Spin waves in quasi-equilibrium spin systems

Using the Landau Fermi liquid theory we have discovered a new regime for the propagation of spin waves in a quasi-equilibrium spin systems. We have determined the dispersion relation for the transverse spin waves and found that one of the modes is gapless. The gapless mode corresponds to the precessional mode of the magnetization in a paramagnetic system in the absence of an external magnetic field. One of the other modes is gapped which is associated with the precession of the spin current around the internal field. The gapless mode has a quadratic dispersion leading to some interesting thermodynamic properties including a $T^{3/2}$ contribution to the specific heat. We also show that these modes make significant contributions to the dynamic structure function.Comment: 4 pages, 3 figure

Incommensurate spin resonance in URu2Si2

We focus on inelastic neutron scattering in $URu_2Si_2$ and argue that observed gap in the fermion spectrum naturally leads to the spin feature observed at energies $\omega_{res} = 4-6 meV$ at momenta at \bQ^* = (1\pm 0.4, 0,0). We discuss how spin features seen in $URu_2Si_2$ can indeed be thought of in terms of {\em spin resonance} that develops in HO state and is {\em not related} to superconducting transition at 1.5K. In our analysis we assume that the HO gap is due to a particle-hole condensate that connects nested parts of the Fermi surface with nesting vector $\bf{Q}^*$. Within this approach we can predicted the behavior of the spin susceptibility at \bQ^* and find it to be is strikingly similar to the phenomenology of resonance peaks in high-T$_c$ and heavy fermion superconductors. The energy of the resonance peak scales with $T_{HO}$ $\omega_{res} \simeq 4 k_BT_{HO}$. We discuss observable consequences spin resonance will have on neutron scattering and local density of states.Comment: 8 pgaes latex, 4 fig

Remarks on the tight-binding model of graphene

We address a simple but fundamental issue arising in the study of graphene, as well as of other systems that have a crystalline structure with more than one atom per unit cell. For these systems, the choice of the tight-binding basis is not unique. For monolayer graphene two bases are widely used in the literature. While the expectation values of operators describing physical quantities should be independent of basis, the form of the operators may depend on the basis, especially in the presence of disorder or of an applied magnetic field. Using the inappropriate form of certain operators may lead to erroneous physical predictions. We discuss the two bases used to describe monolayer graphene, as well as the form of the most commonly used operators in the two bases. We repeat our analysis for the case of bilayer graphene.Comment: 15 pages, 4 figure

X-ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

Magnesium borohydride (Mg(BH4)(2)) is an attractive compound for solid-state hydrogen storage due to its lucratively high hydrogen densities and theoretically low operational temperature. Hydrogen release from Mg(BH4)(2) occurs through several steps. The reaction intermediates formed at these steps have been extensively studied for a decade. In this work, we apply spectroscopic methods that have rarely been used in such studies to provide alternative insights into the nature of the reaction intermediates. The commercially obtained sample was decomposed in argon flow during thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) to differentiate between the H-2-desorption reaction steps. The reaction products were analyzed by powder X-ray diffraction (PXRD), near edge soft X-ray absorption spectroscopy at boron K-edge (NEXAFS), and synchrotron infrared (IR) spectroscopy in mid- and far-IR ranges (SR-FTIR). Up to 12 wt% of H-2 desorption was observed in the gravimetric measurements. PXRD showed no crystalline decomposition products when heated at 260-280 degrees C, the formation of MgH2 above 300 degrees C, and Mg above 320 degrees C. The qualitative analysis of the NEXAFS data showed the presence of boron in lower oxidation states than in (BH4)(-). The NEXAFS data also indicated the presence of amorphous boron at and above 340 degrees C. This study provides additional insights into the decomposition reaction of Mg(BH4)(2)

X‐ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

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Microcoining ripples in metal foils

Experiments, upper bound models, and finite element simulations are used to determine forming loads needed to microcoin surface ripples in thin metal foils. Coining is traditionally performed in a closed die, however enclosing all non-patterned surfaces is difficult to directly scale down to sub-millimeter foils. We find different forming regimes can exist at this small scale in an open pressing configuration. We explore the effects of the metal foil thickness and its work hardening behavior, two primary factors controlling the microcoining ripple forming load. For very thin foils, the load needed to coin a ripple pattern is lower than the load needed to compress the foil so that the open pressing configuration behavior is effectively closed with pattern formation without thickness change. For moderate thickness foils, the load needed to coin significantly drops as the entire foil compresses. For thick foils approaching bulk materials, the pattern will not completely form as the die macroscopically indents into the metal. Work hardening is found to raise the forming load for the thin, effectively closed die scenario, however it is a secondary effect at moderate thickness. This insight is used to microcoin patterns in extremely hard, thin metal foils

Agricultural innovation and adaptation to climate change: empirical evidence from diverse agro-ecologies in South Asia

While impacts of climate change on agricultural systems have been widely researched, there is still limited understanding of what agricultural practices evolves over time in response to both climatic and non-climatic drivers and how actors mobilize their resources, institutions and practices in South Asia. Through eight case studies and a survey of300 households in 15 locations in India, Nepal and Bangladesh, this paper generates empirical evidence on emerging agricultural interventions in contrasting socio-economic, geographical and agro-ecological contexts. The study shows that several farm practices emerge out in response to multiple drivers over time; some of them can be further adjusted to the challenge of climate change by planned adaptation programs. Most actors, however, have considered private risks in the short run. Although there has been some progress in streamlining climate change into strategic planning in different countries of South Asia, policy, research and extension systems lack adequate attention to wider resilience of the system. Based on this analysis, we recommend that adaptation policies should complement farmers’ responses to climate change through informed research and extension systems and pro-poor government policies that improve adaptation and coordinate activities of different actors