Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

The 1937 theoretical discovery of Majorana fermions-whose defining property is that they are their own anti-particles-has since impacted diverse problems ranging from neutrino physics and dark matter searches to the fractional quantum Hall effect and superconductivity. Despite this long history the unambiguous observation of Majorana fermions nevertheless remains an outstanding goal. This review paper highlights recent advances in the condensed matter search for Majorana that have led many in the field to believe that this quest may soon bear fruit. We begin by introducing in some detail exotic 'topological' one- and two-dimensional superconductors that support Majorana fermions at their boundaries and at vortices. We then turn to one of the key insights that arose during the past few years; namely, that it is possible to 'engineer' such exotic superconductors in the laboratory by forming appropriate heterostructures with ordinary s-wave superconductors. Numerous proposals of this type are discussed, based on diverse materials such as topological insulators, conventional semiconductors, ferromagnetic metals and many others. The all-important question of how one experimentally detects Majorana fermions in these setups is then addressed. We focus on three classes of measurements that provide smoking-gun Majorana signatures: tunneling, Josephson effects and interferometry. Finally, we discuss the most remarkable properties of condensed matter Majorana fermions-the non-Abelian exchange statistics that they generate and their associated potential for quantum computation.

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New directions in the pursuit of Majorana fermions in solid state systems.

Electrically induced electron-spin polarization near the edges of a semiconductor channel was detected and imaged with the use of Kerr rotation microscopy The polarization is out-of-plane and has opposite sign for the two edges, consistent with the predictions of the spin Hall effect Measurements of unstrained gallium arsenide and strained indium gallium arsenide samples reveal that strain modifies spin accumulation at zero magnetic field A weak dependence on crystal orientation for the strained samples suggests that the mechanism is the extrinsic spin Hall effect

https://science.sciencemag.org/content/sci/306/5703/1910.full.pdf

Observation of the spin Hall effect in semiconductors.

Superconducting circuits are macroscopic in size but have generic quantum properties such as quantized energy levels, superposition of states, and entanglement, all of which are more commonly associated with atoms. Superconducting quantum bits (qubits) form the key component of these circuits. Their quantum state is manipulated by using electromagnetic pulses to control the magnetic flux, the electric charge or the phase difference across a Josephson junction (a device with nonlinear inductance and no energy dissipation). As such, superconducting qubits are not only of considerable fundamental interest but also might ultimately form the primitive building blocks of quantum computers.

Superconducting quantum bits

A study was made on Nb based Josephson junctions with evaporated Te barriers. As the first step, a Nb-Te-Pb configuration was tested. The smooth surfaced Nb electrode enabled the formation of a pinhole free barrier. Supercurrent density was found to be large compared to that for surface oxidized junctions. As the second step, a Nb-Te-Nb structure was tested. Although the junction uniformity was not as good as that of the Nb-Te-Pb junctions, the aging property was drastically improved. Considerations were made on the origin of the remaining aging. The junctions showed excellent response to 50 GHz signals.

Nb based Te barrier Josephson junctions

We have fabricated a sub-micron-sized structure consisting of an InAs-based 2DEG, two narrow Nb leads and a gate, where the indirect ballistic transport between the non-oppositely superconducting contacts can be controlled by the voltage applied to the gate. This new kind of tunable junction can be used for applications and allows several fundamental questions related to the transport mechanism to be studied. First results of experiments carried out in this respect are presented.

Ballistic reflection at a side gate in a superconductor–semiconductor–superconductor structure

Ferromagnetic contacts on a high-mobilty two-dimensional electron gas (2DEG) in a narrow gap semiconductor with strong spin–orbit interaction are used to investigate spin-polarized electron transport. We demonstrate the use of magnetized contacts to preferentially inject and detect specific spin orientations. Spin dephasing and spin precession effects are studied by temperature and 2DEG channel length dependent measurements.

Spin injection experiment with multiple NiFe/InAs-2DEG/NiFe junctions

Sub-energy gap structures due to multiple Andreev-reflections are observed in a Nb/InAs/Nb junction. These structures appear because the inelastic scattering length for InAs is longer than the length between Nb electrodes. The sub- energygap structures are thought to reflect the pair-potential in InAs induced by the superconducting proximity effect.

Sub-Energygap Structures in a Nb/InAs/Nb Junction

We theoretically investigated superconducting proximitycorrections of the conductance of a mesoscopic metal wire interms of the Kubo-formula. In diagrammatic expression, theelectromagnetic response kernel is quite similar to that ofreproducible conductance fluctuation. However, the proximitycorrection modifies the average conductance of the wire eventhough the latter gives a mesoscopic fluctuation.This Kubo-formula expression is applicable to the analysis ofcharging effects on the proximity correction in asuperconductor(S)/Mesoscopic-normalmetal-wire(N) hybrid systemwith a very small S/N interface. Although the chargingeffect exponentially suppresses Andreev reflections at theS/N interface, the proximity correction survives.We propose an experiment to confirm this charging effect,where a degenerated semiconductor is used as the normal-metalwire. The proximity correction can be picked from the entireconductance as the magnetoconductance of an interferometer,and the charging effect is depicted by a gate control of thesuperconducting island. The semiconductor interferometer ismore promising than a metallic one because the conductancewithout the proximity effect is bigger.

Hideaki Takayanagi

Papers

Nb based Te barrier Josephson junctions

Ballistic reflection at a side gate in a superconductor–semiconductor–superconductor structure

Spin injection experiment with multiple NiFe/InAs-2DEG/NiFe junctions

Sub-Energygap Structures in a Nb/InAs/Nb Junction

Superconducting Proximity Effect and Charging Effect on the Normal-Metal(Semiconductor) Wire