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Domain wall (DW) devices are gaining additional attention for their suitability in emerging neuromorphic computing (NC) 1. Underlying building blocks of NC are neurons and synapses, where neurons process the information and synapses control the information passing from pre-neurons to post-neurons. Such synaptic functions can be achieved by realizing multiple conductance (resistance) states. In DW devices, this virtue can be achieved by controllably stopping the DW at multiple specific positions. However, DW motion is stochastic, and therefore, there is a need to artificially form centers by amending magnetic/geometric properties that can controllably pin the DWs 2.
In the present study, we have proposed and studied the concept of nanoscale interfacial Dzyaloshinskii-Moriya interaction (iDMI) region to pin the DW. When an electrical current is passed through the ferromagnetic (FM) nanowire, the DW moves from the left end of the wire and gets pinned at the pinning site for a large range of iDMI constant, the width of the pinning site, and current density (J) values. The DWs can be depinned from the pinning site by applying a higher J. We found that local iDMI forms a barrier in energy locally and hence pins the DW. Besides, the magnetostatic interaction between DW and the pinning site plays an important role in pinning the DWs. In addition to DW pinning, we observed that the DW oscillates at GHz frequencies at the pinning site 3. Later, we introduced five pinning sites along the FM wire and observed that the DW can be pinned (and depinned) successively at every pinning site. Therefore, multiple resistance states and hence synaptic operations can be achieved. The results of this study will be discussed in full detail during the conference. 


References 
1. A. Sengupta and K. Roy, Applied Physics Express, 11 (2018) 030101. 
2. D. Kumar et al., IEEE Transactions on Magnetics, 55 (2018) 1-8. 
3. D. Kumar et al., Journal of Magnetism and Magnetic Materials, (2020) 166807.

MATERIALS FOR HUMANITY (MH 21)

Domain wall pinning through nanoscale interfacial Dzyaloshinskii-Moriya interaction

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technical paper

Spin-dependent thermoelectric research has drawn overwhelming attention during the last few years because of its potentiality in energy harvesting and communication applications. Waste heat from locomotives and industries can be converted into electrical and electronic signals to power their own electronics. The Anomalous Nernst effect (ANE) one of the thermomagnetic phenomena proposed to be useful for heat to electrical energy conversion even from a curved surface unlike a conventional thermoelectric module based on semiconductors. Here, we have systematically studied the ANE in polycrystalline metallic perovskite Pr0.5Sr0.5CoO3 as a function of temperature and external magnetic fields. The ANE shows a rapid increase at the onset of ferromagnetic transition (TC ~ 229 K) and after showing a broad maximum at lower temperatures, and it decreases linearly below 100 K. Magnetic field dependence of the ANE, quantified by the transverse Seebeck coefficient (Sxy), exhibits hysteresis below TC as like the magnetization. The maximum saturated value of Sxy due to ANE (~ 0.46 μVK¬-1) found near 175 K (below TC) is ~ 400 times higher than the manganite analog Pr0.5Sr0.5MnO3, and larger than other polycrystalline perovskite oxides reported to date. Our analysis suggests that ANE in this sample is most likely due to asymmetric skew scattering of charge carriers. This work may open a new pathway for the perovskite magnetic oxides for energy-saving applications.

Anomalous Nernst Effect in Bulk Pr0.5Sr0.5CoO3

The mutual synchronization of spin-torque oscillators (STOs) is critical for communication, energy harvesting and neuromorphic applications. Short range magnetic coupling-based synchronization has spatial restrictions (few μm), whereas the long-range electrical synchronization using vortex STOs has limited frequency responses in hundreds MHz (<500 MHz), restricting them for on-chip GHz-range applications. Here, we demonstrate electrical synchronization of four non-vortex uniformly-magnetized STOs using a single common current source in both parallel and series configurations at 2.4 GHz band, resolving the frequency-area quandary for designing STO based on-chip communication systems. Under injection locking, synchronized STOs demonstrate an excellent time-domain stability and substantially improved phase noise performance. By integrating the electrically connected eight STOs, we demonstrate the battery-free energy-harvesting system by utilizing the wireless radio-frequency energy to power electronic devices such as LEDs using the setup shown in Fig. 1a. We also demonstrate the capability of this energy harvesting system in holding dc power using a time-varying signal [Fig. 1b], which is useful in harvesting energy from discrete commercial sources such as a WiFi router. Our results highlight the significance of electrical topology (series vs. parallel) while designing an on-chip STOs system.

