The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO
2
) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO
2
, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.
We report on superconductivity in Nd
1−
x
Eu
x
NiO
2
using Eu as a 4f dopant of the parent NdNiO
2
infinite-layer compound. We use an all–in situ molecular beam epitaxy reduction process to achieve the superconducting phase, providing an alternate method to the ex situ CaH
2
reduction process to induce superconductivity in the infinite-layer nickelates. The Nd
1−
x
Eu
x
NiO
2
samples exhibit a step-terrace structure on their surfaces, have a
T
c
onset of 21 K at
x
= 0.25, and have a large upper critical field that may be related to Eu 4f doping.
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