503 research outputs found
Major shifts at the range edge of marine forests: the combined effects of climate changes and limited dispersal
Global climate change is likely to constrain low latitude range edges across many taxa and habitats. Such is the case for NE Atlantic marine macroalgal forests, important ecosystems whose main structuring species is the annual kelp Saccorhiza polyschides. We coupled ecological niche modelling with simulations of potential dispersal and delayed development stages to infer the major forces shaping range edges and to predict their dynamics. Models indicated that the southern limit is set by high winter temperatures above the physiological tolerance of overwintering microscopic stages and reduced upwelling during recruitment. The best range predictions were achieved assuming low spatial dispersal (5 km) and delayed stages up to two years (temporal dispersal). Reconstructing distributions through time indicated losses of similar to 30% from 1986 to 2014, restricting S. polyschides to upwelling regions at the southern edge. Future predictions further restrict populations to a unique refugium in northwestern Iberia. Losses were dependent on the emissions scenario, with the most drastic one shifting similar to 38% of the current distribution by 2100. Such distributional changes might not be rescued by dispersal in space or time (as shown for the recent past) and are expected to drive major biodiversity loss and changes in ecosystem functioning.Electricity of Portugal (Fundo EDP para a Biodiversidade); FCT - Portuguese Science Foundation [PTDC/MAR-EST/6053/2014, EXTANT-EXCL/AAG-GLO/0661/2012, SFRH/BPD/111003/2015
Pair formation in two electron correlated chains
We study two correlated electrons in a nearest neighbour tight- binding
chain, with both on site and nearest neighbour interaction. Both the cases of
parallel and antiparallel spins are considered. In addition to the free
electron band for two electrons, there are correlated bands with positive or
negative energy, depending on wheather the interaction parameters are repulsive
or attractive. Electrons form bound states, with amplitudes that decay
exponentially with separation. Conditions for such states to be filled at low
temperatures are discussed.Comment: To appear in J. Phys: Condens. Matter 15 (2003
Dynamic instabilities in resonant tunneling induced by a magnetic field
We show that the addition of a magnetic field parallel to the current induces
self sustained intrinsic current oscillations in an asymmetric double barrier
structure. The oscillations are attributed to the nonlinear dynamic coupling of
the current to the charge trapped in the well, and the effect of the external
field over the local density of states across the system. Our results show that
the system bifurcates as the field is increased, and may transit to chaos at
large enough fields.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Letter
Delocalization and conductance quantization in one-dimensional systems
We investigate the delocalization and conductance quantization in finite
one-dimensional chains with only off-diagonal disorder coupled to leads. It is
shown that the appearence of delocalized states at the middle of the band under
correlated disorder is strongly dependent upon the even-odd parity of the
number of sites in the system. In samples with inversion symmetry the
conductance equals for odd samples, and is smaller for even parity.
This result suggests that this even-odd behaviour found previously in the
presence of electron correlations may be unrelated to charging effects in the
sample.Comment: submitted to PR
Azbel-Hofstadter model on triangular lattice revisited
In the present paper, the mean of Lyapunov exponents for the Azbel-Hofstadter
model on the triangular lattice is calculated. It is recently proposed that
[Phys. Rev. Lett. {\bf 85}, 4920 (2000)], for the case of the square lattice,
this quantity can be related to the logarithm of the partition function of the
two dimensional Ising model and has a connection to the asymptotic bandwidth.
We find that the correspondence of this quantity to the logarithm of the
partition function of the two dimensional Ising model is not complete for the
triangular lattice. Moreover, the detailed connection between this quantity and
the asymptotic bandwidth is not valid. Thus the conclusions for the mean of
Lyapunov exponents suggested previously depend on the lattice geometry.Comment: RevTeX, 4 page, no figur
Inverse flux quantum periodicity of magnetoresistance oscillations in two-dimensional short-period surface superlattices
Transport properties of the two-dimensional electron gas (2DEG) are
considered in the presence of a perpendicular magnetic field and of a {\it
weak} two-dimensional (2D) periodic potential modulation in the 2DEG plane. The
symmetry of the latter is rectangular or hexagonal. The well-known solution of
the corresponding tight-binding equation shows that each Landau level splits
into several subbands when a rational number of flux quanta pierces the
unit cell and that the corresponding gaps are exponentially small. Assuming the
latter are closed due to disorder gives analytical wave functions and
simplifies considerably the evaluation of the magnetoresistivity tensor
. The relative phase of the oscillations in and
depends on the modulation periods involved. For a 2D modulation
with a {\bf short} period nm, in addition to the Weiss oscillations
the collisional contribution to the conductivity and consequently the tensor
show {\it prominent peaks when one flux quantum passes
through an integral number of unit cells} in good agreement with recent
experiments. For periods nm long used in early experiments, these
peaks occur at fields 10-25 times smaller than those of the Weiss oscillations
and are not resolved
Absorption and wavepackets in optically excited semiconductor superlattices driven by dc-ac fields
Within the one-dimensional tight-binding minibands and on-site
Coloumbic interaction approximation, the absorption spectrum and coherent
wavepacket time evolution in an optically excited semiconductor superlattice
driven by dc-ac electric fields are investigated using the semiconductor Bloch
equations.
The dominating roles of the ratios of dc-Stark to external ac frequency, as
well as ac-Stark to external ac frequency, is emphasized. If the former is an
integer , then also harmonics are present within one Stark
frequency, while the fractional case leads to the formation of excitonic
fractional ladders. The later ratio determines the size and profile of the
wavepacket. In the absence of excitonic interaction it controls the maximum
size wavepackets reach within one cycle, while the interaction produces a
strong anisotropy and tends to palliate the dynamic wavepacket localization.Comment: 14 pages, 7 postscript figure
Landau levels in the case of two degenerate coupled bands: kagome lattice tight-binding spectrum
The spectrum of charged particles hopping on a kagome lattice in a uniform
transverse magnetic field shows an unusual set of Landau levels at low field.
They are unusual in two respects: the lowest Landau levels are paramagnetic so
their energies decrease linearly with increasing field magnitude, and the
spacings between the levels are not equal. These features are shown to follow
from the degeneracy of the energy bands in zero magnetic field. We give a
general discussion of Landau levels in the case of two degenerate bands, and
show how the kagome lattice tight-binding model includes one special case of
this more general problem. We also discuss the consequences of this for the
behavior of the critical temperature of a kagome grid superconducting wire
network, which is the experimental system that originally motivated this work.Comment: 18 pages, 8 figure
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