236 research outputs found
Electronic Ladders with SO(5) Symmetry: Phase Diagrams and Correlations at half-filling
We construct a family of electronic ladder models with SO(5) symmetry which
have exact ground states in the form of finitely correlated wave functions.
Extensions for these models preserving this symmetry are studied using these
states in a variational approach. Within this approach, the zero temperature
phase diagram of these electronic ladders at half filling is obtained,
reproducing the known results in the weak coupling (band insulator) and strong
coupling regime, first studied by Scalapino, Zhang and Hanke. Finally, the
compact form of the variational wave functions allows to compute various
correlation functions for these systems.Comment: RevTeX+epsf macros, 23 pp. including figure
Comparing memory skill maintenance across the lifespan : Preservation in adults, increase in children
Thermodynamics of the (1,1/2) Ferrimagnet in Finite Magnetic Fields
We investigate the specific heat and magnetisation of a ferrimagnet with gS=1
and S=1/2 spins in a finite magnetic field using the transfer matrix DMRG down
to T=0.025J. Ferromagnetic gapless and antiferromagnetic gapped excitations for
H=0 lead to rich thermodynamics for H > 0. While the specific heat is
characterized by a generic double peak structure, magnetisation reveals two
critical fields, Hc1=1.76(1) and Hc2=3.00(1) with square-root behaviour in the
T=0 magnetisation. Simple analytical arguments allow to understand these
experimentally accessible findings.Comment: 5 pages, 7 eps figures, uses RevTeX, submitted to PR
Elementary excitations in the gapped phase of a frustrated S=1/2 spin ladder: from spinons to the Haldane triplet
We use the variational matrix-product ansatz to study elementary excitations
in the S=1/2 ladder with additional diagonal coupling, equivalent to a single
S=1/2 chain with alternating exchange and next-nearest neighbor interaction. In
absence of alternation the elementary excitation consists of two free S=1/2
particles ("spinons") which are solitons in the dimer order. When the
nearest-neighbor exchange alternates, the "spinons" are confined into one S=1
excitation being a soliton in the generalized string order. Variational results
are found to be in a qualitative agreement with the exact diagonalization data
for 24 spins. We argue that such an approach gives a reasonably good
description in a wide range of the model parameters.Comment: RevTeX, 13 pages, 11 embedded figures, uses psfig and multico
Phase diagram and hidden order for generalized spin ladders
We investigate the phase diagram of antiferromagnetic spin ladders with
additional exchange interactions on diagonal bonds by variational and numerical
methods. These generalized spin ladders interpolate smoothly between the
chain with competing nn and nnn interactions, the chain with
alternating exchange and the antiferromagnetic chain. The Majumdar-Ghosh
ground states are formulated as matrix product states and are shown to exhibit
the same type of hidden order as the af chain. Generalized matrix product
states are used for a variational calculation of the ground state energy and
the spin and string correlation functions. Numerical (Lanczos) calculations of
the energies of the ground state and of the low-lying excited states are
performed, and compare reasonably with the variational approach. Our results
support the hypothesis that the dimer and Majumdar-Ghosh points are in the same
phase as the af chain.Comment: 23 pages, REVTEX, 7 figure
Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains
We numerically investigate elementary excitations of the Heisenberg
alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically
coupled to each other. Employing a recently developed efficient Monte Carlo
technique as well as an exact diagonalization method, we verify the spin-wave
argument that the model exhibits two distinct excitations from the ground state
which are gapless and gapped. The gapless branch shows a quadratic dispersion
in the small-momentum region, which is of ferromagnetic type. With the
intention of elucidating the physical mechanism of both excitations, we make a
