1,833 research outputs found
Charge Stripe in an Antiferromagnet: 1d Band of Composite Excitations
With the help of analytical and numerical studies of the - model we
argue that the charge stripe in an antiferromagnetic insulator should be
understood as a system of holon-spin-polaron excitations condensed at the
self-induced antiphase domain wall. The structure of such a charge excitation
is studied in detail with numerical and analytical results for various
quantities being in a very close agreement. An analytical picture of these
excitations occupying an effective 1D stripe band is also in a very good accord
with numerical data. The emerging concept advocates the primary role of the
kinetic energy in favoring the stripe as a ground state. A comparative analysis
suggests the effect of pairing and collective meandering on the energetics of
the stripe formation to be secondary.Comment: 5 pages, 3 figures, proceedings of SCES'01 conference, Ann Arbor,
2001, to be published in Physica
Lateral Distribution for Aligned Events in Muon Groups Deep Underground
The paper concerns the so-called aligned events observed in cosmic rays. The
phenomenon of the alignment of the most energetic subcores of gamma-ray--hadron
() families (particles of the highest energies in the central EAS
core) was firstly found in the "Pamir" emulsion chamber experiment and related
to a coplanar particle production at eV. Here a separation
distribution (distances between pairs of muons) for aligned events has been
analyzed throughout muon groups measured by Baksan Underground Scintillation
Telescope (BUST) for threshold energies TeV during a period of
7.7 years. Only muon groups of multiplicity with inclined
trajectories for an interval of zenith angles were
selected for the analysis. The analysis has revealed that the distribution
complies with the exponential law. Meanwhile the distributions become steeper
with the increase of threshold energy. There has been no difference between the
lateral distribution of all the groups and the distribution of the aligned
groups.Comment: 4 pages, 3 figures. Prepared for the ISVHECRI'2006, Weihai, Chin
On the Nagaoka polaron in the t-J model
It is widely believed that a single hole in the two (or three) dimensional
t-J model, for sufficiently small exchange coupling J, creates a ferromagnetic
bubble around itself, a finite J remnant of the ferromagnetic groundstate at
J=0 (the infinite U Hubbard model), first established by Nagaoka. We
investigate this phenomenon in two dimensions using the density matrix
renormalization group, for system sizes up to 9x9. We find that the polaron
forms for J/t<0.02-0.03 (a somewhat larger value than estimated previously).
Although finite-size effects appear large, our data seems consistent with the
expected 1.1(J/t)^{-1/4} variation of polarion radius. We also test the
Brinkman-Rice model of non-retracing paths in a Neel background, showing that
it is quite accurate, at larger J. Results are also presented in the case where
the
Heisenberg interaction is dropped (the t-J^z model). Finally we discuss a
"dressed polaron" picture in which the hole propagates freely inside a finite
region but makes only self-retracing excursions outside this region.Comment: 7 pages, 9 encapsulated figure
Effects of domain walls on hole motion in the two-dimensional t-J model at finite temperature
The t-J model on the square lattice, close to the t-J_z limit, is studied by
quantum Monte Carlo techniques at finite temperature and in the underdoped
regime. A variant of the Hoshen-Koppelman algorithm was implemented to identify
the antiferromagnetic domains on each Trotter slice. The results show that the
model presents at high enough temperature finite antiferromagnetic (AF) domains
which collapse at lower temperatures into a single ordered AF state. While
there are domains, holes would tend to preferentially move along the domain
walls. In this case, there are indications of hole pairing starting at a
relatively high temperature. At lower temperatures, when the whole system
becomes essentially fully AF ordered, at least in finite clusters, holes would
likely tend to move within phase separated regions. The crossover between both
states moves down in temperature as doping increases and/or as the off-diagonal
exchange increases. The possibility of hole motion along AF domain walls at
zero temperature in the fully isotropic t-J is discussed.Comment: final version, to appear in Physical Review
Competition between local potentials and attractive particle-particle interactions in superlattices
Naturally occuring or man-made systems displaying periodic spatial
modulations of their properties on a nanoscale constitute superlattices. Such
modulated structures are important both as prototypes of simple
nanotechnological devices and as particular examples of emerging spatial
inhomogeneity in interacting many-electron systems. Here we investigate the
effect different types of modulation of the system parameters have on the
ground-state energy and the charge-density distribution of the system. The
superlattices are described by the inhomogeneous attractive Hubbard model, and
the calculations are performed by density-functional and density-matrix
renormalization group techniques. We find that modulations in local electric
potentials are much more effective in shaping the system's properties than
modulations in the attractive on-site interaction. This is the same conclusions
we previously (Phys. Rev. B 71, 125130) obtained for repulsive interactions,
suggesting that it is not an artifact of a specific state, but a general
property of modulated structures.Comment: 8 pages, 2 figure
Coulomb gap in a model with finite charge transfer energy
The Coulomb gap in a donor-acceptor model with finite charge transfer energy
describing the electronic system on the dielectric side of the
metal-insulator transition is investigated by means of computer simulations on
two- and three-dimensional finite samples with a random distribution of equal
amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry
of the model presented with respect to the exchange of donors and acceptors are
derived. In the immediate neighborhood of the Fermi energy the the
density of one-electron excitations is determined solely by
finite size effects and further away from is described by
an asymmetric power law with a non-universal exponent, depending on the
parameter .Comment: 10 pages, 6 figures, submitted to Phys. Rev.
