Integrated Calendar

Hippocampal place cells are considered basic substrates of spatial memory, but the degree to which their ensemble representations of space are stable over long time periods has remained unmeasured. By using an integrated, miniature microscope, and micro-endoscope probes, we performed Ca2+-imaging in behaving mice as they repeatedly explored a familiar environment. This approach allowed us to track the place fields of thousands of CA1 hippocampal neurons over weeks. Spatial coding was highly dynamic, for on each day the neural representation of this environment involved a unique subset of neurons. A minority of the cells (~15–25%) overlapped between any two of these subsets and retained the same place fields. Although this overlap was also dynamic it sufficed to preserve a stable and accurate ensemble representation of space across weeks. These findings raise several important questions: What are the biological mechanisms that drive the turnover in the place cell membership of each day’s coding ensemble? What is the functional relevance of these dynamics to hippocampal memory? Overall, this work reveals a dynamic time-dependent facet of the hippocampal representation of space, and introduces a novel approach for investigating, in a behaving animal, how coding in large neuronal populations changes over long periods of time and as function of experience.

The coupling between the spin of an electron and its momentum is recognized to generate a variety of new phases in condensed matter systems. For example, it has been recently demonstrated that spin-orbit coupling can change the nature of a trivial insulator to endow it with topological properties. Or, in symmetry broken states, spin-orbit coupling permits exotic low energy excitations such as skyrmions in helimagnets and Majorana modes in superconductors. The interplay between superconductivity and spin-orbit effects gives rise to additional surprising features which I will discuss in my talk. For instance, the locking of the spin and orbital degrees of freedom can protect superconductors with unconventional pairing symmetry against disorder. Further, I will show that it stabilizes a condensate of Cooper pairs with finite momentum (a variant of the Fulde-Ferrel-Larkin-Ovchinikov state) up to high magnetic fields. More generally, in the presence of spin-orbit coupling a superconductor not only supports dissipationless spin currents, but also has a peculiar mixed state in which vortices resemble magnetic monopoles.

Molybdenum (Mo) isotopes are efficiently removed under euxinic conditions and consequently may directly record the Mo isotopic composition of contemporaneous seawater in ancient organic-rich shales. Removal of Mo to sediment in other environments (i.e., anoxic and oxic) is less efficient and accompanied by a significant negative isotope fractionation, where Δ98MoSW-SED is typically 1 to 3 ‰ [1,2]. Because Mo in solution occurs primarily as the oxidized molybdate complex MoO42-, it is generally accepted that before the Great Oxidation Event (GOE) at ca. 2.3 Ga the transfer of Mo to the oceans was primarily in detrital form. This is in accordance with some available sedimentary data showing low concentrations and a narrow range of isotopic compositions corresponding to the crustal reservoir [3,4]. As atmospheric oxygen started to rise, Mo was chemically weathered from continental sources and transported to the oceans as molybdate. There, it was removed to sediments via several fractionating mechanisms, depending on the redox conditions. Consequently, Proterozoic and Phanerozoic black shales record a wide range of Mo concentrations and isotopic values, reflecting variations in the isotopic composition of seawater as determined by the mass balance between the different sinks [5,6].
In order to further explore the Mo isotopic record of Earth system redox evolution, we measured Mo concentrations and isotopic compositions of black shales from several Neoarchean and Paleoproterozoic sections (2.7 Ga - Belingwe Fm., Zimbabwe; 2.63 Ga - Jeerinah Fm., Western Australia; 2.52 Ga - Gamohaan Fm., South Africa; 2.32 Ga – Timeball Hill South Africa; 2.15 Ga - Sengoma Argillite Fm., Botswana; 2.06 Ga – Zaonega Fm., Karelia). The data suggest low levels of free O2 up to 400 Myr before the GOE, where elevated Mo concentrations together with large isotopic variations and high δ98Mo values are observed in sections dated 2.72 – 2.5 Ga. Moreover, early euxinic conditions are detected in the 2.63 Ga Jeerinah Formation. The 2.32 Ga Timeball Hill Formation, contemporaneous with the GOE [7], shows a dramatic increase in Mo transport accompanied by very strong fractionation effects, possibly pointing to rapid and large variations in free O2 levels. Post-GOE sections (2.15 – 2.05 Ga) indicate another increase in Mo transport to the ocean and development of widespread euxinia at 2.05 Ga. Overall, we show here that secular evolution of the oceanic Mo cycle tracks redox changes in the oceans and atmosphere and represents a powerful tool for refining our understanding of the Earth redox evolution.

I will discuss two kinds of tidal disruption events. 1) Stars orbiting closely
enough to a massive black hole are tidally compressed into a transient
pancake-shape configuration before the total disruption. I discuss its
implications to X-ray and gravitational wave astronomy. 2) The leading
model for the formation of hyper-velocity stars is the breakup of a binary
as it approaches the massive black hole in the Galactic Center. The large
mass ratio between the black hole and binary allows us to formulate the
problem in the restricted parabolic three-body approximation. I discuss
the ejection and capture dynamics in the framework, and the velocity
distribution in the Galactic halo is discussed. The disruption results are
also used to study irregular satellites around the giant planets
in the Solar system, especially Triton - Neptune's largest moon.