arXiv subjects

Yu-Tsun Shao

Publications and source records attributed to Yu-Tsun Shao.

33 records · Page 2Linked to original sources

arXiv2023

Smooth, homogeneous, high-purity Nb3Sn superconducting RF resonant cavity by seed-free electrochemical synthesis

Workbench-size particle accelerators, enabled by Nb3Sn-based superconducting radio-frequency (SRF) cavities, hold the potential of driving scientific discovery by offering a widely accessible and affordable source of high-energy electrons and X-rays. Thin-film Nb3Sn RF superconductors with high quality factors, high operation temperatures, and high-field potentials are critical for these devices. However, surface roughness, non-stoichiometry, and impurities in Nb3Sn deposited by conventional Sn-vapor diffusion prevent them from reaching their theoretical capabilities. Here we demonstrate a seed-free electrochemical synthesis that pushes the limit of chemical and physical properties in Nb3Sn. Utilization of electrochemical Sn pre-deposits reduces the roughness of converted Nb3Sn by five times compared to typical vapor-diffused Nb3Sn. Quantitative mappings using chemical and atomic probes confirm improved stoichiometry and minimized impurity concentrations in electrochemically synthesized Nb3Sn. We have successfully applied this Nb3Sn to the large-scale 1.3 GHz SRF cavity and demonstrated ultra-low BCS surface resistances at multiple operation temperatures, notably lower than vapor-diffused cavities. Our smooth, homogeneous, high-purity Nb3Sn provides the route toward high efficiency and high fields for SRF applications under helium-free cryogenic operations.

cond-mat.mtrl-sci↗

arXiv2023

Emergent chirality in a polar meron to skyrmion phase transition

Polar skyrmions are predicted to emerge from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. With the discovery of topologically-stable polar skyrmions, it is of both fundamental and practical interest to understand the microscopic nature and the possibility of temperature- and strain-driven phase transitions in ensembles of such polar skyrmions. Here, we explore the reversible transition from a skyrmion state (topological charge of -1) to a two-dimensional, tetratic lattice of merons (with topological charge of -1/2) upon varying the temperature and elastic boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 lifted-off membranes. This topological phase transition is accompanied by a change in chirality, from zero-net chirality (in meronic phase) to net-handedness (in skyrmionic phase). To map these changes microscopically required developing new imaging methods. We show how scanning convergent beam electron diffraction provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while also directly mapping the chirality of each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and theoretical phase-field simulations. These results revealed by our new measurement methods provide the first illustration of systematic control of rich variety of topological dipole textures by altering the mechanical boundary conditions, which may offer a promising way to control their functionalities in ferroelectric nanodevices using the local and spatial distribution of chirality and order.

cond-mat.mes-hall↗

arXiv2022

Pervasive beyond room-temperature ferromagnetism in a doped van der Waals magnet: Ni doped Fe$_5$GeTe$_2$ with $T_{\text{C}}$ up to 478 K

The existence of long range magnetic order in low dimensional magnetic systems, such as the quasi-two-dimensional (2D) van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe$_N$GeTe$_2$ ($N$ = 3, 4, 5; FGT) family is exceptional owing to its vast tunability of magnetic properties. Particularly, a ferromagnetic ordering temperature ($T_{\text{C}}$) above room temperature at $N$ = 5 (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron (Fe) in F5GT, a record high $T_{\text{C}}$ = 478(6) K is achieved. Importantly, pervasive, beyond-room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, in which the structural alteration enhanced 3D magnetic couplings might be critical for enhancing the ferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2022

Disentangling magnetic and grain contrast in polycrystalline FeGe thin films using four-dimensional Lorentz scanning transmission electron microscopy

