Showing 1–18 of 18 results for author: Malik, A

The role of absorption in three-dimensional electron diffraction dynamical structure refinement

Authors: Benjamin Colmey, Tiarnan A. S. Doherty, Shreshth A. Malik, Paul A. Midgley

Abstract: The role of absorption in 3D electron diffraction is established through analytical theory, simulation, and dynamical refinement. A two-beam expression for the absorbed integrated intensity in centrosymmetric crystals is derived, showing that for $t/ξ_g \ll 1$ reflections follow a uniform exponential decay set by the mean absorptive potential $U_0'$. Many-beam simulations of both centrosymmetric a… ▽ More The role of absorption in 3D electron diffraction is established through analytical theory, simulation, and dynamical refinement. A two-beam expression for the absorbed integrated intensity in centrosymmetric crystals is derived, showing that for $t/ξ_g \ll 1$ reflections follow a uniform exponential decay set by the mean absorptive potential $U_0'$. Many-beam simulations of both centrosymmetric and non-centrosymmetric crystals reveal additional reflection-specific anomalous absorption beyond the uniform attenuation set by $U_0'$. Neglecting these effects in dynamical refinement of integrated intensities incurs an error that increases approximately linearly with thickness, with this error becoming more severe near zone axes. Dynamical refinements were performed on CsPbBr$_3$, quartz, and borane, with the inclusion of absorption yielding an improvement in $R_{\mathrm{obs}}$ from $6.4$ to $5.3$ \% for CsPbBr$_3$ and negligible improvements for quartz and borane. Anomalous absorption may therefore be ignored for routine refinement of integrated intensities except in high-$Z$ materials at thicknesses approaching $ξ_g $. △ Less

Submitted 9 August, 2026; v1 submitted 9 February, 2026; originally announced February 2026.

MADE: Benchmark Environments for Closed-Loop Materials Discovery

Authors: Shreshth A Malik, Tiarnan Doherty, Panagiotis Tigas, Muhammed Razzak, Stephen J. Roberts, Aron Walsh, Yarin Gal

Abstract: Existing benchmarks for computational materials discovery primarily evaluate static predictive tasks or isolated computational sub-tasks. While valuable, these evaluations neglect the inherently iterative and adaptive nature of scientific discovery. We introduce MAterials Discovery Environments (MADE), a novel framework for benchmarking end-to-end autonomous materials discovery pipelines. MADE sim… ▽ More Existing benchmarks for computational materials discovery primarily evaluate static predictive tasks or isolated computational sub-tasks. While valuable, these evaluations neglect the inherently iterative and adaptive nature of scientific discovery. We introduce MAterials Discovery Environments (MADE), a novel framework for benchmarking end-to-end autonomous materials discovery pipelines. MADE simulates closed-loop discovery campaigns in which an agent or algorithm proposes, evaluates, and refines candidate materials under a constrained oracle budget, capturing the sequential and resource-limited nature of real discovery workflows. We formalize discovery as a search for thermodynamically stable compounds relative to a given convex hull, and evaluate efficacy and efficiency via comparison to baseline algorithms. The framework is flexible; users can compose discovery agents from interchangeable components such as generative models, filters, and planners, enabling the study of arbitrary workflows ranging from fixed pipelines to fully agentic systems with tool use and adaptive decision making. We demonstrate this by conducting systematic experiments across a family of systems, enabling ablation of components in discovery pipelines, and comparison of how methods scale with system complexity. △ Less

Submitted 23 August, 2026; v1 submitted 28 January, 2026; originally announced January 2026.

Comments: Poster at ICML 2026 Main Conference

Probing imbalanced Weyl nodes in two-dimensional anisotropic Weyl semimetal via optical conductivity

