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The competition between kinetic energy and Coulomb interactions in electronic systems can lead to complex many-body ground states with competing superconducting, charge density wave, and magnetic orders. Here we study the low temperature phases of a strongly interacting zinc-oxide-based high mobility two dimensional electron system that displays a tunable metal-insulator transition. Through a comprehensive analysis of the dependence of electronic transport on temperature, carrier density, in-plane and perpendicular magnetic fields, and voltage bias, we provide evidence for the existence of competing correlated metallic and insulating states with varying degrees of spin polarization. Our system features an unprecedented level of agreement with the state-of-the-art Quantum Monte Carlo phase diagram of the ideal jellium model, including a Wigner crystallization transition at a value of the interaction parameter $r_s\sim 30$ and the absence of a pure Stoner transition. In-plane field dependence of transport reveals a new low temperature state with partial spin polarization separating the spin unpolarized metal and the Wigner crystal, which we examine against possible theoretical scenarios such as an anti-ferromagnetic crystal, Coulomb induced micro-emulsions, and disorder driven puddle formation.

We present systematic investigations of the electrically induced breakdown of the integer quantum Hall effect (QHE) in (Al,Ga)As-based Hall bars of different widths ranging from 7 to 70 $\ensuremath{\mu}\mathrm{m}$. It is striking that, for the narrow Hall bars, the threshold values for the applied voltage that induces the breakdown of the longitudinal zero-resistance state differ appreciably between the upper and lower sides of a quantum Hall (QH) plateau. When moving from the low magnetic field side of the QH plateau to higher magnetic fields, the threshold rises in a strongly superlinear manner until---at the expected integer value for the Landau level (LL) filling factor in the bulk of the two-dimensional electron system (2DES)---the threshold value abruptly drops to low values for the rest of the plateau. With increasing widths of the Hall bars, the zero-resistance state extends slightly further to higher magnetic field values. The threshold values on the low-magnetic field side are almost independent of the Hall bar width, whereas the threshold values on the high magnetic field side scale linearly with the Hall bar width. These observations correspond perfectly with the microscope picture of the QHE where the biased current flows in a dissipationless manner in electrically incompressible regions of the 2DES of locally the same LL filling factor, driven by the respective drop of the Hall voltage over the width of these incompressible regions. Owing to the self-consistent evolution of the electrically incompressible/compressible landscape within the 2DES as a function of magnetic field, a transition from an edge- to a bulk-dominated QH regime is described within a QH plateau. This microscopic picture was derived from sophisticated, long-lasting scanning probe experiments measuring Hall potential profiles on typically 15-$\ensuremath{\mu}$m-wide Hall bars. Conversely, performing systematic electrically induced breakdown measurements like those presented here would allow us to identify the presence of edge- and bulk-dominated regimes of the QHE in a wider variety of samples.

We present an experimental study on microwave illuminated high mobility MgZnO/ZnO based two-dimensional electron systems with different electron densities and, hence, varying Coulomb interaction strength. The photoresponse of the low-temperature dc resistance in perpendicular magnetic field is examined in low and high density samples over a broad range of illumination frequencies. In low density samples a response due to cyclotron resonance (CR) absorption dominates, while high density samples exhibit pronounced microwave-induced resistance oscillations (MIRO). Microwave transmission experiments serve as a complementary means of detecting the CR over the entire range of electron densities and as a reference for the band mass unrenormalized by interactions. Both CR and MIRO-associated features in the resistance permit extraction of the effective mass of electrons but yield two distinct values. The conventional cyclotron mass representing center-of-mass dynamics exhibits no change with density and coincides with the band electron mass of bulk ZnO, while MIRO mass reveals a systematic increase with lowering electron density consistent with renormalization expected in interacting Fermi liquids.

