Finite Excitation Research Articles

Tools for the Quantitative Analysis of Sedimentation Boundaries Detected by Fluorescence Optical Analytical Ultracentrifugation

Fluorescence optical detection in sedimentation velocity analytical ultracentrifugation allows the study of macromolecules at nanomolar concentrations and below. This has significant promise, for example, for the study of systems of high-affinity protein interactions. Here we describe adaptations of the direct boundary modeling analysis approach implemented in the software SEDFIT that were developed to accommodate unique characteristics of the confocal fluorescence detection system. These include spatial gradients of signal intensity due to scanner movements out of the plane of rotation, temporal intensity drifts due to instability of the laser and fluorophores, and masking of the finite excitation and detection cone by the sample holder. In an extensive series of experiments with enhanced green fluorescent protein ranging from low nanomolar to low micromolar concentrations, we show that the experimental data provide sufficient information to determine the parameters required for first-order approximation of the impact of these effects on the recorded data. Systematic deviations of fluorescence optical sedimentation velocity data analyzed using conventional sedimentation models developed for absorbance and interference optics are largely removed after these adaptations, resulting in excellent fits that highlight the high precision of fluorescence sedimentation velocity data, thus allowing a more detailed quantitative interpretation of the signal boundaries that is otherwise not possible for this system.

Quantum-Phase-Transition-Induced Multiferroics and Higgs Mode in Integer Spin Systems in Noncentrosymmetric Lattice with Strong Single-Ion Anisotropy

In integer spin systems with a strong easy-plane type single-ion anisotropy, a paramagnetic ground state is stabilized with a finite excitation gap. Applying a magnetic field or pressure can collapse the excitation gap and induce quantum phase transition into a magnetically ordered state. In a lattice without the inversion symmetry at the magnetic ion site, electric polarization emerges spontaneously by the quantum phase transition. We study how excitation modes evolve with field or pressure and analyze the dynamical correlation functions of the electric polarization. In the case of the pressure-induced order, a longitudinal mode exists in the low-energy region in the vicinity of the quantum critical point. It corresponds to a “Higgs mode” and is observable as an “electromagnon” that is excited by the electric field component of light absorption.

Role ofdelectrons in electronic stopping of slow light ions

In recent energy loss measurements, band structure effects in electronic stopping have been observed for materials with finite excitation thresholds, for example, noble metals such as Cu and Au. To further investigate the influence of the position of the $d$ band relative to the Fermi edge, electronic stopping of hydrogen and helium ions in Ag and Pt was determined. For Ag, the electronic stopping power exhibits a velocity dependence similar to Cu and Au. No particular effect due to the comparatively large $d$-band offset in Ag is found. In the case of Pt, the electronic stopping power is virtually velocity proportional for H${}^{+}$ ions and exhibits a distinct velocity dependence for He${}^{+}$ ions. For hydrogen the results are compatible with modeling the conduction band as a free electron gas with an energy-dependent effective number of electrons. For He${}^{+}$, however, the observed effects point towards an additional energy loss mechanism, e.g., by charge-exchange processes.

Atomic force microscopy of confined liquids using the thermal bending fluctuations of the cantilever

We use atomic force microscopy to measure the distance-dependent solvation forces and the dissipation across liquid films of octamethylcyclotetrasiloxane (OMCTS) confined between a silicon tip and a highly oriented pyrolytic graphite substrate without active excitation of the cantilever. By analyzing the thermal bending fluctuations, we minimize possible nonlinearities of the tip-substrate interaction due to finite excitation amplitudes because these fluctuations are smaller than the typical 1 Å, which is much smaller than the characteristic interaction length. Moreover, we avoid the need to determine the phase lag between cantilever excitation and response, which suffers from complications due to hydrodynamic coupling between cantilever and fluid. Consistent results, and especially high-quality dissipation data, are obtained by analyzing the power spectrum and the time autocorrelation of the force fluctuations. We validate our approach by determining the bulk viscosity of OMCTS using tips with a radius of approximately 1 μm at tip-substrate separations >5 nm. For sharp tips we consistently find an exponentially decaying oscillatory tip-substrate interaction stiffness as well as a clearly nonmonotonic variation of the dissipation for tip-substrate distances up to 8 and 6 nm, respectively. Both observations are in line with the results of recent simulations which relate them to distance-dependent transitions of the molecular structure in the liquid.