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Group behavioural responses of cyprinids to artificial acoustic stimuli: implications for fisheries management
Physical screens are considered a necessary mitigation strategy, restricting access to hazardous areas (e.g. turbine intakes or extraction points) and guiding fish to safer preferred routes. Whilst this precautionary approach is considered fit for purpose, fish interactions at such interfaces have been observed to cause stress, injury and mortalities where high sweeping approach velocities exist. Alternative or collaborative systems may allow for improvements where effectiveness of physical screens alone are limited (e.g. repelling small sized larval/ juvenile stages).
A number of acoustic guidance systems have been deployed to date, however efficiencies of such devices are measured using relatively simplistic metrics (e.g.percentage deflection or attraction). Results are highly variable, with some systems observing little to no deflection. Current approaches testing efficiencies are based on methods which provide limited information surrounding the wild behaviour of migratory fish. Furthermore, many studies investigating fish bioacoustics pay limited attention to the entirety of the acoustic field to which fish are exposed.
The auditory system is exceptionally important to most aquatic species due to its information provisioning, however various challenges need addressed to extrapolate useful informative data which could develop working behavioural deterrents. Life-history strategies, life-stages, species specific hearing capabilities (i.e. directivity index, sensitivity, and critical ratio), and the propensity for a sound to elicit a repeatedly effective behavioural response (i.e. signal detection theory and effects of habituation) are all areas that require thorough investigation.
Utilising model cyprinid species (in possession of morphological hearing specialisations), groups of fish were video recorded under controlled conditions and exposed to differing acoustic stimuli. Behaviours were quantified and analysed, alongside appropriate mapping of tested acoustic fields. This talk summarises past and ongoing experiments of a PhD project investigating group behavioural responses to sound. Results may better inform fisheries engineering design of acoustic behavioural deterrents for conservation and control purposes
Perturbation Theory for Spin Ladders Using Angular-Momentum Coupled Bases
We compute bulk properties of Heisenberg spin-1/2 ladders using
Rayleigh-Schr\"odinger perturbation theory in the rung and plaquette bases. We
formulate a method to extract high-order perturbative coefficients in the bulk
limit from solutions for relatively small finite clusters. For example, a
perturbative calculation for an isotropic ladder yields an
eleventh-order estimate of the ground-state energy per site that is within
0.02% of the density-matrix-renormalization-group (DMRG) value. Moreover, the
method also enables a reliable estimate of the radius of convergence of the
perturbative expansion. We find that for the rung basis the radius of
convergence is , with defining the ratio between
the coupling along the chain relative to the coupling across the chain. In
contrast, for the plaquette basis we estimate a radius of convergence of
. Thus, we conclude that the plaquette basis offers the
only currently available perturbative approach which can provide a reliable
treatment of the physically interesting case of isotropic spin
ladders. We illustrate our methods by computing perturbative coefficients for
the ground-state energy per site, the gap, and the one-magnon dispersion
relation.Comment: 22 pages. 9 figure
Oxidation and fragmentation of plastics in a changing environment; from UV-radiation to biological degradation
Understanding the fate of plastics in the environment is of critical importance for the quantitative assessment of the biological impacts of plastic waste. Specially, there is a need to analyze in more detail the reputed longevity of plastics in the context of plastic degradation through oxidation and fragmentation reactions. Photo-oxidation of plastic debris by solar UV radiation (UVR) makes material prone to subsequent fragmentation. The fragments generated following oxidation and subsequent exposure to mechanical stresses include secondary micro- or nanoparticles, an emerging class of pollutants. The paper discusses the UV-driven photo-oxidation process, identifying relevant knowledge gaps and uncertainties. Serious gaps in knowledge exist concerning the wavelength sensitivity and the dose-response of the photo-fragmentation process. Given the heterogeneity of natural UV irradiance varying from no exposure in sediments to full UV exposure of floating, beach litter or air-borne plastics, it is argued that the rates of UV-driven degradation/fragmentation will also vary dramatically between different locations and environmental niches. Biological phenomena such as biofouling will further modulate the exposure of plastics to UV radiation, while potentially also contributing to degradation and/or fragmentation of plastics independent of solar UVR. Reductions in solar UVR in many regions, consequent to the implementation of the Montreal Protocol and its Amendments for protecting stratospheric ozone, will have consequences for global UV-driven plastic degradation in a heterogeneous manner across different geographic and environmental zones. The interacting effects of global warming, stratospheric ozone and UV radiation are projected to increase UV irradiance at the surface in localized areas, mainly because of decreased cloud cover. Given the complexity and uncertainty of future environmental conditions, this currently precludes reliable quantitative predictions of plastic persistence on a global scale
Coulomb gap in one-dimensional disordered electronic systems
We study a one-dimensional system of spinless electrons in the presence of a
long-range Coulomb interaction (LRCI) and a random chemical potential at each
site. We first present a Tomonaga-Luttinger liquid (TLL) description of the
system. We use the bosonization technique followed by the replica trick to
average over the quenched randomness. An expression for the localization length
of the system is then obtained using the renormalization group method and also
a physical argument. We then find the density of states for different values of
the energy; we get different expressions depending on whether the energy is
larger than or smaller than the inverse of the localization length. We work in
the limit of weak disorder where the localization length is very large; at that
length scale, the LRCI has the effect of reducing the interaction parameter K
of the TLL to a value much smaller than the noninteracting value of unity.Comment: Revtex, 6 pages, no figures; discussions have been expanded in
several place
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