arXiv subjects

Christian Scheppach

Publications and source records attributed to Christian Scheppach.

4 recordsLinked to original sources

arXiv2026

Efficient quantum transport in disordered Floquet networks

We propose a mechanism for fast and efficient quantum transport through disordered networks with variable on-site energies, inspired by photosynthetic complexes. The mechanism relies on an interplay between inter-site couplings of the network and driving by external vibrations. Two design principles are shown to ensure close-to-perfect transport despite the disorder, namely a reflection symmetry in Floquet-Hilbert space and the existence of a dominant doublet or triplet of Floquet states.

quant-ph↗

arXiv2017

Fluctuation analysis in nonstationary conditions: single Ca channel current in cortical pyramidal neurons

Fluctuation analysis is a method which allows measurement of the single channel current of ion channels even when it is too small to be resolved directly with the patch clamp technique. This is the case for voltage-gated Ca2+ channels (VGCCs). They are present in all mammalian central neurons, controlling presynaptic release of transmitter, postsynaptic signaling and synaptic integration. The amplitudes of their single channel currents in a physiological concentration of extracellular Ca2+, however, are small and not well determined. But measurement of this quantity is essential for estimating numbers of functional VGCCs in the membrane and the size of channel-associated Ca2+ signaling domains, and for understanding the stochastic nature of Ca2+ signaling. Here, we recorded the VGCC current in nucleated patches from layer 5 pyramidal neurons in rat neocortex, in physiological external Ca2+ (1-2 mM). The ensemble-averaging of current responses required for conventional fluctuation analysis proved impractical because of the rapid rundown of VGCC currents. We therefore developed a more robust method, using mean current fitting of individual current responses and band-pass filtering. Furthermore, voltage ramp stimulation proved useful. We validated the accuracy of the method by analyzing simulated data. At an external Ca2+ concentration of 1 mM, and a membrane potential of -20 mV, we found that the average single channel current amplitude was about 0.04 pA, increasing to 0.065 pA at 2 mM external Ca2+, and 0.12 pA at 5 mM. The relaxation time constant of the fluctuations was in the range 0.2-0.8 ms. The results are relevant to understanding the stochastic properties of dendritic Ca2+ spikes in neocortical layer 5 pyramidal neurons. With the reported method, single channel current amplitude of native VGCCs can be resolved accurately despite conditions of unstable rundown.

q-bio.NC↗

arXiv2016

High- and low-conductance NMDA receptors are present in layer 4 spiny stellate and layer 2/3 pyramidal neurons of mouse barrel cortex

NMDA receptors are ion channels activated by the neurotransmitter glutamate in the mammalian brain and are important in synaptic function and plasticity, but are also found in extrasynaptic locations and influence neuronal excitability. There are different NMDA receptor subtypes which differ in their single-channel conductance. Recently, synaptic plasticity has been studied in mouse barrel cortex, the primary sensory cortex for input from the animal's whiskers. Pharmacological data imply the presence of low-conductance NMDA receptors in spiny stellate neurons of cortical layer 4, but of high-conductance NMDA receptors in pyramidal neurons of layer 2/3. Here, to obtain complementary electrophysiological information on the functional NMDA receptors expressed in layer 4 and layer 2/3 neurons, single NMDA receptor currents were recorded with the patch-clamp method. Both cell types were found to contain high-conductance as well as low-conductance NMDA receptors. The results are consistent with the reported pharmacological data on synaptic plasticity, and with previous claims of a prominent role of low-conductance NMDA receptors in layer 4 spiny stellate neurons, including broad integration, amplification and distribution of excitation within the barrel in response to whisker stimulation, as well as modulation of excitability by ambient glutamate. However, layer 4 cells also expressed high-conductance NMDA receptors. The presence of low-conductance NMDA receptors in layer 2/3 pyramidal neurons suggests that some of these functions may be shared with layer 4 spiny stellate neurons.

q-bio.NC↗

arXiv2009

Matter Wave Turbulence: Beyond Kinetic Scaling

Turbulent scaling phenomena are studied in an ultracold Bose gas away from thermal equilibrium. Fixed points of the dynamical evolution are characterized in terms of universal scaling exponents of correlation functions. The scaling behavior is determined analytically in the framework of quantum field theory, using a nonperturbative approximation of the two-particle irreducible effective action. While perturbative Kolmogorov scaling is recovered at higher energies, scaling solutions with anomalously large exponents arise in the infrared regime of the turbulence spectrum. The extraordinary enhancement in the momentum dependence of long-range correlations could be experimentally accessible in dilute ultracold atomic gases. Such experiments have the potential to provide insight into dynamical phenomena directly relevant also in other present-day focus areas like heavy-ion collisions and early-universe cosmology.

cond-mat.quant-gas↗