Giant dipole resonance width as a probe for nuclear deformation at finite excitation

The first systematic study of the correlation between the experimental giant dipole resonance (GDR) width and the average deformation $\ensuremath{\langle}\ensuremath{\beta}\ensuremath{\rangle}$ of the nucleus at finite excitation is presented for the mass region $A\ensuremath{\sim}59$ to 208. We show that the width of the GDR ($\ensuremath{\Gamma}$) and the quadrupole deformation of the nucleus do not follow a linear relation, as predicted earlier, owing to the GDR-induced quadrupole moment, and the correlation also depends on the mass of the nuclei. The different empirical values of $\ensuremath{\langle}\ensuremath{\beta}\ensuremath{\rangle}$ extracted from the experimental GDR width match exceptionally well with the thermal shape fluctuation model. As a result, this universal correlation between $\ensuremath{\langle}\ensuremath{\beta}\ensuremath{\rangle}$ and $\ensuremath{\Gamma}$ provides a direct experimental probe to determine the nuclear deformation at finite temperature and angular momentum over the entire mass region.

First-principles study of ring to cage structural crossover in small ZnO Clusters

Density functional, full-potential computations are performed to study the origin and consequences of the ring to cage structural crossover in (ZnO)n (n = 2–16) clusters. The origin of this structural crossover, which is found to occur at n = 10, is studied by investigating the behavior of the Zn–O–Zn bond angle, the Zn–O bond strength, and the number of bonds in the systems. It is argued that 12 is the lowest magic number of ZnO clusters in the ground state, while finite temperature vibrational excitations enhance the relative stability of the (ZnO)9 cluster to make it a magic system at temperatures above about 170 K. The obtained electronic structure of the clusters before and after applying the many-body GW corrections evidence a size-induced redshift originating from the ring to cage structural crossover in the system. The behavior of the electron density bond points of the clusters along with the extrapolated cluster binding energy at very large sizes may indicate the existence of a metastable structure for large ZnO nanostructures, different from the bulk ZnO structure.

Magnetic properties of nanoscale compass-Heisenberg planar clusters

We study a model of spins 1/2 on a square lattice, generalizing the quantum compass model via the addition of perturbing Heisenberg interactions between nearest neighbors, and investigate its phase diagram and magnetic excitations. This model has motivations both from the field of strongly correlated systems with orbital degeneracy and from that of solid-state based devices proposed for quantum computing. We find that the high degeneracy of ground states of the compass model is fragile and changes into twofold degenerate ground states for any finite amplitude of Heisenberg coupling. By computing the spin structure factors of finite clusters with Lanczos diagonalization, we evidence a rich variety of phases characterized by Z2 symmetry, that are either ferromagnetic, C-type antiferromagnetic, or of Neel type, and analyze the effects of quantum fluctuations on phase boundaries. In the ordered phases the anisotropy of compass interactions leads to a finite excitation gap to spin waves. We show that for small nanoscale clusters with large anisotropy gap the lowest excitations are column-flip excitations that emerge due to Heisenberg perturbations from the manifold of degenerate ground states of the compass model. We derive an effective one-dimensional XYZ model which faithfully reproduces the exact structure of these excited states and elucidates their microscopic origin. The low energy column-flip or compass-type excitations are robust against decoherence processes and are therefore well designed for storing information in quantum computing. We also point out that the dipolar interactions between nitrogen-vacancy centers forming a rectangular lattice in a diamond matrix may permit a solid-state realization of the anisotropic compass-Heisenberg model.

Inelastic Electron Tunneling Spectroscopy of a Mn Dimer

A scanning tunneling microscope (STM) can probe the inelastic spin excitations of single magnetic atoms in a surface via spin-flip assisted tunneling. A particular and intriguing case is the Mn dimer case. We show here that the existing theories for inelastic transport spectroscopy do not explain the observed spin transitions when both atoms are equally coupled to the STM tip and the substrate. The hyperfine coupling to the nuclear spins is shown to lead to a finite excitation amplitude, but the physical mechanism leading to the large inelastic signal observed is still unknown. We discuss some other alternatives that break the symmetry of the system and allows for larger excitation probabilities.

