1,367 research outputs found
Spin-orbit induced mixed-spin ground state in NiO perovskites probed by XAS: new insight into the metal to insulator transition
We report on a Ni L edges x-ray absorption spectroscopy (XAS) study
in NiO perovskites. These compounds exhibit a metal to insulator ()
transition as temperature decreases. The L edge presents a clear
splitting in the insulating state, associated to a less hybridized ground
state. Using charge transfer multiplet calculations, we establish the
importance of the crystal field and 3d spin-orbit coupling to create a
mixed-spin ground state. We explain the transition in NiO
perovskites in terms of modifications in the Ni crystal field splitting
that induces a spin transition from an essentially low-spin (LS) to a
mixed-spin state.Comment: 4 pages, 4 figures, accepted as PRB - Rapid Comm. Dez. 200
From band insulator to Mott insulator in one dimension
We derive the phase diagram for the one-dimensional model of a ferroelectric
perovskite recently introduced by Egami, Ishihara and Tachiki [Science, {\bf
261}, 1307 (1993)]. We show that the interplay between covalency, ionicity and
strong correlations results in a spontaneously dimerized phase which separates
the weak-coupling band insulator from the strong-coupling Mott insulator. The
transition from the band insulator to the dimerized phase is identified as an
Ising critical point. The charge gap vanishes at this single point with the
optical conductivity diverging as . The spin
excitations are gapless above the second transition to the Mott insulator
phase.Comment: 4 pages LaTex (RevTex) and 1 postscript figure included by eps
Short-range charge-order in NiO perovskites (=Pr,Nd,Eu) probed by X-ray absorption spectroscopy
The short-range organization around Ni atoms in orthorhombic NiO
(=Pr,Nd,Eu) perovskites has been studied over a wide temperature range by Ni
K-edge x-ray absorption spectroscopy. Our results demonstrate that two
different Ni sites, with different average Ni-O bond lengths, coexist in those
orthorhombic compounds and that important modifications in the Ni nearest
neighbors environment take place across the metal-insulator transition. We
report evidences for the existence of short-range charge-order in the
insulating state, as found in the monoclinic compounds. Moreover, our results
suggest that the two different Ni sites coexists even in the metallic state.
The coexistence of two different Ni sites, independently on the ion,
provides a common ground to describe these compounds and shed new light in the
understanding of the phonon-assisted conduction mechanism and unusual
antiferromagnetism present in all NiO compounds.Comment: 4 pages, 3 figures, accepted PRB - Brief Report Dec.200
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Exploring the concept of 'ideal' university student
This paper contributes to our understanding of the ‘ideal’ university student – a working concept that promotes a more transparent conversation about the explicit, implicit and idealistic expectations of students in higher education. Drawing on Weber’s theory of ideal types, we explore university staff and students’ conceptualisation of the ‘ideal’ student. Informed by 20 focus groups with 75 university staff and students, we focus on how the concept of ‘ideal’ student is perceived, challenged and negotiated. We argue that the ‘ideal’ student has important conceptual and practical implications for higher education, especially the importance of explicitness and the dangers of presumptions. The concept has the potential to bridge differences and manage expectations between lecturers and students, which have been stretched due to consumerism, by offering a platform where expectations of university students are elaborated. We present a working definition of ‘ideal’ university student, which, we argue, encompasses desirability, imperfection and realism
Coulomb parameters and photoemission for the molecular metal TTF-TCNQ
We employ density-functional theory to calculate realistic parameters for an
extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra-
and intermolecular screening in the crystal, we find significant longer-range
Coulomb interactions along the molecular stacks, as well as inter-stack
coupling. We show that the long-range Coulomb term of the extended Hubbard
model leads to a broadening of the spectral density, likely resolving the
problems with the interpretation of photoemission experiments using a simple
Hubbard model only.Comment: 4 pages, 2 figure
Silicon intercalation into the graphene-SiC interface
In this work we use LEEM, XPEEM and XPS to study how the excess Si at the
graphene-vacuum interface reorders itself at high temperatures. We show that