Electrically connected spin-torque oscillators array for 2.4 GHz WiFi band transmission and energy harvesting

Human sweat is one of the most available biofluids and the effect of sweat on the wearable electrodes has always been considered a deteriorating effect for electrode’s conductivity because of the enhanced corrosion behavior of bulk metals in the presence of saline solution. Herein, we demonstrate the first example of stretchable Ag electrodes in which the conductivity can be enhanced by human sweating behavior. The stretchable electrodes were fabricated by directly printing the ink composing of conductive filler silver flakes and rationally designed elastic binder (HPUA, thermoplastic and hydrophilic poly(urethane-acrylate) on hydrophilic textiles. Sweat can reduce the resistance of electrodes in both relaxed and stretched states. Our method uses sweat that is unharmful to human skin as the sintering agent that reduces the resistance of printed stretchable electrodes during wearing, providing a new route in the design of printed stretchable and wearable devices. In summary, we designed printable, photocurable, and textile-based Ag-HPUA electrodes whose resistance can be reduced by human sweat in both original and stretched states. The electrodes compose of Ag flakes and HPUA binder with hydrophilic, photocurable, and high stretchable properties. When the Ag-HPUA encounters the sweat, the lactic acid, and Cl- work synergistically to enhance the conductivity rapidly and permanently by removing the lubricant and sintering the exposed Ag flakes. The affecting factors and mechanism of the reaction between the Ag-HPUA electrode and sweat were well explored.

Printable Elastomeric Electrodes with Sweat-Enhanced Conductivity for Wearables

A recent proposal for a brain-inspired architecture, known as Neuromorphic Computing (NC), has received critical appraisal from many researchers charged with procuring low processing power that could one day rival the human brain. Its basic elements can be broadly classed into neurons and synapses, the latter of which is responsible for memory storage featuring multiply resistive states. Since then, spin-based neuromorphic elements have been a subject of study, amongst which the domain wall (DW) device expends the least energy. Nevertheless, the stochastic behaviour of DW motion complicates the identity of its position and renders its motion uncontrollable. This paper proposes the use of synthetic anti-ferromagnetic pinning sites embedded within a ferromagnetic device to control the position of the DW. We simulate both pinning and de-pinning of DWs at synthetic anti-ferromagnetic regions coupled by an inter-layer exchange interaction of up to -0.3 mJ/m2 and relate an increased magnitude of the inter-layer exchange coupling with a de-pinning current that grows in proportion in the affairs of longer pining sites (> 50 nm) with no dependence established on DMI or further length. On the contrary, we find both increasing and decreasing relationships between DMI and de-pinning current which are distinguished by two stable DW configurations at pinning sites of lower lengths. We also extend our study to accommodate five pinning sites to demonstrate the various resistive states from which its neuromorphic synaptic properties may be derived.

Pinning Domain Walls with Synthetic Antiferromagnets

Artificial electronic synapses with two-terminal floating-gate (FG) configurations have undergone enormous development because of the advantage in terms of easy modulation and quick fabrication. Although synaptic features can be effectively modulated via the FG field effect, the tunability remain limited, especially for the commonly used n-type transition metal dichalcogenides (TMDCs). This paper focuses on a two-terminal MoS2 FG artificial synaptic device, the surface of which has been functionalized by an ultrathin deposition (0.1 nm) of MoO3 on the channel region. The surface functionalization together with the gating effect greatly enhance hysteresis behaviors of the device with expanded memory window from 6 to 14.8 V and significantly increased ON/OFF current ratio from 10 to 107. In addition, essential synaptic functions were successfully emulated in this synaptic two-terminal device, including transition from short-term to long-term potentiation, paired-pulse facilitation/depression, and spike timing-dependent plasticity. These findings promise surface charge transfer doping as an effective method to broaden the functionality of two-dimensional TMDCs-based electronic devices and provide a platform for the development of electric-modulated neuromorphic architectures.

