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1. Single skyrmion true random number generator using local dynamics and interaction between skyrmions

   https://doi.org/10.1038/s41467-022-28334-4  

   Wang, Kang; Zhang, Yiou; Bheemarasetty, Vineetha; Zhou, Shiyu; Ying, See-Chen; Xiao, Gang (February 2022, Nature Communications)

   Abstract Magnetic skyrmions are of great interest to both fundamental research and applications in post-von-Neumann computing devices. The successful implementation of skyrmionic devices requires functionalities of skyrmions with effective controls. Here we show that the local dynamics of skyrmions, in contrast to the global dynamics of a skyrmion as a whole, can be introduced to provide effective functionalities for versatile computing. A single skyrmion interacting with local pinning centres under thermal effects can fluctuate in time and switch between a small-skyrmion and a large-skyrmion state, thereby serving as a robust true random number generator for probabilistic computing. Moreover, neighbouring skyrmions exhibit an anti-correlated coupling in their fluctuation dynamics. Both the switching probability and the dynamic coupling strength can be tuned by modifying the applied magnetic field and spin current. Our results could lead to progress in developing magnetic skyrmionic devices with high tunability and efficient controls. 

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2. Single-electron transistors featuring silicon nitride tunnel barriers prepared by atomic layer deposition

   https://doi.org/10.1109/ULIS.2016.7440045  

   Karbasian, Golnaz; Orlov, Alexei O.; Mukasyan, Alexander S.; Snider, Gregory L. (January 2016, Single-electron transistors featuring silicon nitride tunnel barriers prepared by atomic layer deposition)

   Single electron transistors (SET) featuring metal (Ni) electrodes and silicon nitride dielectric barriers prepared by atomic layer deposition are fabricated and tested. Electrical characterization of the devices reveals electrostatic energy parameters consistent with the parameters of the designed tunnel junctions. In addition, an analysis of temperature dependence of conductance confirms the formation of metal-insulator-metal (MIM) junctions with negligible in-series contribution of any surface native metal oxide. However, the fabricated devices exhibit a very high level of electrical noise, far exceeding the commonly observed shot noise. Experimental investigation reveals the random telegraph signal (RTS) nature of the observed excess noise. The RTS noise in electronic devices is commonly associated with charging of external traps that are electrostatically coupled to the SET island. In the devices under study, however, the defects that result in the observed RTS noise are demonstrated to reside within the tunnel junctions. Our results also indicate the critical importance of interface states and surface preparation for achieving good performance of the SETs fabricated using ALD to form the tunnel barrier. 

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3. What can we learn when fitting a simple telegraph model to a complex gene expression model?

   https://doi.org/10.1371/journal.pcbi.1012118  

   Jiao, Feng; Li, Jing; Liu, Ting; Zhu, Yifeng; Che, Wenhao; Bleris, Leonidas; Jia, Chen (May 2024, PLOS Computational Biology)

   Finley, Stacey D (Ed.)

   In experiments, the distributions of mRNA or protein numbers in single cells are often fitted to the random telegraph model which includes synthesis and decay of mRNA or protein, and switching of the gene between active and inactive states. While commonly used, this model does not describe how fluctuations are influenced by crucial biological mechanisms such as feedback regulation, non-exponential gene inactivation durations, and multiple gene activation pathways. Here we investigate the dynamical properties of four relatively complex gene expression models by fitting their steady-state mRNA or protein number distributions to the simple telegraph model. We show that despite the underlying complex biological mechanisms, the telegraph model with three effective parameters can accurately capture the steady-state gene product distributions, as well as the conditional distributions in the active gene state, of the complex models. Some effective parameters are reliable and can reflect realistic dynamic behaviors of the complex models, while others may deviate significantly from their real values in the complex models. The effective parameters can also be applied to characterize the capability for a complex model to exhibit multimodality. Using additional information such as single-cell data at multiple time points, we provide an effective method of distinguishing the complex models from the telegraph model. Furthermore, using measurements under varying experimental conditions, we show that fitting the mRNA or protein number distributions to the telegraph model may even reveal the underlying gene regulation mechanisms of the complex models. The effectiveness of these methods is confirmed by analysis of single-cell data forE. coliand mammalian cells. All these results are robust with respect to cooperative transcriptional regulation and extrinsic noise. In particular, we find that faster relaxation speed to the steady state results in more precise parameter inference under large extrinsic noise. 

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4. Robust skyrmion mediated reversal of ferromagnetic nanodots of 20 nm lateral dimension with high Ms and observable DMI

   https://doi.org/10.1038/s41598-021-99780-1  

   Rajib, Md Mahadi; Misba, Walid Al; Bhattacharya, Dhritiman; Atulasimha, Jayasimha (December 2021, Scientific Reports)

   Abstract Implementation of skyrmion based energy efficient and high-density data storage devices requires aggressive scaling of skyrmion size. Ferrimagnetic materials are considered to be a suitable platform for this purpose due to their low saturation magnetization (i.e. smaller stray field). However, this method of lowering the saturation magnetization and scaling the lateral size of skyrmions is only applicable where the skyrmions have a smaller lateral dimension compared to the hosting film. Here, we show by performing rigorous micromagnetic simulation that the size of skyrmions, which have lateral dimension comparable to their hosting nanodot can be scaled by increasing saturation magnetization. Also, when the lateral dimension of nanodot is reduced and thereby the skyrmion confined in it is downscaled, there remains a challenge in forming a stable skyrmion with experimentally observed Dzyaloshinskii–Moriya interaction (DMI) values since this interaction has to facilitate higher canting  per spin to complete a 360° rotation along the diameter. In our study, we found that skyrmions can be formed in 20 nm lateral dimension nanodots with high saturation magnetization (1.30–1.70 MA/m) and DMI values (~ 3 mJ/m 2 ) that have been reported to date. This result could stimulate experiments on implementation of highly dense skyrmion devices. Additionally, using this, we show that voltage controlled magnetic anisotropy based switching mediated by an intermediate skyrmion state can be achieved in the soft layer of a ferromagnetic p-MTJ of lateral dimensions 20 nm with sub 1 fJ/bit energy in the presence of room temperature thermal noise with reasonable DMI ~ 3 mJ/m 2 . 

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5. Origins of Fermi Level Pinning for Ni and Ag Metal Contacts on Tungsten Dichalcogenides

   https://doi.org/10.1021/acsnano.3c06494  

   Wang, Xinglu; Hu, Yaoqiao; Kim, Seong Yeoul; Addou, Rafik; Cho, Kyeongjae; Wallace, Robert M (October 2023, ACS Nano)

   Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (EF) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of EF pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure. We conclude that the origins of EF pinning at a covalent contact metal/W-TMD interface, such as Ni/W-TMDs, can be attributed to defects, impurities, and interface reaction products. In contrast, for a van der Waals contact metal/TMD system such as Ag/W-TMDs, the primary factor responsible for EF pinning is the electronic modification of the TMDs resulting from the defects and impurities with the minor impact of metal-induced gap states. The potential strategies for carefully engineering the metal deposition approach are also discussed. This work unveils the origins of EF pinning at metal/TMD interfaces experimentally and theoretically and provides guidance on further enhancing and improving the device performance. 

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