The magnetic properties of dilute magnetic semiconductors (DMSs) are calculated from first-principles by mapping the ab initio results on a classical Heisenberg model. By using the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) method within the local-density approximation, the electronic structure of (Ga, Mn)N and (Ga, Mn)As is calculated. Effective exchange coupling constants J ij 's are determined by embedding two Mn impurities at sites i and j in the CPA medium and using the J ij formula of Liechtenstein et al. [J. Magn. Magn. Mater. 67, 65 (1987)]. It is found that the range of the exchange interaction in (Ga, Mn)N, being dominated by the double exchange mechanism, is very short ranged due to the exponential decay of the impurity wave function in the gap. On the other hand, in (Ga, Mn)As, where p-d exchange mechanism dominates, the interaction range is weaker but long ranged, because the extended valence hole states mediate the ferromagnetic interaction. Curie temperatures (T C 's) of DMSs are calculated by using the mean-field approximation (MFA), the random-phase approximation, and the, in principle exact, Monte Carlo method. It is found that the T C values of (Ga, Mn)N are very low since, due to the short-ranged interaction, percolation of the ferromagnetic coupling is difficult to achieve for small concentrations. The MFA strongly overestimates T C . Even in (Ga, Mn)As, where the exchange interaction is longer ranged, the percolation effect is still important and the MFA overestimates T C by about 50%-100%. Dilute magnetic semiconductors (DMSs), such as (In, Mn)As and (Ga, Mn)As discovered by Munekata et al. and Ohno et al., have been well investigated as hopeful materials for spintronics. 1 Curie temperatures (T C 's) of these DMSs are well established 1-3 and some prototypes of spintronics devices have been produced based on these DMSs. The magnetism in these DMSs are theoretically investigated and it is known that the ferromagnetism in these systems, as well as (Ga, Mn)Sb, can be well described by Zener's p-d exchange interaction, due to the fact that the majority of d states lies energetically in the lower part of the valence band.4 Dietl et al. 5 and MacDonald et al. 6 explained many physical properties of (Ga, Mn)As based on the p-d exchange model, and first-principles calculations by Sato et al. showed that the concentration dependence of T C in (Ga, Mn)As was well understood by the p-d exchange interaction if a correction to the local-density approximation (LDA) is simulated by the LDA+ U method with U = 4 eV. 4 While these p-d exchange systems, in which the d states of Mn impurities are practically localized, are well understood, there exist an even larger class of systems where the d levels lie in the gap exhibiting impurity bands for sufficiently large concentrations. To these impurity band systems belong (Ga, Mn)N, (Ga, Cr)N, (Ga, Cr)As, (Zn, Cr)Te, (Zn, Cr)Se, and many others, as shown by first-principles calculations. 7Most of these systems are controversially discussed ...

all show a sharp peak in the vibrational density of states at 57 2 K, which is absent in the unfilled skutterudite CoSb 3 . Heat capacity measurements on Tl 0:8 Co 4 Sb 11 Sn as compared to CoSb 3 are consistent with the presence of a localized vibrational mode associated with the ''rattling'' thallium atoms in this filled skutterudite compound. Both results are well described by a localized Einstein mode model with an Einstein temperature E of 53 1 K. These data provide perhaps the clearest example of local mode behavior in a concentrated metallic system.

