Integrated Calendar

Populations of hippocampal neurons have been hypothesized to operate collectively to support the stable maintenance of long-term memories. To test this hypothesis, we performed large-scale calcium imaging in the hippocampus of freely behaving mice that repeatedly explored the same environments over weeks. Surprisingly, we discovered that across separate visits to the same familiar environment, hippocampal neurons can collectively switch between multiple distinct spatial representations, without any apparent changes in sensory input or animal’s behavior. The distinct representations were spatially informative and stable over weeks, and switching between them required a complete disconnection of the animal from the environment, demonstrating the coexistence of distinct stable attractors in the hippocampal network. In the second part of the talk, I will present a comparison of the coding properties between hippocampal subfields CA1 and CA3 in novel environments. Place cells in CA3 had more precise and stable spatial tuning than place cells in CA1. Moreover, we showed that in CA3 the tuning of place cells exhibited a higher statistical dependence with their peers compared to in CA1, uncovering an organization of CA3 into cell assemblies. Interestingly, cells with stronger tuning peer-dependence had higher stability but not higher precision, suggesting that distinct mechanisms control these two aspects of the neural code. Overall, our results demonstrate that multiple attractor states can stably coexist in the hippocampus and suggest that a cell-assembly organization in hippocampal CA3 underlies the long-term maintenance of stable spatial codes.

Link:https://weizmann.zoom.us/j/97167587409?pwd=TDFFYWI0ZmF5YXk0TW5oN1ZKSStndz09
Meeting ID: 971 6758 7409
Password: 227875

Coexistence of optical and electrical excitations in semiconductors has a long history of research. This scenario typically involves simultaneous presence of Coulomb-bound electron-hole pairs, known as excitons, and free charge carriers. Conceptually similar to related phenomena in the ultra cold atom gases, exciton-carrier mixtures strongly influence the properties of excited semiconductors and their response to external fields. It involves the formation of bound three-particle states known as charged excitons or trions, Fermi polarons, renormalization and screening effects, as well as the Mott-transition in the high-density regime. These phenomena offer fertile ground to merge the realms of optics and transport, motivated by the availability of excitations that are both electrically tunable and couple strongly to light.
Van der Waals monolayer semiconductors and layered metal-halide perovskites recently emerged as particularly suitable platforms to study exciton-carrier mixtures due to exceptionally strong Coulomb interactions on the order of many 100’s of meV. In this talk, I will discuss a number of intriguing phenomena associated with coupling of excitons to free charge carriers in these systems. I will present experimental evidence for dressing of excited exciton states by continuously tunable Fermi sea. These quasiparticles are reminiscent of two-electron excitations of the negatively charged hydrogen ion and are subject to autoionization - a unique scattering pathway available for excited states. I will further illustrate the impact of free carriers on the exciton transport revealing non-monotonous density dependence and highly efficient propagation of charged exciton complexes. Finally, I will demonstrate electrically tunable trions in hybrid organic-inorganic semiconductors. These three-particle complexes feature unusually large binding energies combined with substantial mobility at elevated temperatures.

In the present seminar, I will introduce how the flat bands in the magic angle twisted bilayer graphene (MATBG) can be understood from the zeroth Landau levels under the twisting generated pseudo magnetic field. These pseudo Landau level wave functions are almost the exact Eigen solutions of the real space Hamiltonian around the AA stacking center and can be further viewed as the analog of the “atomic core level” states in the band structure calculations for the ordinary crystals. In addition, we can use the pseudo zeroth Landau level (PZLL) and the “orthogonalised plane waves” (OPW) made from the PZLL as the two types of basis functions to efficiently reconstruct the entire Moire band structure. Using these PZLL and OPW basis functions, we can describe both the localised and itinerant components in the Moire bands of MATBG and map the MATBG to a “heavy fermion” like system, which can be used to study the orbital magnetism, topology and strongly correlation physics in MATBG.

Heterostructures (HS) of two-dimensional materials offer unlimited possibilities to design new materials for applications to optoelectronics and photonics. In such HS the electronic structure of the individual layers is well retained because of the weak interlayer van der Waals coupling. Nevertheless, new physical properties and functionalities arise beyond those of their constituent blocks, depending on the type and the stacking sequence of layers. In this presentation we use high time resolution ultrafast transient absorption (TA) and two-dimensional electronic spectroscopy (2DES) to resolve the interlayer charge scattering processes in HS.
We first study a WSe2/MoSe2 HS, which displays type II band alignment with a staggered gap, where the valence band maximum and the conduction band minimum are in the same layer. By two-colour pump-probe spectroscopy, we selectively photogenerate intralayer excitons in MoSe2 and observe hole injection in WSe2 on the sub-picosecond timescale, leading to the formation of interlayer excitons (ILX). The temperature dependence of the build-up and decay of interlayer excitons provide insights into the layer coupling mechanisms [1]. By tuning into the ILX emission band, we observe a signal which grows in on a 400 fs timescale, significantly slower than the interlayer charge transfer process. This suggests that photoexcited carriers are not instantaneously converted into the ILX following interlayer scattering, but that rather an intermediate scattering processes take place We then perform 2DES, a method with both high frequency and temporal resolution, on a large-area WS2/MoS2 HS where we unambiguously time resolve both interlayer hole and electron transfer with 34 ± 14 and 69 ± 9 fs time constants, respectively [2]. We simultaneously resolve additional optoelectronic processes including band gap renormalization and intralayer exciton coupling.
Finally, we investigate a graphene/WS2 HS where, for excitation well below the bandgap of WS2, we observe the characteristic signal of the A and B excitons of WS2, indicating ultrafast charge transfer from graphene to the semiconductor [3]. The nonlinear excitation fluence dependence of the TA signal reveals that the underlying mechanism is hot electron/hole transfer, whereby a tail the hot Fermi-Dirac carrier distribution in graphene tunnels through the Schottky barrier. Hot electron transfer is promising for the development of broadband and efficient low-dimensional photodetectors.

[1] Z. Wang et al., Nano Lett. 21, 2165–2173 (2021).
[2] V. Policht et al., Nano Lett. 21, 4738–4743 (2021).
[3] C. Trovatello et al., npj 2D Mater Appl 6, 24 (2022).

Many cognitive computations rely on the nervous system estimating mathematical vectors, but aside from computer models, how brains represent vectors or perform vector operations remains unknown. In this talk, I will describe how the fly brain performs vector arithmetic in the context of spatial navigation. The central features of this vector calculator inside the insect brain may generalize to other nervous systems and other cognitive domains beyond navigation where vector operations are required.