Surface Acoustic Waves (SAW) have been utilized to investigate the properties of a two-dimensional electron system, subjected to a perpendicular magnetic field and monochromatic microwave radiation, in the regime where the so-called microwave-induced zero-resistance states form. Contrary to conventional magneto-transport in Hall bar and van der Pauw geometries, the collimated SAW beam probes only the bulk of the electronic system exposed to this wave. Clear signatures appear in the SAW propagation velocity corroborating that neither contacts, nor sample edges are a root source for their emergence. By virtue of the directional nature of this probing method and with the assistance of theoretical modelling, we were also able to demonstrate that the SAW response depends on the angle between its propagation vector and the orientation of domains which spontaneously form when zero-resistance is observed in transport. This confirms in unprecedented manner the formation of an inhomogeneous phase under these non-equilibrium conditions.

The effect of microwave radiation on the amplitudes of the quantum magnetocapacitance oscillations in field-effect transistors has been studied. It has been found that the microwave-induced variation of the oscillation amplitude depends nonmonotonically on the magnetic field including the amplitude enhancement effect. It has been shown that this behavior is due to the band bending near the edge of the gate. The effect has been attributed to the interference of quantum oscillations with radiation-induced magneto-oscillations of the screening length of a static electric field.

The temperature dependence of the microwave photovoltage has been studied in microwave-induced states of a two-dimensional electron system, which are characterized by an almost dissipationless flow of a low-frequency current. At decreasing temperature, a smooth transition has been found from a bistable state, where the photovoltage demonstrates switching between two levels, which are due to reversals of the spontaneous electric field in a domain structure, to a steady state. The transition occurs as the shift of one of the levels of the bistable photovoltage to the other level accompanied by a decrease in the switching frequency. The results indicate the freezing of the dynamic domain structure in the state corresponding to the more stable configuration of the electric field.

The temperature dependence of the switching frequency of a microwave-induced spontaneous electric field forming a domain structure and the conductance of a doping layer that provides electrons to the two-dimensional electron system have been measured on samples fabricated from the same GaAs/AlGaAs heterostructure. Both quantities have been found to obey the thermally activated dependence (Arrhenius law) with close activation energies. This result indicates a relation between the quantities and confirms a hypothesis that the observed dynamics of the domain structure originates from the dynamical screening of the spontaneous electric field of domains by charges in the doping layer.

Aims The identification of high-risk groups is crucial in public health suicide prevention approaches. This study aims to compare suicide risks of Germans with nine of the largest immigrant populations living in Germany. Methods Based on a German national database of mortality statistics, the number of suicides in Germans and immigrants was available for the study period (2000 - 2017), stratified for gender and age groups. Standard mortality ratios (SMR) for suicide were computed since age distributions differed between populations. Moreover, SMR of immigrant populations were correlated with potential risk and resilience factors. Results The analysed dataset covers a period of 18 years, which translates to over 1.47 billion life years (LY) and 206,056 recorded suicides. 134,971,779 LY (10.1%) and 8,936 (4.3%) suicides were assigned to non-German citizens. SMR, calculated for nine of the largest immigrant populations, were lower compared with the German reference population ranging from 0.24 (Greek nationality) to 0.86 (Russian nationality). SMR in immigrants was highest in adolescents and declined with age. SMR was associated with country of origin (CO) suicide rates as well as with socio-economic factors of immigrant groups in Germany. With the global financial crisis, suicide risk of immigrants from the most affected countries decreased more strongly compared to immigrants from other CO. Conclusions The suicide risk strongly differs between the individual immigrant groups and is associated with risk factors of the respective CO. Therefore, future suicide prevention approaches in immigrants should take CO-specific vulnerabilities into account as well as age-related risk factors.

It has been found on a sample of the GaAs/AlGaAs heterostructure with the two-dimensional electron system that different configurations of domains of a spontaneous electric field are possible within one microwave- induced state with the resistance tending to zero. Transitions between such configurations are observed at the variation of the radiation power and magnetic field. In the general case, the configuration of domains is more complicated than existing models. The fragment of the distribution of the electric field in the sample for one of the observed configurations is in agreement with the rhombic domain structure considered by I. G. Finkler and B. I. Halperin, Phys. Rev. B 79, 085315 (2009).

