591 research outputs found
Topological effects at short antiferromagnetic Heisenberg chains
The manifestations of topological effects in finite antiferromagnetic
Heisenberg chains is examined by density matrix renormalization group technique
in this paper. We find that difference between integer and half-integer spin
chains shows up in ground state energy per site when length of spin chain is
longer than , where is a spin-spin correlation
length, for spin magnitude S up to 5/2. For open chains with spin magnitudes
to S=5, we verify that end states with fractional spin quantum numbers
exist and are visible even when the chain length is much smaller than the
correlation length . The end states manifest themselves in the structure
of the low energy excitation spectrum.Comment: 4 pages, 6 figure
Field-induced gap in the spin-1/2 antiferromagnetic Heisenberg chain: A density matrix renormalization group study
We study the spin-1/2 antiferromagnetic Heisenberg chain in both uniform and
(perpendicular) staggered magnetic fields using the density-matrix
renormalization-group method. This model has been shown earlier to describe the
physics of the copper benzoate materials in magnetic field. In the present
work, we extend the study to more general case for a systematic investigation
of the field-induced gap and related properties of the spin-1/2
antiferromagnetic Heisenberg chain. In particular, we explore the high magnetic
field regime where interesting behaviors in the field-induced gap,
magnetization, and spin correlation functions are found. Careful examination of
the low energy properties and magnetization reveals interesting competing
effects of the staggered and uniform fields. The incommensurate behavior in the
spin correlation functions is demonstrated and discussed in detail. The present
work reproduces earlier results in good agreement with experimental data on
copper benzoate and predicts new interesting field-induced features at very
high magnetic field.Comment: 8 pages, 6 figure
Low-energy properties and magnetization plateaus in a 2-leg mixed spin ladder
Using the density matrix renormalization group technique we investigate the
low-energy properties and the magnetization plateau behavior in a 2-leg mixed
spin ladder consisting of a spin-1/2 chain coupled with a spin-1 chain. The
calculated results show that the system is in the same universality class as
the spin-3/2 chain when the interchain coupling is strongly ferromagnetic, but
the similarity between the two systems is less clear under other coupling
conditions. We have identified two types of magnetization plateau phases. The
calculation of the magnetization distribution on the spin-1/2 and the spin-1
chains on the ladder shows that one plateau phase is related to the partially
magnetized valence-bond-solid state, and the other plateau state contains
strongly coupled S=1 and s=1/2 spins on the rung.Comment: 6 pages with 8 eps figure
DMRG Study of Critical Behavior of the Spin-1/2 Alternating Heisenberg Chain
We investigate the critical behavior of the S=1/2 alternating Heisenberg
chain using the density matrix renormalization group (DMRG). The ground-state
energy per spin and singlet-triplet energy gap are determined for a range of
alternations. Our results for the approach of the ground-state energy to the
uniform chain limit are well described by a power law with exponent p=1.45. The
singlet-triplet gap is also well described by a power law, with a critical
exponent of p=0.73, half of the ground-state energy exponent. The
renormalization group predictions of power laws with logarithmic corrections
can also accurately describe our data provided that a surprisingly large scale
parameter is present in the logarithm.Comment: 6 pages, 4 eps-figure
Transport and Fate of MethyI Iodide and Its Pest Control in Soils
For fumigants, information on transport and fate, as well as pest control, is needed to develop management practices with the fewest human and environmental health risks while offering sufficient pest control efficacy. For this purpose, a 2-D soil chamber (60 cm wide, 60 cm long, and 6 cm thick) with a surface-mounted flux chamber was designed to determine volatilization, spatial and temporal distribution of soil gas-phase concentration, degradation and organism survivability after methyl iodide (MeI) fumigation. Three types of pests (barnyardgrass seed [Echinochloa crus-galli], citrus nematode [Tylenchulus semipenetrans], and fungi [Fusarium oxysporum]) were used to give a broad spectrum of pest control information. After MeI fumigation at a rate of 56.43 kg ha-1 for 24 hr, about 25.8 % of MeI was emitted into air, 6.8 % remained in the soil, and 43.6% degraded in the soil (based on the residual iodide concentration). The uncertainty in the measured MeI degradation using iodide concentration was thought to contribute to the unrecovered MeI (about 23%). Based on the spatial and temporal distribution of soil gas-phase concentration, the concentration-time index (CT) and its distribution was quantified. The citrus nematodes were effectively eliminated even at low CT values (< 30 µg hr ml-1) but all Fusarium oxysporum survived at the applied rate. The response of barnyardgrass seeds spatially varied with the concentration-time index (CT) values in the 2-D soil chamber. To fully control barnyardgrass seeds, CT of greater than 300 µg hr ml-1 was required. Using this experimental approach, different fumigant emission reduction strategies can be tested and mathematical models can be verified to determine which strategies produce least emission to atmosphere while maintaining sufficient pest control efficacy
Transition from band insulator to Mott insulator in one dimension: Critical behavior and phase diagram
We report a systematic study of the transition from a band insulator (BI) to
a Mott insulator (MI) in a one-dimensional Hubbard model at half-filling with
an on-site Coulomb interaction U and an alternating periodic site potential V.
