ReceivedWe examine the ground state of a Heisenberg model with arbitrary spin S on a one-dimensional lattice composed of diamond-shaped units. A unit includes two types of antiferromagnetic exchange interactions which frustrate each other. The system undergoes phase changes when the ratio λ between the exchange parameters varies. In some phases, strong frustration leads to larger local structures or clusters of spins than a dimer. We prove for arbitrary S that there exists a phase with four-spin cluster states, which was previously found numerically for a special value of λ in the S = 1/2 case. For S = 1/2 we show that there are three ground state phases and determine their boundaries.PACS numbers: 75.10.Jm, 75.50.Gg Effects of frustration in quantum antiferromagnets are of great current interest in solid state physics. In a classical system, strong frustration obstructs a simple antiferromagnetic (AF) ordering and produces another magnetic order characteristic of each system, e.g. the 120• structure or a spiral [1]. In a quantum system, the interplay of quantum fluctuations and frustration makes the situation more complicated. There may appear an exotic ground state which does not have magnetic order and has no classical analog. A typical example is the complete dimer state in the Majumdar-Ghosh (MG) model [2].There are two types of frustrated quantum spin systems. One is a system the classical version of which has a locally stable spin configuration in the ground state; i.e. any local deformation for a spin configuration always raises the energy. We say that such a system is elastic. The MG model, the model with linearly decreasing AF interactions [3,4] and the AF Heisenberg model on the triangular lattice are of this type. For the other type, the classical version of a system has ground-state spin configurations which can be locally deformed without raising the energy. Then the set of these configurations is a manifold with dimensions proportional to the system size. We say that a system of this type is floppy. The AF Heisenberg models on the ∆ chain [5], the double chain with diagonal interaction [6] and the kagomé lattice [7] are of this type. Some other floppy spin systems are also seen in [3].Specially interesting is floppy systems. For example, it is argued that the kagomé antiferromagnet has a mysterious peak in a low temperature part of the specific heat [7]. In spite of quite a few theoretical studies on this system, its quantum ground state and the low temperature thermodynamic properties are hardly clarified. The difficulty of the problem originates from the floppiness. In a classical floppy system, the configuration of a local set of spins can be deformed without affecting the other part. In the corresponding quantum system the local set may form a nearly closed state, or a cluster. A part surrounded by such clusters forms another cluster. Thus the total wave function becomes approximately of a direct product form. To confirm this picture, it is important to examine a simple floppy model in whi...
Protons with energies up to ~ 10 15 eV are the main component 1 of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking 2 . Electrons are known to be accelerated to cosmic-ray energies in supernova remnants 3,4 , and the shock waves associated with such remnants, when they hit the surrounding interstellar medium, could also provide the energy to accelerate protons. The signature of such a process would be the decay of pions (π 0 ), which are generated when the protons collide with atoms and molecules in an interstellar cloud: pion decay results in γ-rays with a particular spectral-energy distribution 5,6 . Here we report the observation of cascade showers of optical photons resulting fromγ-rays at energies of ~ 10 12 eV hitting Earth's upper atmosphere, in the direction of the supernova remnant RX J1713.7-3946. The spectrum is a good match to that predicted by pion decay, and cannot be explained by other mechanisms.
Pericytes are believed to originate from either mesenchymal or neural crest cells. It has recently been reported that pericytes play important roles in the central nervous system (CNS) by regulating blood-brain barrier homeostasis and blood flow at the capillary level. However, the origin of CNS microvascular pericytes and the mechanism of their recruitment remain unknown. Here, we show a new source of cerebrovascular pericytes during neurogenesis. In the CNS of embryonic day 10.5 mouse embryos, CD31+F4/80+ hematopoietic lineage cells were observed in the avascular region around the dorsal midline of the developing midbrain. These cells expressed additional macrophage markers such as CD206 and CD11b. Moreover, the CD31+F4/80+ cells phagocytosed apoptotic cells as functionally matured macrophages, adhered to the newly formed subventricular vascular plexus, and then divided into daughter cells. Eventually, these CD31+F4/80+ cells transdifferentiated into NG2/PDGFRβ/desmin-expressing cerebrovascular pericytes, enwrapping and associating with vascular endothelial cells. These data indicate that a subset of cerebrovascular pericytes derive from mature macrophages in the very early phase of CNS vascular development, which in turn are recruited from sites of embryonic hematopoiesis such as the yolk sac by way of blood flow.
A novel nonlinear sigma model method is proposed for the two-dimensional J1-J2 model, which is extended to include plaquette-type distortion. The nonlinear sigma model is properly derived without spoiling the original spin degrees of freedom. The method shows that a single disordered phase continuously extends from a frustrated uniform regime to an unfrustrated distorted regime. By the continuity and Oshikawa's commensurability condition, the disordered ground states for the uniform J1-J2 model are plaquette states with fourfold degeneracy.
To elucidate sleep disturbances in the etiology of peptic ulcers, the prevalence of peptic ulcer disease was compared among shift workers and daytime workers. The subjects (N = 11,657) were employees of various institutions, such as factories, banks, or schools, and had undergone a mass x-ray examination of the upper gastrointestinal tract. With present shift workers (N = 2269), the prevalence of gastric ulcers was 2.38% (N = 54) and that of duodenal ulcers, 1.37% (N = 31). With the past shift workers (N = 2111), the prevalence of gastric ulcers was 1.52% (N = 32) and that of duodenal ulcers, 0.62% (N = 13). On the contrary, with the daytime workers (N = 6525) the prevalence of gastric ulcer was 1.03% (N = 67) and that of duodenal ulcer, 0.69% (N = 45). The working schedule for 752 examinees was unknown. The prevalence of gastric and duodenal ulcers was higher with shift workers than daytime workers. Thus, shift work involving sleep disturbances may play an important role in the development of peptic ulcers.
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