In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur if spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental set-up is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet pairs to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.

The magnetic field B c , in which the electrons become fully spin polarized, is found to be proportional to the deviation of the electron density from the zero-field metal-insulator transition in a two-dimensional electron system in silicon. The tendency of B c to vanish at a finite electron density suggests a ferromagnetic instability in this strongly correlated electron system. DOI: 10.1103/PhysRevLett.87.086801 PACS numbers: 71.30. +h, 73.40.Qv At sufficiently low electron densities, an ideal twodimensional (2D) electron system becomes strongly correlated, because the kinetic energy is overpowered by energy of electron-electron interactions (exchange and correlation energy). The interaction strength is normally described by the Wigner-Seitz radius, r s 1͑͞pn s ͒ 1͞2 a B (where n s is the electron density and a B is the effective Bohr radius in semiconductor), which is equal in the single-valley case to the ratio of the Coulomb and the Fermi energies as calculated for 2D band electrons. There are several possible candidates for the ground state of the system, for example, (i) a Wigner crystal characterized by spatial and spin ordering [1], (ii) a ferromagnetic Fermi liquid with spontaneous spin ordering [2], and (iii) a paramagnetic Fermi liquid [3]. The Wigner crystal is expected to form in a very dilute limit, at r s * 35 [4]. The spin ordering may survive at higher electron densities (lower r s ) up to the threshold determined by ferromagnetic (Stoner) instability [2] as caused by the competition between the exchange energy and the Pauli principle.In the strongly interacting limit ͑r s ¿ 1͒, all results given by theoretical approaches are very approximate, not to mention that in real 2D electron systems, the influence of disorder needs to be taken into account, which complicates the problem drastically. For an ideal 2D electron system, a direct transition from the Wigner crystal to the paramagnetic Fermi liquid was predicted by quantum Monte Carlo calculation [4], although near the transition the energies of all three states are very close to each other. On the other hand, a tendency to spontaneous spin polarization has been found recently in numerical studies of a disordered and interacting electron gas [5]. In strongly disordered 2D systems, the influence of the disorder can dominate the interaction effects leading to a disorder-driven localization, whereas in the least disordered 2D systems, the metalinsulator transition may be caused by interaction effects [6]. The problem of the ground state of strongly interacting 2D systems is therefore far from being solved.In magnetic fields parallel to the 2D electron plane, the spin effects should dominate the properties of a 2D electron system once the orbital quantization is quenched. Indeed, the resistance of a 2D electron gas in silicon metaloxide-semiconductor field-effect transistors (MOSFETs) was found to be isotropic with respect to in-plane magnetic field, B, and rise steeply with the field saturating to a constant value above a critical magnetic fie...

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.

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