Authors: Suheel Ahmad Malik, M. A. H. Ahshan, SK Firoz Islam

Abstract: We present a theoretical investigation of the electronic band structure and optical properties of a two-dimensional anisotropic semimetal that is described by a tilted semi-Dirac type spectrum with a pair of Weyl nodes. We observe that a tilt along the quadratic direction can give rise to an energy imbalance between these nodes, contrary to the effect of tilt along the linear direction. We investi… ▽ More We present a theoretical investigation of the electronic band structure and optical properties of a two-dimensional anisotropic semimetal that is described by a tilted semi-Dirac type spectrum with a pair of Weyl nodes. We observe that a tilt along the quadratic direction can give rise to an energy imbalance between these nodes, contrary to the effect of tilt along the linear direction. We investigate the optical response of such system subjected to an external AC bias, aiming to probe the energy imbalance between the nodes. We show that the anisotropic interband optical conductivity gives a clear signature of imbalanced nodes by exciting electrons at two different chemical potentials at near zero frequency and the difference between these two chemical potentials is the direct measure of the energy imbalance. Subsequently, we also investigate the intraband DC conductivity by using the semi-classical Boltzmann transport theory which reveals that contrary to the tilted Dirac materials, tilt can convert semi-Dirac material from semimetallic phase to metallic phase. Furthermore, we periodically drive the system by external time-periodic perturbation to open up topological gap at those nodes. We also show that the presence of imbalanced Weyl nodes would prevent the semi-Dirac material from switching to Chern topological phase even after opening topological gaps at the nodes as the bulk remains gapless. Such state cannot be probed by the usual anomalous Hall response, as it will be overshadowed by the bulk contribution. Here, we show that those gaps at different chemical potentials can be probed by optical excitation. Finally, we extend our study to nonlinear regime, where we particularly focus on second harmonic generation in an inversion symmetry broken tilted semi-Dirac system. A clear signature of energy imbalanced Weyl nodes can also be detected here. △ Less

Submitted 8 July, 2026; v1 submitted 12 August, 2025; originally announced August 2025.

Comments: 15 pages, 23 figures. (Published version)

Journal ref: Physical Review B 114, 055406 (2026)

Unconventional superlattice ordering in intercalated transition metal dichalcogenide V$_{1/3}$NbS$_2$

Authors: Shannon S. Fender, Noah Schnitzer, Wuzhang Fang, Lopa Bhatt, Dingbin Huang, Amani Malik, Oscar Gonzalez, Veronika Sunko, Lilia S. Xie, David A. Muller, Joseph Orenstein, Yuan Ping, Berit H. Goodge, D. Kwabena Bediako

Abstract: The interplay between symmetry and topology in magnetic materials makes it possible to engineer exotic phases and technologically useful properties. A key requirement for these pursuits is achieving control over local crystallographic and magnetic structure, usually through sample morphology (such as synthesis of bulk crystals versus thin-films) and application of magnetic or electric fields. Here… ▽ More The interplay between symmetry and topology in magnetic materials makes it possible to engineer exotic phases and technologically useful properties. A key requirement for these pursuits is achieving control over local crystallographic and magnetic structure, usually through sample morphology (such as synthesis of bulk crystals versus thin-films) and application of magnetic or electric fields. Here we show that V$_{1/3}$NbS$_2$ can be crystallized in two ordered superlattices, distinguished by the periodicity of out-of-plane magnetic intercalants. Whereas one of these structures is metallic and displays the hallmarks of altermagnetism, the other superlattice, which has not been isolated before in this family of intercalation compounds, is a semimetallic noncollinear antiferromagnet that may enable access to topologically nontrivial properties. This observation of an unconventional superlattice structure establishes a powerful route for tailoring the tremendous array of magnetic and electronic behaviors hosted in related materials. △ Less

Submitted 27 June, 2025; originally announced June 2025.

Comments: 7 pages, 4 figures

Strain Induced Robust Skyrmion lattice at Room Temperature in van der Waals Ferromagnet

Authors: Xinyi Zhou, Iftikhar Ahmed Malik, Ruihuan Duan, Hanqing Shi, Chen Liu, Yan Luo, Yue Sun, Ruixi Chen, Yilin Liu, Shian Xia, Vanessa Li Zhang, Sheng Liu, Chao Zhu, Xixiang Zhang, Yi Du, Zheng Liu, Ting Yu

