Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies, but creation and placement are difficult to control. We describe here a novel paradigm for encoding strain into 2D materials to create and deterministically place SPEs in arbitrary locations with nanometer-scale precision using an atomic force microscope. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs. Because monolayer WSe2 is a direct gap semiconductor, SPE emission at a given wavelength is often intermixed with classical light, reducing the purity of the quantum emission. We show that this undesirable classical emission, arising primarily from defect bound excitonic processes, is significantly suppressed by electrostatic gating or incorporating the WSe2 layer in a simple van der Waals heterostructure, resulting in values of the autocorrelation function g(2)(t=0) as low as 0.07 at low temperature. In addition, the SPE intensity can be strongly modulated by changing the polarity of the gate bias, a feature of technological importance for practical applications.
Current-generated spin in topological insulators (TIs) has been shown to efficiently switch FM magnetization via spin-orbit torque (SOT) with much lower critical currently densities. However, TI bulk are often degenerately doped and can shunt current from the surface states. Here we demonstrate SOT switching from bulk-insulating Bi2Se3, obtained by growth on BiInSe/In2Se3 buffer layers by MBE, with significantly reduced critical current density than conventional “bulk-conducting” Bi2Se3. We further grew epitaxial In2Se3 tunnel barriers on Bi2Se3, and demonstrate its spin sensitivity, towards further minimize current shunting through the FM metal and overall power consumption for magnetization switching.
The spin mixing conductance is an important figure of merit for spin transport across an interface. This is a particularly important number for Spin Orbit Torque Magnetic Random Access Devices, where spin generated in one layer is used to provide the spin torque needed to flip the magnetization in an adjacent layer. Here the spins are generated in either an topological insulator (TI) or an heavy metal (HM). The overall efficiency of such a device depends on both the charge to spin conversion in the spin generation layer and the spin mixing conductance of the interface.
The unique electronic band structure in monolayer transition metal dichalcogenides (TMDs) provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL) and a high degree of circular polarization has been theoretically predicted for resonant excitation of TMDs such as MoS2, MoSe2, WS2 and WSe2, yet rarely observed experimentally. In fact, a wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer TMDs, although the reasons for the disparity are unclear. Here we investigate the room-temperature Pcirc in TMD monolayers synthesized via chemical vapor deposition. In each material system, a wide range of Pcirc and PL intensity values are observed. However, there is a pronounced inverse correlation between Pcirc and PL intensity, which we attribute to sample-dependent variations in the exciton radiative and non-radiative lifetime components. Samples that demonstrate weak PL emission and short exciton relaxation time exhibit a high degree of valley polarization. The short exciton lifetime results in a higher measured polarization by limiting opportunity for depolarizing scattering events. These findings clarify the disparities among previously reported values and suggest a means to engineer valley polarization via controlled introduction of defects and non-radiative recombination sites.
Topological insulators (TIs) exhibit topologically protected metallic surface states populated by massless Dirac fermions with spin-momentum locking – the carrier spin lies in-plane, locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization. Here we show direct electrical detection of this bias current induced spin polarization as a voltage measured on a ferromagnetic (FM) metal tunnel barrier surface contact [1]. The voltage measured at this contact is proportional to the projection of the TI spin polarization onto this axis, and similar data are obtained for two different FM contact structures, Fe/Al2O3 and Co/MgO/graphene. From measurements of the carrier type and sign of the spin voltage for n-Bi2Se3 and p-Sb2Te3, we show that transport measurements can be used to determine the chirality of the spin texture [2]. The chirality inverts as one crosses the Dirac point, so that the carrier spin-momentum locking follows a left-hand rule (clockwise chirality) when the Fermi level is above the Dirac point, and right-hand rule below (counter-clockwise chirality). These results demonstrate simple and direct electrical access to the TI Dirac surface state spin system, provide clear evidence for the spin-momentum locking and bias current-induced spin polarization, and enable utilization of these remarkable properties for future technological applications.
[1] C. H. Li, O. M. J. van ‘t Erve, J. T. Robinson, Y. Liu, L. Li , and B. T. Jonker, Nature Nanotech. 9, 218 (2014).
[2] C. H. Li, O. M. J. van ‘t Erve, Y. Y. Li, L. Li and B. T. Jonker, under review.
Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with many interesting properties capable of producing an all-surface material applicable to sensing, single-atom storage and other quantum-based technologies. Here we report on the optical control of single layers of MX2 such that the photoluminescence (PL) is solely from the trion state. After trion isolation, changes in the Raman spectra were observed: there is a decrease in the intensity of the out of plane mode and an enhancement of the 2LA mode. The effect is reversible, and our results suggest that the changes of the strength of a particular excitonic state are due to surface interactions with ambient environment. In addition, spatial non-uniformity is probed by studying variations of strain and the PL emission as a function of position on our sample. The boundaries of mechanically exfoliated MX2 as well as boundaries intentionally created via fs laser ablation were investigated. The edges exhibit significant Raman shifts as well as remarkably enhanced PL emission compared to their respective central area. Finally, we probe the degree of circular polarization of the emitted PL as a function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. This work was supported by the FP7-REGPOT-2012-2013-1, under grant agreement 316165.
Nonlinear magneto-plasmonics (NMP) describes systems where nonlinear optics, magnetics and plasmonics are all involved. NMP can be referred to as interdisciplinary studies at the intersection of Nonlinear Plasmonics (NP), Magneto- Plasmonics (MP), and nanoscience. In NMP systems, nanostructures are the bases, Surface Plasmons (SPs) work as catalyst due to strong field enhancement effects, and the nonlinear magneto-optical Kerr effect (nonlinear MOKE) plays an important role as a characterization method. Many new effects were discovered recently, which include enhanced magnetization-induced harmonic generation, controlled and enhanced magnetic contrast, magneto-chiral effect, correlation between giant magnetroresistance (GMR) and nonlinear MOKE, etc. We review the structures, experiments, findings, and the applications of NMP.
Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with some interesting properties. First, the low-dimensional hexagonal structure leads to two inequivalent K-points, K and K’, in the brillioun zone. Second, this valley index and spin are intrinsically coupled, and spin-dependent selection rules enable one to independently populate and interrogate a unique K valley with circularly polarized light. Here we probe the degree of circular polarization of the emitted photoluminescence as function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. Monolayer flakes of MoS2 and MoSe2 show a strong depolarization as the excitation energy is increased. However, WS2 maintains significant polarization for high excitation energies, even at room temperature when properly prepared. We discuss the behavior of the polarization in terms of various phonon assisted intervalley scattering processes. This work was supported by NRL and the NRL Nanoscience Institute
Modern thin film growth techniques have enabled the realization of low dimensional semiconductor heterostructures and hybrid metal/semiconductor structures with properties tailored for a variety of device applications. It has been empirically established, for example, that the net magnetic anisotropy exhibited by a simple metal epitaxial film on a semiconductor is strongly affected by the interactions at the metal/semiconductor interface, although the mechanisms have not been systematically addressed and remain an open issue. An understanding of these mechanisms is prerequisite to obtaining a fundamental description of anisotropy, and has significant implications for successful realization of spin sensitive device structures. We consider here how contributions to the net magnetic anisotropy arise from the formation of the interface and early stages of metal film growth. We suggest that these contributions originate in the initial metal adsorption sites and subsequent bond or site filling, and are strongly dependent on the semiconductor surface reconstruction. We consider specifically the cases of the epitaxial growth of Fe films on the various reconstructions of the ZnSe(001) and GaAs(001) surfaces, and attempt to interpret the net magnetic properties in light of the atomic structure of the film and interface.
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