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Register a new userGraphene-WS$_2$ heterostructures for tunable spin injection and spin transport
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We report the first measurements of spin injection in to graphene through a 20 nm thick tungsten disulphide (WS$_2$) layer, along with a modified spin relaxation time ({tau}s) in graphene in the WS$_2$ environment, via spin-valve and Hanle spin-precession measurements, respectively. First, during the spin-injection into graphene through a WS$_2$-graphene interface, we can tune the interface resistance at different current bias and modify the spin injection efficiency, in a correlation with the conductivity-mismatch theory. Temperature assisted tunneling is identified as a dominant mechanism for the charge transport across the interface. Second, we measure the spin transport in graphene, underneath the WS$_2$ crystal and observe a significant reduction in the {tau}s down to 17 ps in graphene in the WS$_2$ covered region, compared to that in its pristine state. The reduced {tau}s indicates the WS$_2$-proximity induced additional dephasing of the spins in graphene.
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We study spin-transport in bilayer-graphene (BLG), spin-orbit coupled to a tungsten di sulfide (WS$_2$) substrate, and measure a record spin lifetime anisotropy ~40-70, i.e. ratio between the out-of-plane $tau_{perp}$ and in-plane spin relaxation time $tau_{||}$. We control the injection and detection of in-plane and out-of-plane spins via the shape-anisotropy of the ferromagnetic electrodes. We estimate $tau_{perp}$ ~ 1-2 ns via Hanle measurements at high perpendicular magnetic fields and via a new tool we develop: Oblique Spin Valve measurements. Using Hanle spin-precession experiments we find a low $tau_{||}$ ~ 30 ps in the electron-doped regime which only weakly depends on the carrier density in the BLG and conductivity of the underlying WS$_2$, indicating proximity-induced spin-orbit coupling (SOC) in the BLG. Such high $tau_{perp}$ and spin lifetime anisotropy are clear signatures of strong spin-valley coupling for out-of-plane spins in BLG/WS$_2$ systems in the presence of SOC, and unlock the potential of BLG/transition metal dichalcogenide heterostructures for developing future spintronic applications.
Graphene supported on a transition metal dichalcogenide substrate offers a novel platform to study the spin transport in graphene in presence of a substrate induced spin-orbit coupling, while preserving its intrinsic charge transport properties. We report the first non-local spin transport measurements in graphene completely supported on a 3.5 nm thick tungsten disulfide (WS$_2$) substrate, and encapsulated from the top with a 8 nm thick hexagonal boron nitride layer. For graphene, having mobility up to 16,000 cm$^2$V$^{-1}$s$^{-1}$, we measure almost constant spin-signals both in electron and hole-doped regimes, independent of the conducting state of the underlying WS$_2$ substrate, which rules out the role of spin-absorption by WS$_2$. The spin-relaxation time $tau_{text{s}}$ for the electrons in graphene-on-WS$_2$ is drastically reduced down to~10 ps than $tau_{text{s}}$ ~ 800 ps in graphene-on-SiO$_2$ on the same chip. The strong suppression of $tau_{text{s}}$ along with a detectable weak anti-localization signature in the quantum magneto-resistance measurements is a clear effect of the WS$_2$ induced spin-orbit coupling (SOC) in graphene. Via the top-gate voltage application in the encapsulated region, we modulate the electric field by 1 V/nm, changing $tau_{text{s}}$ almost by a factor of four which suggests the electric-field control of the in-plane Rashba SOC. Further, via carrier-density dependence of $tau_{text{s}}$ we also identify the fingerprints of the Dyakonov-Perel type mechanism in the hole-doped regime at the graphene-WS$_2$ interface.
Spin-orbit coupling in graphene can be increased far beyond its intrinsic value by proximity coupling to a transition metal dichalcogenide. In bilayer graphene, this effect was predicted to depend on the occupancy of both graphene layers, rendering it gate-tunable by an out-of-plane electric field. We experimentally confirm this prediction by studying magnetotransport in a dual-gated WSe$_2$/bilayer graphene heterostructure. Weak antilocalization, which is characteristic for phase-coherent transport in diffusive samples with spin-orbit interaction, can be strongly enhanced or suppressed at constant carrier density, depending on the polarity of the electric field. From the spin-orbit scattering times extracted from the fits, we calculate the corresponding Rashba and intrinsic spin-orbit parameters. They show a strong dependence on the transverse electric field, which is well described by a gate-dependent layer polarization of bilayer graphene.
In this review we discuss spin and charge transport properties in graphene-based single-layer and few-layer spin-valve devices. We give an overview of challenges and recent advances in the field of device fabrication and discuss two of our fabrication methods in more detail which result in distinctly different device performances. In the first class of devices, Co/MgO electrodes are directly deposited onto graphene which results in rough MgO-to-Co interfaces and favor the formation of conducting pinholes throughout the MgO layer. We show that the contact resistance area product (R$_c$A) is a benchmark for spin transport properties as it scales with the measured spin lifetime in these devices indicating that contact-induced spin dephasing is the bottleneck for spin transport even in devices with large R$_c$A values. In a second class of devices, Co/MgO electrodes are first patterned onto a silicon substrate. Subsequently, a graphene-hBN heterostructure is directly transferred onto these prepatterned electrodes which provides improved interface properties. This is seen by a strong enhancement of both charge and spin transport properties yielding charge carrier mobilities exceeding 20000 cm$^2$/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss several shortcomings in the determination of both quantities which complicates the analysis of both extrinsic and intrinsic spin scattering mechanisms. Furthermore, we show that contacts can be the origin of a second charge neutrality point in gate dependent resistance measurements which is influenced by the quantum capacitance of the underlying graphene layer.
In the framework of first-principles calculations, we investigate the structural and electronic properties of graphene in contact with as well as sandwiched between WS$_2$ and WSe$_2$ monolayers. We report the modification of the band characteristics due to the interaction at the interface and demonstrate that the presence of the dichalcogenides results in quantum spin Hall states in the absence of a magnetic field.
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