Metric fixed point theory and partial impredicativity

We show that the Priess-Crampe & Ribenboim fixed point theorem is provable in [Formula: see text]. Furthermore, we show that Caristi's fixed point theorem for both Baire and Borel functions is equivalent to the transfinite leftmost path principle, which falls strictly between [Formula: see text] and [Formula: see text]. We also exhibit several weakenings of Caristi's theorem that are equivalent to [Formula: see text] and to [Formula: see text]. This article is part of the theme issue 'Modern perspectives in Proof Theory'.

Creation of skyrmions in van der Waals ferromagnet Fe3GeTe2 on (Co/Pd) n superlattice

Magnetic skyrmions are topological spin textures, which usually exist in noncentrosymmetric materials where the crystal inversion symmetry breaking generates the so-called Dzyaloshinskii-Moriya interaction. This requirement unfortunately excludes many important magnetic material classes, including the recently found two-dimensional van der Waals (vdW) magnetic materials, which offer unprecedented opportunities for spintronic technology. Using photoemission electron microscopy and Lorentz transmission electron microscopy, we investigated and stabilized Néel-type magnetic skyrmion in vdW ferromagnetic Fe₃GeTe₂ on top of (Co/Pd) _n in which the Fe₃GeTe₂ has a centrosymmetric crystal structure. We demonstrate that the magnetic coupling between the Fe₃GeTe₂ and the (Co/Pd) _n could create skyrmions in Fe₃GeTe₂ without the need of an external magnetic field. Our results open exciting opportunities in spintronic research and the engineering of topologically protected nanoscale features by expanding the group of skyrmion host materials to include these previously unknown vdW magnets.

Tunable Magnetoelastic Effects in Voltage-Controlled Exchange-Coupled Composite Multiferroic Microstructures

The magnetoelectric properties of exchange-coupled Ni/CoFeB-based composite multiferroic microstructures are investigated. The strength and sign of the magnetoelastic effect are found to be strongly correlated with the ratio between the thicknesses of two magnetostrictive materials. In cases where the thickness ratio deviates significantly from one, the magnetoelastic behavior of the multiferroic microstructures is dominated by the thicker layer, which contributes more strongly to the observed magnetoelastic effect. More symmetric structures with a thickness ratio equal to one show an emergent interfacial behavior which cannot be accounted for simply by summing up the magnetoelastic effects occurring in the two constituent layers. This aspect is clearly visible in the case of ultrathin bilayers, where the exchange coupling drastically affects the magnetic behavior of the Ni layer, making the Ni/CoFeB bilayer a promising next-generation synthetic magnetic system entirely. This study demonstrates the richness and high tunability of composite multiferroic systems based on coupled magnetic bilayers compared to their single magnetic layer counterparts. Furthermore, because of the compatibility of CoFeB with present magnetic tunnel junction-based spintronic technologies, the reported findings are expected to be of great interest for the development of ultralow-power magnetoelectric memory devices.

Anomalous orbital structure in two-dimensional titanium dichalcogenides

Generally, lattice distortions play a key role in determining the electronic ground states of materials. Although it is well known that trigonal distortions are generic to most two dimensional transition metal dichalcogenides, the impact of this structural distortion on the electronic structure and topological properties has not been understood conclusively. Here, by using a combination of polarization dependent X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and atomic multiplet cluster calculations, we have investigated the electronic structure of titanium dichalcogenides TiX₂ (X = S, Se, Te), where the magnitude of the trigonal distortion increase monotonically from S to Se and Te. Our results reveal the presence of an anomalously large crystal field splitting. This unusual kind of crystal field splitting is likely responsible for the unconventional electronic structure of TiX₂ compounds and ultimately controls the degree of the electronic phase protection. Our findings also indicate the drawback of the distorted crystal field picture in explaining the observed electronic ground state and emphasize the key importance of trigonal symmetry, metal-ligand hybridization and electron-electron correlations in defining the electronic structures at the Fermi energy.

