Topologically protected magnetoelectric switching in a multiferroic.

Electric control of magnetism and magnetic control of ferroelectricity can improve the energy efficiency of magnetic memory and data-processing devices1. However, the necessary magnetoelectric switching is hard to achieve, and requires more than just a coupling between the spin and the charge degrees of freedom2-5. Here we show that an application and subsequent removal of a magnetic field reverses the electric polarization of the multiferroic GdMn2O5, thus requiring two cycles to bring the system back to the original configuration. During this unusual hysteresis loop, four states with different magnetic configurations are visited by the system, with one half of all spins undergoing unidirectional full-circle rotation in increments of about 90 degrees. Therefore, GdMn2O5 acts as a magnetic crankshaft that converts the back-and-forth variations of the magnetic field into a circular spin motion. This peculiar four-state magnetoelectric switching emerges as a topologically protected boundary between different two-state switching regimes. Our findings establish a paradigm of topologically protected switching phenomena in ferroic materials.

Discovery of Two-Dimensional Electromagnetic Plasma Waves.

We report the experimental discovery of "superluminal" electromagnetic 2D plasma waves in the electromagnetic response of a high-quality GaAs/AlGaAs two-dimensional electron system on a dielectric substrate. We measure the plasma wave spectrum on samples with different electron density. It is established that, at large two-dimensional densities, there is a strong hybridization between the plasma and the Fabry-Perot light modes. In the presence of a perpendicular magnetic field, the plasma resonance is shown to split into two modes, each corresponding to a particular sense of circular polarization. Experimental results are found to be in good agreement with the theory.

Neurogenic Potential of Implanted Neurospheres Is Regulated by Optogenetic Stimulation of Hippocampal Astrocytes Ex Vivo.

The protocol of optogenetic ChR2-mediated activation of astrocytes was used in a model of artificial neurogenic niche, neurospheres implanted into ex vivo organotypic cultures of mouse hippocampus. The electrophysiological characteristics of the hippocampus and expression of molecules involved in the mechanisms of activation of astrocytes and microglia (GFAP, CD38, C3/C3b, Cx43, CD11b, and CD18) were evaluated. Photoactivation of astrocytes led to activation of neurogenesis and changes in the expression of molecules (Cx43 and CD38) that determine bioavailability of NAD+ to ensure proliferative activity of cells in the neurogenic niche. Implantation of neurospheres into organotypic slices of the hippocampus caused an increase in C3/C3b expression and suppression of the synaptic plasticity of hippocampal neurons.

[Features of the in vitro expression profile of hippocampal neurogenic niche cells during optogenetic stimulation]. / Osobennosti ékspressionnogo profilia kletok v modeli neirogennoi nishi gippokampa in vitro pri optogeneticheskoi stimuliatsii.

In the central nervous system of mammals, there are specialized areas in which neurogenesis - neurogenic niches - is observed in the postnatal period. It is believed that astrocytes in the composition of neurogenic niches play a significant role in the regulation of neurogenesis, and therefore they are considered as a promising "target" for the possible control of neurogenesis, including the use of optogenetics. In the framework of this work, we formed an in vitro model of a neurogenic niche, consisting of cerebral endothelial cells, astrocytes and neurospheres. Astrocytes in the neurogenic niche model expressed canalorodopsin ChR2 and underwent photoactivation. The effect of photoactivated astrocytes on the expression profile of neurogenic niche cells was evaluated using immunocytochemical analysis methods. It was found that intact astrocytes in the composition of the neurogenic niche contribute to neuronal differentiation of stem cells, as well as the activation of astroglia expressing photosensitive proteins, changes the expression of molecules characterized by intercellular interactions of pools of resting and proliferating cells in the composition of the neurogenic niche with the participation of NAD+ (Cx43, CD38, CD157), lactate (MCT1). In particular, the registered changes reflect a violation of the paracrine intercellular interactions of two subpopulations of cells, one of which acts as a source of NAD+, and the second as a consumer of NAD+ to ensure the processes of intracellular signal transduction; a change in the mechanisms of lactate transport due to aberrant expression of the lactate transporter MCT1 in cells forming a pool of cells developing along the neuronal path of differentiation. In general, with photostimulation of niche astrocytes, the total proliferative activity increases mainly due to neural progenitor cells, but not neural stem cells. Thus, optogenetic activation of astrocytes can become a promising tool for controlling the activity of neurogenesis processes and the formation of a local proneurogenic microenvironment in an in vitro model of a neurogenic niche.

