Moiré-Induced Optical Nonlinearities: Single- and Multiphoton Resonances.

Moiré excitons promise a new platform with which to generate and manipulate hybrid quantum phases of light and matter in unprecedented regimes of interaction strength. We explore the properties in this regime, through studies of a Bose-Hubbard model of excitons coupled to cavity photons. We show that the steady states exhibit a rich phase diagram with pronounced bistabilities governed by multiphoton resonances reflecting the strong interexciton interactions. In the presence of an incoherent pumping of excitons we find that the system can realize single- and multiphoton lasers.

P-Band Induced Self-Organization and Dynamics with Repulsively Driven Ultracold Atoms in an Optical Cavity.

We investigate a Bose-Einstein condensate strongly coupled to an optical cavity via a repulsive optical lattice. We detect a stable self-ordered phase in this regime, and show that the atoms order through an antisymmetric coupling to the P band of the lattice, limiting the extent of the phase and changing the geometry of the emergent density modulation. Furthermore, we find a nonequilibrium phase with repeated intense bursts of the intracavity photon number, indicating nontrivial driven-dissipative dynamics.

Topological bands for ultracold atoms.

There have been significant recent advances in realizing band structures with geometrical and topological features in experiments on cold atomic gases. This review summarizes these developments, beginning with a summary of the key concepts of geometry and topology for Bloch bands. Descriptions are given of the different methods that have been used to generate these novel band structures for cold atoms and of the physical observables that have allowed their characterization. The focus is on the physical principles that underlie the different experimental approaches, providing a conceptual framework within which to view these developments. Also described is how specific experimental implementations can influence physical properties. Moving beyond single-particle effects, descriptions are given of the forms of interparticle interactions that emerge when atoms are subjected to these energy bands and of some of the many-body phases that may be sought in future experiments.

Chromaticity separation and the alpha response.

Chromatic gratings can be uncomfortable to view and can evoke a large haemodynamic response. Both the discomfort and the amplitude of the haemodynamic response increase monotonically with the perceptual difference in the colour of the component bars of the grating, as registered by the separation in their chromaticity in the CIE 1976 UCS diagram. Individuals with photosensitive epilepsy exhibit epileptiform EEG activity in response to flickering light of alternate colours. The probability of the epileptiform response again increases monotonically with the separation of the colours in the CIE UCS diagram. We investigated whether alpha power, which is known to reflect the excitation of large populations of neurons, is similarly affected by the separation in chromaticity. Chromatic square-wave gratings with bars that differed in CIE UCS chromaticity were presented, together with a central fixation cross. In 18 non-clinical participants, alpha responses were recorded over the visual cortex (O1, Oz, O2, PO3, POz, PO4, P1, P2) and compared to responses in prefrontal cortex (F1, F2). Gratings comprised bars of two alternate colours that either had a small difference in chromaticity (mean CIE UCS separation of 0.03), a medium difference (mean separation of 0.19), or a large difference (mean separation of 0.43). The colour pairs had chromaticities that lay on the red-green, red-blue, or blue-green borders of the screen gamut. Regardless of the hue, the larger the separation in chromaticity, the greater the alpha desynchronization and the lower the alpha power (p = 0.004), but only in posterior electrodes (p < 0.001). Together this indicates that differences in colour evoke a cortical excitation that increases monotonically with the colour difference. In this respect the alpha response resembles the haemodynamic response.

Superradiance Induced Particle Flow via Dynamical Gauge Coupling.

We study fermions that are gauge coupled to a cavity mode via Raman-assisted hopping in a one-dimensional lattice. For an infinite lattice, we find a superradiant phase with an infinitesimal pumping threshold which induces a directed particle flow. We explore the fate of this flow in a finite lattice with boundaries, studying the nonequilibrium dynamics including fluctuation effects. The short-time dynamics is dominated by superradiance, while the long-time behavior is governed by cavity fluctuations. We show that the steady state in the finite lattice is not unique and can be understood in terms of coherent bosonic excitations above a Fermi surface in real space.

Quantum Quenches in Chern Insulators.