silicon deposited at room temperature onto multilayer graphene films grown on
the SiC(000[`1]) rapidly diffuses to the graphene-SiC interface when heated to
temperatures above 1020. In a sequence of depositions, we have been able to
intercalate ~ 6 ML of Si into the graphene-SiC interface.Comment: 6 pages, 8 figures, submitted to PR
Bloch oscillations of magnetic solitons in anisotropic spin-1/2 chains
We study the quantum dynamics of soliton-like domain walls in anisotropic
spin-1/2 chains in the presence of magnetic fields. In the absence of fields,
domain walls form a Bloch band of delocalized quantum states while a static
field applied along the easy axis localizes them into Wannier wave packets and
causes them to execute Bloch oscillations, i.e. the domain walls oscillate
along the chain with a finite Bloch frequency and amplitude. In the presence of
the field, the Bloch band, with a continuum of extended states, breaks up into
the Wannier-Zeeman ladder -- a discrete set of equally spaced energy levels. We
calculate the dynamical structure factor in the one-soliton sector at finite
frequency, wave vector, and temperature, and find sharp peaks at frequencies
which are integer multiples of the Bloch frequency. We further calculate the
uniform magnetic susceptibility and find that it too exhibits peaks at the
Bloch frequency. We identify several candidate materials where these Bloch
oscillations should be observable, for example, via neutron scattering
measurements. For the particular compound CoCl_2.2H_2O we estimate the Bloch
amplitude to be on the order of a few lattice constants, and the Bloch
frequency on the order of 100 GHz for magnetic fields in the Tesla range and at
temperatures of about 18 Kelvin.Comment: 31 single-spaced REVTeX pages, including 7 figures embedded with eps
Dynamics of pollen beetle (Brassicogethes aeneus) immigration and colonization of oilseed rape (Brassica napus) in Europe
BACKGROUND
Understanding the dynamics of pest immigration into an agroecosystem enables effective and timely management strategies. The pollen beetle (Brassicogethes aeneus) is a primary pest of the inflorescence stages of oilseed rape (Brassica napus). This study investigated the spatial and temporal dynamics of pollen beetle immigration into oilseed rape fields in Denmark and the UK using multiple methods, including optical sensors.
RESULTS
In all fields, pollen beetles were found to be aggregated and beetle density was related to plant growth stage, with more beetles occurring on plants after the budding stage than before inflorescence development. Optical sensors were the most efficient monitoring method, recording pollen beetles 2 and 4 days ahead of water traps and counts from plant scouting, respectively.
CONCLUSION
Optical sensors are a promising tool for early warning of insect pest immigration. The aggregation pattern of pollen beetles post immigration could be used to precisely target control in oilseed rape crops. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry
Evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in EuGdO
Raman scattering studies as functions of temperature, magnetic field, and
Gd-substitution are used to investigate the evolution of magnetic polarons and
spin-carrier interactions through the metal-insulator transition in
EuGdO. These studies reveal a greater richness of phase behavior
than have been previously observed using transport measurements: a
spin-fluctuation-dominated paramagnetic (PM) phase regime for T T
T, a two-phase regime for T T in which magnetic polarons
develop and coexist with a remnant of the PM phase, and an inhomogeneous
ferromagnetic phase regime for T T
Susceptibility of the one-dimensional, dimerized Hubbard model
We show that the zero temperature susceptibility of the one-dimensional,
dimerized Hubbard model at quarter-filling can be accurately determined on the
basis of exact diagonalization of small clusters. The best procedure is to
perform a finite-size scaling of the spin velocity , and to calculate
the susceptibility from the Luttinger liquid relation . We
show that these results are reliable by comparing them with the analytical
results that can be obtained in the weak and strong coupling limits. We have
also used quantum Monte Carlo simulations to calculate the temperature
dependence of the susceptibility for parameters that should be relevant to the
Bechgaard salts. This shows that, used together, these numerical techniques are
able to give precise estimates of the low temperature susceptibility of
realistic one-dimensional models of correlated electrons.Comment: 10 pages, latex, figures available from the authors. To appear in
Phys. Rev. B, Rapid Comm
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