602 research outputs found
A Modelling Approach for Evaluating Agri-Environmental Policies at Sector Level
This paper presents a new approach to evaluate the cost effectiveness of agri-environmental policies at sector level. Policy uptake, cumulative environmental effects and public expenditure are identified as the main determinants of cost-effectiveness. On the basis of the sector-consistent, comparative-static, farm group model FARMIS, the determinants of policy cost-effectiveness at sector level are addressed. Firstly, intensity levels for the FARMIS activities are defined in order to model uptake of agri-environmental policies with FARMIS, secondly, life-cycle assessment data is attached to these intensity levels to determine environmental effects of the policies and thirdly, public expenditure is calculated under consideration of transaction costs. This paper concludes delineating the strengths and limitations of the approach
Impurity spin relaxation in S=1/2 XX chains
Dynamic autocorrelations (\alpha=x,z) of an
isolated impurity spin in a S=1/2 XX chain are calculated. The impurity spin,
defined by a local change in the nearest-neighbor coupling, is either in the
bulk or at the boundary of the open-ended chain. The exact numerical
calculation of the correlations employs the Jordan-Wigner mapping from spin
operators to Fermi operators; effects of finite system size can be eliminated.
Two distinct temperature regimes are observed in the long-time asymptotic
behavior. At T=0 only power laws are present. At high T the x correlation
decays exponentially (except at short times) while the z correlation still
shows an asymptotic power law (different from the one at T=0) after an
intermediate exponential phase. The boundary impurity correlations follow power
laws at all T. The power laws for the z correlation and the boundary
correlations can be deduced from the impurity-induced changes in the properties
of the Jordan-Wigner fermion states.Comment: Final version to be published in Phys. Rev. B. Three references
added, extended discussion of relation to previous wor
COMPLETE SOLUTION OF THE XXZ-MODEL ON FINITE RINGS. DYNAMICAL STRUCTURE FACTORS AT ZERO TEMPERATURE.
The finite size effects of the dynamical structure factors in the XXZ-model
are studied in the euclidean time -representation. Away from the
critical momentum finite size effects turn out to be small except for
the large limit. The large finite size effects at the critical momentum
signal the emergence of infrared singularities in the spectral
-representation of the dynamical structure factors.Comment: PostScript file with 12 pages + 11 figures uuencoded compresse
Low temperature relaxational dynamics of the Ising chain in a transverse field
We present asymptotically exact results for the real time order parameter
correlations of a class of d=1 Ising models in a transverse field at low
temperatures (T) on both sides of the quantum critical point. The correlations
are a product of a T-independent factor determined by quantum effects, and a
T-dependent relaxation function which comes from a classical theory. We confirm
our predictions by a no-free-parameter comparison with numerical studies on the
nearest neighbor spin-1/2 model.Comment: Final version to be published in Physical Review Letters. The
postscript file is also available by anonymous ftp at
ftp://chopin.ucsc.edu/pub/dynamics.ps.g
Dynamic properties of the spin-1/2 XY chain with three-site interactions
We consider a spin-1/2 XY chain in a transverse (z) field with multi-site
interactions. The additional terms introduced into the Hamiltonian involve
products of spin components related to three adjacent sites. A Jordan-Wigner
transformation leads to a simple bilinear Fermi form for the resulting
Hamiltonian and hence the spin model admits a rigorous analysis. We point out
the close relationships between several variants of the model which were
discussed separately in previous studies. The ground-state phases (ferromagnet
and two kinds of spin liquid) of the model are reflected in the dynamic
structure factors of the spin chains, which are the main focus in this study.
First we consider the zz dynamic structure factor reporting for this quantity a
closed-form expression and analyzing the properties of the two-fermion
(particle-hole) excitation continuum which governs the dynamics of transverse
spin component fluctuations and of some other local operator fluctuations. Then
we examine the xx dynamic structure factor which is governed by many-fermion
excitations, reporting both analytical and numerical results. We discuss some
easily recognized features of the dynamic structure factors which are
signatures for the presence of the three-site interactions.Comment: 28 pages, 10 fugure
Disorder Induced Quantum Phase Transition in Random-Exchange Spin-1/2 Chains
We investigate the effect of quenched bond-disorder on the anisotropic
spin-1/2 (XXZ) chain as a model for disorder induced quantum phase transitions.
