Bulk-Rashba Effect with Suppressed Spin Relaxation in a Polar Phase of Bi In O.

The Rashba effect enables control over the spin degree of freedom, particularly in polar materials where the polar symmetry couples to Rashba-type spin splitting. The exploration of this effect, however, has been hindered by the scarcity of polar materials exhibiting the bulk-Rashba effect and rapid spin-relaxation effects dictated by the D'yakonov-Perel mechanism. Here, a polar LiNbO-type R3c phase of Bi In O with x ≈0.

Thermal multiferroics in all-inorganic quasi-two-dimensional halide perovskites.

Multiferroic materials, particularly those possessing simultaneous electric and magnetic orders, offer a platform for design technologies and to study modern physics. Despite the substantial progress and evolution of multiferroics, one priority in the field remains to be the discovery of unexplored materials, especially those offering different mechanisms for controlling electric and magnetic orders. Here we demonstrate the simultaneous thermal control of electric and magnetic polarizations in quasi-two-dimensional halides (K,Rb)MnCl, arising from a polar-antipolar transition, as evidenced using both X-ray and neutron powder diffraction data.

Out-of-plane ferroelectricity and robust magnetoelectricity in quasi-two-dimensional materials.

Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power.

Tunable Negative Thermal Expansion in Layered Perovskites from Quasi-Two-Dimensional Vibrations.

We identify a quasi-two-dimensional (quasi-2D) phonon mode in the layered-perovskite Ca_{3}Ti_{2}O_{7}, which exhibits an acoustic branch with quadratic dispersion. Using first-principles methods, we show this mode exhibits atomic displacements perpendicular to the layered [CaTiO_{3}]_{2} blocks comprising the structure and a negative Grüneisen parameter. Owing to these quasi-2D structural and dynamical features, we find that the mode can be utilized to realize unusual membrane effects, including a tunable negative thermal expansion (NTE) and a rare pressure-independent thermal softening of the bulk modulus.

Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides.

Epitaxial strain can induce collective phenomena and new functionalities in complex oxide thin films. Strong coupling between strain and polar lattice modes can stabilize new ferroelectric phases from nonpolar dielectrics or enhance electric polarizations and Curie temperatures. Recently, strain has also been exploited to induce novel metal-insulator transitions and magnetic reconstructions through its coupling to nonpolar modes, including rotations of BO6 transition-metal octahedra.