1,164 research outputs found
Ultra--cold gases and the detection of the Earth's rotation: Bogoliubov space and gravitomagnetism
The present work analyzes the consequences of the gravitomagnetic effect of
the Earth upon a bosonic gas in which the corresponding atoms have a
non--vanishing orbital angular momentum. Concerning the ground state of the
Bogoliubov space of this system we deduce the consequences, on the pressure and
on the speed of sound, of the gravitomagnetic effect. We prove that the effect
on a single atom is very small, but we also show that for some thermodynamical
properties the consequences scale as a non--trivial function of the number of
particles.Comment: 4 page
High-resolution thermal expansion measurements under Helium-gas pressure
We report on the realization of a capacitive dilatometer, designed for
high-resolution measurements of length changes of a material for temperatures
1.4 K 300 K and hydrostatic pressure 250 MPa. Helium
(He) is used as a pressure-transmitting medium, ensuring
hydrostatic-pressure conditions. Special emphasis has been given to guarantee,
to a good approximation, constant-pressure conditions during temperature
sweeps. The performance of the dilatometer is demonstrated by measurements of
the coefficient of thermal expansion at pressures 0.1 MPa (ambient
pressure) and 104 MPa on a single crystal of azurite,
Cu(CO)(OH), a quasi-one-dimensional spin S = 1/2 Heisenberg
antiferromagnet. The results indicate a strong effect of pressure on the
magnetic interactions in this system.Comment: 8 pages, 7 figures, published in Rev. Sci. Instrum with minor change
String amplitudes in arbitrary dimensions
We calculate gravitational dressed tachyon correlators in non critcal
dimensions. The 2D gravity part of our theory is constrained to constant
curvature. Then scaling dimensions of gravitational dressed vertex operators
are equal to their bare conformal dimensions. Considering the model as d+2
dimensional critical string we calculate poles of generalized Shapiro-Virasoro
amplitudes.Comment: 14 page
Negatively buoyant starting jets
The initial development of negatively buoyant jets has been investigated experimentally and numerically, focusing on the role played by gravity in the evolution of the leading vortex ring. Under the experimental conditions considered in this work, the densimetric Froude number, Fr= ρjU²j/[(ρ₀ − ρj) gD] , which represents the ratio between the jet momentum and the buoyancy forces,
emerges as the most relevant parameter characterizing the dynamics of the flow. Two different flow
regimes have been observed depending on the Froude number: for sufficiently small Fr, the vortex
ring generated initially is pushed radially away by gravity forces before it has time to detach from
the shear layer originating at the orifice. On the other hand, when the Froude number is larger than
a critical value, Fr> Frc∼ 1, the vortex ring detaches from the injection orifice and propagates
downstream into the stagnant ambient followed by a trailing jet until it eventually reaches a
maximum penetration depth. In order to clarify the mechanisms leading to the transition between the
two regimes, and to gain physical understanding of the formation dynamics of negatively buoyant
starting jets, the total and the vortex circulation, as well as the trajectory of the vortex center, have
been measured and compared to the case of neutrally buoyant jets. Finally, based on the
experimental measurements and on the results of the numerical computations, a kinematic model
that successfully describes the evolution of both total circulation and vortex trajectory is
proposed.This work was supported by the Spanish Ministry of Education under Project Nos. DPI2008-06624-C03-02 and ENE2008-0615-C04. This work has been extracted from the
Ph.D. thesis of Marugán-CruzPublicad
Far-field optical microscope with nanometer-scale resolution based on in-plane surface plasmon imaging
A new far-field optical microscopy technique capable of reaching
nanometer-scale resolution has been developed recently using the in-plane image
magnification by surface plasmon polaritons. This microscopy is based on the
optical properties of a metal-dielectric interface that may, in principle,
provide extremely large values of the effective refractive index n up to
100-1000 as seen by the surface plasmons. Thus, the theoretical diffraction
limit on resolution becomes lambda/2n, and falls into the nanometer-scale
range. The experimental realization of the microscope has demonstrated the
optical resolution better than 50 nm for 502 nm illumination wavelength.
However, the theory of such surface plasmon-based far-field microscope
presented so far gives an oversimplified picture of its operation. For example,
the imaginary part of the metal dielectric constant severely limits the
surface-plasmon propagation and the shortest attainable wavelength in most
cases, which in turn limits the microscope magnification. Here I describe how
this limitation has been overcome in the experiment, and analyze the practical
limits on the surface plasmon microscope resolution. In addition, I present
more experimental results, which strongly support the conclusion of extremely
high spatial resolution of the surface plasmon microscope.Comment: 23 pages, 9 figures, will be published in the topical issue on
Nanostructured Optical Metamaterials of the Journal of Optics A: Pure and
Applied Optics, Manuscript revised in response to referees comment
Stability and electronic structure of the complex KPtCl structure-type hydrides
The stability and bonding of the ternary complex KPtCl structure
hydrides is discussed using first principles density functional calculations.