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinkii-Moriya interaction (DMI) is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. One such material is sputter-deposited B20 FeGe on Si, which has been actively investigated as the basis for low-power, high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal FeGe, probing the magnetism of sputtered B20 FeGe is more challenging because the sub-micron crystal grains add confounding contrast. We address the challenge of disentangling magnetic and grain contrast by applying 4-dimensional Lorentz scanning transmission electron microscopy using an electron microscope pixel array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic range detector becomes extremely important. Despite the small grain size in sputtered B20 FeGe on Si, using this approach we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.

cond-mat.mtrl-sci↗

arXiv2022

Nonvolatile Electric-Field Control of Inversion Symmetry

In condensed-matter systems, competition between ground states at phase boundaries can lead to significant changes in material properties under external stimuli, particularly when these ground states have different crystal symmetries. A key scientific and technological challenge is to stabilize and control coexistence of symmetry-distinct phases with external stimuli. Using BiFeO3 (BFO) layers confined between layers of the dielectric TbScO3 as a model system, we stabilize the mixed-phase coexistence of centrosymmetric and non-centrosymmetric BFO phases with antipolar, insulating and polar, semiconducting behavior, respectively at room temperature. Application of in-plane electric (polar) fields can both remove and introduce centrosymmetry from the system resulting in reversible, nonvolatile interconversion between the two phases. This interconversion between the centrosymmetric insulating and non-centrosymmetric semiconducting phases coincides with simultaneous changes in the non-linear optical response of over three orders of magnitude, a change in resistivity of over five orders of magnitude, and a change in the polar order. Our work establishes a materials platform allowing for novel cross-functional devices which take advantage of changes in optical, electrical, and ferroic responses.

cond-mat.mtrl-sci↗

arXiv2021

Very-High Dynamic Range, 10,000 frames/second Pixel Array Detector for Electron Microscopy

Precision and accuracy of quantitative scanning transmission electron microscopy (STEM) methods such as ptychography, and the mapping of electric, magnetic and strain fields depend on the dose. Reasonable acquisition time requires high beam current and the ability to quantitatively detect both large and minute changes in signal. A new hybrid pixel array detector (PAD), the second-generation Electron Microscope Pixel Array Detector (EMPAD-G2), addresses this challenge by advancing the technology of a previous generation PAD, the EMPAD. The EMPAD-G2 images continuously at a frame-rates up to 10 kHz with a dynamic range that spans from low-noise detection of single electrons to electron beam currents exceeding 180 pA per pixel, even at electron energies of 300 keV. The EMPAD-G2 enables rapid collection of high-quality STEM data that simultaneously contain full diffraction information from unsaturated bright field disks to usable Kikuchi bands and higher-order Laue zones. Test results from 80 to 300 keV are presented, as are first experimental results demonstrating ptychographic reconstructions, strain and polarization maps. We introduce a new information metric, the Maximum Usable Imaging Speed (MUIS), to identify when a detector becomes electron-starved, saturated or its pixel count is mismatched with the beam current.

physics.ins-det↗

arXiv2021

Data-Driven Electron Microscopy: Electron Diffraction Imaging of Materials Structural Properties

Transmission electron diffraction is a powerful and versatile structural probe for the characterization of a broad range of materials, from nanocrystalline thin films to single crystals. With recent developments in fast electron detectors and efficient computer algorithms, it now becomes possible to collect unprecedently large datasets of diffraction patterns (DPs) and process DPs to extract crystallographic information to form images or tomograms based on crystal structural properties, giving rise to data-driven electron microscopy. Critical to this kind of imaging is the type of crystallographic information being collected, which can be achieved with a judicious choice of electron diffraction techniques, and the efficiency and accuracy of DP processing, which requires the development of new algorithms. Here, we review recent progress made in data collection, new algorithms, and automated electron DP analysis. These progresses will be highlighted using application examples in materials research. Future opportunities based on smart sampling and machine learning are also discussed.

