Modeling spin transport in electrostatically-gated lateral-channel silicon devices: role of interfacial spin relaxation

Using a two-dimensional finite-differences scheme to model spin transport in silicon devices with lateral geometry, we simulate the effects of spin relaxation at interfacial boundaries, i.e. the exposed top surface and at an electrostatically-controlled backgate with SiO_2 dielectric. These gate-voltage-dependent simulations are compared to previous experimental results and show that strong spin relaxation due to extrinsic effects yield an Si/SiO_2 interfacial spin lifetime of ~ 1ns, orders of magnitude lower than lifetimes in the bulk Si, whereas relaxation at the top surface plays no substantial role. Hall effect measurements on ballistically injected electrons gated in the transport channel yield the carrier mobility directly and suggest that this reduction in spin lifetime is only partially due to enhanced interfacial momentum scattering which induces random spin flips as in the Elliott effect. Therefore, other extrinsic mechanisms such as those caused by paramagnetic defects should also be considered in order to explain the dramatic enhancement in spin relaxation at the gate interface over bulk values

PERGESERAN NEGARA HUKUM KE ‘NEGARA HIMBAUAN’: MENAKAR DAMPAK REGULASI PENANGANAN COVID-19 TERHADAP PEREKONOMIAN DAN KEUANGAN INDONESIA

AbstractThis article discusses the shifting phenomenon from rule of law to ‘rule of persuasion’ by analysing regulations concerning Covid-19 mitigation through large-scale social restrictions (PSBB) and their impact on Indonesian economy and financial sector. Analysis of PSBB regulations shows that the regulations do not have criminal and law enforcement provisions that could lead legal uncertainty. PSBB regulations are simply a persuasion model. This has led to the shifting from rule of law to ‘law rule of persuasion’. As a result, on one hand, law enforcement related to PSBB regulations would not be effective and could make the Covid-19 pandemic prolonged, and on the other hand, legal uncertainty itself as well as the Covid-19 pandemic would have serious implications for Indonesian economy and financial sector. It is recommended that in the future, any regulations, especially at the level of statutes (acts), should seriously consider the establishment of legal certainty through criminal provisions and law enforcement and anticipate properly the impact such regulations and Covid-19 on Indonesian economy and financial sector.Keywords: legal uncertainty; rule of law; ‘rule of persuasion’AbstrakArtikel ini membahas fenomena pergeseran negara ke ‘negara himbauan’ dengan melakukan analisis terhadap regulasi terkait penangangan Covid-19 melalui pembatasan sosial berskala besar (PSBB) dan dampaknya terhadap keuangan dan perekonomian Indonesia. Analisis terhadap regulasi PSBB menunjukan bahwa regulasi tidak memiliki ketentuan pidana dan aspek penegakan hukumnya yang dapat menimbulkan ketidakpastian hukum. Regulasi PSBB sekedar merupakan model himbauan yang telah menegaskan adanya fenomena pergeseran dari negara hukum ke ‘negara himbauan’. Akibatnya, di satu sisi, penegakan hukum terhadap regulasi PSBB tidak akan berjalan efektif dan bisa membuat pandemi Covid-19 berkepanjangan, dan di sisi lainnya, ketidakpastian hukum sebagaimana juga pandemic Covid-19 akan memiliki implikasi serius bagi perekonomian dan keuangan Indonesia. Direkomendasikan agar kedepannya, regulasi apapun, utamanya di tingkat undang-undang, harus secara sungguh memperhatikan aspek kepastian hukum melalui pengaturan dalam ketentuan pidana dan penegakan hukumnya dan mengantisipasi secara tepat dampak regulasi dan Covid-19 bagi perekonomian dan keuangan Indonesia. Kata kunci: ketidakpastian hukum; negara hukum; ‘negara himbauan’

Spin relaxation and decoherence of two-level systems

We revisit the concepts of spin relaxation and spin decoherence of two level (spin-1/2) systems. From two toy-models, we clarify two issues related to the spin relaxation and decoherence: 1) For an ensemble of two-level particles each subjected to a different environmental field, there exists an ensemble relaxation time $T_1^*$ which is fundamentally different from $T_1$. When the off-diagonal coupling of each particle is in a single mode with the same frequency but a random coupling strength, we show that $T_1^*$ is finite while the spin relaxation time of a single spin $T_1$ and the usual ensemble decoherence time $T_2^*$ are infinite. 2) For a two-level particle under only a random diagonal coupling, its relaxation time $T_1$ shall be infinite but its decoherence time $T_2$ is finite.Comment: 5 pages, 2 figure

