1,823 research outputs found
On the robustness of acoustic black hole spectra
We study the robustness of the spectrum emitted by an acoustic black hole by
considering series of stationary flows that become either subsonic or
supersonic, i.e. when the horizon disappears. We work with the superluminal
Bogoliubov dispersion of Bose--Einstein condensates. We find that the spectrum
remains remarkably Planckian until the horizon disappears. When the flow is
everywhere supersonic, new pair creation channels open. This will be the
subject of a forthcoming work.Comment: 4 pages, 2 figure, jpconf.cls; to appear in the proceedings of the
Spanish Relativity Meeting ERE201
Instability of the superfluid flow as black-hole lasing effect
We show that the instability leading to the decay of the one-dimensional
superfluid flow through a penetrable barrier are due to the black-hole lasing
effect. This dynamical instability is triggered by modes resonating in an
effective cavity formed by two horizons enclosing the barrier. The location of
the horizons is set by , with being the local fluid
velocity and sound speed, respectively. We compute the critical velocity
analytically and show that it is univocally determined by the horizons
configuration. In the limit of broad barriers, the continuous spectrum at the
origin of the Hawking-like radiation and of the Landau energetic instability is
recovered.Comment: 18 pages, 3 figure
Black hole lasers in Bose-Einstein condensates
We consider elongated condensates that cross twice the speed of sound. In the
absence of periodic boundary conditions, the phonon spectrum possesses a
discrete and finite set of complex frequency modes that induce a laser effect.
This effect constitutes a dynamical instability and is due to the fact that the
supersonic region acts as a resonant cavity. We numerically compute the complex
frequencies and density-density correlation function. We obtain patterns with
very specific signatures. In terms of the gravitational analogy, the flows we
consider correspond to a pair of black hole and white hole horizons, and the
laser effect can be conceived as a self-amplified Hawking radiation. This is
verified by comparing the outgoing flux at early time with the standard black
hole radiation.Comment: iopams, 37 pages, 14 figures, 1 table; for associated gif animations,
see http://people.sissa.it/~finazzi/bec_bhlasers/movies/ or
http://iopscience.iop.org/1367-2630/12/9/095015/media. Published on New. J.
Phys. (http://iopscience.iop.org/1367-2630/12/9/095015/). V2: few new
comments, modified figure
Modification of magnetic and transport properties of manganite layers in Au/La_0.67Sr_0.33MnO_3/SrTiO_3 interfaces
The effect of gold capping on magnetic and transport properties of optimally
doped manganite thin films is studied. An extraordinary suppression of
conductivity and magnetic properties occurs in epitaxial (001)
La_0.67Sr_0.33MnO_3 (LSMO) films grown on SrTiO_3 upon deposition of 2 nm of
Au: in the case of ultrathin films of LSMO (4 nm thick) the resistivity
increases by four orders of magnitude while the Curie temperature decreases by
180 K. Zero-field 55Mn nuclear magnetic resonance reveals a significant
reduction of ferromagnetic double-exchange mechanism in manganite films upon
the gold capping. We find evidence for the formation of a 1.9-nm thick magnetic
"dead-layer" at the Au/LSMO interface, associated with the creation of
interfacial non double-exchange insulating phases.Comment: 4 figure
Bulk Cr tips for scanning tunneling microscopy and spin-polarized scanning tunneling microscopy
A simple, reliable method for preparation of bulk Cr tips for Scanning
Tunneling Microscopy (STM) is proposed and its potentialities in performing
high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are
investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to
what happens with conventional W tips, rest atoms of the Si(111)-7x7
reconstruction can be routinely observed, probably due to a different
electronic structure of the tip apex. SP-STM measurements of the Cr(001)
surface showing magnetic contrast are reported. Our results reveal that the
peculiar properties of these tips can be suited in a number of STM experimental
situations
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