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  1. Large exchange-driven intrinsic circular dichroism of a chiral 2D hybrid perovskite

    Abstract In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals. Comparison with ab initio theory reveals the large circular dichroism arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenonmore » driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands, resulting in a Cotton effect. Our study suggests that previous data for spin-coated films largely underestimate the optical chirality and provides quantitative insights into the intrinsic optical properties of chiral perovskites for chiroptical and spintronic applications.« less
  2. Highly Efficient Room‐Temperature Spin‐Orbit‐Torque Switching in a Van der Waals Heterostructure of Topological Insulator and Ferromagnet

    Abstract All‐Van der Waals (vdW)‐material‐based heterostructures with atomically sharp interfaces offer a versatile platform for high‐performing spintronic functionalities at room temperature. One of the key components is vdW topological insulators (TIs), which can produce a strong spin‐orbit‐torque (SOT) through the spin‐momentum locking of their topological surface state (TSS). However, the relatively low conductance of the TSS introduces a current leakage problem through the bulk states of the TI or the adjacent ferromagnetic metal layers, reducing the interfacial charge‐to‐spin conversion efficiency ( q ICS ). Here, a vdW heterostructure is used consisting of atomically‐thin layers of a bulk‐insulating TI Sn‐doped Bimore » 1.1 Sb 0.9 Te 2 S 1 and a room‐temperature ferromagnet Fe 3 GaTe 2, to enhance the relative current ratio on the TSS up to ≈20%. The resulting q ICS reaches ≈1.65 nm −1 and the critical current density J c ≈0.9 × 10 6  Acm −2 at 300 K, surpassing the performance of TI‐based and heavy‐metal‐based SOT devices. These findings demonstrate that an all‐vdW heterostructure with thickness optimization offers a promising platform for efficient current‐controlled magnetization switching at room temperature.« less
  3. Electrically Controlled All‐Antiferromagnetic Tunnel Junctions on Silicon with Large Room‐Temperature Magnetoresistance

    Abstract Antiferromagnetic (AFM) materials are a pathway to spintronic memory and computing devices with unprecedented speed, energy efficiency, and bit density. Realizing this potential requires AFM devices with simultaneous electrical writing and reading of information, which are also compatible with established silicon‐based manufacturing. Recent experiments have shown tunneling magnetoresistance (TMR) readout in epitaxial AFM tunnel junctions. However, these TMR structures are not grown using a silicon‐compatible deposition process, and controlling their AFM order required external magnetic fields. Here it is shown three‐terminal AFM tunnel junctions based on the noncollinear antiferromagnet PtMn 3 , sputter‐deposited on silicon. The devices simultaneously exhibitmore » electrical switching using electric currents, and electrical readout by a large room‐temperature TMR effect. First‐principles calculations explain the TMR in terms of the momentum‐resolved spin‐dependent tunneling conduction in tunnel junctions with noncollinear AFM electrodes.« less
  4. Enhanced magnetic susceptibility in Ti 3 C 2 T x MXene with Co and Ni incorporation

    Magnetic nanomaterials are sought to provide new functionalities for applications ranging from information processing and storage to energy generation and biomedical imaging. MXenes are a rapidly growing family of two-dimensional transition metal carbides and nitrides with versatile chemical and structural diversity, resulting in a variety of interesting electronic and optical properties. However, strategies for producing MXenes with tailored magnetic responses remain underdeveloped and challenging. Herein, we incorporate elemental Ni and Co into Ti3C2Tx MXene by mixing with dilute metal chloride solutions. We achieve a uniform distribution of Ni and Co, confirmed by X-ray fluorescence (XRF) mapping with nanometer resolution, withmore » Ni and Co concentrations of approximately 2 and 7 at% relative to the Ti concentration. The magnetic susceptibility of these Ni- and Co-incorporated Ti3C2Tx MXenes is one to two orders of magnitude larger than pristine Ti3C2Tx, illustrating the potential for dilute metal incorporation to enhance linear magnetic responses at room temperature.« less
  5. Ergodicity, lack thereof, and the performance of reservoir computing with memristive networks

    Abstract Networks composed of nanoscale memristive components, such as nanowire and nanoparticle net-works, have recently received considerable attention because of their potential use as neuromorphic devices. In this study, we explore ergodicity in memristive networks, showing that the performance on machine leaning tasks improves when these networks are tuned to operate at the edge between two global stability points. We find this lack of ergodicity is associated with the emergence of memory in the system. We measure the level of ergodicity using the Thirumalai-Mountain metric, and we show that in the absence of ergodicity, two different memristive network systems showmore » improved performance when utilized as reservoir computers (RC). We highlight that it is also important to let the system synchronize to the input signal in order for the performance of the RC to exhibit improvements over the baseline.« less
  6. Photo‐Induced Charge State Dynamics of the Neutral and Negatively Charged Silicon Vacancy Centers in Room‐Temperature Diamond