Abstract: Manipulating topological magnetic orders of two-dimensional (2D) magnets by strain, once achieved, offers enormous potential for future low-power flexible spintronic applications. In this work, by placing Fe3GaTe2 (FGaT), a room-temperature 2D ferromagnet, on flexible substrate, we demonstrate a field-free and robust formation of skyrmion lattice induced by strain. By applying a minimal strain of… ▽ More Manipulating topological magnetic orders of two-dimensional (2D) magnets by strain, once achieved, offers enormous potential for future low-power flexible spintronic applications. In this work, by placing Fe3GaTe2 (FGaT), a room-temperature 2D ferromagnet, on flexible substrate, we demonstrate a field-free and robust formation of skyrmion lattice induced by strain. By applying a minimal strain of ~0.80% to pre-annealed FGaT flakes, the Magnetic Force Microscopy (MFM) tip directly triggers the transition from maze-like domains to an ordered skyrmion lattice while scanning the sample surface. The skyrmion lattice is rather stable against extensive cyclic mechanical testing (stretching, bending, and twisting over 2000 cycles each). It also exhibited stability across a wide range of magnetic fields (~2.9 kOe) and temperatures (~ 323 K), as well as long-term retention stability, highlighting its robustness and field free stabilization. The strain effect reduces the lattice symmetry and enhances the Dzyaloshinskii-Moriya interaction (DMI) of FGaT, thus stabilizing the skyrmion lattice. Our findings highlight the potential of FGaT for integrating magnetic skyrmions into future low-power-consumption flexible spintronics devices. △ Less

Submitted 13 May, 2025; originally announced May 2025.

Spontaneous Enhancement of Dzyaloshinskii-Moriya Interaction via Field-Cooling-Induced Interface Engineering in 2D van der Waals Ferromagnetic ternary Tellurides

Authors: Shian Xia, Yan Luo, Iftikhar Ahmed Malik, Xinyi Zhou, Keying Han, Yue Sun, Haoyun Lin, Hanqing Shi, Yingchun Cheng, Vanessa Li Zhang, Yi Du, Sheng Liu, Chao Zhu, Ting Yu

Abstract: The emergence of two-dimensional (2D) van der Waals (vdW) ferromagnets has opened new avenues for exploring topological spin textures and their applications in next-generation spintronics. Among these materials, Fe3GaTe2 (FGaT) emerges as a model system due to its room-temperature skyrmion phases, which are stabilized by strong Dzyaloshinskii-Moriya interaction (DMI). However, the atomistic origin… ▽ More The emergence of two-dimensional (2D) van der Waals (vdW) ferromagnets has opened new avenues for exploring topological spin textures and their applications in next-generation spintronics. Among these materials, Fe3GaTe2 (FGaT) emerges as a model system due to its room-temperature skyrmion phases, which are stabilized by strong Dzyaloshinskii-Moriya interaction (DMI). However, the atomistic origins of DMI in centrosymmetric vdW lattices remain elusive. Here, we report a spontaneous DMI enhancement mechanism driven by FC in FGaT and its analog Fe3GeTe2 (FGeT). Combining Raman spectroscopy and scanning transmission electron microscopy (STEM), we have observed the irreversible precipitation of FeTe2 in annealed FGaT. The resulting FeTe2/FGaT heterostructure is considered to break the symmetry and significantly enhance the DMI. Furthermore, similar phenomenon has been observed in the family ferromagnetic material FGeT as well. Additionally, the precipitation of FeTe2 varies significantly with different thicknesses of FGaT, aligning closely with the reported behavior of skyrmions. This discovery provides new insights into the mechanisms behind the origin of the DMI in ternary tellurides, paving the way for advanced spintronic applications. △ Less

Submitted 17 May, 2025; v1 submitted 11 May, 2025; originally announced May 2025.

Micromorphic FE$^2$ Simulation of Plastic Deformations of Foam Structures

Authors: Alexander Malik, Geralf Hütter, Martin Abendroth, Bjoern Kiefer

Abstract: Capturing and predicting the effective mechanical properties of highly porous cellular media still represents a significant challenge for the research community, due to their complex structural interdependencies and known size effects. Micromorphic theories are often applied in this context to model the inelastic deformation behavior of foam-like structures, in particular to incorporate such size… ▽ More Capturing and predicting the effective mechanical properties of highly porous cellular media still represents a significant challenge for the research community, due to their complex structural interdependencies and known size effects. Micromorphic theories are often applied in this context to model the inelastic deformation behavior of foam-like structures, in particular to incorporate such size effect into the investigation of structure-property correlations. This raises the problems of formulating appropriate constitutive relations for the numerous non-classical stress measures and determining the corresponding material parameters, which are usually difficult to assess experimentally. The present contribution therefore alternatively employs a concurrent micromorphic multi-scale approach within the Direct FE$^2$ framework to simulate the complex irreversible behavior of foam-like porous solids. The predictions of Cosserat (micropolar) and a fully-micromorphic theory are compared with conventional FE$^2$ results and direct numerical simulations (DNS) for complex loading scenarios with elastic, elastic-plastic, and creep deformations. △ Less

Submitted 16 February, 2024; originally announced March 2024.

