The isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material, as well as graphene-based spintronic devices Here we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other 2D materials

Graphene Spintronics

The CHIANTI atomic database contains atomic energy levels, wavelengths, radiative transition probabilities, and collisional excitation data for a large number of ions of astrophysical interest. CHIANTI also includes a suite of IDL routines to calculate synthetic spectra and carry out plasma diagnostics. Version 5 has been released, which includes several new features, as well as new data for many ions. The new features in CHIANTI are as follows: the inclusion of ionization and recombination rates to individual excited levels as a means to populate atomic levels; data for Kα and Kβ emission from Fe II to Fe XXIV; new data for high-energy configurations in Fe XVII to Fe XXIII; and a complete reassessment of level energies and line identifications in the X-ray range, multitemperature particle distributions, and photoexcitation from any user-defined radiation field. New data for ions already in the database, as well as data for ions not present in earlier versions of the database, are also included. Version 5 of CHIANTI represents a major improvement in the calculation of line emissivities and synthetic spectra in the X-ray range and expands and improves theoretical spectra calculations in all other wavelength ranges.

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CHIANTI-An Atomic Database for Emission Lines. VII. New Data for X-Rays and Other Improvements

Carbon-based materials are seen as promising candidates for applications such as spintronics and as spin qubits, as their electron spins are thought to be exceptionally stable. In particular, it was assumed that the effect of electron spin coupling to its orbital motion — a source for spin decoherence — is negligible. Kuemmeth et al. have now disproved this assumption. Based on a detailed set of electronic transport measurements on high-quality, clean, single-walled carbon nanotubes, they observe direct signatures of electron spin–orbit coupling. The findings may lead to new design principles for the realization of qubits in nanotubes. And the observed spin–orbit coupling may prove to be a valuable tool as a mechanism for all-electrical control of spins in carbon nanotubes. Based on a detailed set of electronic transport measurements on high-quality, clean single walled carbon nanotubes, direct signatures of electron spin-orbit coupling are observed. The findings may lead to new design principles for the realization of qubits in nanotubes. Furthermore, the observed spin-orbit coupling may prove to be a valuable tool as a mechanism for all-electrical control of spins in carbon nanotubes. Electrons in atoms possess both spin and orbital degrees of freedom. In non-relativistic quantum mechanics, these are independent, resulting in large degeneracies in atomic spectra. However, relativistic effects couple the spin and orbital motion, leading to the well-known fine structure in their spectra. The electronic states in defect-free carbon nanotubes are widely believed to be four-fold degenerate1,2,3,4,5,6,7,8,9,10, owing to independent spin and orbital symmetries, and also to possess electron–hole symmetry11. Here we report measurements demonstrating that in clean nanotubes the spin and orbital motion of electrons are coupled, thereby breaking all of these symmetries. This spin–orbit coupling is directly observed as a splitting of the four-fold degeneracy of a single electron in ultra-clean quantum dots. The coupling favours parallel alignment of the orbital and spin magnetic moments for electrons and antiparallel alignment for holes. Our measurements are consistent with recent theories12,13 that predict the existence of spin–orbit coupling in curved graphene and describe it as a spin-dependent topological phase in nanotubes. Our findings have important implications for spin-based applications in carbon-based systems, entailing new design principles for the realization of quantum bits (qubits) in nanotubes and providing a mechanism for all-electrical control of spins14 in nanotubes.

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Coupling of spin and orbital motion of electrons in carbon nanotubes

The CHIANTI spectral code consists of an atomic database and a suite of computer programs to calculate the optically thin spectrum of astrophysical objects and carry out spectroscopic plasma diagnostics. The database includes atomic energy levels, wavelengths, radiative transition probabilities, collision excitation rate coefficients, and ionization and recombination rate coefficients, as well as data to calculate free-free, free-bound, and two-photon continuum emission. Version 7 has been released, which includes several new ions, significant updates to existing ions, as well as Chianti-Py, the implementation of CHIANTI software in the Python programming language. All data and programs are freely available at http://www.chiantidatabase.org, while the Python interface to CHIANTI can be found at http://chiantipy.sourceforge.net.

/pdf/chianti-an-atomic-database-for-emission-lines-xii-version-7-4y5jdtauvc.pdf

Chianti—an atomic database for emission lines. xii. version 7 of the database

Atomic polarization phenomena impinge upon a number of areas and processes in physics. The dielectric constant and refractive index of any gas are examples of macroscopic properties that are largely determined by the dipole polarizability. When it comes to microscopic phenomena, the existence of alkaline-earth anions and the recently discovered ability of positrons to bind to many atoms are predominantly due to the polarization interaction. An imperfect knowledge of atomic polarizabilities is presently looming as the largest source of uncertainty in the new generation of optical frequency standards. Accurate polarizabilities for the group I and II atoms and ions of the periodic table have recently become available by a variety of techniques. These include refined many-body perturbation theory and coupled-cluster calculations sometimes combined with precise experimental data for selected transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index measurements in microwave cavities, ab initio calculations of atomic structures using explicitly correlated wavefunctions, interferometry with atom beams and velocity changes of laser cooled atoms induced by an electric field. This review examines existing theoretical methods of determining atomic and ionic polarizabilities, and discusses their relevance to various applications with particular emphasis on cold-atom physics and the metrology of atomic frequency standards.

https://iopscience.iop.org/article/10.1088/0953-4075/43/20/202001/pdf

Theory and applications of atomic and ionic polarizabilities

NIST Atomic Spectra Database (version 2.0)

In a recombining plasma the metastable states are known to accumulate population thereby slowing down the recombination process. We show that an account of the doubly excited autoionizing states, formed due to collisional recombination of metastable ions, results in a significant acceleration of recombination. A fully time-dependent collisional-radiative (CR) modeling for stripped ions of carbon recombining in a cold dense plasma demonstrates an order of magnitude faster recombination of He-like ions. The CR model used in calculations is discussed in detail.

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Accelerated recombination due to resonant deexcitation of metastable states

We review the first “virtual workshop” designed to compare NLTE emissivities produced by widely differing types of atomic physics codes. A small set of significant results, illustrating the progress that is still to be achieved, is presented. We conclude with some lessons learned and possible avenues for future progress.

Review of the NLTE emissivities code comparison virtual workshop

Plasma ionization composition and level population calculations require, in particular, the cross sections of direct ionization from each quantum state into each state which may be generated by means of removal of any electron. We analysed published data and propose here an empirical formula for cross sections of direct electron-impact ionization of positive atomic ions. The cross sections given by this formula are in satisfactory agreement with those calculated in the distorted-wave (DW) approximation; therefore, we believe that for any direct state-to-state ionization channel this formula provides a reasonably accurate prediction of the DW result. Comparisons with published data and with the Lotz formula are reported as well.

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Empirical formula for cross section of direct electron-impact ionization of ions

On the basis of average experimental data we demonstrate scaling laws of electron-impact multiple ionization cross sections and propose expressions for the cross sections for arbitrary atoms and ions.

Yu. Ralchenko

Papers

NIST Atomic Spectra Database (version 2.0)

Accelerated recombination due to resonant deexcitation of metastable states

Review of the NLTE emissivities code comparison virtual workshop

Empirical formula for cross section of direct electron-impact ionization of ions

A scaling of multiple ionization cross sections