arXiv subjects

Y. Zou

Publications and source records attributed to Y. Zou.

32 records · Page 2Linked to original sources

arXiv2019

Atomically-thin micas as proton conducting membranes

Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons. For thicker two-dimensional (2D) materials, proton conductivity diminishes exponentially so that, for example, monolayer MoS2 that is just three atoms thick is completely impermeable to protons. This seemed to suggest that only one-atom-thick crystals could be used as proton conducting membranes. Here we show that few-layer micas that are rather thick on the atomic scale become excellent proton conductors if native cations are ion-exchanged for protons. Their areal conductivity exceeds that of graphene and hBN by one-two orders of magnitude. Importantly, ion-exchanged 2D micas exhibit this high conductivity inside the infamous gap for proton-conducting materials, which extends from 100 C to 500 C. Areal conductivity of proton-exchanged monolayer micas can reach above 100 S cm-2 at 500 C, well above the current requirements for the industry roadmap. We attribute the fast proton permeation to 5 A-wide tubular channels that perforate micas' crystal structure which, after ion exchange, contain only hydroxyl groups inside. Our work indicates that there could be other 2D crystals with similar nm-scale channels, which could help close the materials gap in proton-conducting applications.

cond-mat.mes-hall↗

arXiv2019

Observation of indirect ionization of W7+ in an electron-beam ion-trap plasma

In this work, visible and extreme ultraviolet spectra of W7+ are measured using the high-temperature superconducting electron-beam ion trap (EBIT) at the Shanghai EBIT Laboratory under extremely low-energy conditions (lower than the nominal electron-beam energy of 130 eV). The relevant atomic structure is calculated using the flexible atomic code package based on the relativistic configuration interaction method. The GRASP2K code, in the framework of the multiconfiguration Dirac-Hartree-Fock method, is employed as well for calculating the wavelength of the M1 transition in the ground configuration of W7+. A line from the W7+ ions is observed at a little higher electron-beam energy than the ionization potential for W4+, making this line appear to be from W5+. A hypothesis for the charge-state evolution of W7+ is proposed based on our experimental and theoretical results; that is, the occurrence of W7+ ions results from indirect ionization caused by stepwise excitation between some metastable states of lower-charge-state W ions, at the nominal electron-beam energy of 59 eV.

physics.atom-ph↗

arXiv2016

Explosive transitions in complex networks' structure and dynamics: percolation and synchronization

Percolation and synchronization are two phase transitions that have been extensively studied since already long ago. A classic result is that, in the vast majority of cases, these transitions are of the second-order type, i.e. continuous and reversible. Recently, however, explosive phenomena have been reported in com- plex networks' structure and dynamics, which rather remind first-order (discontinuous and irreversible) transitions. Explosive percolation, which was discovered in 2009, corresponds to an abrupt change in the network's structure, and explosive synchronization (which is concerned, instead, with the abrupt emergence of a collective state in the networks' dynamics) was studied as early as the first models of globally coupled phase oscillators were taken into consideration. The two phenomena have stimulated investigations and de- bates, attracting attention in many relevant fields. So far, various substantial contributions and progresses (including experimental verifications) have been made, which have provided insights on what structural and dynamical properties are needed for inducing such abrupt transformations, as well as have greatly enhanced our understanding of phase transitions in networked systems. Our intention is to offer here a monographic review on the main-stream literature, with the twofold aim of summarizing the existing results and pointing out possible directions for future research.

nlin.AO↗

arXiv2016

Investigation of M1 transitions of the ground-state configuration of In-like Tungsten

Three visible lines of M1 transitions from In-like W were recorded using the Shanghai permanent magnet electron beam ion trap. The experimental wavelengths were measured as 493.84 $\pm$ 0.15, 226.97 $\pm$ 0.13 and 587.63 $\pm$ 0.23 nm (vacuum wavelengths). These results are in good agreement with theoretical predictions obtained using large-scale Relativistic Many-Body Perturbation Theory, in the form of the Flexible Atomic Code.

physics.atom-ph↗

arXiv2015

The Bellerophon state: a novel coherent phase of globally coupled oscillators

From rhythmic physiological processes to the collective behaviors of technological and natural networks, coherent phases of interacting oscillators are the foundation of the events' coordination leading a system to behave cooperatively. We unveil the existence of a new of such states, occurring in globally coupled nonidentical oscillators in the proximity of the point where the transition from the system's incoherent to coherent phase converts from explosive to continuous. In such a state, oscillators form quantized clusters, where they are neither phase- nor frequency-locked. Oscillators' instantaneous speeds are different within the clusters, but they form a characteristic cusped pattern and, more importantly, they behave periodically in time so that their average values are the same. Given its intrinsic specular nature with respect to the recently introduced Chimera states, the phase is termed the {\it Bellerophon} state. We provide analytical and numerical description of the microscopic and macroscopic details of {\it Bellerophon} states, thus furnishing practical hints on how to seek for the new phase in a variety of experimental and natural systems.

nlin.CD↗

arXiv2015

Dual Fano and Lorentzian line profile poperties of autoionizing states

Ott et al. (Science (340, 716 (2013)) successfully transferred Fano profile into Lorentzian lineshape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward of quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e. electron ion recombination with photon emission, making sure the resonant autoionizing state is not perturbed by the laser fields. Our result implies that excitation of the state through different paths can lead to different Fano profiles for the same resonant state. This allows more options for the combination of laser fields and lead to more opportunities for quantum phase control. Our result also indicates the breakdown of the classical two step picture for dielectronic recombination.