We explore the nonequilibrium response of Chern insulators. Focusing on the Haldane model, we study the dynamics induced by quantum quenches between topological and nontopological phases. A notable feature is that the Chern number, calculated for an infinite system, is unchanged under the dynamics following such a quench. However, in finite geometries, the initial and final Hamiltonians are distinguished by the presence or absence of edge modes. We study the edge excitations and describe their impact on the experimentally observable edge currents and magnetization. We show that, following a quantum quench, the edge currents relax towards new equilibrium values, and that there is light-cone spreading of the currents into the interior of the sample.

Cortical excitability and the shape of the haemodynamic response.

Individual differences in the temporal dynamics of the haemodynamic response can reflect cortical excitation and can reveal underlying cortical physiology. Here, we show differences in the shape of the haemodynamic response that are dependent on stimulus parameters. Two sets of visual stimuli were used varying in parameters that are known to manipulate the haemodynamic response in the visual cortex. We measured the oxyhaemoglobin response using near infrared spectroscopy. The first set of stimuli comprised chromatic square-wave gratings that varied with respect to the separation in the CIE UCS chromaticities of the alternating bars. The gratings with large separations in chromaticity evoked an oxyhaemoglobin response with greater amplitude, consistent with greater activation of the visual cortex. The second set of stimuli comprised horizontal achromatic gratings that (1) were static, (2) drifted at a constant velocity towards fixation, or (3) reversed direction every half spatial cycle to create a vertical vibrating motion. Although the three types of grating had a similar effect on the amplitude of the oxyhaemoglobin response, the moving gratings (2 and 3) evoked a steeper decrease in oxyhaemoglobin concentration after stimulus-offset. The steeper slope appears to reflect the post-stimulus undershoot and the slope may provide a correlate of cortical excitability when the amplitude of the haemodynamic response has saturated.

Designing topological bands in reciprocal space.

Motivated by new capabilities to realize artificial gauge fields in ultracold atomic systems, and by their potential to access correlated topological phases in lattice systems, we present a new strategy for designing topologically nontrivial band structures. Our approach is simple and direct: it amounts to considering tight-binding models directly in reciprocal space. These models naturally cause atoms to experience highly uniform magnetic flux density and lead to topological bands with very narrow dispersion, without fine-tuning of parameters. Further, our construction immediately yields instances of optical Chern lattices, as well as band structures with Chern numbers of magnitude larger than one.

Topological Kondo effect with Majorana fermions.

The Kondo effect is a striking consequence of the coupling of itinerant electrons to a quantum spin with degenerate energy levels. While degeneracies are commonly thought to arise from symmetries or fine-tuning of parameters, the recent emergence of Majorana fermions has brought to the fore an entirely different possibility: a topological degeneracy that arises from the nonlocal character of Majorana fermions. Here we show that nonlocal quantum spins formed from these degrees of freedom give rise to a novel topological Kondo effect. This leads to a robust non-Fermi liquid behavior, known to be difficult to achieve in the conventional Kondo context. Focusing on mesoscopic superconductor devices, we predict several unique transport signatures of this Kondo effect, which would demonstrate the nonlocal quantum dynamics of Majorana fermions and validate their potential for topological quantum computation.

Probing fractional topological insulators with magnetic edge perturbations.

We discuss detection strategies for fractional topological insulators (FTIs) realizing time-reversal invariant analogues of fractional quantum Hall systems in the Laughlin universality class. Focusing on transport measurements, we study the effect of magnetic perturbations on the edge modes. We find that the modes show unexpected robustness against magnetic backscattering for moderate couplings and edge interactions, allowing for various phase transitions signaling the FTI phase. We also describe protocols for extracting the universal integer m characterizing the phase and the edge interaction parameter from the conductance of setups with magnets and a quantum point contact.

Emotive interference during cognitive processing in major depression: an investigation of lower alpha 1 activity.

BACKGROUND: Individuals with Major Depressive Disorder (MDD) tend to be more susceptible to distraction by negative emotional material than their non-depressed counterparts. This extends to an enhanced vulnerability to interference from mood-congruent stimuli during cognitive processing. The current study investigated the electrophysiological correlates of competing cognitive and emotional processing demands in MDD. METHODS: Event-related alpha activity within the lower alpha 1 band was examined during the online information retention phase of a non-emotive WM task with extraneous emotional stimuli (positive, negative and neutral) presented as background images. EEG activity over posterior parietal cortex was compared between 15 acutely depressed and 16 never depressed right-handed women. RESULTS: A valence specific dissociation in lower alpha 1 activity was observed between the two groups, consistent with greater attentional resource allocation to positive distracters in control participants and to negative distracters in MDD participants. No group differences were seen when neutral distracters were displayed. CONCLUSIONS: These results demonstrate that activity within the lower alpha 1 band is sensitive to competing emotional and cognitive processing demands and highlight the importance of posterior parietal regions in depression-related susceptibility to affective distractibility during cognitive processing.

