arXiv subjects

J. Stein

Publications and source records attributed to J. Stein.

7 recordsLinked to original sources

arXiv2025

KOMPASS: the new cold neutron triple-axis-spectrometer specialized for polarization analysis

KOMPASS is a polarized triple-axis cold neutron spectrometer recently installed at the FRM II neutron source. The instrument is designed to operate exclusively with polarized neutrons and is specialized in longitudinal polarization analysis using Helmholtz coils and spherical zero-field neutron polarimetry using a Cryopad device. The advanced guide system polarizes and focuses flexibly in the scattering plane. A first, fixed parabolic focusing part contains a series of three polarizing supermirror V-cavities that produce a highly polarized beam. By exchanging straight and parabolic front-end guide sections, the resolution of the instrument can be optimized to meet experimental requirements. Large, double-focusing monochromator and analyzer units with pyrolytic graphite crystals enable efficient and adaptive energy selection, and a compact supermirror cavity analyzes the final neutron polarization. Alternatively, or in combination with this cavity, a Heusler polarization analyzer can be used. KOMPASS offers full- and half-polarized configurations with or without secondary energy analysis and provides a wide range of polarization options. Therefore, KOMPASS is well suited for various studies of static and dynamic magnetic correlations with energy transfers on the neutron energy loss side up to ~12 meV.

physics.ins-det↗

arXiv2021

A Generalization of the Ornstein-Uhlenbeck Process: Theoretical Results, Simulations and Parameter Estimation

In this work, we study the class of stochastic process that generalizes the Ornstein-Uhlenbeck processes, hereafter called by \emph{Generalized Ornstein-Uhlenbeck Type Process} and denoted by GOU type process. We consider them driven by the class of noise processes such as Brownian motion, symmetric $α$-stable Lévy process, a Lévy process, and even a Poisson process. We give necessary and sufficient conditions under the memory kernel function for the time-stationary and the Markov properties for these processes. When the GOU type process is driven by a Lévy noise we prove that it is infinitely divisible showing its generating triplet. Several examples derived from the GOU type process are illustrated showing some of their basic properties as well as some time series realizations. These examples also present their theoretical and empirical autocorrelation or normalized codifference functions depending on whether the process has a finite or infinite second moment. We also present the maximum likelihood estimation as well as the Bayesian estimation procedures for the so-called \emph{Cosine process}, a particular process in the class of GOU type processes. For the Bayesian estimation method, we consider the power series representation of Fox's H-function to better approximate the density function of a random variable $α$-stable distributed. We consider four goodness-of-fit tests for helping to decide which \emph{Cosine process} (driven by a Gaussian or an $α$-stable noise) best fit real data sets. Two applications of GOU type model are presented: one based on the Apple company stock market price data and the other based on the cardiovascular mortality in Los Angeles County data.

math.ST↗

arXiv2019

Epileptiform spikes in specific left temporal and mesial temporal structures disrupt verbal episodic memory encoding

Patients diagnosed with epilepsy experience cognitive dysfunction that may be due to a transient cognitive/memory impairment (TCI/TMI) caused by spontaneous epileptiform spikes. We asked in a cohort of 166 adult patients with medically refractory focal epilepsy if spikes in specific neuroanatomical regions during verbal episodic memory encoding would significantly decrease the probability of recall. We found using a naïve Bayesian machine learning model that the probability of correct word recall decreased significantly by 11.9% when spikes occurred in left Brodmann area 21 (BA)21, (p<0.001), 49.7% in left BA38 (p=0.01), and 32.2% in right BA38 (p<0.001), and 21.4% in left BA36 (p<0.01). We also examined the influence of the seizure-onset zone and the language dominant hemisphere on this effect. Our results demonstrate that spontaneous epileptiform spikes produce a large effect TCI/TMI in brain regions known to be important in semantic processing and episodic memory. Thus memory impairment in patients with epilepsy may be attributable to cellular events associated with abnormal inter-ictal electrical events.

q-bio.NC↗

arXiv2019

High gamma and beta band oscillations in left ventral posterior parietal cortex are regionally dissociated during verbal episodic encoding and recall

