Time-periodic corner states from Floquet higher-order topology

The recent discoveries of higher-order topological insulators (HOTIs) have shifted the paradigm of topological materials, previously limited to topological states at boundaries of materials, to include topological states at boundaries of boundaries, such as corners. So far, all HOTI realisations have been based on static systems described by time-invariant Hamiltonians, without considering the time-variant situation. There is growing interest in Floquet systems, in which time-periodic driving can induce unconventional phenomena such as Floquet topological phases and time crystals. Recent theories have attempted to combine Floquet engineering and HOTIs, but there has been no experimental realisation so far. Here we report on the experimental demonstration of a two-dimensional (2D) Floquet HOTI in a three-dimensional (3D) acoustic lattice, with modulation along a spatial axis serving as an effective time-dependent drive. Acoustic measurements reveal Floquet corner states with double the period of the underlying drive; these oscillations are robust, like time crystal modes, except that the robustness arises from topological protection. This shows that space-time dynamics can induce anomalous higher-order topological phases unique to Floquet systems.

Thermal Properties of Magnesium Hydroxide Using First-Principles Method

The thermal properties of the magnesium hydroxide crystalline powder samples, which were prepared by us, were investigated by first-principles method. The calculated results show that the forbidden band of phonon appears in both 14～19 THz and 33～102 THz. There are 3 acoustic lattice wave branches and 12 optical ones, where two horizontal acoustic branches are degeneracy and two horizontal optical ones are degeneracyhe. Moreover, The constant-volume specific heat quickly increases at low temperature; but it tends to be flat at high temperature. Besides, the Debye temperature quickly increases to 483K from temperature 0K to 10K; but from temperature 100K to 1000K, it is linear with temperature approximately, whose increasing rate is about 1.333; at temperature of 1000K, the Debye temperature is 1930K, reaching its maximum.