[Astroglia-mediated regulation of cell development in the model of neurogenic niche in vitro treated with Aß1-42]. / Astrotsit-oposredovannye mekhanizmy reguliatsii neirogeneza v modeli neirogennoi nishi in vitro pri deistviin Aß1-42.

Neurogenesis is a complex process which governs embryonic brain development and is importants for brain plasticity throughout the whole life. Postnatal neurogenesis occurs in neurogenic niches that regulate the processes of proliferation and differentiation of stem and progenitor cells under the action of stimuli that trigger the mechanisms of neuroplasticity. Cells of glial and endothelial origin are the key regulators of neurogenesis. It is known that physiological neurogeneses is crucial for memory formation, whereas reparative neurogenesis provides partial repair of altered brain structure and compensation of neurological deficits caused by brain injury. Dysregulation of neurogenesis is a characteristics of various neurodevelopmental and neurodegenerative diseases, particularly, Alzheimer's disease which is very important medical and social problem. In the in vitro model of the neurogenic niche using hippocampal neurospheres as a source of stem/progenitor cells and astrocytes, we studied effects of astrocyte activation on the expression of markers of different stages of cell proliferation and differentiation. We found that aberrant mechanisms of development of stem and progenitor cells, caused by the beta-amyloid (Aß1-42), can be partially restored by targeted activation of GFAP-expressing cells in the neurogenic niche.

[Early changes in hyppocampal neurogenesis induced by soluble Ab1-42 oligomers]. / Rannie izmeneniia v gippokampal'nom neirogeneze, indutsirovannye rastvorimymi formami oligomerov Ab1-42.

Alzheimer's disease is characterized by the loss of neurons, the accumulation of intracellular neurofibrillary tangles and extracellular amyloid plaques in the brain. However, there are contradicting data on differences in neurogenesis at the onset of the disease or before the formation of amyloid plaques. As awareness of the importance of the pre-symptom phase in neurodegenerative diseases grows in the context of early diagnosis and pathogenesis, we analyzed the critical periods of adult hippocampal neurogenesis at an early stage under the action of soluble Ab1-42 beta-amyloid. The proliferation, migration and neuronal cells survival were evaluated in mice with an injection of soluble amyloid beta-oligomers. It was found that the injection of Ab1-42 oligomers causes a decrease in cell proliferation in the mouse hippocampus. Despite the preservation of the neuroblast pool in animals after beta-amyloid injection, the process of radial migration is disrupted, and an increase in apoptosis in the neurogenic niche was revealed. Thus, our results demonstrate damage of neurogenesis critical stages: the progenitor cells, neuroblast migration, the integration of immature neurons, and the survival of neurons under application of soluble beta-amyloid oligomers. The obtained data indicate decline in proliferation rate in the subgranular zone, that is accompanied by ectopic differentiation and disturbed migration, producing, apparently, abnormal neurons that have lower survival rates. That could lead to a decrease in mature neurons numbers and the number of cells in the granular layer of the dentate gyrus.

Switching of Magnons by Electric and Magnetic Fields in Multiferroic Borates.

