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MATERIALS FOR HUMANITY (MH 21)

Multispectral Liquid Crystal Spatial Light Modulator based on Fabry-Perot Cavity Resonances

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technical paper

Symmetry is fundamental to topological phases. In the presence of a gauge field, spatial symmetries will be projectively represented, which may alter their algebraic structure and generate novel physics. We show that the Z2 projectively represented translational symmetry operators adopt a distinct anticommutation relation. As a result, each energy band is twofold degenerate, and carries a varying spinor structure for translation operators in momentum space, which cannot be flattened globally. Moreover, combined with other internal or external symmetries, they give rise to exotic band topologies. Particularly, with the inherent time-reversal symmetry, a single fourfold Dirac point must be enforced at the Brillouin zone corner. By breaking one primitive translation, the Dirac semimetal is shifted into a special topological insulator phase, where the edge bands have a Möbius twist. Our work opens an arena of research for exploring topological phases protected by projectively represented space groups.

Z2-projective translational symmetry protected topological phases

Conductance state of a single atom, which exclusively affects the charge transferring rate, plays a key role in determining the electronic properties of the single atom/molecule devices [1, 2]. Therefore, it is significantly important to precisely characterize and deliberately switch the conductance states of single atoms/molecules. In this study, we systematically investigated the dependence of the oxygen adatom (Oad) conductance switching on its charge states by noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM) at 78 K. We firstly clarify the Oad charge and conductance states on rutile TiO2(110) using simultaneous nc-AFM/STM measurement (Figure 1). It suggests that the Oad- has much higher conductance than Oad2-, and the Oad conductance switching between high and low levels can be controlled by deliberately manipulating the Oad charge states through ramping the bias voltage. The charge and conductance states switching of the Oad can be attributed to the tunneling electrons transferring from the tip into the Oad or from the Oad to the substrate polaron [3,4,5]. Our study provides a good reference for the systematic investigation of the atomic conductance, and has a potential overriding effect on revolutionizing the investigation of the single atom/molecule devices.

Charge state dependence of oxygen adatom conductance switching

The combination of inversion symmetry breaking and spin-orbit coupling gives rise to symmetry-dependent spin physics, such as spin-orbit torque (SOT). The lack of vertical inversion symmetry in multilayer structures gives rise to a conventional SOT (or in-plane SOT), which can be used to switch the perpendicular magnetization with the assistance of an external magnetic field. To realize the magnetic field-free switching of the perpendicular magnetization, one solution is to generate an unconventional SOT (or out-of-plane SOT) by using the lack of lateral inversion/mirror symmetry. In the presentation, I will present our recent progress in engineering the out-of-plane SOT through the crystal symmetry at the interface of CuPt/CoPt bilayer.

Perpendicular magnetization switching through crystal symmetry-engineered spin-orbit torque

The spin-orbit torque (SOT) has been used to efficiently manipulate the magnetization in the ferromagnet (FM)/heavy metal (HM) heterostructures for non-volatile storage applications. Engineering of the interface in FM/HM has attracted great attention due to its crucial role in the spin transfer and generation. However, the underlying mechanism for the interfacial contribution to SOT generation is still a subject of debate. Interfacial transparency has been widely used to illustrate the interfacial contribution to the SOT efficiency, for example, the Pt/Co bilayer has a larger interfacial transparency and thus a larger SOT efficiency than Pt/Py bilayer despite the same spin Hall effect of Pt. However, this explanation has been questioned since the Pt/Co interface also has a large spin memory loss, which can lead to a substantial drop of the SOT efficiency [1]. The orbital hybridization is another phenomenon at the FM/HM interface, which is particularly prominent for Pt/Co. Theoretical studies have predicted a strong effect of hybridization on the SOT generated at the Pt/Co interface [2,3], which has not been proved experimentally.
In this work, we perform the STFMR on the epitaxially grown Pt/Ni81Fe19 (as known as permalloy, Py) and Pt/Co/Py heterostructures with (111) crystalline orientation to investigate both the strength and components of their SOT. We demonstrate that inserting an ultra-thin Co layer between epitaxial Pt and Py substantially enhances both the in-plane (IP) and out-of-plane (OP) damping-like SOTs, both of which can be attributed to the strong interfacial hybridization between Co and Pt.