perturbation approach from the decoupled-dimer limit. The gapless branch is
directly related to spin 1's, while the gapped branch originates from
cooperation of the two kinds of spins.Comment: 7 pages, 7 Postscript figures, RevTe
Frustrated quantum Heisenberg ferrimagnetic chains
We study the ground-state properties of weakly frustrated Heisenberg
ferrimagnetic chains with nearest and next-nearest neighbor antiferromagnetic
exchange interactions and two types of alternating sublattice spins S_1 > S_2,
using 1/S spin-wave expansions, density-matrix renormalization group, and
exact- diagonalization techniques. It is argued that the zero-point spin
fluctuations completely destroy the classical commensurate- incommensurate
continuous transition. Instead, the long-range ferrimagnetic state disappears
through a discontinuous transition to a singlet state at a larger value of the
frustration parameter. In the ferrimagnetic phase we find a disorder point
marking the onset of incommensurate real-space short-range spin-spin
correlations.Comment: 16 pages (LaTex 2.09), 6 eps figure
Magnetic excitations in SrCu2O3: a Raman scattering study
We investigated temperature dependent Raman spectra of the one-dimensional
spin-ladder compound SrCu2O3. At low temperatures a two-magnon peak is
identified at 3160+/-10 cm^(-1) and its temperature dependence analyzed in
terms of a thermal expansion model. We find that the two-magnon peak position
must include a cyclic ring exchange of J_cycl/J_perp=0.09-0.25 with a coupling
constant along the rungs of J_perp approx. 1215 cm^(-1) (1750 K) in order to be
consistent with other experiments and theoretical results.Comment: 4 pages, 3 figure
Schwermetallmonitoring mittels transgener Hefen
Schwermetallionen spielen bei der Reinigung von Abwässern der Galvanik-Industrie eine entscheidende Rolle. Hier muss die Abwasserbehandlung mit Verfahren gekoppelt sein, die diese toxischen Metallionen vom Abwasser trennen. Dazu ist eine kontinuierliche Messung der spezifischen Verbindungen, d.h. der Schwermetallionen-Konzentration, essentielle Voraussetzung, um mit möglichst geringem Materialeinsatz einen optimalen Reinigungseffekt zu erzielen.
Für solche Messungen wird derzeit vor allem die Atomadsorptionsspektrometrie eingesetzt, die zwar sehr exakt, aber auch sehr kostenintensiv ist und nur die absoluten und nicht die biologisch verfügbaren Konzentrationen an Schwermetallbelastungen misst. Zunehmend besteht daher die Forderung nach Messsystemen, die durch eine schnelle, kostengünstige und umweltschonende Schadstofferfassung, speziell von Schwermetallionen, charakterisiert ist, bei der der apparative Aufwand möglichst gering sein soll. Diese Forderungen erfüllen Biosensoren, die durch den Einsatz von gentechnisch veränderten Mikroorganismen, speziell Hefen, auch für solche Aufgabenstellungen genutzt werden können
Diagonal Ladders: A New Class of Models for Strongly Coupled Electron Systems
We introduce a class of models defined on ladders with a diagonal structure
generated by plaquettes. The case corresponds to the necklace
ladder and has remarkable properties which are studied using DMRG and recurrent
variational ansatzes. The AF Heisenberg model on this ladder is equivalent to
the alternating spin-1/spin-1/2 AFH chain which is known to have a
ferrimagnetic ground state (GS). For doping 1/3 the GS is a fully doped (1,1)
stripe with the holes located mostly along the principal diagonal while the
minor diagonals are occupied by spin singlets. This state can be seen as a Mott
insulator of localized Cooper pairs on the plaquettes. A physical picture of
our results is provided by a model of plaquettes coupled diagonally
with a hopping parameter . In the limit we recover the
original model on the necklace ladder while for weak hopping parameter
the model is easily solvable. The GS in the strong hopping regime is
essentially an "on link" Gutzwiller projection of the weak hopping GS. We
generalize the model to diagonal ladders with and the 2D
square lattice. We use in our construction concepts familiar in Statistical
Mechanics as medial graphs and Bratelli diagrams.Comment: REVTEX file, 22 pages (twocolumn), 35 figures inserted in text. 12
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