Signatures of Correlated Defects in an Ultraclean Wigner Crystal in the Extreme Quantum Limit.

Low-disorder two-dimensional electron systems in the presence of a strong, perpendicular magnetic field terminate at very small Landau level filling factors in a Wigner crystal (WC), where the electrons form an ordered array to minimize the Coulomb repulsion. The nature of this exotic, many-body, quantum phase is yet to be fully understood and experimentally revealed. Here we probe one of WC's most fundamental parameters, namely, the energy gap that determines its low-temperature conductivity, in record mobility, ultrahigh-purity, two-dimensional electrons confined to GaAs quantum wells. The WC domains in these samples contain ≃1000 electrons. The measured gaps are a factor of three larger than previously reported for lower quality samples, and agree remarkably well with values predicted for the lowest-energy, intrinsic, hypercorrelated bubble defects in a WC made of flux-electron composite fermions, rather than bare electrons. The agreement is particularly noteworthy, given that the calculations are done for disorder-free composite fermion WCs, and there are no adjustable parameters. The results reflect the exceptionally high quality of the samples, and suggest that composite fermion WCs are indeed more stable compared to their electron counterparts.

Moving Crystal Phases of a Quantum Wigner Solid in an Ultra-High-Quality 2D Electron System.

In low-disorder, two-dimensional electron systems (2DESs), the fractional quantum Hall states at very small Landau level fillings (ν) terminate in a Wigner solid (WS) phase, where electrons arrange themselves in a periodic array. The WS is typically pinned by the residual disorder sites and manifests an insulating behavior, with nonlinear current-voltage (I-V) and noise characteristics. We report here measurements on an ultralow-disorder, dilute 2DES, confined to a GaAs quantum well. In the ν<1/5 range, superimposed on a highly insulating longitudinal resistance, the 2DES exhibits a developing fractional quantum Hall state at ν=1/7, attesting to its exceptional high quality and dominance of electron-electron interaction in the low filling regime. In the nearby insulating phases, we observe remarkable nonlinear I-V and noise characteristics as a function of increasing current, with current thresholds delineating three distinct phases of the WS: a pinned phase (P1) with very small noise, a second phase (P2) in which dV/dI fluctuates between positive and negative values and is accompanied by very high noise, and a third phase (P3) where dV/dI is nearly constant and small, and noise is about an order of magnitude lower than in P2. In the depinned (P2 and P3) phases, the noise spectrum also reveals well-defined peaks at frequencies that vary linearly with the applied current, suggestive of washboard frequencies. We discuss the data in light of a recent theory that proposes different dynamic phases for a driven WS.

Delocalization and Universality of the Fractional Quantum Hall Plateau-to-Plateau Transitions.

Disorder and electron-electron interaction play essential roles in the physics of electron systems in condensed matter. In two-dimensional, quantum Hall systems, extensive studies of disorder-induced localization have led to the emergence of a scaling picture with a single extended state, characterized by a power-law divergence of the localization length in the zero-temperature limit. Experimentally, scaling has been investigated via measuring the temperature dependence of plateau-to-plateau transitions between the integer quantum Hall states (IQHSs), yielding a critical exponent κ≃0.42. Here we report scaling measurements in the fractional quantum Hall state (FQHS) regime where interaction plays a dominant role. Our Letter is partly motivated by recent calculations, based on the composite fermion theory, that suggest identical critical exponents in both IQHS and FQHS cases to the extent that the interaction between composite fermions is negligible. The samples used in our experiments are two-dimensional electron systems confined to GaAs quantum wells of exceptionally high quality. We find that κ varies for transitions between different FQHSs observed on the flanks of Landau level filling factor ν=1/2 and has a value close to that reported for the IQHS transitions only for a limited number of transitions between high-order FQHSs with intermediate strength. We discuss possible origins of the nonuniversal κ observed in our experiments.

Anisotropic Two-Dimensional Disordered Wigner Solid.

