13,372 research outputs found
False vacuum decay: effective one-loop action for pair creation of domain walls
An effective one-loop action built from the soliton field itself for the
two-dimensional (2D) problem of soliton pair creation is proposed. The action
consists of the usual mass term and a kinetic term in which the simple
derivative of the soliton field is replaced by a covariant derivative. In this
effective action the soliton charge is treated no longer as a topological
charge but as a Noether charge. Using this effective one-loop action, the
soliton-antisoliton pair production rate is calculated and one recovers Stone's
exponential factor and the prefactor of Kiselev, Selivanov and Voloshin. The
results are also valid straightforwardly to the problem of pair creation rate
of domain walls in dimensions greater than 2.Comment: 12 pages, Late
Space VLBI Observations of 3C371
We present the first space VLBI observations of 3C~371, carried out at a
frequency of 4.8 GHz. The combination of the high resolution provided by the
orbiting antenna Highly Advanced Laboratory for Communications and Astronomy
(HALCA) and the high sensitivity of the VLBA allows imaging of the jet of
3C~371 with an angular resolution of approximately 0.26 mas, which for this
relatively nearby source corresponds to 0.4 h pc. Comparison
between two epochs separated by 66 days reveals no apparent motions in the
inner 7 mas jet structure above an upper limit of c. This
value, the absence of detectable counterjet emission from the presumably
symmetric jet, plus the presence of extended double-lobe structure, are
consistent with the knots in the jet being stationary features such as standing
shocks. The jet intensity declines with the angular distance from the core as
. This is more gradual than that derived for 3C~120,
, for which there is evidence for strong intereactions between
the jet and ambient medium. This suggests that in 3C~371 there is a greater
level of {\it in situ} acceleration of electrons and amplification of magnetic
field. We interpret sharp bends in the jet at sites of off-center knots as
further evidence for the interaction between the jet and external medium, which
may also be responsible for the generation of standing recollimation shocks.
These recollimation shocks may be responsible for the presumably stationary
components. The radio properties of 3C~371 are intermediate between those of
other radio galaxies with bright cores and those of BL Lacertae objects.Comment: 5 pages, 4 figures. Accepted for publication in Ap
Incidence of bullying and victimisation among adolescents in New Zealand
It has been established that bullying and victimisation have negative outcomes for those involved. However, this problem has received little research attention in New Zealand samples, particularly with longitudinal designs. The incidence of four types bullying was assessed in a large adolescent New Zealand sample including; traditional bullying inside the school, bullying outside the school, bullying via text message and bullying via the internet. The same categorisation of victimisation was also assessed. The overall rates of bullying and victimisation appeared elevated relative to international samples but traditional school-based bullying was more frequent than text or internet bullying. No gender differences were found. Differences for ethnic group differences were found only for specific types of bullying with Māori students reporting more traditional school and text bullying, and more text-based victimisation than other ethnic groups
Absorption in atomic wires
The transfer matrix formalism is implemented in the form of the multiple
collision technique to account for dissipative transmission processes by using
complex potentials in several models of atomic chains. The absorption term is
rigorously treated to recover unitarity for the non-hermitian hamiltonians. In
contrast to other models of parametrized scatterers we assemble explicit
potentials profiles in the form of delta arrays, Poschl-Teller holes and
complex Scarf potentials. The techniques developed provide analytical
expressions for the scattering and absorption probabilities of arbitrarily long
wires. The approach presented is suitable for modelling molecular aggregate
potentials and also supports new models of continuous disordered systems. The
results obtained also suggest the possibility of using these complex potentials
within disordered wires to study the loss of coherence in the electronic
localization regime due to phase-breaking inelastic processes.Comment: 14 pages, 15 figures. To appear in Phys. Rev.
Universal power law in the orientational relaxation in thermotropic liquid crystals
We observe a surprisingly general power law decay at short to intermediate
times in orientational relaxation in a variety of model systems (both calamitic
and discotic, and also discrete) for thermotropic liquid crystals. As all these
systems transit across the isotropic-nematic phase boundary, two power law
relaxation regimes, separated by a plateau, emerge giving rise to a step-like
feature (well-known in glassy liquids) in the single-particle second-rank
orientational time correlation function. In contrast to its probable dynamical
origin in supercooled liquids, we show that the power law here can originate
from the thermodynamic fluctuations of the orientational order parameter,
driven by the rapid growth in the second-rank orientational correlation length.Comment: Submitted to Physical Review Letter
Failure of the work-Hamiltonian connection for free energy calculations
Extensions of statistical mechanics are routinely being used to infer free
energies from the work performed over single-molecule nonequilibrium
trajectories. A key element of this approach is the ubiquitous expression
dW/dt=\partial H(x,t)/ \partial t which connects the microscopic work W
performed by a time-dependent force on the coordinate x with the corresponding
Hamiltonian H(x,t) at time t. Here we show that this connection, as pivotal as
it is, cannot be used to estimate free energy changes. We discuss the
implications of this result for single-molecule experiments and atomistic
molecular simulations and point out possible avenues to overcome these
limitations
Nonlocality of Majorana modes in hybrid nanowires
Spatial separation of Majorana zero modes distinguishes trivial from topological midgap states and is key to topological protection in quantum computing applications. Although signatures of Majorana zero modes in tunneling spectroscopy have been reported in numerous studies, a quantitative measure of the degree of separation, or nonlocality, of the emergent zero modes has not been reported. Here, we present results of an experimental study of nonlocality of emergent zero modes in superconductor-semiconductor hybrid nanowire devices. The approach takes advantage of recent theory showing that nonlocality can be measured from splitting due to hybridization of the zero mode in resonance with a quantum dot state at one end of the nanowire. From these splittings as well as anticrossing of the dot states, measured for even and odd occupied quantum dot states, we extract both the degree of nonlocality of the emergent zero mode, as well as the spin canting angles of the nonlocal zero mode. Depending on the device measured, we obtain either a moderate degree of nonlocality, suggesting a partially separated Andreev subgap state, or a highly nonlocal state consistent with a well-developed Majorana modeThis research was supported by Microsoft, the Danish National Research Foundation, the European Commission, and the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2015-65706-P, No. FIS2015-64654-P, and No. FIS2016-80434-P (AEI/FEDER, EU), the Ramón y Cajal programme Grant No. RYC-2011-09345, and the María de Maeztu Programme for Units of Excellence in R&D (Grant No. MDM-2014-0377). C.M.M. acknowledges support from the Villum Foundation. M.-T.D. acknowledges support from State Key Laboratory of High Performance Computing, Chin
A Proposal for a Multi-Drive Heterogeneous Modular Pipe- Inspection Micro-Robot
This paper presents the architecture used to develop a micro-robot for narrow pipes inspection. Both the electromechanical design and the control scheme will be described. In pipe environments it is very useful to have a method to retrieve information of the state of the inside part of the pipes in order to detect damages, breaks and holes. Due to the di_erent types of pipes that exists, a modular approach with di_erent types of modules has been chosen in order to be able to adapt to the shape of the pipe and to chose the most appropriate gait. The micro-robot has been designed for narrow pipes, a _eld in which there are not many prototypes. The robot incorporates a camera module for visual inspection and several drive modules for locomotion and turn (helicoidal, inchworm, two degrees of freedom rotation). The control scheme is based on semi-distributed behavior control and is also described. A simulation environment is also presented for prototypes testing
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