اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدOxygen atoms and molecules at Lanthanum-Strontium Manganite surfaces
195
0
0.0
(
0
)
تأليف
Walter A. Harrison
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
A localized description, rather than energy bands, is appropriate for the manganite substrate. Empty substrate levels lower in energy than occupied oxygen levels indicate need for further terms beyond the Local Density Approximation. So also does van-der-Waals interaction between the two. Methods to include both are suggested by related, exactly soluble, two-electron problems. The descriptions of the electronic structure of the molecule and a LaSrMnO3 (LSM) substrate are greatly simplified to allow incorporation of these effects and to treat a range of problems involving the interactions between oxygen atoms, or oxygen molecules, and such a substrate. These include elastic impacts, impacts with electronic transitions, and impacts with phonon excitation. They provide for capture of the atoms or molecules by the surface, leaving the neutral molecule strongly bound over a Mn(4+) site. It is found that oxygen vacancies in LSM diffuse as a neutral species, and can appear at the surface. Bound molecules tend to avoid sites next to vacancies but, if there, should drop one atom into the vacancy leaving the remaining triplet oxygen atom bound to the resulting ideal surface, with no need for spin flips nor successive ionization steps.
قيم البحث
اقرأ أيضاً
We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-delta}$. By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermogravimetric analysi
s, we have synthesized a series of samples with precisely controlled $delta=0.00-0.49$. These samples show three structural phases at $delta=0.00-0.15$, $approx0.25$, $approx0.5$, and two-phase behavior for other oxygen contents. The stoichiometric material with $delta=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_{rm C}=274$ K. The increase of $delta$ to 0.15 is followed by a linear decrease of $T_{rm C}$ to $approx$ 160 K and a metal-insulator transition near the boundary of the cubic structure range. Further increase of $delta$ results in formation of a tetragonal $2a_ptimes 2a_p times 4a_p$ phase for $deltaapprox 0.25$ and a brownmillerite phase for $deltaapprox0.5$. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at $T_{rm m}approx230$ and 120 K, respectively. At higher temperatures, the $2a_ptimes 2a_p times 4a_p$ phase is $G$-type antiferromagnetic between 230 K and $approx$360 K. Low temperature magnetic properties of this system for $delta<1/3$ can be described in terms of a mixture of Co$^{3+}$ ions in the low-spin state and Co$^{4+}$ ions in the intermediate-spin state and a possible spin transition of Co$^{3+}$ to the intermediate-spin state above $T_{rm C}$. For $delta>1/3$, there appears to be a combination of Co$^{2+}$ and Co$^{3+}$ ions, both in the high-spin state with dominating antiferromagnetic interactions.
An earlier analysis of manganese oxides in various charge states indicated that free-atom term values and universal coupling gave a reasonable account of the cohesion. This approach is here extended to LaxSr(1-x)MnO3 in a perovskite structure, and a
wide range of properties, with comparable success, including the cohesion, as a function of x. Magnetic and electronic properties are treated in terms of the same parameters and the cluster orbitals used for cohesion. This includes an estimate of the Neel and Curie-Weiss temperatures for SrMnO3, an antiferromagnetic insulator, and the magnitude of a Jahn-Teller distortion in LaMnO3 which makes it also insulating with (100) ferromagnetic planes (due to a novel double-exchange for the distorted state), antiferromagnetically stacked, as observed. We estimate the Neel temperature and its volume dependence, and the ferromagnetic Curie-Weiss temperature which applies between the Neel and Jahn-Teller temperatures. We expect hopping conductivity when there is doping (0<x<1) and estimate it in the context of small-polaron theory. It is in accord with experiment between the Neel and Jahn-Teller temperatures, but below the Neel temperature the conduction appears to be band-like, for which we estimate a hole mass as enhanced in large-polaron theory. We see that above the Jahn-Teller temperature LaMnO3 should be metallic as observed, and paramagnetic with a ferromagnetic Curie-Weiss constant which we estimate. Many of these predictions are not so accurate, but are sufficiently close to provide a clear understanding of all of these properties in terms of a simple theory and parameters known at the outset. We provide also these parameters for Fe, Co, and Ca so that formulae for the properties can readily be evaluated for similar systems.
Ferromagnetic/metallic manganese perovskites, such as La2/3Sr1/3MnO3 (LSMO)are promising materials for the design and implementation of novel spintronic devices working at room temperature. However, their implementation in practical applications has
been severely hampered due to the breakdown of their magnetotransport properties at temperatures well below their magnetic transition temperature. This breakdown has been usually associated to surface and interface related problems but its physical origin has not been clearly established yet. In this work we investigate the interface between La2/3Sr1/3MnO3 (LSMO) thin films and different capping layers by means of x-ray linear dichroism and transport measurements. Our data reveal that, irrespective to the capping material, LSMO/capping layer bilayers exhibit an antiferromegnetic/insulating phase at the interface, likely to originate from a preferential occupancy of Mn 3d 3z2-r2 eg orbitals. This phase, which extends ca. 2 unit cells, is also observed in an uncapped LSMO reference sample thus, pointing to an intrinsic interfacial phase separation phenomenon, likely to be promoted by the structural disruption and symmetry breaking at the LSMO free surface/interface. These experimental observations strongly suggest that the structural disruption at the LSMO interfaces play a major role on the observed depressed magnetotransport properties in manganite-based magnetic tunneling junctions and it is at the origin of the so-called dead layer.
Any evolving system can change of state via thermal mechanisms (hopping a barrier) or via quantum tunneling. Most of the time, efficient classical mechanisms dominate at high temperatures. This is why an increase of the temperature can initiate the c
hemistry. We present here an experimental investigation of O-atom diffusion and reactivity on water ice. We explore the 6-25 K temperature range at sub-monolayer surface coverages. We derive the diffusion temperature law and observe the transition from quantum to classical diffusion. Despite of the high mass of O, quantum tunneling is efficient even at 6 K. As a consequence, the solid-state astrochemistry of cold regions should be reconsidered and should include the possibility of forming larger organic molecules than previously expected.
The oxygen stoichiometry has a large influence on the physical and chemical properties of complex oxides. Most of the functionality in e.g. catalysis and electrochemistry depends in particular on control of the oxygen stoichiometry. In order to under
stand the fundamental properties of intrinsic surfaces of oxygen-deficient complex oxides, we report on in situ temperature dependent scanning tunnelling spectroscopy experiments on pristine oxygen deficient, epitaxial manganite films. Although these films are insulating in subsequent ex situ in-plane electronic transport experiments at all temperatures, in situ scanning tunnelling spectroscopic data reveal that the surface of these films exhibits a metal-insulator transition (MIT) at 120 K, coincident with the onset of ferromagnetic ordering of small clusters in the bulk of the oxygen-deficient film. The surprising proximity of the surface MIT transition temperature of nonstoichiometric films with that of the fully oxygenated bulk suggests that the electronic properties in the surface region are not significantly affected by oxygen deficiency in the bulk. This carries important implications for the understanding and functional design of complex oxides and their interfaces with specific electronic properties for catalysis, oxide electronics and electrochemistry.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
