A Chemical turnstile

A chemical turnstile is a device for transporting small, well-characterised doses of atoms from one location to another. A working turnstile has yet to be built, despite the numerous technological applications available for such a device. The key difficulty in manufacturing a chemical turnstile is finding a medium which will trap and transport atoms. Here we propose that ferroelastic twin walls are suitable for this role. Previous work shows that twin walls can act as two-dimensional trapping planes within which atomic transport is fast. We report simulations showing that a stress-induced reorientation of a twin wall can occur. This behaviour is ideal for chemical turnstile applications.Comment: 2 pages, 3 figure

Avalanche criticality in the martensitic transition of Cu67.64Zn16.71Al15.65 shape-memory alloy: a calorimetric and acoustic emission study

The first-order diffusionless structural transition in Cu67.64Zn16.71Al15.65 is characterized by jerky propagation of phase fronts related to the appearance of avalanches. In this paper, we describe a full analysis of this avalanche behavior using calorimetric heat-flux measurements and acoustic emission measurements. Two different propagation modes, namely, smooth front propagation and jerky avalanches, were observed in extremely slow measurements with heating and cooling rates as low as a few 10−3 K/h. Avalanches show criticality where each avalanche leads to a spike in the heat flux. Their statistical analysis leads to a power law [P(E)∼E−ε, where P(E)dE is the probability to observe an avalanche with energy E in an interval between E and E+dE] with an energy exponent of ε=2.15±0.15 in excellent agreement with the results of acoustic emission measurements. Avalanches appear to be more common for heating rates faster than 5×10−3 K/h whereas smooth front propagation occurs in all calorimetric measurements and (almost) exclusively for slower heating rates. Repeated cooling runs were taken after a waiting time of 1 month (and an intermediate heating run). Correlations between the avalanche sequences of the two cooling runs were found for the strongest avalanche peaks but not for the full sequence of avalanches. The memory effect is hence limited to strong avalanches

Polar domain walls trigger magnetoelectric coupling

Interface physics in oxide heterostructures is pivotal in material's science. Domain walls (DWs) in ferroic systems are examples of naturally occurring interfaces, where order parameter of neighboring domains is modified and emerging properties may develop. Here we show that electric tuning of ferroelastic domain walls in SrTiO3 leads to dramatic changes of the magnetic domain structure of a neighboring magnetic layer (La1/2Sr1/2MnO3) epitaxially clamped on a SrTiO3 substrate. We show that by exploiting the resposiveness of DWs nanoregions to external stimuli, even in absence of any domain contribution, prominent and adjustable macroscopic reactions of neighboring layers can be obtained. We conclude that polar DWs, known to exist in other materials, can be used to trigger tunable responses and may lead to new ways for manipulation of interfacial emerging properties

Robust templates for domain boundary engineering in ErMnO3

Emerging properties of domain boundaries define the emerging field of domain boundary engineering. For many applications, the domain boundary acts as template onto which the desired properties, such as (super-) conductivity, polarity, ferroelectricity, magnetism, are imposed. This requires for most applications that the domain structures remain unchanged under appropriate chemical doping. Hassanpour et al (2016 New J. Phys. 18 043015) have now shown, for the first time, that the magnetic and electric domain structures remain indeed robust against charge carrier doping (Ca2+ and Zr4+) of the workbench multi-ferroic ErMnO3. This opens the way into novel functionalities based on the nanostructure of ErMnO3

Correlations between Elastic, Calorimetric, and Polar Properties of Ferroelectric PbSc0.5Ta0.5O3 (PST)

Calorimetric, elastic, and polar properties of ferrolectric lead scandium tantalate PbSc0.5Ta0.5O3 (PST) with 65% cation order have been investigated in the vicinity of the paraelectric-ferroelectric transition at Ttrans = 295K. Comparison of temperature dependencies of the excess specific heat and elastic properties indicate that both anomalies stem from ther- mal fluctuations of order parameters in three dimensions. These fluctuations are consistent with tweed microstructure. This transition is driven by several coupled thermodynamic order parameters, as evidenced by a strongly non-linear scaling of the excess entropy with the squared ferroelectric polarization.National Natural Science Foundation of China (51850410520, 51320105014 and 51621063

