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1. Timing Selector: Using Transient Switching Dynamics to Solve the Sneak Path Issue of Crossbar Arrays

   https://doi.org/10.1002/smsc.202100072  

   Rao, Mingyi ; Song, Wenhao ; Kiani, Fatemeh ; Asapu, Shiva ; Zhuo, Ye ; Midya, Rivu ; Upadhyay, Navnidhi ; Wu, Qing ; Barnell, Mark ; Lin, Peng ; et al ( October 2021 , Small Science)

   Sneak path current is a fundamental issue and a major roadblock to the wide application of memristor crossbar arrays. Traditional selectors such as transistors compromise the 2D scalability and 3D stack‐ability of the array, while emerging selectors with highly nonlinear current–voltage relations contradict the requirement of a linear current–voltage relation for efficient multiplication by directly using Ohm's law. Herein, the concept of a timing selector is proposed and demonstrated, which addresses the sneak path issue with a voltage‐dependent delay time of its transient switching behavior, while preserving a linear current–voltage relationship for computation. Crossbar arrays with silver‐based diffusive memristors as the timing selectors are built and the operation principle and operational windows are experimentally demonstrated. The timing selector enables large memristor crossbar arrays that can be used to solve large‐dimension real‐world problems in machine intelligence and neuromorphic computing.

    

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2. Ultra-fast robust compressive sensing based on memristor crossbars

   https://doi.org/10.1109/ICASSP.2017.7952333  

   Liu, Sijia ; Ren, Ao ; Wang, Yanzhi ; Varshney, Pramod K. ( March 2017 , 2017 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP))

   In this paper, we propose a new approach for robust compressive sensing (CS) using memristor crossbars that are constructed by recently invented memristor devices. The exciting features of a memristor crossbar, such as high density, low power and great scalability, make it a promising candidate to perform large-scale matrix operations. To apply memristor crossbars to solve a robust CS problem, the alternating directions method of multipliers (ADMM) is employed to split the original problem into subproblems that involve the solution of systems of linear equations. A system of linear equations can then be solved using memristor crossbars with astonishing O(1) time complexity. We also study the impact of hardware variations on the memristor crossbar based CS solver from both theoretical and practical points of view. The resulting overall complexity is given by O(n), which achieves O(n2.5) speed-up compared to the state-of-the-art software approach. Numerical results are provided to illustrate the effectiveness of the proposed CS solver. 

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3. Algorithm-hardware co-optimization of the memristor-based framework for solving SOCP and homogeneous QCQP problems

   https://doi.org/10.1109/ASPDAC.2017.7858420  

   Ren, Ao ; Liu, Sijia ; Cai, Ruizhe ; Wen, Wujie ; Varshney, Pramod K. ; Wang, Yanzhi ( January 2017 , 2017 22nd Asia and South Pacific Design Automation Conference (ASP-DAC))

   A memristor crossbar, which is constructed with memristor devices, has the unique ability to change and memorize the state of each of its memristor elements. It also has other highly desirable features such as high density, low power operation and excellent scalability. Hence the memristor crossbar technology can potentially be utilized for developing low-complexity and high-scalability solution frameworks for solving a large class of convex optimization problems, which involve extensive matrix operations and have critical applications in multiple disciplines. This paper, as the first attempt towards this direction, proposes a novel memristor crossbar-based framework for solving two important convex optimization problems, i.e., second-order cone programming (SOCP) and homogeneous quadratically constrained quadratic programming (QCQP) problems. In this paper, the alternating direction method of multipliers (ADMM) is adopted. It splits the SOCP and homogeneous QCQP problems into sub-problems that involve the solution of linear systems, which could be effectively solved using the memristor crossbar in O(1) time complexity. The proposed algorithm is an iterative procedure that iterates a constant number of times. Therefore, algorithms to solve SOCP and homogeneous QCQP problems have pseudo-O(N) complexity, which is a significant reduction compared to the state-of-the-art software solvers (O(N3.5)-O(N4)). 

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4. Engineering large perpendicular magnetic anisotropy in amorphous ferrimagnetic gadolinium cobalt alloys

   https://doi.org/10.1039/D3TC00332A  

   Srinivasan, Karthik ; Chen, Yulan ; Cestarollo, Ludovico ; Dare, Darrah K. ; Wright, John G. ; El-Ghazaly, Amal ( April 2023 , Journal of Materials Chemistry C)

   Amorphous magnetic alloys with large perpendicular magnetic anisotropy (PMA) have emerged as a suitable material choice for spintronic memory and high-frequency non-reciprocal devices on-chip. Unlike ferromagnets, ferrimagnets offer faster switching dynamics, lower net saturation magnetization, minimal stray field and a lower net angular momentum. Ferrimagnetic thin films of Gd x Co 1− x sputter deposited as heterostructures of Ta/Pt/Gd x Co 1− x (t)/Pt on Si/SiO 2 have bulk-like PMA for thicknesses of 5–12 nm and room-temperature magnetic compensation for x = 28–32%. Preferential oxygenation of GdCo has been found to increase the effective anisotropy energy density by an order of magnitude and produce near-ideal remanence ratios. X-ray photoelectron spectroscopy accurately quantifies the metal-oxidation ratio, which shows that an oxygen-rich and Co-deficient stoichiometry (Gd 21 Co 28 O 51 ) likely weakens the ferromagnetic exchange interaction between Co–Co and contributes additional antiferromagnetic exchange through superexchange-like interactions between Gd and Co via O, resulting in a stronger out-of-plane magnetization. Even greater PMA and giant-anisotropy field of 11 kOe are achieved in super-lattices of the Gd 21 Co 28 O 51 heterostructure. The combination of ferrimagnetic ordering in amorphous Gd x Co 1− x and its affordance of pathways for engineering large PMA will enable the design of integrated high-frequency devices beyond 30 GHz and ultrafast energy efficient memory devices with switching speeds down to tens of picoseconds. 

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5. Hybrid molecular beam epitaxy of germanium-based oxides

   https://doi.org/10.1038/s43246-022-00290-y  

   Liu, Fengdeng ; Truttmann, Tristan K. ; Lee, Dooyong ; Matthews, Bethany E. ; Laraib, Iflah ; Janotti, Anderson ; Spurgeon, Steven R. ; Chambers, Scott A. ; Jalan, Bharat ( December 2022 , Communications Materials)

   Abstract Germanium-based oxides such as rutile GeO 2 are garnering attention owing to their wide band gaps and the prospects of ambipolar doping for application in high-power devices. Here, we present the use of germanium tetraisopropoxide (GTIP), a metal-organic chemical precursor, as a source of germanium for the demonstration of hybrid molecular beam epitaxy for germanium-containing compounds. We use Sn 1- x Ge x O 2 and SrSn 1- x Ge x O 3 as model systems to demonstrate our synthesis method. A combination of high-resolution X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy confirms the successful growth of epitaxial rutile Sn 1- x Ge x O 2 on TiO 2 (001) substrates up to x  = 0.54 and coherent perovskite SrSn 1- x Ge x O 3 on GdScO 3 (110) substrates up to x  = 0.16. Characterization and first-principles calculations corroborate that germanium occupies the tin site, as opposed to the strontium site. These findings confirm the viability of the GTIP precursor for the growth of germanium-containing oxides by hybrid molecular beam epitaxy, thus providing a promising route to high-quality perovskite germanate films. 

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