90 research outputs found
In-situ Investigation of the Early Stage of TiO2 epitaxy on (001) SrTiO3
We report on a systematic study of the growth of epitaxial TiO2 films
deposited by pulsed laser deposition on Ti-terminated (001) SrTiO3 single
crystals. By using in-situ reflection high energy electron diffraction, low
energy electron diffraction, x-ray photoemission spectroscopy and scanning
probe microscopy, we show that the stabilization of the anatase (001) phase is
preceded by the growth of a pseudomorphic Sr-Ti-O intermediate layer, with a
thickness between 2 and 4 nm. The data demonstrate that the formation of this
phase is related to the activation of long range Sr migration from the
substrate to the film. The role of interface Gibbs energy minimization, as a
driving force for Sr diffusion, is discussed. Our results enrich the phase
diagram of the Sr-Ti-O system under epitaxial strain opening the roudeficient
SrTiO phase.Comment: 8 pages, 7 figure
Charge density waves enhance the electronic noise of manganites
The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin
films in the temperature range from room temperature to 160 K are reported. It
is shown that both the broadband 1/f noise properties and the dependence of
resistance on electric field are consistent with the idea of a collective
electrical transport, as in the classical model of sliding charge density
waves. On the other hand, the observations cannot be reconciled with standard
models of charge ordering and charge melting. Methodologically, it is proposed
to consider noise-spectra analysis as a unique tool for the identification of
the transport mechanism in such highly correlated systems. On the basis of the
results, the electrical transport is envisaged as one of the most effective
ways to understand the nature of the insulating, charge-modulated ground states
in manganites.Comment: 6 two-column pages, 5 figure
Effect of Housing Quality on the Mental Health of University Students during the COVID-19 Lockdown
COVID-19 outbreak imposed rapid and severe public policies that consistently impacted the lifestyle habits and mental health of the general population. Despite vaccination, lockdown restrictions are still considered as potential measures to contrast COVID-19 variants spread in several countries. Recent studies have highlighted the impacts of lockdowns on the population\u2019s mental health; however, the role of the indoor housing environment where people spent most of their time has rarely been considered. Data from 8177 undergraduate and graduate students were collected in a large, cross-sectional, web-based survey, submitted to a university in Northern Italy during the first lockdown period from 1 April to 1 May 2020. Logistic regression analysis showed significant associations between moderate and severe depression symptomatology (PHQ-9 scores 65 15), and houses with both poor indoor quality and small dimensions (OR = 4.132), either medium dimensions (OR = 3.249) or big dimensions (OR = 3.522). It was also found that, regardless of housing size, poor indoor quality is significantly associated with moderate\u2013severe depressive symptomatology. Further studies are encouraged to explore the long-term impact of built environment parameter modifications on mental health, and therefore support housing and public health policies
Magnetic properties of pseudomorphic epitaxial films of Pr_{0.7}Ca_{0.3}MnO_3 under different biaxial tensile stresses
In order to analyse the effect of strain on the magnetic properties of
narrow-band manganites, the temperature and field dependent susceptibilities of
about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001)
and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on
(001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative
of the possible coexistence of antiferromagnetic and ferromagnetic phases. On
the contrary, ultrathin films grown on (110) substrates show a robust
ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn
atom along the easy axis. On the base of high resolution reciprocal space
mapping analyses performed by x-ray diffraction, the different behaviours are
discussed in terms of the crystallographic constraints imposed by the epitaxy
of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the
tensile strain on the film c-axis, lying within the substrate plane, favours
the ferromagnetic phase, possibly by allowing a mixed occupancy and
hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to
shed some physics of inhomogeneous states in manganites and on the nature of
their ferromagnetic insulating state.Comment: 9 pages, 9 figure
Electronic phase separation at LaAlO3/SrTiO3 interfaces tunable by oxygen deficiency
Electronic phase separation is crucial for the fascinating macroscopic
properties of the LaAlO3/SrTiO3 (LAO/STO) paradigm oxide interface, including