Correlated phases of bosons in the flat lowest band of the dice lattice.

We study correlated phases occurring in the flat lowest band of the dice-lattice model at flux density one-half. We discuss how to realize this model, also referred to as the T(3) lattice, in cold atomic gases. We construct the projection of the model to the lowest dice band, which yields a Hubbard Hamiltonian with interaction-assisted hopping processes. We solve this model for bosons in two limits. In the limit of large density, we use Gross-Pitaevskii mean-field theory to reveal time-reversal symmetry breaking vortex lattice phases. At low density, we use exact diagonalization to identify three stable phases at fractional filling factors ν of the lowest band, including a classical crystal at ν = 1/3, a supersolid state at ν = 1/2, and a Mott insulator at ν = 1.

Z(2) topological insulators in ultracold atomic gases.

We describe how optical dressing can be used to generate band structures for ultracold atoms with nontrivial Z(2) topological order. Time-reversal symmetry is preserved by simple conditions on the optical fields. We first show how to construct optical lattices that give rise to Z(2) topological insulators in two dimensions. We then describe a general method for the construction of three-dimensional Z(2) topological insulators. A central feature of our approach is a new way to understand Z(2) topological insulators starting from the nearly free electron limit.

Optical flux lattices for ultracold atomic gases.

We show that simple laser configurations can give rise to "optical flux lattices," in which optically dressed atoms experience a periodic effective magnetic flux with high mean density. These potentials lead to narrow energy bands with nonzero Chern numbers. Optical flux lattices will greatly facilitate the achievement of the quantum Hall regime for ultracold atomic gases.

Local tensor network for strongly correlated projective States.

The success of tensor network approaches in simulating strongly correlated quantum systems crucially depends on whether the many-body states that are relevant for the problem can be encoded in a local tensor network. Despite numerous efforts, strongly correlated projective states, including fractional quantum Hall states, have not yet found a local tensor network representation. Here we show that one can encode the calculation of averages of local operators in a Grassmann tensor network which is local. Our construction is explicit and allows the use of physically motivated trial wave functions as starting points in tensor network variational calculations.

Effects of 2G and 3G mobile phones on performance and electrophysiology in adolescents, young adults and older adults.

OBJECTIVE: This study examined sensory and cognitive processing in adolescents, young adults and older adults, when exposed to 2nd (2G) and 3rd (3G) generation mobile phone signals. METHODS: Tests employed were the auditory 3-stimulus oddball and the N-back. Forty-one 13-15 year olds, forty-two 19-40 year olds and twenty 55-70 year olds were tested using a double-blind cross-over design, where each participant received Sham, 2G and 3G exposures, separated by at least 4 days. RESULTS: 3-Stimulus oddball task: Behavioural: accuracy and reaction time of responses to targets were not affected by exposure. Electrophysiological: augmented N1 was found in the 2G condition (independent of age group). N-back task: Behavioural: the combined groups performed less accurately during the 3G exposure (compared to Sham), with post hoc tests finding this effect separately in the adolescents only. Electrophysiological: delayed ERD/ERS responses of the alpha power were found in both 3G and 2G conditions (compared to Sham; independent of age group). CONCLUSION: Employing tasks tailored to each individual's ability level, this study provides support for an effect of acute 2G and 3G exposure on human cognitive function. SIGNIFICANCE: The subtlety of mobile phone effect on cognition in our study suggests that it is important to account for individual differences in future mobile phone research.

Individualized alpha activity and frontal asymmetry in major depression.