The pathogenesis of post-traumatic stress disorder (PTSD) is incompletely understood. We hypothesize that disruptions in mother-child relations may be a key contributor to development of PTSD. A normal and healthy separation-individuation process requires adaptations of self- and interactive contingency in both the mother and her child, especially in early childhood development. Anxious mothers are prone to overprotection, which may hinder the individuation process in their children. We examined long-term stress hormones and other stress markers in subjects three generations removed from the Holocaust, to assess the long-term consequences of inherited behavioral and physiological responses to prior stress and trauma. Jewish subjects who recalled overprotective parental behavior had higher hairsteroid-concentrations and dampened limbic-hypothalamic-pituitary-adrenal (LHPA) axis reactivity compared to German and Russian-German subjects with overprotective parents. We suggest that altered LHPA axis activity in maternally overprotected Jewish subjects may indicate a transmitted pathomechanism of "frustrated individuation" resulting from cross-generational anti-Semitic experiences. Thus measurements of hairsteroid-concentrations and parenting practices may have clinical value for diagnosis of PTSD. We propose that this apparent inherited adaptivity of LHPA axis activity could promote higher individual stress resistance, albeit with risk of an allostatic overload.

published

Half-filled Landau levels host an emergent Fermi-liquid which displays an instability towards pairing, culminating in a gapped even-denominator fractional quantum Hall ground state. While this pairing may be probed by tuning the polarization of carriers in competing orbital and spin degrees of freedom, sufficiently high quality platforms offering such tunability remain few. Here we explore the ground states at filling factor $\nu$ = 5/2 in ZnO-based two-dimensional electron systems through a forced intersection of opposing spin branches of Landau levels taking quantum numbers $N$ = 1 and 0. We reveal a cascade of phases with distinct magnetotransport features including a gapped phase polarized in the $N$ = 1 level and a compressible phase in N = 0, along with an unexpected Fermi-liquid, a second gapped, and a strongly anisotropic nematic-like phase at intermediate polarizations when the levels are near degeneracy. The phase diagram is produced by analyzing the proximity of the intersecting levels and highlights the excellent reproducibility and controllability ZnO offers for exploring exotic fractionalized electronic phases.

Here we report on a transparent method to characterize individual layers in a double-layer electron system which forms in a wide quantum well and to determine their electron densities. The technique relies on the simultaneous measurement of the capacitances between the electron system and gates located on either side of the well. Modifications to the electron wave function due to the population of the second subband and appearance of an additional electron layer can be detected. The magnetic field dependence of these capacitances is dominated by quantum corrections caused by the occupation of Landau levels in the nearest electron layer. The technique should be equally applicable to other implementations of a double layer electron system., submitted to Journal of Applied Physics

Hintergrund Eine familienfokussierte Sichtweise auf die psychosozialen Folgen einer Krebserkrankung gewinnt zunehmend an Bedeutung.

We report on Hall field-induced resistance oscillations (HIRO) in a 60 nm-wide GaAs/AlGaAs quantum well with an \emph{in situ} grown back gate, which allows tuning the carrier density $n$. At low $n$, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with $n^{-1/2}$, as expected. Remarkably, population of the second subband (SB2) significantly enhances HIRO, while their frequency now scales as $n^{-1}$. We demonstrate that in this two-subband regime HIRO still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, while that of SB1 remains essentially unchanged. The enhancement of HIRO manifests an unexpected, step-like increase of the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the two-subband regime., Comment: 5 pages, 5 figures

We have studied the absorption of monochromatic microwave radiation in high-quality two-dimensional electron systems for the frequency span from 10 to 380 GHz using a bolometric method. For frequencies above 100 GHz the absorption exhibits an anomalous magnetic field dependence. Minima form at harmonics of the cyclotron resonance frequency. The results contrast previously reported data for other frequency ranges. Quasiclassical memory effects originating from the non-Markovian dynamics of electrons in a disorder potential containing short-range scatterers on top of a smooth potential background favorably account for the observed behavior.

