This article overviews recent theoretical work that explored a mechanism of the magnetism-induced negative thermal expansion (NTE) observed in inverse-perovskite antiferromagnets Mn3 AN ( A=Zn, Ga, etc). These compounds exhibit crystal-volume expansion upon cooling, triggered by a non-coplanar antiferromagnetic order. In spite of intensive studies, however, the mechanism has been remained as a puzzle for 50 years. This puzzle was recently solved by considering microscopic electronic and magnetic structures and couplings between magnetism and the crystal lattice in real materials. The clarified mechanism turned out to be applicable to even magnets with crystal structures other than the inverse-perovskite. Hence, the work provides important guiding principles to research for new NTE materials and will contribute to the development of this research field.
Due to invention of Chirped Pulse Amplification (CPA) by Gérard Mourou and Donna Strickland, ultrashort light pulse of attosecond duration and ultra-high intensity field reaching relativistic proton acceleration are now available. Recent progresses in the ultrafast and high intensity science opened by CPA and optical phase control are reviewed.
Barium disilicide (BaSi2) has shown potential as a photo-absorber material for thin-film solar cells. The electronic properties, optical absorption coefficient, and defect properties of BaSi2 were investigated using calculations based on the density functional theory. The calculations show charge transfer from Ba to Si atoms and covalent bond formation in Si4 tetrahedra, which confirm that BaSi2 is a member of the Zintl phase. The density of states and partial charge distributions for BaSi2 revealed that the electronic states of BaSi2 can be explained based on the electronic states of Si4 and Ba atoms. The high optical absorption coefficient can be attributed to three direct transitions with energy gaps, which are slightly larger than the indirect band gap. Si vacancy, Ba substituted for Si antisite, and Si interstitial defects are dominant, but do not create a significant number of carriers. The calculated Fermi level is pinned in the middle of the band gap, indicating that bipolar doping is feasible; thus, it is easier to form p–n junctions.
When a photon induces an interband transition in a solid, the excited electron (hole) experiences a spatial shift if the crystal lacks an inversion symmetry, leading to a generation of directional photocurrent. This phenomenon has been known as bulk or anomalous photovoltaic effect for years, and is recently reformulated with the viewpoint of quantum mechanics as the change in the geometrical phase upon photoexcitation. It is also predicted that this so-called shift current of electron (hole) can be less-dissipative and ultrafast. We demonstrate the emergence of shift current in several organic and inorganic ferroelectrics by using extensive optical spectroscopies, whose excitation spectra nicely compared to the ones from first-principles band calculations.