Demonstration of one-parameter scaling at the Dirac point in graphene

We numerically calculate the conductivity $\sigma$ of an undoped graphene sheet (size $L$) in the limit of vanishingly small lattice constant. We demonstrate one-parameter scaling for random impurity scattering and determine the scaling function $\beta(\sigma)=d\ln\sigma/d\ln L$. Contrary to a recent prediction, the scaling flow has no fixed point ($\beta>0$) for conductivities up to and beyond the symplectic metal-insulator transition. Instead, the data supports an alternative scaling flow for which the conductivity at the Dirac point increases logarithmically with sample size in the absence of intervalley scattering -- without reaching a scale-invariant limit.Comment: 4 pages, 5 figures; v2: introduction expanded, data for Gaussian model extended to larger system sizes to further demonstrate single parameter scalin

Magnetic-field and chemical-potential effects on the low-energy separation

We show that in the presence of a magnetic field the usual low-energy separation of the Hubbard chain is replaced by a ``$c$'' and ``$s$'' separation. Here $c$ and $s$ refer to small-momentum and low-energy independent excitation modes which couple both to charge and spin. Importantly, we find the exact generators of these excitations both in the electronic and pseudoparticle basis. In the limit of zero magnetic field these generators become the usual charge and spin fluctuation operators. The $c$ and $s$ elementary excitations are associated with the $c$ and $s$ pseudoparticles, respectively. We also study the separate pseudoparticle left and right conservation laws. In the presence of the magnetic field the small-momentum and low-energy excitations can be bosonized. However, the suitable bosonization corresponds to the $c$ and $s$ pseudoparticle modes and not to the usual charge and spin fluctuations. We evaluate exactly the commutator between the electronic-density operators. Its spin-dependent factor is in general non diagonal and depends on the interaction. The associate bosonic commutation relations characterize the present unconventional low-energy separation.Comment: 29 pages, latex, submitted to Phys. Rev.

Effect of antimony on the eutectic reaction of heavy section spheroidal graphite castings

There is a strong demand for heavy section castings made of spheroidal graphite with a fully ferritic matrix, e.g. for manufacturing hubs for windmills. Such castings with slow solidification process are prone to graphite degeneration that leads to a dramatic decrease of the mechanical properties of the cast parts. Chunky graphite is certainly the most difficult case of graphite degeneracy, though it has long been known that the limited and controlled addition of antimony may help eliminate it. The drawback of this remedy is that too large Sb additions lead to other forms of degenerate graphite, and also that antimony is a pearlite promoter. As part of an investigation aimed at mastering low level additions to cast iron melts before casting, solidification of large blocks with or without Sb added was followed by thermal analysis. Comparison of the cooling curves and of the microstructures of these different castings gives suggestions to understand the controlling nucleation and growth mechanisms for chunky graphite cells

Dynamical charge and spin density wave scattering in cuprate superconductor

We show that a variety of spectral features in high-T_c cuprates can be understood from the coupling of charge carriers to some kind of dynamical order which we exemplify in terms of fluctuating charge and spin density waves. Two theoretical models are investigated which capture different aspects of such dynamical scattering. The first approach leaves the ground state in the disordered phase but couples the electrons to bosonic degrees of freedom, corresponding to the quasi singular scattering associated with the closeness to an ordered phase. The second, more phenomological approach starts from the construction of a frequency dependent order parameter which vanishes for small energies. Both theories capture scanning tunneling microscopy and angle-resoved photoemission experiments which suggest the protection of quasiparticles close to the Fermi energy but the manifestation of long-range order at higher frequencies.Comment: 27 pages, 13 figures, to appear in New J. Phy

Conditional nationally determined contributions in the Paris Agreement: foothold for equity or Achilles heel?

The Paris Agreement’s success depends on parties’ implementation of their Nationally Determined Contributions (NDCs) towards the Paris Agreement’s goals. In these climate action plans, most developing countries make their mitigation and adaptation contributions conditional upon receiving international support (finance, technology transfer and/or capacity building). While provision of support for NDC implementation could enhance equity among countries, the feasibility of NDC implementation might be challenged by the large number of conditional NDCs. This paper addresses the implications of this tension based on an analysis of all 168 NDCs. We find that feasibility is challenged because conditions applied to NDCs are often not well defined. Moreover, the costs of implementing all conditional contributions are too high to be covered by existing promises of support from developed countries, even if the entire annual $100 billion of climate finance were earmarked for NDC implementation. Consistent with principles of equity and the prioritization in the Paris Agreement, a higher proportion of Least Developed Countries (LDCs) and Small Island Developing States (SIDS) have conditional NDCs than do other countries. However, differences between the distribution of countries requesting support and those currently receiving support, in particular among middle-income countries, demonstrates potential tensions between feasibility and equity. The article concludes with recommendations on how cost estimates and updated NDCs can be strengthened to ensure support for NDC implementation is targeted more equitably and cost-effectively. Key policy insights -Support requested by developing countries to implement conditional NDCs far exceeds existing funding pledges. -Differences between existing patterns of financial assistance, and those implied by requests under conditional NDCs, mean that supporting NDCs may require a significant shift in provider countries’ priorities for allocating climate finance. This may challenge feasibility. -The Paris Agreement’s provisions on prioritizing LDCs and SIDS offer valuable guidance in making difficult choices on allocating support. -To increase the likelihood of attracting support, developing countries (assisted by capacity building as needed), should include credible cost estimates in future NDCs and formulate investment plans. -By outlining plans to mobilize support in their NDCs, developed countries can reassure developing countries that raising the ambition of NDCs is feasibl

