Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

This article reviews recent theoretical and experimental advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical systems. In the presence of effective photon-photon interactions induced by the optical nonlinearity of the medium, a many-photon system can behave collectively as a quantum fluid with a number of novel features stemming from its intrinsically nonequilibrium nature. A rich variety of recently observed photon hydrodynamical effects is presented, from the superfluid flow around a defect at low speeds, to the appearance of a Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of topological excitations such as quantized vortices and dark solitons at the surface of large impenetrable obstacles. While the review is mostly focused on a specific class of semiconductor systems that have been extensively studied in recent years (planar semiconductor microcavities in the strong light-matter coupling regime having cavity polaritons as elementary excitations), the very concept of quantum fluids of light applies to a broad spectrum of systems, ranging from bulk nonlinear crystals, to atomic clouds embedded in optical fibers and cavities, to photonic crystal cavities, to superconducting quantum circuits based on Josephson junctions. The conclusive part of the article is devoted to a review of the future perspectives in the direction of strongly correlated photon gases and of artificial gauge fields for photons. In particular, several mechanisms to obtain efficient photon blockade are presented, together with their application to the generation of novel quantum phases.

Quantum fluids of light

The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.

https://physics.nyu.edu/kentlab/Papers/2012/NatureMaterials_11_372_2012.pdf

Current-induced torques in magnetic materials

Spin–orbit torques in heavy metal/ferromagnetic layers have a complex dependence on the magnetization direction. This dependence can be exploited to increase the efficiency of spin–orbit torques.

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Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures

Spin Hall effects convert charge currents into spin currents and vice versa even in nonmagnetic conductors due to spin orbit coupling. This enables spin Hall effects to be utilized both for the generation and detection of spin currents and magnetization dynamics. This paper reviews the experimental characterization of these effects in metallic systems, which have so far shown the highest efficiency in using spin Hall effects for charge-to-spin interconversion. The advantages and disadvantages of complimentary measurement techniques are discussed and in addition an outlook of the possible impact on applications is presented.

Spin Hall Effects in Metals

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. Bose–Einstein condensation, the formation of a collective quantum state of identical particles, called bosons, is observed at room temperature in a gas of magnons, which are a type of magnetic excitation. Bose–Einstein condensation1,2 is one of the most fascinating phenomena predicted by quantum mechanics. It involves the formation of a collective quantum state composed of identical particles with integer angular momentum (bosons), if the particle density exceeds a critical value. To achieve Bose–Einstein condensation, one can either decrease the temperature or increase the density of bosons. It has been predicted3,4 that a quasi-equilibrium system of bosons could undergo Bose–Einstein condensation even at relatively high temperatures, if the flow rate of energy pumped into the system exceeds a critical value. Here we report the observation of Bose–Einstein condensation in a gas of magnons at room temperature. Magnons are the quanta of magnetic excitations in a magnetically ordered ensemble of magnetic moments. In thermal equilibrium, they can be described by Bose–Einstein statistics with zero chemical potential and a temperature-dependent density. In the experiments presented here, we show that by using a technique of microwave pumping it is possible to excite additional magnons and to create a gas of quasi-equilibrium magnons with a non-zero chemical potential. With increasing pumping intensity, the chemical potential reaches the energy of the lowest magnon state, and a Bose condensate of magnons is formed.

Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping

Spin currents—the flow of angular momentum without the simultaneous transfer of electrical charge—play an enabling role in the field of spintronics1, 2, 3, 4, 5, 6, 7, 8. Unlike the charge current, the spin current is not a conservative quantity within the conduction carrier system. This is due to the presence of the spin–orbit interaction that couples the spin of the carriers to angular momentum in the lattice. This spin–lattice coupling9 acts also as the source of damping in magnetic materials, where the precessing magnetic moment experiences a torque towards its equilibrium orientation; the excess angular momentum in the magnetic subsystem flows into the lattice. Here we show that this flow can be reversed by the three-magnon splitting process and experimentally achieve the enhancement of the spin current emitted by the interacting spin waves. This mechanism triggers angular momentum transfer from the lattice to the magnetic subsystem and modifies the spin-current emission. The finding illustrates the importance of magnon–magnon interactions for developing spin-current based electronics.

/pdf/controlled-enhancement-of-spin-current-emission-by-three-43wv8p8a2i.pdf

Controlled enhancement of spin-current emission by three-magnon splitting

The room-temperature dynamics of a magnon gas driven by short microwave pumping pulses is studied. An overpopulation of the lowest energy level of the system following the pumping is observed. Using the sensitivity of the Brillouin light scattering technique to the coherence degree of the scattering magnons we demonstrate the spontaneous emergence of coherence of the magnons at the lowest level, if their density exceeds a critical value. This finding is clear proof of the quantum nature of the observed phenomenon and direct evidence of Bose-Einstein condensation of magnons at room temperature.

Observation of spontaneous coherence in Bose-Einstein condensate of magnons.

The thermalization of parametrically pumped magnons caused by nonlinear multimagnon scattering processes and leading to the magnon Bose-Einstein condensation is investigated experimentally with high temporal resolution. The threshold pumping power necessary for the thermalization is determined. For pumping powers above this threshold the thermalization time has been found to decrease rapidly with power reaching the value down to 50 ns, which is much smaller than the magnon lifetime.

Thermalization of a parametrically driven magnon gas leading to Bose-Einstein condensation.

A gas of magnons in magnetic films differs from all other known systems demonstrating Bose-Einstein condensation (BEC), since it possesses two energetically degenerate lowest-energy quantum states with non-zero wave vectors ±kBEC. Therefore, BEC in this system results in a spontaneously formed two-component Bose-Einstein condensate described by a linear combination of two spatially non-uniform wave-functions ∝exp(±ikBECz), while condensates found in other physical systems are characterized by spatially uniform wave-functions. Here we report a study of BEC of magnons with sub-micrometer spatial resolution. We experimentally confirm the existence of the two wave-functions and show that their interference results in a non-uniform ground state of the condensate with the density oscillating in space. Additionally, we observe stable topological defects in the condensate. By comparing the experimental results with predictions of a theoretical model based on the Ginzburg-Landau equation, we identify these defects as quantized vortices.

/pdf/spatially-non-uniform-ground-state-and-quantized-vortices-in-2xz5zpa71e.pdf

O. Dzyapko

Papers

Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping

Controlled enhancement of spin-current emission by three-magnon splitting

Observation of spontaneous coherence in Bose-Einstein condensate of magnons.

Thermalization of a parametrically driven magnon gas leading to Bose-Einstein condensation.

Spatially non-uniform ground state and quantized vortices in a two-component Bose-Einstein condensate of magnons