Artificial synapses based on two-dimensional van der Waals materials for brain-inspired computing

2D transition-metal dichalcogenides (2D TMDs) exhibit a multitude of exotic properties. In particular, if 2D van der Waals magnets exist, they would be ideal atomically thin building blocks for 2D spintronics [1]. Theories have predicted intrinsic magnetism in 2D VX2, such as vanadium diselenide and vanadium ditelluride. Bonilla et al. reported strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates [2]. We show however, that 2D VSe2 is not intrinsically ferromagnetic, but displays evidence of spin frustration [3]. Nevertheless, a magnetic transition in 2D VSe2 can be induced at the contamination-free interface between Co and VSe2 via interface hybridization [4]. Promotion of ferromagnetism in 2D VSe2 is accompanied by antiferromagnetic coupling to Co and a reduction in the spin moment of Co. Consistent results are obtained for 2D VTe2 [5].

We have addressed the conflicting reports on the ferromagnetism of clean monolayer VSe2. We studied the controllable formation of 1D defect line patterns in VSe2 monolayers using STM and q-plus AFM techniques [6]. We found that the reconstructed VSe2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the DFT calculations. This work possibly resolves the controversy on whether monolayer VSe2 is intrinsically ferromagnetic, and highlights the importance of controlling surface defects in 2D crystals.

References

[1] W. Zhang, P. K. J. Wong, R. Zhu, A. T. S. Wee, InfoMat. (2019) 1, 1–17.
[2] M. Bonilla et al., Nat. Nano. (2018) 13, 289-293. 
[3] P. K. J. Wong et al., Adv. Mat. (2019) 31, 1901185.
[4] W. Zhang et al., ACS Nano (2019) 13, 8997-9004 
[5] P. K. J. Wong et al., ACS Nano (2019) 13, 11, 12894–12900
[6] R. Chua et al., Adv. Mater. (2020) 200069 


Is 2D Vanadium Diselenide Ferromagnetic?

Topological phonons in crystalline materials have been attracting great interest. However, most cases studied so far are direct generalizations of the topological states from electronic systems. Here, we reveal a novel class of topological phonons -- the symmetry-enforced nodal-chain phonons, which manifest features unique for phononic systems. We show that with D2d little co-group at a non-time-reversal-invariant-momentum point, the phononic nodal chain is guaranteed to exist owing to the vector basis symmetry of phonons, which is a unique character distinct from electronic and other systems. Combined with the spinless character, this makes the proposed nodal-chain phonons enforced by symmorphic crystal symmetries. We further screen all 230 space groups, and find five candidate groups. Interestingly, the nodal chains in these five groups exhibit two different patterns: for tetragonal systems, they are one-dimensional along the fourfold axis; for cubic systems, they form a three-dimensional network structure. Based on first-principles calculations, we identify K2O as a realistic material hosting almost ideal nodal-chain phonons. We show that the effect of LO-TO splitting, another unique feature for phonons, helps to expose the nodal-chain phonons in K2O in a large energy window. In addition, all the five candidate groups have spacetime inversion symmetry, so the nodal chains also feature a quantized π Berry phase. This leads to drumhead surface phonon modes that must exist on multiple surfaces of a sample.

