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Locking the spins in a superconductor
In Cooper pairs—pairs of electrons responsible for the exotic properties of superconductors—the two electrons' spins typically point in opposite directions. A strong-enough external magnetic field will destroy superconductivity by making the spins point in the same direction. Lu et al. observed a two-dimensional superconducting state in the material MoS2 that was surprisingly immune to a magnetic field applied in the plane of the sample (see the Perspective by Suderow). The band structure of MoS2 and its spin-orbit coupling conspired to create an effective magnetic field that reinforced the electron pairing, with spins aligned perpendicular to the sample.
Abstract
The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field Bc2 far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced Bc2 is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.
