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

In superconductor–semiconductor–superconductor (Su–Sm–Su) junctions with superlattice structure, differential resistance as a function of voltage shows oscillatory behavior under the irradiation of radio-frequency (rf) waves with specific frequency regardless of the superconducting materials and the junction geometries. We interpret quantitatively this newly discovered phenomenon in terms of the coupling of superconducting quasiparticles with long-wavelength acoustic phonons indirectly excited by the rf waves. We propose that the strong coupling causes the formation of novel composite particles which we named Andreev polarons.

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Transport properties of Andreev polarons in a superconductor–semiconductor–superconductor junction with superlattice structure

Multiphoton Absorption and SQUID Switching Current Behaviors in Superconducting Flux-Qubit Experiments

We have studied low temperature transport characteristics of an independently contacted double quantum dot–quantum wire coupled system. Each quantum dot is connected to three leads; source, drain and wire. Fano resonances associated with one dot appear in the conductance of the other dot, which are gradually suppressed as the inter-dot tunnel coupling via the wire is reduced. When the dot-wire tunnel coupling is lost, conductance modulation similar to the Fano resonances reappear, which is ascribed to purely inter-dot electrostatic coupling effect.

Interplay between electrostatic and tunnel couplings in an independently contacted double quantum dot quantum wire coupled device

Observation of charged excitons in a back-gated undoped quantum well in the integer and the fractional quantum-Hall regimes is reported in magnetic fields depending on the electron density. We find anomalies of the photoluminescence at integer and fractional filling factors.

Charged excitons in a back-gated undoped quantum well in the integer and the fractional quantum-hall regimes

Unconventional superconductors involve not only gauge-symmetry breaking but also orbital- and spin-symmetry breaking. Superconducting Sr2RuO4 is known as a spin-triplet chiral p-wave and a topological superconductor with broken time-reversal symmetry (BTRS). Kerr-effect and muon-spin-rotation (pSR) measurements have shown that the bulk superconducting state of Sr2RuO4 features BTRS in the orbital part; hence, it is called the chiral state. BTRS in the response of superconducting junctions or SQUIDs would appear as the shifts of magnetic interference patterns. However, it is problematic to distinguish whether the shift originates in the residual magnetic field (trapped vortex) or the effects of BTRS. Here, we show that the magnetic interference patterns of a SQUID based on Sr2RuO4 are explicitly asymmetric with respect to the direction of both the bias current and the applied magnetic field; namely, there is no inversion symmetry. This indicates that the superconducting state of Sr2RuO4 undoubtedly breaks the time-reversal symmetry of the SQUID.

http://iopscience.iop.org/article/10.1088/1742-6596/568/2/022020/pdf

Hideaki Takayanagi

Papers

Transport properties of Andreev polarons in a superconductor–semiconductor–superconductor junction with superlattice structure

Multiphoton Absorption and SQUID Switching Current Behaviors in Superconducting Flux-Qubit Experiments

Interplay between electrostatic and tunnel couplings in an independently contacted double quantum dot quantum wire coupled device

Charged excitons in a back-gated undoped quantum well in the integer and the fractional quantum-hall regimes

Broken time-reversal symmetry in a SQUID based on chiral superconducting Sr2RuO4