arXiv subjects

D. Ray

Publications and source records attributed to D. Ray.

48 records · Page 3Linked to original sources

arXiv2015

Nuclear energy density functionals: what we can learn about/from their global performance?

A short review of recent results on the global performance of covariant energy density functionals is presented. It is focused on the analysis of the accuracy of the description of physical observables of ground and excited states as well as to related theoretical uncertainties. In addition, a global analysis of pairing properties is presented and the impact of pairing on the position of two-neutron drip line is discussed.

nucl-th↗

arXiv2015

Quantized Transport for a Skyrmion Moving on a Two-Dimensional Periodic Substrate

We study the dynamics and velocity-force curves of a skyrmion moving over a two-dimensional periodic substrate using simulations of a particle-based skyrmion model, focusing on the role of the non-dissipative Magnus term. In the overdamped limit, the skyrmion depins into a sliding state with a Hall angle of zero. When a Magnus term is included, the Hall angle is nonzero in the absence of a substrate. On a periodic substrate, the Hall angle varies with the drive amplitude. Due to the substrate symmetry the Hall angle does not change continuously with drive, but forms a series of discrete steps at rational ratios of the skyrmion velocity components perpendicular and parallel to the drive direction, when the skyrmion motion locks to symmetry directions of the substrate for fixed intervals of the drive amplitude. On each step, the Hall angle is constant and the skyrmion motion is orderly. Transitions between locked phases generate pronounced cusps in the velocity-force curves, as well as regions of negative differential mobility. The number of observable locking steps increases when the relative strength of the Magnus term increases. We find an overshoot phenomena where the Hall angle exceeds the clean limit value, as well as an acceleration effect where the skyrmion moves faster over a substrate than through a clean sample. These effects are robust for different types of periodic substrates. With a simple model for a skyrmion interacting with a single pin, we can capture the behavior of the Hall angle. The Magnus term induces a curvature in the skrymion orbit as it moves through the pin, resulting in a side step phenomenon that decreases with increasing drive. When the Magnus term is large, the range of impact parameters that permit the skyrmion to be trapped by the pin is small, which is a reasons that strong Magnus force effects reduce the pinning in skyrmion systems.

cond-mat.str-el↗

arXiv2014

Collective Transport Properties of Driven Skyrmions with Random Disorder

We use particle-based simulations to examine the static and driven collective phases of skyrmions interacting with random quenched disorder. We show that non-dissipative effects due to the Magnus term reduce the depinning threshold and strongly affect the skyrmion motion and the nature of the dynamic phases. The quenched disorder causes the Hall angle to become drive-dependent in the moving skyrmion phase, while different flow regimes produce distinct signatures in the transport curves. For weak disorder, the skyrmions form a pinned crystal and depin elastically, while for strong disorder the system forms a pinned amorphous state that depins plastically. At high drives the skyrmions can dynamically reorder into a moving crystal, with the onset of reordering determined by the strength of the Magnus term.

cond-mat.str-el↗

arXiv2014

Pinning, Ordering, and Dynamics of Vortices in Conformal Crystal and Gradient Pinning Arrays

We numerically investigate magnetization, pinning, ordering, and dynamics of vortices interacting with pinning arrangements which have a density gradient. We focus on conformal crystal structures obtained by conformally transforming a spatially uniform periodic array, as well as non-conformal gradient structures and structures with quasiperiodic order. The conformal structures feature a density gradient and local ordering. Using magnetization simulations we find that conformal pinning arrays exhibit enhanced pinning compared to non-conformal gradient arrays as well as compared to random, periodic, and quasiperiodic arrays, for a broad range of fields. The effectiveness of conformal arrays arises from the continuum of length scales introduced into the arrays by the conformal transformation, allowing for a broad range of local commensuration effects. At higher vortex fillings above the range of conformal effectiveness, we show that a non-conformal rectangular gradient array exhibits strong pinning due to a novel commensuration effect and vortex ordering. Using transport simulations where vortices are driven along the gradient and at an angle to the gradient, we confirm the effectiveness of conformal pinning at increasing the critical current. For a rotated drive, the gradient arrays produce a strong vortex guidance effect in the direction perpendicular to the gradient.

cond-mat.supr-con↗

arXiv2014

Global performance of covariant energy density functionals: ground state observables of even-even nuclei and the estimate of theoretical uncertainties

Covariant density functional theory (CDFT) is a modern theoretical tool for the description of nuclear structure phenomena. The current investigation aims at the global assessment of the accuracy of the description of the ground state properties of even-even nuclei. We also estimate {\it theoretical uncertainties} defined here as the spreads of predictions within four covariant energy density functionals (CEDF) in known regions of the nuclear chart and their propagation towards the neutron drip line. Large-scale axial relativistic Hartree-Bogoliubov (RHB) calculations are performed for all $Z\leq 104$ even-even nuclei between the two-proton and two-neutron drip lines with four modern covariant energy density functionals such as NL3*, DD-ME2, DD-ME$δ$ and DD-PC1. The physical observables of interest include the binding energies, two-particle separation energies, charge quadrupole deformations, isovector deformations, charge radii, neutron skin thicknesses and the positions of the two-proton and two-neutron drip lines. The predictions for the two-neutron drip line are also compared in a systematic way with the ones obtained in non-relativistic models. As an example, the data set of the calculated properties of even-even nuclei obtained with DD-PC1 CEDF is provided as Supplemental Material with this article (file ddpc1.pdf).