The interplay between the Fermi sea anisotropy, electron-electron interaction, and localization phenomena can give rise to exotic many-body phases. An exciting example is an anisotropic two-dimensional (2D) Wigner solid (WS), where electrons form an ordered array with an anisotropic lattice structure. Such a state has eluded experiments up to now as its realization is extremely demanding: First, a WS entails very low densities where the Coulomb interaction dominates over the kinetic (Fermi) energy. Attaining such low densities while keeping the disorder low is very challenging. Second, the low-density requirement has to be fulfilled in a material that hosts an anisotropic Fermi sea. Here, we report transport measurements in a clean (low-disorder) 2D electron system with anisotropic effective mass and Fermi sea. The data reveal that at extremely low electron densities, when the r_{s} parameter, the ratio of the Coulomb to the Fermi energy, exceeds ≃38, the current-voltage characteristics become strongly nonlinear at small dc biases. Several key features of the nonlinear characteristics, including their anisotropic voltage thresholds, are consistent with the formation of a disordered, anisotropic WS pinned by the ubiquitous disorder potential.

Correlated States of 2D Electrons near the Landau Level Filling ν=1/7.

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors (ν≲1/6) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultrahigh-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density n=6.1×10^{10}/cm^{2} and mobility µ=25×10^{6} cm^{2}/V s, we find a deep minimum in the longitudinal magnetoresistance (R_{xx}) at ν=1/7 when T≃104 mK. There is also a clear sign of a developing minimum in R_{xx} at ν=2/13. While insulating phases are still predominant when ν≲1/6, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence ν=p/(2mp±1) even in the very low Landau level filling limit. The magnetic-field-dependent activation energies deduced from the relation R_{xx}∝e^{E_{A}/2kT} corroborate this view and imply the presence of pinned Wigner solid states when ν≠p/(2mp±1). Similar results are seen in another sample with a lower density, further generalizing our observations.

Uso de bebidas energizantes y síntomas de insomnio en estudiantes de medicina de una universidad peruana / Use of energy drinks and insomnia symptoms in medical students from a Peruvian university

OBJETIVO: Determinar la asociación entre el consumo de bebidas energizantes y síntomas de insomnio en estudiantes de medicina de una universidad en Lima, Perú. MATERIALES Y MÉTODO: El tipo de estudio realizado es transversal analítico. La población objetivo fueron los estudiantes de medicina de una universidad peruana ubicada en Lima sur de segundo y quinto año. Hubo 289 participantes en el estudio. Se utilizó una encuesta compuesta por un cuestionario sobre el consumo de bebidas energizantes y el Insomnia Severity Index, del cual se determinó un punto de corte de 15 para considerar la presencia de síntomas de insomnio. El análisis multivariado crudo y ajustado se realizó usando la regresión de Poisson con varianza robusta ajustado para sexo, edad, consumo de café, y antecedente de ansiedad y depresión, para obtener el PR (Razón de Prevalencias) con un intervalo de confianza de 95%. RESULTADOS Y DISCUSIÓN: La prevalencia de síntomas de insomnio en la muestra estudiada fue de 21,80%, mientras que la de consumo de bebidas energizantes fue de 39,45%. Se encontró asociación significativa (p=0,008) entre el consumo de este tipo de bebidas y la presencia de síntomas de insomnio. Además, se encontró que los estudiantes que consumen bebidas energizantes tuvieron 1,78 veces más probabilidad de presentar síntomas de insomnio (IC95%: 1,13-2,82), en comparación con los que no consumieron bebidas energizantes (p=0,013). CONCLUSIONES: Existe asociación entre el consumo de bebidas energizantes y síntomas de insomnio.

OBJECTIVE: To determine the association between the consumption of energy drinks and symptoms of insomnia in medical students of a university in Lima, Peru. Materials and METHODS: This study is cross-sectional analytical. The target population was the second-and fifth-year medical students of a private Peruvian university in southern Lima. There were 289 participants in this study. For this study, a survey composed of a questionnaire about the consumption of energy drinks and the Insomnia Severity Index were used, from which a cut-off point of 15 was determined to consider the presence of insomnia symptoms. The multivariate crude and adjusted analysis were done with Poisson regression with robust variance adjusted for sex, age, coffee consumption, previous diagnosis of depression and previous diagnosis of anxiety; to calculate the PR (Prevalence Ratio) with a 95% CI. RESULTS AND DISCUSSION: The prevalence of insomnia symptoms in the sample studied was 21,80%, while the consumption of energy drinks was 39,45%. A significant association was found (p = 0.008) between the consumption of this type of drinks and the presence of insomnia symptoms. In addition, it was found that students who consumed energy drinks were 1,78 times more likely to have symptoms of insomnia (95% CI: 1,13-2,82), compared to those who did not consume energy drinks (p=<0,013). CONCLUSIONS: There is an association between the consumption of energy drinks and symptoms of insomnia.