Self-Organized Ordering of Nanostructures Produced by Ion-Beam Sputtering

We study the self-organized ordering of nanostructures produced by ion-beam sputtering (IBS) of targets amorphizing under irradiation. By introducing a model akin to models of pattern formation in aeolian sand dunes, we extend consistently the current continuum theory of erosion by IBS. We obtain new non-linear effects responsible for the in-plane ordering of the structures, whose strength correlates with the degree of ordering found in experiments. Our results highlight the importance of redeposition and surface viscous flow to this nanopattern formation process.Comment: 4 pages, 2 figure

Fitossociologia de um campo cerrado no Pantanal da Nhecolândia, Corumbá, MS.

Os campos cerrados há muito tempo vêm sendo ocupados para a criação extensiva de gado bovino no Pantanal de Mato Grosso do Sul. Apesar da grande importância dessa fisionomia para a pecuária na região, ainda são poucos os estudos de estrutura e composição florística nesse tipo de fisionomia. Desse modo, este trabalho teve como objetivo realizar um levantamento florístico e fitossociológico das espécies lenhosas de uma área de campo cerrado na fazenda Nhumirim, sub-região da Nhecolândia. As amostragens da vegetação foram feitas pelo método de quadrantes móveis, sendo incluídos somente indivíduos com diâmetro ao nível do solo (DAS) ≥ 1 cm. Os parâmetros fitossociológicos foram estimados com auxílio do programa FITOPAC 1, além das similaridades florística (Índice de Jaccard) e proporcional da área. Registrou-se 187 indivíduos pertencentes a 23 espécies, 22 gêneros e 17 famílias. A densidade total foi de 1.670 indivíduos/hectare. A altura média dos indivíduos foi de 1,6 m, com amplitude de 0,4 m a 7,5 m. As espécies com maior Índice Valor de Importância e número de indivíduos foram Annona dioica, Byrsonima cydoniifolia e Mouriri elliptica (81, 39 e 13 indivíduos, respectivamente). A área estudada apresentou um baixo índice de similaridade florística quando comparada com as áreas de campo cerrado nas sub-regiões de Poconé e Paiaguás no Pantanal (MS e MT, respectivamente). No entanto, a similaridade proporcional foi maior, evidenciando semelhança na estrutura das formações vegetais comparadas, três das cinco áreas comparadas apresentaram índices acima de 30%, sendo Byrsonima cydoniifolia a principal espécie que contribuiu para a essa similaridade. Tanto no Paraná como em São Paulo os valores para ambos os índices foram mais baixos do que os encontrados na comparação com as sub-regiões do Pantanal. Na área de campo cerrado estudada Annona dioica, Byrsonima cydoniifolia, Mouriri elliptica foram as espécies mais importantes, sendo a área considerada pobre em relação à riqueza de espécies quando comparada com outras áreas de campo cerrado.bitstream/item/76172/1/BP119.pd

Theory of Spin Fluctuations in Striped Phases of Doped Antiferromagnetic Cuprates

We study the properties of generalized striped phases of doped cuprate planar quantum antiferromagnets. We invoke an effective, spatially anisotropic, non-linear sigma model in two space dimensions. Our theoretical predictions are in quantitative agreement with recent experiments in La_{2-x}Sr_xCuO_4 with $0 \leq x \leq 0.018$. We focus on (i) the magnetic correlation length, (ii) the staggered magnetization at $T=0$ and (iii) the N\'eel temperature, as functions of doping, using parameters determined previously and independently for this system. These results support the proposal that the low doping (antiferromagnetic) phase of the cuprates has a striped configuration.Comment: 4 pages, Revtex. To appear in the Proceedings of the International Conference "Stripes, Lattice Instabilities and High Tc Superconductivity", (Rome, Dec. 1996

On the Stability and Single-Particle Properties of Bosonized Fermi Liquids

We study the stability and single-particle properties of Fermi liquids in spatial dimensions greater than one via bosonization. For smooth non-singular Fermi liquid interactions we obtain Shankar's renormalization- group flows and reproduce well known results for quasi-particle lifetimes. We demonstrate by explicit calculation that spin-charge separation does not occur when the Fermi liquid interactions are regular. We also explore the relationship between quantized bosonic excitations and zero sound modes and present a concise derivation of both the spin and the charge collective mode equations. Finally we discuss some aspects of singular Fermi liquid interactions.Comment: 13 pages plus three postscript figures appended; RevTex 3.0; BUP-JBM-