4,011 research outputs found
Flexstab on the IBM 360
FLEXSTAB, an array of computer programs developed on CDC equipment, has been converted to operate on the IBM 360 computation system. Instructions for installing, validating, and operating FLEXSTAB on the IBM 360 are included. Hardware requirements are itemized and supplemental materials describe JCL sequences, the CDC to IBM conversion, the input output subprograms, and the interprogram data flow
Dominant particle-hole contributions to the phonon dynamics in the spinless one-dimensional Holstein model
In the spinless Holstein model at half-filling the coupling of electrons to
phonons is responsible for a phase transition from a metallic state at small
coupling to a Peierls distorted insulated state when the electron-phonon
coupling exceeds a critical value. For the adiabatic case of small phonon
frequencies, the transition is accompanied by a phonon softening at the
Brillouin zone boundary whereas a hardening of the phonon mode occurs in the
anti-adiabatic case. The phonon dynamics studied in this letter do not only
reveal the expected renormalization of the phonon modes but also show
remarkable additional contributions due to electronic particle-hole
excitations.Comment: 7 pages, 4 figures and 1 table included; v2: discussion of Luttinger
liquid parameters adde
Phase separation in the Edwards model
The nature of charge transport within a correlated background medium can be
described by spinless fermions coupled to bosons in the model introduced by
Edwards. Combining numerical density matrix renormalization group and
analytical projector-based renormalization methods we explore the ground-state
phase diagram of the Edwards model in one dimension. Below a critical boson
frequency any long-range order disappears and the system becomes metallic. If
the charge carriers are coupled to slow quantum bosons the Tomonaga-Luttinger
liquid is attractive and finally makes room for a phase separated state, just
as in the t-J model. The phase boundary separating repulsive from the
attractive Tomonaga-Luttinger liquid is determined from long-wavelength charge
correlations, whereas fermion segregation is indicated by a vanishing inverse
compressibility. On approaching phase separation the photoemission spectra
develop strong anomalies.Comment: 6 pages, 5 figures, final versio
Coexistence of superconductivity and charge-density waves in a two-dimensional Holstein model at half-filling
In one dimension the coupling of electrons to phonons leads to a transition
from a metallic to a Peierls distorted insulated state if the coupling exceeds
a critical value. On the other hand, in two dimensions the electron-phonon
interaction may also lead to the formation of Cooper pairs. This competition of
superconductivity and charge order (in conjunction with a lattice distortion)
is studied in this letter by means of the projector-based renormalization
method (PRM). Increasing the electron-phonon interaction, we find a crossover
behavior between a purely superconducting state and a charge-density wave where
a well-defined parameter range of coexistence of superconductivity and lattice
distortion exists.Comment: 11 pages, 2 figure
Constructive procedures to solve 2-dimensional bin packing problems with irregular pieces and guillotine cuts
This paper presents an approach for solving a new real problem in cutting and packing. At its core is an innovative mixed integer programme model that places irregular pieces and defines guillotine cuts. The two-dimensional irregular shape bin packing problem with guillotine constraints arises in the glass cutting industry, for example, the cutting of glass for conservatories. Almost all cutting and packing problems that include guillotine cuts deal with rectangles only, where all cuts are orthogonal to the edges of the stock sheet and a maximum of two angles of rotation are permitted. The literature tackling packing problems with irregular shapes largely focuses on strip packing i.e. minimizing the length of a single fixed width stock sheet, and does not consider guillotine cuts. Hence, this problem combines the challenges of tackling the complexity of packing irregular pieces with free rotation, guaranteeing guillotine cuts that are not always orthogonal to the edges of the stock sheet, and allocating pieces to bins. To our knowledge only one other recent paper tackles this problem. We present a hybrid algorithm that is a constructive heuristic that determines the relative position of pieces in the bin and guillotine constraints via a mixed integer programme model. We investigate two approaches for allocating guillotine cuts at the same time as determining the placement of the piece, and a two phase approach that delays the allocation of cuts to provide flexibility in space usage. Finally we describe an improvement procedure that is applied to each bin before it is closed. This approach improves on the results of the only other publication on this problem, and gives competitive results for the classic rectangle bin packing problem with guillotine constraint
Spectral signatures of the BCS-BEC crossover in the excitonic insulator phase of the extended Falicov-Kimball model
We explore the spontaneous formation of an excitonic insulator state at the
semimetal-semiconductor transition of mixed-valence materials in the framework
of the spinless Falicov-Kimball model with direct - electron hopping.
Adapting the projector-based renormalization method, we obtain a set of
renormalization differential equations for the extended Falicov-Kimball model
parameters and finally derive analytical expressions for the order parameter,
as well as for the renormalized - and -electron dispersions, momentum
distributions, and wave-vector resolved single-particle spectral functions. Our
numerical results proved the valence transition picture, related to the
appearance of the excitonic insulator phase, in the case of overlapping and
bands. Thereby the photoemission spectra show significant differences
between the weak-to-intermediate and intermediate-to-strong Coulomb attraction
regimes, indicating a BCS-BEC transition of the excitonic condensate.Comment: final version, minor corrections in the text, references update
Multiwavelength Observations of Small-Scale Reconnection Events triggered by Magnetic Flux Emergence in the Solar Atmosphere
The interaction between emerging magnetic flux and the pre-existing ambient
field has become a "hot" topic for both numerical simulations and
high-resolution observations of the solar atmosphere. The appearance of
brightenings and surges during episodes of flux emergence is believed to be a
signature of magnetic reconnection processes. We present an analysis of a
small-scale flux emergence event in NOAA 10971, observed simultaneously with
the Swedish 1-m Solar Telescope on La Palma and the \emph{Hinode} satellite
during a joint campaign in September 2007. Extremely high-resolution G-band,
H, and \ion{Ca}{2} H filtergrams, \ion{Fe}{1} and \ion{Na}{1}
magnetograms, EUV raster scans, and X-ray images show that the emerging region
was associated with chromospheric, transition region and coronal brightenings,
as well as with chromospheric surges. We suggest that these features were
caused by magnetic reconnection at low altitude in the atmosphere. To support
this idea, we perform potential and linear force-free field extrapolations
using the FROMAGE service. The extrapolations show that the emergence site is
cospatial with a 3D null point, from which a spine originates. This magnetic
configuration and the overall orientation of the field lines above the emerging
flux region are compatible with the structures observed in the different
atmospheric layers, and remain stable against variations of the force-free
field parameter. Our analysis supports the predictions of recent 3D numerical
simulations that energetic phenomena may result from the interaction between
emerging flux and the pre-existing chromospheric and coronal field.Comment: In press for Ap
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