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Predictive drift compensation of multi-frame STEM via live scan modification
Authors:
Matthew Mosse,
Jonathan J. P. Peters,
Eoin Moynihan,
James A. Gott,
Ana M. Sanchez,
Michele Conroy,
Lewys Jones
Abstract:
Scanning transmission electron microscopy (STEM) is widely used tool for materials characterisation. However, being a scanned technique, STEM is susceptible to sample, stage or beam drift, manifesting as distortions within images or movement in the field-of-view during multi-frame imaging. Often this is corrected post-acquisition using image registration of multiple frames, but drift reduces the u…
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Scanning transmission electron microscopy (STEM) is widely used tool for materials characterisation. However, being a scanned technique, STEM is susceptible to sample, stage or beam drift, manifesting as distortions within images or movement in the field-of-view during multi-frame imaging. Often this is corrected post-acquisition using image registration of multiple frames, but drift reduces the usable area common to all frames. Here we present a method to mitigate sample drift by analysing past frames to predict the sampling-grid points for the immediately future frame. We present this correction across two time-scales and two lengthscales. By offsetting the scan-grid framewise we remove long-range drift, and offsetting pixelwise we minimise intra-image warping. Examples are presented for both atomic-resolution imaging and lower-magnification in-situ video capture. The framework is general to raster, serpentine, interlaced and other scan patterns, as well as sequential or scan-rotation series STEM.
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Submitted 22 April, 2026;
originally announced April 2026.
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Spectral Spacetime Entropy for Quasifree Theories
Authors:
Joshua Y. L. Jones,
Yasaman K. Yazdi
Abstract:
Motivated by the necessity to UV-regularise entanglement entropy, we present a spectral method for calculating the entropy of quasifree states, for both bosonic and fermionic field theories. This construction is defined in spacetime rather than on a hypersurface, enabling the covariant regularisation of entropies, and its calculation in generic spacetime regions. We derive these formulae, which ha…
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Motivated by the necessity to UV-regularise entanglement entropy, we present a spectral method for calculating the entropy of quasifree states, for both bosonic and fermionic field theories. This construction is defined in spacetime rather than on a hypersurface, enabling the covariant regularisation of entropies, and its calculation in generic spacetime regions. We derive these formulae, which have previously appeared in the literature, in a new manner and highlight certain aspects of them, such as their connection to the density matrix and its eigenvalues. The spacetime nature of the formulation makes it particularly apt in the context of semiclassical and quantum gravity and in connection to black hole entropy. Another useful property of the formulation is its application to settings where no notion of a Cauchy surface exists, such as in the causal set theory approach to quantum gravity. We show example applications of the formulae which demonstrate their ability to reproduce known results. We also show a calculation in a causal set in $1+1$ dimensions which makes use of several of the unique and useful features of the formalism. In this last example, we obtain a novel result of a slightly modified entanglement entropy scaling coefficient, giving a possible signature of spacetime discreteness.
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Submitted 18 February, 2026;
originally announced February 2026.
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Relaxation approach to quantum-mechanical modeling of ferroelectric and antiferroelectric phase transitions
Authors:
Nikhilesh Maity,
Sergey Lisenkov,
Arlies Valdespino,
Milan Haddad,
Lewys Jones,
Amit Kumar,
Nazanin Bassiri-Gharb,
Inna Ponomareva
Abstract:
Ferroelectrics and antiferroelectrics are the electric counterparts of ferromagnets and antiferromagnets. These materials undergo temperature- and electric-fieldinduced phase transitions that give rise to their characteristic hysteresis loops. Modeling such hysteresis loops and associated phase transitions enables both a deeper fundamental understanding and reliable property predictions for this i…
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Ferroelectrics and antiferroelectrics are the electric counterparts of ferromagnets and antiferromagnets. These materials undergo temperature- and electric-fieldinduced phase transitions that give rise to their characteristic hysteresis loops. Modeling such hysteresis loops and associated phase transitions enables both a deeper fundamental understanding and reliable property predictions for this important class of materials. To date, modeling has largely relied on classical approaches, often remaining qualitative and/or empirical. Traditional interpretation of these transitions rests on two assumptions: (i) they are activated Arrhenius-type processes and (ii) they occur well within the classical regime. Here, we demonstrate that a model can instead be built on two \orthogonal" assumptions: (i) the phase transitions are relaxational processes and (ii) they require a quantum mechanical treatment. Applying this model to both antiferroelectrics and ferroelectrics overcomes the limitations of traditional models and enables efficient first-principles simulations of phase transitions. The success of our unconventional approach highlights the significance of quantum mechanics in transitions long regarded as purely classical. We anticipate that this framework will be applicable to a broad range of phase transitions, including magnetic, elastic, multiferroic, and electronic, along with modeling of quantum tunneling, rates of chemical reactions, and others.
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Submitted 26 June, 2026; v1 submitted 13 November, 2025;
originally announced November 2025.
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Beyond Contrast Transfer: Spectral SNR as a Dose-Aware Metric for STEM Phase Retrieval
Authors:
Georgios Varnavides,
Julie Marie Bekkevold,
Stephanie M Ribet,
Mary C Scott,
Lewys Jones,
Colin Ophus
Abstract:
The contrast transfer function (CTF) is widely used to evaluate phase retrieval methods in scanning transmission electron microscopy (STEM), including center-of-mass imaging, parallax imaging, direct ptychography, and iterative ptychography. However, the CTF reflects only the maximum usable signal, neglecting the effects of finite electron fluence and the Poisson-limited nature of detection. As a…
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The contrast transfer function (CTF) is widely used to evaluate phase retrieval methods in scanning transmission electron microscopy (STEM), including center-of-mass imaging, parallax imaging, direct ptychography, and iterative ptychography. However, the CTF reflects only the maximum usable signal, neglecting the effects of finite electron fluence and the Poisson-limited nature of detection. As a result, it can significantly overestimate practical performance, especially in low-dose regimes. Here, we employ the spectral signal-to-noise ratio (SSNR), as a dose-aware statistical framework to evaluate the recoverable signal as a function of spatial frequency. Using numerical reconstructions of white-noise objects, we show that center-of-mass, parallax, and direct ptychography exhibit dose-independent SSNRs, with close-form analytic expressions. In contrast, iterative ptychography exhibits a surprising dose dependence: at low fluence, its SSNR converges to that of direct ptychography; at high fluence, it saturates at a value consistent with the maximum detective quantum efficiency predicted by recent quantum Fisher information bounds. The results highlight the limitations of CTF-based evaluation and motivate SSNR as a more accurate, dose-aware metric for assessing STEM phase retrieval methods.
