6,538 research outputs found
Renyi Entropy and Parity Oscillations of the Anisotropic Spin-s Heisenberg Chains in a Magnetic Field
Using the density matrix renormalization group, we investigate the Renyi
entropy of the anisotropic spin-s Heisenberg chains in a z-magnetic field. We
considered the half-odd integer spin-s chains, with s=1/2,3/2 and 5/2, and
periodic and open boundary conditions. In the case of the spin-1/2 chain we
were able to obtain accurate estimates of the new parity exponents
and that gives the power-law decay of the
oscillations of the Renyi entropy for periodic and open boundary
conditions, respectively. We confirm the relations of these exponents with the
Luttinger parameter , as proposed by Calabrese et al. [Phys. Rev. Lett. 104,
095701 (2010)]. Moreover, the predicted periodicity of the oscillating term was
also observed for some non-zero values of the magnetization . We show that
for the amplitudes of the oscillations are quite small, and get
accurate estimates of and become a
challenge. Although our estimates of the new universal exponents
and for the spin-3/2 chain are not so
accurate, they are consistent with the theoretical predictions.Comment: revised version, accepted to PRB. 9 pages, 3 Figures, 4 Table
Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders
Using the Density Matrix Renormalization Group and two-leg ladders, we
investigate an electronic two-orbital Hubbard model including plaquette
diagonal hopping amplitudes. Our goal is to search for regimes where charges
added to the undoped state form pairs, presumably a precursor of a
superconducting state.For the electronic density , i.e. the undoped
limit, our investigations show a robust antiferromagnetic ground
state, as in previous investigations. Doping away from and for large
values of the Hund coupling , a ferromagnetic region is found to be stable.
Moreover, when the interorbital on-site Hubbard repulsion is smaller than the
Hund coupling, i.e. for in the standard notation of multiorbital Hubbard
models, our results indicate the coexistence of pairing tendencies and
ferromagnetism close to . These results are compatible with previous
investigations using one dimensional systems. Although further research is
needed to clarify if the range of couplings used here is of relevance for real
materials, such as superconducting heavy fermions or pnictides, our theoretical
results address a possible mechanism for pairing that may be active in the
presence of short-range ferromagnetic fluctuations.Comment: 8 pages, 4 Fig
Minimum Length from Quantum Mechanics and Classical General Relativity
We derive fundamental limits on measurements of position, arising from
quantum mechanics and classical general relativity. First, we show that any
primitive probe or target used in an experiment must be larger than the Planck
length, . This suggests a Planck-size {\it minimum ball} of uncertainty in
any measurement. Next, we study interferometers (such as LIGO) whose precision
is much finer than the size of any individual components and hence are not
obviously limited by the minimum ball. Nevertheless, we deduce a fundamental
limit on their accuracy of order . Our results imply a {\it device
independent} limit on possible position measurements.Comment: 8 pages, latex, to appear in the Physical Review Letter
Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys
Inelastic and elastic neutron scattering have been used to study a single
crystal of the NiMnAl Heusler alloy over a broad
temperature range. The paper reports the first experimental determination of
the low-lying phonon dispersion curves for this alloy system. We find that the
frequencies of the TA modes are relatively low. This branch exhibits an
anomaly (dip) at a wave number , which softens with
decreasing temperature. Associated with this anomalous dip at , an
elastic central peak scattering is also present. We have also observed
satellites due to the magnetic ordering.Comment: 6 pages, 6 figures. Accepted for publication in the Physical Review
Avaliação do potencial de estirpes de rizóbio em fixar nitrogênio associadas ao feijão-caupi em Petrolina-PE.
O feijão-caupi é uma cultura leguminosa de grande importância para a alimentação humana, constituindo-se em uma fonte de proteína para a maioria da população rural. A eficiência de diferentes estirpes de rizóbio em fixar o nitrogênio atmosférico em associação com o feijão-caupi foi testada ao nível de campo na Estação Experimental da Caatinga, da Embrapa Semi-Árido, localizada no município de Petrolina-PE (09o23'S e 40o30'W, altitude: 350 m). O experimento foi implantado em condições de sequeiro, com delineamento de blocos ao acaso e seis repetições, constando de seis tratamentos, sendo quatro estirpes de rizóbio - BR 3267, BR 3299, BR 33o1 e BR 3302, uma dose de nitrogênio mineral - 50 kg ha-1 e uma testemunha. Aos trinta dias após o plantio, o número e peso seco de nódulos, peso de matéria seca e concentração de nitrogênio na parte aérea da planta foram determinados. Todas as estirpes superaram significativamente o tratamento testemunha, tanto em número como em peso seco de nódulos, denotando uma alta eficiência em nodular as raízes de feijão-caupi em relação às estirpes de rizóbio existentes no solo da área experimental
Unfolding Rates for the Diffusion-Collision Model
In the diffusion-collision model, the unfolding rates are given by the
likelihood of secondary structural cluster dissociation. In this work, we
introduce an unfolding rate calculation for proteins whose secondary structural
elements are -helices, modeled from thermal escape over a barrier which
arises from the free energy in buried hydrophobic residues. Our results are in
good agreement with currently accepted values for the attempt rate.Comment: Shorter version of cond-mat/0011024 accepted for publication in PR
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