We investigate structural, magnetic, and electronic properties of SrFeAsF as a new parent for superconductors using state-of-the-art density-functional theory method. Calculated results show that striped antiferromagnetic order is the magnetic ground
state in the Fe layer and interlayer magnetic interaction is tiny. Calculated As and Sr positions are in agreement with experiment. There are only two uniaxially-dispersed bands near the Fermi level. The valent charge is mainly in the Fe and F layers, and the magnetic moment is confined to the Fe atoms. Inter-Fe-spin couplings is due to superexchange through As atoms. These are useful to understanding the SrFeAsF and should have helpful implications to doped samples.
We propose a natural two-speed model for the phase dynamics of Si(111) 7$times$7 phase transition to high temperature unreconstructed phase. We formulate the phase dynamics by using phase-field method and adaptive mesh refinement. Our simulated resul
ts show that a 7$times$7 island decays with its shape kept unchanged, and its area decay rate is shown to be a constant increasing with its initial area. LEEM experiments concerned are explained, which confirms that the dimer chains and corner holes are broken first in the transition, and then the stacking fault is remedied slowly. This phase-field method is a reliable approach to phase dynamics of surface phase transitions.
A ternary ferrimagnetic half-metal, constructed through substituting 25% Fe for Mn in zincblende semiconductor MnTe, is predicted in terms of accurate first-principles calculations. It has a large half-metallic (HM) gap of 0.54eV and its ferrimagneti
c order is very stable against other magnetic fluctuations. The HM ferrimagnetism is formed because the complete moment compensation in the antiferromagnetic MnTe is replaced by an uncomplete one in the Fe-substituted MnTe. This should make a novel approach to new HM materials. The half-metal could be fabricated because Fe has good affinity with Mn, and useful for spintronics.
We propose a two-dimensional phase-field-crystal model for the (2$times$1)-(1$times$1) phase transitions of Si(001) and Ge(001) surfaces. The dimerization in the 2$times$1 phase is described with a phase-field-crystal variable which is determined by
solving an evolution equation derived from the free energy. Simulated periodic arrays of dimerization variable is consistent with scanning-tunnelling-microscopy images of the two dimerized surfaces. Calculated temperature dependence of the dimerization parameter indicates that normal dimers and broken ones coexist between the temperatures describing the charactristic temperature width of the phase-transition, $T_L$ and $T_H$, and a first-order phase transition takes place at a temperature between them. The dimerization over the whole temperature is determined. These results are in agreement with experiment. This phase-field-crystal approach is applicable to phase-transitions of other reconstructed surface phases, especially semiconductor $ntimes$1 reconstructed surface phases.
High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high tem
perature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V_N). Calculated results are in agreement with experimental observations and facts of real Cr-doped AlN samples and their synthesis. Our first-principles results are useful to elucidating the mechanism for the ferromagnetism and exploring high-performance Cr-doped AlN diluted magnetic semiconductors.