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Submitted 1 September, 2025; v1 submitted 25 July, 2025;
originally announced July 2025.
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Relaxing Direct Ptychography Sampling Requirements via Parallax Imaging Insights
Authors:
Georgios Varnavides,
Julie Marie Bekkevold,
Stephanie M Ribet,
Mary C Scott,
Lewys Jones,
Colin Ophus
Abstract:
Direct ptychography enables the retrieval of information encoded in the phase of an electron wave passing through a thin sample by deconvolving the interference effects of a converged probe with known aberrations. Under the weak phase object approximation, this permits the optimal transfer of information using non-iterative techniques. However, the achievable resolution of the technique is traditi…
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Direct ptychography enables the retrieval of information encoded in the phase of an electron wave passing through a thin sample by deconvolving the interference effects of a converged probe with known aberrations. Under the weak phase object approximation, this permits the optimal transfer of information using non-iterative techniques. However, the achievable resolution of the technique is traditionally limited by the probe step size -- setting stringent Nyquist sampling requirements. At the same time, parallax imaging has emerged as a dose-efficient phase-retrieval technique which relaxes sampling requirements and enables scan-upsampling. Here, we formulate parallax imaging as a quadratic approximation to part of the direct ptychography kernel and use this insight to enable upsampling in direct ptychography. We validate our analytical results numerically using simulated and experimental reconstructions.
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Submitted 1 September, 2025; v1 submitted 24 July, 2025;
originally announced July 2025.
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Defects in the $β$-Ga$_2$O$_3$($\bar201$)/HfO$_2$ MOS system and the effect of thermal treatments
Authors:
Khushabu. S. Agrawal,
Paolo LaTorraca,
Jonas Valentijn,
Roberta Hawkins,
Adam A. Gruszecki,
Joy Roy,
Vasily Lebedev,
Lewys Jones,
Robert M. Wallace,
Chadwin D. Young,
Paul K. Hurley,
Karim Cherkaoui
Abstract:
We have investigated the properties of the $β$-Ga$_2$O$_3$($\bar201$)/HfO$_2$/Cr/Au MOS (metal-oxide-semiconductor) system after annealing (450$^\circ$C) in different ambient conditions (forming gas, N$_2$ and O$_2$). Defect properties have been analyzed using an approach combining experimental impedance measurements with physics-based simulations of the capacitance-voltage (C-V) and conductance-v…
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We have investigated the properties of the $β$-Ga$_2$O$_3$($\bar201$)/HfO$_2$/Cr/Au MOS (metal-oxide-semiconductor) system after annealing (450$^\circ$C) in different ambient conditions (forming gas, N$_2$ and O$_2$). Defect properties have been analyzed using an approach combining experimental impedance measurements with physics-based simulations of the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of $β$-Ga$_2$O$_3$/HfO$_2$ MOS capacitors. This approach enabled us to detect two defect bands in HfO$_2$ characterized by thermal ionization energies of ~1.1eV (acceptor-like) and ~2eV (donor-like) attributed to a polaronic self-trapping state and an oxygen vacancy in HfO$_2$, respectively. This study demonstrates how thermal treatments affect the energy distributions and densities of the observed defects. The adopted methodology also enabled the extraction of the spatial distribution of defects across the HfO$_2$ thickness and Cr/HfO$_2$ interface. The high concentration of oxygen vacancies close to the Cr/HfO$_2$ interface extracted from experimental and simulated electrical data is confirmed by in-situ XPS analysis which shows how Cr is scavenging oxygen from the HfO$_2$ and creating the donor band confined near the Cr/HfO$_2$ interface. This donor band density is observed to be reduced after annealing as per simulation and unchanged for different annealing conditions. We speculate this may be due to the formation of dense films and polyforms of HfO$_2$ under different ambient as revealed by high-resolution TEM images.
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Submitted 24 February, 2025;
originally announced February 2025.
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Franck-Condon electron emission from polar semiconductor photocathodes
Authors:
W. Andreas Schroeder,
L. A. Angeloni,
I-J. Shan,
L. B. Jones
Abstract:
A presented analytical formulation of (optical)phonon-mediated and momentum-resonant Franck-Condon emission of photoexcited electrons from polar semiconductors is shown to be very consistent with (i) the observed emission properties of a Cesiated GaAs(001) photocathode at 808nm [J. Phys. D: Appl. Phys. 54, 205301 (2021)] and (ii) the measured spectral emission properties of a GaN(0001) photocathod…
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A presented analytical formulation of (optical)phonon-mediated and momentum-resonant Franck-Condon emission of photoexcited electrons from polar semiconductors is shown to be very consistent with (i) the observed emission properties of a Cesiated GaAs(001) photocathode at 808nm [J. Phys. D: Appl. Phys. 54, 205301 (2021)] and (ii) the measured spectral emission properties of a GaN(0001) photocathode from just below its bandgap energy to 5eV. The theoretical analysis in the parabolic band approximation predicts the form of both the quantum efficiency and mean transverse energy of photoemission as a function of the photocathode's electron affinity and the electron temperature in the vicinity of its emission face. The good agreement between theory and experimental data also suggests that sub-10nm rms surface roughness effects are not significant for polar semiconductor photocathodes.
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Submitted 14 June, 2025; v1 submitted 22 December, 2024;
originally announced December 2024.
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Principles for a Distinguished Global Vacuum: Entropy and the Vacuum State in Causal Set Theory
Authors:
Joshua Y. L. Jones
Abstract:
Using the framework of real scalar field theory on causal sets, the intimate relation of the Sorkin-Johnston vacuum to entropic purity is elucidated. It is shown that taking a set of sensible principles, and the most natural assumption on the space of fields, leaves the Sorkin-Johnston state as the only candidate for the global vacuum of a quasifree theory.
Using the framework of real scalar field theory on causal sets, the intimate relation of the Sorkin-Johnston vacuum to entropic purity is elucidated. It is shown that taking a set of sensible principles, and the most natural assumption on the space of fields, leaves the Sorkin-Johnston state as the only candidate for the global vacuum of a quasifree theory.
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Submitted 10 December, 2024;
originally announced December 2024.