nucl-th↗

arXiv2014

Casimir Effect in Active Matter Systems

We numerically examine run-and-tumble active matter particles in Casimir geometries composed of two finite parallel walls. We find that there is an attractive force between the two walls of a magnitude that increases with increasing run length. The attraction exhibits an unusual exponential dependence on the wall separation, and it arises due to a depletion of swimmers in the region between the walls by a combination of the motion of the particles along the walls and a geometric shadowing effect. This attraction is robust as long as the wall separation is comparable to or smaller than the swimmer run length, and is only slightly reduced by the inclusion of steric interactions between swimmers. We also examine other geometries and find regimes in which there is a crossover from attraction to repulsion between the walls as a function of wall separation and run length.

cond-mat.soft↗

arXiv2013

Vortex Transport and Pinning Effectiveness in Conformal Pinning Arrays

We examine the current driven dynamics for vortices interacting with conformal crystal pinning arrays and compare to the dynamics of vortices driven over random pinning arrays. We find that the pinning is enhanced in the conformal arrays over a wide range of fields, consistent with previous results from flux gradient-driven simulations. At fields above this range, the effectiveness of the pinning in the moving vortex state can be enhanced in the random arrays compared to the conformal arrays, leading to crossing of the velocity-force curves.

cond-mat.supr-con↗

arXiv2013

Comparing the Dynamics of Skyrmions and Superconducting Vortices

Vortices in type-II superconductors have attracted enormous attention as ideal systems in which to study nonequilibrium collective phenomena, since the self-ordering of the vortices competes with quenched disorder and thermal effects. Dynamic effects found in vortex systems include depinning, nonequilibrium phase transitions, creep, structural order-disorder transitions, and melting. Understanding vortex dynamics is also important for applications of superconductors which require the vortices either to remain pinned or to move in a controlled fashion. Recently, topological defects called skyrmions have been realized experimentally in chiral magnets. Here we highlight similarities and differences between skyrmion dynamics and vortex dynamics. Many of the previous ideas and experimental setups that have been applied to superconducting vortices can also be used to study skyrmions. We also discuss some of the differences between the two systems, such as the potentially large contribution of the Magnus force in the skyrmion system that can dramatically alter the dynamics and transport properties.

cond-mat.str-el↗

arXiv2013

Nuclear landscape in covariant density functional theory

The neutron and proton drip lines represent the limits of the nuclear landscape. While the proton drip line is measured experimentally up to rather high $Z$-values, the location of the neutron drip line for absolute majority of elements is based on theoretical predictions which involve extreme extrapolations. The first ever systematic investigation of the location of the proton and neutron drip lines in the covariant density functional theory has been performed by employing a set of the state-of-the-art parametrizations. Calculated theoretical uncertainties in the position of two-neutron drip line are compared with those obtained in non-relativistic DFT calculations. Shell effects drastically affect the shape of two-neutron drip line. In particular, model uncertainties in the definition of two-neutron drip line at $Z\sim 54, N=126$ and $Z\sim 82, N=184$ are very small due to the impact of spherical shell closures at N=126 and 184.

nucl-th↗

arXiv2013

Vortex States in Archimedean Tiling Pinning Arrays

We numerically study vortex ordering and pinning in Archimedean tiling substrates composed of square and triangular plaquettes. The two different plaquettes become occupied at different vortex densities, producing commensurate peaks in the magnetization at non-integer matching fields. We find that as the field increases, in some cases the fraction of occupied pins can decrease due to the competition between fillings of the different plaquette types. We also identify a number of different types of vortex orderings as function of field at integer and non-integer commensurate fillings.

cond-mat.supr-con↗

arXiv2012

Strongly Enhanced Vortex Pinning by Conformal Crystal Arrays

Conformal crystals are non-uniform structures created by a conformal transformation of regular two-dimensional lattices. We show that gradient-driven vortices interacting with a conformal pinning array exhibit substantially stronger pinning effects over a much larger range of field than found for random or periodic pinning arrangements. The pinning enhancement is partially due to matching of the critical flux gradient with the pinning gradient, but the preservation of the sixfold ordering in the conformally transformed hexagonal lattice plays a crucial role. Our results can be generalized to a wide class of gradient-driven interacting particle systems such as colloids on optical trap arrays.

cond-mat.supr-con↗

arXiv2009

Field-free orientation of CO molecules by femtosecond two-color laser fields

We report the first experimental observation of non-adiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction and molecular frame photoemission.

physics.chem-ph↗