Competing Higher Order Topological Superconducting Phases in Triangular Lattice Magnet-Superconductor Hybrid Systems

Authors: Ka Ho Wong, Jacopo Gliozzi, Mark R. Hirsbrunner, Arbaz Malik, Barry Bradlyn, Taylor L. Hughes, Dirk K. Morr

Abstract: We demonstrate that a plethora of higher order topological phases emerge in magnet-superconductor hybrid (MSH) systems through the interplay of a stacked magnetic structure and an underlying triangular surface lattice; the latter being of great current experimental interest. Such lattices offer the ability to create three main types of edge terminations -- called x-, y- and y'-edges -- of MSH isla… ▽ More We demonstrate that a plethora of higher order topological phases emerge in magnet-superconductor hybrid (MSH) systems through the interplay of a stacked magnetic structure and an underlying triangular surface lattice; the latter being of great current experimental interest. Such lattices offer the ability to create three main types of edge terminations -- called x-, y- and y'-edges -- of MSH islands that, in turn, give rise to a complex phase diagrams exhibiting various regions of HOTSC phases. We identify the single adatom chain, as well as a pair of adjacent adatom chains (called a double-chain), as the basic building blocks for the emergence of HOTSC phases. Of particular interest are those HOTSC phase which arise from a competition between the topology of single and double-chain blocks, which are absent for square latices. △ Less

Submitted 8 December, 2023; v1 submitted 6 December, 2023; originally announced December 2023.

A modified Ehlers model for the description of inelastic behavior of porous structures

Authors: Martin Abendroth, Alexander Malik, Bjoern Kiefer

Abstract: This paper describes a modification of Ehlers' model for the inelastic behavior of granular media. The modified model can be applied for describing the inelastic behavior of porous media. The key feature is a subtle change of the yield potential, which allows the correct orientation of the triangular-shaped yield surface cross sections depending on the hydrostatic stress state. The model is incorp… ▽ More This paper describes a modification of Ehlers' model for the inelastic behavior of granular media. The modified model can be applied for describing the inelastic behavior of porous media. The key feature is a subtle change of the yield potential, which allows the correct orientation of the triangular-shaped yield surface cross sections depending on the hydrostatic stress state. The model is incorporated into a general framework for isotropic plasticity. An elastic predictor/corrector algorithm is employed to solve the constitutive equations. The necessary derivatives for a Newton update are also given in detail. The model is calibrated using stress, and strain data obtained from finite element simulations of a generic highly porous open-cell Wheire-Phelan foam. △ Less

Submitted 14 July, 2023; originally announced July 2023.

Dipole-moment induced capacitance in nanoscale molecular junctions

Authors: Ankur Malik, Ritu Gupta, Prakash Chandra Mondal

Abstract: Nanoscale molecular junctions are celebrated nanoelectronic devices for mimicking several electronic functions including rectifiers, sensors, wires, switches, transistors, and memory but capacitive behavior is nearly unexplored. Capacitors are crucial energy storage devices that store energy in the form of electrical charges. A capacitor utilizes two electrical conductors separated by a dielectric… ▽ More Nanoscale molecular junctions are celebrated nanoelectronic devices for mimicking several electronic functions including rectifiers, sensors, wires, switches, transistors, and memory but capacitive behavior is nearly unexplored. Capacitors are crucial energy storage devices that store energy in the form of electrical charges. A capacitor utilizes two electrical conductors separated by a dielectric material. However, many oxides-based dielectrics are well-studied for integrating capacitors, however, capacitors comprised of thin-film molecular layers are not well-studied. The present work describes electrochemically grafted thin films of benzimidazole (BENZ) grown on patterned ITO electrodes on which a 50 nm Al is deposited to fabricate large-scale (500 x 500 micron2) molecular junctions. The nitrogen and sulfur-containing molecular junctions, ITO/BENZ/Al act as a parallel-plate capacitor with a maximum capacitance of ~59.6 to 4.79 microFcm-2. The present system can be an excellent platform for molecular charge storage for future energy applications. △ Less

Submitted 28 March, 2023; originally announced March 2023.

