arXiv subjects

Jian-Min Zuo

Publications and source records attributed to Jian-Min Zuo.

32 records · Page 2Linked to original sources

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↗

arXiv2020

Molecular beam epitaxy of the magnetic kagome metal FeSn on LaAlO3 (111)

Materials with a layered Kagome lattice are expected to give rise to novel physics arising from band structures with topological properties, spin liquid behavior and the formation of skyrmions. Until now, most work on Kagome materials has been performed on bulk samples due to difficulties in thin film synthesis. Here, by using molecular beam epitaxy, layered Kagome-structured FeSn films are synthesized on (111) oriented LaAlO3 substrate. Both in-situ and ex-situ characterizations indicate these films are highly crystalline and c-axis oriented, with atomically smooth surfaces. However, the films grow as disconnected islands, with lateral dimensions on the micron scale. By patterning Pt electrodes using a focused electron beam, longitudinal and transverse resistance of single islands have been measured in magnetic fields. Our work opens a pathway for exploring mesoscale transport properties in thin films of Kagome materials and related devices.

cond-mat.mtrl-sci↗

arXiv2020

Doped NiO: the Mottness of a charge transfer insulator

The evolution of the electronic structures of strongly correlated insulators with doping has long been a central fundamental question in condensed matter physics; it is also of great practical relevance for applications. We have studied the evolution of NiO under hole {\em and} electron doping using high-quality thin film and a wide range of experimental and theoretical methods. The evolution is in both cases very smooth with dopant concentration. The band gap is asymmetric under electron and hole doping, consistent with a charge-transfer insulator picture, and is reduced faster under hole than electron doping. For both electron and hole doping, occupied states are introduced at the top of the valence band. The formation of deep donor levels under electron doping and the inability to pin otherwise empty states near the conduction band edge is indicative that local electron addition and removal energies are dominated by a Mott-like Hubbard $U$-interaction even though the global bandgap is predominantly a charge-transfer type gap.

cond-mat.str-el↗

arXiv2020

Discovery of two-dimensional anisotropic superconductivity at KTaO$_3$ (111) interfaces

The unique electronic structure found at interfaces between materials can allow unconventional quantum states to emerge. Here we observe superconductivity in electron gases formed at interfaces between (111) oriented KTaO$_3$ and insulating overlayers of either EuO or LaAlO$_3$. The superconducting transition temperature, approaching 2.2 K, is about one order of magnitude higher than that of the LaAlO$_3$/SrTiO$_3$ system. Strikingly, similar electron gases at (001) KTaO$_3$ interfaces remain normal down to 25 mK. The critical field and current-voltage measurements indicate that the superconductivity is two dimensional. Higher mobility EuO/KTaO$_3$ (111) samples show a large in-plane anisotropy in transport properties at low temperatures prior to onset of superconductivity, suggesting the emergence of a stripe like phase where the superconductivity is nearly homogeneous in one direction, but strongly modulated in the other.

cond-mat.supr-con↗

arXiv2019

Point group symmetry of cadmium arsenide thin films determined by convergent beam electron diffraction

Cadmium arsenide (Cd3As2) is one of the first materials to be discovered to belong to the class of three-dimensional topological semimetals. Reported room temperature crystal structures of Cd3As2 reported differ subtly in the way the Cd vacancies are arranged within its antifluorite-derived structure, which determines if an inversion center is present and if Cd3As2 is a Dirac or Weyl semimetal. Here, we apply convergent beam electron diffraction (CBED) to determine the point group of Cd3As2 thin films grown by molecular beam epitaxy. Using CBED patterns from multiple zone axes, high-angle annular dark-field images acquired in scanning transmission electron microscopy, and Bloch wave simulations, we show that Cd3As2 belongs to the tetragonal 4/mmm point group, which is centrosymmetric. The results show that CBED can distinguish very subtle differences in the crystal structure of a topological semimetal, a capability that will be useful for designing materials and thin film heterostructures with topological states that depend on the presence of certain crystal symmetries.

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↗

arXiv2016

Peak separation method for sub-lattice strain analysis at atomic resolution: Application to InAs/GaSb superlattice

We report on a direct measurement of cation and anion sub-lattice strain in an InAs/GaSb type-II strained layer superlattice (T2SLs) using atomic resolution Z-contrast imaging and advanced image processing. Atomic column positions in InAs and GaSb are determined by separating the cation and anion peak intensities. Analysis of the InAs/GaSb T2SLs reveals the compressive strain in the nominal GaSb layer and tensile strain at interfaces between constituent layers, which indicate In incorporation into the nominal GaSb layer and the formation of GaAs like interfaces, respectively. The results are compared with the model-dependent X-ray diffraction measurements in terms of interfacial chemical intermixing and strain. Together, these techniques provide a robust measurement of atomic-scale strain which is vital to determine T2SL properties.