The physics of itinerant electrons in condensed matter is by and large governed by repulsive Coulomb forces. However, attractive interactions may emerge and prevail in determining the ground state despite the pervasive Coulomb repulsion. A notable example is electron pairing and superconductivity. The interplay of attractive and repulsive interactions may also instigate spontaneous symmetry lowering and clustering of charges in geometric patterns even without net attraction. Both types of attractive interaction triggered physics—pairing and charge ordering—are at play in two-dimensional electron systems exposed to a quantizing magnetic field. The charge ordering has been concluded indirectly from transport behaviour. Here we report the observation of negative permittivity present solely when bubble and stripe phases form. In conjunction with a theoretical model, the negative permittivity provides evidence for the underlying attractive exchange-correlation energy which sufficiently countervails Coulomb repulsion at small distances to enable and mediate charge clustering.

In a two-dimensional electron system, microwave radiation may induce giant resistance oscillations. Their origin has been debated controversially and numerous mechanisms based on very different physical phenomena have been invoked. However none of them have been unambiguously experimentally identified, since they produce similar effects in transport studies. The capacitance of a two-subband system is sensitive to a redistribution of electrons over energy states, since it entails a shift of the electron charge perpendicular to the plane. In such a system microwave induced magnetocapacitance oscillations have been observed. They can only be accounted for by an electron distribution function oscillating with energy due to Landau quantization, one of the quantum mechanisms proposed for the resistance oscillations., submitted to Phys.Rev.Lett., corresponding author S. I. Dorozhkin

BackgroundThe concept of mental toughness has gained increasing importance among non-elite athletes for its psychological importance and explanatory power for a broad range of health-related behaviors. On the flip side, no study has focused so far on the psychological origins of mental toughness. Therefore, the aims of the present study were three-fold: to explore, to what extent psychological profiles of preschoolers at the age of five years predicted mental toughness scores and sleep disturbances at the age of 14 years, and to explore possible gender differences.MethodNine years after their first assessment at the age of five years (preschoolers), a total of 77 adolescents (mean age: 14.35 years; SD = 1.22; 42% females) took part in the present follow-up study. At baseline, both parents and teachers completed the Strengths and Difficulties Questionnaire (SDQ), covering internalizing and externalizing problems, hyperactivity, negative peer relationships, and prosocial behavior. At follow-up, participants completed a booklet of questionnaires covering socio-demographic data, mental toughness, and sleep disturbances.ResultsPreschoolers with high prosocial behavior and low internalizing and externalizing problems, as rated by parents and teachers, at the age of 14 years self-reported higher mental toughness and lower sleep disturbances. At the age of 14 years, and relative to their male counterparts, female participants reported lower MT scores and higher sleep disturbances.ConclusionsThe pattern of results suggests that mental toughness traits during adolescence have their origins during pre-school years.Disclosure of interestThe authors have not supplied their declaration of competing interest.

Since 1990, the integer quantum Hall effect has provided the electrical resistance standard, and there has been a firm belief that the measured quantum Hall resistances are described only by fundamental physical constants—the elementary charge e and the Planck constant h . The metrological application seems not to rely on detailed knowledge of the microscopic picture of the quantum Hall effect; however, technical guidelines are recommended to confirm the quality of the sample to confirm the exactness of the measured resistance value. In this paper, we give our present understanding of the microscopic picture, derived from systematic scanning force microscopy investigations on GaAs/(AlGa)As quantum Hall samples, and relate these to the technical guidelines.

Microwave radiation has a dramatic effect on the magneto-resistance of two-dimensional electron systems, even reducing it below zero. It is thought that this is the result of the formation of distinct current domains. Direct experimental evidence for these domains is now presented for the first time.