cond-mat.mtrl-sci↗

arXiv2021

A room temperature polar ferromagnetic metal

The advent of long-range magnetic order in non-centrosymmetric compounds has stimulated interest in the possibility of exotic spin transport phenomena and topologically protected spin textures for applications in next-generation spintronics. This work reports a novel wurtzite-structure polar magnetic metal, identified as AA'-stacked (Fe0.5Co0.5)5-xGeTe2, which exhibits a Neel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temperature. Atomic resolution imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies and ushers in a promising new framework for spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Ferroelectricity in Polar ScAlN/GaN Epitaxial Semiconductor Heterostructures

Room temperature ferroelectricity is observed in lattice-matched ~18% ScAlN/GaN heterostructures grown by molecular beam epitaxy on single-crystal GaN substrates. The epitaxial films have smooth surface morphologies and high crystallinity. Pulsed current-voltage measurements confirm stable and repeatable polarization switching in such ferroelectric/semiconductor structures at several measurement conditions, and in multiple samples. The measured coercive field values are Ec~0.7 MV/cm at room temperature, with remnant polarization Pr~10 μC/cm2 for ~100 nm thick ScAlN layers. These values are substantially lower than comparable ScAlN control layers deposited by sputtering. Importantly, the coercive field of MBE ScAlN is smaller than the critical breakdown field of GaN, offering the potential for low voltage ferroelectric switching. The low coercive field ferroelectricity of ScAlN on GaN heralds the possibility of new forms of electronic and photonic devices with epitaxially integrated ferroelectric/semiconductor heterostructures that take advantage of the GaN electronic and photonic semiconductor platform, where the underlying semiconductors themselves exhibit spontaneous and piezoelectric polarization.

cond-mat.mtrl-sci↗

arXiv2021

Emergence of a noncollinear magnetic state in twisted bilayer CrI3

The emergence of two-dimensional (2D) magnetic crystals and moiré engineering has opened the door for devising new magnetic ground states via competing interactions in moiré superlattices. Although a suite of interesting phenomena, including multi-flavor magnetic states, noncollinear magnetic states, moiré magnon bands and magnon networks, has been predicted, nontrivial magnetic ground states in twisted bilayer magnetic crystals have yet to be realized. Here, by utilizing the stacking-dependent interlayer exchange interactions in CrI3, we demonstrate in small-twist-angle bilayer CrI3 a noncollinear magnetic ground state. It consists of both antiferromagnetic (AF) and ferromagnetic (FM) domains and is a result of the competing interlayer AF coupling in the monoclinic stacking regions of the moiré superlattice and the energy cost for forming AF-FM domain walls. Above the critical twist angle of ~ 3°, the noncollinear state transitions abruptly to a collinear FM ground state. We further show that the noncollinear magnetic state can be controlled by gating through the doping-dependent interlayer AF interaction. Our results demonstrate the possibility of engineering moiré magnetism in twisted bilayer magnetic crystals, as well as gate-voltage-controllable high-density magnetic memory storage.

cond-mat.mtrl-sci↗

arXiv2021

Cepstral Scanning Transmission Electron Microscopy Imaging of Severe Lattice Distortions

The development of four-dimensional (4D) scanning transmission electron microscopy (STEM) using fast detectors has opened-up new avenues for addressing some of long-standing challenges in electron imaging. One of these challenges is how to image severely distorted crystal lattices, such as at a dislocation core. Here we introduce a new 4D-STEM technique, called Cepstral STEM, for imaging disordered crystals using electron diffuse scattering. Local fluctuations of diffuse scattering are captured by scanning electron nanodiffraction (SEND) using a coherent probe. The harmonic signals in electron diffuse scattering are detected through Cepstral analysis and used for imaging. By integrating Cepstral analysis with 4D-STEM, we demonstrate that information about the distortive part of electron scattering potential can be separated and imaged at nm spatial resolution. We apply our technique to the analysis of a dislocation core in SiGe and lattice distortions in high entropy alloy.

