ladders with even numbers of leg, but has no gap for odd-numbered leg systems. Theories [3,4] Very recently, superconductivity of T c ϳ 10 K was observed in the highly doped sample of the Ca substitution for Sr, when high pressure of ϳ3 GPa was applied [18].In a previous paper [19], we have confirmed the existence of the energy gaps in spin excitation spectra in both Cu sites by NMR͞NQR measurements, as we have observed Cu-NMR͞NQR signals of the chain and ladder site separately. We assign the gap energy of D 140 K for the CuO 2 chains and D 470 K for the Cu 2 O 3 ladder in Sr 14 Cu 24 O 41 . The value of 140 K is close to the 11 meV peak and D 470 K is close to the 35 meV peak for inelastic neutron experiments [20,21]. The difference in the spin gap between the chain and the ladder is attributed to the large exchange coupling in the ladder and the small one in the chain, which may be owing to the different atomic bond configurations; namely 180 ± Cu-O-Cu bond for the ladder and the nearly 90 ± Cu-O-Cu bond for the chain [12,13].In this Letter, we report carrier doping effects on the spin gaps in both sites in Sr 142x A x Cu 24 O 41 (A Ca, Y, and La). The main results are as follows. The spin gap for the ladder increases with Y and La substitutions for Sr, decreases with increasing x in Sr 142x Ca x Cu 24 O 41 , and seems to collapse around x 13 of the Ca substitution. We will discuss hole-doping effects on the gap energy of spin excitation spectra in terms of magnon and holedepairing excitations.Polycrystalline samples were prepared by a conventional solid state reaction under ambient pressure [15 -17]. NMR and NQR were measured by a conventional phase coherent pulse method.

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