Long-lived magnetoexcitons in 2D-fermion system

The paper addresses the experimental technique that, when applied to a 2D-electron system in the integer quantum Hall regime with filling factor ν = 2 (the Hall insulating state), allows resonant excitation of magnetoexcitons, their detection, control of an ensemble of long-lived triplet excitons and investigation of their radiationless decay related to exciton spin relaxation into the ground state. The technique proposed enables independent control of photoexcited electrons and Fermi-holes using photoinduced resonance reflection spectra as well as estimate with a reasonable degree of accuracy the resulting density of photoinduced electron-hole pairs bound into magnetoexcitons. The mere existence of triplet excitons was directly established by inelastic light scattering spectra which were analyzed to determine the value of singlet-triplet exciton splitting. It was found that the lifetimes of triplet excitons conditioned by electron spin relaxation in highly perfect GaAs/AlGaAs heterostructures with highly mobile 2D electrons are extremely long exceeding 100  μs at T < 1 K. The paper presents a qualitative explanation of the long-spin relaxation lifetimes which are unprecedented for translation-invariant 2D systems. This enabled us to create sufficiently high concentrations of triplet magnetoexcitons, electrically neutral excitations following Bose–Einstein statistics, in a Fermi electron system and investigate their collective properties. At sufficiently high densities of triplet magnetoexcitons and low temperatures, T < 1 K, the degenerate magnetofermionic system exhibits condensation of the triplet magnetoexcitons into a qualitatively new collective state with unusual properties which occurs in the space of generalized moments (magnetic translation vectors). The occurrence of a condensed phase is accompanied with a significant decrease in the viscosity of the photoexcited system, which is responsible for electron spin transport at macroscopic distances, as well as with the effects of threshold enhancement of the system response to the external action of the electromagnetic field and emergence of a new intensive radiative recombination channel

Models of Deformation Dependences of Total Integrated Intensity of Dynamical Diffraction in Single Crystals for Various Diffraction Conditions

В работе с помощью теории Чуховского—Петрашеня для деформационной зависимости (ДЗ) интегральной интенсивности динамической дифракции (ИИДД) в кристаллах без дефектов показан характер изменения ДЗ ИИДД с толщиной кристаллов и с вариацией других условий дифракции. На этой основе, а также при использовании ряда экспериментов с реальными дефектными кристаллами и результатов теории полной интегральной интенсивности динамической дифракции (ПИИДД) в кристаллах с дефектами без изгиба построена аналитическая модель ДЗ ПИИДД в кристаллах с дефектами, пригодная для диагностики параметров структурных дефектов в кристаллах.В роботі за допомогою теорії Чуховського—Петрашеня для деформаційної залежности (ДЗ) інтеґральної інтенсивности динамічної дифракції (ІІДД) у кристалах без дефектів показано характер зміни ДЗ ІІДД із товщиною кристалу та з варіяцією інших умов дифракції. На цій основі, а також при використанні ряду експериментів із реальними дефектними кристалами і результатів теорії повної інтеґральної інтенсивности динамічної дифракції (ПІІДД) у кристалах з дефектами без вигину побудовано аналітичну модель ДЗ ПІІДД у кристалах з дефектами, придатну для діягностики параметрів структурних дефектів у кристалах.The paper shows the pattern of change in the deformation dependences (DD) of integrated intensity of dynamical diffraction (IIDD) with crystal thickness and with variation of other diffraction conditions by means of the Chukhovskii—Petrashen theory for the DD of IIDD in defect-free crystals. Relying on this and numerous other experiments with real defective crystals as well as the results of total integrated intensity of dynamical diffraction (TIIDD) in crystals with defects without bend, an analytical model of the DD of TIIDD in crystals with defects is developed, which is feasible for the diagnostics of structural defects in crystals

Noise Enhanced Signaling in STDP Driven Spiking-Neuron Network

Population spike signaling is widely observed both in intact brain and neuronal cultures. Experimental evidence suggests that a locally applied electrical stimulus can shape the network architecture and thus the neuronal response. However, there is no clue on how this process can be controlled. Here we study a realistic model of a culture of cortical-like neurons with spike timing dependent plasticity. We show that a stimulus applied at a corner of the culture can rebuild synaptic couplings. Then the network eventually switches from a turbulent-like asynchronous spiking to an ordered population spike signaling mode. The structural analysis shows that the stimulus potentiates centrifugal couplings, which promotes spiking waves traveling outwards the stimulus location. This phenomenon can be catalyzed by noise of an intermediate strength. We predict that matured cultures with high connectivity are more susceptible to reconfiguration and generation of a population spike response than young cultures with low connectivity. We also report on an intermittent synchronization causing switches between two quasi-stable states: generation of time-locked population spikes and turbulent spiking. In the turbulent mode the stimulus excites patches of spiking activity randomly traveling in the network. Such a regime can be implemented through a large scale looping of couplings backwards to the stimulus location. We anticipate that the robust mechanisms of shaping the network architecture discussed here can also be effective in more complex preparations and studies of the relationship between network structure and function

Noise Enhanced Signaling in STDP Driven Spiking-Neuron Network

Population spike signaling is widely observed both in intact brain and neuronal cultures. Experimental evidence suggests that a locally applied electrical stimulus can shape the network architecture and thus the neuronal response. However, there is no clue on how this process can be controlled. Here we study a realistic model of a culture of cortical-like neurons with spike timing dependent plasticity. We show that a stimulus applied at a corner of the culture can rebuild synaptic couplings. Then the network eventually switches from a turbulent-like asynchronous spiking to an ordered population spike signaling mode. The structural analysis shows that the stimulus potentiates centrifugal couplings, which promotes spiking waves traveling outwards the stimulus location. This phenomenon can be catalyzed by noise of an intermediate strength. We predict that matured cultures with high connectivity are more susceptible to reconfiguration and generation of a population spike response than young cultures with low connectivity. We also report on an intermittent synchronization causing switches between two quasi-stable states: generation of time-locked population spikes and turbulent spiking. In the turbulent mode the stimulus excites patches of spiking activity randomly traveling in the network. Such a regime can be implemented through a large scale looping of couplings backwards to the stimulus location. We anticipate that the robust mechanisms of shaping the network architecture discussed here can also be effective in more complex preparations and studies of the relationship between network structure and function