The posterior parietal cortex (PPC) has a unique role in memory retrieval: fMRI and electrocorticography studies suggest that within the ventral PPC (VPC) specifically, there is an anterior-posterior functional divergence between externally-oriented and internally-oriented attention to memory (AtoM). However, the role of VPC during verbal episodic encoding, and the relationship between encoding- and retrieval-related activity, is less understood. Here we show that activation within a subregion of VPC is doubly dissociated between its anterior and posterior parts, during encoding compared to recall in a free recall task. We found that regional activation defined by increased high gamma power and decreased beta power oscillations during encoding and recall correlated with recall success. During word encoding, iEEG sites that showed this correlation were located anterior to those that showed deactivation. Conversely, during word recall, sites that showed stronger correlations between activity and number of words recalled were located more posteriorly. Our results demonstrate the significance of high gamma and beta oscillations suggesting a push-pull relationship between attention to external stimuli and internal memories within left ventral PPC. Knowledge of this divergence of function along the anterior-posterior axis within left ventral PPC may prove useful for guiding brain stimulation strategies.

q-bio.NC↗

arXiv2014

Domain dynamics in the multiferroic phase of MnWO4

By using broadband linear and nonlinear dielectric spectroscopy we studied the magnetoelectric dynamics in the chiral antiferromagnet MnWO4. In the multiferroic phase the dielectric response is dominated by the dynamics of domains and domain walls which is strongly dependent on the stimulating electric field. The mean switching time reaches values in the minute range in the middle of the multiferroic temperature regime at T=10 K but unexpectedly decays again on approaching the lower, first-order phase boundary at T_N1=7.6K. The switchability of the ferroelectric domains denotes a pinning-induced threshold and can be described considering a growth-limited scenario with an effective growth dimension of d=1.8. The rise of the effective dynamical coercive field on cooling below the TN2 is much stronger compared to the usual ferroelectrics and can be described by a power law E_c ~ν^1/2. The latter questions the feasibility of fast-switching devices based on this type of material.

cond-mat.str-el↗

arXiv2001

On the Thermonuclear Runaway in Type Ia Supernovae: How to run away?

Type Ia Supernovae are thought to be thermonuclear explosions of massive white dwarfs (WD). We present the first study of multi-dimensional effects during the final hours prior to the thermonuclear runaway which leads to the explosion. The calculations utilize an implicit, 2-D hydro code.Mixing and the ignition process are studied in detail. We find that the initial chemical structure of the WD is changed but the material is not fully homogenized. The exploding WD sustains a central region with a low C/O ratio. This implies that the explosive nuclear burning will begin in a partially C-depleted environment. The thermonuclear runaway happens in a well defined region close to the center. It is induced by compressional heat when matter is brought inwards by convective flows. We find no evidence for multiple spot or strong off-center ignition. Convective velocities are of the order of 100 km/sec which is well above the effective burning speeds in SNe Ia previously expected right after the runaway. For about 0.5 to 1 sec, the speed of the burning front will neither be determined by the laminar speed nor the Rayleigh-Taylor instabilities but by convective flows produced prior to the runaway. The consequences are discussed for our under- standing of the detailed physics of the flame propagation, the deflagration detonation transition, and the nucleosynthesis in the central layers. Our results strongly suggest the pre-conditioning of the progenitor as a key-factor for our understanding of the diversity in SNeIa.

astro-ph↗

arXiv1999

The Role of Kinetic Energy Flux in the Convective Urca Process

The previous analysis of the convective Urca neutrino loss process in degenerate, convective, quasi-static, carbon-burning cores by Barkat and Wheeler omitted specific consideration of the role of the kinetic energy flux. The arguments of Barkat and Wheeler that steady-state composition gradients exist are correct, but chemical equilibrium does not result in net cooling. Barkat and Wheeler included a "work" term that effectively removed energy from the total energy budget that could only have come from the kinetic energy, which must remain positive. Consideration of the kinetic energy in the thermodynamics of the convective Urca process shows that the convective Urca neutrinos reduce the rate of increase of entropy that would otherwise be associated with the input of nuclear energy and slow down the convective current, but, unlike the "thermal" Urca process do not reduce the entropy or temperature.

astro-ph↗