Soft Manifold Dynamics Behind Negative Thermal Expansion

Pressure-induced softening as a common feature of framework structures that have negative thermal expansion

Results of a series of molecular dynamics simulations of cubic siliceous zeolites suggest that pressure-induced softening -- the phenomenon in which a material becomes progressively more compressible under pressure -- is likely to be a common feature of framework materials that show negative thermal expansion. The correlation between the negative thermal expansion and the pressure-induced softening is investigated on the basis of thermodynamics.

Graded anharmonic crystals as genuine thermal diodes: Analytical description of rectification and negative differential thermal resistance

We address the heat flow study starting from microscopic models of matter: we develop an approach and investigate some anharmonic graded mass crystals, with weak interparticle interactions. We calculate the thermal conductivity, and show the existence of rectification and negative differential thermal resistance. Our formalism allows us to understand the mechanism behind the phenomena, and shows that the properties of graded materials make them genuine thermal diodes.

Acoustic-Soft Optical Mode Coupling Leads to Negative Thermal Expansion of GeTe near the Ferroelectric Phase Transition

GeTe is a well-known ferroelectric and thermoelectric material that undergoes a structural phase transition from a rhombohedral to the rocksalt structure at $\sim 600-700$ K. We present a first principles approach to calculate the thermal expansion of GeTe in the rhombohedral phase up to the Curie temperature. We find the minimum of the Helmholtz free energy with respect to each structural parameter in a manner similar to the traditional Gr{ü}neisen theory, and account for the temperature dependence of elastic constants. We obtain the temperature variation of the structural parameters of rhomb…

On the microscopic mechanisms behind hyperferroelectricity

Hyperferroelectrics are receiving a growing interest thanks to their unique property to retain a spontaneous polarization even in presence of a depolarizing field. Nevertheless, general microscopic mechanisms driving hyperferroelectricity, which is ascribed to the softening of a polar $LO$ mode, are still missing. Here, by means of phonons calculations and force constants analysis in two class of hyperferroelectrics, the ABO$_3$-LiNbO3-type systems and the prototypical hexagonal-ABC systems, we unveiled common features in the dynamical properties of a hyperferroelectric behind such $LO$ instab…

脆性断裂背后的弹性不稳定性

Elastic instability behind brittle fracture

研究无缺陷软弹性固体中脆性裂纹的形核机制，指出其源于涉及大转动和软化的几何敏感非线性弹性分岔，并用相场方法刻画亚连续尺度的应变局部化与裂纹形成。

Colossal pressure-induced softening in scandium fluoride

The counter-intuitive phenomenon of pressure-induced softening in materials is likely to be caused by the same dynamical behaviour that produces negative thermal expansion. Through a combination of molecular dynamics simulation on an idealised model and neutron diffraction at variable temperature and pressure, we show the existence of extraordinary and unprecedented pressure-induced softening in the negative thermal expansion material scandium fluoride, ScF$_3$, with values of the pressure-derivative of the bulk modulus $B$, $B^\prime = \partial B / \partial P$, reaching as low as $-40 \pm 1$.