physics.atom-ph↗

arXiv2015

Almost-sure hedging with permanent price impact

We consider a financial model with permanent price impact. Continuous time trading dynamics are derived as the limit of discrete rebalancing policies. We then study the problem of super-hedging a European option. Our main result is the derivation of a quasi-linear pricing equation. It holds in the sense of viscosity solutions. When it admits a smooth solution, it provides a perfect hedging strategy.

q-fin.PR↗

arXiv2014

Fermi liquid breakdown and evidence for superconductivity in YFe$_2$Ge$_2$

In the d-electron system YFe$_2$Ge$_2$, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity $C/T \sim 100~\mathrm{mJ/(molK^2)}$ and an anomalous power law temperature dependence of the electrical resistivity $ρ\simeq ρ_0 + AT^{3/2}$ signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions, accompanied by DC diamagnetic screening fractions of up to 80\% suggest that pure samples of YFe$_2$Ge$_2$ superconduct below $1.8~\mathrm{K}$.

cond-mat.str-el↗

arXiv2012

Power-laws in recurrence networks from dynamical systems

Recurrence networks are a novel tool of nonlinear time series analysis allowing the characterisation of higher-order geometric properties of complex dynamical systems based on recurrences in phase space, which are a fundamental concept in classical mechanics. In this Letter, we demonstrate that recurrence networks obtained from various deterministic model systems as well as experimental data naturally display power-law degree distributions with scaling exponents $γ$ that can be derived exclusively from the systems' invariant densities. For one-dimensional maps, we show analytically that $γ$ is not related to the fractal dimension. For continuous systems, we find two distinct types of behaviour: power-laws with an exponent $γ$ depending on a suitable notion of local dimension, and such with fixed $γ=1$.

nlin.CD↗

arXiv2011

Transport and Thermodynamic Evidence for a Marginal Fermi Liquid State in ZrZn$_2$

Measurements of low temperature transport and thermodynamic properties have been used to characterize the non-Fermi liquid state of the itinerant ferromagnet ZrZn$_2$. We observe a $T^{5/3}$ temperature dependence of the electrical resistivity at zero field, which becomes $T^2$ like in an applied field of 9 T. In zero field we also measured the thermal conductivity, and we see a novel linear in $T$ dependence of the difference between the thermal and electrical resistivities. Heat capacity measurements, also at zero field, reveal an upturn in the electronic contribution at low temperatures when the phonon term is subtracted. Taken together, we argue that these properties are consistent with a marginal Fermi liquid state which is predicted by a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions. We compare our data to quantitative predictions and establish this model as a compelling theoretical framework for understanding ZrZn$_2$.

cond-mat.str-el↗

arXiv2009

Complex Network Approach for Recurrence Analysis of Time Series

We propose a novel approach for analysing time series using complex network theory. We identify the recurrence matrix calculated from time series with the adjacency matrix of a complex network, and apply measures for the characterisation of complex networks to this recurrence matrix. By using the logistic map, we illustrate the potentials of these complex network measures for detecting dynamical transitions. Finally we apply the proposed approach to a marine palaeo-climate record and identify subtle changes of the climate regime.

nlin.CD↗

arXiv2009

Recurrence networks - A novel paradigm for nonlinear time series analysis

This paper presents a new approach for analysing structural properties of time series from complex systems. Starting from the concept of recurrences in phase space, the recurrence matrix of a time series is interpreted as the adjacency matrix of an associated complex network which links different points in time if the evolution of the considered states is very similar. A critical comparison of these recurrence networks with similar existing techniques is presented, revealing strong conceptual benefits of the new approach which can be considered as a unifying framework for transforming time series into complex networks that also includes other methods as special cases. It is demonstrated that there are fundamental relationships between the topological properties of recurrence networks and the statistical properties of the phase space density of the underlying dynamical system. Hence, the network description yields new quantitative characteristics of the dynamical complexity of a time series, which substantially complement existing measures of recurrence quantification analysis.

nlin.CD↗

arXiv2005

Multiscale analysis of re-entrant production lines: An equation-free approach

The computer-assisted modeling of re-entrant production lines, and, in particular, simulation scalability, is attracting a lot of attention due to the importance of such lines in semiconductor manufacturing. Re-entrant flows lead to competition for processing capacity among the items produced, which significantly impacts their throughput time (TPT). Such production models naturally exhibit two time scales: a short one, characteristic of single items processed through individual machines, and a longer one, characteristic of the response time of the entire factory. Coarse-grained partial differential equations for the spatio-temporal evolution of a "phase density" were obtained through a kinetic theory approach in Armbruster et al. [2]. We take advantage of the time scale separation to directly solve such coarse-grained equations, even when we cannot derive them explicitly, through an equation-free computational approach. Short bursts of appropriately initialized stochastic fine-scale simulation are used to perform coarse projective integration on the phase density. The key step in this process is lifting: the construction of fine-scale, discrete realizations consistent with a given coarse-grained phase density field. We achieve this through computational evaluation of conditional distributions of a "phase velocity" at the limit of large item influxes.

math.DS↗

arXiv2003

Measurement of Decay Parameters for $Ξ^{-} \to Λπ^{-}$ Decay

Based on 1.35 million polarized $Ξ^{-}$ events, we measure the parameter $ϕ_Ξ$ to be $-1.61^{\circ} \pm 2.66^{\circ} \pm 0.37^{\circ}$ for $Ξ^{-} \to Λπ^{-}$ decay. New results for the parameters $β_Ξ$ and $γ_Ξ$ are also presented. Assuming that the CP-violating phase-shift difference is negligible, we deduce the strong phase-shift difference between the P-wave and S-wave amplitudes of the $Λπ$ final state to be $3.17^{\circ} \pm 5.28^{\circ} \pm 0.73^{\circ}$. This strong phase-shift difference reduces the theoretical uncertainty in estimating the level of CP violation in $Ξ$-hyperon decay.

hep-ex↗