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

The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

Phase transitions to quantum condensed phases—such as Bose–Einstein condensation (BEC), superfluidity, and superconductivity—have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 109 times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase. Bose–Einstein condensation (BEC), a form of matter first postulated in 1924, has famously been demonstrated in dilute atomic gases at ultra-low temperatures. Much effort is now being devoted to exploring solid-state systems in which BEC can occur. In theory semiconductor microcavities, where photons are confined and coupled to electronic excitations leading to the creation of polaritons, could allow BEC at standard cryogenic temperatures. Kasprzak et al. now present experiments in which polaritons are excited in such a microcavity. Above a critical polariton density, spontaneous onset of a macroscopic quantum phase occurs, indicating a solid-state BEC. BEC should also be possible at higher temperatures if coupling of light with solid excitations is sufficiently strong. Demokritov et al. have achieved just that, BEC at room temperature in a gas of magnons, which are a type of magnetic excitation. This paper presents a comprehensive set of experiments in which polaritons are excited in a semiconductor microcavity. Above a critical density of polaritons, massive occupation of the ground state at 19 K is observed and various pieces of experimental evidence point to a spontaneous onset of a macroscopic quantum phase.

Bose-Einstein condensation of exciton polaritons

Femtosecond filamentation in transparent media

We propose a dual non-perturbative description for maximally extended Schwarzschild Anti-de-Sitter spacetimes. The description involves two copies of the conformal field theory associated to the AdS spacetime and an initial entangled state. In this context we also discuss a version of the information loss paradox and its resolution.

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Eternal black holes in anti-de Sitter

http://iopscience.iop.org/article/10.1070/PU1970v013n02ABEH004239/pdf

Electron-hole Drops in Semiconductors

The concept of a coherent exciton state is formulated. It is shown that for this state, a macroscopic wave function can be introduced such that it satisfies a nonlinear equation of the type familiar in the phenomenological theory of a superfluid liquid. The corresponding nondissipative flux is the flux of energy. For excitons interacting with an electromagnetic field, a coupled system of Maxwell equations and Ginzburg–Pitaevskii-type equations (phenomenological theory of Bose liquid) is obtained.

https://robots.iopscience.iop.org/article/10.3367/UFNe.2017.10.038227/pdf

Coherent states of excitons

Superconductivity in nonmetallic systems

Hard excitation of stimulated polariton polariton scattering in semiconductor microcavities

The effect of the interband matrix elements of the electrons' Coulomb interaction on the exciton spectrum and the nature of the phase transition into the \"excitonic insulator\" state are investigated in a model of a semiconductor with the following features: the maximum of the valence band is located at the center of the Brillouin zone, and the conduction band minima occur at the edges of the Brillouin zone' the model is also characterized by a narrow energy gap. It is shown that if the exciton binding , . energy is close enough to the width of the energy gap, the system becomes thermodynamlCally unstable and undergoes a first-order phase transition. The two-particle excitation (exciton) spectrum in the new phase is also separated from the ground state by an energy gap.

Leonid Veniaminovich Keldysh

Papers

Electron-hole Drops in Semiconductors

Coherent states of excitons

Superconductivity in nonmetallic systems

Hard excitation of stimulated polariton polariton scattering in semiconductor microcavities

Nature of the Phase Transition under the Conditions of an "Excitonic" Instability in the Electronic Spectrum of a Crystal