Session V17: 2D Devices: Spin Transport, Spin Orbit Coupling

Optimizing Spin Generation in 2D Materials: Topological Insulators and Graphene

Novel two-dimensional electronic systems with Dirac-like dispersion present unique opportunities for spintronic applications. In this seminar I will discuss two specific examples. First we examine the potential of topological insulators as spin-source materials. Using a new spin-polarized tunneling method [1], giant charge-spin conversion efficiency in topological insulators is revealed, well exceeding that in conventional magnetic tunnel junctions.[2] Through a comparative study between Bi2Se3 and (Bi,Sb)2Te3, we verify the topological-surface-state origin of the observed giant spin signals and further extract the energy dependence of the effective spin polarization in Bi2Se3.[2] Next we explore the potential of interfacial exchange interaction in 2D materials for spin control and spin generation. Using graphene as a prototype, we demonstrate that its coupling to a model magnetic insulator (EuS) produces a substantial magnetic exchange field (\textgreater 14 T), which yields orders-of-magnitude enhancement in the spin signal originated from the Zeeman spin-Hall effect.[3] Furthermore, the strong exchange field lifts the spin degeneracy of graphene in the quantum Hall regime, which may lead to interesting spin-polarized edge transport and thus open up new application space for classical and quantum information processing. [1] Luqiao Liu, Ching-Tzu Chen, J. Z. Sun, Nature Physics 10, 561--566 (2014). [2] Luqiao Liu, A. Richardella, Ion Garate, Yu Zhu, N. Samarth, and Ching-Tzu Chen, Physical Review B 91, 235437 (2015). [3] Peng Wei, Sunwoo Lee, Florian Lemaitre, Lucas Pinel, Davide Cutaia , Wujoon Cha , Donald Heiman, James Hone, Jagadeesh S. Moodera, Ching-Tzu Chen, Giant Interfacial Exchange Field in a 2D Material/Magnetic-Insulator Heterostructure: Graphene/EuS, arXiv:1510.05920.   

A theoretical design of graphene-based spin field-effect transistors

The search for a feasible design of graphene-based materials for spintronics applications has been intensified in recent years. Encouraged by recent experimental achievements, here we propose a new scheme to realize graphene-based spin field-effect transistors. The new design is constituted of a half-hydrogenated graphene nanoroad embedded in a fully-hydrogenated graphene. Using first-principles density function theory calculations, we demonstrate that such a design can convert non-magnetic pristine graphene into a bipolar ferromagnetic semiconductor. More importantly, the magnetism of such a nanoroad is very robust: independent of its width and orientation. We also discuss the stability of such nanoroads, as well as a simple design of an all-electric controlled device for generation and detection of a fully spin-polarized electric current.   

Giant Rashba spin splitting in Bi bilayer induced by a 2D ferroelectric substrate

Based on density functional theory calculations, we discover that a Bi layer when placed on the top of a recently predicted 2D ferroelectric material with spontaneous out-of-plane electric polarization can exhibit giant Rashba-type spin splitting of over 200 meV, while the whole system still remains semiconducting. In addition, the magnitude of the Rashba spin splitting can be tuned by switching the diploe orientation of the 2D ferroelectric substrate. This finding provides a promising 2D material system for spintronics.   

Spin-orbit interactions in two-dimensional holes in quasi-triangular wells: variational calculations

Spin-orbit (SO) interactions in semiconductors are key to the realization of semiconductor spintronic devices and quantum information processing. Low-dimensional holes are strongly SO-coupled systems, as such, they offer the promise of all-electrical spin control which can lead to more efficient electronic devices. However the spin properties of holes are highly complex, and heavily influencd by the nature of the confining potential. So far, calculations on two-dimensional holes in semiconductor heterojunctions have mostly been numerical and material-specific. In this work, we develop variational-based methods, which are easy to use and applicable to various materials, to quantify SO interactions in two-dimensional holes confined in self-consistent quasi-triangular wells. In particular, we calculate the SO hole spin-splittings and effective masses in common semiconductor materials such as GaAs, Ge, InSb, InAs, and Si. Our results show that the strength of SO interactions is very sensitive to the material type and that in zincblende materials with a bulk inversion asymmetry (BIA), the dominant contribution to the SO interaction is still the structure inversion asymmetry (SIA) term corresponding to the confinement potential.   

