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The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic-angle twisted bilayer graphene (MATBG)(1,2) crucially depend on the interlayer twist angle, theta. Although control of the global theta with a precision of about 0.1 degrees has been demonstrated(1-7), little information is available on the distribution of the local twist angles. Here we use a nanoscale on-tip scanning superconducting quantum interference device (SQUID-on-tip)(8) to obtain tomographic images of the Landau levels in the quantum Hall state(9) and to map the local theta variations in hexagonal boron nitride (hBN)-encapsulated MATBG devices with relative precision better than 0.002 degrees and a spatial resolution of a few moire periods. We find a correlation between the degree of theta disorder and the quality of the MATBG transport characteristics and show that even state-of-the-art devices-which exhibit correlated states, Landau fans and superconductivity-display considerable local variation in theta of up to 0.1 degrees, exhibiting substantial gradients and networks of jumps, and may contain areas with no local MATBG behaviour. We observe that the correlated states in MATBG are particularly fragile with respect to the twist-angle disorder. We also show that the gradients of theta generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which profoundly alter the quantum Hall state by forming edge channels in the bulk of the sample and may affect the phase diagram of the correlated and superconducting states. We thus establish the importance of theta disorder as an unconventional type of disorder enabling the use of twist-angle gradients for bandstructure engineering, for realization of correlated phenomena and for gate-tunable built-in planar electric fields for device applications.

SQUID-on-tip tomographic imaging of Landau levels in magic-angle graphene provides nanoscale maps of local twist-angle disorder and shows that its properties are fundamentally different from common types of disorder.

The neurotransmitter dopamine is required for the reinforcement of actions by rewarding stimuli(1). Neuroscientists have tried to define the functions of dopamine in concise conceptual terms(2), but the practical implications of dopamine release depend on its diverse brain-wide consequences. Although molecular and cellular effects of dopaminergic signalling have been extensively studied(3), the effects of dopamine on larger-scale neural activity profiles are less well-understood. Here we combine dynamic dopamine-sensitive molecular imaging(4) and functional magnetic resonance imaging to determine how striatal dopamine release shapes local and global responses to rewarding stimulation in rat brains. We find that dopamine consistently alters the duration, but not the magnitude, of stimulus responses across much of the striatum, via quantifiable postsynaptic effects that vary across subregions. Striatal dopamine release also potentiates a network of distal responses, which we delineate using neurochemically dependent functional connectivity analyses. Hot spots of dopaminergic drive notably include cortical regions that are associated with both limbic and motor function. Our results reveal distinct neuromodulatory actions of striatal dopamine that extend well beyond its sites of peak release, and that result in enhanced activation of remote neural populations necessary for the performance of motivated actions. Our findings also suggest brain-wide biomarkers of dopaminergic function and could provide a basis for the improved interpretation of neuroimaging results that are relevant to learning and addiction.

Molecular and functional magnetic resonance imaging in the rat reveals distinct neuromodulatory effects of striatal dopamine that extend beyond peak release sites and activate remote neural populations necessary for performing motivated actions.

Tandem repeat elements such as the diverse class of satellite repeats occupy large parts of eukaryotic chromosomes, mostly at centromeric, pericentromeric, telomeric and subtelomeric regions(1). However, some elements are located in euchromatic regions throughout the genome and have been hypothesized to regulate gene expression in cis by modulating local chromatin structure, or in trans via transcripts derived from the repeats(2-4). Here we show that a satellite repeat in the mosquito Aedes aegypti promotes sequence-specific gene silencing via the expression of two PIWI-interacting RNAs (piRNAs). Whereas satellite repeats and piRNA sequences generally evolve extremely quickly(5-7), this locus was conserved for approximately 200 million years, suggesting that it has a central function in mosquito biology. piRNA production commenced shortly after egg laying, and inactivation of the more abundant piRNA resulted in failure to degrade maternally deposited transcripts in the zygote and developmental arrest. Our results reveal a mechanism by which satellite repeats regulate global gene expression in trans via piRNA-mediated gene silencing that is essential for embryonic development.

A conserved satellite repeat in the mosquito Aedes aegypti encodes PIWI-interacting RNAs that promote sequence-specific gene silencing in trans and have an essential role in embryonic development.