The Larmor clock and anomalous spin dephasing in silicon

Drift-diffusion theory - which fully describes charge transport in semiconductors - is also universally used to model transport of spin-polarized electrons in the presence of longitudinal electric fields. By transforming spin transit time into spin orientation with precession (a technique called the "Larmor clock") in current-sensing vertical-transport intrinsic Si devices, we show that spin diffusion (and concomitant spin dephasing) can be greatly enhanced with respect to charge diffusion, in direct contrast to predictions of spin Coulomb-drag diffusion suppression.Comment: minor edits and updated ref

Field-induced negative differential spin lifetime in silicon

We show that the electric field-induced thermal asymmetry between the electron and lattice systems in pure silicon substantially impacts the identity of the dominant spin relaxation mechanism. Comparison of empirical results from long-distance spin transport devices with detailed Monte-Carlo simulations confirms a strong spin depolarization beyond what is expected from the standard Elliott-Yafet theory already at low temperatures. The enhanced spin-flip mechanism is attributed to phonon emission processes during which electrons are scattered between conduction band valleys that reside on different crystal axes. This leads to anomalous behavior, where (beyond a critical field) reduction of the transit time between spin-injector and spin-detector is accompanied by a counterintuitive reduction in spin polarization and an apparent negative spin lifetime

Spin flip scattering at Al surfaces

Non-local measurements are performed on a multi terminal device to $in-situ$ determine the spin diffusion length and in combination with resistivity measurements also the spin relaxation time in Al films. By varying the thickness of Al we determine the contribution to spin relaxation from surface scattering. From the temperature dependence of the spin diffusion length it is established that the spin relaxation is impurity dominated at low temperature. A comparison of the spin and momentum relaxation lengths for different thicknesses reveals that the spin flip scattering at the surfaces is weak compared to that within the bulk of the Al films.Comment: 11 pages, 5 figure

Ab initio investigation of Elliott-Yafet electron-phonon mechanism in laser-induced ultrafast demagnetization

The spin-flip (SF) Eliashberg function is calculated from first-principles for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization. This is used to compute the SF probability and demagnetization rate for laser-created thermalized as well as non-equilibrium electron distributions. Increased SF probabilities are found for thermalized electrons, but the induced demagnetization rate is extremely small. A larger demagnetization rate is obtained for {non-equilibrium} electron distributions, but its contribution is too small to account for femtosecond demagnetization.Comment: 5 pages, 3 figures, to appear in PR

The dominant spin relaxation mechanism in compound organic semiconductors

Despite the recent interest in "organic spintronics", the dominant spin relaxation mechanism of electrons or holes in an organic compound semiconductor has not been conclusively identified. There have been sporadic suggestions that it might be hyperfine interaction caused by background nuclear spins, but no confirmatory evidence to support this has ever been presented. Here, we report the electric-field dependence of the spin diffusion length in an organic spin-valve structure consisting of an Alq3 spacer layer, and argue that this data, as well as available data on the temperature dependence of this length, contradict the notion that hyperfine interactions relax spin. Instead, they suggest that the Elliott-Yafet mechanism, arising from spin-orbit interaction, is more likely the dominant spin relaxing mechanism.Comment: Accepted for publication in Physical Review

Chiral orbital current and anomalous magnetic moment in gapped graphene

We present a low-energy effective-mass theory to describe chiral orbital current and anomalous magnetic moment in graphenes with band gap and related materials. We show that a Bloch electron generally contains an anomalous current density due to inter-band matrix elements, which describes a chiral current circulation associated with magnetic moment. In gapped graphenes, the chiral current is opposite between different valleys, and corresponding magnetic moment accounts for valley splitting of Landau levels. In gapped bilayer graphene, in particular, the valley-dependent magnetic moment is responsible for divergence of paramagnetic susceptibility at the band bottom, and full valley polarization is achieved in relatively small magnetic field range. The formulation also applies to the gapped surface states of three-dimensional topological insulator, where the anomalous current is related to the magneto-electric response in spatially-modulated potential.Comment: 10 pages, 3 figure

Impurity induced spin-orbit coupling in graphene

We study the effect of impurities in inducing spin-orbit coupling in graphene. We show that the sp3 distortion induced by an impurity can lead to a large increase in the spin-orbit coupling with a value comparable to the one found in diamond and other zinc-blende semiconductors. The spin-flip scattering produced by the impurity leads to spin scattering lengths of the order found in recent experiments. Our results indicate that the spin-orbit coupling can be controlled via the impurity coverage.Comment: 4 pages, 6 figure