Microscopic origin of the ferromagnetic (FM) exchange coupling in two Cr trihalides, CrCl 3 and CrI 3 , their common aspects and differences, are investigated on the basis of density functional theory combined with realistic modeling approach for the analysis of interatomic exchange interactions. For these purposes, we perform a comparative study based on the pseudopotential and linear muffin-tin orbital methods by treating the effects of electron exchange and correlation in generalized gradient approximation (GGA) and local spin density approximation (LSDA), respectively.The results of ordinary band structure calculations are used in order to construct the minimal tight-binding type models describing the behavior of the magnetic Cr 3d and ligand p bands in the basis of localized Wannier functions, and evaluate the effective exchange coupling (J eff ) between two Cr sublattices employing four different technique: (i) Brute force total energy calculations;(ii) Second-order Green's function perturbation theory for infinitesimal spin rotations of the LSDA (GGA) potential at the Cr sites; (iii) Enforcement of the magnetic force theorem in order to treat both Cr and ligand spins on a localized footing; (iv) Constrained total-energy calculations with an external field, treated in the framework of self-consistent linear response theory. We argue that the ligand states play crucial role in the ferromagnetism of Cr trihalides, though their contribution to J eff strongly depends on additional assumptions, which are traced back to fundamentals of adiabatic spin dynamics. Particularly, by neglecting ligand spins in the Green's function method, J eff can easily become antiferromagnetic, while by treating them as localized, one can severely overestimate the FM coupling. The best considered approach is based on the constraint method, where the ligand states are allowed to relax in response to each instantaneous reorientation of the Cr spins, controlled by the external field. Furthermore, the differences of the electronic structure of Cr trihalides in GGA and LSDA, and their impact on the exchange coupling are discussed in details, as well as the possible roles played by the on-site Coulomb repulsion U . * SOLOVYEV.Igor@nims.go.jp

The activation gaps for fractional quantum Hall states at filling fractions ν = n/(2n + 1) are computed for heterojunction, square quantum well, as well as parabolic quantum well geometries, using an interaction potential calculated from a self-consistent electronic structure calculation in the local density approximation. The finite thickness is estimated to make ∼30% correction to the gap in the heterojunction geometry for typical parameters, which accounts for roughly half of the discrepancy between the experiment and theoretical gaps computed for a pure two dimensional system. Certain model interactions are also considered. It is found that the activation energies behave qualitatively differently depending on whether the interaction is of longer or shorter range than the Coulomb interaction; there are indications that fractional Hall states close to the Fermi sea are destabilized for the latter.

Park et al. Reply: Morf [1] has questioned the value of the "thickness parameter" l used in our work [2], which enters the model interaction V ͑r͒ e 2 ͑͞r 2 1 l 2 ͒ 1͞2 employed to simulate the effect of nonzero thickness and was fixed with the help of the n 1͞3 gap in Ref. [3]. While we had characterized l 117 Å as "reasonable," as it is on the order of the transverse extent of the electron wave function, Morf has looked into the question in greater depth and concluded that it is too large by a factor of 2. To resolve the issue, we have now determined the effective interelectron interaction in a self-consistent local-density-approximation (LDA) calculation and used it to recompute the gaps. There is no adjustable parameter in the calculation now; the inputs are the electron density and known material constants. The resulting gaps are shown in Fig. 1 and the corresponding effective masses in Fig. 2, also including results for n 5͞11. While the modification in the interaction potential due to nonzero transverse thickness reduces the discrepancy between experiment and the zero-thickness results roughly by half, it is by itself not able to fully account for the experimental gaps or the effective mass, contrary to our earlier conclusion. It was shown in Ref.[2] that a short-range softening of the interaction can cause a negative intercept for a straight line fit through the gaps and also a composite fermion (CF) mass enhancement as the half-filled Landau

No abstract

No abstract

The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (α(V)) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for α(V) of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained α(V) is in good agreement with experiment. The values of α(V) for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.

The phonon spectra of diamond, Si, Ge, and ␣-Sn are systematically investigated by using real-space interatomic force constants ͑IFC's͒ up to the 25th shell. The calculations are performed employing density functional perturbation theory, a pseudopotential plane-wave approach, and the local density approximation for the exchange-correlation potential. Moreover, analytical expressions are derived for the phonon frequencies at some high-symmetry points, in terms of the IFC's up to second nearest neighbors. Compared to those of Si, Ge, and ␣-Sn, the IFC's of diamond are found to be larger in magnitude and those of the first and second shells show significantly different relative strengths. The reasons behind the peculiar features of the phonon spectra of diamond are identified. Contrary to a common belief, the flatness of some transverse acoustical phonon branches ͑of Si, Ge, and ␣-Sn͒ appears only after including IFC's up to the eighth shell. The calculated phonon spectra are in good agreement with the available experimental data, for the four considered systems.

No abstract