456 research outputs found
Deviations from Matthiessen rule and resistivity saturation effects in Gd and Fe
According to earlier first-principles calculations, the spin-disorder
contribution to the resistivity of rare-earth metals in the paramagnetic state
is strongly underestimated if Matthiessen's rule is assumed to hold. To
understand this discrepancy, the resistivity of paramagnetic Fe and Gd is
evaluated by taking into account both spin and phonon disorder. Calculations
are performed using the supercell approach within the linear muffin-tin orbital
method. Phonon disorder is modeled by introducing random displacements of the
atomic nuclei, and the results are compared with the case of fictitious
Anderson disorder. In both cases the resistivity shows a nonlinear dependence
on the square of the disorder potential, which is interpreted as a resistivity
saturation effect. This effect is much stronger in Gd than in Fe. The
non-linearity makes the phonon and spin-disorder contributions to the
resistivity non-additive, and the standard procedure of extracting the
spin-disorder resistivity by extrapolation from high temperatures becomes
ambiguous. An "apparent" spin-disorder resistivity obtained through such
extrapolation is in much better agreement with experiment compared to the
results obtained by considering only spin disorder. By analyzing the spectral
function of the paramagnetic Gd in the presence of Anderson disorder, the
resistivity saturation is explained by the collapse of a large area of the
Fermi surface due to the disorder-induced mixing between the electronic and
hole sheets.Comment: 9 pages, 7 figure
Interaction induced Dirac fermions from quadratic band touching in bilayer graphene
We revisit the effect of local interactions on the quadratic band touching
(QBT) of Bernal stacked bilayer graphene models using renormalization group
(RG) arguments and quantum Monte Carlo simulations of the Hubbard model. We
present an RG argument which predicts, contrary to previous studies, that weak
interactions do not flow to strong coupling even if the free dispersion has a
QBT. Instead they generate a linear term in the dispersion, which causes the
interactions to flow back to weak coupling. Consistent with this RG scenario,
in unbiased quantum Monte Carlo simulations of the Hubbard model we find
compelling evidence that antiferromagnetism turns on at a finite , despite
the hopping problem having a QBT. The onset of antiferromagnetism takes
place at a continuous transition which is consistent with a dynamical critical
exponent as expected for 2+1 d Gross-Neveu criticality. We conclude that
generically in models of bilayer graphene, even if the free dispersion has a
QBT, small local interactions generate a Dirac phase with no symmetry breaking
and that there is a finite-coupling transition out of this phase to a
symmetry-broken state
Spectral signatures of thermal spin disorder and excess Mn in half-metallic NiMnSb
Effects of thermal spin disorder and excess Mn on the electronic spectrum of
half-metallic NiMnSb are studied using first-principles calculations.
Temperature-dependent spin disorder, introduced within the vector disordered
local moment model, causes the valence band at the point to broaden
and shift upwards, crossing the Fermi level and thereby closing the
half-metallic gap above room temperature. The spectroscopic signatures of
excess Mn on the Ni, Sb, and empty sites (Mn, Mn,
and Mn) are analyzed. Mn is spectroscopically
invisible. The relatively weak coupling of Mn and Mn
spins to the host strongly deviates from the Heisenberg model, and the spin of
Mn is canted in the ground state. While the half-metallic gap is
preserved in the collinear ground state of Mn, thermal spin
disorder of the weakly coupled Mn spins destroys it at low
temperatures. This property of Mn may be the source of the
observed low-temperature transport anomalies.Comment: 5 pages, 7 figures, updated version with minor revisions and an
additional figure, accepted in Phys. Rev. B (Rapid Communication
Spontaneous Currents in Spinless Fermion Lattice Models at the Strong-Coupling Limit
What kind of lattice Hamiltonian manifestly has an ordered state with
spontaneous orbital currents? We consider interacting spinless fermions on an
array of square plaquettes, connected by weak hopping; the array geometry may
be a 2 x 2L ladder, a 2 x 2 x 2L "tube", or a 2L x 2L square grid. At half
filling, we derive an effective Hamiltonian in terms of pseudospins, of which
one component represents orbital currents, and find the conditions sufficient
for orbital current long-range order. We consider spinfull variants of the
aforesaid spinless models and make contact with other spinfull models in the
literature purported to possess spontaneous currents.Comment: added two new references following recent communicatio
Gossip Codes for Fingerprinting: Construction, Erasure Analysis and Pirate Tracing
This work presents two new construction techniques for q-ary Gossip codes
from tdesigns and Traceability schemes. These Gossip codes achieve the shortest
code length specified in terms of code parameters and can withstand erasures in
digital fingerprinting applications. This work presents the construction of
embedded Gossip codes for extending an existing Gossip code into a bigger code.
It discusses the construction of concatenated codes and realisation of erasure
model through concatenated codes.Comment: 28 page
Gradient Based Hybridization of PSO
Particle Swarm Optimization (PSO) has emerged as a powerful metaheuristic
global optimization approach over the past three decades. Its appeal lies in
its ability to tackle complex multidimensional problems that defy conventional
algorithms. However, PSO faces challenges, such as premature stagnation in
single-objective scenarios and the need to strike a balance between exploration
and exploitation. Hybridizing PSO by integrating its cooperative nature with
established optimization techniques from diverse paradigms offers a promising
solution. In this paper, we investigate various strategies for synergizing
gradient-based optimizers with PSO. We introduce different hybridization
principles and explore several approaches, including sequential decoupled
hybridization, coupled hybridization, and adaptive hybridization. These
strategies aim to enhance the efficiency and effectiveness of PSO, ultimately
improving its ability to navigate intricate optimization landscapes. By
combining the strengths of gradient-based methods with the inherent social
dynamics of PSO, we seek to address the critical objectives of intelligent
exploration and exploitation in complex optimization tasks. Our study delves
into the comparative merits of these hybridization techniques and offers
insights into their application across different problem domains
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