187,941 research outputs found
Energy Density Functional analysis of shape evolution in N=28 isotones
The structure of low-energy collective states in proton-deficient N=28
isotones is analyzed using structure models based on the relativistic energy
density functional DD-PC1. The relativistic Hartree-Bogoliubov model for
triaxial nuclei is used to calculate binding energy maps in the
- plane. The evolution of neutron and proton single-particle
levels with quadrupole deformation, and the occurrence of gaps around the Fermi
surface, provide a simple microscopic interpretation of the onset of
deformation and shape coexistence. Starting from self-consistent constrained
energy surfaces calculated with the functional DD-PC1, a collective Hamiltonian
for quadrupole vibrations and rotations is employed in the analysis of
excitation spectra and transition rates of Ar, S, and Si.
The results are compared to available data, and previous studies based either
on the mean-field approach or large-scale shell-model calculations. The present
study is particularly focused on S, for which data have recently been
reported that indicate pronounced shape coexistence.Comment: 31 pages, 11 figures. arXiv admin note: text overlap with
arXiv:1102.419
Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates
The electronic nematic order characterized by broken rotational symmetry has
been suggested to play an important role in the phase diagram of the high
temperature cuprates. We study the interplay between the electronic nematic
order and a spin density wave order in the presence of a magnetic field. We
show that a cooperation of the nematicity and the magnetic field induces a
finite coupling between the spin density wave and spin-triplet staggered flux
orders. As a consequence of such a coupling, the magnon gap decreases as the
magnetic field increases, and it eventually condenses beyond a critical
magnetic field leading to a field-induced spin density wave order. Both
commensurate and incommensurate orders are studied, and the experimental
implications of our findings are discussed.Comment: 5 pages, 3 figure
Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors
We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors
under a modified piezoresponse force microscope. We obtained domain evolution
images during polarization switching process and observed that domain
nucleation occurs at particular sites. This inhomogeneous nucleation process
should play an important role in an early stage of switching and under a high
electric field. We found that the number of nuclei is linearly proportional to
log(switching time), suggesting a broad distribution of activation energies for
nucleation. The nucleation sites for a positive bias differ from those for a
negative bias, indicating that most nucleation sites are located at
ferroelectric/electrode interfaces
Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite
Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown
impressive power conversion efficiencies of above 20%. However, the microscopic
mechanism of the high photovoltaic performance is yet to be fully understood.
Particularly, the dynamics of CH3NH3+ cations and their impact on relevant
processes such as charge recombination and exciton dissociation are still
poorly understood. Here, using elastic and quasi-elastic neutron scattering
techniques and group theoretical analysis, we studied rotational modes of the
CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K)
and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-fold
rotational symmetry of the C-N axis (C4) along with three-fold rotation around
the C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation is
present. Around room temperature, the characteristic relaxation times for the
C4 rotation is found to be ps while for the C3 rotation ps. The -dependent
rotational relaxation times were fitted with Arrhenius equations to obtain
activation energies. Our data show a close correlation between the C4
rotational mode and the temperature dependent dielectric permittivity. Our
findings on the rotational dynamics of CH3NH3+ and the associated dipole have
important implications on understanding the low exciton binding energy and slow
charge recombination rate in CH3NH3PbI3 which are directly relevant for the
high solar cell performance
The STAR MAPS-based PiXeL detector
The PiXeL detector (PXL) for the Heavy Flavor Tracker (HFT) of the STAR
experiment at RHIC is the first application of the state-of-the-art thin
Monolithic Active Pixel Sensors (MAPS) technology in a collider environment.
Custom built pixel sensors, their readout electronics and the detector
mechanical structure are described in detail. Selected detector design aspects
and production steps are presented. The detector operations during the three
years of data taking (2014-2016) and the overall performance exceeding the
design specifications are discussed in the conclusive sections of this paper
How Many Cooks Spoil the Soup?
In this work, we study the following basic question: "How much parallelism
does a distributed task permit?" Our definition of parallelism (or symmetry)
here is not in terms of speed, but in terms of identical roles that processes
have at the same time in the execution. We initiate this study in population
protocols, a very simple model that not only allows for a straightforward
definition of what a role is, but also encloses the challenge of isolating the
properties that are due to the protocol from those that are due to the
adversary scheduler, who controls the interactions between the processes. We
(i) give a partial characterization of the set of predicates on input
assignments that can be stably computed with maximum symmetry, i.e.,
, where is the minimum multiplicity of a state in
the initial configuration, and (ii) we turn our attention to the remaining
predicates and prove a strong impossibility result for the parity predicate:
the inherent symmetry of any protocol that stably computes it is upper bounded
by a constant that depends on the size of the protocol.Comment: 19 page
Ground state energy of unitary fermion gas with the Thomson Problem approach
The dimensionless universal coefficient defines the ratio of the
unitary fermions energy density to that for the ideal non-interacting ones in
the non-relativistic limit with T=0. The classical Thomson Problem is taken as
a nonperturbative quantum many-body arm to address the ground state energy
including the low energy nonlinear quantum fluctuation/correlation effects.
With the relativistic Dirac continuum field theory formalism, the concise
expression for the energy density functional of the strongly interacting limit
fermions at both finite temperature and density is obtained. Analytically, the
universal factor is calculated to be . The energy gap is
\Delta=\frac{{5}{18}{k_f^2}/(2m).Comment: Identical to published version with revisions according to comment
Superconductivity induced by oxygen deficiency in Sr-doped LaOFeAs
We synthesized Sr-doped sample with single phase,
and systematically studied the effect of oxygen deficiency in the Sr-doped
LaOFeAs system. It is found that substitution of Sr for La indeed induces the
hole carrier evidenced by positive thermoelectric power (TEP), but no bulk
superconductivity is observed. The superconductivity can be realized by
annealing the as-grown sample in vacuum to produce the oxygen deficiency. With
increasing the oxygen deficiency, the superconducting transition temperature
() increases and maximum reaches about 26 K the same as that in
La(O,F)FeAs. TEP dramatically changes from positive to negative in the
nonsuperconducting as-grown sample to the superconducting samples with oxygen
deficiency. While is always negative for all samples (even for Sr-doped
as grown sample). It suggests that the is
still electron-type superconductor.Comment: 4 pages, 4 figure
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