Photostriction is predicted for SnS and SnSe monolayers, two-dimensional ferroelectrics with rectangular unit cells (the lattice vector a1 is larger than a2) and an intrinsic dipole moment parallel to a1. Photostriction in these two-dimensional materials is found to be related to the structural change induced by a screened electric polarization in the photoexcited electronic state (i.e., a converse piezoelectric effect) that leads to a compression of a1 and a comparatively smaller increase of a2 for a reduced unit cell area. The structural change documented here is ten times larger than that observed in BiFeO3, making monochalcogenide monolayers an ultimate platform for this effect. This structural modification should be observable under experimentally feasible densities of photexcited carriers on samples that have been grown already, having a potential usefulness for light-induced, remote mechano-opto-electronic applications.A truly novel opto-mechanical coupling in twodimensional (2D) ferroelectric materials awaits to be discovered.Photostriction -the creation of nonthermal strain upon illumination [1-4]-has been welldocumented in three-dimensional ferroelectrics such as SbSI [5] and BiFeO 3 [6,7]. It has been suggested to be driven by the large voltage build-up caused by a photovoltaic effect and the resulting converse piezoelectricity [8], and it may be useful for applications such as remotely-switchable memory devices [9] and lightinduced actuators [10]. The earliest studied photostrictive material, SbSI, transitions from a ferroelectric onto a paraelectric at a critical temperature T c < 300 K. As photostrictive effects are larger in the ferroelectric phase, T c can be increased above 300 K on SbSI ceramics which have smaller domain sizes and display a non-uniform stoichiometry nevertheless. The photostriction response time increases with sample thickness due to a reduced penetration depth, being a few seconds on bulk samples [5]. On the other hand, ferroelectric films show photostriction within a few picoseconds [11][12][13][14][15], and even when the photoexcited electron-hole pair is localized [16,17].The growing interest on the interactions of light with 2D materials [18-21] makes a study of illumination leading to non-trivial structural deformations an interesting and timely endeavor. As long as a photoexcited state induces some amount of charge redistribution -which is a quite reasonable physical assumption-any material is expected to change shape as the structure is let to relieve the stress induced by the photoexcited carriers. Here, the surprising result is the rather large magnitude of such structural change for 2D ferroelectrics, that originates from an inverse piezoelectric effect upon illumination.Two-dimensional ferroelectrics in the group IV monochalcogenide family (GeS, GeSe, SnS, SnSe, among others) [22][23][24][25][26][27][28][29][30][31] undergo a ferroelectric-to-paraelectric transition with a transition temperature that is tunable by atomic number [25]. Following the numerical approach...

Utilizing a material in thermoelectric applications requires a mechanical, thermal, and lattice stability as well a high figure of merit (ZT). In this work, we present the structural, electronic, magnetic, mechanical, thermodynamic, dynamic, and thermoelectric properties of CoFeYGe (Y = Ti, Cr) quaternary Heusler compounds using the density functional theory (DFT). The calculated mechanical properties and phonon dispersions reveal that the structures of these compounds are stable. Both CoFeCrGe and CoFeTiGe compounds show a ferromagnetic and ferrimagnetic halfmetallic behavior with band gaps of 0.41 and 0.38 eV, respectively. The lattice thermal conductivity (κL) exhibits low values that reach 3.01 W/(m.K) (3.47 W/(m.K)) for CoFeCrGe (CoFeTiGe) at 1100 K. The optical phonon modes have a large contribution of 60.2% (70.9 %) to κL value for CoFeCrGe (CoFeTiGe). High ZT values of 0.71 and 0.65 were obtained for CoFeCrGe and CoFeTiGe, respectively. Based on our calculations, CoFeCrGe and CoFeTiGe combine both good spintronic and thermoelectric behaviors that may be used in spin injection applications.

No abstract

No abstract