Comments: 12 Pages, 5 Figures

ACM Class: F.2.2

Nanoscale molecular electrochemical supercapacitors

Authors: Ritu Gupta, Ankur Malik, Vincent Vivier, Prakash Chandra Mondal

Abstract: Due to the shorter channel length allowing faster ion/charge movement, nanoscale molecular thin films can be attractive electronic components for next-generation high-performing energy storage devices. However, controlling chemical functionalization and achieving stable electrode-molecule interfaces at the nanoscale via covalent functionalization for low-voltage operational, ultrafast charging/dis… ▽ More Due to the shorter channel length allowing faster ion/charge movement, nanoscale molecular thin films can be attractive electronic components for next-generation high-performing energy storage devices. However, controlling chemical functionalization and achieving stable electrode-molecule interfaces at the nanoscale via covalent functionalization for low-voltage operational, ultrafast charging/discharging remains a challenge. Herein, we present a simple, controllable, scalable, low-cost, and versatile electrochemical grafting approach to modulate chemical and electronic properties of graphite rods (GRs) that are extracted from low-cost EVEREADY cells (1.5 US $ for 10 cells of 1.5 V). On the ANT-modified GR (ANT/GR), the total capacitance unveils 350-fold enhancement as compared to an unmodified GR tested with 0.1 M H2SO4 electrolyte ensured by both potentiostatic and galvanostatic measurements. Such enhancement in capacitance is attributed to the contribution from the electrical double layer and Faradaic charge transfer. Due to higher conductivity, anthracene molecular layers possess more azo groups (-N=N-) over pyrene, and naphthalene molecular films during the electrochemical grafting, which is key to capacitance improvements. The ultra-low-loading nanofilms expose high surface area leading to extremely high energy density. The nanoscale molecular films (~ 23 nm thickness) show exceptional galvanostatic charge-discharge cycling stability (10,000) that operates at low potential. Electrochemical impedance spectroscopy was performed along with the DC measurements to unravel in-depth charge storage performances. Electrochemically grafted molecular films on GR show excellent balance in capacitance and electrical conductivity, high diffusion coefficient toward ferrocene, and can easily be synthesized in good yield on rigid to flexible electrodes. △ Less

Submitted 26 March, 2023; originally announced March 2023.

Comments: 18 pages, 6 Figures

ACM Class: F.2.2; I.2.7

Higher Order Topological Superconductivity in Magnet-Superconductor Hybrid Systems

Authors: Ka Ho Wong, Mark R. Hirsbrunner, Jacopo Gliozzi, Arbaz Malik, Barry Bradlyn, Taylor L. Hughes, Dirk K. Morr

Abstract: Quantum engineering of topological superconductors and of the ensuing Majorana zero modes might hold the key for realizing a new paradigm for the implementation of topological quantum computing and topology-based devices. Magnet-superconductor hybrid (MSH) systems have proven to be experimentally versatile platforms for the creation of topological superconductivity by custom-designing the complex… ▽ More Quantum engineering of topological superconductors and of the ensuing Majorana zero modes might hold the key for realizing a new paradigm for the implementation of topological quantum computing and topology-based devices. Magnet-superconductor hybrid (MSH) systems have proven to be experimentally versatile platforms for the creation of topological superconductivity by custom-designing the complex structure of their magnetic layer. Here, we demonstrate that higher order topological superconductivity (HOTSC) can be realized in two-dimensional MSH systems by using stacked magnetic structures. We show that the sensitivity of the HOTSC to the particular magnetic stacking opens an unprecedented ability to tune the system between trivial and topological phases using atomic manipulation techniques. We propose that the realization of HOTSC in MSH systems, and in particular the existence of the characteristic Majorana corner modes, allows for the implementation of a measurement-based protocols for topological quantum computing. △ Less

Submitted 27 October, 2022; originally announced October 2022.

