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Heteroepitaxial growth of T-Nb2O5 on SrTiO3
There is a growing interest in exploiting the functional properties of niobium oxides in general and of the T-Nb2O5 polymorph in particular. Fundamental investigations of the properties of niobium oxides are, however, hindered by the availability of materials with sufficient structural perfection. It is expected that high-quality T-Nb2O5 can be made using heteroepitaxial growth. Here, we investigated the epitaxial growth of T-Nb2O5 on a prototype perovskite oxide, SrTiO3. Even though there exists a reasonable lattice mismatch in one crystallographic direction, these materials have a significant difference in crystal structure: SrTiO3 is cubic, whereas T-Nb2O5 is orthorhombic. It is found that this difference in symmetry results in the formation of domains that have the T-Nb2O5 c-axis aligned with the SrTiO3 s in-plane directions. Hence, the number of domain orientations is four and two for the growth on (100)s- and (110)s-oriented substrates, respectively. Interestingly, the out-of-plane growth direction remains the same for both substrate orientations, suggesting a weak interfacial coupling between the two materials. Despite challenges associated with the heteroepitaxial growth of T-Nb2O5, the T-Nb2O5 films presented in this paper are a significant improvement in terms of structural quality compared to their polycrystalline counterparts
Spontaneous polarization in NaNbO film on NdGaO and DyScO substrates
Pure NaNbO is an antiferroelectric material at room temperature that
irreversibly transforms to a ferroelectric polar state when subjected to an
external electrical field or lattice strain. Experimentally, it has been
observed that NaNbO films grown on NdGaO exhibit an electrical
polarization along the [001] direction, whereas films on
DyScO substrates exhibit a polarization along the [011]
direction. These effects have been attributed to the realization of different
lattice symmetries in the films due to the incorporation of lattice strain
imposed by the use of oxide substrates with different lattice parameters.
However, the underlying atomistic mechanisms of the resulting phase symmetry in
the films are hardly clear, given that NaNbO features a diverse and
complex phase diagram. In turn, these also impede a straightforward tailoring
and optimization of the resulting macroscopic properties on different
substrates. To clarify this issue, we perform all-electron first-principles
calculations for several potential NaNbO polymorphs under stress and
strain. The computed properties, including the ferroelectric polarization,
reveal that an orthorhombic phase is realized on NdGaO
substrates since this is the only phase with an out-of-plane polarization under
a compressive strain. Conversely, the monoclinic phase is consistent for
the samples grown on DyScO substrate, since this phase exhibits a
spontaneous in-plane polarization along [011] under tensile
strain.Comment: 9 pages, 5 figures, and supplementary material
Strain engineering of ferroelectric domains in KxNa1−xNbO3 epitaxial layers
The application of lattice strain through epitaxial growth of oxide films on lattice mismatched
perovskite-like substrates strongly influences the structural properties of ferroelectric
domains and their corresponding piezoelectric behavior. The formation of different
ferroelectric phases can be understood by a strain-phase diagram, which is calculated
within the framework of the Landau–Ginzburg–Devonshire theory. In this paper, we illustrate
the opportunity of ferroelectric domain engineering in the KxNa1−xNbO3 lead-free
material system. In particular, the following examples are discussed in detail: (i) Different
substrates (NdGaO3, SrTiO3, DyScO3, TbScO3, and GdScO3) are used to systematically
tune the incorporated epitaxial strain from compressive to tensile. This can be exploited
to adjust the NaNbO3 thin film surface orientation and, concomitantly, the vector of
electrical polarization, which rotates from mainly vertical to exclusive in-plane orientation.
(ii) In ferroelectric NaNbO3, thin films grown on rare-earth scandate substrates, highly
regular stripe domain patterns are observed. By using different film thicknesses, these
can be tailored with regard to domain periodicity and vertical polarization component.