Decoupling Carrier Concentration and Electron-Phonon Coupling in Oxide Heterostructures Observed with Resonant Inelastic X-Ray Scattering

We report the observation of multiple phonon satellite features in ultrathin superlattices of the form nSrIrO_{3}/mSrTiO_{3} using resonant inelastic x-ray scattering (RIXS). As the values of n and m vary, the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the RIXS cross section, we extract the variation in the electron-phonon coupling strength as a function of n and m. Combined with the negligible carrier doping into the SrTiO_{3} layers, these results indicate that the tuning of the electron-phonon coupling can be effectively decoupled from doping. This work both showcases a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling, which is often associated with superconductivity in SrTiO_{3}-based systems.

Disentangled Cooperative Orderings in Artificial Rare-Earth Nickelates

Coupled transitions between distinct ordered phases are important aspects behind the rich phase complexity of correlated oxides that hinder our understanding of the underlying phenomena. For this reason, fundamental control over complex transitions has become a leading motivation of the designer approach to materials. We have devised a series of new superlattices by combining a Mott insulator and a correlated metal to form ultrashort period superlattices, which allow one to disentangle the simultaneous orderings in RENiO_{3}. Tailoring an incommensurate heterostructure period relative to the bulk charge ordering pattern suppresses the charge order transition while preserving metal-insulator and antiferromagnetic transitions. Such selective decoupling of the entangled phases resolves the long-standing puzzle about the driving force behind the metal-insulator transition and points to the site-selective Mott transition as the operative mechanism. This designer approach emphasizes the potential of heterointerfaces for selective control of simultaneous transitions in complex materials with entwined broken symmetries.

Direct Detection of Pure ac Spin Current by X-Ray Pump-Probe Measurements

Despite recent progress in spin-current research, the detection of spin current has mostly remained indirect. By synchronizing a microwave waveform with synchrotron x-ray pulses, we use the ferromagnetic resonance of the Py (Ni_{81}Fe_{19}) layer in a Py/Cu/Cu_{75}Mn_{25}/Cu/Co multilayer to pump a pure ac spin current into the Cu_{75}Mn_{25} and Co layers, and then directly probe the spin current within the Cu_{75}Mn_{25} layer and the spin dynamics of the Co layer by x-ray magnetic circular dichroism. This element-resolved pump-probe measurement unambiguously identifies the ac spin current in the Cu_{75}Mn_{25} layer.

Correlation-Driven Insulator-Metal Transition in Near-Ideal Vanadium Dioxide Films

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO_{2}. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO_{2} films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (±0.3  K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

Magnetic Interactions at the Nanoscale in Trilayer Titanates

We report on the phase diagram of competing magnetic interactions at the nanoscale in engineered ultrathin trilayer heterostructures of LaTiO_{3}/SrTiO_{3}/YTiO_{3}, in which the interfacial inversion symmetry is explicitly broken. Combined atomic layer resolved scanning transmission electron microscopy with electron energy loss spectroscopy and electrical transport have confirmed the formation of a spatially separated two-dimensional electron liquid and high density two-dimensional localized magnetic moments at the LaTiO_{3}/SrTiO_{3} and SrTiO_{3}/YTiO_{3} interfaces, respectively. Resonant soft x-ray linear dichroism spectroscopy has demonstrated the presence of orbital polarization of the conductive LaTiO_{3}/SrTiO_{3} and localized SrTiO_{3}/YTiO_{3} electrons. Our results provide a route with prospects for exploring new magnetic interfaces, designing a tunable two-dimensional d-electron Kondo lattice, and potential spin Hall applications.

Above-bandgap voltages from ferroelectric photovoltaic devices

In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.

Psychosocial predictors of lifestyle management in adults with epilepsy

The purposes of the work described in this article were to (1) describe a model of predictive relationships among psychosocial variables and lifestyle management, and (2) test the model among people with epilepsy. The variables selected for the model were based on social cognitive theory and the results of previous studies examining psychosocial predictors of self-management among people with chronic physical health conditions. Variables included in the model were self-efficacy, outcome expectancies, depressive symptoms, and social support. Participants for the study were recruited from epilepsy treatment facilities in Boston, MA, and Atlanta, GA, USA. Half of the participants were female, 81% were white, and their mean age was 43.1 years. As predicted by social cognitive theory, self-efficacy was related to lifestyle management and explained 23% of its variation. Depressive symptoms were related to both self-efficacy and social support. Social support was related to self-efficacy. These findings suggest that lifestyle management is influenced by a number of relationships between psychosocial variables, particularly by self-efficacy.

