147 research outputs found
Level statistics of XXZ spin chains with a random magnetic field
The level-spacing distribution of a spin 1/2 XXZ chain is numerically studied
under random magnetic field. We show explicitly how the level statistics
depends on the lattice size L, the anisotropy parameter , and the mean
amplitude of the random magnetic field h. In the energy spectrum, quantum
integrability competes with nonintegrability derived from the randomness, where
the XXZ interaction is modified by the parameter . When ,
the level-spacing distribution mostly shows Wigner-like behavior, while when
=0, Poisson-like behavior appears although the system is nonintegrable
due to randomness. Poisson-like behavior also appears for in the
large h limit. Furthermore, the level-spacing distribution depends on the
lattice size L, particularly when the random field is weak.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
Unexpected non-Wigner behavior in level-spacing distributions of next-nearest-neighbor coupled XXZ spin chains
The level-spacing distributions of XXZ spin chains with next-nearest-neighbor
couplings are studied under periodic boundary conditions. We confirm that
integrable XXZ spin chains mostly have the Poisson distribution as expected. On
the contrary, the level-spacing distributions of next-nearest-neighbor coupled
XXZ chains are given by non-Wigner distributions. It is against the
expectations, since the models are nonintegrable.Comment: 4 pages, 4 figures, to be published in Physical Review
Palaeoecological study of a Weichselian wetland site in the Netherlands suggests a link with Dansgaard-Oeschger climate oscillation
Botanical microfossils, macroremains and oribatid mites of a Weichselian interstadial deposit in the central Netherlands point to a temporary, sub-arctic wetland in a treeless landscape. Radiocarbon dates and OSL dates show an age between ca. 54.6 and 46.6 ka cal BP. The vegetation succession, starting as a peat-forming wetland that developed into a lake, might well be linked with a Dansgaard-Oeschger climatic cycle. We suggest that during the rapid warming at the start of a D-O cycle, relatively low areas in the landscape became wetlands where peat was formed. During the more gradual temperature decline that followed, evaporation diminished; the wetlands became inundated and lake sediments were formed. During subsequent sub-arctic conditions the interstadial deposits were covered with wind-blown sand. Apart from changes in effective precipitation also the climate-related presence and absence of permafrost conditions may have played a role in the formation of the observed sedimentological sequence from sand to peat, through lacustrine sediment, with coversand on top. The Wageningen sequence may correspond with D-O event 12, 13 or 14. Some hitherto not recorded microfossils were described and illustrated
Palaeoecological study of a Weichselian wetland site in the Netherlands suggests a link with Dansgaard-Oeschger climate oscillation
Incoherent tunnelling through two quantum dots with Coulomb interaction
The Ohmic conductance and current through two quantum dots in series is
investigated for the case of incoherent tunnelling. A generalised master
equation is employed to include the discrete nature of the energy levels.
Regions of negative differential conductance can occur in the I-V
characteristics. Transport is dominated by matching energy levels, even when
they do not occur at the charge degeneracy points.Comment: RevTeX + epsf.sty + 13 figure
Dietary Mannan Oligosaccharides Modulate Gut Microbiota, Increase Fecal Bile Acid Excretion, and Decrease Plasma Cholesterol and Atherosclerosis Development
Functional Genomics of Systemic Disorder
Level Statistics of XXZ Spin Chains with Discrete Symmetries: Analysis through Finite-size Effects
Level statistics is discussed for XXZ spin chains with discrete symmetries
for some values of the next-nearest-neighbor (NNN) coupling parameter. We show
how the level statistics of the finite-size systems depends on the NNN coupling
and the XXZ anisotropy, which should reflect competition among quantum chaos,
integrability and finite-size effects. Here discrete symmetries play a central
role in our analysis. Evaluating the level-spacing distribution, the spectral
rigidity and the number variance, we confirm the correspondence between
non-integrability and Wigner behavior in the spectrum. We also show that
non-Wigner behavior appears due to mixed symmetries and finite-size effects in
some nonintegrable cases.Comment: 19 pages, 6 figure
Semiclassical theory for many-body Fermionic systems
We present a treatment of many-body Fermionic systems that facilitates an
expression of the well-known quantities in a series expansion of the Planck's
constant. The ensuing semiclassical result contains to a leading order of the
response function the classical time correlation function of the observable
followed by the Weyl-Wigner series, on top of these terms are the
periodic-orbit correction terms. The treatment given here starts from linear
response assumption of the many-body theory and in its connection with
semiclassical theory, it makes no assumption of the integrability of classical
dynamics underlying the one-body quantal system. Applications of the framework
are also discussed.Comment: 18 pages, Te
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