Symmetry-Enforced Nodal Chain Phonons

2D transition metal dichalcogenides (TMDCs) have potential for quantum applications but progress has been slow due to contact problems. In particular, quantum transport studies are difficult as low-quality contacts are exacerbated at cryogenic temperatures where quantum devices are typically operated. Here, we use indium alloy to form contacts with chemical vapour deposition grown single-layer and bilayer WS2 devices. The devices exhibit ultra-low contact resistances and Schottky barrier heights at temperatures as low as 3 K, which in turn enables high carrier mobilities and measurements of resonant quantum tunnelling. Density functional theory calculations provide further insights into the intricacies of the metal/semiconductor interface and quantum transport in the material. Our results unlock the potential for 2D semiconductors in low temperature applications such as in quantum information processing.

Quantum Transport in Two-Dimensional WS2 with High-Efficiency Carrier Injection through Indium Alloy Contacts

We experimentally demonstrate that absorption of one of the photons from the entangled pair can be switched on and off by controlling the Pancharatnam-Berry phase of the other photon, the portion of the light does not go through the absorber. By using the so-called quantum coherent perfect absorption effect [1], we can expect near-perfect switching of the remote photon. The polarization entangled state between ‘idler’ and ‘signal’ photons is generated through spontaneous parametric down conversion process in the non-linear BBO crystal. The signal photon is sent to the interferometer where it excites the standing wave in the middle of the interferometer. The standing wave interacts with a thin absorber by the mechanism of coherent perfect absorption: if the absorber is at the anti-node of the standing wave, strong light-matter interaction takes place and the photon is fully absorbed; if the absorber is at the node of the standing wave, the photon is fully transmitted. The idler photon passes through the combination of three wave plates and acquires Pancharatnam-Berry phase due to the cyclic evolution of the photon polarization in Poincare sphere [2]. The introduced Pancharatnam-Berry phase of the idler photon, that does not go through the interferometer, enables the phase shift inside the interferometer, thus affect the interaction of the signal photon and the absorber. We perform two-photon polarization-sensitive measurements and demonstrate the probability of the signal photon dissipation by the absorber depends on the PB phase applied to the idler photon. 
References
[1] T. Roger, S. Vezzoli, E. Bolduc, J. Valente, J. J. F. Heitz, J. Jeffers, C. Soci, J. Leach, C. Couteau, N. I. Zheludev, and D. Faccio, Nat. Commun. 6, 7031 (2015).
[2] E. Cohen, H. Larocque, F. Bouchard, F. Nejadsattari, Y. Gefen, and E. Karimi, Nat. Rev. Phys. 1, 437–449 (2019).

Non-Local Control of Quantum coherent perfect absorption

Despite being a bona-fide bulk insulator, diamond develops an intriguing two-dimensional (2D) p-type surface conductivity when its surface is terminated by hydrogen and exposed to appropriate surface adsorbate layer such as atmospheric water as a result of the surface transfer doping process. Consequently, the surface of diamond presents a versatile platform for exploiting some of the extraordinary physical and chemical properties of diamond, leading to applications such as chemical/biological sensing and the development of high-power and high-frequency field effect transistors (FETs).

In this talk, I will describe our recent work on the surface transfer doping of diamond by transition metal oxides (TMOs). Specifically, I will show that by interfacing diamond with MoO3 or V2O5 a 2D hole conducting layer with metallic transport behaviours arises on diamond. Magnetotransport studies at low temperature reveal phase coherent transport in the 2D channel with a transition from weak localisation to weak antilocalisation as temperature drops, and are analysed in the context of spin-orbit coupling induced by Rashba effect (Figure 1). The obtained spin-orbit interaction is a few folds higher than that in the air-doped channel or ionic liquid gated channel with similar hole densities, suggesting that the local carrier density could in fact be much higher than that obtained by Hall-effect measurements. We also demonstrate that this surface conducting channel can be exploited to build diamond surface electronic devices such as metal-oxide semiconductor FETs (MOSFETs). Lastly, the prospects for constructing novel quantum devices on diamond surface by making use of this highly tunable 2D conducting layer on diamond are also explored.

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Artificial synapses based on two-dimensional van der Waals materials for brain-inspired computing