arXiv subjects

Mingqiang Li

Publications and source records attributed to Mingqiang Li.

27 records · Page 2Linked to original sources

arXiv2019

Atomic-environment-dependent thickness of ferroelastic domain walls

Domain walls are of increasing interest in ferroelectrics because of their unique properties and potential applications in future nanoelectronics. However, the thickness of ferroelastic domain walls remains elusive due to the challenges in experimental characterization. Here, we determine the atomic structure of ferroelastic domain walls and precisely measure the polarization and domain wall thickness at picometer scale using annular bright field imaging in an aberration-corrected scanning transmission electron microscope. We find that the domain wall thickness in PbZr0.2Ti0.8O3 and PbTiO3 thin films is typically about one-unit cell, across which the oxygen octahedron distortion behavior is in excellent agreement with first principles calculations. Remarkably, wider domain walls about two-unit cells in thickness are also observed for those domains walls are coupled with dislocations and underwent a compressive strain. These results suggest that the thickness of ferroelastic domain walls highly depends on their atomic environments. This study can help us to understand the past debatable experimental results and provide further insights into control of domain walls via strain engineering for their possible applications in nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2019

Atomic Imaging of Mechanically Induced Topological Transition of Ferroelectric Vortices

Ferroelectric vortices formed through complex lattice-charge interactions have great potential in applications for future nanoelectronics such as memories. For practical applications, it is crucial to manipulate these topological states under external stimuli. Here, we apply mechanical loads to locally manipulate the vortices in a PbTiO3-SrTiO3 superlattice via atomically resolved in situ scanning transmission electron microscopy. The vortices undergo a transition to the a-domain with in-plane polarization under external compressive stress and spontaneously recover after removal of the stress. We reveal the detailed transition process at the atomic scale and reproduce this numerically using phase-field simulations. These findings provide new pathways to control the exotic topological ferroelectric structures for future nanoelectronics and also valuable insights into understanding of lattice-charge interactions at nanoscale.

cond-mat.mtrl-sci↗

arXiv2018

Atomic Origin of Ti Deficient Dislocation in SrTiO3 Bicrystal and Their Electronic Structures

Dislocations in perovskite oxides have important impacts on their physical and chemical properties, which are determined by their unique atomic environments. In the present study, the structure of dislocations in a 10° low-angle grain boundary of SrTiO3 (STO) is characterized by spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) and spectroscopy. In contrast to previous studies, the deficiency instead of enrichment of titanium (Ti) is observed at the dislocation cores mainly due to the Sr substitution and under occupancy of Ti. The presence of oxygen vacancies and partially reduced Ti are also detected at the Ti deficient dislocations cores. These findings indicate the atomic structure of dislocations can be very different even they have the same Burges vectors. Controllable elemental segregation in the dislocations and grain boundaries via bicrystal engineering should be very useful for design of devices with novel functions.

cond-mat.mtrl-sci↗

arXiv2017

Anomalous Hall effect and magnetic orderings in nanothick V$_5$S$_8$

The rise of graphene marks the advent of two-dimensional atomic crystals, which have exhibited a cornucopia of intriguing properties, such as the integer and fractional quantum Hall effects, valley Hall effect, charge density waves and superconductivity, to name a few. Yet, magnetism, a property of extreme importance in both science and technology, remains elusive. There is a paramount need for magnetic two-dimensional crystals. With the availability of many magnetic materials consisting of van der Waals coupled two-dimensional layers, it thus boils down to the question of how the magnetic order will evolve with reducing thickness. Here we investigate the effect of thickness on the magnetic ordering in nanothick V$_5$S$_8$. We uncover an anomalous Hall effect, by which the magnetic ordering in V$_5$S$_8$ down to 3.2 nm is probed. With decreasing thickness, a breakdown of antiferromagnetism is evident, followed by a spin-glass-like state. For thinnest samples, a weak ferromagnetic ordering emerges. The results not only show an interesting effect of reducing thickness on the magnetic ordering in a potential candidate for magnetic two-dimensional crystals, but demonstrate the anomalous Hall effect as a useful characterization tool for magnetic orderings in two-dimensional systems.

cond-mat.mes-hall↗

arXiv2017

Ultrafast Epitaxial Growth of Metre-Sized Single-Crystal Graphene on Industrial Cu Foil

A foundation of the modern technology that uses single-crystal silicon has been the growth of high-quality single-crystal Si ingots with diameters up to 12 inches or larger. For many applications of graphene, large-area high-quality (ideally of single-crystal) material will be enabling. Since the first growth on copper foil a decade ago, inch-sized single-crystal graphene has been achieved. We present here the growth, in 20 minutes, of a graphene film of 5 x 50 cm2 dimension with > 99% ultra-highly oriented grains. This growth was achieved by: (i) synthesis of sub-metre-sized single-crystal Cu(111) foil as substrate; (ii) epitaxial growth of graphene islands on the Cu(111) surface; (iii) seamless merging of such graphene islands into a graphene film with high single crystallinity and (iv) the ultrafast growth of graphene film. These achievements were realized by a temperature-driven annealing technique to produce single-crystal Cu(111) from industrial polycrystalline Cu foil and the marvellous effects of a continuous oxygen supply from an adjacent oxide. The as-synthesized graphene film, with very few misoriented grains (if any), has a mobility up to ~ 23,000 cm2V-1s-1 at 4 K and room temperature sheet resistance of ~ 230 ohm/square. It is very likely that this approach can be scaled up to achieve exceptionally large and high-quality graphene films with single crystallinity, and thus realize various industrial-level applications at a low cost.

