Controllable spin and charge transport in two-dimensional monochalcogenide semiconductors for spintronic devices

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In addition to their charge, electrons have an intrinsic angular momentum, called 'spin', that is coupled to a magnetic moment. Manipulation of electron spin orientation in polarized ensembles provides a basis for new logic devices and circuits with potential advantages over present-day charge-based designs. It is widely believed that whenever spin encodes logic state, semiconductor materials with the longest spin lifetime are the most suitable choice for transport channels between injection and detection contacts. However, once a logic operation is completed, residual spins can and will interfere with future operations. If a device can be designed with a controllable spin lifetime such that injected spins do not relax during transport in a logic operation but upon completion vanish from the channel by an induced fast equilibration mechanism, it will have a significant advantage over the current spin logic technology. The innovation of the proposed research lies in making use of semiconductor materials having anisotropic spin relaxation. The impact of the proposed activity is in the potential of spin-based semiconductor devices and circuits to circumvent the fundamental limitations of charge-based electronics. In addition, graduate students will be trained in interdisciplinary research involving semiconductor device design, processing, measurement, magnetism, low-temperature techniques; and outreach activities will extend to broaden understanding of 2-dimensional semiconductors to elementary and high-school students, parents, and the general public by developing demonstrations of the scientific contents of this research.Spins are initially aligned parallel or antiparallel to the quantization axis at injection. After the completion of a logic operation, spin transpors to other parts of the device and a clocked voltage pulse on an electrostatic gate generates an electric field that induces a Bychkov-Rashba effective magnetic field. This magnetic field is non-colinear to the spin axis and thus coherently rotates the spins onto an orthogonal axis where strong relaxation eliminates their polarization. The logic environment is then reset. Through theoretical symmetry analysis it has been determined, several two-dimensional materials that are based on graphene-like honeycomb lattice have the requisite anisotropic spin-orbit properties. The most promising candidate material system is the group-III metal-monochalcogenide monolayers, called III-VI-ene ('three-six-ene'). In these inversion asymmetric materials (such as GaSe, InS, etc), the spin-orbit-induced wave-vector- dependent Dresselhaus effective magnetic field is oriented perpendicular to the plane, making spin up/down the natural eigenstates, immune to Dyakonov-Perel relaxation via precessional dephasing upon momentum scattering. To control spin lifetime, a Bychkov-Rashba field resulting from electrostatic gating that is always oriented in-plane can be used to rotate spins toward the plane so that precessional dephasing leads to efficient depolarization. This proposal deals to exploit the unique properties of three-six-enes (specifically, exfoliated GaSe) that intimately combines the symmetry-based theoretical understanding of the fundamental electronic structure in these materials with an experimental plan to demonstrate the predicted anisotropic relaxation effects in spin- and charge-transport devices. The focus is on understanding the experimental consequences of complex electronic structure and develop a scheme for implementation of the unique properties of three-six-enes in spin-enabled circuits for engineering applications.