365 research outputs found
Multi-wavelength properties of IGR J05007-7047 (LXP 38.55) and identification as a Be X-ray binary pulsar in the LMC
We report on the results of a 40 d multi-wavelength monitoring of the
Be X-ray binary system IGR J05007-7047 (LXP 38.55). During that period the
system was monitored in the X-rays using the Swift telescope and in the optical
with multiple instruments. When the X-ray luminosity exceeded erg/s
we triggered an XMM-Newton ToO observation. Timing analysis of the photon
events collected during the XMM-Newton observation reveals coherent X-ray
pulsations with a period of 38.551(3) s (1 {\sigma}), making it the 17
known high-mass X-ray binary pulsar in the LMC. During the outburst, the X-ray
spectrum is fitted best with a model composed of an absorbed power law () plus a high-temperature black-body (kT 2 keV) component. By
analysing 12 yr of available OGLE optical data we derived a 30.776(5) d
optical period, confirming the previously reported X-ray period of the system
as its orbital period. During our X-ray monitoring the system showed limited
optical variability while its IR flux varied in phase with the X-ray
luminosity, which implies the presence of a disk-like component adding cooler
light to the spectral energy distribution of the system.Comment: 11 pages, 11 figures, Accepted for publication in MNRA
Inverse flux quantum periodicity of magnetoresistance oscillations in two-dimensional short-period surface superlattices
Transport properties of the two-dimensional electron gas (2DEG) are
considered in the presence of a perpendicular magnetic field and of a {\it
weak} two-dimensional (2D) periodic potential modulation in the 2DEG plane. The
symmetry of the latter is rectangular or hexagonal. The well-known solution of
the corresponding tight-binding equation shows that each Landau level splits
into several subbands when a rational number of flux quanta pierces the
unit cell and that the corresponding gaps are exponentially small. Assuming the
latter are closed due to disorder gives analytical wave functions and
simplifies considerably the evaluation of the magnetoresistivity tensor
. The relative phase of the oscillations in and
depends on the modulation periods involved. For a 2D modulation
with a {\bf short} period nm, in addition to the Weiss oscillations
the collisional contribution to the conductivity and consequently the tensor
show {\it prominent peaks when one flux quantum passes
through an integral number of unit cells} in good agreement with recent
experiments. For periods nm long used in early experiments, these
peaks occur at fields 10-25 times smaller than those of the Weiss oscillations
and are not resolved
Discovery of SXP265, a Be/X-ray binary pulsar in the Wing of the Small Magellanic Cloud
We identify a new candidate for a Be/X-ray binary in the XMM-Newton slew
survey and archival Swift observations that is located in the transition region
of the Wing of the Small Magellanic Cloud and the Magellanic Bridge. We
investigated and classified this source with follow-up XMM-Newton and optical
observations. We model the X-ray spectra and search for periodicities and
variability in the X-ray observations and the OGLE I-band light curve. The
optical counterpart has been classified spectroscopically, with data obtained
at the SAAO 1.9 m telescope, and photometrically, with data obtained using
GROND at the MPG 2.2 m telescope. The X-ray spectrum is typical of a high-mass
X-ray binary with an accreting neutron star. We detect X-ray pulsations, which
reveal a neutron-star spin period of P = (264.516+-0.014) s. The source likely
shows a persistent X-ray luminosity of a few 10^35 erg/s and in addition type-I
outbursts that indicate an orbital period of ~146 d. A periodicity of 0.867 d,
found in the optical light curve, can be explained by non-radial pulsations of
the Be star. We identify the optical counterpart and classify it as a
B1-2II-IVe star. This confirms SXP 265 as a new Be/X-ray binary pulsar
originating in the tidal structure between the Magellanic Clouds.Comment: 11 pages, 12 figures, accepted for publication in MNRA
Valley-dependent tunneling through electrostatically created quantum dots in heterostructures of graphene with hexagonal boron nitride
Kelvin probe force microscopy (KPFM) has been employed to probe charge
carriers in a graphene/hexagonal boron nitride (hBN) heterostructure [Nano
Lett, 21, 5013 (2021)]. We propose an approach for operating valley filtering
based on the KPFM-induced potential instead of using external or induced
