Bilayer Borophenes Establish a New Upper Limit for Elemental Superconducting Transition Temperatures.

Elemental superconductors serve as essential model systems because their compositional simplicity enables fundamental mechanisms to be probed with minimal extrinsic complexity. Their superconducting transition temperatures (T_{c}), however, are generally low, with the notable exception of scandium, which reaches 36 K at 260 GPa-the highest T_{c} reported for any elemental material. Although bulk boron is semiconducting at ambient pressure, two-dimensional boron monolayers (borophenes) exhibit rich structural polymorphism, and several phases are predicted to be superconducting with T_{c} values of 3.7-27.6 K, limited by relatively weak electron-phonon coupling. Here, we show that borophene bilayers, stabilized by interlayer B-B bonds, unexpectedly enhance the electronic density of states near the Fermi level and promote cooperative electron-phonon interactions, in which p_{x,y} states couple to in-plane phonons and p_{z} states to out-of-plane modes. A high-throughput search of more than 9000 bilayer configurations identifies an AA-stacked, low-energy v_{1/7} structure with T_{c}≈68  K, setting a new record for elemental superconductivity.