Im Rahmen einer randomisierten Kontrollgruppenstudie wird die Effektivitat des Hausbesuchsprogramms „Pro Kind“ uberpruft. 755 multipel risikobelastete Erstgebarende wurden bis Ende 2009 in Niedersachsen, Bremen und Sachsen akquiriert und einer von zwei Untersuchungsgruppen zugewiesen. Die Teilnehmerinnen konnen die Regelversorgung in Anspruch nehmen, Frauen in der Treatmentgruppe erhalten daruber hinaus uber einen Zeitraum von zweieinhalb Jahren regelmasig Hausbesuche von Familienbegleiterinnen. Primare Projektziele der Begleitung sind die Verbesserung des mutterlichen Gesundheitsverhaltens bereits in der Schwangerschaft und die Geburt eines gesunden Kindes, langerfristig soll die kindliche Gesundheit und Entwicklung verbessert werden. Bisher sind keine signifikanten Gruppenunterschiede in den primaren Zielvariablen der Schwangerschaftsphase nachweisbar. In Interimsanalysen zur kindlichen Entwicklung im Alter von sechs und zwolf Monaten zeigt sich eine tendenziell positivere kognitive Entwicklung der Kinder in der Treatmentgruppe. Die Ergebnisse werden hinsichtlich methodischer und praktischer Aspekte kritisch diskutiert.

ZusammenfassungVerfahren der Adipositaschirurgie stellen eine Therapieoption dar bei extrem adipösen Erwachsenen und zunehmend auch bei Jugendlichen mit morbider Adipositas, bei denen die konservativen Therapieansätze (Ernährungsanalyse und -schulung, Steigerung der körperlichen Aktivität, psychologische und familientherapeutische Intervention) nicht zum gewünschten Erfolg geführt haben. Die wenigen bisher zur Verfügung stehenden Daten zur Adipositaschirurgie im Jugendalter berichten nicht nur über eine signifikante postoperative BMI-Reduktion, sondern zeigen auch eine deutliche Verbesserung von präoperativ bereits bestehenden Begleiterkrankungen und dem psychosozialen Wohlbefinden. Die Indikationsstellung zur Adipositaschirurgie sowie obligate präoperative Untersuchungen sollten in speziellen Adipositas-Zentren erfolgen. Für die prä- und postoperative Betreuung ist ein interdisziplinäres Team bestehend aus Pädiater/Internist, (Kinder-)Chirurg, Anästhesist, (Kinder-)Psychiater bzw. -psychologe und Ökotrophologe wünschenswert.

ZusammenfassungLebensstile beinhalten nicht nur Ernährungs-, Bewegungsund Fernsehgewohnheiten. Sie stellen vielmehr ein Muster aus Präferenzen dar, die die Auswahl sämtlicher Verhaltensweisen im Alltag steuern. Wir zeigen im Beitrag Ergebnisse einer bevölkerungsrepräsentativen Befragung von Erwachsenen im Alter von 18 bis 25 Jahren. Lebensstile ermitteln wir, indem wir aus einer Vielzahl von Alltagsaktivitäten Lebensstilorientierungs-Skalen bilden, die als die Präferenzen der Befragten interpretiert werden. Im ersten Teil der Auswertungen zeigen sich Zusammenhänge zwischen eher passiven Lebensstilorientierungen und ungünstigen Ernährungsgewohnheiten. Im zweiten Teil werden Unterschiede in den Lebensstilorientierungen und den Ernährungsgewohnheiten zwischen normalgewichtigen, übergewichtigen und adipösen Probanden geprüft. Hier zeigen sich niedrige Ausprägungen bei aktiven Lebensstilorientierungen und höhere bei passiven Lebensstilorientierungen für übergewichtige und adipöse junge Erwachsene.

Magnetic-field dependences of the conductivity of a two-dimensional electron system obtained by contact and contactless measurements in the regime of microwave-induced giant magnetoresistance oscillations have been comparatively analyzed. The contactless technique for studying the conductivity of two-dimensional electrons is based on measuring the attenuation of the RF signal propagating along a coplanar waveguide manufactured using lithography on the sample surface. It has been found that Shubnikov-de Haas oscillations of conductivity are observed in both techniques, whereas the microwave-induced giant magnetoresistance oscillations appear only in the contact measurements. This contradiction indicates that the contact and/or boundary regions of the two-dimensional system with a strong potential gradient play an important role for the observation of the induced magnetoresistance oscillations.