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All-optical method to directly measure the pressure-volume-temperature equation of state of fluids in the diamond anvil cell
Authors:
J. E. Proctor,
C. E. A. Robertson,
L. J. Jones,
J. Phillips,
K. Watson,
Y. Dabburi,
B. Moss
Abstract:
We have developed a new all-optical method to directly measure the pressure-volume-temperature (PVT) equation of state (EOS) of fluids and transparent solids in the diamond anvil high pressure cell by measuring the volume of the sample chamber. Our method combines confocal microscopy and white light interference with a new analysis method which exploits the mutual dependence of sample density and…
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We have developed a new all-optical method to directly measure the pressure-volume-temperature (PVT) equation of state (EOS) of fluids and transparent solids in the diamond anvil high pressure cell by measuring the volume of the sample chamber. Our method combines confocal microscopy and white light interference with a new analysis method which exploits the mutual dependence of sample density and refractive index: Experimentally, the refractive index determines the measured sample chamber thickness (and therefore the measured sample volume/density), yet the sample density is by far the dominant factor in determining the variation in refractive index with pressure. Our analysis method allows us to obtain a set of values for the density and refractive index which are mutually consistent, and agree with the experimental data within error. We have conducted proof-of-concept experiments on a variety of samples (H$_{2}$O, CH$_{4}$, C$_{2}$H$_{6}$, C$_{3}$H$_{8}$, KCl and NaCl) at ambient temperature, and at high temperatures up to just above 500 K. Our proof-of-concept data demonstrate that our method is able to reproduce known fluid and solid EOS within error. Furthermore, we demonstrate that our method allows us to directly and routinely measure the PVT EOS of simple fluids at GPa pressures up to, at least, 514 K (the highest temperature reached in our study). A reasonable estimation of the known sources of error in our volume determinations indicates that the error is currently $\pm$ 2.7% at high temperature, and that it is feasible to reduce it to ca. $\pm$ 1% in future work.
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Submitted 15 August, 2024; v1 submitted 10 July, 2024;
originally announced July 2024.
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Vertically Graded Fe-Ni Alloys with Low Damping and a Sizeable Spin-Orbit Torque
Authors:
Rachel E. Maizel,
Shuang Wu,
Purnima P. Balakrishnan,
Alexander J. Grutter,
Christy J. Kinane,
Andrew J. Caruana,
Prabandha Nakarmi,
Bhuwan Nepal,
David A. Smith,
Youngmin Lim,
Julia L. Jones,
Wyatt C. Thomas,
Jing Zhao,
F. Marc Michel,
Tim Mewes,
Satoru Emori
Abstract:
Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to $\sim$1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10 nm thick single-layer ferromagnet to attain both low…
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Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to $\sim$1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10 nm thick single-layer ferromagnet to attain both low damping and a sizable SOT. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of $α_\mathrm{eff} < 5\times10^{-3}$ in the FeNi alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable anti-damping SOT efficiency of $|θ_\mathrm{DL}| \approx 0.05$, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices.
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Submitted 19 December, 2024; v1 submitted 14 June, 2024;
originally announced June 2024.
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Ultra-fast Digital DPC Yielding High Spatio-Temporal Resolution for Low-Dose Phase Characterisation
Authors:
Julie Marie Bekkevold,
Jonathan J. P. Peters,
Ryo Ishikawa,
Naoya Shibata,
Lewys Jones
Abstract:
In the scanning transmission electron microscope, both phase imaging of beam-sensitive materials and characterisation of a material's functional properties using in-situ experiments are becoming more widely available. As the practicable scan speed of 4D-STEM detectors improves, so too does the temporal resolution achievable for both differential phase contrast (DPC) and ptychography. However, the…
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In the scanning transmission electron microscope, both phase imaging of beam-sensitive materials and characterisation of a material's functional properties using in-situ experiments are becoming more widely available. As the practicable scan speed of 4D-STEM detectors improves, so too does the temporal resolution achievable for both differential phase contrast (DPC) and ptychography. However, the read-out burden of pixelated detectors, and the size of the gigabyte to terabyte sized data sets, remain a challenge for both temporal resolution and their practical adoption. In this work, we show that a high-fidelity DPC phase reconstruction can be achieved from both annular segmented detectors or pixelated arrays with relatively few elements using signal digitisation. Unlike conventional analog data, even at the fastest scan speeds, phase reconstructions from digitised DPC-segment images yield reliable data. Finally, dose fractionation by fast scanning and multi-framing allows for post-process binning of frame streams to balance signal-to-noise ratio and temporal resolution for low-dose phase imaging for in-situ experiments.
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Submitted 16 August, 2024; v1 submitted 10 May, 2024;
originally announced May 2024.
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Ferroelectricity at the extreme thickness limit in the archetypal antiferroelectric PbZrO$_3$
Authors:
Nikhilesh Maity,
Milan Haddad,
Nazanin Bassiri-Gharb,
Amit Kumar,
Lewys Jones,
Sergey Lisenkov,
Inna Ponomareva
Abstract:
Size-driven transition of an antiferroelectric into a polar ferroelectric or ferrielectric state is a strongly debated issue from both experimental and theoretical perspectives. While critical thickness limits for such transitions have been explored, a bottom-up approach in the ultrathin limit considering few atomic layers could provide insight into the mechanism of stabilization of the polar phas…
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Size-driven transition of an antiferroelectric into a polar ferroelectric or ferrielectric state is a strongly debated issue from both experimental and theoretical perspectives. While critical thickness limits for such transitions have been explored, a bottom-up approach in the ultrathin limit considering few atomic layers could provide insight into the mechanism of stabilization of the polar phases over the antipolar phase seen in bulk PbZrO$_3$. Here, we use first-principles density functional theory to predict the stability of polar phases in Pt/PbZrO$_3$/Pt nanocapacitors. In a few atomic layer thick slabs of PbZrO$_3$ sandwiched between Pt electrodes, we find that the polar phase originating from the well established R3c phase of bulk PbZrO$_3$ is energetically favorable over the antipolar phase originating from the Pbam phase of bulk PbZrO$_3$. The famous triple-well potential of antiferroelectric PbZrO$_3$ is modified in the nanocapacitor limit in such a way as to swap the positions of the global and local minima, stabilizing the polar phase relative to the antipolar one. The size effect is decomposed into the contributions from dimensionality reduction, surface charge screening, and interfacial relaxation, which reveals that it is the creation of well-compensated interfaces that stabilizes the polar phases over the antipolar ones in nanoscale PbZrO$_3$.
