arXiv subjects

Ragnar Fleischmann

Publications and source records attributed to Ragnar Fleischmann.

13 recordsLinked to original sources

arXiv2023

Branched flows in active random walks and the formation of ant trail patterns

Branched flow governs the transition from ballistic to diffusive motion of waves and conservative particle flows in spatially correlated random or complex environments. It occurs in many physical systems from micrometer to interstellar scales. In living matter systems, however, this transport regime is usually suppressed by dissipation and noise. In this article we demonstrate that, nonetheless, noisy active random walks, characterizing many living systems like foraging animals, and chemotactic bacteria, can show a regime of branched flow. To this aim we model the dynamics of trail forming ants and use it to derive a scaling theory of branched flows in active random walks in random bias fields in the presence of noise. We also show how trail patterns, formed by the interaction of ants by depositing pheromones along their trajectories, can be understood as a consequence of branched flow.

physics.bio-ph↗

arXiv2019

Husimi function for electrons moving in magnetic fields

Husimi functions allow one to obtain sensible and useful phase space probability distributions from quantumechanical wavefunctions or classical wave fields, linking them to (semi-)classical methods and intuition. They have been used in several fields of physics, including electronic transport. We show that applying Husimi functions to ballistic electron dynamics in magnetic fields needs special consideration in order for them to obey gauge invariance and energy conservation. We therefore extend the Husimi function formalism to allow for magnetic fields making use of magnetic translation operators. We demonstrate the application in tight-binding magneto-transport calculations in graphene nanodevices, highlighting connections with Klein tunneling. In continuation of recent work, with this paper we further pave the way for using the Husimi function to unravel quantum transport phenomena in nanodevices.

cond-mat.mes-hall↗

arXiv2019

Branched Flow

In many physical situations involving diverse length scales, waves or rays representing them travel through media characterized by spatially smooth, random, modest refactive index variations. "Primary" diffraction (by individual sub-wavelength features) is absent. Eventually the weak refraction leads to imperfect focal "cusps". Much later, a statistical regime characterized by momentum diffusion is manifested. An important intermediate regime is often overlooked, one that is diffusive only in an ensemble sense. Each realization of the ensemble possesses dramatic ray limit structure that guides the waves (in the same sense that ray optics is used to design lens systems). This structure is a universal phenomenon called branched flow. Many important phenomena develop in this intermediate regime. Here we give examples and some of the physics of this emerging field.

physics.class-ph↗

arXiv2019

Phase space analysis of quantum transport in electronic nanodevices

Electronic transport in nanodevices is commonly studied theoretically and numerically within the Landauer-B\"uttiker formalism: a device is characterized by its scattering properties to and from reservoirs connected by perfect semi-infinite leads, and transport quantities are derived from the scattering matrix. In some respects, however, the device becomes a "black box" as one only analyses what goes in and out. Here we use the Husimi function as a complementary tool for quantitatively understanding transport in graphene nanodevices. It is a phase space representation of the scattering wavefunctions that allows to link the scattering matrix to a more semiclassical and intuitive description and gain additional insight in to the transport process. In this article we use the Husimi function to analyze some of the fascinating electronic transport properties of graphene, \emph{Klein tunneling} and \emph{intervalley scattering}, in two exemplary graphene nanodevices. By this we demonstrate the usefulness of the Husimi function in electronic nanodevices and present novel results e.g. on Klein tunneling outside the Dirac regime and intervalley scattering at a pn-junction and a tilted graphene edge.

cond-mat.mes-hall↗

arXiv2019

Estimating Lyapunov exponents in billiards

Dynamical billiards are paradigmatic examples of chaotic Hamiltonian dynamical systems with widespread applications in physics. We study how well their Lyapunov exponent, characterizing the chaotic dynamics, and its dependence on external parameters can be estimated from phase space volume arguments, with emphasis on billiards with mixed regular and chaotic phase spaces. We show that in the very diverse billiards considered here the leading contribution to the Lyapunov exponent is inversely proportional to the chaotic phase space volume, and subsequently discuss the generality of this relationship. We also extend the well established formalism by Dellago, Posch, and Hoover to calculate the Lyapunov exponents of billiards to include external magnetic fields and provide a software implementation of it.

nlin.CD↗

arXiv2017

Robustness of ballistic transport in antidot superlattices

The magneto-resistance of antidot lattices shows pronounced peaks, which became a hallmark of ballistic electron transport. While most studies agree that they reflect the interplay of regular and chaotic motion in the quasi-classical dynamics, the exact mechanism has been surprisingly controversial. Inspired by recent experiments on graphene antidot lattices showing that the effect survives strong impurity scattering, we give a new explanation of the peaks linked to a fundamental relation between collision times and accessible phase space volumes, accounting for their robustness. Due to the fundamental nature of the mechanism described it will be relevant in many mesoscopic transport phenomena.

