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arXiv2014

Spin and orbital magnetic moments of size-selected iron, cobalt, and nickel clusters and their link to the bulk phase diagrams

Spin and orbital magnetic moments of cationic iron, cobalt, and nickel clusters have been determined from x-ray magnetic circular dichroism spectroscopy. In the size regime of $n = 10 - 15$ atoms, these clusters show strong ferromagnetism with maximized spin magnetic moments of 1~$μ_B$ per empty $3d$ state because of completely filled $3d$ majority spin bands. The only exception is $\mathrm{Fe}_{13}^+$ where an unusually low average spin magnetic moment of $0.73 \pm 0.12$~$μ_B$ per unoccupied $3d$ state is detected; an effect, which is neither observed for $\mathrm{Co}_{13}^+$ nor $\mathrm{Ni}_{13}^+$.\@ This distinct behavior can be linked to the existence and accessibility of antiferromagnetic, paramagnetic, or nonmagnetic phases in the respective bulk phase diagrams of iron, cobalt, and nickel. Compared to the experimental data, available density functional theory calculations generally seem to underestimate the spin magnetic moments significantly. In all clusters investigated, the orbital magnetic moment is quenched to $5 - 25$\,\% of the atomic value by the reduced symmetry of the crystal field. The magnetic anisotropy energy is well below 65 $μ$eV per atom.

physics.atm-clus↗

arXiv2016

A local order metric for condensed phase environments

We introduce a local order metric (LOM) that measures the degree of order in the neighborhood of an atomic or molecular site in a condensed medium. The LOM maximizes the overlap between the spatial distribution of sites belonging to that neighborhood and the corresponding distribution in a suitable reference system. The LOM takes a value tending to zero for completely disordered environments and tending to one for environments that match perfectly the reference. The site averaged LOM and its standard deviation define two scalar order parameters, $S$ and $δS$, that characterize with excellent resolution crystals, liquids, and amorphous materials. We show with molecular dynamics simulations that $S$, $δS$ and the LOM provide very insightful information in the study of structural transformations, such as those occurring when ice spontaneously nucleates from supercooled water or when a supercooled water sample becomes amorphous upon progressive cooling.

physics.comp-ph↗

arXiv2018

Calculating magnetic interactions in organic electrides

We present our calculation results for organic magnetic electrides. In order to identify the `cavity' electrons, we use maximally-localized Wannier functions and `empty atom' technique. The estimation of magnetic coupling is then performed based on magnetic force linear response theory. Both short- and long-range magnetic interactions are calculated with a single self-consistent calculation of a primitive cell. With this scheme we investigate four different organic electrides whose magnetic properties have been partly unknown or under debate. Our calculation results unveil the nature of magnetic moment and their interactions, and justify or defy the validity of preassumed spin models. Our work not only provides useful insight to understand magnetic electrides but also suggests a new paradigm to study the related materials.

cond-mat.mtrl-sci↗

arXiv2014

Semiclassical origin of anomalous shell effect for tetrahedral deformation in radial power-law potential model

Shell structures in single-particle energy spectra are investigated against regular tetrahedral type deformation using radial power-law potential model. Employing a natural way of shape parametrization which interpolates sphere and regular tetrahedron, we find prominent shell effects at rather large tetrahedral deformations, which bring about shell energies much larger than the cases of spherical and quadrupole type shapes. We discuss the semiclassical origin of these anomalous shell structures using periodic orbit theory.

nucl-th↗

arXiv2018

Energy transfer mechanisms in a dipole chain: From energy equipartition to the formation of breathers

We study the energy transfer in a classical dipole chain of $N$ interacting rigid rotating dipoles. The underlying high--dimensional potential energy landscape is analyzed in particular by determining the equilibrium points and their stability in the common plane of rotation. Starting from the minimal energy configuration, the response of the chain to excitation of a single dipole is investigated. Using both the linearized and the exact Hamiltonian of the dipole chain, we detect an approximate excitation energy threshold between a weakly and a strongly nonlinear dynamics. In the weakly nonlinear regime, the chain approaches in the course of time the expected energy equipartition among the dipoles. For excitations of higher energy, strongly localized excitations appear whose trajectories in time are either periodic or irregular, relating to the well-known discrete or chaotic breathers, respectively. The phenomenon of spontaneous formation of domains of opposite polarization and phase locking is found to commonly accompany the time evolution of the chaotic breathers. Finally, the sensitivity of the dipole chain dynamics to the initial conditions is studied as a function of the initial excitation energy by computing a fast chaos indicator. The results of this study confirm the aforementioned approximate threshold value for the initial excitation energy, below which the dynamics of the dipole chain is regular and above which it is chaotic.