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Submitted 9 December, 2024; v1 submitted 21 February, 2024;
originally announced February 2024.
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3D-imaging of Printed Nanostructured Networks using High-resolution FIB-SEM Nanotomography
Authors:
Cian Gabbett,
Luke Doolan,
Kevin Synnatschke,
Laura Gambini,
Emmet Coleman,
Adam G. Kelly,
Shixin Liu,
Eoin Caffrey,
Jose Munuera,
Catriona Murphy,
Stefano Sanvito,
Lewys Jones,
Jonathan N. Coleman
Abstract:
Networks of solution-processed nanomaterials are important for multiple applications in electronics, sensing and energy storage/generation. While it is known that network morphology plays a dominant role in determining the physical properties of printed networks, it remains difficult to quantify network structure. Here, we utilise FIB-SEM nanotomography to characterise the morphology of nanostruct…
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Networks of solution-processed nanomaterials are important for multiple applications in electronics, sensing and energy storage/generation. While it is known that network morphology plays a dominant role in determining the physical properties of printed networks, it remains difficult to quantify network structure. Here, we utilise FIB-SEM nanotomography to characterise the morphology of nanostructured networks. Nanometer-resolution 3D-images were obtained from printed networks of graphene nanosheets of various sizes, as well as networks of WS2 nanosheets, silver nanosheets and silver nanowires. Important morphological characteristics, including network porosity, tortuosity, pore dimensions and nanosheet orientation were extracted and linked to network resistivity. By extending this technique to interrogate the structure and interfaces within vertical printed heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.
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Submitted 26 January, 2023;
originally announced January 2023.
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Temperature-Dependent Dynamic Disproportionation in LiNiO$_2$
Authors:
Andrey D. Poletayev,
Robert J. Green,
Jack E. N. Swallow,
Lijin An,
Leanne Jones,
Grant Harris,
Peter Bencok,
Ronny Sutarto,
Jonathon P. Cottom,
Benjamin J. Morgan,
Robert A. House,
Robert S. Weatherup,
M. Saiful Islam
Abstract:
Nickelate materials offer diverse functionalities for energy and computing applications. Lithium nickel oxide (LiNiO$_2$) is an archetypal layered nickelate, but the electronic structure of this correlated material is not yet fully understood. Here we investigate the temperature-dependent speciation and spin dynamics of Ni ions in LiNiO$_2$. Our ab initio simulations predict that Ni ions dispropor…
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Nickelate materials offer diverse functionalities for energy and computing applications. Lithium nickel oxide (LiNiO$_2$) is an archetypal layered nickelate, but the electronic structure of this correlated material is not yet fully understood. Here we investigate the temperature-dependent speciation and spin dynamics of Ni ions in LiNiO$_2$. Our ab initio simulations predict that Ni ions disproportionate into three states, which dynamically interconvert and whose populations vary with temperature. These predictions are verified using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and resonant inelastic x-ray scattering at the Ni L$_{3,2}$-edge. Charge-transfer multiplet calculations consistent with disproportionation reproduce all experimental features. Together, our experimental and computational results support a model of dynamic disproportionation that explains diverse physical observations of LiNiO$_2$, including magnetometry, thermally activated electronic conduction, diffractometry, core-level spectroscopies, and the stability of ubiquitous antisite defects. This unified understanding of the fundamental material properties of LiNiO$_2$ is important for applications of nickelate materials as battery cathodes, catalysts, and superconductors.
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Submitted 2 July, 2024; v1 submitted 16 November, 2022;
originally announced November 2022.
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Tackling Disorder in $γ$-Ga$_2$O$_3$
Authors:
Laura E. Ratcliff,
Takayoshi Oshima,
Felix Nippert,
Benjamin M. Janzen,
Elias Kluth,
Rüdiger Goldhahn,
Martin Feneberg,
Piero Mazzolini,
Oliver Bierwagen,
Charlotte Wouters,
Musbah Nofal,
Martin Albrecht,
Jack E. N. Swallow,
Leanne A. H. Jones,
Pardeep K. Thakur,
Tien-Lin Lee,
Curran Kalha,
Christoph Schlueter,
Tim D. Veal,
Joel B. Varley,
Markus R. Wagner,
Anna Regoutz
Abstract:
Ga$_2$O$_3$ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga$_2$O$_3$ offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. $γ$-Ga$_2$O$_3$ presents a particular challenge across synthesis, characterisation, and theory due to its inherent…
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Ga$_2$O$_3$ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga$_2$O$_3$ offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. $γ$-Ga$_2$O$_3$ presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure -- electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the $γ$-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of $γ$-Ga$_2$O$_3$. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by PLE spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure.
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Submitted 9 May, 2022;
originally announced May 2022.
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Site-specific surface atom valence band structure via X-ray standing wave excited photoemission
Authors:
Yanna Chen,
Leighton O. Jones,
Tien-Lin Lee,
Anusheela Das,
Martin A. Mosquera,
Denis T. Keane,
George C. Schatz,
Michael J. Bedzyk
Abstract:
X-ray standing wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for 1/2 monolayer (ML) Pt grown on a SrTiO3 (001) surface. The XSW induced modulations in the core level (CL) and VB photoemission from the surface and substrate atoms were monitored for three hkl substrate Bragg reflections. The XSW CL analysis shows the Pt to have an fcc-like cube-on-…
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X-ray standing wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for 1/2 monolayer (ML) Pt grown on a SrTiO3 (001) surface. The XSW induced modulations in the core level (CL) and VB photoemission from the surface and substrate atoms were monitored for three hkl substrate Bragg reflections. The XSW CL analysis shows the Pt to have an fcc-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Overall, this work represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.
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Submitted 11 January, 2022;
originally announced January 2022.