Spontaneous Valley Polarization of Itinerant Electrons.

Memory or transistor devices based on an electron's spin rather than its charge degree of freedom offer certain distinct advantages and comprise a cornerstone of spintronics. Recent years have witnessed the emergence of a new field, valleytronics, which seeks to exploit an electron's valley index rather than its spin. An important component in this quest would be the ability to control the valley index in a convenient fashion. Here we show that the valley polarization can be switched from zero to 1 by a small reduction in density, simply tuned by a gate bias, in a two-dimensional electron system. This phenomenon, which is akin to Bloch spin ferromagnetism, arises fundamentally as a result of electron-electron interaction in an itinerant, dilute electron system. Essentially, the kinetic energy favors an equal distribution of electrons over the available valleys, whereas the interaction between electrons prefers single-valley occupancy below a critical density. The gate-bias-tuned transition we observe is accompanied by a sudden, twofold change in sample resistance, making the phenomenon of interest for potential valleytronic transistor device applications. Our observation constitutes a quintessential demonstration of valleytronics in a very simple experiment.

Fractional Quantum Hall Effect Energy Gaps: Role of Electron Layer Thickness.

The fractional quantum Hall effect stands as a quintessential manifestation of an interacting two-dimensional electron system. One of the fractional quantum Hall effect's most fundamental characteristics is the energy gap separating the incompressible ground state from its excitations. Yet, despite nearly four decades of investigations, a quantitative agreement between the theoretically calculated and experimentally measured energy gaps is lacking. Here we report a systematic experimental study that incorporates very high-quality two-dimensional electron systems confined to GaAs quantum wells with fixed density and varying well widths. The results demonstrate a clear decrease of the energy gap as the electron layer is made thicker and the short-range component of the Coulomb interaction is weakened. We also provide a quantitative comparison between the measured energy gaps and the available theoretical calculations that takes into account the role of finite layer thickness and Landau level mixing. All the measured energy gaps fall below the calculations, but as the electron layer thickness increases, the results of experiments and calculations come closer. Accounting for the role of disorder in a phenomenological manner, we find better overall agreement between the measured and calculated energy gaps, although some puzzling discrepancies remain.

Ultra-high-quality two-dimensional electron systems.

Two-dimensional electrons confined to GaAs quantum wells are hallmark platforms for probing electron-electron interactions. Many key observations have been made in these systems as sample quality has improved over the years. Here, we present a breakthrough in sample quality via source-material purification and innovation in GaAs molecular beam epitaxy vacuum chamber design. Our samples display an ultra-high mobility of 44 × 106 cm2 V-1 s-1 at an electron density of 2.0 × 1011 cm-2. These results imply only 1 residual impurity for every 1010 Ga/As atoms. The impact of such low impurity concentration is manifold. Robust stripe and bubble phases are observed, and several new fractional quantum Hall states emerge. Furthermore, the activation gap (Δ) of the fractional quantum Hall state at the Landau-level filling (ν) = 5/2, which is widely believed to be non-Abelian and of potential use for topological quantum computing, reaches Δ ≈ 820 mK. We expect that our results will stimulate further research on interaction-driven physics in a two-dimensional setting and substantially advance the field.

Observation of spontaneous ferromagnetism in a two-dimensional electron system.

What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter [Formula: see text], defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when [Formula: see text] surpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, when [Formula: see text] exceeds [Formula: see text] However, our sample makes a transition to an insulating state at [Formula: see text], preceding the onset of the spontaneous ferromagnetism, implying that besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.