The cohesion is dominated by ionic contributions, but ligand field effects are
important, and are responsible for the 18-electron rule. Similarities to oxides
are discussed in terms of the electronic structure. However, phonon
calculations for SrRuH also show differences, particularly in the
polarizability of the RuH octahedra. Nevertheless, the yet to be made
compounds PbRuH and BeFeH are possible ferroelectrics. The
electronic structure and magnetic properties of the decomposition product,
FeBe are reported. Implications of the results for H storage are discussed
Simulation of crack propagation in alumina with ab-initio based polarizable force field
We present an effective atomic interaction potential for crystalline
alpha-Al2O3 generated by the program potfit. The Wolf direct, pairwise
summation method with spherical truncation is used for electrostatic
interactions. The polarizability of oxygen atoms is included by use of the
Tangney-Scandolo interatomic force field approach. The potential is optimized
to reproduce the forces, energies and stresses in relaxed and strained
configurations as well as {0001}, {10-10} and {11-20} surfaces of Al2O3.
Details of the force field generation are given, and its validation is
demonstrated. We apply the developed potential to investigate crack propagation
in alpha-Al2O3 single crystals.Comment: 8 pages, 5 figure
Quasiparticle scattering time in superconducting films: from dirty to clean limit
We study the quasiparticle energy relaxation processes in superconducting Nb
films of different thicknesses corresponding to different electron mean free
paths in a state far from equilibrium, that is the highly dissipative flux-flow
state driven up to the instability point. From the measured current-voltage
curves we derive the vortex critical velocity for several temperatures.
From the values, the quasiparticle energy relaxation time
is evaluated within the Larkin-Ovchinnikov model and
numerical calculations of the quasiparticle energy relaxation rates are carried
out to support the experimental findings. Besides the expected constant
behavior of for the dirty samples, we observe a strong
temperature dependence of the quasiparticle energy relaxation time in the clean
samples. This feature is associated with the increasing contribution from the
electron-phonon scattering process as the dirty limit is approached from the
clean regime
Economic analysis of condition monitoring systems for offshore wind turbine sub-systems
The use of condition monitoring systems on wind turbines has increased dramatically in recent times. However, their use is mostly restricted to vibration based monitoring systems for the gearbox, generator and drive train. There are many forms and types of condition monitoring systems now available for wind turbines. A survey of commercially available condition monitoring systems and their associated costs has been undertaken for the blades, drive train and tower. This paper considers what value can be obtained from these systems if they are used correctly. This is achieved by running simulations on an operations and maintenance model for a 20 year life cycle wind farm. The model uses Hidden Markov Models to represent both the actual system state and the observed state. The costs for system failures are derived, as are possible reductions in these costs due to early detection. Various scenarios are simulated including the addition of condition monitoring systems to the drive train and blade and tower monitoring. Finally, the efficacy of these systems is examined and its effect on operation costs
Nano granular metallic Fe - oxygen deficient TiO composite films: A room temperature, highly carrier polarized magnetic semiconductor
Nano granular metallic iron (Fe) and titanium dioxide (TiO) were
co-deposited on (100) lanthanum aluminate (LaAlO) substrates in a low
oxygen chamber pressure using a pulsed laser ablation deposition (PLD)
technique. The co-deposition of Fe and TiO resulted in 10 nm
metallic Fe spherical grains suspended within a TiO matrix. The
films show ferromagnetic behavior with a saturation magnetization of 3100 Gauss
at room temperature. Our estimate of the saturation magnetization based on the
size and distribution of the Fe spheres agreed well with the measured value.
The film composite structure was characterized as p-type magnetic semiconductor
at 300 K with a carrier density of the order of . The
hole carriers were excited at the interface between the nano granular Fe and
TiO matrix similar to holes excited in the metal/n-type
semiconductor interface commonly observed in Metal-Oxide-Semiconductor (MOS)
devices. From the large anomalous Hall effect directly observed in these films
it follows that the holes at the interface were strongly spin polarized.
Structure and magneto transport properties suggested that these PLD films have
potential nano spintronics applications.Comment: 6 pages in Latex including 8 figure
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