Role of interfacial orbital hybridization in spin-orbit torque generation in Pt-based heterostructures

Ultrashort-laser-induced terahertz (THz) emission is a contact-less probe for charge current generation, which has been recently highlighted in electronics by exploring ultrafast charge dynamics. Here we show it is also applicable in magnetics by exploring spin dynamics. In particular, the generation and transport of an ultrashort spin current, preferably reaching a frequency up to THz, is the core of modern spintronics. First, we demonstrate a picosecond-long spin current generation in a Weyl semimetal Td-WTe2 thin film. The anisotropic dependence of the circular photogalvanic effect in the THz emission indicates that the spin current is generated along the crystalline low-symmetric a-axis, which can be further modulated by the helicity of the laser excitation. Moreover, the generated spins are polarized along the surface normal, which potentially provides an efficient means to manipulate magnetic devices with perpendicular magnetic anisotropy. Since the prior studies have been limited to the static measurements with in-plane spin orientation, our work provides an insight into dynamic feature of the anisotropic spin current and shows a potential for the THz-spintronic devices with Weyl semimetals. Second, we further show the transport of an ultrashort spin current using THz emission. In a magnetic multiplayer Bi2Se3/NiO/NiFe, we show that a spin current generated in the ferromagnetic NiFe layer is transferred to the Bi2Se3 layer through the antiferromagnetic insulator NiO layer. The magnetic and insulating features of NiO enables magnons, i.e., the quanta of spin waves, to carry spin currents without moving charges, which therefore can avoid substantial energy dissipation caused by Joule heating and subsequently provides an efficient means to manipulate the magnetization. Our works, which are relevant to the high-speed and energy-efficient control of spintronic devices, will invigorate magnon-based memory and logic devices.

Exploring generation and transport of ps-long spin currents with terahertz probes

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and
unique advantage of immunity against external field perturbations. To act as the charge-to-spin conversion source in energy-saving spintronic devices, it is of great importance for the AFM material to possess a large spin torque efficiency (ξDL). In this work, using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/NiFe(Py) bilayer system, we systemically study the ξDL of AFM Mn2Au films with different crystal structures. Compared with polycrystalline Mn2Au with effective ξDL < 0.051, we show a much larger ξDL of ~0.333 in single-crystal Mn2Au, which arises from the large spin Hall conductivity instead of electrical resistivity. Moreover, with a further contribution of interfacial effects, the effective ξDL of single-crystalline Mn2Au/Py system increases to 0.731, which is more than two times larger than the value of ~0.22 reported for the Mn2Au/CoFeB system. By utilizing the large ξDL of Mn2Au in a perpendicularly magnetized MnGa/Mn2Au system, energy-efficient deterministic magnetization switching with a current density at ~10^6 A cm−2 is achieved. Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

We describe a simple experimental method to detect electron paramagnetic resonance (EPR) in polycrystalline 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample, the standard g-marker for EPR spectroscopy, without using a cavity resonator or a prefabricated waveguide. It is shown that microwave (MW) current injected into a layer of silver paint coated on an insulating DPPH sample can excite the paramagnetic resonance in DPPH. As the applied dc magnetic field H is swept, the high-frequency resistance of the Ag-paint layer, measured at room temperature with a single port impedance analyzer in the MW frequency range f = 1 to 2.5 GHz, exhibits a sharp peak at a critical value of the dc field (H = Hr) while the reactance exhibits a dispersion-like behavior around the same field value for a given f. Hr increases linearly with f. We interpret the observed features in the impedance to EPR in DPPH driven by the Oersted magnetic field arising from the MW current in the Ag-paint layer. We also confirm the occurrence of EPR in DPPH independently using a co-planar waveguide based broadband technique in the frequency range 2 – 4 GHz. This technique has the potential to investigate other EPR active inorganic and organic compounds.

Electrically Detected Paramagnetic Resonance in Ag-paint Coated Polycrystalline DPPH

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.

Domain wall pinning through nanoscale interfacial Dzyaloshinskii-Moriya interaction

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.

Multispectral Liquid Crystal Spatial Light Modulator based on Fabry-Perot Cavity Resonances

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Z2-projective translational symmetry protected topological phases

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