Thermal and Quantum Melting Phase Diagrams for a Magnetic-Field-Induced Wigner Solid.

A sufficiently large perpendicular magnetic field quenches the kinetic (Fermi) energy of an interacting two-dimensional (2D) system of fermions, making them susceptible to the formation of a Wigner solid (WS) phase in which the charged carriers organize themselves in a periodic array in order to minimize their Coulomb repulsion energy. In low-disorder 2D electron systems confined to modulation-doped GaAs heterostructures, signatures of a magnetic-field-induced WS appear at low temperatures and very small Landau level filling factors (ν≃1/5). In dilute GaAs 2D hole systems, on the other hand, thanks to the larger hole effective mass and the ensuing Landau level mixing, the WS forms at relatively higher fillings (ν≃1/3). Here we report our measurements of the fundamental temperature vs filling phase diagram for the 2D holes' WS-liquid thermal melting. Moreover, via changing the 2D hole density, we also probe their Landau level mixing vs filling WS-liquid quantum melting phase diagram. We find our data to be in good agreement with the results of very recent calculations, although intriguing subtleties remain.

Precise Experimental Test of the Luttinger Theorem and Particle-Hole Symmetry for a Strongly Correlated Fermionic System.

A fundamental concept in physics is the Fermi surface, the constant-energy surface in momentum space encompassing all the occupied quantum states at absolute zero temperature. In 1960, Luttinger postulated that the area enclosed by the Fermi surface should remain unaffected even when electron-electron interaction is turned on, so long as the interaction does not cause a phase transition. Understanding what determines the Fermi surface size is a crucial and yet unsolved problem in strongly interacting systems such as high-T_{c} superconductors. Here we present a precise test of the Luttinger theorem for a two-dimensional Fermi liquid system where the exotic quasiparticles themselves emerge from the strong interaction, namely, for the Fermi sea of composite fermions (CFs). Via direct, geometric resonance measurements of the CFs' Fermi wave vector down to very low electron densities, we show that the Luttinger theorem is obeyed over a significant range of interaction strengths, in the sense that the Fermi sea area is determined by the density of the minority carriers in the lowest Landau level. Our data also address the ongoing debates on whether or not CFs obey particle-hole symmetry, and if they are Dirac particles. We find that particle-hole symmetry is obeyed, but the measured Fermi sea area differs quantitatively from that predicted by the Dirac model for CFs.

[The gut-brain axis in attention deficit hyperactivity disorder: the role of the microbiota]. / El eje intestino-cerebro en el trastorno por deficit de atencion/hiperactividad: papel de la microbiota.

INTRODUCTION: Attention deficit hyperactivity disorder (ADHD) has a complex aetiology, mainly attributed to a number of susceptibility genes and environmental factors. Genetic association studies, however, have been inconsistent and have identified genetic variants with a moderate effect that explain a small proportion of the estimated inheritability of the disorder (< 10%). Recent studies suggest that the gut microbiota and diet play an important role in the development and symptoms of different mental disorders. Nevertheless, no clear evidence exists on the issue. This project proposes an alternative approach to identify mechanisms by which the intestinal microbial ecosystem and diet could contribute to the presence of ADHD. AIM: To identify biomarkers for ADHD by examining the gut microbiota. SUBJECTS AND METHODS: We conducted a cross-sectional study of adult patients with ADHD (n = 100) and control subjects (n = 100). Measures of ADHD evaluation and eating habits were performed in both groups. Samples of faecal material were obtained from which to extract bacterial DNA, then used to characterise the participants' gut microbiota. A meta-genomic association study was later performed to attempt to correlate the bacterial composition of the intestine with the clinical subtypes of the disorder. RESULTS AND CONCLUSIONS: Comparing the gut microbiota profiles of subjects with ADHD and controls is expected to help account for the clinical heterogeneity of the disorder and identify new mechanisms involved in its development.

