12,690 research outputs found
Ag and N acceptors in ZnO: ab initio study of acceptor pairing, doping efficiency, and the role of hydrogen
Efficiency of ZnO doping with Ag and N shallow acceptors, which substitute
respectively cations and anions, was investigated. First principles
calculations indicate a strong tendency towards formation of nearest neighbor
Ag-N pairs and N-Ag-N triangles. Binding of acceptors stems from the formation
of quasi-molecular bonds between dopants, and has a universal character in
semiconductors. The pairing increases energy levels of impurities, and thus
lowers doping efficiency. In the presence of donors, pairing is weaker or even
forbidden. However, hydrogen has a tendency to form clusters with Ag and N,
which favors the Ag-N aggregation and lowers the acceptor levels of such
complexes.Comment: 10 pages, 4 figure
The Formation and Fragmentation of Disks around Primordial Protostars
The very first stars to form in the Universe heralded an end to the cosmic
dark ages and introduced new physical processes that shaped early cosmic
evolution. Until now, it was thought that these stars lived short, solitary
lives, with only one extremely massive star, or possibly a very wide binary
system, forming in each dark matter minihalo. Here we describe numerical
simulations that show that these stars were, to the contrary, often members of
tight multiple systems. Our results show that the disks that formed around the
first young stars were unstable to gravitational fragmentation, possibly
producing small binary and higher-order systems that had separations as small
as the distance between the Earth and the Sun.Comment: This manuscript has been accepted for publication in Science. This
version has not undergone final editing. Please refer to the complete version
of record at http://www.sciencemag.org
Formation and evolution of primordial protostellar systems
We investigate the formation of the first stars at the end of the cosmic dark
ages with a suite of three-dimensional, moving mesh simulations that directly
resolve the collapse of the gas beyond the formation of the first protostar at
the centre of a dark matter minihalo. The simulations cover more than 25 orders
of magnitude in density and have a maximum spatial resolution of 0.05 R_sun,
which extends well below the radius of individual protostars and captures their
interaction with the surrounding gas. In analogy to previous studies that
employed sink particles, we find that the Keplerian disc around the primary
protostar fragments into a number of secondary protostars, which is facilitated
by H2 collisional dissociation cooling and collision-induced emission. The
further evolution of the protostellar system is characterized by strong
gravitational torques that transfer angular momentum between the secondary
protostars formed in the disc and the surrounding gas. This leads to the
migration of about half of the secondary protostars to the centre of the cloud
in a free-fall time, where they merge with the primary protostar and enhance
its growth to about five times the mass of the second most massive protostar.
By the same token, a fraction of the protostars obtain angular momentum from
other protostars via N-body interactions and migrate to higher orbits. On
average, only every third protostar survives until the end of the simulation.
However, the number of protostars present at any given time increases
monotonically, suggesting that the system will continue to grow beyond the
limited period of time simulated here.Comment: 19 pages, 13 figures, accepted for publication in MNRAS, movies of
the simulations may be downloaded at http://www.mpa-garching.mpg.de/~tgrei
High-field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2/2H2O
We report results of 51V NMR experiments on a high-quality powder sample of
volborthite Cu3V2O7(OH)2/2H2O, a spin-1/2 Heisenberg antiferromagnet on a
distorted kagome lattice. Following the previous experiments in magnetic fields
below 12 T, the NMR measurements have been extended to higher fields up to
31 T. In addition to the two already known ordered phases (phases I and II), we
found a new high-field phase (phase III) above 25 T, at which a second
magnetization step has been observed. The transition from the paramagnetic
phase to the antiferromagnetic phase III occurs at 26 K, which is much higher
than the transition temperatures from the paramagnetic to the lower field
phases I (B < 4.5 T) and II (4.5 < B < 25 T). At low temperatures, two types of
the V sites are observed with different relaxation rates and line shapes in
phase III as well as in phase II. Our results indicate that both phases II and
III exhibit a heterogeneous spin state consisting of two spatially alternating
Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting
with the other showing a conventional static order. The magnetization of the
latter system exhibits a sudden increase upon entering into phase III,
resulting in the second magnetization step at 26 T.We discuss the possible spin
structure in phase III.Comment: 9 pages, 12 figure
Composite CDMA - A statistical mechanics analysis
Code Division Multiple Access (CDMA) in which the spreading code assignment
to users contains a random element has recently become a cornerstone of CDMA
research. The random element in the construction is particular attractive as it
provides robustness and flexibility in utilising multi-access channels, whilst
not making significant sacrifices in terms of transmission power. Random codes
are generated from some ensemble, here we consider the possibility of combining
two standard paradigms, sparsely and densely spread codes, in a single
composite code ensemble. The composite code analysis includes a replica
symmetric calculation of performance in the large system limit, and
investigation of finite systems through a composite belief propagation
algorithm. A variety of codes are examined with a focus on the high
multi-access interference regime. In both the large size limit and finite
systems we demonstrate scenarios in which the composite code has typical
performance exceeding sparse and dense codes at equivalent signal to noise
ratio.Comment: 23 pages, 11 figures, Sigma Phi 2008 conference submission -
submitted to J.Stat.Mec
Specific Heat Study on a Novel Spin-Gapped System : (CH_3)_2NH_2CuCl_3
Specific heat measurements down to 120mK have been performed on a
quasi-one-dimensional spin-gapped system (CH)NHCuCl in
a magnetic field up to 8 T. This compound has a characteristic magnetization
curve which shows a gapless ground state and a plateau at 1/2 of the saturation
value. We have observed a spontaneous antiferromagnetic ordering and a
field-induced one below and above the 1/2 plateau field range, respectively.
The field versus temperature phase diagram is quite unusual and completely
different from those of the other quantum spin systems investigated so far. In
the plateau field range, a double-structure in the specific heat is observed,
reflecting the coexistence of ferromagnetic and antiferromagnetic excitations.
These behaviors are discussed on the basis of a recently proposed novel quantum
spin chain model consisting of weakly coupled ferromagnetic and
antiferromagnetic dimers.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp
Magnetization Process of Kagome-Lattice Heisenberg Antiferromagnet
The magnetization process of the isotropic Heisenberg antiferromagnet on the
kagome lattice is studied. Data obtained from the numerical-diagonalization
method are reexamined from the viewpoint of the derivative of the magnetization
with respect to the magnetic field. We find that the behavior of the derivative
at approximately one-third of the height of the magnetization saturation is
markedly different from that for the cases of typical magnetization plateaux.
The magnetization process of the kagome-lattice antiferromagnet reveals a new
phenomenon, which we call the "magnetization ramp".Comment: 4 pages, 5figures, accepted in J. Phys. Soc. Jpn
Measuring the Lyapunov exponent using quantum mechanics
We study the time evolution of two wave packets prepared at the same initial
state, but evolving under slightly different Hamiltonians. For chaotic systems,
we determine the circumstances that lead to an exponential decay with time of
the wave packet overlap function. We show that for sufficiently weak
perturbations, the exponential decay follows a Fermi golden rule, while by
making the difference between the two Hamiltonians larger, the characteristic
exponential decay time becomes the Lyapunov exponent of the classical system.
We illustrate our theoretical findings by investigating numerically the overlap
decay function of a two-dimensional dynamical system.Comment: 9 pages, 6 figure
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