20,058 research outputs found
A Riccati type PDE for light-front higher helicity vertices
This paper is based on a curious observation about an equation related to the
tracelessness constraints of higher spin gauge fields. The equation also occurs
in the theory of continuous spin representations of the Poincar\'e group.
Expressed in an oscillator basis for the higher spin fields, the equation
becomes a non-linear partial differential operator of the Riccati type acting
on the vertex functions. The consequences of the equation for the cubic vertex
is investigated in the light-front formulation of higher spin theory. The
classical vertex is completely fixed but there is room for off-shell quantum
corrections.Comment: 27 pages. Updated to published versio
Spin-Peierls transition in the Heisenberg chain with finite-frequency phonons
We study the spin-Peierls transition in a Heisenberg spin chain coupled to optical bond phonons. Quantum Monte Carlo results for systems with up to N=256 spins show unambiguously that the transition occurs only when the spin-phonon coupling α exceeds a critical value α_c. Using sum rules, we show that the phonon spectral function has divergent (for N→∞) weight extending to zero frequency for α<α_c. The phonon correlations decay with distance r as 1/r. This behavior is characteristic for all 0<α<α_c and the q=π phonon does not soften (to zero frequency) at α=α_c.First author draf
BIOBREED – a new project on marker assisted population breeding in wheat with resistance to common bunt
The paper describes the BIOBREED project
Comment on ``Quantum Phase Transition of the Randomly Diluted Heisenberg Antiferromagnet on a Square Lattice''
In Phys. Rev. Lett. 84, 4204 (2000) (cond-mat/9905379), Kato et al. presented
quantum Monte Carlo results indicating that the critical concentration of
random non-magnetic sites in the two-dimensional antiferromagnetic Heisenberg
model equals the classical percolation density; pc=0.407254. The data also
suggested a surprising dependence of the critical exponents on the spin S of
the magnetic sites, with a gradual approach to the classical percolation
exponents as S goes to infinity. I here argue that the exponents in fact are
S-independent and equal to those of classical percolation. The apparent
S-dependent behavior found by Kato et al. is due to temperature effects in the
simulations as well as a quantum effect that masks the true asymptotic scaling
behavior for small lattices.Comment: Comment on Phys. Rev. Lett. 84, 4204 (2000), by K. Kato et al.; 1
page, 1 figur
Magnetoresistance and negative differential resistance in Ni/Graphene/Ni vertical heterostructures driven by finite bias voltage: A first-principles study
Using the nonequilibrium Green function formalism combined with density
functional theory, we study finite-bias quantum transport in Ni/Gr_n/Ni
vertical heterostructures where graphene layers are sandwiched between two
semi-infinite Ni(111) electrodes. We find that recently predicted "pessimistic"
magnetoresistance of 100% for junctions at zero bias voltage , persists up to V, which makes such devices
promising for spin-torque-based device applications. In addition, for parallel
orientations of the Ni magnetizations, the junction exhibits a pronounced
negative differential resistance as the bias voltage is increased from
V to V. We confirm that both of these nonequilibrium effects
hold for different types of bonding of Gr on the Ni(111) surface while
maintaining Bernal stacking between individual Gr layers.Comment: 6 pages, 5 figures, PDFLaTeX; Figure labels correcte
Impurity effects at finite temperature in the two-dimensional S=1/2 Heisenberg antiferromagnet
We discuss effects of various impurities on the magnetic susceptibility and
the specific heat of the quantum S=1/2 Heisenberg antiferromagnet on a
two-dimensional square lattice. For impurities with spin S_i > 0 (here S_i=1/2
in the case of a vacancy or an added spin, and S_i=1 for a spin coupled
ferromagnetically to its neighbors), our quantum Monte Carlo simulations
confirm a classical-like Curie susceptibility contribution S_i^2/4T, which
originates from an alignment of the impurity spin with the local N\'eel order.
In addition, we find a logarithmically divergent contribution, which we
attribute to fluctuations transverse to the local N\'eel vector. We also study
frustrated and nonfrustrated bond impurities with S_i=0. For a simple intuitive
picture of the impurity problem, we discuss an effective few-spin model that
can distinguish between the different impurities and reproduces the
leading-order simulation data over a wide temperature range.Comment: 15 pages, 14 figures, submitted to PRB. v2, published version with
cosmetic change
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