622 research outputs found
A magnetic model for the incommensurate I phase of spin-Peierls systems
A magnetic model is proposed for describing the incommensurate I phase of
spin-Peierls systems. Based on the harmonicity of the lattice distortion, its
main ingredient is that the distortion of the lattice adjusts to the average
magnetization such that the system is always gapful. The presence of dynamical
incommensurabilities in the fluctuation spectra is also predicted. Recent
experimental results for CuGeO_3 obtained by NMR, ESR and light scattering
absorption are well understood within this model.Comment: 8 pages, 3 figures, Latex with EPL style files all include
Maladaptive Planning and the Pro-Innovation Bias: Considering the Case of Automated Vehicles
This article argues that a more critical approach to innovation policy within planning is needed and offers recommendations for achieving this. These recommendations entail rethinking the values, focus, speed, and legitimacy of innovations. It takes a critical perspective on how contemporary societies treat rapid innovation as having necessarily positive results in the achievement of objectives such as sustainability and justice. This critical perspective is needed because innovation can both contribute to and drive a form of maladaptive planning: a collective approach to reality that imposes constant and rapid changes to societal configurations due to an obsession with the new and with too little rapport with the problems in place or that it creates. A maladaptive direction for transport planning is used as a sectorial illustration of the broader conceptual ideas presented: for both sustainability and social justice reasons, it would be desirable to see peak car occurring. However, the car industry is presenting driving automation as an innovation with the potential to restore the vitality of the private vehicles market while creating effective means to dismiss alternatives to car dominance
Finite Temperature DMRG Investigation of the Spin-Peierls Transition in CuGeO
We present a numerical study of thermodynamical properties of dimerized
frustrated Heisenberg chains down to extremely low temperatures with
applications to CuGeO. A variant of the finite temperature density matrix
renormalization group (DMRG) allows the study of the dimerized phase previously
unaccessible to ab initio calculations. We investigate static dimerized systems
as well as the instability of the quantum chain towards lattice dimerization.
The crossover from a quadratic response in the free energy to the distortion
field at finite temperature to nonanalytic behavior at zero temperature is
studied quantitatively. Various physical quantities are derived and compared
with experimental data for CuGeO such as magnetic dimerization, critical
temperature, susceptibility and entropy.Comment: LaTeX, 5 pages, 5 eps figures include
Raman Response of Magnetic Excitations in Cuprate Ladders and Planes
An unified picture for the Raman response of magnetic excitations in cuprate
spin-ladder compounds is obtained by comparing calculated two-triplon Raman
line-shapes with those of the prototypical compounds SrCu2O3 (Sr123),
Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the
two-leg ladder contains Heisenberg exchange couplings J_parallel and J_perp
plus an additional four-spin interaction J_cyc. Within this model Sr123 and
Sr14 can be described by x:=J_parallel/J_perp=1.5, x_cyc:=J_cyc/J_perp=0.2,
J_perp^Sr123=1130 cm^-1 and J_perp^Sr14=1080 cm^-1. The couplings found for
La6Ca8 are x=1.2, x_cyc=0.2, and J_perp^La6Ca8=1130 cm^-1. The unexpected sharp
two-triplon peak in the ladder materials compared to the undoped
two-dimensional cuprates can be traced back to the anisotropy of the magnetic
exchange in rung and leg direction. With the results obtained for the isotropic
ladder we calculate the Raman line-shape of a two-dimensional square lattice
using a toy model consisting of a vertical and a horizontal ladder. A direct
comparison of these results with Raman experiments for the two-dimensional
cuprates R2CuO4 (R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O(6+delta) yields a good
agreement for the dominating two-triplon peak. We conclude that short range
quantum fluctuations are dominating the magnetic Raman response in both,
ladders and planes. We discuss possible scenarios responsible for the
high-energy spectral weight of the Raman line-shape, i.e. phonons, the
triple-resonance and multi-particle contributions.Comment: 10 pages, 6 figure
Spin-Peierls transition of the first order in S=1 antiferromagnetic Heisenberg chains
We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model
coupled to a lattice distortion by a quantum Monte Carlo method. Investigating
the ground state energy of the static bond-alternating chain, we find that the
instability to a dimerized chain depends on the value of the spin-phonon
coupling, unlike the case of S=1/2. The spin state is the dimer state or the
uniform Haldane state depending on whether the lattice distorts or not,
respectively. At an intermediate value of the spin-phonon coupling, we find the
first-order transition between the two states. We also find the coexistence of
the two states.Comment: 7 pages, 12 eps figures embedded in the text; corrected typos,
replaced figure
Excitations of the field-induced soliton lattice in CuGeO3
Here we report the first inelastic neutron scattering study of the magnetic
excitations in the incommensurate phase of a spin-Peierls material. The results
on CuGeO3 provide direct evidence of a finite excitation gap, two sharp
magnetic excitation branches and a very low-lying excitation which is
identified as a phason mode, the Goldstone mode of the incommensurate soliton
lattice.Comment: 5 pages, revtex, 4 figures (*.eps), win-zippe
Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System
We investigate thermodynamic properties of a one-dimensional S=1/2
antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum
Monte Carlo method. In particular we study how spin and lattice dimerize as a
function of the temperature, which gives a fundamental process of the
spin-Peierls transition in higher dimensions. The degree of freedom of the
lattice is taken into account adiabatically and the thermal distribution of the
lattice distortion is obtained by the thermal bath algorithm. We find that the
dimerization develops as the temperature decreases and it converges to the
value of the dimerization of the ground state at T=0. Furthermore we find that
the coupling constants of spins fluctuate quite largly at high temperature and
there thermodynamic properties deviate from those of the uniform chain. Doping
of non-magnetic impurities causes cut of the chain into short chains with open
boundary. We investigate thermodynamic properties of open chains taking
relaxation of the lattice into consideration. We find that strong bonds locate
at the edges and a defect of the bond alternation appears in the chain with odd
number of sites, which causes enhancement of the staggered magnetic order. We
find a spreaded staggered structure which indicates that the defect moves
diffusively in the chain even at very low temperature.Comment: 7 pages, 17 figures; added comments on section 2 and 3, corrected
typo
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