366 research outputs found
Design of an integrated hybrid plasmonic-photonic device for all-optical switching and reading of spintronic memory
We introduce a novel integrated hybrid plasmonic-photonic device for
all-optical switching and reading of nanoscale ferrimagnet bits. The racetrack
memory made of synthetic ferrimagnetic material with a perpendicular magnetic
anisotropy is coupled on to a photonic waveguide onto the indium phosphide
membrane on silicon platform. The device which is composed of a double V-shaped
gold plasmonic nanoantenna coupled with a photonic crystal cavity can enable
switching and reading of the magnetization state in nanoscale magnetic bits by
enhancing the absorbed energy density and polar magneto-optical Kerr effect
(PMOKE) locally beyond the diffraction limit. Using a three-dimensional
finite-difference time-domain method, we numerically show that our device can
switch and read the magnetization state in targeted bits down to ~100 nm in the
presence of oppositely magnetized background regions in the racetrack with
widths of 30 to 120 nm, clearly outperforming a bare photonic waveguide. Our
hybrid device tackles the challenges of nonlinear absorption in the waveguide,
weak PMOKE, and size mismatch between spintronics and integrated photonics.
Thus, it provides missing link between the integrated photonics and nanoscale
spintronics, expediting the development of ultrafast and energy efficient
advanced on-chip applications
Emergent Phenomena Induced by Spin-Orbit Coupling at Surfaces and Interfaces
Spin-orbit coupling (SOC) describes the relativistic interaction between the
spin and momentum degrees of freedom of electrons, and is central to the rich
phenomena observed in condensed matter systems. In recent years, new phases of
matter have emerged from the interplay between SOC and low dimensionality, such
as chiral spin textures and spin-polarized surface and interface states. These
low-dimensional SOC-based realizations are typically robust and can be
exploited at room temperature. Here we discuss SOC as a means of producing such
fundamentally new physical phenomena in thin films and heterostructures. We put
into context the technological promise of these material classes for developing
spin-based device applications at room temperature
Charge-Spin Conversion and Electronic Transport in Two-Dimensional Materials and van der Waals Heterostructures
Applications related to artificial intelligence (AI), 5G communication, cloud computing, Internet of Things (IoT) will necessitate wide range of data collection, communication and processing. Current charge-based technology using conventional materials suffers adverse effects with down-scaling the device size and has limited efficiency in meeting the future demands for computation and data storage. The exploration of alternative device technology along with new materials is important to enhance computing performance and energy efficiency. In this thesis, I investigated new materials for future memory and logic technologies.\ua0 Recently developed 2D materials such as graphene, semiconductors, and semimetals exhibit remarkable new properties that promise faster and energy efficient non-volatile memory and logic functionalities. For non-volatile memory technologies, increasing efforts are being directed towards exploiting charge-spin conversion phenomena in high spin-orbit coupling (SOC) materials to realize all-electric magnetic memory. Interestingly, magnetic memory devices have been demonstrated on an industrial scale; however, the moderate efficiency and fundamental limitations of the conventional materials employed limit their use in consumer electronics. This thesis addresses some of these critical challenges and presents charge-spin conversion mechanisms in layered high SOC materials such as topological insulators, semimetals, and two-dimensional (2D) materials heterostructures. At the same time, this thesis contributes in the direction of integrating memory and logic devices by investigating 2D semiconductor devices with sub-20 nm narrow channel width and memristive switching in field-effect transistors using 2D semiconductors with graphene contacts. Such 2D semiconductors have enormous prospects for next-generation high-performance and energy-efficient nanoscale field-effect transistors and integration with memory technologies. These studies of charge and spin transport in 2D materials and heterostructures can open the door for nanometer-scale memory, logic and sensing technologies
Gunn Effect in Silicon Nanowires: Charge Transport under High Electric Field
Gunn (or Gunn-Hilsum) Effect and its associated negative differential
resistivity (NDR) emanates from transfer of electrons between two different
energy bands in a semiconductor. If applying a voltage (electric field)
transfers electrons from an energy sub band of a low effective mass to a second
one with higher effective mass, then the current drops. This manifests itself
as a negative slope or NDR in the I-V characteristics of the device which is in
essence due to the reduction of electron mobility. Recalling that mobility is
inversely proportional to electron effective mass or curvature of the energy
sub band. This effect was observed in semiconductors like GaAs which has direct
bandgap of very low effective mass and its second indirect sub band is about
300 meV above the former. More importantly a self-repeating oscillation of
spatially accumulated charge carriers along the transport direction occurs
which is the artifact of NDR, a process which is called Gunn oscillation and
was observed by J. B. Gunn. In sharp contrast to GaAs, bulk silicon has a very
high energy spacing (~1 eV) which renders the initiation of transfer-induced
NDR unobservable. Using Density Functional Theory (DFT), semi-empirical 10
orbital () Tight Binding (TB) method and Ensemble Monte Carlo
(EMC) simulations we show for the first time that (a) Gunn Effect can be
induced in narrow silicon nanowires with diameters of 3.1 nm under 3 % tensile
strain and an electric field of 5000 V/cm, (b) the onset of NDR in I-V
characteristics is reversibly adjustable by strain and (c) strain can modulate
the value of resistivity by a factor 2.3 for SiNWs of normal I-V
characteristics i.e. those without NDR. These observations are promising for
applications of SiNWs in electromechanical sensors and adjustable microwave
oscillators.Comment: 18 pages, 6 figures, 63 reference
Graphene spin circuits and spin-orbit phenomena in van der Waals heterostructures with topological insulators
Spintronics offers an alternative approach to conventional charge-based information processing by using the electron spin for next-generation non-volatile memory and logic technologies. To realize such technologies, it is necessary to develop spin-polarized current sources, spin interconnects, charge-to-spin conversion processes, and gate-tunable spintronic functionalities. The recently emerged two-dimensional (2D) and topological materials represent a promising platform to realize such spin-based phenomena. Due to its small spin-orbit coupling (SOC), graphene was predicted to preserve electron spin coherence for a long time, making it an ideal material for spin communication. In contrast, topological insulators (TIs) have high SOC and develop a nontrivial band structure with insulating bulk but conducting spin-polarized surface states. Combining these materials in van der Waals heterostructures has been predicted to give rise to unique proximity-induced spin-orbit phenomena that may be used for electrical control of spin polarization.In this thesis, we experimentally prove that the large-area chemical vapor deposited (CVD) graphene is an excellent material choice for the realization of robust spin interconnects, which are capable of spin communication over channel lengths exceeding 34 μm. Utilizing such graphene, we realize a spin summation operation in multiterminal devices and employ it to construct a prototype spin majority logic gate operating with pure spin currents. In topological insulators, we electrically detect the spin-momentum locking and reveal how the bulk and surface conducting channels affect the charge-to-spin conversion efficiency. Finally, by combining graphene and TIs in hybrid devices, we confirm the emergence of a strong proximity-induced SOC with a Rashba spin texture in graphene. We further show that in such heterostructures a spin-charge conversion capability is induced in graphene via the spin-galvanic effect at room temperature and reveal its strong tunability in magnitude and sign by the gate voltage. These findings demonstrate the robust performance of graphene as a spin interconnect for emerging spin-logic architectures and present all-electrical and gate-tunable spintronic devices based on graphene-TI heterostructures, paving the way for next-generation spin-based computing
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