cond-mat.mtrl-sci↗

arXiv2021

Electron ptychography achieves atomic-resolution limits set by lattice vibrations

Transmission electron microscopes use electrons with wavelengths of a few picometers, potentially capable of imaging individual atoms in solids at a resolution ultimately set by the intrinsic size of an atom. Unfortunately, due to imperfections in the imaging lenses and multiple scattering of electrons in the sample, the image resolution reached is 3 to 10 times worse. Here, by inversely solving the multiple scattering problem and overcoming the aberrations of the electron probe using electron ptychography to recover a linear phase response in thick samples, we demonstrate an instrumental blurring of under 20 picometers. The widths of atomic columns in the measured electrostatic potential are now no longer limited by the imaging system, but instead by the thermal fluctuations of the atoms. We also demonstrate that electron ptychography can potentially reach a sub-nanometer depth resolution and locate embedded atomic dopants in all three dimensions with only a single projection measurement.

cond-mat.mtrl-sci↗

arXiv2020

The Exit-Wave Power-Cepstrum Transform for Scanning Nanobeam Electron Diffraction: Robust Strain Mapping at Subnanometer Resolution and Subpicometer Precision

Scanning nanobeam electron diffraction (NBED) with fast pixelated detectors is a valuable technique for rapid, spatially resolved mapping of lattice structure over a wide range of length scales. However, intensity variations caused by dynamical diffraction and sample mistilts can hinder the measurement of diffracted disk centers as necessary for quantification. Robust data processing techniques are needed to provide accurate and precise measurements for complex samples and non-ideal conditions. Here we present an approach to address these challenges using a transform, called the exit wave power cepstrum (EWPC), inspired by cepstral analysis in audio signal processing. The EWPC transforms NBED patterns into real-space patterns with sharp peaks corresponding to inter-atomic spacings. We describe a simple analytical model for interpretation of these patterns that cleanly decouples lattice information from the intensity variations in NBED patterns caused by tilt and thickness. By tracking the inter-atomic spacing peaks in EWPC patterns, strain mapping is demonstrated for two practical applications: mapping of ferroelectric domains in epitaxially strained PbTiO3 films and mapping of strain profiles in arbitrarily oriented core-shell Pt-Co nanoparticle fuel-cell catalysts. The EWPC transform enables lattice structure measurement at sub-pm precision and sub-nm resolution that is robust to small sample mistilts and random orientations.

cond-mat.mtrl-sci↗

arXiv2019

The Paracrystalline Nature of Lattice Distortion in a High Entropy Alloy

Severe lattice distortion is suggested for high entropy alloys (HEAs), however, evidence for such effect so far is lacking, and the nature of distortion is yet to be understood. Here, we reveal the distortion in an fcc HEA, Al0.1CrFeCoNi, by direct imaging using electron nanodiffraction. Information about crystal symmetry, lattice strain and atomic distortion are data-mined and mapped from many (~10^4) diffraction patterns. Application to the HEA reveals two embodiments of distortion, nm-sized mosaic blocks of paracrystals and strained nano-clusters. Their interaction gives rise to fractal strain field across nanoscopic to mesoscopic scales. As lattice distortion impedes dislocation motion and contributes to strengthening, results here thus provide critical insights about the complex nature of distortion in a HEA.

cond-mat.mtrl-sci↗

arXiv2016

Lattice rotation vortex at the monoclinic ferroelectric domain boundary in relaxor ferroelectric crystal

We present the evidence of lattice rotation vortices having an average radius of ~7 nm at the monoclinic ferroelectric domain boundary of (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-xPT, x=0.08). Maps of crystal orientations, domain configurations, symmetry breaking are obtained using scanning convergent beam electron diffraction (SCBED). Such measurements suggest the merging of 2D and 1D topological defects, with implications for domain-switching mechanisms in relaxor ferroelectric crystals, and the possibility of a new form of nanoscale ferroelectric devices.

cond-mat.mes-hall↗