Lateralized differences in frontal alpha power in individuals with major depressive disorder (MDD) are thought to reflect an aberrant affective processing style. However research into anterior alpha asymmetry and MDD has often produced conflicting results. The current study aimed to investigate whether individualized alpha bandwidths provide a more sensitive measure of anterior alpha asymmetry in MDD than the traditional fixed 8-13 Hz alpha band. Resting EEG was recorded from 34 right-handed female participants (18 controls, 16 MDD). Each participant's Individual Alpha Frequency was used to delineate a broad individualized alpha band and three individualized narrow alpha sub-bands: lower alpha1, lower alpha 2 and upper alpha. Activity within the broad and narrow individualized bandwidths and within the traditional fixed alpha band were used to compare a) controls and acutely depressed individuals and b) medicated and unmedicated MDD participants. Individualizing and subdividing the alpha bandwidth did not add appreciably to the sensitivity of anterior alpha asymmetry in MDD as no significant differences in lateralized alpha power between controls and MDD participants were observed in any alpha bandwidth. This finding was consistent under two reference schemes and across multiple scalp locations. Within the MDD group, antidepressant use was associated with significantly greater right than left hemispheric power in the lower alpha 1 band. The relevance of this finding is discussed in relation to the electrophysiological correlates of antidepressant medication use, lateralized differences in affective processing and treatment resistant MDD.

Upper alpha activity during working memory processing reflects abnormal inhibition in major depression.

BACKGROUND: EEG studies examining 'resting' state (i.e. non-task) state brain activity in major depressive disorder (MDD) have reported numerous abnormalities within the alpha bandwidth. These findings are discussed extensively within affective disorders literature but their relationship to functional aspects of depressive psychopathology remains unclear. Investigating alpha modulation during active cognitive processing may provide a more targeted means of relating aberrant alpha activity to specific aspects of depression symptomatology. Alpha activity is reliably modulated during working memory (WM) processing and WM impairments are a common neuropsychological consequence of MDD. Moreover, it has been suggested that alpha activity reflects internally mediated inhibitory process and attenuated inhibition has been suggested to contribute to WM inefficacy. AIM: The current investigation examined whether alpha was modulated differently in MDD participants during WM processing and whether the pattern of alpha activity was consistent with impairments in inhibitory processes. METHOD: Event related synchronisation (ERS) within the upper alpha band over the retention interval of a modified Sternberg WM task was examined in 15 acutely depressed and 15 never depressed right-handed female participants. RESULTS: MDD participants displayed greater upper alpha ERS than controls during the online information maintenance component of WM processing. This was evident over left, but not right, parieto-occipital cortex. CONCLUSION: The results are consistent with increased inhibition of extraneous material during WM processing in depression. This may reflect a neurobiological compensation strategy whereby additional neural resources are required to achieve comparable performance accuracy during effortful cognitive processing in MDD.

Effects of 2G and 3G mobile phones on human alpha rhythms: Resting EEG in adolescents, young adults, and the elderly.

The present study was conducted to determine whether adolescents and/or the elderly are more sensitive to mobile phone (MP)-related bioeffects than young adults, and to determine this for both 2nd generation (2G) GSM, and 3rd generation (3G) W-CDMA exposures. To test this, resting alpha activity (8-12 Hz band of the electroencephalogram) was assessed because numerous studies have now reported it to be enhanced by MP exposure. Forty-one 13-15 year olds, forty-two 19-40 year olds, and twenty 55-70 year olds were tested using a double-blind crossover design, where each participant received Sham, 2G and 3G exposures, separated by at least 4 days. Alpha activity, during exposure relative to baseline, was recorded and compared between conditions. Consistent with previous research, the young adults' alpha was greater in the 2G compared to Sham condition, however, no effect was seen in the adolescent or the elderly groups, and no effect of 3G exposures was found in any group. The results provide further support for an effect of 2G exposures on resting alpha activity in young adults, but fail to support a similar enhancement in adolescents or the elderly, or in any age group as a function of 3G exposure.

Critical supercurrents and self-organization in quantum Hall bilayers.

We present a theory of interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. In agreement with recent experiments [Phys. Rev. B 80, 165120 (2009); Phys. Rev. B 78, 075302 (2008)], we find that the critical current is proportional to the sample area and is comparable in magnitude to observed values. This reflects the formation of a Bean critical state as a result of current injection at the boundary. We predict a crossover to a critical current proportional to the square-root of the area in smaller samples. We also predict a peak in the critical current as the electron density varies at fixed layer separation.