17,013 research outputs found
On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars
The Fourier Transform method is a popular tool to derive the rotational
velocities of stars from their spectral line profiles. However, its domain of
validity does not include line-profile variables with time-dependent profiles.
We investigate the performance of the method for such cases, by interpreting
the line-profile variations of spotted B stars, and of pulsating B tars, as if
their spectral lines were caused by uniform surface rotation along with
macroturbulence. We perform time-series analysis and harmonic least-squares
fitting of various line diagnostics and of the outcome of several
implementations of the Fourier Transform method. We find that the projected
rotational velocities derived from the Fourier Transform vary appreciably
during the pulsation cycle whenever the pulsational and rotational velocity
fields are of similar magnitude. The macroturbulent velocities derived while
ignoring the pulsations can vary with tens of km/s during the pulsation cycle.
The temporal behaviour of the deduced rotational and macroturbulent velocities
are in antiphase with each other. The rotational velocity is in phase with the
second moment of the line profiles. The application of the Fourier method to
stars with considerable pulsational line broadening may lead to an appreciable
spread in the values of the rotation velocity, and, by implication, of the
deduced value of the macroturbulence. These two quantities should therefore not
be derived from single snapshot spectra if the aim is to use them as a solid
diagnostic for the evaluation of stellar evolution models of slow to moderate
rotators.Comment: 13 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Pitfall of the Detection Rate Optimized Bit Allocation within template protection and a remedy
One of the requirements of a biometric template protection system is that the protected template ideally should not leak any information about the biometric sample or its derivatives. In the literature, several proposed template protection techniques are based on binary vectors. Hence, they require the extraction of a binary representation from the real- valued biometric sample. In this work we focus on the Detection Rate Optimized Bit Allocation (DROBA) quantization scheme that extracts multiple bits per feature component while maximizing the overall detection rate. The allocation strategy has to be stored as auxiliary data for reuse in the verification phase and is considered as public. This implies that the auxiliary data should not leak any information about the extracted binary representation. Experiments in our work show that the original DROBA algorithm, as known in the literature, creates auxiliary data that leaks a significant amount of information. We show how an adversary is able to exploit this information and significantly increase its success rate on obtaining a false accept. Fortunately, the information leakage can be mitigated by restricting the allocation freedom of the DROBA algorithm. We propose a method based on population statistics and empirically illustrate its effectiveness. All the experiments are based on the MCYT fingerprint database using two different texture based feature extraction algorithms
Dielectric properties of charge ordered LuFe2O4 revisited: The apparent influence of contacts
We show results of broadband dielectric measurements on the charge ordered,
proposed to be mul- tiferroic material LuFe2O4. The temperature and frequency
dependence of the complex permittivity as investigated for temperatures above
and below the charge-oder transition near T_CO ~ 320 K and for frequencies up
to 1 GHz can be well described by a standard equivalent-circuit model
considering Maxwell-Wagner-type contacts and hopping induced AC-conductivity.
No pronounced contribution of intrinsic dipolar polarization could be found and
thus the ferroelectric character of the charge order in LuFe2O4 has to be
questioned.Comment: 4 pages, 3 figure
Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates
We present a theoretical analysis of the selective darkening method for
implementing quantum controlled-NOT (CNOT) gates. This method, which we
recently proposed and demonstrated, consists of driving two
transversely-coupled quantum bits (qubits) with a driving field that is
resonant with one of the two qubits. For specific relative amplitudes and
phases of the driving field felt by the two qubits, one of the two transitions
in the degenerate pair is darkened, or in other words, becomes forbidden by
effective selection rules. At these driving conditions, the evolution of the
two-qubit state realizes a CNOT gate. The gate speed is found to be limited
only by the coupling energy J, which is the fundamental speed limit for any
entangling gate. Numerical simulations show that at gate speeds corresponding
to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and
increases further for lower gate speeds. In addition, the effect of
higher-lying energy levels and weak anharmonicity is studied, as well as the
scalability of the method to systems of multiple qubits. We conclude that in
all these respects this method is competitive with existing schemes for
creating entanglement, with the added advantages of being applicable for qubits
operating at fixed frequencies (either by design or for exploitation of
coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure
Magneto-resistance in a lithography defined single constrained domain wall spin valve
We have measured domain wall magnetoresistance in a single lithographically constrained domain wall. An H-shaped Ni nano-bridge was fabricated by e-beam lithography with the two sides being single magnetic do- mains showing independent magnetic switching. The connection between the sides constraining the domain wall when the sides line up anti-parallel. The magneto-resistance curve clearly identifies the magnetic con- figurations that are expected from a spin valve-like structure. The value of the magneto-resistance at room temperature is around 0.1% or 0.4 Â. This value is shown to be in agreement with a theoretical formulation based on spin accumulation. Micromagnetic simulations show it is possible to reduce the size of the domain wall further by shortening the length of the bridge
Kinetic cross coupling between non-conserved and conserved fields in phase field models
We present a phase field model for isothermal transformations of two
component alloys that includes Onsager kinetic cross coupling between the
non-conserved phase field and the conserved concentration field. We also
provide the reduction of the phase field model to the corresponding macroscopic
description of the free boundary problem. The reduction is given in a general
form. Additionally we use an explicit example of a phase field model and check
that the reduced macroscopic description, in the range of its applicability, is
in excellent agreement with direct phase field simulations. The relevance of
the newly introduced terms to solute trapping is also discussed
Metal-catalyst-free growth of carbon nanotubes and their application in field-effect transistors
The metal-catalyst-free growth of carbon nanotubes (CNTs) using chemical vapor deposition and the application in field-effect transistors (FETs) is demonstrated. The CNT growth process used a 3-nm-thick Ge layer on SiO2 that was subsequently annealed to produce Ge nanoparticles. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. The synthesized CNTs are used to fabricate CNTFETs and the best device has a state-of-the-art on/off current ratio of 3Ă—108 and a steep sub-threshold slope of 110 mV/dec
H-theorem for classical matter around a black hole
We propose a classical solution for the kinetic description of matter falling
into a black hole, which permits to evaluate both the kinetic entropy and the
entropy production rate of classical infalling matter at the event horizon. The
formulation is based on a relativistic kinetic description for classical
particles in the presence of an event horizon. An H-theorem is established
which holds for arbitrary models of black holes and is valid also in the
presence of contracting event horizons
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