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Entanglement Entropy of Disjoint Spacetime Intervals in Causal Set Theory
Authors:
Callum F. Duffy,
Joshua Y. L. Jones,
Yasaman K. Yazdi
Abstract:
A more complete understanding of entanglement entropy in a covariant manner could inform the search for quantum gravity. We build on work in this direction by extending previous results to disjoint regions in $1+1$D. We investigate the entanglement entropy of a scalar field in disjoint intervals within the causal set framework, using the spacetime commutator and correlator, $i\mathbfΔ$ and…
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A more complete understanding of entanglement entropy in a covariant manner could inform the search for quantum gravity. We build on work in this direction by extending previous results to disjoint regions in $1+1$D. We investigate the entanglement entropy of a scalar field in disjoint intervals within the causal set framework, using the spacetime commutator and correlator, $i\mathbfΔ$ and $\mathbf{W}$ (or the Pauli-Jordan and Wightman functions), respectively. A new truncation scheme for disjoint causal diamonds is presented, which follows from the single diamond truncation scheme. We investigate setups including two and three disjoint causal diamonds, as well as a single causal diamond that shares a boundary with a larger global causal diamond. In all the cases that we study, our results agree with the expected area laws. In addition, we study the mutual information in the two disjoint diamonds setup. The ease of our calculations indicate our methods to be a useful tool for numerically studying such systems. We end with a discussion of some of the strengths and future applications of the spacetime formulation we use in our entanglement entropy computations, both in causal set theory and in the continuum.
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Submitted 8 April, 2022; v1 submitted 14 October, 2021;
originally announced October 2021.
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Procedure for 3D atomic resolution reconstructions using atom-counting and a Bayesian genetic algorithm
Authors:
Annick De Backer,
Sandra Van Aert,
Peter D. Nellist,
Lewys Jones
Abstract:
We introduce a Bayesian genetic algorithm for reconstructing atomic models of nanoparticles from a single projection using Z-contrast imaging. The number of atoms in a projected atomic column obtained from annular dark field scanning transmission electron microscopy (ADF STEM) images serves as an input for the initial three-dimensional (3D) model. The novel algorithm minimizes the energy of the st…
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We introduce a Bayesian genetic algorithm for reconstructing atomic models of nanoparticles from a single projection using Z-contrast imaging. The number of atoms in a projected atomic column obtained from annular dark field scanning transmission electron microscopy (ADF STEM) images serves as an input for the initial three-dimensional (3D) model. The novel algorithm minimizes the energy of the structure while utilizing a priori information about the finite precision of the atom-counting results and neighbor-mass relations. The results show excellent prospects for obtaining reliable reconstructions of beam-sensitive nanoparticles during dynamical processes from images acquired with sufficiently low incident electron doses.
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Submitted 30 March, 2022; v1 submitted 12 May, 2021;
originally announced May 2021.
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Increasing Spatial Fidelity and SNR of 4D-STEM using Multi-frame Data Fusion
Authors:
Colum M. O'Leary,
Benedikt Haas,
Christoph T. Koch,
Peter D. Nellist,
Lewys Jones
Abstract:
4D-STEM, in which the 2D diffraction plane is captured for each 2D scan position in the scanning transmission electron microscope (STEM) using a pixelated detector, is complementing and increasingly replacing existing imaging approaches. However, at present the speed of those detectors, although having drastically improved in the recent years, is still 100 to 1,000 times slower than the current PM…
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4D-STEM, in which the 2D diffraction plane is captured for each 2D scan position in the scanning transmission electron microscope (STEM) using a pixelated detector, is complementing and increasingly replacing existing imaging approaches. However, at present the speed of those detectors, although having drastically improved in the recent years, is still 100 to 1,000 times slower than the current PMT technology operators are used to. Regrettably, this means environmental scanning-distortion often limits the overall performance of the recorded 4D data. Here we present an extension of existing STEM distortion correction techniques for the treatment of 4D-data series. Although applicable to 4D-data in general, we use electron ptychography and electric-field mapping as model cases and demonstrate an improvement in spatial-fidelity, signal-to-noise ratio (SNR), phase-precision and spatial-resolution.
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Submitted 5 July, 2021; v1 submitted 4 March, 2021;
originally announced March 2021.
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Unlocking the Origin of Compositional Fluctuations in InGaN Light Emitting Diodes
Authors:
Tara P. Mishra,
Govindo J. Syaranamual,
Zeyu Deng,
Jing Yang Chung,
Li Zhang,
Sarah A Goodman,
Lewys Jones,
Michel Bosman,
Silvija Gradečak,
Stephen J. Pennycook,
Pieremanuele Canepa
Abstract:
The accurate determination of the compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content $In_{x}Ga_{1-x}N$ light-emitting diodes, the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at sub-nanometer resolution with multiscale computational models to obtain a statistic…
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The accurate determination of the compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content $In_{x}Ga_{1-x}N$ light-emitting diodes, the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at sub-nanometer resolution with multiscale computational models to obtain a statistical distribution of the compositional fluctuations in $In_{x}Ga_{1-x}N$ quantum wells (QWs). Employing a multiscale computational model, we show the tendency of intrinsic compositional fluctuation in $In_{x}Ga_{1-x}N$ QWs at different Indium concentration and in the presence of strain. We have developed a systematic formalism based on the autonomous detection of compositional fluctuation in observed and simulated EELS maps. We have shown a direct comparison between the computationally predicted and experimentally observed compositional fluctuations. We have found that although a random alloy model captures the distribution of compositional fluctuations in relatively low In ($\sim$ 18%) content $In_{x}Ga_{1-x}N$ QWs, there exists a striking deviation from the model in higher In content ($\geq$ 24%) QWs. Our results highlight a distinct behavior in carrier localization driven by compositional fluctuations in the low and high In-content InGaN QWs, which would ultimately affect the performance of LEDs. Furthermore, our robust computational and atomic characterization method can be widely applied to study materials in which nanoscale compositional fluctuations play a significant role on the material performance.
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Submitted 6 February, 2021;
originally announced February 2021.