Nonequilibrium phenomena in damaged media and their effects on the elastic properties

Concrete, particularly if damaged, exhibits a peculiar nonlinear elastic behavior, which is mainly due to the coupling between nonequilibrium and nonlinear features, the two of which are intrinsically connected. More specifically, the formulation of a constitutive equation able to properly predict the dynamic behavior of damaged concrete is made difficult by the concomitant presence of two mechanisms: The modification of the microstructure of the medium and the transition to a new elastic state caused by a finite amplitude excitation (conditioning). Memory of that new state is kept when the excitation is removed, before relaxation back to the original elastic state takes place. Indeed, besides accounting for linear and nonlinear parameters, a realistic constitutive equation to be used in reliable prediction models should take into account nonequilibrium effects. Specific parameters, sensitive to finite amplitude excitations, should be introduced to provide information about conditioning effects. In this paper, experimental results indicating that nonlinearity of damaged concrete is memory-dependent will be presented and the implications of such findings in the development of physical models, with relevant outcomes for the characterization of hysteretical features, will be discussed.

Importance of Thermally Induced Magnetic Excitations in First-Principles Simulations of Elastic Properties of Transition Metal Alloys

We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe72Cr16Ni12face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.

DMRG study of fractional quantum Hall effect and valley skyrmions in graphene

The ground state and low-energy excitations of graphene and its bilayer are investigated by the density matrix renormalization group (DMRG) method. We analyze the effect of Coulomb interaction between the electrons including valley degrees of freedoms. The obtained results show finite charge excitation gap at various fractional fillings νn = 1/3, 2/5, 2/3 in the n = 0 and 1 Landau levels of single-layer graphene (SLG) and n = 2 Landau level of bilayer graphene (BLG). The lowest charge excitations at ν = 1/3, and 1 in SLG are valley skyrmions.

Beating spatio-temporal coupling: implications for pulse shaping and coherent control experiments

Diffraction of finite sized laser beams imposes a limit on the control that can be exerted over ultrafast pulses. This limit manifests as spatio-temporal coupling induced in standard implementations of pulse shaping schemes. We demonstrate the influence this has on coherent control experiments that depend on finite excitation, sample, and detection volumes. Based on solutions used in pulse stretching experiments, we introduce a double-pass scheme that reduces the errors produced through spatio-temporal coupling by at least one order of magnitude. Finally, employing single molecules as nanoscale probes, we prove that such a double pass scheme is capable of artifact-free pulse shaping at dimensions two orders of magnitude smaller than the diffraction limit.

Majorana fermions in spin-orbit-coupled ferromagnetic Josephson junctions

We study all possible Majorana modes in two-dimensional spin-orbit coupled ferromagnetic superconductor-normal state-superconductor (SNS) Josephson junctions and propose experiments to detect them. With the S region in a non-trivial topological phase and a superconducting phase difference $\phi = \pi$ across the junction, two delocalized Majorana fermions with no excitation gap appear in the N region. In addition, if S and N belong to different topological phases and have well-separated the Fermi surfaces, localized Majorana fermions with a finite excitation gap also emerge at both SN interfaces for all $\phi$.

Three-dimensional finite element simulation of closed delaminations in composite materials

Early stage delaminations in composite materials tend to be closed at rest. Inspection with traditional linear ultrasonic techniques generally fails to diagnose and locate such imperfections. However, if undetected and left untreated, incipient defects may gradually grow within the material and eventually lead to failure of the component. Kissing bonds or clapping contacts inherently demand a non-linear diagnostic method, applying a finite excitation amplitude that is able to overcome an activation threshold to open and close the contact. In order to obtain a better understanding and analysis of the macroscopic non-linear behavior that can be observed at the component level, we developed and investigated the results of a finite element model for a composite material containing a single circular delamination. The model makes use of local node splitting and the non-linear constitutive behavior is implemented by means of spring-damper elements at the delamination interface. The results of this parametric study allow a better insight in the behavior of the excited delamination in experimental conditions, including the appearance of localized subharmonics and harmonics of the excitation frequency. Based on the developed model, two different detection and localization techniques (using either a single frequency or a sweep excitation) were demonstrated to determine position, shape, depth and orientation of one or multiple delaminations.

Superbalanced steady state free precession

Steady state free precession (SSFP) signal theory is commonly derived in the limit of quasi-instantaneous radiofrequency (RF) excitation. SSFP imaging protocols, however, are frequently set up with minimal pulse repetition times and RF pulses can thus constitute a considerable amount to the actual pulse repetition time. As a result, finite RF pulse effects can lead to 10-20% signal deviation from common SSFP theory in the transient and in the steady state which may impair the accuracy of SSFP-based quantitative imaging techniques. In this article, a new and generic approach for intrinsic compensation of finite RF pulse effects is introduced. Compensation is based on balancing relaxation effects during finite RF excitation, similar to flow or motion compensation of gradient moments. RF pulse balancing, in addition to the refocusing of gradient moments with balanced SSFP, results in a superbalanced SSFP sequence free of finite RF pulse effects in the transient and in the steady state; irrespective of the RF pulse duration, flip angles, relaxation times, or off-resonances. Superbalancing of SSFP sequences can be used with all quantitative SSFP techniques where finite RF pulse effects are expected or where elongated RF pulses are used.