    Abstract The silicon vacancy (SiV) center in diamond is drawing much attention due to its optical and spin properties, attractive for quantum information processing and sensing. Comparatively little is known, however, about the dynamics governing SiV charge state interconversion mainly due to challenges associated with generating, stabilizing, and characterizing all possible charge states, particularly at room temperature. Here, multi‐color confocal microscopy and density functional theory are used to examine photo‐induced SiV recombination — from neutral, to single‐, to double‐negatively charged — over a broad spectral window in chemical‐vapor‐deposition (CVD) diamond under ambient conditions. For the SiV 0 to SiV more » transition, a linear growth of the photo‐recombination rate with laser power at all observed wavelengths is found, a hallmark of single photon dynamics. Laser excitation of SiV , on the other hand, yields only fractional recombination into SiV 2‒ , a finding that is interpreted in terms of a photo‐activated electron tunneling process from proximal nitrogen atoms.« less
  7. On-surface magnetocaloric effect for a van der Waals Gd( iii ) 2D MOF grown on Si

    [Gd(MeCOO)(PhCOO) 2 ], a 2D MOF is reported and characterized, the material shows a magnetocaloric effect both in bulk and chemisorbed on a Silicon substrate. This opens up the possibilities for on-surface cooling devices.
  8. Photovoltaic effect on silicon-alumina-ferromagnet tunnel junction providing insights about spin-dependent molecular spintronics solar cells

    The study focuses on the observation of the photovoltaic (PV) effect on Si/AlOx/FM semiconductor-insulator-ferromagnetic metal (SIFM) structure. Utilization of ~10 nm NiFe film as the top FM layer was permeable for sufficient light radiation necessary for reaching the silicon substrate for the generation of electron-hole pairs upon photoexcitation. The effect of light intensity and magnetic field was studied on the SIFM's PV response. We also investigated the role of silicon doping and the AlOx tunnel barrier between Si and FM for exploring suitable band bending necessary for separating the electron-hole pairs. Increasing the dopant density in Si and a damagedmore » AlOx tunnel barrier quenched the PV effect. FMIFM was also studied to gain deeper mechanistic insights into the spin-dependent photovoltaic effect observed on FM/AlOx/FM tunnel junction-based molecular spintronics devices. Bridging of magnetic molecules between the Si and FM electrodes of SIFM increased the overall device current by establishing additional parallel conduction channels along with the AlOx tunnel barrier. However, SIFM with molecular conduction channels did not produce a PV effect. This study reported the PV effect on well-designed SIFM and opened possibilities of exploring new systems. More importantly, this paper provided insights into the role of molecule-induced exchange coupling in transforming an ordinary cheap and widely available ferromagnet into a semiconductor-like material capable of showing PV.« less
  9. 2024 roadmap on 2D topological insulators

    Abstract 2D topological insulators promise novel approaches towards electronic, spintronic, and quantum device applications. This is owing to unique features of their electronic band structure, in which bulk-boundary correspondences enforces the existence of 1D spin-momentum locked metallic edge states – both helical and chiral – surrounding an electrically insulating bulk. Forty years since the first discoveries of topological phases in condensed matter, the abstract concept of band topology has sprung into realization with several materials now available in which sizable bulk energy gaps – up to a few hundred meV – promise to enable topology for applications even at room-temperature.more » Further, the possibility of combing 2D TIs in heterostructures with functional materials such as multiferroics, ferromagnets, and superconductors, vastly extends the range of applicability beyond their intrinsic properties. While 2D TIs remain a unique testbed for questions of fundamental condensed matter physics, proposals seek to control the topologically protected bulk or boundary states electrically, or even induce topological phase transitions to engender switching functionality. Induction of superconducting pairing in 2D TIs strives to realize non-Abelian quasiparticles, promising avenues towards fault-tolerant topological quantum computing. This roadmap aims to present a status update of the field, reviewing recent advances and remaining challenges in theoretical understanding, materials synthesis, physical characterization and, ultimately, device perspectives.« less
  10. Topological Spin Textures in an Insulating van der Waals Ferromagnet

    Abstract Generation and control of topological spin textures constitutes one of the most exciting challenges of modern spintronics given their potential applications in information storage technologies. Of particular interest are magnetic insulators, which due to low damping, absence of Joule heating and reduced dissipation can provide energy‐efficient spin‐textures platform. Here, it is demonstrated that the interplay between sample thickness, external magnetic fields, and optical excitations can generate a prolific paramount of spin textures, and their coexistence in insulating CrBr 3 van der Waals (vdW) ferromagnets. Using high‐resolution magnetic force microscopy and large‐scale micromagnetic simulation methods, the existence of a largemore » region in T‐B phase diagram is demonstrated where different stripe domains, skyrmion crystals, and magnetic domains exist and can be intrinsically selected or transformed to each‐other via a phase‐switch mechanism. Lorentz transmission electron microscopy unveils the mixed chirality of the magnetic textures that are of Bloch‐type at given conditions but can be further manipulated into Néel‐type or hybrid‐type via thickness‐engineering. The topological phase transformation between the different magnetic objects can be further inspected by standard photoluminescence optical probes resolved by circular polarization indicative of an existence of exciton‐skyrmion coupling mechanism. The findings identify vdW magnetic insulators as a promising framework of materials for the manipulation and generation of highly ordered skyrmion lattices relevant for device integration at the atomic level.« less
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