Giant fluctuations in the 2D-electron recombination radiation were studied in structures with a single or double GaAs quantum well under quantum Hall effect conditions. It is established that, if these conditions are exactly satisfied, the amplitude of the 2D-electron photoluminescence (PL) intensity is several orders of magnitude higher than the noise level, with the noise having a normal (Poisson) distribution. The fluctuations in the PL line intensity are accompanied by a jumpwise change in the line positions. Analogous jumps were also observed in the spectra of inelastic light scattering by 2D electrons in structures with a single GaAs quantum well. The fluctuation processes are correlated over macroscopic distances. The characteristic correlation length is 1–2 mm. The spectral density of giant fluctuations was found to exhibit narrow peaks. The ratios of the frequencies of these peaks are equal to those of Fibonacci numbers. The appearance of such frequencies in the fluctuation spectrum indicates that the fluctuations studied bear a resemblance to processes occurring in open dissipative dynamic systems. The methods developed in the theory of these systems can, in principle, be used to study giant fluctuations.

Recent experiments on quantum Hall bilayers in the vicinity of total filling factor 1 (νT=1) have revealed the possibility of a superfluidic exciton condensate. We report on our experimental work involving the νT=1 exciton condensate in independently contacted bilayer two-dimensional electron systems. We reproduce the previously reported zero bias resonant tunneling peak, a quantized Hall drag resistivity, and in counter-flow configuration, the near vanishing of both ρxx and ρxy resistivity components. At balanced electron densities in the layers, we find for both drag and counter-flow current configurations, thermally activated transport with a monotonic increase of the activation energy for d/ℓB < 1.65 with activation energies up to 0.4 K. In the imbalanced system the activation energies show a striking asymmetry around the balance point, implying that the gap to charge excitations is considerably different in the separate layers that form the bilayer condensate. This indicates that the measured activation energy is neither the binding energy of the excitons, nor their condensation energy. We establish a phase diagram of the excitonic condensate showing the enhancement of this state at slight imbalances.

The spectra of resonant microwave absorption in single strips of two-dimensional electrons with a large length-to-width ratio have been analyzed using the optical detection method. Deviations from the linear behavior in the region of low quasimomenta have been observed in the measured wavenumber dependences of the plasma-excitation frequencies. These deviations are caused by the effect of the logarithmic term on the dispersion law of a one-dimensional plasmon. Comparison of the experimental results with theoretical calculations confirms the applicability of the logarithmic term to the description of the one-dimensional plasmon for wavenumbers that are tens and hundreds times lower than the inverse strip width.

In a two-dimensional electron system subject to microwaves and a magnetic field, photovoltages emerge. They can be separated into two components originating from built-in electric fields and electric field domains arising from spontaneous symmetry breaking. The latter occurs in the zero resistance regime only and manifests itself in pulsed behavior, synchronous across the sample. The pulses show sign reversal. This implies a flip of the field in each domain, consistent with the existence of two equally probable electric field domain configurations due to the spontaneous symmetry breaking.

We have investigated the influence of the polarization of the incident radiation on the recently discovered microwave-induced resistance oscillations in state-of-the-art highest purity 2D electron systems. A quasi-optical setup allows us to tune in situ between different circular as well as linear polarization states. We find that the microwave-induced zero resistance and the resistance oscillations are notably immune to changes in the polarization. This observation is discrepant with a number of proposed theories. Deviations for different polarizations only occur for a bolometric contribution to the resistance associated with the resonant heating at the cyclotron resonance itself.