Kondo physics in the presence of Rashba spin-orbit interactions

Recent theoretical studies have shown that Rashba spin-orbit interactions in a two-dimensional electron gas (2DEG) affect the thermodynamics of the impurity Kondo effect only through changes in the host density of states [1]. These changes are generally modest [1], but yield exponential enhancement of the Kondo temperature $T_K$ [2] if the 2DEG can be tuned to a helical regime in which all electrons at the Fermi surface have the same relation between the directions of their spin and momentum. It has been proposed to access the helical regime using irradiation with circularly polarized light, giving rise to an effective Zeeman splitting of the conduction band without any direct splitting of the impurity level. We show that under this scenario, the impurity contribution to the system's net angular momentum is a universal function of the Zeeman energy divided by a temperature scale that (surprisingly at first sight) is not $T_K$, but rather is proportional to $T_K$ divided by the impurity hybridization width. This universal scaling can be understood via a perturbative treatment of irradiation-induced changes in the electron densities of states. [1] R. Zitko and J. Bonca, Phys. Rev. B 84, 193411 (2011). [2] A. Wong, S. E. Ulloa, N. Sandler, and K. Ingersent, arXiv:1509.08433.   

Measurement of Spin Torques in WTe2/Ferromagnet Bilayers

WTe$_2$ is a semimetallic transition metal dichalcogenide (TMD) stable in the $T_d$ crystal structure. The strong spin-orbit coupling, metallic conduction, and crystalline layered structure of the material make it interesting for both fundamental and applied spintronics research, but measurements of the spin transport properties (e.g., the spin Hall conductivity) are lacking. Here we report measurements of current induced spin torques in WTe$_2$/Ferromagnet bilayers, detected using spin torque ferromagnetic resonance. We will attempt to distinguish whether these torques arise from interfacial spin-orbit coupling or the spin Hall effect in the TMD. We study these torques as a function of TMD layer number, from bulk to few-layer, and correlate our results with layer-number dependent charge transport measurements.   

Spin relaxation in hole-doped transition metal dichalcogenides with the crystal defects

We theoretically investigate the electronic spin relaxation effect in the hole-doped monolayer and bilayer transition-metal dichalcogenides in the presence of the crystal defects. We simulate lattice vacancies in the multi-orbital tight-binding model obtained by the first-principle method and actually estimate the spin relaxation rate by using the tight-binding model. In the monolayer, the spin-relaxation time is found to be much longer than the momentum relaxation time, and this is attributed to the fact that the spin hybridization in the band structure is suppressed by the mirror reflection symmetry. The bilayer TMD has a much shorter spin relaxation time in contrast because of the stronger spin hybridization due to the absence of the mirror symmetry.   

Induced spin orbit coupling in graphene by proximity to transition metal dichalcogenides monolayer

Proximity effects resulting from depositing a graphene layer on a substrate may induce spin depend interactions that change the topological properties of graphene. A suitable candidate to study this effect is a transition metal dichalcogenides substrate. A 2D layer of these materials has a large SOC that in turn induces a sizable effect near the graphene Dirac points. Graphene and 2D TMDs are nearly commensurate lattices, producing an interesting moir\'{e} pattern when adhered to one another. We study theoretically the electronic structure of graphene-TMD systems using a tight binding formalism. We find that graphene exhibits a strong proximity SOC, in addition to other perturbations that strongly affect the states; the linear dispersion near the neutrality point becomes gapped. Based on symmetries allowed by the heterostructure, we find the effective Hamiltonian to describe the low energy states. We find that diagonal SOC and staggered potential terms characterize the wave functions, akin to the structure in TMDs. A relative voltage between the layers enhances the proximity SOC in graphene, providing a tunable effect that may impact the optoelectronic properties of the system.   

