Phonon plasmon interaction in ternary group-III-nitrides

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 101, 041909 (2012) and may be found at https://doi.org/10.1063/1.4739415.Phonon-plasmon-coupling in the ternary group-III-nitrides InGaN and AlGaN is investigated experimentally and theoretically. Based on the observation of broadening and shifting of the A1(LO) mode in AlGaN upon Si-doping, a lineshape analysis was performed to determine the carrier concentration. The results obtained by this method are in excellent agreement to those from Hall measurements, confirming the validity of the employed model. Finally, neglecting phonon and plasmon damping, the Raman shift of the A1(LO) mode in dependence of the carrier concentration for AlGaN and InGaN is calculated. This enables a fast and contactless determination of carrier concentrations in the future.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Mice with Reduced NMDA Receptor Expression Display Behaviors Related to Schizophrenia

AbstractN-methyl-D-aspartate receptors (NMDARs) represent a subclass of glutamate receptors that play a critical role in neuronal development and physiology. We report here the generation of mice expressing only 5% of normal levels of the essential NMDAR1 (NR1) subunit. Unlike NR1 null mice, these mice survive to adulthood and display behavioral abnormalities, including increased motor activity and stereotypy and deficits in social and sexual interactions. These behavioral alterations are similar to those observed in pharmacologically induced animal models of schizophrenia and can be ameliorated by treatment with haloperidol or clozapine, antipsychotic drugs that antagonize dopaminergic and serotonergic receptors. These findings support a model in which reduced NMDA receptor activity results in schizophrenic-like behavior and reveals how pharmacological manipulation of monoaminergic pathways can affect this phenotype

Long-term warming alters the composition of Arctic soil microbial communities

Despite the importance of Arctic soils in the global carbon cycle, we know very little of the impacts of warming on the soil microbial communities that drive carbon and nutrient cycling in these ecosystems. Over a 2-year period, we monitored the structure of soil fungal and bacterial communities in organic and mineral soil horizons in plots warmed by greenhouses for 18 years and in control plots. We found that microbial communities were stable over time but strongly structured by warming. Warming led to significant reductions in the evenness of bacterial communities, while the evenness of fungal communities increased significantly. These patterns were strongest in the organic horizon, where temperature change was greatest and were associated with a significant increase in the dominance of the Actinobacteria and significant reductions in the Gemmatimonadaceae and the Proteobacteria. Greater evenness of the fungal community with warming was associated with significant increases in the ectomycorrhizal fungi, Russula spp., Cortinarius spp., and members of the Helotiales suggesting that increased growth of the shrub Betula nana was an important mechanism driving this change. The shifts in soil microbial community structure appear sufficient to account for warming-induced changes in nutrient cycling in Arctic tundra as climate warm

Electronic structure and magnetism of equiatomic FeN

In order to investigate the phase stability of equiatomic FeN compounds and the structure-dependent magnetic properties, the electronic structure and total energy of FeN with NaCl, ZnS and CsCl structures and various magnetic configurations are calculated using the first-principles TB-LMTO-ASA method. Among all the FeN phases considered, the antiferromagnetic NaCl structure with q=(00pi) is found to have the lowest energy at the theoretical equilibrium volume. However, the FM NaCl phase lies only 1mRyd higher. The estimated equilibrium lattice constant for nonmagnetic ZnS-type FeN agrees quite well with the experimental value, but for the AFM NaCl phase the estimated value is 6.7% smaller than that observed experimentally. For ZnS-type FeN, metastable magnetic states are found for volumes larger than the equilibrium value. On the basis of an analysis of the atom- and orbital-projected density of states and orbital-projected Crystal Orbital Hamilton Population, the iron-nitrogen interactions in NM ZnS, AFM NaCl and FM CsCl structures are discussed. The leading Fe-N interactions is due to the d-p iron-nitrogen hybridization, while considerable s-p and p-p hybridizations are also observed in all three phases. The iron magnetic moment in FeN is found to be highly sensitive to the nearest-neighboring Fe-N distance. In particular, the magnetic moment shows an abrupt drop from a value of about 2 muB to zero with the reduction of the Fe-N distance for the ZnS and CsCl structures.Comment: 12 pages, 6 figure

Spontaneous separation of two-component Fermi gases in a double-well trap

The two-component Fermi gas in a double-well trap is studied using the density functional theory and the density profile of each component is calculated within the Thomas-Fermi approximation. We show that the two components are spatially separate in the two wells once the repulsive interaction exceeds the Stoner point, signaling the occurrence of the ferromagnetic transition. Therefore, the double-well trap helps to explore itinerant ferromagnetism in atomic Fermi gases, since the spontaneous separation can be examined by measuring component populations in one well.Comment: 6 pages, 6 figures, to appear in ep

Field-dependent AC susceptibility of itinerant ferromagnets

Whereas dc measurements of magnetic susceptibility, $\chi$, fail to distinguish between local and weak itinerant ferromagnets, radio-frequency (rf) measurements of $\chi$ in the ferromagnetic state show dramatic differences between the two. We present sensitive tunnel-diode resonator measurements of $\chi$ in the weak itinerant ferromagnet ZrZn$_2$ at a frequency of 23 MHz. Below Curie temperature, $T_C \approx 26$ K, the susceptibility is seen to increase and pass through a broad maximum at approximately 15 K in zero applied dc magnetic field. Application of a magnetic field reduces the amplitude of the maximum and shifts it to lower temperatures. The existence and evolution this maximum with applied field is not predicted by either the Stoner or self-consistent renormalized (SCR) spin fluctuations theories. For temperatures below $T_C$ both theories derive a zero-field limit expression for $\chi$. We propose a semi-phenomenological model that considers the effect of the internal field from the polarized fraction of the conduction band on the remaining, unpolarized conduction band electrons. The developed model accurate describes the experimental data

Complex itinerant ferromagnetism in noncentrosymmetric Cr11Ge19

The noncentrosymmetric ferromagnet Cr11Ge19 has been investigated by electrical transport, AC and DC magnetization, heat capacity, x-ray diffraction, resonant ultrasound spectroscopy, and first principles electronic structure calculations. Complex itinerant ferromagnetism in this material is indicated by nonlinearity in conventional Arrott plots, unusual behavior of AC susceptibility, and a weak heat capacity anomaly near the Curie temperature (88 K). The inclusion of spin wave excitations was found to be important in modeling the low temperature heat capacity. The temperature dependence of the elastic moduli and lattice constants, including negative thermal expansion along the c axis at low temperatures, indicate strong magneto-elastic coupling in this system. Calculations show strong evidence for itinerant ferromagnetism and suggest a noncollinear ground state may be expected

Young tableau reconstruction via minors

The tableau reconstruction problem, posed by Monks (2009), asks the following. Starting with a standard Young tableau $T$, a 1-minor of $T$ is a tableau obtained by first deleting any cell of $T$, and then performing jeu de taquin slides to fill the resulting gap. This can be iterated to arrive at the set of $k$-minors of $T$. The problem is this: given $k$, what are the values of $n$ such that every tableau of size $n$ can be reconstructed from its set of $k$-minors? For $k=1$, the problem was recently solved by Cain and Lehtonen. In this paper, we solve the problem for $k=2$, proving the sharp lower bound $n \geq 8$. In the case of multisets of $k$-minors, we also give a lower bound for arbitrary $k$, as a first step toward a sharp bound in the general multiset case.Comment: 24 pages, 18 figure