The nature of the superconducting state in quasi-one-dimensional organic conductors has remained controversial since its discovery. Here we present results of (77)Se NMR Knight shift (K(s)) experiments in (TMTSF)(2)PF(6) under 7 kbar of pressure with a magnetic field aligned along the most conducting a axis. We find no noticeable shift in K(s) upon cooling through the superconducting transition. Since K(s) directly probes the spin susceptibility chi(s), the fact that chi(s) remains unchanged through the superconducting transition strongly suggests spin-triplet superconductivity.

We have discovered a phenomenon where the orbital pair breaking effect is reduced, if not eliminated. It appears as a striking enhancement in the upper critical field H(c2) for (TMTSF)2PF6 and a strong upward curvature in the critical field versus temperature in the region of pressure-temperature phase space near the superconductor-spin density wave insulator boundary. A simple model based on self-consistently dividing the superconductor into layers explains the observations remarkably well and provides a unique way around orbital frustration and toward higher critical fields.

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We have measured the thermoelectric signal along the a axis in (TMTSF)2PF6 at 10 kbar as a function of the orientation of the applied magnetic field. Resonantlike Nernst signals were found with a dramatic sign change as the field was rotated through the "Lebed magic angles." The sign change indicates that the electrical current is "locked in" to the magic angle (interchain) directions for field alignment close to, but on either side of, the magic angles. The amplitude of signals near these angles is many orders of magnitude larger than expected from conventional Boltzmann transport theory.

It has been suggested that the strong upward curvature and large enhancement of the b axis critical fields in (TMTSF) 2 PF 6 are the result of a magnetic field-induced dimensional crossover ͑FIDC͒ effect. In this paper we present a critical test of the FIDC for this material at a pressure of 5.7 kbar. Decoupled two-dimensional layers should exhibit a cusp in H c2 vs angle near H parallel to the layers. Rather we see no cusp in H c2 () and the anisotropy decreases as temperature is reduced. Our data, in this pressure regime, are more consistent with a recently proposed insulator-superconductor slab model.The Bechgaard salts ͑quasi-one-dimensional systems͒ continue to serve as model systems for many theories of interacting electrons in reduced dimensions. Among them, (TMTSF) 2 PF 6 is one of the most intriguing systems, since it exhibits many different physical properties, from insulating to unconventional superconducting, 1 depending on the applied magnetic field, the temperature, and the pressure. Recently, it has stirred interest as a possible triplet superconductor. 2-9 Magnetic fields destroy the superconducting state by two independent mechanisms. There is a spin effect from Zeeman splitting, which restricts the critical field of a spin singlet superconductor to the Pauli limit H P ͑Ref. 10͒ (& B H P ϭ1.76kT c ). There is also an orbital effect associated with the induced currents generated to screen the external field, which is an effective pair breaker for both singlet and triplet superconductors. The main subject of this paper is the origin of the apparent lack of orbital pair breaking in (TMTSF) 2 X, as evidenced by a large enhancement of the upper critical field, which persists up to four times the Pauli limit with strong upward curvature in the temperature dependence. 2,5 While pair breaking due to the Zeeman effect can be lifted either by triplet pairing or, to some extent, by the formation of Larkin-Ovchinnikov-Fulde-Ferrell ͑LOFF͒ state, 11-13 a field-induced dimensional crossover ͑FIDC͒ effect proposed by Lebed and others 14,15 allows Cooper pairs to overcome the orbital pairbreaking effect. A possible role of the LOFF state in (TMTSF) 2 PF 6 is very slim as noted previously. 3,5 Moreover, unusually high critical fields and the absence of a resonant frequency shift in nuclear magnetic resonance ͑NMR͒ Knight shift experiments strongly suggest spin triplet pairing. 16 Therefore, to describe the unusual superconductivity in (TMTSF) 2 PF 6 , both triplet superconductivity and strong suppression of orbital superconducting frustration are essential.The band structure of this highly anisotropic quasi-onedimensional system can often be simplified to a tight-binding form, E(k)ϭϪ2t a cos(k a a)Ϫ2t b cos(k b b)Ϫ2t c cos(k c c) with the transfer energy integrals given as 4t a :4t b :4t c ϭ1:0.1:0.003 eV. 1 The anisotropy in conductivity is a : b : c ϭ10 5 :10 3 :1. The resulting Fermi surface consists of a pair of slightly warped sheets, opened along the k b and k c directions.When a magnetic field is applied a...