665 research outputs found
Magnetically ordered state at correlated oxide interfaces: the role of random oxygen defects
Using an effective one-band Hubbard model with disorder, we consider magnetic
states of the correlated oxide interfaces, where effective hole self-doping and
a magnetially ordered state emerge due to electronic and ionic reconstructions.
By employing the coherent potential approximation, we analyze the effect of
random oxygen vacancies on the two-dimensional magnetism. We find that the
random vacancies enhance the ferromagnetically ordered state and stabilize a
robust magnetization above a critical vacancy concentration of about c=0.1. In
the strong-correlated regime, we also obtain a nonmonotonic increase of the
magnetization upon an increase of vacancy concentration and a substantial
increase of the magnetic moments, which can be realized at oxygen reduced
high-Tc cuprate interfaces.Comment: 8 pages, 2 figures, submitted to J Supercond Novel Magnetism (ICSM12
conference contribution
Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons
Ferromagnetism and superconductivity are in most cases adverse. However,
recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3.
We analyze the magnetic state within density functional theory and provide
evidence that magnetism is not an intrinsic property of the two-dimensional
electron liquid at the interface. We demonstrate that the robust ferromagnetic
state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer.
This allows for the notion that areas with increased density of oxygen
vacancies produce ferromagnetic puddles and account for the previous
observation of a superparamagnetic behavior in the superconducting state.Comment: 5 pages, 4 figures, to appear in Physical Review B (Rapid
Communications
Interface hole-doping in cuprate-titanate superlattices
The electronic structure of interfaces between YBaCuO and
SrTiO is studied using local spin density approximation (LSDA) with
intra-atomic Coulomb repulsion (LSDA+U). We find a metallic state in
cuprate/titanate heterostructures with the hole carriers concentrated
substantially in the CuO-layers and in the first interface TiO and SrO
planes. This effective interface doping appears due to the polarity of
interfaces, caused by the first incomplete copper oxide unit cell.
Interface-induced high pre-doping of CuO-layers is a key mechanism
controlling the superconducting properties in engineered field-effect devices
realized on the basis of cuprate/titanate superlattices.Comment: 5 pages, 5 figure
Oxygen vacancies at titanate interfaces: two-dimensional magnetism and orbital reconstruction
We show that oxygen vacancies at titanate interfaces induce a complex
multiorbital reconstruction which involves a lowering of the local symmetry and
an inversion of t2g and eg orbitals resulting in the occupation of the eg
orbitals of Ti atoms neighboring the O vacancy. The orbital reconstruction
depends strongly on the clustering of O vacancies and can be accompanied by a
magnetic splitting between the local eg orbitals with lobes directed towards
the vacancy and interface dxy orbitals. The reconstruction generates a
two-dimensional interface magnetic state not observed in bulk SrTiO3. Using
generalized gradient approximation (LSDA) with intra-atomic Coulomb repulsion
(GGA+U), we find that this magnetic state is common for titanate surfaces and
interfaces.Comment: 13 pages, 18 figures, to appear in Physical Review
Thermodynamics of the \phi^4 theory in tadpole approximation
Relying on the Luttinger-Ward theorem we derive a thermodynamically
selfconsistent and scale independent approximation of the thermodynamic
potential for the scalar theory in the tadpole approximation. The
resulting thermodynamic potential as a function of the temperature is similar
to the one of the recently proposed screened perturbation theory.Comment: 6 pages, including 1 eps figur
Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007
We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around 4.7 μm, we determine the excitation conditions in the line-forming region. We find 12C/13C = 3.5+2.0−1.5, consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of 2.2 × 10−6≤MCO≤ 2.7 × 10−6 M⊙ of CO ejecta outside the dust, forming a high-velocity wind of 500 ± 80 km s−1. We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor
Modelling the spectral energy distribution of the red giant in RS Ophiuchi: Evidence for irradiation
We present an analysis of optical and infrared spectra of the recurrent nova RS Oph obtained during between 2006 and 2009. The best fit to the optical spectrum for 2006 September 28 gives Teff = 3900 K for log g = 2.0, while for log g = 0.0 we find Teff = 4700 K, and a comparison with template stellar spectra provides Teff ~ 4500 K. The observed spectral energy distribution (SED), and the intensities of the emission lines, vary on short (≲1 d) time-scales, due to disc variability. We invoke a simple one-component model for the accretion disc, and a model with a hot boundary layer, with high (~3.9 × 10-6M⊙ yr-1) and low (~2 × 10-8M⊙ yr-1) accretion rates, respectively. Fits to the accretion disc-extracted infrared spectrum (2008 July 15) yield effective temperatures for the red giant of Teff = 3800 ± 100 K (log g = 2.0) and Teff = 3700 ± 100 K (log g = 0.0). Furthermore, using a more sophisticated approach, we reproduced the optical and infrared SEDs of the red giant in the RS Oph system with a twocomponent model atmosphere, in which 90 per cent of the surface has Teff = 3600 K and 10 per cent has Teff = 5000 K. Such structure could be due to irradiation of the red giant by the white dwarf. © 2015 The Authors
Metallicity and effective temperature of the secondary of RS Oph
The recurrent nova RS Oph undergoes nova eruptions every ~ 10-20years as a
result of thermonuclear runaway on the surface of a white dwarf close to the
Chandrasekhar limit. Both the progress of the eruption, and its aftermath,
depend on the (poorly known) composition of the red giant in the RS Oph system.
Our aim is to understand better the effect of the giant secondary on the
recurrent nova eruption. Synthetic spectra were computed for a grid of M-giant
model atmospheres having a range of effective temperatures 3200 < Teff < 4400
K, gravities 0 < log g < 1 and abundances -4 <[Fe/H] < 0.5, and fit to infrared
spectra of RS Oph as it returned to quiescence after its 2006 eruption. We have
modelled the infrared spectrum in the range 1.4-2.5 micron to determine
metallicity and effective temperature of the red giant. We find Teff = 4100 +/-
100 K, log g = 0.0 +/- 0.5, [Fe/H] = 0.0 +/- 0.5, [C/H] = -0.8 +/- 0.2, [N/H] =
+0.6 +/- 0.3 in the atmosphere of the secondary, and demonstrate that that
inclusion of some dust `veiling' in the spectra cannot improve our fits.Comment: 6 pages, 5 figs, 1 table, to appear in A&
Holstein polaron in the presence of disorder
Non-local, inhomogeneous and retarded response observed in experiments is
reproduced by introducing the Inhomogeneous Momentum Average (IMA) method to
study single polaron problems with disorder in the on-site potential and/or
spatial variations of the electron-phonon couplings and/or phonon frequencies.
We show that the electron-phonon coupling gives rise to an additional
inhomogeneous, strongly retarded potential, which makes instant approximations
questionable. The accuracy of IMA is demonstrated by comparison with results
from the approximation free Diagrammatic Monte Carlo (DMC) method. Its
simplicity allows for easy study of many problems that were previously
unaccessible. As an example, we show how inhomogeneities in the electron-phonon
coupling lead to nonlocal, retarded response in scanning tunneling microscopy
(STM) images.Comment: 4 pages, 3 figure
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