Electric manipulation of magnetic properties is a key problem of materials research. To fulfill the requirements of modern electronics, these processes must be shifted to high frequencies. In multiferroic materials, this may be achieved by electric and magnetic control of their fundamental excitations. Here we identify magnetic vibrations in multiferroic iron borates that are simultaneously sensitive to external electric and magnetic fields. Nearly 100% modulation of the terahertz radiation in an external field is demonstrated for SmFe_{3}(BO_{3})_{4}. High sensitivity can be explained by a modification of the spin orientation that controls the excitation conditions in multiferroic borates. These experiments demonstrate the possibility to alter terahertz magnetic properties of materials independently by external electric and magnetic fields.

Observation of the universal magnetoelectric effect in a 3D topological insulator.

The electrodynamics of topological insulators (TIs) is described by modified Maxwell's equations, which contain additional terms that couple an electric field to a magnetization and a magnetic field to a polarization of the medium, such that the coupling coefficient is quantized in odd multiples of α/4π per surface. Here we report on the observation of this so-called topological magnetoelectric effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semitransparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant α=e2/2hc (in SI units) when a linearly polarized THz radiation of a certain frequency passes through the two surfaces of a strained HgTe 3D TI. These experiments give insight into axion electrodynamics of TIs and may potentially be used for a metrological definition of the three basic physical constants.

Universal Faraday Rotation in HgTe Wells with Critical Thickness.

The universal value of the Faraday rotation angle close to the fine structure constant (α≈1/137) is experimentally observed in thin HgTe quantum wells with a thickness on the border between trivial insulating and the topologically nontrivial Dirac phases. The quantized value of the Faraday angle remains robust in the broad range of magnetic fields and gate voltages. Dynamic Hall conductivity of the holelike carriers extracted from the analysis of the transmission data shows a theoretically predicted universal value of σ_{xy}=e^{2}/h, which is consistent with the doubly degenerate Dirac state. On shifting the Fermi level by the gate voltage, the effective sign of the charge carriers changes from positive (holes) to negative (electrons). The electronlike part of the dynamic response does not show quantum plateaus and is well described within the classical Drude model.

[Transparent evolution of the energy/matter interactions on earth: from gas whirlwind to technogenic civilization].

The paper presents the idea of transparent evolution through the long-term reaction of the planet Earth on the external flow of radiant energy from the Sun. Due to limitations of matter on Earth, as well as on any other planet, the continuous pumping flow of radiant energy was shown to lead to cyclization and transport of substance on emerging gradients. The evolution of energy-matter interaction follows the path of capturing and transferring more energy by the fewer matter, i.e., the path of growth of the amount of energy used by each unit mass. For this indicator, the least effective mass transfer is a simple mass transfer as vortices of gases, in the gradients of temperature and pressure, which occurred on the primary surface of the planet. A long-term natural selection related to the accumulation of water on the planet has played a special role in developing the interaction of energy and matter. Phase transformations (ice, water, vapor) and mechanical transfers are the most common energy-matter processes. Based on water cycles, cyclic transports and transformations, chemical transformation of substances became possible developing over time into a biological transformation. This kind of the interaction of energy and matter is most efficient. In particular, during photosynthesis the energy of our star "is captured and utilized" in the most active part of the spectrum of its radiation. In the process of biological evolution of heterotrophs, a rise (by a factor of hundreds) in the coefficient that characterizes the intensity of energy exchange from protozoa to mammals is most illustratory. The development and the current dominance of humans as the most energy-using active species in capturing the energy and meaningful organization of its new flows especially on the basis of organic debris of former biospheres is admirable, but quite natural from the energy positions. In the course of technological evolution of humankind, the measure of the intensity of energy for homoeothermic (warm-blooded) animals has increased 20 times, based on the process energy used by the "average" inhabitant of the world. Thus, the victory of our species in planetary evolution is easy to fit into the mainstream of evolution through energy-matter interactions: multiple growth of star energy was used to transform the matter on the surface of the irradiated planet.

Electric field control of terahertz polarization in a multiferroic manganite with electromagnons.

