Efficient variational approach to dynamics of a spatially extended bosonic Kondo model

We develop an efficient variational approach to studying dynamics of a localized quantum spin coupled to a bath of mobile spinful bosons. We use parity symmetry to decouple the impurity spin from the environment via a canonical transformation and reduce the problem to a model of the interacting bosonic bath. We describe coherent time evolution of the latter using bosonic Gaussian states as a variational ansatz. We provide full analytical expressions for equations describing variational time evolution that can be applied to study in- and out-of-equilibrium phenomena in a wide class of quantum impurity problems. In the accompanying paper [Y. Ashida {\it et al.}, Phys. Rev. Lett. 123, 183001 (2019)], we present a concrete application of this general formalism to the analysis of the Rydberg Central Spin Model, in which the spin-1/2 Rydberg impurity undergoes spin-changing collisions in a dense cloud of two-component ultracold bosons. To illustrate new features arising from orbital motion of the bath atoms, we compare our results to the Monte Carlo study of the model with spatially localized bosons in the bath, in which random positions of the atoms give rise to random couplings of the standard central spin model.Comment: 15 pages, 6 figures. See also Phys. Rev. Lett. 123, 183001 (2019) [arXiv:1905.08523

Quantum Rydberg Central Spin Model

We consider dynamics of a Rydberg impurity in a cloud of ultracold bosonic atoms in which the Rydberg electron can undergo spin-changing collisions with surrounding atoms. This system realizes a new type of the quantum impurity problem that compounds essential features of the Kondo model, the Bose polaron, and the central spin model. To capture the interplay of the Rydberg-electron spin dynamics and the orbital motion of atoms, we employ a new variational method that combines an impurity-decoupling transformation with a Gaussian ansatz for the bath particles. We find several unexpected features of this model that are not present in traditional impurity problems, including interaction-induced renormalization of the absorption spectrum that eludes simple explanations from molecular bound states, and long-lasting oscillations of the Rydberg-electron spin. We discuss generalizations of our analysis to other systems in atomic physics and quantum chemistry, where an electron excitation of high orbital quantum number interacts with a spinful quantum bath.Comment: 6 pages, 5 figures. See also Phys. Rev. A 100, 043618 (2019) [arXiv:1905.09615

Mean-field approximations of networks of spiking neurons with short-term synaptic plasticity

Low-dimensional descriptions of neural network dynamics are an effective tool for bridging different scales of organization of brain structure and function. Recent advances in deriving mean-field descriptions for networks of coupled oscillators have sparked the development of a new generation of neural mass models. Of notable interest are mean-field descriptions of all-to-all coupled quadratic integrate-and-fire (QIF) neurons, which have already seen numerous extensions and applications. These extensions include different forms of short-term adaptation (STA) considered to play an important role in generating and sustaining dynamic regimes of interest in the brain. It is an open question, however, whether the incorporation of pre-synaptic forms of synaptic plasticity driven by single neuron activity would still permit the derivation of mean-field equations using the same method. Here, we discuss this problem using an established model of short-term synaptic plasticity at the single neuron level, for which we present two different approaches for the derivation of the mean-field equations. We compare these models with a recently proposed mean-field approximation that assumes stochastic spike timings. In general, the latter fails to accurately reproduce the macroscopic activity in networks of deterministic QIF neurons with distributed parameters. We show that the mean-field models we propose provide a more accurate description of the network dynamics, although they are mathematically more involved. Using bifurcation analysis, we find that QIF networks with pre-synaptic short-term plasticity can express regimes of periodic bursting activity as well as bi-stable regimes. Together, we provide novel insight into the macroscopic effects of short-term synaptic plasticity in spiking neural networks, as well as two different mean-field descriptions for future investigations of such networks.Comment: 15 pages, 7 figure

NMR Investigation and Conformational Analysis of a Synthetic Hexasaccharide

The structure of the hexasaccharide 1 has been examined by a spectroscopic investigation using one- and two-dimensional NMR spectroscopy. All 1H and 13C signals of the saccharide part were assigned. NOESY and ROESY experiments allowed to discuss the flexibility of the molecule

Equivalence of students\u27 scores on timed and untimed anatomy practical examinations.

Untimed examinations are popular with students because there is a perception that first impressions may be incorrect, and that difficult questions require more time for reflection. In this report, we tested the hypothesis that timed anatomy practical examinations are inherently more difficult than untimed examinations. Students in the Doctor of Physical Therapy program at Thomas Jefferson University were assessed on their understanding of anatomic relationships using multiple-choice questions. For the class of 2012 (n = 46), students were allowed to circulate freely among 40 testing stations during the 40-minute testing session. For the class of 2013 (n = 46), students were required to move sequentially through the 40 testing stations (one minute per item). Students in both years were given three practical examinations covering the back/upper limb, lower limb, and trunk. An identical set of questions was used for both groups of students (untimed and timed examinations). Our results indicate that there is no significant difference between student performance on untimed and timed examinations (final percent scores of 87.3 and 88.9, respectively). This result also held true for students in the top and bottom 20th percentiles of the class. Moreover, time limits did not lead to errors on even the most difficult, higher-order questions (i.e., items with P-values \u3c 0.70). Thus, limiting time at testing stations during an anatomy practical examination does not adversely affect student performance

Insensitivity of alkenone carbon isotopes to atmospheric CO₂ at low to moderate CO₂ levels

Atmospheric pCO2 is a critical component of the global carbon system and is considered to be the major control of Earth’s past, present and future climate. Accurate and precise reconstructions of its concentration through geological time are, therefore, crucial to our understanding of the Earth system. Ice core records document pCO2 for the past 800 kyrs, but at no point during this interval were CO2 levels higher than today. Interpretation of older pCO2 has been hampered by discrepancies during some time intervals between two of the main ocean-based proxy methods used to reconstruct pCO2: the carbon isotope fractionation that occurs during photosynthesis as recorded by haptophyte biomarkers (alkenones) and the boron isotope composition (δ11B) of foraminifer shells. Here we present alkenone and δ11B-based pCO2 reconstructions generated from the same samples from the Plio-Pleistocene at ODP Site 999 across a glacial-interglacial cycle. We find a muted response to pCO2 in the alkenone record compared to contemporaneous ice core and δ11B records, suggesting caution in the interpretation of alkenone-based records at low pCO2 levels. This is possibly caused by the physiology of CO2 uptake in the haptophytes. Our new understanding resolves some of the inconsistencies between the proxies and highlights that caution may be required when interpreting alkenone-based reconstructions of pCO2