Electronic transport in the quantum spin Hall state due to the presence of adatoms in graphene

Heavy adatoms, even at low concentrations, are predicted to turn a graphene sheet into a topological insulator with substantial gap. The adatoms mediate the spin-orbit coupling that is fundamental to the quantum spin Hall effect. The adatoms act as local spin-orbit scatterer inducing hopping processes between distant carbon atoms giving origin to transverse spin currents. Although there are effective models that describe spectral properties of such systems with great detail, quantitative theoretical work for the transport counterpart is still lacking. We developed a multiprobe recursive Green's function technique with spin resolution to analyze the transport properties for large geometries. We use an effective tight-binding Hamiltonian to describe the problem of adatoms randomly placed at the center of the honeycomb hexagons, which is the case for most transition metals. Our choice of current and voltage probes is favorable to experiments since it filters the contribution of only one spin orientation, leading to a quantized spin Hall conductance of $e^2/h$. We also discuss the electronic propagation in the system by imaging the local density of states and the electronic current densities.   

Current-Controlled Spin Flip in Magnetically-Substituted Graphene Nanoribbons: Toward the Realization of Graphene-Based Spintronic Devices

We examine the possibility of using graphene nanoribbons with directly substituted chromium atoms as spintronic device. Using density functional theory, we simulate a voltage bias across a constructed graphene nanoribbon in a device setup, where a magnetic dimer has been substituted into the lattice. Using a first principles approach, we calculate the electronic and magnetic properties as a function of Hubbard U, voltage, and magnetic configuration. Through a calculation of the energy of each magnetic configuration, we can determine that initial antiferromagnetic ground state flips to a ferromagnetic state with applied bias. Mapping this transition point to the calculated conductance for the system reveals that there is a distinct change in conductance through the graphene nanoribbon, which indicates the possibility of a spin valve. We also show that this corresponds to a distinct change in the induced magnetization within the graphene. Our goal is to show that graphene, while already being used in electronic, may also have spintronic capabilities as well.   

The spin orbit coupling and magnetization in graphene$\backslash$YIG and WTe2$\backslash$graphene$\backslash$YIG

Quantum anomalous Hall effect (QAHE) may occur in graphene if there are both exchange field and Rashba spin-orbit coupling (SOC). Since pristine graphene is not magnetic and has extremely weak SOC, these two ingredients need to be induced externally through the proximity effect or electric field. Recently experiment found the anomalous Hall effect in graphene when it is supported on yttrium-iron-garnet (YIG), indicating the proximity-induced spin polarization in graphene. However QAHE has not been observed due to small Rashba SOC. In this work, we explore the means that may lead to strong enhancement of Rashba SOC, through first-principles calculations for graphene$\backslash$YIG and WTe2$\backslash$graphene$\backslash$YIG. We find that the Rashba SOC strength is only 1.1 meV for graphene on YIG, whereas the exchange splitting is sizeable, 15 meV. The coverage of a WTe2 layer on graphene$\backslash$YIG enhances the Rashba SOC but lowers the magnetization. The presence of electric field may offer a balance between these two quantities and the physical origins will be discussed.   

Measurement of current-generated torques in transition metal dichalcogenide / ferromagnet bilayers

We present measurements of current-generated torques in ferromagnet / transition metal dichalcogenide (TMD) bilayers for a wide range of semi-conducting TMDs, including MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$. TMDs present a unique opportunity to study interfacial spin-orbit torques at the two dimensional limit due to a wide range in material properties and large spin-orbit coupling. Thin TMD films are either grown by chemical vapor deposition or exfoliated from readily available TMD crystals and are incorporated into ferromagnet / TMD bilayers by either evaporation or off-axis sputtering of the ferromagnet to avoid damage to the TMD surface. Measurements of the current-generated torque are made by spin transfer ferromagnetic resonance and the magneto-optical Kerr effect. Dependence on layer number, spin-orbit coupling strength, mobility and gate dependence will be explored.