50 research outputs found
Frustrated impurity spins in ordered two-dimensional quantum antiferromagnets
Dynamical properties of an impurity spin coupled symmetrically to sublattices
of ordered 2D Heisenberg quantum antiferromagnet (i.e., frustrated impurity
spin) are discussed at (existence of a small interaction stabilizing
the long range order at is implied). We continue our study on this
subject started in Phys.Rev.B 72, 174419 (2005), where spin-1/2 defect is
discussed and the host spins fluctuations are considered within the spin-wave
approximation (SWA). In the present paper we i) go beyond SWA and ii) study
impurities with spins . It is demonstrated that in contrast to defects
coupled to sublattices asymmetrically longitudinal host spins fluctuations play
important role in the frustrated impurity dynamics. The spectral function, that
is proportional to within SWA, acquires new terms proportional to
and . It is observed that the spin-1/2 impurity
susceptibility has the same structure as that obtained within SWA: the Lorenz
peak and the non-resonant term. The difference is that the width of the peak
becomes larger being proportional to rather than ,
where is the dimensionless coupling parameter. We show that transverse
static susceptibility acquires a new negative logarithmic contribution. In
accordance with previous works we find that host spins fluctuations lead to an
effective one-ion anisotropy on the impurity site. Then defects with
appears to be split. We observe strong reduction of the value of the splitting
due to longitudinal host spins fluctuations. We demonstrate that the dynamical
impurity susceptibility contains Lorenz peaks corresponding to transitions
between the levels, and the non-resonant term.Comment: 17 pages, 7 figures, to appear in PR
Hidden long range order in Heisenberg Kagome antiferromagnets
We give a physical picture of the low-energy sector of the spin 1/2
Heisenberg Kagome antiferromagnet (KAF). It is shown that Kagome lattice can be
presented as a set of stars which are arranged in a triangular lattice and
contain 12 spins. Each of these stars has two degenerate singlet ground states
which can be considered in terms of pseudospin. As a result of interaction
between stars we get Hamiltonian of the Ising ferromagnet in magnetic field. So
in contrast to the common view there is a long range order in KAF consisting of
definite singlet states of the stars.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Late pleistocene sedimentation history of the Shirshov Ridge, Bering Sea
The analysis of the lithology, grain-size distribution, clay minerals, and geochemistry of Upper
Pleistocene sediments from the submarine Shirshov Ridge (Bering Sea) showed that the main source area was
the Yukon–Tanana terrane of Central Alaska. The sedimentary materials were transported by the Yukon
River through Beringia up to the shelf break, where they were entrained by a strong northwestward-flowing
sea current. The lithological data revealed several pulses of ice-rafted debris deposition, roughly synchronous
with Heinrich events, and periods of weaker bottom-current intensity. Based on the geochemical results, we
distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. The
results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and
glacial cycles in Alaska
The uncoupling of respiration in plant mitochondria: Keeping reactive oxygen and nitrogen species under control
Plant mitochondrial respiration involves operation of various alternate pathways. These pathways participate, both directly and indirectly, in the maintenance of mitochondrial functions though they do not contribute to energy production being uncoupled from the generation of an electrochemical gradient across the mitochondrial membrane and thus from ATP production. Recent findings suggest that uncoupled respiration is involved in reactive oxygen species (ROS) and nitric oxide (NO) scavenging, regulation and homeostasis. Here we discuss specific roles and possible functions of the uncoupled mitochondrial respiration in ROS and NO metabolism. The mechanisms of expression and regulation of the NDA, NDB and NDC type non-coupled NADH and NADPH dehydrogenases, the alternative oxidase (AOX), and the uncoupling protein (UCP) are examined in relation to their involvement in the establishment of the stable far-from-equilibrium state of plant metabolism. The role of uncoupled respiration in controlling the levels of ROS and NO as well as inducing signaling events is considered. Secondary functions of uncoupled respiration include its role in protection from stress factors and roles in biosynthesis and catabolism. It is concluded that uncoupled mitochondrial respiration plays an important role in providing rapid adaptation of plants to changing environmental factors via regulation of ROS and NO