Laserlike Vibrational Instability in Rectifying Molecular Conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.

Holography of AdS vacuum bubbles

We consider the fate of AdS vacua connected by tunneling events. A precise holographic dual of thin-walled Coleman-de Luccia bounces is proposed in terms of Fubini instantons in an unstable CFT. This proposal is backed by several qualitative and quantitative checks, including the precise calculation of the instanton action appearing in evaluating the decay rate. Big crunches manifest themselves as time dependent processes which reach the boundary of field space in a finite time. The infinite energy difference involved is identified on the boundary and highlights the ill-defined nature of the bulk setup. We propose a qualitative scenario in which the crunch is resolved by stabilizing the CFT, so that all attempts at crunching always end up shielded from the boundary by the formation of black hole horizons. In all these well defined bulk processes the configurations have the same asymptotics and are finite energy excitations.

Two-Stage Ordering of Spins in Dipolar Spin Ice on the Kagome Lattice

Spin ice, a peculiar thermal state of a frustrated ferromagnet on the pyrochlore lattice, has a finite entropy density and excitations carrying magnetic charge. By combining analytical arguments and Monte Carlo simulations, we show that spin ice on the two-dimensional kagome lattice orders in two stages. The intermediate phase has ordered magnetic charges and is separated from the paramagnetic phase by an Ising transition. The transition to the low-temperature phase is of the three-state Potts or Kosterlitz-Thouless type, depending on the presence of defects in the charge order.

Ground state properties and Coulomb dissociation of the deformed halo nucleusNe31

The recently observed large cross sections for the Coulomb dissociation of $^{31}\mathrm{Ne}$ nucleus indicate that this nucleus takes a halo structure in the ground state. We analyze these experimental data using the particle-rotor model that takes into account the rotational excitation of the core nucleus $^{30}\mathrm{Ne}$. We show that the experimental data can be well reproduced when the quadrupole deformation parameter of $^{30}\mathrm{Ne}$ is around ${\ensuremath{\beta}}_{2}=0.2~0.3$, at which the ground state takes the spin and parity of ${I}^{\ensuremath{\pi}}=3/{2}^{\ensuremath{-}}$. This state corresponds to the Nilsson level [330 1/2] in the adiabatic limit of the particle-rotor model. On the other hand, the state corresponding to the Nilsson level [321 3/2] with ${\ensuremath{\beta}}_{2}~0.55$ can be excluded when the finite excitation energy of the core is taken into account, even though this configuration is a candidate for the ground state of $^{31}\mathrm{Ne}$ in the Nilsson model analysis. We discuss briefly also a possibility of the ${I}^{\ensuremath{\pi}}=1/{2}^{+}$ configuration with ${\ensuremath{\beta}}_{2}~1$ and ${\ensuremath{\beta}}_{2}~\ensuremath{-}0.4$.

Dynamics of a Bose–Einstein condensate in a symmetric triple-well trap

We present a complete analysis of the dynamics of a Bose–Einstein condensate trapped in a symmetric triple-well potential. Our classical analogue treatment, based on a time-dependent variational method using SU(3) coherent states, includes the parameter dependence analysis of the equilibrium points and their local stability, which is closely related to the condensate collective behaviour. We also consider the effects of off-site interactions, and how these ‘cross-collisions’ may become relevant for a large number of trapped bosons. Even in the presence of cross-collisional terms, the model still features an integrable sub-regime, known as the twin-condensate dynamics, which corresponds to invariant surfaces in the classical phase space. However, the quantum dynamics preserves the twin-condensate defining characteristics only partially, thus breaking the invariance of the associated quantum subspace. Moreover, the periodic geometry of the trapping potential allowed us to investigate the dynamics of finite angular momentum collective excitations, which can be suppressed by the emergence of chaos. Finally, using the generalized purity associated with the su(3) algebra, we were able to quantify the dynamical classicality of a quantum evolved system, as compared to the corresponding classical trajectory.