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D. Ray

Publications and source records attributed to D. Ray.

At least 37 records · Page 2Linked to original sources
arXiv2022

Improved Limit on Tensor Currents in the Weak Interaction from $^8$Li $\beta$ Decay

The electroweak interaction in the Standard Model (SM) is described by a pure vector-axial-vector structure, though any Lorentz-invariant component could contribute. In this work, we present the most precise measurement of tensor currents in the low-energy regime by examining the $\beta$-$\bar{\nu}$ correlation of trapped $^{8}$Li ions with the Beta-decay Paul Trap. We find $a_{\beta\nu} = -0.3325 \pm 0.0013_{stat} \pm 0.0019_{syst}$ at $1\sigma$ for the case of coupling to right-handed neutrinos $(C_T=-C_T')$, which is consistent with the SM prediction.

nucl-ex↗
arXiv2021

Few-femtosecond resolved imaging of laser-driven nanoplasma expansion

The free expansion of a planar plasma surface is a fundamental non-equilibrium process relevant for various fields but as-yet experimentally still difficult to capture. The significance of the associated spatiotemporal plasma motion ranges from astrophysics and controlled fusion to laser machining, surface high-harmonic generation, plasma mirrors, and laser-particle acceleration. Here, we show that x-ray coherent diffractive imaging can surpass existing approaches and enables the quantitative real-time analysis of the sudden free expansion of nanoplasmas. For laser-ionized SiO$_2$ nanospheres, we resolve the formation of the emerging nearly self-similar plasma profile evolution and expose the so far inaccessible shell-wise expansion dynamics including the associated startup delay and rarefaction front velocity. Our results establish time-resolved diffractive imaging as an accurate quantitative diagnostic platform for tracing and characterizing plasma expansion and indicate the possibility to resolve various laser-driven processes including shock formation and wave-breaking phenomena with unprecedented resolution.

physics.plasm-ph↗
arXiv2021

Viscoelastic flow past an infinite plate with suction and constant heat flux

While studying the viscoelastic flow past an infinite plate with suction and constant heat flux between fluid and plate, Raptis and Tziyanidis gave the solution of a pair of equations for velocity and temperature as functions of distance. They then gave some approximate solutions. This letter shows that the approximations are not justified and presents an exact analytical study.

physics.flu-dyn↗
arXiv2021

Scalar meson field in a conformally flat space

Among several authors, who studied massive and massless scalar meson fields in general relativity, attempts to obtain a complete set of solutions for a conformally flat metric $e^ψ\left({dx^1}^2 + {dx^2}^2 + {dx^3}^2 - {dx^4}^2\right)$ were made by Ray for massive and massless mesons and Gursay for massless mesons. Both of them concluded that $ψ$ must be a function of $K_0\left({dx^1}^2 + {dx^2}^2 + {dx^3}^2 -{dx^4}^2\right) + K_1 x^1 + K_2 x^2 + K_3 x^3 + K_4 x^4$, where, where $K_0,~K_1,~ K_2,~K_3,~K_4$ are all constants. Both Ray and Gursay, however, overlooked an important particular case, which is studied here. As a by-product certain equations obtained by Auria and Regge in connection with "Gravitational theories with asymptotic flat Instantons," are solved under less restrictive assumptions.

gr-qc↗
arXiv2020

Self-Similar Magnetohydrodynamics

For the solution of the full set of magnetohydrodynamics (MHD) equations in the presence of gravity due to a central point-mass, a self-similar theory for a general polytrope has already suggested a set of exact time-dependent solutions by analytical methods for a, (gamma = 4\over3) polytrope, since (gamma = 4/3) is the simplest to treat. In the present paper while going for a complete set of self-similar solutions, we find that (gamma = 4/3) is the only physically-possible polytrope and that then, pressure is independent of the scalar function A, and depends on angle and time only. We also obtain a specific form of time-dependence for the self-similar variable.