cond-mat.mtrl-sci↗

arXiv2016

Rare-Region Onset of Superconductivity in Granular Systems

The critical behavior of disordered systems-from metals (1) to magnets (2) and superconductors (3)-is often dominated by the behavior of rare regions of a correlated phase, which control the inception and dynamics of the phase transition. Yet, despite significant theoretical (3,4,5) and experimental (6,7,8,9) interest, there has been little direct evidence of the presence of these regions, or of their role in initiating transitions. Here, we provide direct evidence for rare-region effects at the onset of superconductivity in granular superconducting islands. By considering the strong diameter-dependence of the transition, as well as observations of large fluctuations in the transition temperature as island diameters decrease, we are able to show that superconducting order first appears in unusually large grains- i.e. rare regions- within each island and, due to proximity coupling, spreads to other grains. This work thus provides a quantitative, local understanding of the onset of correlated order in strongly disordered systems.

cond-mat.supr-con↗

arXiv2014

Surface atomic diffusion processes observed at milliseconds time resolution using environmental TEM

Significant progress has been made in spatial resolution using environmental transmission electron microscopes (ETEM), which now enables atomic resolution visualization of structural transformation under variable temperature and gas environments close to materials real operational conditions. Structural transformations are observed by recording images or diffraction patterns at various time intervals using a video camera or by taking snap shots using electron pulses. While time resolution at 15 ns has been reported using pulsed electron beams, the time interval that can be recorded by this technique is currently very limited. For longer recording, however, time resolution inside ETEM has been limited by electron cameras to ~1/30 seconds for a long time. Using the recently developed direct electron detection technology, we have significantly improved the time resolution of ETEM to 2.5 ms (milliseconds) for full frame or 0.625 ms for 0.25 frames.

cond-mat.mtrl-sci↗

arXiv2014

Interfacial microscopic mechanism of free energy minimization in Omega precipitate formation

Precipitate strengthening of light metals underpins a large segment of industry.Yet, quantitative understanding of physics involved in precipitate formation is often lacking, especially, about interfacial contribution to the energetics of precipitate formation.Here, we report an intricate strain accommodation and free energy minimization mechanism in the formation of Omega precipitates (Al2Cu)in the Al_Cu_Mg_Ag alloy. We show that the affinity between Ag and Mg at the interface provides the driving force for lowering the heat of formation, while substitution between Mg, Al and Cu of different atomic radii at interfacial atomic sites alters interfacial thickness and adjust precipitate misfit strain. The results here highlight the importance of interfacial structure in precipitate formation, and the potential of combining the power of atomic resolution imaging with first-principles theory for unraveling the mystery of physics at nanoscale interfaces.

cond-mat.mtrl-sci↗

arXiv2010

Electrical transport in small bundles of single-walled carbon nanotubes: intertube interaction and effects of tube deformation

We report a combined electronic transport and structural characterization study of small carbon nanotube bundles in field-effect transistors (FET). The atomic structures of the bundles are determined by electron diffraction using an observation window built in the FET. The single-walled nanotube bundles exhibit electrical transport characteristics sensitively dependent on the structure of individual tubes, their arrangements in the bundle, deformation due to intertube interaction, and the orientation with respect to the gate electric field. Our ab-initio simulation shows that tube deformation in the bundle induces a bandgap opening in a metallic tube. These results show the importance of intertube interaction in electrical transport of bundled nanotubes.

cond-mat.mes-hall↗

arXiv2010

Comment on "Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography"

In a recent letter (F. Carbone, P. Baum, P. Rudolf, et al., Physical Review Letters 100, 2008), graphite is reported to undergo a c-axis contraction on the time scale of few picoseconds (ps) after ultrafast pulsed laser excitation. The velocity of lattice contraction depended on the laser fluence. Furthermore, the lattice contraction is followed by large, non-thermal, lattice expansion of several picometers (pm) after few hundreds of ps. These results were interpreted based on the position of the (0014) diffraction spot in the glancing angle diffraction geometry off the graphite surface using ultrafast electron diffraction (UED). The lattice contraction and expansion corresponded to upward and downward movements of the diffraction spot respectively. Here, we show that ultrafast pulsed laser excitation of graphite produces large transient electric fields (TEFs). The TEFs deflect the electron beam in a manner similar to these reported and the neglect of TEFs likely led to erroneous conclusions.

cond-mat.mtrl-sci↗

arXiv2007

Electronic reconstruction at SrMnO3-LaMnO3 superlattice interfaces

We use resonant soft x-ray scattering to study electronic reconstruction at the interface between the Mott insulator LaMnO3 and the "band" insulator SrMnO3. Superlattices of these two insulators were shown previously to have both ferromagnetism and metallic tendencies [Koida et al., Phys. Rev. B 66, 144418 (2002)]. By studying a judiciously chosen superlattice reflection we show that the interface density of states exhibits a pronounced peak at the Fermi level, similar to that predicted by Okamoto et al. [Phys. Rev. B 70, 241104(R) (2004)]. The intensity of this peak correlates with the conductivity and magnetization, suggesting it is the driver of metallic behavior. Our study demonstrates a general strategy for using RSXS to probe the electronic properties of heterostructure interfaces.

cond-mat.str-el↗