4,548 research outputs found
Thermodynamic Study of Excitations in a 3D Spin Liquid
In order to characterize thermal excitations in a frustrated spin liquid, we
have examined the magnetothermodynamics of a model geometrically frustrated
magnet. Our data demonstrate a crossover in the nature of the spin excitations
between the spin liquid phase and the high-temperature paramagnetic state. The
temperature dependence of both the specific heat and magnetization in the spin
liquid phase can be fit within a simple model which assumes that the spin
excitations have a gapped quadratic dispersion relation.Comment: 5 figure
Field-induced breakdown of the quantum Hall effect
A numerical analysis is made of the breakdown of the quantum Hall effect
caused by the Hall electric field in competition with disorder. It turns out
that in the regime of dense impurities, in particular, the number of localized
states decreases exponentially with the Hall field, with its dependence on the
magnetic and electric field summarized in a simple scaling law. The physical
picture underlying the scaling law is clarified. This intra-subband process,
the competition of the Hall field with disorder, leads to critical breakdown
fields of magnitude of a few hundred V/cm, consistent with observations, and
accounts for their magnetic-field dependence \propto B^{3/2} observed
experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.
Hyperglycaemia and Ischaemia Impair Wound Healing via Toll-like Receptor 4 Pathway Activation in vitro and in an Experimental Murine Model
OBJECTIVE: Diabetes mellitus has reached epidemic proportions. Foot ulceration is a multifactorial complication of diabetes associated with marked morbidity and mortality. Innate immune Toll-like receptor 4 (TLR4) mediated inflammation has been implicated in the systemic pathogenesis of diabetes and may contribute to impairment of wound healing. This study investigates the effect of high glucose and hypoxic conditions on TLR4 activation and signalling in vitro and in vivo. METHODS: Fibroblasts cultured at physiological glucose concentration (5.5 mM) were exposed to glucose concentrations from 0 mM to 25 mM, with duplicates placed in a hypoxic chamber. TLR4 inhibition was assessed in the 25 mM glucose groups. Diabetes was induced in wild type (WT) and TLR4 knockout (KO) C57BL/6 mice by intraperitoneal injection of low dose streptozocin (STZ). Hindlimb ischaemia was induced by femoral artery ligation four weeks post streptozocin, and a full thickness 4 mm skin wound inflicted below the knee. Wound healing was assessed via digital planimetry on days 3, 7, and 14 post surgery. RESULTS: Hypoxic and high glucose (25 mM) conditions led to an increase in TLR4 protein expression, apoptosis, and interleukin (IL)-6 release. Inhibition with a TLR4 neutralising antibody and specific TLR4 antagonist ameliorated the effects of high glucose and ischaemia (p < .05). In vivo, wound healing was significantly impaired in the diabetic ischaemic group at day 14 (p < .05). Diabetic ischaemic wounds in TLR4 KO mice exhibited significantly improved healing rates compared with those in WT mice at all time points. CONCLUSION: Hypoxia stimulates upregulation of TLR4 protein expression and this effect is exaggerated by hyperglycaemia. In TLR4 KO mice, there is a significant improvement in the healing of diabetic ischaemic wounds compared with WT. It is suggested that a synergistic effect between hypoxia and hyperglycaemia impairing wound healing exists, through TLR4 mediated inflammation
Nighttime chemistry at a high altitude site above Hong Kong
Nighttime reactions of nitrogen oxides influence ozone, volatile organic compounds, and aerosol and are thus important to the understanding of regional air quality. Despite large emissions and rapid recent growth of nitrogen oxide concentrations, there are few studies of nighttime chemistry in China. Here we present measurements of nighttime nitrogen oxides, NO3 and N2O5, from a coastal mountaintop site in Hong Kong adjacent to the megacities of the Pearl River Delta region. This is the first study of nighttime chemistry from a site within the residual layer in China. Key findings include the following. First, highly concentrated urban NOx outflow from the Pearl River Delta region was sampled infrequently at night, with N2O5 mixing