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Yuqiao Guo1 Changzheng Wu1

1, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, , China

Free-standing 2D crystals mechanically cleaved from van der Waals solids exhibit unprecedented high crystal quality and macroscopic continuity which not only renovate the cognition that 2D long-range crystalline order cannot exist at finite temperature but also reveal the dramatic effect of dimensional crossovers on intrinsic physical properties. However, current progress is primarily limited to those typical strongly-anisotropic layered lattices and for the predominated 3D non-layered lattices whether their isolated atomic planes could survive in principle and if so how to two-dimensionalize those isotropic solids from their high symmetry parent crystals remains fundamental question and significant challenge. Herein, we theoretically proposed the quantum electronic stability of freestanding rock-salt ZrN nanosheet with critical-thickness of three atomic bilayers based on the electronic growth model. Along the way, we further experimentally realized ZrN vdW-like crystal via a bilayer-by-bilayer growth mode under CaH2-assisted topotactic reaction. The ZrN vdW-like crystals can be mechanically exfoliated and transferred in the same way as natural layered vdW materials and the step height in exfoliated layers is in good agreement with the height of critical-thickness calculated. The exfoliated high quality ZrN nanosheets with a few nanometres thick exhibits dimensional crossover effect of emerging spin density wave and 2D superconductivity with the unconventional upper critical field beyond the Pauli paramagnetic limit, which suggests a role for spin fluctuations in the pairing mechanism in dimensionally confined superconductors. The existence of critical thickness stability for 2D freestanding lattice and mild topotactic transformation process can be the general strategy to design and fabricate other high-quality 2D crystals in isotropic lattice, which further enrich the arena of artificial 2D materials and their confined new physics.

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