Y 0:5 Ca 0:5 BaCo 4 O 7 contains kagomé layers of Co ions, whose spins are strongly coupled, with a CurieWeiss temperature of ÿ2200 K. At low temperature, T 1:2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three-dimensional long-range antiferromagnetic order indicates negligible coupling between the kagomé layers. The scattering intensities are consistent with high spin S 3=2 states of Co 2 in the kagomé layers and low spin S 0 states for Co 3 ions on interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of effectively short range, staggered chiral spin order. DOI: 10.1103/PhysRevLett.98.067201 PACS numbers: 75.25.+z, 61.12.ÿq, 75.40.Cx, 75.50.Ee The topology of many crystal structures has an important influence on the collective behavior of interacting magnetic moments. In lattices with triangular networks the antiferromagnetic (AF) coupling between all spins cannot be satisfied simultaneously owing to geometrical frustration that strongly reduces the ordering temperature and disturbs the settling of the system into a long-range ordered state. Exotic phenomena such as spin-ice and spinliquid phases can emerge from magnetic interactions that are incompatible with the underlying crystal geometry. A hallmark of frustration is the large degeneracy of complex, noncollinear ground states of finite entropy and the appearance of chiral correlations [1]. According to the MerminWagner theorem [2], in low dimensions, the AF order that would reveal the ground state properties is suppressed at finite temperatures. The famous case of spins on the twodimensional kagomé lattice that are simply AF coupled to only nearest neighbor comprises high geometrical frustration and low dimensionality and still challenges theoretical understanding as well as experimental observations and analysis.Here, we present diffuse neutron scattering results with polarization analysis on a new compound Y 0:5 Ca 0:5 BaCo 4 O 7 , in which the Co spins (S 3=2) in noninteracting kagomé layers appear to realize ideally the kagomé AF with only 2D nearest neighbor interactions, which allows an unprecedented approach to its unusual ground state properties.Considering the classical Heisenberg model of AF coupled nearest neighbor spins on the kagomé lattice, the ground state is highly degenerate and characterized by competing chiral spin structures. The spins have a relative orientation of 120 degrees in each triangular unit, so that the local sum of spins is zero. However, there are two competing structures that show either uniform or staggered chirality, see Fig. 1. According to predictions for the classical Heisenberg AF the staggered chiral structure is favored [3,4], because the existence of local zero energy modes, so-called weathervane defects, which are common spin rotations on hexagons, cause a larger degeneracy; such entropical selecti...

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Inelastic neutron scattering measurements on thermoelectric Zn 4 Sb 3 reveal a dominant soft local phonon mode at 5.3(1) meV. The form factor of this local mode is characteristic for dumbbells vibrating preferably along the dumbbell axis and can be related to a vibration of Sb dimers along the c axis. The Lorentzian width of the mode corresponds to short phonon lifetimes of 0.39(2) ps and yields an estimate of the thermal conductivity that agrees quantitatively with recent steady state measurements. Heat capacity measurements are consistent with an Einstein mode model describing the local Sb-dimer rattling mode with an Einstein temperature of 62(1) K. Our study suggests that soft localized phonon modes in crystalline solids are not restricted to cagelike structures and are likely to be a universal feature of good thermoelectric materials.

We present a detailed study of magnetism in LuFe2O4, combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of TN involves a metamagnetic transition separating an antiferro-and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above TN is explained in terms of near degeneracy of the magnetic phases.

The ambitious instrument suite for the future European Spallation Source whose civil construction started recently in Lund, Sweden, demands a set of diverse and challenging requirements for the neutron detectors. For instance, the unprecedented high flux expected on the samples to be investigated in neutron diffraction or reflectometry experiments requires detectors that can handle high counting rates, while the investigation of sub-millimeter protein crystals will only be possible with large-area detectors that can achieve a position resolution as low as 200 µm. This has motivated an extensive research and development campaign to advance the state-of-the-art detector and to find new technologies that can reach maturity by the time the ESS will operate at full potential. This paper presents the key detector requirements for three of the Time-of-Flight (TOF) diffraction instrument concepts selected by the Scientific Advisory Committee to advance into the phase of preliminary engineering design. We discuss the detector technologies commonly employed at the existing similar instruments and their major challenges for ESS. The detector technologies selected by the instrument teams to collect the diffraction patterns are also presented. Analytical calculations, Monte-Carlo simulations, and real experimental data are used to develop a generic method to estimate the event rate in the diffraction detectors. We apply this method to make predictions for the future diffraction instruments, and thus provide additional information that can help the instrument teams with the optimisation of the detector designs.

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