TITLE: El eje intestino-cerebro en el trastorno por deficit de atencion/hiperactividad: papel de la microbiota.Introduccion. El trastorno por deficit de atencion/hiperactividad (TDAH) presenta una etiologia compleja, atribuida principalmente a multiples genes de susceptibilidad y factores ambientales. No obstante, los estudios geneticos de asociacion han sido inconsistentes, identificando variantes geneticas de efecto moderado que explican una pequeña proporcion de la heredabilidad estimada del trastorno (< 10%). Recientes estudios sugieren que la microbiota intestinal y la dieta desempeñan un papel importante en el desarrollo y los sintomas de diferentes trastornos mentales. Sin embargo, en la actualidad no existe una claridad absoluta al respecto. El presente proyecto propone un abordaje alternativo para identificar mecanismos a traves de los cuales el ecosistema microbiano intestinal y la dieta podrian contribuir a la presencia del TDAH. Objetivo. Identificar biomarcadores para el TDAH a traves del estudio de la microbiota intestinal. Sujetos y metodos. Estudio transversal de pacientes adultos con TDAH (n = 100) y de individuos control (n = 100). En ambos grupos se tomaran medidas de evaluacion de TDAH y habitos alimentarios. Se obtendran muestras fecales para la extraccion del ADN bacteriano, que permitiran caracterizar la microbiota intestinal de los participantes, para posteriormente realizar un estudio de asociacion metagenomico e intentar correlacionar la composicion bacteriana intestinal con subtipos clinicos del trastorno. Resultados y conclusiones. Se espera que la comparacion de los perfiles de microbiota intestinal entre sujetos con TDAH y controles ayude a explicar la heterogeneidad clinica del trastorno e identificar nuevos mecanismos implicados en su desarrollo.

Transference of Fermi Contour Anisotropy to Composite Fermions.

There has been a surge of recent interest in the role of anisotropy in interaction-induced phenomena in two-dimensional (2D) charged carrier systems. A fundamental question is how an anisotropy in the energy-band structure of the carriers at zero magnetic field affects the properties of the interacting particles at high fields, in particular of the composite fermions (CFs) and the fractional quantum Hall states (FQHSs). We demonstrate here tunable anisotropy for holes and hole-flux CFs confined to GaAs quantum wells, via applying in situ in-plane strain and measuring their Fermi wave vector anisotropy through commensurability oscillations. For strains on the order of 10^{-4} we observe significant deformations of the shapes of the Fermi contours for both holes and CFs. The measured Fermi contour anisotropy for CFs at high magnetic field (α_{CF}) is less than the anisotropy of their low-field hole (fermion) counterparts (α_{F}), and closely follows the relation α_{CF}=sqrt[α_{F}]. The energy gap measured for the ν=2/3 FQHS, on the other hand, is nearly unaffected by the Fermi contour anisotropy up to α_{F}∼3.3, the highest anisotropy achieved in our experiments.

Control of overweight and obesity in childhood through education in meal time habits. The 'good manners for a healthy future' programme.

OBJECTIVE: Our aim is to determine the effect of paced eating, exposure to an educational programme that promotes healthy eating habits and allowing the satiety reflex to limit food intake in controlling weight gain in healthy adolescents. METHODS: Fifty-four healthy individuals consisting of 18 adolescent girls and 36 boys aged 12 ± 2 years were given recommendations for reducing eating rate without changing diet or meal size according to the educational programme 'good manners for a healthy future'. Each participant was provided with a 30-s portable hourglass to pace time between bites. Individuals using and not using the hourglass were placed either into an 'adhering' or a 'non-adhering' group, respectively. Control data were obtained from a similar population. RESULTS: Initially, the adhering group had higher weight compared with the non-adhering group (64.1 ± 13.2 vs. 56.2 ± 11.7 kg). Control group weight was no different from the study group at baseline (56.3 ± 10.3 kg). Weight in the adhering group decreased after the first semester of participation by 2.0 ± 5.7% and after a year by 3.4 ± 4.8%, while the non-adhering group gained weight by 5.8 ± 4.5% and 12.6 ± 8.3%. The control group increased weight after a year by 8.2 ± 6.5%. In total, 18 non-adhering and 14 adhering adolescents completed the study. CONCLUSIONS: This 1-year study shows a statistically significant association between rate of food intake and weight control in adherence to an educational programme directed at developing healthy eating habits. The proposed behavioural training may serve as an option for weight control in adolescents.