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Register a new userCritical Collapse of the Exchange Enhanced Spin Splitting in 2-D Systems
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D.R. Leadley
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The critical filling factor v_c where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of v_c at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange energy, leading to the empirical relationship v_c = g* n t h / 2 m_0. This is valid for all the samples studied, where the density n and single particle lifetime t both vary by more than an order of magnitude and g* the exchange enhanced g-factor has a weak dependence on density. For each sample the spin gap energy shows a linear increase with magnetic field. Experiments in tilted magnetic field show the spin gap is the sum of the bare Zeeman energy and an exchange term. This explains why measurements of the enhanced g-factor from activation energy studies in perpendicular field and the coincidence method in tilted fields have previously disagreed.
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Electron exchange and correlations emerging from the coupling between ionic vibrations and electrons are addressed. Spin-dependent electron-phonon coupling originates from the spin-orbit interaction, and it is shown that such electron-phonon coupling introduces exchange splitting between the spin channels in the structure. By application of these results to a model for a chiral molecular structure mounted between metallic leads, the chirality induced spin selectivity is found to become several tens of percents using experimentally feasible parameters.
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Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition metal dichalcogenide (TMDC) with broken inversion symmetry possesses two degenerate yet inequivalent valleys, offering unique opportunities for valley control through helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest, (~ 0.2 meV/T). Here we show greatly enhanced valley spitting in monolayer WSe2, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magneto-reflectance measurements. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing MEF of a magnetic insulator can induce magnetic order, and valley and spin polarization in TMDCs, which may enable valleytronic and quantum computing applications.
The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy and density functional theory. Our spin-resolved data reveals that, while the macroscopic magnetization of the sample vanishes at the Curie temperature $T_C$, the experimentally-determined exchange splitting of the O 2$p$ band persists at least up to $T_{C}$ if the picture of fluctuating spin-blocks is assumed. We discuss possible temperature-related spectral changes by analyzing ferromagnetic and antiferromagnetic phases, directional effect due to spin orbit coupling, as well as effects due to sample aging. Our calculations with a Hubbard $U$ term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands, shining a new light on the interpretation of previous optical and photoemission spectroscopic results.
The ratio of the Zeeman splitting to the cyclotron energy ($M=Delta E_Z / hbar omega_c$), which characterizes the relative strength of the spin-orbit interaction in crystals, is examined for the narrow gap IV-VI semiconductors PbTe, SnTe, and their alloy Pb$_{1-x}$Sn$_x$Te on the basis of the multiband $kcdot p$ theory. The inverse mass $alpha$, the g-factor $g$, and $M$ are calculated numerically by employing the relativistic empirical tight-binding band calculation. On the other hand, a simple but exact formula of $M$ is obtained for the six-band model based on the group theoretical analysis. It is shown that $M<1$ for PbTe and $M>1$ for SnTe, which are interpreted in terms of the relevance of the interband couplings due to the crystalline spin-orbit interaction. It is clarified both analytically and numerically that $M=1$ just at the band inversion point, where the transition from trivial to nontrivial topological crystalline insulator occurs. By using this property, one can detect the transition point only with the bulk measurements. It is also proposed that $M$ is useful to evaluate quantitatively a degree of the Dirac electrons in solids.
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