nlin.CD↗

arXiv2017

Random Matrix Theory Approach to Chaotic Coherent Perfect Absorbers

We employ Random Matrix Theory in order to investigate coherent perfect absorption (CPA) in lossy systems with complex internal dynamics. The loss strength $\gamma_{\rm CPA}$ and energy $E_{\rm CPA}$, for which a CPA occurs are expressed in terms of the eigenmodes of the isolated cavity -- thus carrying over the information about the chaotic nature of the target -- and their coupling to a finite number of scattering channels. Our results are tested against numerical calculations using complex networks of resonators and chaotic graphs as CPA cavities.

cond-mat.dis-nn↗

arXiv2016

Low-Temperature Linear Thermal Rectifiers Based on Coriolis forces

We demonstrate that a three-terminal harmonic symmetric chain in the presence of a Coriolis force, produced by a rotating platform which is used to place the chain, can produce thermal rectification. The direction of heat flow is reconfigurable and controlled by the angular velocity $Ω$ of the rotating platform. A simple three terminal triangular lattice is used to demonstrate the proposed principle.

cond-mat.mes-hall↗

arXiv2013

Statistics of Extreme Waves in Random Media

Waves traveling through random media exhibit random focusing that leads to extremely high wave intensities even in the absence of nonlinearities. Although such extreme events are present in a wide variety of physical systems and the statistics of the highest waves is important for their analysis and forecast, it remains poorly understood in particular in the regime where the waves are highest. We suggest a new approach that greatly simplifies the mathematical analysis and calculate the scaling and the distribution of the highest waves valid for a wide range of parameters.

nlin.CD↗

arXiv2013

Continuous Dynamic Photostimulation - inducing in-vivo-like fluctuating conductances with Channelrhodopsins

Central neurons operate in a regime of constantly fluctuating conductances, induced by thousands of presynaptic cells. Channelrhodopsins have been almost exclusively used to imprint a fixed spike pattern by sequences of brief depolarizations. Here we introduce continuous dynamic photostimulation (CoDyPs), a novel approach to mimic in-vivo like input fluctuations noninvasively in cells transfected with the weakly inactivating channelrhodopsin variant ChIEF. Even during long-term experiments, cultured neurons subjected to CoDyPs generate seemingly random, but reproducible spike patterns. In voltage clamped cells CoDyPs induced highly reproducible current waveforms that could be precisely predicted from the light-conductance transfer function of ChIEF. CoDyPs can replace the conventional, flash-evoked imprinting of spike patterns in in-vivo and in-vitro studies, preserving natural activity. When combined with non-invasive spike-detection, CoDyPs allows the acquisition of order of magnitudes larger data sets than previously possible, for studies of dynamical response properties of many individual neurons.

q-bio.NC↗

arXiv2013

The nature of self-localization of Bose-Einstein condensates in deep optical lattices

We analyze the nature of a novel type of self-trapping transition called self-localization (SL) of Bose-Einstein condensates in one-dimensional optical lattices in the presence of weak local dissipation. SL has recently been observed in several studies based upon the discrete nonlinear Schrödinger equation (DNLS), however, its origin is hitherto an open question. We show that SL is based upon a self-trapping crossover in the system. Furthermore, we establish that the origin of the crossover is the Peierls-Nabarro barrier, an energy threshold describing the stability of self-trapped states. Beyond the mean-field description the crossover becomes even sharper which is also reflected by a sudden change of the coherence of the condensate. While we expect that the crossover can be readily studied in current experiments in deep optical lattices, our results allow for the preparation of robust and long-time coherent quantum states.

cond-mat.quant-gas↗

arXiv2013

Scaling Theory of Heat Transport in Quasi-1D Disordered Harmonic Chains

We introduce a variant of the Banded Random Matrix ensemble and show, using detailed numerical analysis and theoretical arguments, that the phonon heat current in disordered quasi-one-dimensional lattices obeys a one-parameter scaling law. The resulting beta-function indicates that an anomalous Fourier law is applicable in the diffusive regime, while in the localization regime the heat current decays exponentially with the sample size. Our approach opens a new way to investigate the effects of Anderson localization in heat conduction, based on the powerful ideas of scaling theory.

cond-mat.dis-nn↗

arXiv2009

Exponentially Fragile PT-Symmetry in Lattices with Localized Eigenmodes

We study the effect of localized modes in lattices of size N with parity-time (PT) symmetry. Such modes are arranged in pairs of quasi-degenerate levels with splitting delta exp{-N/xi}, where ξis their localization length. The level "evolution" with respect to the PT breaking parameter gamma shows a cascade of bifurcations during which a pair of real levels becomes complex. The spontaneous PT symmetry breaking occurs at gamma min(delta), thus resulting in an exponentially narrow exact PT phase. As N/xi decreases, it becomes more robust with gamma (1/N)^2 and the distribution P(gamma) changes from log-normal to semi-Gaussian. Our theory can be tested in the frame of optical lattices.

cond-mat.other↗