(iii) A featured potassium concentration of x = 0.9 of KxNa1−xNbO3 thin films grown on
(110) NdScO3 substrates favors the coexistence of two equivalent, monoclinic, but
differently oriented ferroelectric phases. A complicated herringbone domain pattern is
experimentally observed which consists of alternating MC and a1a2 domains. The coexistence
of different types of ferroelectric domains leads to polarization discontinuities
at the domain walls, potentially enabling high piezoelectric responses. In each of these
examples, the experimental results are in excellent agreement with predictions based on
the linear elasticity theory
Spatio-temporal coherent control of thermal excitations in solids
X-ray reflectivity (XRR) measurements of femtosecond laser-induced transient
gratings are applied to demonstrate the spatio-temporal coherent control of
thermally induced surface deformations on ultrafast timescales. Using gracing
incidence X-ray diffraction we unambiguously measure the amplitude of transient
surface deformations with sub-\AA{} resolution. Understanding the dynamics of
femtosecond TG excitations in terms of superposition of acoustic and thermal
gratings makes it possible to develop new ways of coherent control in X-ray
diffraction experiments. Being the dominant source of TG signal, the
long-living thermal grating with spatial period can be canceled by a
second, time-delayed TG excitation shifted by . The ultimate speed
limits of such an ultrafast X-ray shutter are inferred from the detailed
analysis of thermal and acoustic dynamics in TG experiments
Self-stabilization of the equilibrium state in ferroelectric thin films
(K,Na)NbO3 is a lead-free and sustainable ferroelectric material with electromechanical parameters comparable to Pb(Zr,Ti)O3 (PZT) and other lead-based solid solutions. It is therefore a promising candidate for caloric cooling and energy harvesting applications. Specifically, the structural transition from the low-temperature Mc- to the high-temperature c-phase displays a rich hierarchical order of domains and superdomains, that forms at specific strain conditions. The relevant length scales are few tens of nanometers for the domain and few micrometers for the superdomain size, respectively. Phase-field calculations show that this hierarchical order adds to the total free energy of the solid. Thus, domains and their formation has a strong impact on the functional properties relevant for electrocaloric cooling or energy harvesting applications. However, monitoring the formation of domains and superdomains is difficult and requires both, high spatial and high temporal resolution of the experiment. Synchrotron-based time-resolved X-ray diffraction methods in combination with scanning imaging X-ray microscopy is applied to resolve the local dynamics of the domain morphology with sub-micrometer spatial and nanosecond temporal resolution. In this regime, the material displays a novel self-stabilization mechanism of the domain morphology, which may be a general property of first-order phase transitions
Reconciling the theoretical and experimental electronic structure of NbO2
Metal-insulator transition materials such as NbO2 have generated much
excitement in recent years for their potential applications in computing and
sensing. NbO2 has generated considerable debate over the nature of the phase
transition, and the values for the band gap/band widths in the insulating
phase. We present a combined theoretical and experimental study of the band gap
and electronic structure of the insulating phase of NbO2. We carry out
ab-initio density functional theory plus U calculations, directly determining U
and J parameters for both the Nb 4d and O 2p subspaces through the recently
introduced minimum-tracking linear response method. We find a fundamental bulk
band gap of 0.80 eV for the full DFT+U+J theory. We also perform calculations
and measurements for a (100) oriented thin film. Scanning tunnelling
spectroscopy measurements show that the surface band gap varies from 0.75 eV to
1.35 eV due to an excess of oxygen in and near the surface region of the film.
Slab calculations indicate metallicity localised at the surface region caused
by an energy level shift consistent with a reduction in Coulomb repulsion. We
demonstrate that this effect in combination with the simple, low cost DFT+U+J
method can account for the band widths and p-d gap observed in X-ray
photoelectron spectroscopy experiments. Overall, our results indicate the
possible presence of a 2D anisotropic metallic layer at the (100) surface of
NbO2.Comment: 11 pages, 5 figures, plus 3 pages of Supporting Informatio
Bajetta, Coatalen, and Gibson (eds.), Elizabeth I's Foreign Correspondence: Letters, Rhetoric and Politics (Palgrave Macmillan, 2014)
Review of Carlo M. Bajetta, Guillaume Coatalen, and Jonathon Gibson, eds., Elizabeth I's Foreign Correspondence: Letters, Rhetoric and Politics (London: Palgrave Macmillan, 2014)
Alex Owen. The Darkened Room: Women, Power and Spiritualism in Late Victorian England. Philadelphia: University of Pennsylvania Press. 1990. Pp. xxi, 314. 19.95 paper.
Angestelle vor Gericht: Ein Beitrag zur Verrechtlichung von Arbeitsbeziehungen in Deutschland und Frankreich um 1900
Haupt H-G. Angestelle vor Gericht: Ein Beitrag zur Verrechtlichung von Arbeitsbeziehungen in Deutschland und Frankreich um 1900. In: Münkel D, Schwarzkopf J, eds. Geschichte als Experiment. Frankfurt: Campus; 2004: 226-240