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Ligand Directed Self-Assembly of Bulk Organic-Semiconductor/Quantum-Dot Blend Films Enables Near Quantitative Harvesting of Triplet Excitons
Authors:
Jesse Allardice,
Victor Gray,
Simon Dowland,
Daniel T. W. Toolan,
Michael P. Weir,
James Xiao,
Zhilong Zhang,
Jurjen F. Winkel,
Anthony J. Petty II,
John Anthony,
Richard Friend,
Anthony J. Ryan,
Richard A. L. Jones,
Neil C. Greenham,
Akshay Rao
Abstract:
Singlet fission (SF), an exciton multiplication process occurring in organic semiconductors, offers a way to break the Shockley-Queisser limit in single-bandgap photovoltaics (PV). If the triplet excitons generated by SF can be transferred to inorganic quantum dots (QDs), where they radiatively recombine, SF based photon multiplication is achieved, converting a single high-energy photon into two l…
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Singlet fission (SF), an exciton multiplication process occurring in organic semiconductors, offers a way to break the Shockley-Queisser limit in single-bandgap photovoltaics (PV). If the triplet excitons generated by SF can be transferred to inorganic quantum dots (QDs), where they radiatively recombine, SF based photon multiplication is achieved, converting a single high-energy photon into two low-energy photons. Such a SF photon multiplication film (SF-PMF) could raise the efficiency of the best Si-PV from 26.7% to 32.5%. But a precise nanoscale morphology is required within such a film consisting of the appropriate morphology for the organic phase, allowing for efficient SF, within which the QD emitters are well dispersed on a tens of nm length scale to enable efficient harvesting of the triplets. However, it has been a long-standing problem that the individual components in organic-QD blends have a tendency to aggregate and phase separate, due to a mismatch of their size, shape and surface energies. Here, we demonstrate a QD surface engineering approach using an electronically active, highly soluble semiconductor ligand that is matched to the SF material, which allows us to direct the self-assembly process yielding solution processed films with well-dispersed QDs and minimal aggregation, as characterised by X-ray and neutron scattering and electron microscopy. Steady state and time-resolved optical spectroscopy show that the films support efficient SF (190% yield) in the organic phase and quantitative triplet energy transfer across the organic-QD interface, resulting in 95% of the triplet excitons being harvested by the QDs. Our results establish the SF-PMF as a highly promising architecture to harness the SF process to enhance PV efficiencies, and also provide a highly versatile approach to overcome challenges in the blending of organic semiconductors with QDs.
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Submitted 12 September, 2020;
originally announced September 2020.
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Influence of Polymorphism on the Electronic Structure of Ga$_2$O$_3$
Authors:
Jack E. N. Swallow,
Christian Vorwerk,
Piero Mazzolini,
Patrick Vogt,
Oliver Bierwagen,
Alexander Karg,
Martin Eickhoff,
Jörg Schörmann,
Markus R. Wagner,
Joseph W. Roberts,
Paul R. Chalker,
Matthew J. Smiles,
Philip A. E. Murgatroyd,
Sara A. Razek,
Zachary W. Lebens-Higgins,
Louis F. J. Piper,
Leanne A. H. Jones,
Pardeep Kumar Thakur,
Tien-Lin Lee,
Joel B. Varley,
Jürgen Furthmüller,
Claudia Draxl,
Tim D. Veal,
Anna Regoutz
Abstract:
The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy efficient power electronic devices. Ga$_2$O$_3$ has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main $β$-phase. Here, three polymorphs of Ga$_2$O$_3$, $α$, $β$ and $\varepsilon$, are investigated using X-ray diffraction, X-ra…
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The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy efficient power electronic devices. Ga$_2$O$_3$ has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main $β$-phase. Here, three polymorphs of Ga$_2$O$_3$, $α$, $β$ and $\varepsilon$, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga$_2$O$_3$ polymorphs as materials at the heart of future electronic device generations.
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Submitted 22 September, 2020; v1 submitted 27 May, 2020;
originally announced May 2020.
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Synthesis of WTe2 thin films and highly-crystalline nanobelts from pre-deposited reactants
Authors:
John B. Mc Manus,
Cansu Ilhan,
Bastien Balsamo,
Clive Downing,
Conor P. Cullen,
Tanja Stimpfel-Lidner,
Graeme Cunningham,
Lisanne Peters,
Lewys Jones,
Daragh Mullarkey,
Igor V. Shvets,
Georg S. Duesberg,
Niall McEvoy
Abstract:
Tungsten ditelluride is a layered transition metal dichalcogenide (TMD) that has attracted increasing research interest in recent years. WTe2 has demonstrated large non-saturating magnetoresistance, potential for spintronic applications and promise as a type-II Weyl semimetal. The majority of works on WTe2 have relied on mechanically-exfoliated flakes from chemical vapour transport (CVT) grown cry…
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Tungsten ditelluride is a layered transition metal dichalcogenide (TMD) that has attracted increasing research interest in recent years. WTe2 has demonstrated large non-saturating magnetoresistance, potential for spintronic applications and promise as a type-II Weyl semimetal. The majority of works on WTe2 have relied on mechanically-exfoliated flakes from chemical vapour transport (CVT) grown crystals for their investigations. While producing high-quality samples, this method is hindered by several disadvantages including long synthesis times, high-temperature anneals and an inherent lack of scalability. In this work, a synthesis method is demonstrated that allows the production of large-area polycrystalline films of WTe2. This is achieved by the reaction of pre-deposited films of W and Te at a relatively low temperature of 550 degC. Sputter X-ray photoelectron spectroscopy reveals the rapid but self-limiting nature of the oxidation of these WTe2 films in ambient conditions. The WTe2 films are composed of areas of micrometre sized nanobelts that can be isolated and offer potential as an alternative to CVT-grown samples. These nanobelts are highly crystalline with low defect densities indicated by TEM and show promising initial electrical results.
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Submitted 24 April, 2020;
originally announced April 2020.
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Low temperature growth and optical properties of α-Ga2O3 deposited on sapphire by plasma enhanced atomic layer deposition
Authors:
J. W. Roberts,
P. R. Chalker,
B. Ding,
R. A. Oliver,
J. T. Gibbon,
L. A. H. Jones,
V. R. Dhanak,
L. J. Phillips,
J. D. Major,
F. C-P. Massabuau
Abstract:
Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystal…
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Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystallinity. At temperatures below 200°C amorphous Ga2O3 films were deposited. Between 250°C and 350°C the films became predominantly α-Ga2O3. Above 350°C the deposited films showed a mixture of α-Ga2O3 and ε-Ga2O3 phases. Plasma power and O2 flow rate were observed to have less influence over the resultant phases present in the films. However, both parameters could be tuned to alter the strain of the film. Ultraviolet transmittance measurements on the Ga2O3 films showed that the bandgaps ranges from 5.0 eV to 5.2 eV with the largest bandgap of 5.2 eV occurring for the α-Ga2O3 phase deposited at 250°C.
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Submitted 19 August, 2019;
originally announced August 2019.