Recent experiments on quantum Hall bilayers in the vicinity of total filling factor 1 ( ν T = 1 ) have revealed many exciting observations characteristic of a superfluidic exciton condensate. We report on our experimental work involving the ν T = 1 exciton condensate in independently contacted bilayer two-dimensional electron systems. We observe previously reported phenomena as a zero-bias resonant tunneling peak, a quantized Hall drag resistivity, and in counter-flow configuration, the near vanishing of both ρ xx and ρ xy resistivity components. At balanced electron densities in the layers, we find for both drag and counter-flow current configurations, thermally activated transport with a monotonic increase of the activation energy for d / l B 1.65 with activation energies up to 0.4 K. In the imbalanced system the activation energies show a striking asymmetry around the balance point, implying that the gap to charge excitations is considerably different in the separate layers that form the bilayer condensate. This indicates that the measured activation energy is neither the binding energy of the excitons, nor their condensation energy.

We report on our experimental work involving the T = 1 exciton condensate in independently contacted bilayer two-dimensional electron systems. A phase diagram between the weakly coupled ( U = 1/2, L = 1/2) phase and the strongly coupled T = 1 phase is experimentally investigated as a function of d /B and electron density imbalance n ( [n L - n U]/n T) using a drag current configuration. For imbalanced electron layer densities, small interlayer imbalances lead to strengthening of the excitonic phase. Here density imbalance leads to a phase boundary in the drag layer which is symmetric around zero imbalance. Surprisingly this behavior is not seen for drive,xx , where an unusual asymmetric behavior around zero imbalance occurs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The kinetics of the indirect recombination of electrons and holes in wide single quantum wells in a strong electric field has been analyzed. It has been shown that the recombination time increases exponentially up to 20 μs due to the spatial separation of oppositely charged particles. The results of a theoretical model predicting the behavior of the recombination time as a function of the applied field are in good agreement with experimental data.

The spectra of edge magnetoplasma excitations in two-dimensional (2D) electron disks have been analyzed by the method of optical detection of resonant microwave absorption. The magnetic dispersion of an edge magnetoplasmon in samples with a high 2D electron density is found to be poorly reproduced by existing theoretical models. Analysis of the magnetic-field dependence of the linewidth of resonant microwave absorption for samples with various 2D electron densities shows that the inverse width of the main mode of resonant microwave absorption is universally proportional to the Hall resistance of 2D electrons.

The recombination spectra of indirect excitons and double electron-hole layers in a wide single quantum well in an electric field are studied. It is found that electrons and holes in the wide well become spatially separated in a sufficiently strong electric field. This leads to a substantial reorganization of the radiative recombination spectrum and to a significant increase in the carrier lifetime. It is shown that the total charge of the electron-hole system can be changed by varying the photoexcitation frequency and the applied electric field, thus passing from the neutral case of indirect excitons to the case of charged double electron-hole layers. The concentration of excess carriers in the well is measured as a function of the electric field strength. The behavior of the excited states of indirect heavy-hole and light-hole excitions is studied for a neutral excitonic system in a strong electric field. It is shown that the electric-field dependences allow the excited states of indirect excitons with a light hole to be distinguished from the excited states with a heavy hole.

In the spectrum of photovoltage oscillations that are periodic with respect to the magnetic field and appear on Hall structures under microwave irradiation, two frequency components of oscillations are observed and analyzed. The appearance of these two frequencies in photovoltage oscillations is explained by the existence of two trajectories of edge magnetoplasmons and by the effects of the interference of collective excitations on these trajectories. The effects of the temperature, microwave radiation frequency, and magnetic field strength on the mean free path of edge magnetoplasmons are analyzed.

Recombination radiation and inelastic light scattering spectra of 2D electrons are studied simultaneously in structures with a single GaAs quantum well in the quantum Hall effect regime. It is found that the intensity of intersubband inelastic light scattering (the SDE mode) at the filling factor ν = 2 exhibits fluctuations in phase with fluctuations of the photoluminescence intensity of 2D electrons of the ground subband. It is shown that, simultaneously with fluctuations of the scattered light and recombination radiation intensities, the spectral positions of these lines change stepwise by 0.3–0.5 meV. The jumps found in the spectral positions of lines are observed only in narrow intervals of magnetic fields corresponding to the quantum Hall effect and are presumably associated with zero-point fluctuations (Casimir-Lifshits force).