cond-mat.mtrl-sci↗

arXiv2015

CDStore: Toward Reliable, Secure, and Cost-Efficient Cloud Storage via Convergent Dispersal

We present CDStore, which disperses users' backup data across multiple clouds and provides a unified multi-cloud storage solution with reliability, security, and cost-efficiency guarantees. CDStore builds on an augmented secret sharing scheme called convergent dispersal, which supports deduplication by using deterministic content-derived hashes as inputs to secret sharing. We present the design of CDStore, and in particular, describe how it combines convergent dispersal with two-stage deduplication to achieve both bandwidth and storage savings and be robust against side-channel attacks. We evaluate the performance of our CDStore prototype using real-world workloads on LAN and commercial cloud testbeds. Our cost analysis also demonstrates that CDStore achieves a monetary cost saving of 70% over a baseline cloud storage solution using state-of-the-art secret sharing.

cs.CR↗

arXiv2014

STAIR Codes: A General Family of Erasure Codes for Tolerating Device and Sector Failures

Practical storage systems often adopt erasure codes to tolerate device failures and sector failures, both of which are prevalent in the field. However, traditional erasure codes employ device-level redundancy to protect against sector failures, and hence incur significant space overhead. Recent sector-disk (SD) codes are available only for limited configurations. By making a relaxed but practical assumption, we construct a general family of erasure codes called \emph{STAIR codes}, which efficiently and provably tolerate both device and sector failures without any restriction on the size of a storage array and the numbers of tolerable device failures and sector failures. We propose the \emph{upstairs encoding} and \emph{downstairs encoding} methods, which provide complementary performance advantages for different configurations. We conduct extensive experiments on STAIR codes in terms of space saving, encoding/decoding speed, and update cost. We demonstrate that STAIR codes not only improve space efficiency over traditional erasure codes, but also provide better computational efficiency than SD codes based on our special code construction. Finally, we present analytical models that characterize the reliability of STAIR codes, and show that the support of a wider range of configurations by STAIR codes is critical for tolerating sector failure bursts discovered in the field.

cs.IT↗

arXiv2013

On the Confidentiality of Information Dispersal Algorithms and Their Erasure Codes

\emph{Information Dispersal Algorithms (IDAs)} have been widely applied to reliable and secure storage and transmission of data files in distributed systems. An IDA is a method that encodes a file $F$ of size $L=|F|$ into $n$ unrecognizable pieces $F_1$, $F_2$, ..., $F_n$, each of size $L/m$ ($m<n$), so that the original file $F$ can be reconstructed from any $m$ pieces. The core of an IDA is the adopted non-systematic $m$-of-$n$ erasure code. This paper makes a systematic study on the \emph{confidentiality} of an IDA and its connection with the adopted erasure code. Two levels of confidentiality are defined: \emph{weak confidentiality} (in the case where some parts of the original file $F$ can be reconstructed explicitly from fewer than $m$ pieces) and \emph{strong confidentiality} (in the case where nothing of the original file $F$ can be reconstructed explicitly from fewer than $m$ pieces). For an IDA that adopts an arbitrary non-systematic erasure code, its confidentiality may fall into weak confidentiality. To achieve strong confidentiality, this paper explores a sufficient and feasible condition on the adopted erasure code. Then, this paper shows that Rabin's IDA has strong confidentiality. At the same time, this paper presents an effective way to construct an IDA with strong confidentiality from an arbitrary $m$-of-$(m+n)$ erasure code. Then, as an example, this paper constructs an IDA with strong confidentiality from a Reed-Solomon code, the computation complexity of which is comparable to or sometimes even lower than that of Rabin's IDA.

cs.IT↗

arXiv2012

C-Codes: Cyclic Lowest-Density MDS Array Codes Constructed Using Starters for RAID 6

The distance-3 cyclic lowest-density MDS array code (called the C-Code) is a good candidate for RAID 6 because of its optimal storage efficiency, optimal update complexity, optimal length, and cyclic symmetry. In this paper, the underlying connections between C-Codes (or quasi-C-Codes) and starters in group theory are revealed. It is shown that each C-Code (or quasi-C-Code) of length $2n$ can be constructed using an even starter (or even multi-starter) in $(Z_{2n},+)$. It is also shown that each C-Code (or quasi-C-Code) has a twin C-Code (or quasi-C-Code). Then, four infinite families (three of which are new) of C-Codes of length $p-1$ are constructed, where $p$ is a prime. Besides the family of length $p-1$, C-Codes for some sporadic even lengths are also presented. Even so, there are still some even lengths (such as 8) for which C-Codes do not exist. To cover this limitation, two infinite families (one of which is new) of quasi-C-Codes of length $2(p-1)$ are constructed for these even lengths.

cs.IT↗