1,042 research outputs found
Analytic Relations between Localizable Entanglement and String Correlations in Spin Systems
We study the relation between the recently defined localizable entanglement
and generalized correlations in quantum spin systems. Differently from the
current belief, the localizable entanglement is always given by the average of
a generalized string. Using symmetry arguments we show that in most spin 1/2
and spin 1 systems the localizable entanglement reduces to the spin-spin or
string correlations, respectively. We prove that a general class of spin 1
systems, which includes the Heisenberg model, can be used as perfect quantum
channel. These conclusions are obtained in analytic form and confirm some
results found previously on numerical grounds.Comment: 5 pages, RevTeX
Spin Chains in an External Magnetic Field. Closure of the Haldane Gap and Effective Field Theories
We investigate both numerically and analytically the behaviour of a spin-1
antiferromagnetic (AFM) isotropic Heisenberg chain in an external magnetic
field. Extensive DMRG studies of chains up to N=80 sites extend previous
analyses and exhibit the well known phenomenon of the closure of the Haldane
gap at a lower critical field H_c1. We obtain an estimate of the gap below
H_c1. Above the lower critical field, when the correlation functions exhibit
algebraic decay, we obtain the critical exponent as a function of the net
magnetization as well as the magnetization curve up to the saturation (upper
critical) field H_c2. We argue that, despite the fact that the SO(3) symmetry
of the model is explicitly broken by the field, the Haldane phase of the model
is still well described by an SO(3) nonlinear sigma-model. A mean-field theory
is developed for the latter and its predictions are compared with those of the
numerical analysis and with the existing literature.Comment: 11 pages, 4 eps figure
Paleomagnetic record of basaltic volcanism from Pukaki and Onepoto maar lake cores, Auckland Volcanic Field, New Zealand
The Auckland Volcanic Field contains several maars that formed after the last interglacial and subsequently filled with sediment. Two of these maars, Pukaki and Onepoto, were recently cored as part of the Auckland Maar Lakes Project. The tephra stratigraphy of the cores indicates that sediment accumulated relatively slowly in both maars until the Holocene when ocean waters breached the craters and they filled up quite rapidly. Using u-channels, we collected 23 m of pre-Holocene lacustrine sediment from the Pukaki 1-01 core and 15 m from the Onepoto core. Paleomagnetic measurements were performed on these at the University of California, Davis. Environmental magnetic records from both cores provide insights in particular about the eruptive history of the Auckland Volcanic Field. The lack of a tephrostratigraphic control in the lower portion of the cores, and the lack of similar trends in the magnetic parameters, prevented a complete core correlation. The main finding is that local basaltic tephra layers visible in the cores show up as spikes in the concentration dependent magnetic parameters, suggesting that other spikes represent tephra layers that are not as easily discerned
Low mass star formation and subclustering in the HII regions RCW 32, 33 and 27 of the Vela Molecular Ridge. A photometric diagnostics to identify M-type stars
Most stars born in clusters and recent results suggest that star formation
(SF) preferentially occurs in subclusters. Studying the morphology and SF
history of young clusters is crucial to understanding early SF. We identify the
embedded clusters of young stellar objects (YSOs) down to M stars, in the HII
regions RCW33, RCW32 and RCW27 of the Vela Molecular Ridge. Our aim is to
characterise their properties, such as morphology and extent of the clusters in
the three HII regions, derive stellar ages and the connection of the SF history
with the environment. Through public photometric surveys such as Gaia, VPHAS,
2MASS and Spitzer/GLIMPSE, we identify YSOs with IR, Halpha and UV excesses, as
signature of circumstellar disks and accretion. In addition, we implement a
method to distinguish M dwarfs and giants, by comparing the reddening derived
in several optical/IR color-color diagrams, assuming suitable theoretical
models. Since this diagnostic is sensitive to stellar gravity, the procedure
allows us to identify pre-main sequence stars. We find a large population of
YSOs showing signatures of circumstellar disks with or without accretion. In
addition, with the new technique of M-type star selection, we find a rich
population of young M stars with a spatial distribution strongly correlated to
the more massive population. We find evidence of three young clusters, with
different morphology. In addition, we identify field stars falling in the same
region, by securely classifying them as giants and foreground MS stars. We
identify the embedded population of YSOs, down to about 0.1 Msun, associated
with the HII regions RCW33, RCW32 and RCW27 and the clusters Vela T2, Cr197 and
Vela T1, respectively, showing very different morphologies. Our results suggest
a decreasing SF rate in Vela T2 and triggered SF in Cr197 and Vela T1.Comment: Accepted for publication in A&A; 20 pages, 22 figures, 6 table
Long-distance entanglement and quantum teleportation in XX spin chains
Isotropic XX models of one-dimensional spin-1/2 chains are investigated with
the aim to elucidate the formal structure and the physical properties that
allow these systems to act as channels for long-distance, high-fidelity quantum
teleportation. We introduce two types of models: I) open, dimerized XX chains,
and II) open XX chains with small end bonds. For both models we obtain the
exact expressions for the end-to-end correlations and the scaling of the energy
gap with the length of the chain. We determine the end-to-end concurrence and
show that model I) supports true long-distance entanglement at zero
temperature, while model II) supports {\it ``quasi long-distance''}
entanglement that slowly falls off with the size of the chain. Due to the
different scalings of the gaps, respectively exponential for model I) and
algebraic in model II), we demonstrate that the latter allows for efficient
qubit teleportation with high fidelity in sufficiently long chains even at
moderately low temperatures.Comment: 9 pages, 6 figure
Papillomavirus E5: the smallest oncoprotein with many functions
Papillomaviruses (PVs) are established agents of human and animal cancers. They infect cutaneous and mucous epithelia. High Risk (HR) Human PVs (HPVs) are consistently associated with cancer of the uterine cervix, but are also involved in the etiopathogenesis of other cancer types. The early oncoproteins of PVs: E5, E6 and E7 are known to contribute to tumour progression. While the oncogenic activities of E6 and E7 are well characterised, the role of E5 is still rather nebulous. The widespread causal association of PVs with cancer makes their study worthwhile not only in humans but also in animal model systems. The Bovine PV (BPV) system has been the most useful animal model in understanding the oncogenic potential of PVs due to the pivotal role of its E5 oncoprotein in cell transformation. This review will highlight the differences between HPV-16 E5 (16E5) and E5 from other PVs, primarily from BPV. It will discuss the targeting of E5 as a possible therapeutic agent
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