1,568 research outputs found
Decorous lower bounds for minimum linear arrangement
Minimum Linear Arrangement is a classical basic combinatorial optimization problem from the 1960s, which turns out to be extremely challenging in practice. In particular, for most of its benchmark instances, even the order of magnitude of the optimal solution value is unknown, as testified by the surveys on the problem that contain tables in which the best known solution value often has one more digit than the best known lower bound value. In this paper, we propose a linear-programming based approach to compute lower bounds on the optimum. This allows us, for the first time, to show that the best known solutions are indeed not far from optimal for most of the benchmark instances
Phase Diagram of the 1D Kondo Lattice Model
We determine the boundary of the fully polarized ferromagnetic ground state
in the one dimensional Kondo lattice model at partial conduction electron band
filling by using a newly developed infinite size DMRG method which conserves
the total spin quantum number. The obtained paramagnetic to ferromagnetic phase
boundary is below for the whole range of band filling. By this
we solve the controversy in the phase diagram over the extent of the
ferromagnetic region close to half filling.Comment: 6 pages, 4 EPS figures. Presented at MOS9
Coherence length in superconductors from weak to strong coupling
We study the evolution of the superconducting coherence length from
weak to strong coupling, both within a s-wave and a d-wave lattice model. We
show that the identification of with the Cooper-pair size
in the weak-coupling regime is meaningful only for a fully-gapped (e.g.,
s-wave) superconductor. Instead in the d-wave superconductor, where
diverges, we show that is properly defined as the
characteristic length scale for the correlation function of the modulus of the
superconducting order parameter. The strong-coupling regime is quite
intriguing, since the interplay between particle-particle and particle-hole
channel is no more negligible. In the case of s-wave pairing, which allows for
an analytical treatment, we show that is of order of the lattice
spacing at finite densities. In the diluted regime diverges, recovering
the behavior of the coherence length of a weakly interacting effective bosonic
system. Similar results are expected to hold for d-wave superconductors.Comment: 11 pages, 5 figures. Two appendices and new references adde
Charge and spin inhomogeneity as a key to the physics of the high Tc cuprates
We present a coherent scenario for the physics of cuprate superconductors,
which is based on a charge-driven inhomogeneity, i.e. the ``stripe phase''. We
show that spin and charge critical fluctuations near the stripe instability of
strongly correlated electron systems provide an effective interaction between
the quasiparticles, which is strongly momentum, frequency, temperature and
doping dependent. This accounts for the various phenomena occurring in the
overdoped, optimally and underdoped regimes both for the normal and the
superconductive phase.Comment: 6 pages, 1 enclosed figure, proceedings of LT2
Quantum Ising model in a transverse random field: A density-matrix renormalization group analysis
The spin-1/2 quantum Ising chain in a transverse random magnetic field is
studied by means of the density-matrix renormalization group. The system
evolves from an ordered to a paramagnetic state as the amplitude of the random
field is increased. The dependence of the magnetization on a uniform magnetic
field in the z direction and the spontaneous magnetization as a function of the
amplitude of the transverse random magnetic field are determined. The behavior
of the spin-spin correlation function both above and at criticality is studied.
The scaling laws for magnetization and correlation functions are tested against
previous numerical and renormalization-group results.Comment: 5 pages with 7 figures inside them, proper format of authors' names
use
Effective medium theory for superconducting layers: A systematic analysis including space correlation effects
We investigate the effects of mesoscopic inhomogeneities on the
metal-superconductor transition occurring in several two-dimensional electron
systems. Specifically, as a model of systems with mesoscopic inhomogeneities,
we consider a random-resistor network, which we solve both with an exact
numerical approach and by the effective medium theory. We find that the width
of the transition in these two-dimensional superconductors is mainly ruled by
disorder rather than by fluctuations. We also find that "tail" features in
resistivity curves of interfaces between LaAlO3 or LaTiO3 and SrTiO3 can arise
from a bimodal distribution of mesoscopic local Tc's and/or substantial space
correlations between the mesoscopic domains.Comment: 12 pages, 10 figure
Railway Rolling Stock Planning: Robustness Against Large Disruptions
In this paper we describe a two-stage optimization model for determining robust rolling stock circulations for passenger trains. Here robustness means that the rolling stock circulations can better deal with large disruptions of the railway system. The two-stage optimization model is formulated as a large mixed-integer linear programming (MILP) model. We first use Benders decomposition to determine optimal solutions for the LP-relaxation of this model. Then we use the cuts that were generated by the Benders decomposition for computing heuristic robust solutions for the two-stage optimization model. We call our method Benders heuristic. We evaluate our approach on the real-life rolling stock-planning problem of Netherlands Railways, the main operator of passenger trains in the Netherlands. The computational results show that, thanks to Benders decomposition, the LP-relaxation of the two-stage optimization problem can be solved in a short time for a representative number of disruption scenarios. In addition, they demonstrate that the robust rolling stoc
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