Heat transport by phonons and the generation of heat by fast phonon processes in ferroelastic materials

Thermal conductivity of ferroelastic device materials can be reversibly controlled by strain. The nucleation and growth of twin boundaries reduces thermal conductivity if the heat flow is perpendicular to the twin wall. The twin walls act as phonon barriers whereby the thermal conductivity decreases linearly with the number of such phonon barriers. Ferroelastic materials also show elasto-caloric properties with a high frequency dynamics. The upper frequency limit is determined by heat generation on a time scale, which is some 5 orders of magnitude below the typical bulk phonon times. Some of these nano-structural processes are irreversible under stress release (but remain reversible under temperature cycling), in particular the annihilation of needle domains that are a key indicator for ferroelastic behaviour in multiferroic materials

Anisotropy and universality: Critical Binder cumulant of the two-dimensional Ising model

We reanalyze transfer matrix and Monte Carlo results for the critical Binder cumulant U* of an anisotropic two-dimensional Ising model on a square lattice in a square geometry with periodic boundary conditions. Spins are coupled between nearest neighboring sites and between next-nearest neighboring sites along one of the lattice diagonals. We find that U* depends only on the asymptotic critical long-distance features of the anisotropy, irrespective of its realization through ferromagnetic or antiferromagnetic next-nearest neighbor couplings. We modify an earlier renormalization-group calculation to obtain a quantitative description of the anisotropy dependence of U*. Our results support our recent claim towards the validity of universality for critical phenomena in the presence of a weak anisotropy.Comment: 4 pages, 2 figures; one reference and some clarifications adde

Strain-induced interface reconstruction in epitaxial heterostructures

We investigate in the framework of Landau theory the distortion of the strain fields at the interface of two dissimilar ferroelastic oxides that undergo a structural cubic-to-tetragonal phase transition. Simple analytical solutions are derived for the dilatational and the order parameter strains that are globally valid over the whole of the heterostructure. The solutions reveal that the dilatational strain exhibits compression close to the interface which may in turn affect the electronic properties in that region.Comment: 7 pages, 5 figures, to be published in Physical Review

First-principles reinvestigation of bulk WO3_{3}

Using first-principles calculations, we analyze the structural properties of tungsten trioxide WO$_{3}$. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. We show that the hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations, and tilts of oxygen octahedra) structural instabilities. Although the ferroelectric instability is the largest, the instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a $\textit{P}$2$_{1}$/$\textit{c}$ ground state. This nonpolar $\textit{P}$2$_{1}$/$\textit{c}$ phase is only different from the experimentally reported $\textit{Pc}$ ground state by the absence of a very tiny additional ferroelectric distortion. The calculations performed on a stoichiometric compound so suggest that the low-temperature phase of WO$_{3}$ is not intrinsically ferroelectric and that the experimentally observed ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed $\textit{R3m}$ and $\textit{R3c}$ ferroelectric metastable phases with large polarizations and low energies close to the $\textit{P}$2$_{1}$/$\textit{c}$ ground state, which makes WO$_{3}$ $\textbf{a}$ potential antiferroelectric material. The relative stability of various phases is discussed in terms of the anharmonic couplings between different structural distortions, highlighting a very complex interplay.The work was supported by the ARC project AIMED and the F.R.S-FNRS PDR projects HiT4FiT and MaRePeThe. Calculations have been performed within the PRACE project TheDeNoMo and relied on the Céci facilities funded by F.R.S-FNRS (Grant No. 2.5020.1) and Tier-1 supercomputer of the Fédération Wallonie-Bruxelles funded by the Walloon Region (Grant No. 1117545). E.K.H.S. is grateful for support to EPSRC and the Leverhulme trust. H.H. thanks the AVERROES-ERASMUS Mundus project