the coexistence of superconductivity and ferromagnetism. We investigate this
phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the
soft-X-ray energy range, where the enhanced probing depth combined with
resonant photoexcitation allow access to fundamental electronic structure
characteristics (momentum-resolved spectral function, dispersions and ordering
of energy bands, Fermi surface) of buried interfaces. Our experiment uses X-ray
irradiation of the LAO/STO interface to tune its oxygen deficiency, building up
a dichotomic system where mobile weakly correlated Ti t2g-electrons co-exist
with localized strongly correlated Ti eg-ones. The ARPES spectra dynamics under
X-ray irradiation shows a gradual intensity increase under constant Luttinger
count of the Fermi surface. This fact identifies electronic phase separation
(EPS) where the mobile electrons accumulate in conducting puddles with fixed
electronic structure embedded in an insulating host phase, and allows us to
estimate the lateral fraction of these puddles. We discuss the physics of EPS
invoking a theoretical picture of oxygen-vacancy clustering, promoted by the
magnetism of the localized Ti eg-electrons, and repelling of the mobile
t2g-electrons from these clusters. Our results on the irradiation-tuned EPS
elucidate the intrinsic one taking place at the stoichiometric LAO/STO
interfaces.Comment: In review with Phys. Rev. Material
Symmetry breaking at the (111) interfaces of SrTiO hosting a 2D-electron system
We used x-ray absorption spectroscopy to study the orbital symmetry and the
energy band splitting of (111) LaAlO/SrTiO and
LaAlO/EuTiO/SrTiO heterostructures, hosting a quasi
two-dimensional electron system (q2DES), and of a Ti-terminated (111)
SrTiO single crystal, also known to form a q2DES at its surface. We
demonstrate that the bulk tetragonal Ti-3d D crystal field is
turned into trigonal D crystal field in all cases. The symmetry
adapted a and e orbitals are non-degenerate in energy and
their splitting, \Delta, is positive at the bare STO surface but negative in
the heterostructures, where the a orbital is lowest in energy.
These results demonstrate that the interfacial symmetry breaking induced by
epitaxial engineering of oxide interfaces has a dramatic effect on their
electronic properties, and it can be used to manipulate the ground state of the
q2DES.Comment: 6 pages article, plus 5 pages supplementary informatio
Observation of a two-dimensional electron gas at the surface of annealed SrTiO3 single crystals by scanning tunneling spectroscopy
Scanning tunneling spectroscopy suggests the formation of a two dimensional
electron gas (2DEG) on the TiO2 terminated surface of undoped SrTiO3 single
crystals annealed at temperature lower than 400 {\deg}C in ultra high vacuum
conditions. Low energy electron diffraction indicates that the 2D metallic
SrTiO3 surface is not structurally reconstructed, suggesting that non-ordered
oxygen vacancies created in the annealing process introduce carriers leading to
an electronic reconstruction. The experimental results are interpreted in a
frame of competition between oxygen diffusion from the bulk to the surface and
oxygen loss from the surface itself.Comment: 9 pages, 3 figures, submitted to Physical Review
Energy and symmetry of excitations in undoped layered cuprates measured by Cu resonant inelastic x-ray scattering
We measured high resolution Cu edge resonant inelastic x-ray scattering
(RIXS) of the undoped cuprates LaCuO, SrCuOCl, CaCuO
and NdBaCuO. The dominant spectral features were assigned to
excitations and we extensively studied their polarization and scattering
geometry dependence. In a pure ionic picture, we calculated the theoretical
cross sections for those excitations and used them to fit the experimental data
with excellent agreement. By doing so, we were able to determine the energy and
symmetry of Cu-3 states for the four systems with unprecedented accuracy and
confidence. The values of the effective parameters could be obtained for the
single ion crystal field model but not for a simple two-dimensional cluster
model. The firm experimental assessment of excitation energies carries
important consequences for the physics of high superconductors. On one
hand, having found that the minimum energy of orbital excitation is always
eV, i.e., well above the mid-infrared spectral range, leaves to
magnetic excitations (up to 300 meV) a major role in Cooper pairing in
cuprates. On the other hand, it has become possible to study quantitatively the
effective influence of excitations on the superconducting gap in cuprates.Comment: 22 pages, 11 figures, 1 tabl
Effect of the aerenchymatous helophyte Glyceria maxima on the sulfate-reducing communities in two contrasting riparian grassland soils
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