nlin.CD↗

arXiv2013

Proton ejection from molecular hydride clusters exposed to strong X-ray pulses

Clusters consisting of small molecules containing hydrogen do eject fast protons when illuminated by short X-ray pulses. A suitable overall charging of the cluster controlled by the X-ray intensity induces electron migration from the surface to the bulk leading to efficient segregation of the protons and to a globally hindered explosion of the heavy atoms even outside the screened volume. We investigate this peculiar effect systematically along the iso-electronic sequence of methane over ammonia and water to the atomic limit of neon as a reference. In contrast to core-shell systems where the outer shell is sacrificed to reduce radiation damage, the intricate proton dynamics of hydride clusters allows one to keep the entire backbone of heavy atoms intact.

physics.atm-clus↗

arXiv2013

Magnetically generated spin-orbit coupling for ultracold atoms

We present a new technique for producing two- and three-dimensional Rashba-type spin-orbit couplings for ultracold atoms without involving light. The method relies on a sequence of pulsed inhomogeneous magnetic fields imprinting suitable phase gradients on the atoms. For sufficiently short pulse durations, the time-averaged Hamiltonian well approximates the Rashba Hamiltonian. Higher order corrections to the energy spectrum are calculated exactly for spin-1/2 and perturbatively for higher spins. The pulse sequence does not modify the form of rotationally symmetric atom-atom interactions. Finally, we present a straightforward implementation of this pulse sequence on an atom chip.

cond-mat.quant-gas↗

arXiv2013

Time delay in the recoiling valence-photoemission of Ar endohedrally confined in C$_{60}$

Effects of confinement and electron correlations on the relative time delay between the 3s and 3p photoemissions of Ar confined endohedrally in C$_{60}$ are investigated using the time dependent local density approximation - a method that is also found to mostly agree with recent time delay measurements between the 3s and 3p subshells in atomic Ar. At energies in the neighborhood of 3p Cooper minimum, correlations with C$_{60}$ electrons are found to induce opposite temporal effects in the emission of Ar 3p hybridized symmetrically versus that of Ar 3p hybridized antisymmetrically with C$_{60}$. A recoil-type interaction model mediated by the confinement is found to best describe the phenomenon.

physics.atom-ph↗

arXiv2013

Vibrationally Resolved Decay Width of Interatomic Coulombic Decay in HeNe

We investigate the ionization of HeNe from below the He 1s3p excitation to the He ionization threshold. We observe HeNe$^+$ ions with an enhancement by more than a factor of 60 when the He side couples resonantly to the radiation field. These ions are an experimental proof of a two-center resonant photoionization mechanism predicted by Najjari et al. [Phys. Rev. Lett. 105, 153002 (2010)]. Furthermore, our data provide electronic and vibrational state resolved decay widths of interatomic Coulombic decay (ICD) in HeNe dimers. We find that the ICD lifetime strongly increases with increasing vibrational state.

physics.atm-clus↗

arXiv2014

Prevailing features of X-ray induced molecular electron spectra revealed with fullerenes

Intense X-ray photo-absorption from short and intense pulses by a molecule triggers complicated electron and subsequently ion dynamics leading to photo-electron spectra which are difficult to interpret. Illuminating fullerenes offers a way to separate out the electron dynamics. Moreover, the fullerene cage confines spatially the origin of photo and Auger electrons. Together with the sequential nature of the photo processes at intensities available at X-ray free electron lasers, this allows for a remarkably detailed interpretation of the photo-electron spectra as we will demonstrate. The general features derived can serve as a paradigm for less well-defined situations in other large molecules or clusters.

physics.atm-clus↗

arXiv2018

Long-term monitoring of the internal energy distribution of isolated cluster systems

A method is presented to monitor the internal energy distribution of cluster anions via delayed electron detachment by pulsed photoexcitation and demonstrated on Co$_4{}^-$ in an electrostatic ion beam trap. In cryogenic operation, we calibrate the detachment delay to internal energy. By laser frequency scans, at room temperature, we reconstruct the time-dependent internal energy distribution of the clusters. The mean energies of ensembles from a cold and a hot ion source both approach thermal equilibrium. Our data yield a radiative emission law and the absorptivity of the cluster for thermal radiation.

physics.atm-clus↗

arXiv2018

Heteronuclear Limit of Strong-Field Ionization: Fragmentation of HeH$^+$ by Intense Ultrashort Laser Pulses

