The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect, which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called 'orbital' effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17 T at a temperature of 0.1 K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.

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Historic Prussian blue (PB) pigment is easily obtained as an insoluble precipitate in quantitative yield from an aqueous mixture of Fe 3+ and [Fe II (CN) 6 ] 4− (Fe 2+ and [Fe III (CN) 6 ] 3−). It has been found that the PB pigment is inherently an agglomerate of 10-20 nm nanoparticles, based on powder x-ray diffraction (XRD) line broadenings and transmission electron microscopy (TEM) images. The PB pigment has been revived as both organic-solvent-soluble and water-soluble nanoparticle inks. Through crystal surface modification with aliphatic amines, the nanoparticles are stably dispersed from the insoluble agglomerate into usual organic solvents to afford a transparent blue solution. Identical modification with [Fe(CN) 6 ] 4− yields water-soluble PB nanoparticles. A similar ink preparation is applicable to Ni-PBA and Co-PBA (nickel and cobalt hexacyanoferrates). The PB (blue), Ni-PBA (yellow), and Co-PBA (red) nanoparticles function as three primary colour inks.

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor λ-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T, and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe 3+ ions. We further argue that the Fe 3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state Superconductivity is usually destroyed by diamagnetic currents induced in the presence of strong magnetic fields. This effect has orbital character and prevails in most conventional "s-wave" superconductors that involve singlet state of the Cooper pairs. In addition, superconductivity can also be suppressed by the Pauli pair breaking mechanism: here the external field destroys the spinsinglet state of the Cooper pair, imposing the so-called Clogston-Chandrasekhar paramagnetic limit [1,2]. Nevertheless, and despite these well known physical limitations, S. Uji et al. [3] have recently reported the observation of a magnetic-field induced superconducting phase (FISC) in the quasi-two-dimensional organic conductor λ-(BETS) 2 FeCl 4 for fields exceeding 18 tesla, applied parallel to the conducting layers. This is particularly remarkable since this compound, at zero field, is an antiferromagnetic insulator (AI) below T p ∼ = 8.5K [4]. The AI state is suppressed by the application of magnetic fields above 10 tesla at low temperatures [5].The present work was motivated by the apparent increase in the critical temperature of the FISC above 18 T with increasing magnetic field (Ref. [3]). Here, for instance, in the case of spin-triplet superconductivity, there would be in principle, no limit on the upper critical field. The presence of Fe 3+ magnetic moments, which coexist with the FISC state, adds further appeal to the triplet state model. To clarify the nature of the FISC, we have studied the λ-(BETS) 2 FeCl 4 compound at low temperatures in steady, tilted magnetic fields up to 42 tesla. Our main result is the observation of reentrance towards the metallic state at a temperature-dependent critical field. We obtain a temperature-magnetic field phase diagram for the FISC state, which we interpret in terms of the Jacarino-Peter (JP) field compensation effect [6]. This implies that the Cooper pairs condense into a spin-singlet state. We argue further that the Fe 3+ magnetic state is indeed necessary to stabilize the singlet superconducting state by suppression of diamagnetic currents in the associated in-plane high magnetic fields.λ-(BETS) 2 FeCl 4 (where BETS stands for Bis(ethylenedithio)tetraselenafulvalene) crystallizes in a triclinic unit cell. The BETS planar molecules are stacked along the crystallographic a-axis, and constitute conducting planes parallel to the a-c plane. These conducting layers alternate along the b-axis...