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Memories of fearful events can last a lifetime. The prelimbic (PL) cortex, a subregion of prefrontal cortex, plays a critical role in fear memory retrieval over time. Most studies have focused on acquisition, consolidation, and retrieval of recent memories, but much less is known about the neural mechanisms of remote memory. Using a new knock-in mouse for activity-dependent genetic labeling (TRAP2), we demonstrate that neuronal ensembles in the PL cortex are dynamic. PL neurons TRAPed during later memory retrievals are more likely to be reactivated and make larger behavioral contributions to remote memory retrieval compared to those TRAPed during learning or early memory retrieval. PL activity during learning is required to initiate this time-dependent reorganization in PL ensembles underlying memory retrieval. Finally, while neurons TRAPed during earlier and later retrievals have similar broad projections throughout the brain, PL neurons TRAPed later have a stronger functional recruitment of cortical targets. 

Exotic superconductivity, such as high TC, topological, and heavy-fermion superconductors, often rely on phase sensitive measurements to determine the underlying pairing. Here we investigate the proximity-induced superconductivity in nanowires of SnTe, where a s±is′ superconducting state is produced that lacks the time-reversal and valley-exchange symmetry of the parent SnTe. A systematic breakdown of three conventional characteristics of Josephson junctions -- the DC Josephson effect, the AC Josephson effect, and the magnetic diffraction pattern -- fabricated from SnTe nanowire weak links elucidates this novel superconducting state. Further, the AC Josephson effect reveals evidence of a Majorana bound state, tuned by a perpendicular magnetic field. This work represents the definitive phase-sensitive measurement of novel s±is′ superconductivity, providing a new route to the investigation of fractional vortices, topological superconductivity, topological phase transitions, and new types of Josephson-based devices. 

Advances in low-dimensional superconductivity are often realized through improvements in material quality. Apart from a small group of organic materials, there is a near absence of clean-limit two-dimensional (2D) superconductors, which presents an impediment to the pursuit of numerous long-standing predictions for exotic superconductivity with fragile pairing symmetries. We developed a bulk superlattice consisting of the transition metal dichalcogenide (TMD) superconductor 2H-niobium disulfide (2H-NbS₂) and a commensurate block layer that yields enhanced two-dimensionality, high electronic quality, and clean-limit inorganic 2D superconductivity. The structure of this material may naturally be extended to generate a distinct family of 2D superconductors, topological insulators, and excitonic systems based on TMDs with improved material properties.