pseudo-magnetic fields in strained graphene. Employing a tight-binding model,
we investigate the parameters and rules leading to valley filtering in the
presence of a graphene quantum dot (GQD) created by the KPFM tip. This model
leads to a resolution of different transport channels in reciprocal space,
where the electron transmission probability at each Dirac cone (= -K and
= +K) is evaluated separately. The results show that U0 and the Fermi
energy control (or invert) the valley polarization, if electrons are
allowed to flow through a given valley. The resulting valley filtering is
allowed only if the signs of and are the same. If they are
different, the valley filtering is destroyed and might occur only at some
resonant states affected by . Additionally, there are independent valley
modes characterizing the conductance oscillations near the vicinity of the
resonances, whose strength increases with and are similar to those
occurring in resonant tunneling in quantum antidots and to the Fabry-Perot
oscillations. Using KPFM, to probe the charge carriers, and graphene-based
structures to control valley transport, provides an efficient way for attaining
valley filtering without involving external or pseudo-magnetic fields as in
previous proposals
Magnetic Kronig-Penney model for Dirac electrons in single-layer graphene
The properties of Dirac electrons in a magnetic superlattice (SL) on graphene
consisting of very high and thin (delta-function) barriers are investigated. We
obtain the energy spectrum analytically and study the transmission through a
finite number of barriers. The results are contrasted with those for electrons
described by the Schrodinger equation. In addition, a collimation of an
incident beam of electrons is obtained along the direction perpendicular to
that of the SL. We also highlight the analogy with optical media in which the
refractive index varies in space.Comment: 21 pages, 13 figures, to appear in New Journal of Physic
Edge helicons and repulsion of fundamental edge magnetoplasmons in the quantum Hall regime
A quasi-microscopic treatment of edge magnetoplasmons (EMP) is presented for
very low temperatures and confining potentials smooth on the scale of the
magnetic length but sufficiently steep at the edges such that Landau
level (LL) flattening can be discarded. The profile of the unperturbed electron
density is sharp and the dissipation taken into account comes only from
electron intra-edge and intra-LL transitions due to scattering by acoustic
phonons. For wide channels and filling factors and 2, there exist
independent EMP modes spatially symmetric and antisymmetric with respect to the
edge. Some of these modes, named edge helicons, can propagate nearly undamped
even when the dissipation is strong. Their density profile changes
qualitatively during propagation and is given by a rotation of a complex vector
function. For the Coulomb coupling between the LLs leads to a
repulsion of the uncoupled fundamental LL modes: the new modes have very
different group velocities and are nearly undamped. The theory accounts well
for the experimentally observed plateau structure of the delay times as well as
for the EMP's period and decay rates.Comment: 12 pages, 6 figure
On the Theory of Magnetotransport in a Periodically Modulated Two-Dimensional Electron Gas
A semiclassical theory based on the Boltzmann transport equation for a
two-dimensional electron gas modulated along one direction with weak
electrostatic or magnetic modulations is proposed. It is shown that
oscillations of the magnetoresistivity corresponding to the
current driven along the modulation lines observed at moderately low magnetic
fields, can be explained as classical geometric resonances reflecting the
commensurability of the period of spatial modulations and the cyclotron radius
of electrons.Comment: 5 pages, 1 figure, text and 1 figure adde
Confined magnetic guiding orbit states
We show how snake-orbit states which run along a magnetic edge can be
confined electrically. We consider a two-dimensional electron gas (2DEG)
confined into a quantum wire, subjected to a strong perpendicular and steplike
magnetic field . Close to this magnetic step new, spatially confined
bound states arise as a result of the lateral confinement and the magnetic
field step. The number of states, with energy below the first Landau level,
increases as becomes stronger or as the wire width becomes larger. These
bound states can be understood as an interference between two
counter-propagating one-dimensional snake-orbit states.Comment: 4 pages, 4 figure
- …