Materials Graph Transformer predicts the outcomes of inorganic reactions with reliable uncertainties

Authors: Shreshth A. Malik, Rhys E. A. Goodall, Alpha A. Lee

Abstract: A common bottleneck for materials discovery is synthesis. While recent methodological advances have resulted in major improvements in the ability to predicatively design novel materials, researchers often still rely on trial-and-error approaches for determining synthesis procedures. In this work, we develop a model that predicts the major product of solid-state reactions. The cardinal feature of t… ▽ More A common bottleneck for materials discovery is synthesis. While recent methodological advances have resulted in major improvements in the ability to predicatively design novel materials, researchers often still rely on trial-and-error approaches for determining synthesis procedures. In this work, we develop a model that predicts the major product of solid-state reactions. The cardinal feature of this approach is the construction of fixed-length, learned representations of reactions. Precursors are represented as nodes on a `reaction graph', and message-passing operations between nodes are used to embody the interactions between precursors in the reaction mixture. Through an ablation study, it is shown that this framework not only outperforms less physically-motivated baseline methods but also more reliably assesses the uncertainty in its predictions. △ Less

Submitted 3 September, 2020; v1 submitted 30 July, 2020; originally announced July 2020.

Journal ref: Chemistry of Materials 2021 33 (2), 616-624

A new phenomenological definition of entropy and application to black holes

Authors: Taha A Malik, Rafael Lopez-Mobilia

Abstract: Typically, the entropy of an isolated system in equilibrium is calculated by counting the number of accessible microstates, or in more general cases by using the Gibbs formula. In irreversible processes entropy spontaneously increases and this is understood from statistical arguments. We propose a new measure of entropy based on the level of irreversibility of a process. This formulation agrees in… ▽ More Typically, the entropy of an isolated system in equilibrium is calculated by counting the number of accessible microstates, or in more general cases by using the Gibbs formula. In irreversible processes entropy spontaneously increases and this is understood from statistical arguments. We propose a new measure of entropy based on the level of irreversibility of a process. This formulation agrees in first approximation with the usual methods of calculating entropy and can be readily applied in the case of a black hole in the semiclassical regime. △ Less

Submitted 15 April, 2020; originally announced April 2020.

Comments: 7 pages, 3 figures

Electric-field-controllable high-spin SrRuO3 driven by a solid ionic junction

Authors: Jingdi Lu, Liang Si, Xiefei Yao, Chengfeng Tian, Jing Wang, Qinghua Zhang, Zhengxun Lai, Iftikhar Ahmed Malik, Xin Liu, Peiheng Jiang, Kejia Zhu, Youguo Shi, Zhenlin Luo, Lin Gu, Karsten Held, Wenbo Mi, Zhicheng Zhong, Ce-Wen Nan, Jinxing Zhang

Abstract: Controlling magnetism and spin structures in strongly correlated systems by using electric field is of fundamental importance but challenging. Here, a high-spin ruthenate phase is achieved via a solid ionic chemical junction at SrRuO3/SrTiO3 interface with distinct formation energies and diffusion barriers of oxygen vacancies, analogue to electronic band alignment in semiconductor heterojunction.… ▽ More Controlling magnetism and spin structures in strongly correlated systems by using electric field is of fundamental importance but challenging. Here, a high-spin ruthenate phase is achieved via a solid ionic chemical junction at SrRuO3/SrTiO3 interface with distinct formation energies and diffusion barriers of oxygen vacancies, analogue to electronic band alignment in semiconductor heterojunction. Oxygen vacancies trapped within this interfacial SrRuO3 reconstruct Ru-4d electronic structure and orbital occupancy, leading to an enhanced magnetic moment. Furthermore, an interfacial magnetic phase can be switched reversibly by electric-field-rectifying oxygen migration in a solid-state ionic gating device, providing a framework for atomic design of functionalities in strongly correlated oxides using a way of solid chemistry. △ Less

Submitted 24 March, 2020; originally announced March 2020.

Journal ref: Phys. Rev. B 101, 214401 (2020)

Interface-driven unusual anomalous Hall effect in MnxGa/Pt bilayers: No correlation with chiral spin structures

Authors: Kangkang Meng, Lijun Zhu, Zhenhu Jin, Enke Liu, Xupeng Zhao, Iftikhar Ahmed Malik, Zhenguo Fu, Yong Wu, Jun Miao, Xiaoguang Xu, Jinxing Zhang, Jianhua Zhao, Yong Jiang

Abstract: The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create chiral spin structures (e.g. skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological H… ▽ More The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create chiral spin structures (e.g. skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological Hall effect (THE). In this work, however, we have present an unusual anomalous Hall effect (UAHE) in MnxGa/Pt bilayers and demonstrated that the features extremely similar to THE can be generated without involving any chiral spin structures. The low temperature magnetic force microscopy has been used to explore the magnetic field-dependent behavior of spin structures, and the UAHE as a function of magnetic field does not peak near the maximal density of magnetic bubbles. The results unambiguously evidence that the UAHE in MnxGa/Pt bilayers shows no correlation with chiral spin structures but is driven by the modified interfacial properties. The bump/dip features of Hall signals cannot be taken as an unambiguous signature for the emergence of chiral spin structures, and a wealth of underlying and interesting physics need explored. △ Less