We employ both the zero-temperature density matrix renormalization group (DMRG)
method to determine the gap and critical behavior of the system and the
finite-temperature transfer matrix renormalization group method to evaluate the
thermodynamic properties. We find two critical points at U = and U =
that separate the BI and MI phases for a given V. A charge-neutral
spin-singlet exciton band develops in the BI phase (U<) and drops below
the band gap when U exceeds a special point Ue. The exciton gap closes at the
first critical point while the charge and spin gaps persist and coincide
between <U< where the system is dimerized. Both the charge and spin
gaps collapse at U = when the transition to the MI phase occurs. In the
MI phase (U>) the charge gap increases almost linearly with U while the
spin gap remains zero. These findings clarify earlier published results on the
same model, and offer insights into several important issues regarding an
appropriate scaling analysis of DMRG data and a full physical picture of the
delicate nature of the phase transitions driven by electron correlation. The
present work provides a comprehensive understanding for the critical behavior
and phase diagram for the transition from BI to MI in one-dimensional
correlated electron systems with a periodic alternating site potential.Comment: long version, 10 figure
Spin-1 Antiferromagnetic Heisenberg Chains in an External Staggered Field
We present in this paper a nonlinear sigma-model analysis of a spin-1
antiferromagnetic Heisenberg chain in an external commensurate staggered
magnetic field. After rediscussing briefly and extending previous results for
the staggered magnetization curve, the core of the paper is a novel
calculation, at the tree level, of the Green functions of the model. We obtain
precise results for the elementary excitation spectrum and in particular for
the spin gaps in the transverse and longitudinal channels. It is shown that,
while the spectral weight in the transverse channel is exhausted by a single
magnon pole, in the longitudinal one, besides a magnon pole a two-magnon
continuum appears as well whose weight is a stedily increasing function of the
applied field, while the weight of the magnon decreases correspondingly. The
balance between the two is governed by a sum rule that is derived and
discussed. A detailed comparison with the present experimental and numerical
(DMRG) status of the art as well as with previous analytical approaches is also
made.Comment: 23 pages, 3 figures, LaTe
Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations
The phase diagram of a spin-1/2 model is investigated by means of
exact diagonalizations on finite samples. This model is a generalization of the
model on the square lattice with two different nearest-neighbor
couplings and may be also viewed as an array of coupled Heisenberg
chains. The results suggest that the resonnating valence bond state predicted
by Nersesyan and Tsvelik [Phys. Rev. B {\bf 67}, 024422 (2003)] for is realized and extends beyond the limit of small interchain coupling
along a curve nearly coincident with the line where the energy per spin is
maximum. This line is likely bordered on both side by a columnar dimer long
range order. This columnar order could extends for which correspond
to the model.Comment: 14 pages, 21 figures, final versio
Missing OH source in a suburban environment near Beijing: observed and modelled OH an HO2 concentrations in summer 2006
Measurements of ambient OH and HO<sub>2</sub> radicals were performed by laser induced fluorescence (LIF) during CAREBeijing2006 (Campaigns of Air Quality Research in Beijing and Surrounding Region 2006) at the suburban site Yufa in the south of Beijing in summer 2006. On most days, local air chemistry was influenced by aged air pollution that was advected by a slow, almost stagnant wind from southern regions. Observed daily maxima of OH and HO<sub>2</sub><sup>*</sup> were in the range of (4–17) × 10<sup>6</sup> cm<sup>−3</sup> and (2–24) × 10<sup>8</sup> cm<sup>−3</sup>, respectively. During daytime, OH reactivities were generally high (10–30 s<sup>−1</sup>) and mainly contributed by VOCs and their oxidation products. The comparison of modelled and measured HO<sub>x</sub> concentrations reveals a systematic underprediction of OH as a function of NO. A large discrepancy of a factor 2.6 is found at the lowest NO concentration encountered (0.1 ppb), whereas the discrepancy becomes insignificant above 1 ppb NO. This study extends similar observations from the Pearl-River Delta (PRD) in South China to a more urban environment. The OH discrepancy at Yufa can be resolved, if NO-independent additional OH recycling is assumed in the model. The postulated Leuven Isoprene Mechanism (LIM) has the potential to explain the gap between modelled and measured OH at Beijing taking into account conservative error estimates, but still lacks experimental confirmation. This and the hereby unresolved discrepancy at PRD suggest that other VOCs besides isoprene might be involved in the required, additional OH recycling. Fast primary production of RO<sub>x</sub> radicals up to 7 ppb h<sup>−1</sup> was determined at Beijing which was dominated by the photolysis of O<sub>3</sub>, HONO, HCHO, and dicarbonyls. For a special case, 20 August, when the plume of Beijing city was encountered, a missing primary HO<sub>x</sub> source (≈ 3 ppb h<sup>−1</sup>) was determined under high NO<sub>x</sub> conditions similar to other urban areas like Mexico city. CAREBeijing2006 emphasizes the important role of OVOCs as a radical source and sink, and the need for further investigation of the chemical degradation of VOCs in order to better understand radical chemistry in VOC-rich air
Optical nanofibers and spectroscopy
We review our recent progress in the production and characterization of
tapered optical fibers with a sub-wavelength diameter waist. Such fibers
exhibit a pronounced evanescent field and are therefore a useful tool for
highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist
or of the medium surrounding. We use a carefully designed flame pulling process
that allows us to realize preset fiber diameter profiles. In order to determine
the waist diameter and to verify the fiber profile, we employ scanning electron
microscope measurements and a novel accurate in situ optical method based on
harmonic generation. We use our fibers for linear and non-linear absorption and
fluorescence spectroscopy of surface-adsorbed organic molecules and investigate
their agglomeration dynamics. Furthermore, we apply our spectroscopic method to
quantum dots on the surface of the fiber waist and to caesium vapor surrounding
the fiber. Finally, towards dispersive measurements, we present our first
results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B.
Changes according to referee suggestions: changed title, clarification of
some points in the text, added references, replacement of Figure 13
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