ratios up to 8 ppbv (1 min average) or 12 ppbv (1 s average) in nighttime aged air masses. Second, the average N2O5 uptake coefficient was determined from a best fit to the available steady state lifetime data as γ(N2O5) = 0.014 ± 0.007. Although this determination is uncertain due to the difficulty of separating N2O5 losses from those of NO3, this value is in the range of previous residual layer determinations of N2O5 uptake coefficients in polluted air in North America. Third, there was a significant contribution of biogenic hydrocarbons to NO3 loss inferred from canister samples taken during daytime. Finally, daytime N2O5 mixing ratios were in accord with their predicted photochemical steady state. Heterogeneous uptake of N2O5 in fog is determined to be an important production mechanism for soluble nitrate, even during daytime. Key Points Large (up to 12 ppbv N2O5) but infrequent nocturnal NOx outflow from the Pearl River Delta Average N2O5 uptake coefficients 0.014 ± 0.007, in line with residual layer measurements in the U.S. Daytime N2O5 follows predicted steady state but rapidly produces soluble nitrate in fog.Department of Civil and Environmental Engineerin
Towards a field theory of the fractional quantum Hall states
We present a Chern-Simons theory of the fractional quantum Hall effect in
which flux attachment is followed by a transformation that effectively attaches
the correlation holes. We extract the correlated wavefunctions, compute the
drift and cyclotron currents (due to inhomogeneous density), exhibit the Read
operator, and operators that create quasi-particles and holes. We show how the
bare kinetic energy can get quenched and replaced by one due to interactions.
We find that for the low energy theory has neutral quasiparticles
and give the effective hamiltonian and constraints.Comment: Published versio
Stripe State in the Lowest Landau Level
The stripe state in the lowest Landau level is studied by the density matrix
renormalization group (DMRG) method. The ground state energy and pair
correlation functions are systematically calculated for various
pseudopotentials in the lowest Landau level. We show that the stripe state in
the lowest Landau level is realized only in a system whose width perpendicular
to the two-dimensional electron layer is smaller than the order of magnetic
length.Comment: 4 pages, 6 figures, to appear in J. Phys. Soc. Jpn. vol.73 No.1
(2004
Fractional Quantum Hall States in Narrow Channels
A model system is considered where two dimensional electrons are confined by
a harmonic potential in one direction, and are free in the other direction.
Ground state in strong magnetic fields is investigated through numerical
diagonalization of the Hamiltonian. It is shown that the fractional quantum
Hall states are realized even in the presence of the external potential under
suitable conditions, and a phase diagram is obtained.Comment: 8 pages, 2 figures (not included
Enhanced magnetocaloric effect in frustrated magnets
The magnetothermodynamics of strongly frustrated classical Heisenberg
antiferromagnets on kagome, garnet, and pyrochlore lattices is examined. The
field induced adiabatic temperature change (dT/dH)_S is significantly larger
for such systems compared to ordinary non-frustrated magnets and also exceeds
the cooling rate of an ideal paramagnet in a wide range of fields. An
enhancement of the magnetocaloric effect is related to presence of a
macroscopic number of soft modes in frustrated magnets below the saturation
field. Theoretical predictions are confirmed with extensive Monte Carlo
simulations.Comment: 7 page
Zener Tunneling Between Landau Orbits in a High-Mobility Two-Dimensional Electron Gas
Magnetotransport in a laterally confined two-dimensional electron gas (2DEG)
can exhibit modified scattering channels owing to a tilted Hall potential.
Transitions of electrons between Landau levels with shifted guiding centers can
be accomplished through a Zener tunneling mechanism, and make a significant
contribution to the magnetoresistance. A remarkable oscillation effect in weak
field magnetoresistance has been observed in high-mobility 2DEGs in
GaAs-AlGaAs heterostructures, and can be well explained by the
Zener mechanism.Comment: 5 pages, 4 figures. Text slightly shortened, figures resize
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