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31
2024
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Chinese Academy of Sciences latest Nature!
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Softening of transverse optical (TO) phonons, which can trigger the ferroelectric phase transition, can often be achieved by enhancing long-range Coulomb interactions on short-range binding forces, for example, by increasing the Born effective charge.However, it is affected by the depolarization effect, because the induced ferroelectricity is suppressed on the size reduction of the high-density nanoscale electronic material.
On October 30, 2024, Luo Military Commission, Deng Huixiong and Wei Su-Huai of Ningbo Dongfang University of Technology of the Institute of Semiconductors of the Chinese Academy of Sciences jointly communicatedNaturemagazine published online entitled“Softening of the optical phonon by reduced interatomic bonding strength without depolarization”The research paper,The study foundWithout depolarization, optical phonons are softened by reducing the interatomic bond strength.
The significant reduction of TO phonon frequency is of great significance in condensed matter because it is related to ferroelectric phase transition, phase change materials, high dielectric constant media and better pyroelectricity. The TO phonon softening is considered to be entirely dependent on the anomalously strong long-range Coulomb interaction or resonance bonding caused by the giant Born dynamic charge Z * dominating the short-range bonding force. Due to the strong coupling with the electric field, this makes the relevant properties very sensitive to the details of the domain structure, defects and boundary conditions.In particular, ferroelectric perovskites produce depolarization effects due to incomplete screening of surface charges, which inhibits ferroelectricity or significantly reduces the dielectric constant in ultra-thin films, thereby limiting the potential benefits to high-density nanoelectronics.
The strong long-range Coulomb interaction also limits the insulator band gap (Eg) and the static dielectric constant (Es) the decoupling of the inverse relationship.However, recently in the wide band gap (Eg= 10.6 eV) An unexpectedly large static dielectric constant of about 271 was found in the rock salt BeO (rs-BeO)E0（E0is the free space dielectric constant), and has a very low TO frequency, although the static dielectric constant in the more stable zinc sphalerite structure BeO(zb-BeO) is very small (3.16E0）。The latter meets the general expectation of a large band gap of 9.3 eV. This unexpected phenomenon motivates people to explore other ways to soften the TO mode to overcome these undesirable effects of strong long-range Coulomb interaction.
Here, the researchers propose another way to drive the softening of TO phonons. It is shown that the abnormal soft TO phonons in the ultra-wide band gap BeO of the rock salt structure are mainly due to the Coulomb repulsion caused by the electron cloud overlap. The short-range bond interaction is greatly reduced, which is caused by the Coulomb repulsion between two adjacent oxygen ions, these two oxygen ions are arranged in an octahedral form around a very small Be ion. It is further demonstrated that strain-induced perovskite BaZrO epitaxially grown on lattice-mismatched SiO2/Si substrates3and ultra-thin HfO2and ZrO2Strong ferroelectricity appears in the film, which is due to the softening of TO phonons due to the reduction of the short-range bonding strength of the biaxial strain-induced tensile bond.These findings reveal a unified theory of enhanced ferroelectricity in ultrathin films without depolarization fields by tuning chemical bonds using ionic radius differences, strain, doping, and lattice distortion.
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