The spectrum of two-dimensional 2D electrons subjected to a weak 2D potential and a perpendicular magnetic field is composed of Landau bands with a fractal internal pattern of subbands and minigaps referred to as Hofstadter's butterfly. The Hall conductance may serve as a spectroscopic tool as each filled sub-band contributes a specific quantized value. Advances in sample fabrication now finally offer access to the regime away from the limiting case of a very weak potential. Complex behavior of the Hall conductance is observed and assigned to Landau band coupling induced rearrangements within the butterfly.

The spectra of magnetoplasma excitations in two-dimensional electron disks and rings are studied by optical detection of resonance microwave absorption. For ring-shaped structures, two types of edge magnetoplasma modes localized along the inner and outer boundaries of the ring are observed. It is shown that the interaction between these modes leads to a strong modification of their magnetic-field dependences as compared to disks. In addition to the longitudinal edge magnetoplasma excitations, transverse plasma modes associated with the electron density oscillations along the ring radius are revealed. The spectra of magnetoplasma excitations are calculated in terms of the electrodynamic theory for both ring-shaped and disk-shaped structures. The classification of all modes of collective magnetoplasma excitations observed in the experiment is performed on the basis of the comparison between experimental and theoretical results.

The spin unpolarised to spin polarised v=2/3 phase transition is studied in narrow quantum wells. The phase transition is mapped in the filling factor (ν), density (n)-plane and is well described assuming a Image dependence of the Composite Fermion effective mass. At high currents the huge longitudinal resistance anomaly appears along the phase boundary. A model is presented that treats the current induced nuclear spin polarisation as an additional source of disorder, which drastically alters the initial domain structure present at the phase boundary. Time and current dependent measurements at constant magnetic field indicate that a large alternating Hall field is necessary to maintain the high resistance state.

We present magneto-transport measurements in tilted magnetic fields on high mobility (μ=300.000 cm 2 / V s ) AlAs two-dimensional electron systems. Our samples show integer and fractional quantum Hall effects. In titled magnetic fields, at the crossings of Landau levels with opposite spin, the resistance shows a spike if the crossing occurs near integer filling or is significantly enhanced otherwise. The position of the spike displays a hysteresis below 350 mK that reaches more than 0.1 T at 20 mK . From the position of the crossing in the (Btot,Bperp)-plane the difference in exchange energy of the crossing levels is determined and found to be a non-linear function of the perpendicular magnetic field.

Using a new MBE growth technique, we fabricate a two-dimensional electron system which is bent around an atomically sharp 90° corner. In the quantum Hall regime under tilted magnetic fields, we can measure equilibration between both co- and counter-propagating edge channels of arbitrary filling factor ratio. We present here 4-point magnetotransport characterization of the corner junction with unequal filling factor combinations which can all be explained using the standard Landauer–Buttiker edge channel picture. The success of this description confirms the realization of the first non-planar quantum Hall edge geometry.

Spectra of collective and magnetic excitations in symmetric double quantum wells with strong tunnel coupling were studied by inelastic scattering. Gaps in the spectrum of collective and single-particle excitations associated with tunnel interwell splitting were measured. New excitation modes, namely, tunnel Bernstein modes, were detected in a magnetic field. Based on the dispersion and magnetic-field dependences of the energies of inelastic light scattering lines, a classification of excitations and magnetic excitations was given.

A new class of single-particle excitations in tunnel-coupled electron bilayers is investigated by inelastic light scattering. The dispersion law and the dependence of the energies of these excitations on the degree of unbalance between the layers have been measured. A new spectroscopic method is proposed for determining the degree of unbalance between bilayers.

A manifestation of retardation effects, which were predicted theoretically more than 35 years ago, is revealed for the first time in the plasma excitation spectrum of a two-dimensional electron system with a high electron mobility. It is shown that a significant decrease in the resonant plasma frequency due to a hybridization of the plasma and light modes is observed in zero magnetic field. An unusual dependence of the frequency of the hybrid cyclotron-plasmon mode on the magnetic field has been observed in a perpendicular magnetic field. The experimental results are in good quantitative agreement with the theory.