3,260 research outputs found
Exact and LDA entanglement of tailored densities in an interacting one-dimensional electron system
We calculate the `exact' potential corresponding to a one-dimensional
interacting system of two electrons with a specific, tailored density. We use
one-dimensional density-functional theory with a local-density approximation
(LDA) on the same system and calculate densities and energies, which are
compared with the `exact' ones. The `interacting-LDA system' corresponding to
the LDA density is then found and its potential compared with the original one.
Finally we calculate and compare the spatial entanglement of the electronic
systems corresponding to the interacting-LDA and original interacting system.Comment: 7 pages, 4 figure
SXP 323 - an unusual X-ray binary system in the Small Magellanic Cloud
Spectroscopic observations taken with the VLT/UVES telescope/instrument are
presented of the unusual Small Magellanic Cloud (SMC) X-ray binary system SXP
323 = AX J0051-733. This system shows a clear modulation at 0.71d in long term
optical photometry which has been proposed as the binary period of this system.
The high resolution optical spectra, taken at a range of phases during the
0.71d cycle, rule out this possibility. Instead it is suggested that this
long-term effect is due to Non Radial Pulsations (NRP) in the Be star companion
to SXP 323. In addition, the spectra show clear evidence for major changes in
the (V/R) ratio of the double peaks of the Balmer lines indicative of
asymmetries in the circumstellar disk. The complex structure of the
interstellar lines are also discussed in the context of the SMC structure.Comment: Accepted in MNRA
The Be/X-ray Transient V0332+53: Evidence for a tilt between the orbit and the equatorial plane?
We present optical and infrared observations of BQ Cam, the optical
counterpart to the Be/X-ray transient system V0332+53. BQ Cam is shown to be an
O8-9Ve star, which places V0332+53 at a distance of ~7 kpc. H-alpha
spectroscopy and infrared photometry are used to discuss the evolution of the
circumstellar envelope. Due to the low inclination of the system, parameters
are strongly constrained. We find strong evidence for a tilt of the orbital
plane with respect to the circumstellar disc (pressumably on the equatorial
plane). Even though the periastron distance is only ~ 10 R_*, during the
present quiescent state the circumstellar disc does not extend to the distance
of periastron passage. Under these conditions, X-ray emission is effectively
prevented by centrifugal inhibition of accretion. The circumstellar disc is
shown to be optically dense at optical and infrared wavelengths, which together
with its small size, is taken as an indication of tidal truncation.Comment: 9 pages, 3 figures, uses mn.sty and epsfig Accepted for publication
in MNRA
Geometry induced entanglement transitions in nanostructures
We model quantum dot nanostructures using a one-dimensional system of two
interacting electrons. We show that strong and rapid variations may be induced
in the spatial entanglement by varying the nanostructure geometry. We
investigate the position-space information entropy as an indicator of the
entanglement in this system. We also consider the expectation value of the
Coulomb interaction and the ratio of this expectation to the expectation of the
confining potential and their link to the entanglement. We look at the first
derivative of the entanglement and the position-space information entropy to
infer information about a possible quantum phase transition.Comment: 3 pages, 2 figures, to appear in Journal of Applied Physic
The X-ray Transient XTE J2012+381
We present optical and infrared observations of the soft X-ray transient
(SXT) XTE J2012+381 and identify the optical counterpart with a faint red star
heavily blended with a brighter foreground star. The fainter star is coincident
with the radio counterpart and appears to show weak H alpha emission and to
have faded between observations. The RXTE/ASM lightcurve of XTE J2012+381 is
unusual for an SXT in that after an extended linear decay, it settled into a
plateau state for about 40 days before undergoing a weak mini-outburst. We
discuss the nature of the object and suggest similarities to long orbital
period SXTs.Comment: 5 pages, 7 postscript figures included, uses mn.sty. Accepted for
publication in MNRA
Long-term Properties of Accretion Disks in X-ray Binaries: II. Stability of Radiation-Driven Warping
A significant number of X-ray binaries are now known to exhibit long-term
``superorbital'' periodicities on timescales of 10 - 100 days. Several
physical mechanisms have been proposed that give rise to such periodicities, in
particular warping and/or precession of the accretion disk. Recent theoretical
work predicts the stability to disk warping of X-ray binaries as a function of
the mass ratio, binary radius, viscosity and accretion efficiency, and here we
examine the constraints that can be placed on such models by current
observations.
In paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray
datasets (CGRO, RXTE), focusing on the remarkable, smooth variations in the
superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to
investigate the stability of the superorbital periodicities in the neutron star
X-ray binaries Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability
predictions with observation. We find that the period and nature of
superorbital variations in these sources is consistent with the predictions of
warping theory.
We also use a dynamic lightcurve analysis to examine the behaviour of Her X-1
as it enters and leaves the 1999 Anomalous Low State (ALS). This reveals a
significant phase shift some 15 cycles before the ALS, which indicates a change
in the disk structure or profile leading into the ALS.Comment: 12 pages, 14 figures, Re-submitted to MNRAS after referee's comment
Hubbard model as an approximation to the entanglement in nanostructures
We investigate how well the one-dimensional Hubbard model describes the entanglement of particles trapped in a string of quantum wells. We calculate the average single-site entanglement for two particles interacting via a contact interaction and consider the effect of varying the interaction strength and the interwell distance. We compare the results with the ones obtained within the one-dimensional Hubbard model with on-site interaction. We suggest an upper bound for the average single-site entanglement for two electrons in M wells and discuss analytical limits for very large repulsive and attractive interactions. We investigate how the interplay between interaction and potential shape in the quantum-well system dictates the position and size of the entanglement maxima and the agreement with the theoretical limits. Finally, we calculate the spatial entanglement for the quantum-well system and compare it to its average single-site entanglement
The entanglement of few-particle systems when using the local-density approximation
In this chapter we discuss methods to calculate the entanglement of a system
using density-functional theory. We firstly introduce density-functional theory
and the local-density approximation (LDA). We then discuss the concept of the
`interacting LDA system'. This is characterised by an interacting many-body
Hamiltonian which reproduces, uniquely and exactly, the ground state density
obtained from the single-particle Kohn-Sham equations of density-functional
theory when the local-density approximation is used. We motivate why this idea
can be useful for appraising the local-density approximation in many-body
physics particularly with regards to entanglement and related quantum
information applications. Using an iterative scheme, we find the Hamiltonian
characterising the interacting LDA system in relation to the test systems of
Hooke's atom and helium-like atoms. The interacting LDA system ground state
wavefunction is then used to calculate the spatial entanglement and the results
are compared and contrasted with the exact entanglement for the two test
systems. For Hooke's atom we also compare the entanglement to our previous
estimates of an LDA entanglement. These were obtained using a combination of
evolutionary algorithm and gradient descent, and using an LDA-based
perturbative approach. We finally discuss if the position-space information
entropy of the density---which can be obtained directly from the system density
and hence easily from density-functional theory methods---can be considered as
a proxy measure for the spatial entanglement for the test systems.Comment: 12 pages and 5 figures
Effect of confinement potential geometry on entanglement in quantum dot-based nanostructures
We calculate the spatial entanglement between two electrons trapped in a
nanostructure for a broad class of confinement potentials, including single and
double quantum dots, and core-shell quantum dot structures.
By using a parametrized confinement potential, we are able to switch from one
structure to the others with continuity and to analyze how the entanglement is
influenced by the changes in the confinement geometry. We calculate the
many-body wave function by `exact' diagonalization of the time independent
Schr\"odinger equation. We discuss the relationship between the entanglement
and specific cuts of the wave function, and show that the wave function at a
single highly symmetric point could be a good indicator for the entanglement
content of the system. We analyze the counterintuitive relationship between
spatial entanglement and Coulomb interaction, which connects maxima (minima) of
the first to minima (maxima) of the latter. We introduce a potential quantum
phase transition which relates quantum states characterized by different
spatial topology. Finally we show that by varying shape, range and strength of
the confinement potential, it is possible to induce strong and rapid variations
of the entanglement between the two electrons. This property may be used to
tailor nanostructures according to the level of entanglement required by a
specific application.Comment: 10 pages, 8 figures and 1 tabl
Disc loss and renewal in A0535+26
This paper presents observations of the Be/X-ray binary system A0535+26
revealing the first observed loss of its circumstellar disc, demonstrated by
the loss of its JHK infrared excess and optical/IR line emission. However
optical/IR spectroscopy reveals the formation of a new inner disc with
significant density and emission strength at small radii; the disc has proven
to be stable over 5 months in this intermediate state.Comment: 4 pages, 4 figures, accepted by MNRAS, uses mn.st
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