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Direct Imaging of Charge Redistribution due to Bonding at Atomic Resolution via Electron Ptychography
Authors:
Gerardo T. Martinez,
Benjamin X. Shi,
Timothy C. Naginey,
Lewys Jones,
Colum M. O'Leary,
Timothy J. Pennycook,
Rebecca J. Nicholls,
Jonathan R. Yates,
Peter D. Nellist
Abstract:
Phase imaging in electron microscopy is sensitive to the local potential, including charge redistribution from bonding. We demonstrate that electron ptychography provides the necessary sensitivity to detect this subtle effect by directly imaging the charge redistribution in single layer boron nitride. Residual aberrations can be measured and corrected post-collection, and simultaneous atomic numbe…
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Phase imaging in electron microscopy is sensitive to the local potential, including charge redistribution from bonding. We demonstrate that electron ptychography provides the necessary sensitivity to detect this subtle effect by directly imaging the charge redistribution in single layer boron nitride. Residual aberrations can be measured and corrected post-collection, and simultaneous atomic number contrast imaging provides unambiguous sub-lattice identification. Density functional theory calculations confirm the detection of charge redistribution.
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Submitted 15 October, 2023; v1 submitted 30 July, 2019;
originally announced July 2019.
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Proposal for a three-dimensional magnetic measurement method with nanometer-scale depth resolution
Authors:
Devendra Negi,
Lewys Jones,
Juan-Carlos Idrobo,
Jan Rusz
Abstract:
We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2~nm.
We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2~nm.
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Submitted 29 September, 2018;
originally announced October 2018.
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Electron transport properties of sub-3-nm diameter copper nanowires
Authors:
Sarah L. T. Jones,
Alfonso Sanchez-Soares,
John J. Plombon,
Ananth P. Kaushik,
Roger E. Nagle,
James S. Clarke,
James C. Greer
Abstract:
Density functional theory and density functional tight-binding are applied to model electron transport in copper nanowires of approximately 1 nm and 3 nm diameters with varying crystal orientation and surface termination. The copper nanowires studied are found to be metallic irrespective of diameter, crystal orientation and/or surface termination. Electron transmission is highly dependent on cryst…
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Density functional theory and density functional tight-binding are applied to model electron transport in copper nanowires of approximately 1 nm and 3 nm diameters with varying crystal orientation and surface termination. The copper nanowires studied are found to be metallic irrespective of diameter, crystal orientation and/or surface termination. Electron transmission is highly dependent on crystal orientation and surface termination. Nanowires oriented along the [110] crystallographic axis consistently exhibit the highest electron transmission while surface oxidized nanowires show significantly reduced electron transmission compared to unterminated nanowires. Transmission per unit area is calculated in each case, for a given crystal orientation we find that this value decreases with diameter for unterminated nanowires but is largely unaffected by diameter in surface oxidized nanowires for the size regime considered. Transmission pathway plots show that transmission is larger at the surface of unterminated nanowires than inside the nanowire and that transmission at the nanowire surface is significantly reduced by surface oxidation. Finally, we present a simple model which explains the transport per unit area dependence on diameter based on transmission pathways results.
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Submitted 20 September, 2016;
originally announced September 2016.
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Effect of strain, thickness, and local surface environment on electron transport properties of oxygen-terminated copper thin films
Authors:
Alfonso Sanchez-Soares,
Sarah L. T. Jones,
John J. Plombon,
Ananth P. Kaushik,
Roger E. Nagle,
James S. Clarke,
James C. Greer
Abstract:
Electron transport is studied in surface oxidized single-crystal copper thin films with a thickness of up to 5.6 nm by applying density functional theory and density functional tight binding methods to determine electron transport properties within the ballistic regime. The variation of the electron transmission as a function of film thickness as well as the different contributions to the overall…
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Electron transport is studied in surface oxidized single-crystal copper thin films with a thickness of up to 5.6 nm by applying density functional theory and density functional tight binding methods to determine electron transport properties within the ballistic regime. The variation of the electron transmission as a function of film thickness as well as the different contributions to the overall electron transmission as a function of depth into the the films is examined. Transmission at the oxidized copper film surfaces is found to be universally low. Films with thickness greater than 2.7 nm exhibit a similar behavior in local transmission per unit area with depth from the film surface; transmission per unit area initially increases rapidly and then plateaus at a depth of approximately 0.35-0.5 nm away from the surface, dependent on surface facet. Unstrained films tend to exhibit a higher transmission per unit area than corresponding films under tensile strain.
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Submitted 30 September, 2016; v1 submitted 20 September, 2016;
originally announced September 2016.
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Interface-induced Polarization in SrTiO$_3$-LaCrO$_3$ Superlattices
Authors:
Ryan B. Comes,
Steven R. Spurgeon,
Steve M. Heald,
Despoina M. Kepaptsoglou,
Lewys Jones,
Phuong Vu Ong,
Mark E. Bowden,
Quentin M. Ramasse,
Peter V. Sushko,
Scott A. Chambers
Abstract:
Epitaxial interfaces and superlattices comprised of polar and non-polar perovskite oxides have generated considerable interest because they possess a range of desirable properties for functional devices. In this work, emergent polarization in superlattices of SrTiO$_3$ (STO) and LaCrO$_3$ (LCO) is demonstrated. By controlling the interfaces between polar LCO and non-polar STO, polarization is indu…
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Epitaxial interfaces and superlattices comprised of polar and non-polar perovskite oxides have generated considerable interest because they possess a range of desirable properties for functional devices. In this work, emergent polarization in superlattices of SrTiO$_3$ (STO) and LaCrO$_3$ (LCO) is demonstrated. By controlling the interfaces between polar LCO and non-polar STO, polarization is induced throughout the STO layers of the superlattice. Using x-ray absorption near-edge spectroscopy and aberration-corrected scanning transmission electron microscopy displacements of the Ti cations off-center within TiO6 octahedra along the superlattice growth direction are measured. This distortion gives rise to built-in potential gradients within the STO and LCO layers, as measured by in situ x-ray photoelectron spectroscopy. Density functional theory models explain the mechanisms underlying this behavior, revealing the existence of both an intrinsic polar distortion and a built-in electric field, which are due to alternately positively and negatively charged interfaces in the superlattice. This study paves the way for controllable polarization for carrier separation in multilayer materials and highlights the crucial role that interface structure plays in governing such behavior.
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Submitted 8 April, 2016;
originally announced April 2016.
