26,204 research outputs found
The single-electron transport in a three-ion magnetic molecule modulated by a transverse field
We study single-electron transport in a three-ion molecule with strong
uniaxial anisotropy and in the presence of a transverse magnetic field. Two
magnetic ions are connected to each other through a third, nonmagnetic ion. The
magnetic ions are coupled to ideal metallic leads and a back gate voltage is
applied to the molecule, forming a field-effect transistor. The microscopic
Hamiltonian describing this system includes inter-ion hopping, on-site
repulsions, and magnetic anisotropies. For a range of values of the parameters
of the Hamiltonian, we obtain an energy spectrum similar to that of
single-molecule magnets in the giant-spin approximation where the two states
with maximum spin projection along the uniaxial anisotropy axis are well
separated from other states. In addition, upon applying an external in-plane
magnetic field, the energy gap between the ground and first excited states of
the molecule oscillates, going to zero at certain special values of the field,
in analogy to the diabolical points resulting from Berry phase interference in
the giant spin model. Thus, our microscopic model provides the same
phenomenological behavior expected from the giant spin model of a
single-molecule magnet but with direct access to the internal structure of the
molecule, thus making it more appropriate for realistic electronic transport
studies. To illustrate this point, the nonlinear electronic transport in the
sequential tunneling regime is evaluated for values of the field near these
degeneracy points. We show that the existence of these points has a clear
signature in the I-V characteristics of the molecule, most notably the
modulation of excitation lines in the differential conductance.Comment: 10 pages, 13 figure
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Age-related changes in blood-brain barrier integrity in C57BL/6J mice
The blood-brain barrier (BBB) is formed by the endothelial cells of the brain microvasculature, which control the molecular traffic between the blood and brain to maintain the neural microenvironment
Stellar black holes: cosmic history and feedback at the dawn of the universe
Significant historic cosmic evolution for the formation rate of stellar black
holes is inferred from current theoretical models of the evolution of massive
stars, the multiple observations of compact stellar remnants in the near and
distant universe, and the cosmic chemical evolution. The mean mass of stellar
black holes, the fraction of black holes/neutron stars, and the fraction of
black hole high mass X-ray binaries (BH-HMXBs)/solitary black holes increase
with redshift. The energetic feedback from large populations of BH-HMXBs form
in the first generations of star burst galaxies has been overlooked in most
cosmological models of the reionization epoch of the universe. The powerful
radiation, jets, and winds from BH-HMXBs heat the intergalactic medium over
large volumes of space and keep it ionized until AGN take over. It is concluded
that stellar black holes constrained the properties of the faintest galaxies at
high redshifts. I present here the theoretical and observational grounds for
the historic cosmic evolution of stellar black holes. Detailed calculations on
their cosmic impact are presented elsewhere (Mirabel, Dijkstra, Laurent, Loeb,
Pritchard, 2011).Comment: 9 pages, 1 color figure. Invited talk at the IAU Symp. 275, Jets at
all scales. Held in Buenos Aires on 13-17 September 2010. To be published by
Cambridge University Press. Eds. G. Romero, R. Sunyaev and T. Bellon
Recurrent microblazar activity in Cygnus X-1?
Recurrent flaring events at X- and soft gamma-ray energies have been recently
reported for the galactic black hole candidate Cygnus X-1. The observed fluxes
during these transient outbursts are far higher than what is observed in
``normal'' episodes. Here we suggest that the origin of this radiation is
non-thermal and produced by inverse Compton interactions between relativistic
electrons in the jet and external photon fields, with a dominant contribution
from the companion star field. The recurrent and relatively rapid variability
could be explained by the precession of the jet, which results in a variable
Doppler amplification.Comment: 4 pages, 5 figures, Accepted for publication in Astronomy &
Astrophysics Letter
Low Temperature AC Conductivity in BSCCO (2212)
We report measurements of anamolously large dissipative conductivities in
BiSrCaCuO(2212) at low temperatures. We have measured the complex conductivity
of BSCCO thin films at 100-600 GHz as a function of doping from the underdoped
to the overdoped state. At low temperatures there exists a residual dissipative
conductivity which scales with the T=0 superfluid density as the doping is
varied. This residual dissipative conductivity is larger than the possible
contribution from a thermal population of quasiparticles at the d-wave gap
nodes.Comment: Submitted to the Proceedings of the 22nd International Low
Temperature Physics Conference. To be published in Physica B
(http://www.elsevier.nl/locate/physb); 2 Pages with 2 Figure
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