Strain-induced polarization rotation in epitaxial (001) BiFeO3 thin films

Direct measurement of the remanent polarization of high quality (001)-oriented epitaxial BiFeO3 thin films shows a strong strain dependence, even larger than conventional (001)-oriented PbTiO3 films. Thermodynamic analysis reveals that a strain-induced polarization rotation mechanism is responsible for the large change in the out-of-plane polarization of (001) BiFeO3 with biaxial strain while the spontaneous polarization itself remains almost constant.

Strain control of domain-wall stability in epitaxial BiFeO3 (110) films

We have studied the stability of domains and domain walls in multiferroic BiFeO3 thin films using a combination of piezoelectric force microscopy and phase-field simulations. We have discovered that a film-substrate misfit strain may result in a drastically different thermodynamic stability of two parallel domain walls with the same orientation. A fundamental understanding of the underlying physics, the stress distribution in a domain structure, leads to a novel approach to control the ferroelastic domain stability in the multiferroic BiFeO3 system.

Electric field-induced magnetization switching in epitaxial columnar nanostructures

We present direct evidence for room-temperature magnetization reversal induced by an electric field in epitaxial ferroelectric BiFeO3-ferrimagnetic CoFe2O4 columnar nanostructures. Piezoelectric force microscopy and magnetic force microscopy were used to locally image the coupled piezoelectric-magnetic switching. Quantitative analyses give a perpendicular magnetoelectric susceptibility of approximately 1.0 x 10(-2) G cm/V. The observed effect is due to the strong elastic coupling between the two ferric constituents as the result of the three-dimensional heteroepitaxy.

Monitoring steam sterilization practices in primary care settings

Steam sterilization is a common method for processing reusable instruments in ambulatory care settings. Ensuring that sterilization practices are consistent with accepted standards is a quality-of-care parameter as well as an infection control issue. Factors that must be considered in improving performance in this important area are: operator training and continued competency, selection of chemical and biological media and methods, work flow controls, documentation methods and standards, written policies and procedures to support practices, and ongoing monitoring of the system and practices with feedback to responsible persons.

Functional abnormalities in isolated left bundle branch block. The effect of interventricular asynchrony

Eighteen patients with isolated left bundle branch block (LBBB) were compared with 10 normal control subjects. Apexcardiograms, phonocardiograms, electrocardiograms, two-dimensional and dual M-mode echocardiograms, and radionuclide ventriculograms (RNV) were performed. There were no differences in the timing of right ventricular events between LBBB and normal subjects; however, striking delays in left ventricular systolic and diastolic events were apparent in the LBBB group. The delay was associated with shortening of left ventricular diastole and resultant increase in the ratio of right to left ventricular diastolic time in LBBB (1.2 +/- 0.08) compared with normal (1.0 +/- 0.06), p less than 0.0001. First heart sound (S1) amplitude, expressed as the ratio S1/S2, was decreased in LBBB compared with normal (0.67 +/- 0.2 compared with 1.34 +/- 0.25, p less than 0.01), in part due to wide separation of the valvular contributors to S1. The abnormal interventricular septal motion in LBBB corresponded to periods of asynchrony in contraction, ejection, end systole, and end diastole between right and left ventricles. Radionuclide ventriculograms revealed decreased regional ejection fraction of the septum in LBBB (40 +/- 16%) compared with 67 +/- 7% in normal subjects (p less than 0.001), while the apical and lateral regional ejection fractions were similar in the two groups. This loss of septal contribution resulted in a reduction in global ejection fraction in LBBB compared to normals (54 +/- 7% compared with 62 +/- 5%, p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)