All-electrical control of a dynamic magnetoelectric effect is demonstrated in a classical multiferroic manganite DyMnO3, a material containing coupled antiferromagnetic and ferroelectric orders. Because of intrinsic magnetoelectric coupling with electromagnons a linearly polarized terahertz light rotates upon passing through the sample. The amplitude and the direction of the polarization rotation are defined by the orientation of ferroelectric domains and can be switched by static voltage. These experiments allow the terahertz polarization to be tuned using the dynamic magnetoelectric effect.

Giant magneto-optical faraday effect in HgTe thin films in the terahertz spectral range.

We report the observation of a giant Faraday effect, using terahertz (THz) spectroscopy on epitaxial HgTe thin films at room temperature. The effect is caused by the combination of the unique band structure and the very high electron mobility of HgTe. Our observations suggest that HgTe is a high-potential material for applications as optical isolator and modulator in the THz spectral range.

Magnetic and magnetoelectric excitations in multiferroic manganites.

We review the recent experimental findings concerning the magnetic and magnetoelectric excitations in multiferroic manganites with special focus on orthorhombic RMnO(3) (R = rare earth). The dynamics of these materials has attracted special attention recently due to the existence of novel magnetoelectric excitations which were termed electromagnons. In addition to the strong electric activity of the electromagnons, a series of other excitations of magnetic and magnetoelectric nature is observed in the same frequency range as the electromagnons. We summarize the systematics of the existing modes and the current understanding of the underlying mechanisms.

Evidence for electroactive excitation of the spin cycloid in TbMnO3.

Terahertz electromagnetic excitations in the multiferroic TbMnO3 single crystals are investigated across the magnetic field induced rotation of the magnetic spin cycloid. In addition to the electromagnon along the a axis, the detailed polarization analysis of the experimental spectra suggests the existence of an electroactive excitation for ac electric fields of the electromagnetic wave along the crystallographic c axis. This excitation is possibly the electroactive eigenmode of the spin-cycloid in TbMnO3, which has been predicted within the inverse Dzyaloshinskii-Moriya mechanism of magnetoelectric coupling.

Electrically excited inverse electron spin resonance in a split-ring metamaterial resonator.

Bianisotropic properties of the metamaterial built from split-ring resonators are utilized to excite electron-spin resonance in a gadolinium gallium garnet by the electric field of light. Surprisingly, the observed electron-spin resonance signal is seen as a maximum in the field-dependent transmittance, which indicates strong modifications of the metamaterial parameters in the coupled regime. In the geometry where both modes, electron-spin resonance and split-ring resonance, are active, the anticrossing regime can be observed. These effects can be explained using the classical model of two coupled oscillators and the bianisotropic properties of split-ring resonators.

Magnetic and magnetoelectric excitations in TbMnO3.

Magnetic and magnetoelectric excitations in the multiferroic TbMnO3 have been investigated at terahertz frequencies. Using different experimental geometries we can clearly separate the electroactive excitations (electromagnons) from the magnetoactive modes, i.e., antiferromagnetic resonances. Two antiferromagnetic resonances were found to coincide with electromagnons. This indicates that both excitations belong to the same mode and the electromagnons can be excited by a magnetic ac field as well. In spite of the 90 degrees rotation of the magnetic cycloid in external magnetic fields, the electromagnons are observable for electric ac fields parallel to the a axis only.

Dynamical Dzyaloshinsky-Moriya interaction in KCuF3.

The spin dynamics of the prototypical quasi-one-dimensional antiferromagnetic Heisenberg spin S=1/2 chain KCuF3 is investigated by electron spin resonance spectroscopy. Our analysis shows that the peculiarities of the spin dynamics require a new dynamical form of the antisymmetric anisotropic spin-spin interaction. This dynamical Dzyaloshinsky-Moriya interaction is related to strong oscillations of the bridging fluorine ions perpendicular to the crystallographic c axis. This new mechanism allows us to resolve consistently the controversies in observation of the magnetic and structural properties of this orbitally ordered perovskite compound.