physics.flu-dyn↗
arXiv2019

The possibility of $^{14}$C cluster as a building block of medium mass nuclei

The possibility of the $^{14}$C cluster being a basic building block of medium mass nuclei is discussed. Although $α$ cluster structures have been widely discussed in the light $N\approx Z$ mass region, the neutron to proton ratio deviates from unity in the nuclei near $β$-stability line and in neutron-rich nuclei. Thus, more neutron-rich objects with $N>Z$ could become the building blocks of cluster structures in such nuclei. The $^{14}$C nucleus is strongly bound and can be regarded as such a candidate. In addition, the path to the lowest shell-model configuration at short relative distances is closed for the $^{14}$C+$^{14}$C structure contrary to the case of the $^{12}$C+$^{12}$C structure; this allows to keep appreciable separation distance between the $^{14}$C clusters. The recent development of antisymmetrized quasi-cluster model (AQCM) allows us to utilize $jj$-coupling shell model wave function for each cluster in a simplified way. The AQCM results for the $^{14}$C+$^{14}$C structure in $^{28}$Mg are compared with the ones of cranked relativistic mean field (CRMF) calculations. Although theoretical frameworks of these two models are quite different, they give similar results for the nucleonic densities and rotational properties of the structure under investigation. The existence of linear chain three $^{14}$C cluster structure in $^{42}$Ar has also been predicted in AQCM. These results confirm the role of the $^{14}$C cluster as a possible building block of cluster structures in medium mass nuclei.

nucl-th↗
arXiv2019

Rotational excitations in near neutron-drip line nuclei: the birth and death of particle-bound rotational bands and the extension of nuclear landscape beyond spin zero neutron drip line

Two new mechanisms active in rotating nuclei located in the vicinity of neutron drip line have been discovered. Strong Coriolis interaction acting on high-$j$ orbitals transforms particle-unbound (resonance) nucleonic configurations into particle-bound ones with increasing angular momentum. The point of the transition manifests the birth of particle-bound rotational bands. Alternative possibility of the transition from particle-bound to resonance rotational band (the death of particle-bound rotational bands) with increasing spin also exists but it is less frequent in the calculations. The birth of particle-bound rotational bands provides a mechanism for the extension of nuclear landscape to neutron numbers which are larger than those of the neutron drip line in non-rotating nuclei.

nucl-th↗
arXiv2018

Nonequilibrium Phases and Segregation for Skyrmions on Periodic Pinning Arrays

Using particle-based simulations, we examine the collective dynamics of skyrmions interacting with periodic pinning arrays, focusing on the impact of the Magnus force on the sliding phases. As a function of increasing pinning strength, we find a series of distinct dynamical phases, including an interstitial flow phase, a moving disordered state, a moving crystal, and a segregated cluster state. The transitions between these states produce signatures in the skyrmion lattice structure, the skyrmion Hall angle, the velocity fluctuation distributions, and the velocity-force curves. The moving clustered state is similar to the segregated state recently observed in continuum-based simulations with strong quenched disorder. The segregation arises from the drive dependence of the skyrmion Hall angle, and appears in the strong pinning limit when the skyrmions have nonuniform velocities, causing different portions of the sample to have different effective skyrmion Hall angles. We map the evolution of the dynamic phases as a function of the system density, the ratio of the Magnus force to the dissipative term, and the ratio of the number of skyrmions to the number of pinning sites.

cond-mat.mes-hall↗
arXiv2018

Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray/x-ray pump-probe spectroscopy

The capability of generating two intense, femtosecond x-ray pulses with controlled time delay opens the possibility of performing time-resolved experiments for x-ray induced phenomena. We have applied this capability to study the photoinduced dynamics in diatomic molecules. In molecules composed of low-Z elements, \textit{K}-shell ionization creates a core-hole state in which the main decay mode is an Auger process involving two electrons in the valence shell. After Auger decay, the nuclear wavepackets of the transient two-valence-hole states continue evolving on the femtosecond timescale, leading either to separated atomic ions or long-lived quasi-bound states. By using an x-ray pump and an x-ray probe pulse tuned above the \textit{K}-shell ionization threshold of the nitrogen molecule, we are able to observe ion dissociation in progress by measuring the time-dependent kinetic energy releases of different breakup channels. We simulated the measurements on N$_2$ with a molecular dynamics model that accounts for \textit{K}-shell ionization, Auger decay, and the time evolution of the nuclear wavepackets. In addition to explaining the time-dependent feature in the measured kinetic energy release distributions from the dissociative states, the simulation also reveals the contributions of quasi-bound states.

physics.atom-ph↗
arXiv2017

Recent progress in the studies of neutron rich and high-$Z$ systems within the covariant density functional theory