The laser-induced fragmentation dynamics of this most fundamental polar molecule HeH$^+$ are measured using an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for nonionizing dissociation, single ionization and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems leading to a more general picture of strong-field molecular dynamics and facilitating interpolation to systems between the two extreme cases.

physics.chem-ph↗

arXiv2018

Climbing the rotational ladder to chirality

Molecular chirality is conventionally understood as space-inversion-symmetry breaking in the equilibrium structure of molecules. Less well known is that achiral molecules can be made chiral through extreme rotational excitation. Here, we theoretically demonstrate a clear strategy for generating rotationally-induced chirality (RIC): An optical centrifuge rotationally excites the phosphine molecule (PH$_3$) into chiral cluster states that correspond to clockwise ($R$-enantiomer) or anticlockwise ($L$-enantiomer) rotation about axes almost coinciding with single P-H bonds. Application of a strong dc electric field during the centrifuge pulse favors the production of one rotating enantiomeric form over the other, creating dynamically chiral molecules with $permanently$ oriented rotational angular momentum. This essential step toward characterizing RIC promises a fresh perspective on chirality as a fundamental aspect of nature.

physics.chem-ph↗

arXiv2018

Floquet Mechanism for Non-Abelian Fractional Quantum Hall States

Three-body correlations, which arise between spin-polarized electrons in the first excited Landau level, are believed to play a key role in the emergence of enigmatic non-Abelian fractional quantum Hall (FQH) effects. Inspired by recent advances in Floquet engineering, we investigate periodic driving of anisotropic two-body interactions as a route for controllably creating and tuning effective three-body interactions in the FQH regime. We develop an analytic formalism to describe this Floquet-FQH protocol, which is distinct from previous approaches that instead focus on bandstructure engineering via modulation of single-particle hopping terms. By systematically analyzing the resulting interactions using generalized pseudopotentials, we show that our Floquet-FQH approach leads to repulsive as well as attractive three-body interactions that are highly tunable and support a variety of non-Abelian multicomponent FQH states. Finally, we propose an implementation of the protocol in optically dressed ultracold polar molecules with modulated Rabi frequencies.

cond-mat.str-el↗

arXiv2018

Breakdown of the spectator concept in low-electron-energy resonant decay processes

We suggest that low energy electrons, released by resonant decay processes, experience substantial scattering on the electron density of excited electrons, which remain a spectator during the decay. As a result, the angular emission distribution is altered significantly. This effect is expected to be a common feature of low energy secondary electron emission. In this letter, we exemplify our idea by examining the spectator resonant interatomic Coulombic decay (sRICD) of Ne dimers. Our theoretical predictions are confirmed by a corresponding coincidence experiment.

physics.atm-clus↗

arXiv2018

Photoelectron circular dichroism with two overlapping laser pulses of carrier frequencies $ω$ and $2ω$ linearly polarized in two mutually-orthogonal directions

Using a model methane-like chiral system, we theoretically demonstrate a possibility to access photoelectron circular dichroism (PECD) by a single experiment with two overlapping laser pulses of carrier frequencies $ω$ and $2ω$, which are linearly polarized in two mutually-orthogonal directions. Depending on the relative phase, the resulting electric field can be tailored to have two different rotational directions in the upper and lower hemispheres along the polarization of the $ω$-pulse. We predict a strong forward/backward asymmetry in the emission of photoelectrons from randomly oriented samples, which has an opposite sign in the upper and lower hemispheres. The predicted PECD effect is phase- and enantiomer-sensitive, providing new insight in this fascinating fundamental phenomenon. The effect can be optimized by varying relative intensities of the pulses.

physics.atm-clus↗

arXiv2018

Three-Dimensional Shapes of Spinning Helium Nanodroplets

A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion have been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large dataset allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.

physics.atm-clus↗

arXiv2018

Spin excitations in a 4f-3d heterodimer on MgO

We report on the magnetic properties of HoCo dimers as a model system for the smallest intermetallic transition metal-lanthanide compound. The dimers are adsorbed on ultrathin MgO(100) films grown on Ag(100). New for $4f$ elements, we detect inelastic excitations with scanning tunneling microscopy and prove by their behaviour in applied magnetic field that they are spin-excitations. In combination with density functional theory and spin Hamiltonian analysis we determine the magnetic level distribution, as well as sign and magnitude of the exchange interaction between the two atoms. In contrast to typical $4f-3d$ bulk compounds, we find ferromagnetic coupling in the dimer.

physics.atm-clus↗