Submitted 19 July, 2019; v1 submitted 17 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. B 100, 184410 (2019)

Hysteresis-Free Nanosecond Pulsed Electrical Characterization of Top-Gated Graphene Transistors

Authors: Enrique A. Carrion, Andrey Y. Serov, Sharnali Islam, Ashkan Behnam, Akshay Malik, Feng Xiong, Massimiliano Bianchi, Roman Sordan, Eric Pop

Abstract: We measure top-gated graphene field effect transistors (GFETs) with nanosecond-range pulsed gate and drain voltages. Due to high-k dielectric or graphene imperfections, the drain current decreases ~10% over time scales of ~10 us, consistent with charge trapping mechanisms. Pulsed operation leads to hysteresis-free I-V characteristics, which are studied with pulses as short as 75 ns and 150 ns at t… ▽ More We measure top-gated graphene field effect transistors (GFETs) with nanosecond-range pulsed gate and drain voltages. Due to high-k dielectric or graphene imperfections, the drain current decreases ~10% over time scales of ~10 us, consistent with charge trapping mechanisms. Pulsed operation leads to hysteresis-free I-V characteristics, which are studied with pulses as short as 75 ns and 150 ns at the drain and gate, respectively. The pulsed operation enables reliable extraction of GFET intrinsic transconductance and mobility values independent of sweep direction, which are up to a factor of two higher than those obtained from simple DC characterization. We also observe drain-bias-induced charge trapping effects at lateral fields greater than 0.1 V/um. In addition, using modeling and capacitance-voltage measurements we extract charge trap densities up to 10^12 1/cm^2 in the top gate dielectric (here Al2O3). Our study illustrates important time- and field-dependent imperfections of top-gated GFETs with high-k dielectrics, which must be carefully considered for future developments of this technology △ Less

Submitted 31 March, 2014; originally announced April 2014.

Comments: to appear in IEEE Transactions on Electron Devices (2014)

Proceedings of the 35th Annual Australian/New Zealand Condensed Matter and Materials Meeting

Authors: K. Radhanpura, S. Hargreaves, R. A. Lewis, H. Krüger, E. Rey, P. -Z. Si, T. Söhnel, V. Jovic, J. B. Metson, G. I. N. Waterhouse, A. A. Abiona, W. J. Kemp, A. P. Byrne, M. C. Ridgeway, H. Timmers, J. D. Cashion, W. P. Gates, T. L. Greaves, O. Dorjkhaidav, E. Constable, L. G. Gladkis, J. M. Scarvell, P. N. Smith, C. J. Hamer, O. Rojas , et al. (20 additional authors not shown)

Abstract: The 35th Australian/New Zealand Annual Condensed Matter and Materials Meeting was held at the Charles Sturt University campus in Wagga Wagga, NSW, Australia from the 1st to the 4th of February 2011. The conference was attended by 92 delegates from a range of universities across Australia, New Zealand and further afield. There were a total of 9 invited and 21 contributed talks during the three da… ▽ More The 35th Australian/New Zealand Annual Condensed Matter and Materials Meeting was held at the Charles Sturt University campus in Wagga Wagga, NSW, Australia from the 1st to the 4th of February 2011. The conference was attended by 92 delegates from a range of universities across Australia, New Zealand and further afield. There were a total of 9 invited and 21 contributed talks during the three days of scientific sessions, as well as 2 poster sessions with a total of 49 poster presentations. All presenters were invited to submit a manuscript for publication in the conference proceedings. The length limits where six pages for invited papers and four pages for contributed papers. Each manuscript was reviewed by two anonymous referees and 18 papers were accepted for publication. The accepted manuscripts are also available at the online publication section of the Australian Institute of Physics national web site (http://www.aip.org.au/). △ Less

Submitted 18 July, 2011; originally announced July 2011.

Comments: A.P. Micolich (Editor), ISBN 978-0-646-55969-8 (2011). 18 papers from 35th Australian/New Zealand Annual Condensed Matter and Materials Meeting, 85 pages