The analysis of statistical and systematic uncertainties and their propogation to nuclear extremes has been performed. Two extremes of nuclear landscape (neutron-rich nuclei and superheavy nuclei) have been investigated. For the first extreme, we focus on the ground state properties. For the second extreme, we pay a particular attention to theoretical uncertainties in the description of fission barriers of superheavy nuclei and their evolution on going to neutron-rich nuclei.

nucl-th↗
arXiv2017

Assessing theoretical uncertainties in fission barriers of superheavy nuclei

Theoretical uncertainties in the predictions of inner fission barrier heights in superheavy elements have been investigated in a systematic way for a set of state-of-the-art covariant energy density functionals which represent major classes of the functionals used in covariant density functional theory. They differ in basic model assumptions and fitting protocols. Both systematic and statistical uncertainties have been quantified where the former turn out to be larger. Systematic uncertainties are substantial in superheavy elements and their behavior as a function of proton and neutron numbers contains a large random component. The benchmarking of the functionals to the experimental data on fission barriers in the actinides allows to reduce the systematic theoretical uncertainties for the inner fission barriers of unknown superheavy elements. However, even then they on average increase on moving away from the region where benchmarking has been performed. In addition, a comparison with the results of non-relativistic approaches is performed in order to define full systematic theoretical uncertainties over the state-of-the-art models. Even for the models benchmarked in the actinides, the difference in the inner fission barrier height of some superheavy elements reaches $5-6$ MeV. This uncertainty in the fission barrier heights will translate into huge (many tens of the orders of magnitude) uncertainties in the spontaneous fission half-lives.

nucl-th↗
arXiv2017

Emitter-site selective photoelectron circular dichroism of trifluoromethyloxirane

The angle-resolved inner-shell photoionization of R-trifluoromethyloxirane, C3H3F3O, is studied experimentally and theoretically. Thereby, we investigate the photoelectron circular dichroism (PECD) for nearly-symmetric O 1s and F 1s electronic orbitals, which are localized on different molecular sites. The respective dichroic $β_{1}$ and angular distribution $β_{2}$ parameters are measured at the photoelectron kinetic energies from 1 to 16 eV by using variably polarized synchrotron radiation and velocity map imaging spectroscopy. The present experimental results are in good agreement with the outcome of ab initio electronic structure calculations. We report a sizable chiral asymmetry $β_{1}$ of up to about 9% for the K-shell photoionization of oxygen atom. For the individual fluorine atoms, the present calculations predict asymmetries of similar size. However, being averaged over all fluorine atoms, it drops down to about 2%, as also observed in the present experiment. Our study demonstrates a strong emitter- and site-sensitivity of PECD in the one-photon inner-shell ionization of this chiral molecule.

physics.chem-ph↗
arXiv2016

From superdeformation to extreme deformation and clusterization in the N~Z nuclei of the A~40 mass region

A systematic search for extremely deformed structures in the N~Z nuclei of the A~40 mass region has been performed for the first time in the framework of covariant density functional theory. At spin zero such structures are located at high excitation energies which prevents their experimental observation. The rotation acts as a tool to bring these exotic shapes to the yrast line or its vicinity so that their observation could become possible with future generation of $γ-$tracking (or similar) detectors such as GRETA and AGATA. The major physical observables of such structures (such as transition quadrupole moments as well as kinematic and dynamic moments of inertia), the underlying single-particle structure and the spins at which they become yrast or near yrast are defined. The search for the fingerprints of clusterization and molecular structures is performed and the configurations with such features are discussed. The best candidates for observation of extremely deformed structures are identified. For several nuclei in this study (such as $^{36}$Ar), the addition of several spin units above currently measured maximum spin of $16\hbar$ will inevitably trigger the transition to hyper- and megadeformed nuclear shapes.

nucl-th↗
arXiv2016

Pinning, Flux Diodes, and Ratchets for Vortices Interacting with Conformal Pinning Arrays