We find non-universal behavior of the average correlation functions for weak
disorder, followed by a quantum phase transition into a strongly disordered
phase with only short-range xy-correlations. We find no evidence for the
universal strong-disorder fixed point predicted by the real-space
renormalization group, suggesting a qualitatively different view of the
relationship between quantum fluctuations and disorder.Comment: 4 pages, 4 postscript figures, needs RevTeX
Charge and spin dynamics in the one-dimensional and models
The impact of the spin-flip terms on the (static and dynamic) charge and spin
correlations in the Luttinger-liquid ground state of the 1D model is
assessed by comparison with the same quantities in the 1D model, where
spin-flip terms are absent. We employ the recursion method combined with a
weak-coupling or a strong-coupling continued-fraction analysis. At
we use the Pfaffian representation of dynamic spin correlations. The changing
nature of the dynamically relevant charge and spin excitations on approach of
the transition to phase separation is investigated in detail. The
charge excitations (but not the spin excitations) at the transition have a
single-mode nature, whereas charge and spin excitations have a complicated
structure in the model. In the model, phase separation is
accompanied by N\'eel long-range order, caused by the condensation of electron
clusters with an already existing alternating up-down spin configuration
(topological long-range order). In the model, by contrast, the spin-flip
processes in the exchange coupling are responsible for continued strong spin
fluctuations (dominated by 2-spinon excitations) in the phase-separated state.Comment: 11 pages (RevTex). 14 Figures available from author
Efficient and perfect state transfer in quantum chains
We present a communication protocol for chains of permanently coupled qubits
which achieves perfect quantum state transfer and which is efficient with
respect to the number chains employed in the scheme. The system consists of
uncoupled identical quantum chains. Local control (gates, measurements) is only
allowed at the sending/receiving end of the chains. Under a quite general
hypothesis on the interaction Hamiltonian of the qubits a theorem is proved
which shows that the receiver is able to asymptotically recover the messages by
repetitive monitoring of his qubits.Comment: 6 pages, 2 figures; new material adde
Quantum algorithm for simulating the dynamics of an open quantum system
In the study of open quantum systems, one typically obtains the decoherence
dynamics by solving a master equation. The master equation is derived using
knowledge of some basic properties of the system, the environment and their
interaction: one basically needs to know the operators through which the system
couples to the environment and the spectral density of the environment. For a
large system, it could become prohibitively difficult to even write down the
appropriate master equation, let alone solve it on a classical computer. In
this paper, we present a quantum algorithm for simulating the dynamics of an
open quantum system. On a quantum computer, the environment can be simulated
using ancilla qubits with properly chosen single-qubit frequencies and with
properly designed coupling to the system qubits. The parameters used in the
simulation are easily derived from the parameters of the system+environment
Hamiltonian. The algorithm is designed to simulate Markovian dynamics, but it
can also be used to simulate non-Markovian dynamics provided that this dynamics
can be obtained by embedding the system of interest into a larger system that
obeys Markovian dynamics. We estimate the resource requirements for the
algorithm. In particular, we show that for sufficiently slow decoherence a
single ancilla qubit could be sufficient to represent the entire environment,
in principle.Comment: 5 figures, two table
Finite Temperature and Dynamical Properties of the Random Transverse-Field Ising Spin Chain
We study numerically the paramagnetic phase of the spin-1/2 random
transverse-field Ising chain, using a mapping to non-interacting fermions. We
extend our earlier work, Phys. Rev. 53, 8486 (1996), to finite temperatures and
to dynamical properties. Our results are consistent with the idea that there
are ``Griffiths-McCoy'' singularities in the paramagnetic phase described by a
continuously varying exponent , where measures the
deviation from criticality. There are some discrepancies between the values of
obtained from different quantities, but this may be due to
corrections to scaling. The average on-site time dependent correlation function
decays with a power law in the paramagnetic phase, namely
, where is imaginary time. However, the typical
value decays with a stretched exponential behavior, ,
where may be related to . We also obtain results for the full
probability distribution of time dependent correlation functions at different
points in the paramagnetic phase.Comment: 10 pages, 14 postscript files included. The discussion of the typical
time dependent correlation function has been greatly expanded. Other papers
of APY are available on-line at http://schubert.ucsc.edu/pete
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