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Breaking of macroscopic centric symmetry in paraelectric phases of ferroelectric materials and implications for flexoelectricity
Authors:
Alberto Biancoli,
Chris M. Fancher,
Jacob L. Jones,
Dragan Damjanovic
Abstract:
A centrosymmetric stress cannot induce a polar response in centric materials, piezoelectricity is, for example, possible only in non-centrosymmetric structures. An exception is meta-materials with shape asymmetry, which may be polarized by stress even when the material is centric. In this case the mechanism is flexoelectricity, which relates polarization to a strain gradient. The flexoelectric res…
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A centrosymmetric stress cannot induce a polar response in centric materials, piezoelectricity is, for example, possible only in non-centrosymmetric structures. An exception is meta-materials with shape asymmetry, which may be polarized by stress even when the material is centric. In this case the mechanism is flexoelectricity, which relates polarization to a strain gradient. The flexoelectric response scales inversely with size, thus a large effect is expected in nanoscale materials. Recent experiments in polycrystalline, centrosymmetric perovskites [e.g., (Ba,Sr)TiO3] have indicated values of flexoelectric coefficients that are orders of magnitude higher than theoretically predicted, promising practical applications based on bulk materials. We show that materials with unexpectedly large flexoelectric response exhibit breaking of the macroscopic centric symmetry through inhomogeneity induced by the high temperature processing. The emerging electro-mechanical coupling is significant and may help to resolve the controversy surrounding the large apparent flexoelectric coefficients in this class of materials.
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Submitted 16 July, 2017; v1 submitted 22 September, 2015;
originally announced September 2015.
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Evidence of field induced inter-ferroelectric transformation as the dominant driving mechanism for anomalous piezoresponse in the morphotropic phase boundary piezoelectric system PbTiO3-BiScO3
Authors:
Lalitha K. V.,
Chris M. Fancher,
Jacob L. Jones,
Rajeev Ranjan
Abstract:
The contributory mechanisms associated with high piezoelectric response in piezoelectric ceramics have been examined by in-situ electric field dependent high energy synchro x-ray diffraction study. A comparative study of electric field induced lattice strain and the propensity for non-180o domain switching on two closeby compositions of a high performance piezoelectric alloy (1-x)PbTiO3-(x)BiScO3,…
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The contributory mechanisms associated with high piezoelectric response in piezoelectric ceramics have been examined by in-situ electric field dependent high energy synchro x-ray diffraction study. A comparative study of electric field induced lattice strain and the propensity for non-180o domain switching on two closeby compositions of a high performance piezoelectric alloy (1-x)PbTiO3-(x)BiScO3, one within the morphotropic phase boundary (MPB) region exhibiting d33 of 425 pC/N and another just outside the MPB region exhibiting d33 of 260 pC/N, unravelled that, inspite of the MPB specimen exhibiting considerably high piezoelectric response, its lattice strain and domain switching propensity is considerably less as compared to the non-MPB specimen. These new experimental observations contradict the commonly held view that anomalous piezoelectric response in MPB based piezoelectrics arise due to enhanced propensity for domain switching. Our results show the dominant mechanism contributing to the anomalous piezo-response is field induced interferroelectric transformation.
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Submitted 24 August, 2015;
originally announced August 2015.
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Strong interrelationship between anomalous electric-field induced lattice strain along non-polar direction and domain reorientation in pseudorhombohedral piezoelectric ceramic BiScO3-PbTiO3
Authors:
Lalitha KV,
Chris M. Fancher,
Jacob L. Jones,
Rajeev Ranjan
Abstract:
The lattice strain and domain switching behaviour was investigated as a function of cyclic field and grain orientation for a pseudorhombohedral composition of the high Curie point piezoelectric system xBiScO3 - (1-x)PbTiO3 (x = 0.40) by in-situ electric field diffraction technique with high energy synchrotron x-rays. Along the field direction, the system exhibts five time large strain along 100 as…
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The lattice strain and domain switching behaviour was investigated as a function of cyclic field and grain orientation for a pseudorhombohedral composition of the high Curie point piezoelectric system xBiScO3 - (1-x)PbTiO3 (x = 0.40) by in-situ electric field diffraction technique with high energy synchrotron x-rays. Along the field direction, the system exhibts five time large strain along 100 as compared to the 111 direction. A one-to-one correspondence between the 200 lattice strain and the 111 domain switching suggests a strong correlation between the two phenomena.
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Submitted 4 August, 2015;
originally announced August 2015.
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Influence of antisymmetric exchange interaction on quantum tunneling of magnetization in a dimeric molecular magnet Mn6
Authors:
S. Bahr,
C. J. Milios,
L. F. Jones,
E. K. Brechin,
V. Mosser,
W. Wernsdorfer
Abstract:
We present magnetization measurements on the single molecule magnet Mn6, revealing various tunnel transitions inconsistent with a giant-spin description. We propose a dimeric model of the molecule with two coupled spins S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We show that this simplifie…
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We present magnetization measurements on the single molecule magnet Mn6, revealing various tunnel transitions inconsistent with a giant-spin description. We propose a dimeric model of the molecule with two coupled spins S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We show that this simplified model of the molecule explains the experimentally observed tunnel transitions and that the antisymmetric exchange interaction between the spins gives rise to tunneling processes between spin states belonging to different spin multiplets.
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Submitted 3 October, 2008;
originally announced October 2008.
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Self-motile colloidal particles: from directed propulsion to random walk
Authors:
Jonathan R. Howse,
Richard A. L. Jones,
Anthony J. Ryan,
Tim Gough,
Reza Vafabakhsh,
Ramin Golestanian
Abstract:
The motion of an artificial micro-scale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times, it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substa…
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The motion of an artificial micro-scale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times, it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substantially enhanced diffusion coefficient. Our results suggest strategies for designing artificial chemotactic systems.
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Submitted 29 June, 2007;
originally announced June 2007.
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Finite size scaling of the correlation length above the upper critical dimension
Authors:
Jeff L. Jones,
A. P. Young
Abstract:
We show numerically that correlation length at the critical point in the five-dimensional Ising model varies with system size L as L^{5/4}, rather than proportional to L as in standard finite size scaling (FSS) theory. Our results confirm a hypothesis that FSS expressions in dimension d greater than the upper critical dimension of 4 should have L replaced by L^{d/4} for cubic samples with period…
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We show numerically that correlation length at the critical point in the five-dimensional Ising model varies with system size L as L^{5/4}, rather than proportional to L as in standard finite size scaling (FSS) theory. Our results confirm a hypothesis that FSS expressions in dimension d greater than the upper critical dimension of 4 should have L replaced by L^{d/4} for cubic samples with periodic boundary conditions. We also investigate numerically the logarithmic corrections to FSS in d = 4.
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Submitted 7 December, 2004;
originally announced December 2004.