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Register a new userElectrical Spin Pumping of Quantum Dots at Room Temperature
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We report electrical control of the spin polarization of InAs/GaAs self-assembled quantum dots (QDs) at room temperature. This is achieved by electrical injection of spin-polarized electrons from an Fe Schottky contact. The circular polarization of the QD electroluminescence shows that a 5% electron spin polarization is obtained in the InAs QDs at 300 K, which is remarkably insensitive to temperature. This is attributed to suppression of the spin relaxation mechanisms in the QDs due to reduced dimensionality. These results demonstrate that practical regimes of spin-based operation are clearly attainable in solid state semiconductor devices.
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To understand and optimize optical spin initialization in room temperature CdSe nanocrystal quantum dots (NCQDs) we studied the dependence of the time-resolved Faraday rotation signal on pump energy $E_p$ in a series of NCQD samples with different sizes. In larger NCQDs, we observe two peaks in the spin signal vs. $E_p$, whereas in smaller NQCDs, only a single peak is observed before the signal falls to a low, broad plateau at higher energies. We calculate the spin-dependent oscillator strengths of optical transitions using a simple effective mass model to understand these results. The observed $E_p$ dependence of the spin pumping efficiency (SPE) arises from the competition between the heavy hole (hh), light hole (lh) and split-off (so) band contributions to transitions to the conduction band. The two latter contributions lead to an electron spin polarization in the opposite direction from the former. At lower $E_p$ the transitions are dominated by the hh band, giving rise to the low energy peaks. At higher $E_p$, the increasing contributions from the lh and so bands lead to a reduction in SPE. The different number of peaks in larger and smaller NCQDs is attributed to size-dependence of the ordering of the valence band states.
We inject spin-polarized electrons from an Fe/MgO tunnel barrier contact into n-type Ge(001) substrates with electron densities 2e16 < n < 8e17 cm-3, and electrically detect the resulting spin accumulation using three-terminal Hanle measurements. We observe significant spin accumulation in the Ge up to room temperature. We observe precessional dephasing of the spin accumulation (the Hanle effect) in an applied magnetic field for both forward and reverse bias (spin extraction and injection), and determine spin lifetimes and corresponding diffusion lengths for temperatures of 225 K to 300 K. The room temperature spin lifetime increases from {tau}s = 50 ps to 123 ps with decreasing electron concentration, as expected from electron spin resonance work on bulk Ge. The measured spin resistance-area product is in good agreement with values predicted by theory for samples with carrier densities below the metal-insulator transition (MIT), but 100x larger for samples above the MIT. These data demonstrate that the spin accumulation measured occurs in the Ge, although dopant-derived interface or band states may enhance the measured spin voltage above the MIT. We estimate the polarization in the Ge to be on the order of 1%.
Non-local carrier injection/detection schemes lie at the very foundation of information manipulation in integrated systems. This paradigm consists in controlling with an external signal the channel where charge carriers flow between a source and a well separated drain. The next generation electronics may operate on the spin of carriers instead of their charge and germanium appears as the best hosting material to develop such a platform for its compatibility with mainstream silicon technology and the long electron spin lifetime at room temperature. Moreover, the energy proximity between the direct and indirect bandgaps allows for optical spin injection and detection within the telecommunication window. In this letter, we demonstrate injection of pure spin currents (textit{i.e.} with no associated transport of electric charges) in germanium, combined with non-local spin detection blocks at room temperature. Spin injection is performed either electrically through a magnetic tunnel junction (MTJ) or optically, exploiting the ability of lithographed nanostructures to manipulate the distribution of circularly-polarized light in the semiconductor. Pure spin current detection is achieved using either a MTJ or the inverse spin-Hall effect (ISHE) across a platinum stripe. These results broaden the palette of tools available for the realization of opto-spintronic devices.
To mitigate climate change, our global society is harnessing direct (solar irradiation) and indirect (wind/water flow) sources of renewable electrical power generation. Emerging direct sources include current-producing thermal gradients in thermoelectric materials, while quantum physics-driven processes to convert quantum information into energy have been demonstrated at very low temperatures. The magnetic state of matter, assembled by ordering the electrons quantum spin property, represents a sizeable source of built-in energy. We propose to create a direct source of electrical power at room temperature (RT) by utilizing magnetic energy to harvest thermal fluctuations on paramagnetic (PM) centers. Our spin engine rectifies current fluctuations across the PM centers spin states according to the electron spin by utilizing so-called spinterfaces with high spin polarization. As a rare experimental event, we demonstrate how this path can generate 0.1nW at room temperature across a 20 micron-wide spintronic device called the magnetic tunnel junction, assembled using commonplace Co, C and MgO materials. The presence of this path in our experiment, which also generates very high spintronic performance, is confirmed by analytical and ab-initio calculations. Device downscaling, and the ability for other materials systems than the spinterface to select a transport spin channel at RT widens opportunities for routine device reproduction. The challenging control over PM centers within the tunnel barriers nanotransport path may be addressed using oxide- and organic-based nanojunctions. At present densities in MRAM products, this spin engine could lead to always-on areal power densities well beyond that generated by solar irradiation on earth. Further developing this concept can fundamentally alter our energy-driven societys global economic, social and geopolitical constructs.
Disordered films have gained intense interest because of their possibility for spintronics applications by benefiting from other exotic transport properties. Here, we have fabricated disordered Gd-alloyed Bi_x Se_(1-x) (BSG) thin films by magnetron sputtering methods and have investigated their magneto-transport and spin-torque properties. Structural characterizations show a mainly amorphous feature for the 8nm thick BSG film, while Bi rich crystallites are developed inside the 16nm thick BSG film. The bulk resistivity of BSG film is found to be relatively high, up to 6x10^4 uOhm.cm, with respect to the resistivity of the polycrystalline Bi_x Se_(1-x) film. Temperature dependent resistivity measurements display the evident character of a variable range hopping transport from 80K to 300K. Spin pumping transport characterizations have been performed on the BSG(t)/CoFeB(5 nm) bilayer structures with different thickness of BSG (t= 6, 8, 12, 16 nm). The possible various origins of the spin-to-charge conversion are related to extrinsic effects. Our study provides a new experimental direction, beyond crystalline solids, to the search for strong SOC systems in amorphous solids and other engineered random systems.
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