A conformal pinning array can be created by conformally transforming a uniform triangular pinning lattice to produces a new structure in which the six-fold ordering of the original lattice is conserved but where there is a spatial gradient in the density of pinning sites. Here we examine several aspects of vortices interacting with conformal pinning arrays and how they can be used to create a flux flow diode effect for driving vortices in different directions across the arrays. Under the application of an ac drive, a pronounced vortex ratchet effect occurs where the vortices flow in the easy direction of the array asymmetry. When the ac drive is applied perpendicular to the asymmetry direction of the array, it is possible to realize a transverse vortex ratchet effect where there is a generation of a dc flow of vortices perpendicular to the ac drive due to the creation of a noise correlation ratchet by the plastic motion of the vortices. We also examine vortex transport simulations in experiments and compare the pinning effectiveness of conformal arrays to uniform triangular pinning arrays. We find that a triangular array generally pins the vortices more effectively at the first matching field and below, while the conformal array is more effective at higher fields where interstitial vortex flow occurs.

cond-mat.supr-con↗
arXiv2015

The neutron drip line: single-particle degrees of freedom and pairing properties as sources of theoretical uncertainties

The sources of theoretical uncertainties in the prediction of the two-neutron drip line are analyzed in the framework of covariant density functional theory. We concentrate on single-particle and pairing properties as potential sources of these uncertainties. The major source of these uncertainties can be traced back to the differences in the underlying single-particle structure of the various covariant energy density functionals (CEDF). It is found that the uncertainties in the description of single-particle energies at the two-neutron drip line are dominated by those existing already in known nuclei. Only approximately one third of these uncertainties are due to the uncertainties in the isovector channel of CEDF's. Thus, improving the CEDF description of single-particle energies in known nuclei will also reduce the uncertainties in the prediction of the position of two-neutron drip line. The predictions of pairing properties in neutron rich nuclei depend on the CEDF. Although pairing properties affect moderately the position of the two-neutron drip line they represent only a secondary source for the uncertainties in the definition of the position of the two-neutron drip line.

nucl-th↗
arXiv2015

Magnus-Induced Ratchet Effects for Skyrmions Interacting with Asymmetric Substrates

When a particle is driven with an ac force over an asymmetric potential, it can undergo a ratchet effect that produces a net dc motion of the particle. Ratchet effects have been observed in numerous systems such as superconducting vortices on asymmetric pinning substrates. Skyrmions, stable topological spin textures with particle-like properties, have many similarities to vortices but their behavior is strongly influenced by non-disspative effects arising from a Magnus term in their equation of motion. We show using numerical simulations that pronounced ratchet effects can occur for ac driven skyrmions moving over asymmetric quasi-one-dimensional substrates. We find a new type of ratchet effect called a Magnus-induced transverse ratchet that arises when the ac driving force is applied perpendicular rather than parallel to the asymmetry direction of the substrate. This transverse ratchet effect only occurs when the Magnus term is finite, and the threshold ac amplitude needed to induce it decreases as the Magnus term becomes more prominent. Ratcheting skyrmions follow ordered orbits in which the net displacement parallel to the substrate asymmetry direction is quantized. Skyrmion ratchets represent a new ac current-based method for controlling skyrmion positions and motion for spintronic applications.

cond-mat.mes-hall↗
arXiv2015

Reversible Ratchet Effects for Vortices in Conformal Pinning Arrays

A conformal transformation of a uniform triangular pinning array produces a structure called a conformal crystal which preserves the six-fold ordering of the original lattice but contains a gradient in the pinning density. Here we use numerical simulations to show that vortices in type-II superconductors driven with an ac drive over gradient pinning arrays produce the most pronounced ratchet effect over a wide range of parameters for a conformal array, while square gradient or random gradient arrays with equivalent pinning densities give reduced ratchet effects. In the conformal array, the larger spacing of the pinning sites in the direction transverse to the ac drive permits easy funneling of interstitial vortices for one driving direction, producing the enhanced ratchet effect. In the square array, the transverse spacing between pinning sites is uniform, giving no asymmetry in the funneling of the vortices as the driving direction switches, while in the random array, there are numerous easy-flow channels present for either direction of drive. We find multiple ratchet reversals in the conformal arrays as a function of vortex density and ac amplitude, and correlate the features with a reversal in the vortex ordering, which is greater for motion in the ratchet direction. The enhanced conformal pinning ratchet effect can also be realized for colloidal particles moving over a conformal array, indicating the general usefulness of conformal structures for controlling the motion of particles.

cond-mat.supr-con↗