Linear External Field Research Articles

Transferring orbital angular momentum to an electron beam reveals toroidal and chiral order

Orbital angular momentum (OAM) and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics. Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices. Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. Instead, we find that the presence of an electric toroidal moment in a ferrorotational phase transfers measurable torque and OAM to a localized electron beam in the ballistic limit. We record these torque transfers from a high-energy electron beam using a momentum-resolved detector. This approach provides a high-sensitivity method to detect polarization fields and their more complex order parameters and topologies. In addition to toroidal order, we also demonstrate high-precision measurements of vorticity and chirality for polar vortexlike phases.

Dynamics of dissipative Landau-Zener transitions in an anisotropic three-level system.

We investigate the dynamics of Landau-Zener (LZ) transitions in an anisotropic, dissipative three-level LZ model (3-LZM) using the numerically accurate multiple Davydov D2Ansatz in the framework of the time-dependent variational principle. It is demonstrated that a non-monotonic relationship exists between the Landau-Zener transition probability and the phonon coupling strength when the 3-LZM is driven by a linear external field. Under the influence of a periodic driving field, phonon coupling may induce peaks in contour plots of the transition probability when the magnitude of the system anisotropy matches the phonon frequency. The 3-LZM coupled to a super-Ohmic phonon bath and driven by a periodic external field exhibits periodic population dynamics in which the period and amplitude of the oscillations decrease with the bath coupling strength.

Schrödinger-Cat States in Landau-Zener-Stückelberg-Majorana Interferometry: A Multiple Davydov Ansatz Approach.

Employing the time-dependent variational principle combined with the multiple Davydov D2 Ansatz, we investigate Landau-Zener (LZ) transitions in a qubit coupled to a photon mode with various initial photon states at zero temperature. Thanks to the multiple Davydov trial states, exact photonic dynamics taking place in the course of the LZ transition is also studied efficiently. With the qubit driven by a linear external field and the photon mode initialized with Schrödinger-cat states, asymptotic behavior of the transition probability beyond the rotating-wave approximation is uncovered for a variety of initial states. Using a sinusoidal external driving field, we also explore the photon-assisted dynamics of Landau-Zener-Stückelberg-Majorana interferometry. Transition pathways involving multiple energy levels are unveiled by analyzing the photon dynamics.

Transferring Orbital Angular Momentum to an Electron Beam Reveals Toroidal and Chiral Order

Orbital angular momentum and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics(1-4). Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices(5). Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. To develop a mechanism that can control switching in polarization vortices, we utilize a high-energy electron beam and show that transverse currents are generated by polar order in the ballistic limit. We find that the presence of an electric toroidal moment in a ferro-rotational phase transfers a measurable torque and orbital angular momentum to the electron beam. Furthermore, we find that the complex polarization patterns, observed in these heterostructures, are microscopically chiral with a non-trivial axial component of the polarization. This chirality opens the door for the coupling of ferroelectric and optical properties.

Evolution of bunch envelope and emittance in photo-injectors with application to the AWAKE experiment

We investigate on time evolution of envelopes and emittance of bunched beams in photo-injectors. We carry out a detailed beam dynamic study with focus on calculations of space charge forces. We take the average of the forces over the whole ensemble of particles. This results in space charge coefficients that are largely independent of the bunch distribution. By implementing these coefficients in the beam envelope equations we find a set of modified equations as a model for describing the evolution of the bunch envelope. Our equations crucially decrease the consumed time in particle-in-cell codes for designing today’s photo-injectors. The feasibility and accuracy of the model have been benchmarked against the simulation results from existing particle-in-cell codes with very good agreement. We apply our model for the design of the ultra-short bunch photo-injector of the AWAKE experiment. Moreover, by using assumption of linear external fields we develop the governing equations of the bunch emittances in photo-injectors. These equations allow us to partly quantitatively and partly qualitatively study the effects of external and space charge fields on emittance growth phenomena.

Topology optimized and 3D printed polymer-bonded permanent magnets for a predefined external field

Topology optimization offers great opportunities to design permanent magnetic systems that have specific external field characteristics. Additive manufacturing of polymer-bonded magnets with an end-user 3D printer can be used to manufacture permanent magnets with structures that had been difficult or impossible to manufacture previously. This work combines these two powerful methods to design and manufacture permanent magnetic systems with specific properties. The topology optimization framework is simple, fast, and accurate. It can also be used for the reverse engineering of permanent magnets in order to find the topology from field measurements. Furthermore, a magnetic system that generates a linear external field above the magnet is presented. With a volume constraint, the amount of magnetic material can be minimized without losing performance. Simulations and measurements of the printed systems show very good agreement.

Sudden death of particle-pair Bloch oscillation and unidirectional propagation in a one-dimensional driven optical lattice

We study the dynamics of bound pairs in the extended Hubbard model driven by a linear external field. It is shown that two interacting bosons or singlet fermions with nonzero on-site and nearest-neighbor interaction strengths can always form bound pairs in the absence of an external field. There are two bands of bound pairs, one of which may have incomplete wave vectors when it has an overlap with the scattering band, referred to as an imperfect band. In the presence of the external field, the dynamics of the bound pair in the perfect band exhibits distinct Bloch-Zener oscillation (BZO), while in the imperfect band the oscillation presents a sudden death. The pair becomes uncorrelated after the sudden death and the BZO never comes back. Such dynamical behaviors are robust even for the weak-coupling regime and thus can be used to characterize the phase diagram of the bound states.

Separation of quadratic and linear external field effects in high J quantum beats

We discuss quantum beats in electronically excited molecular states with high rotational angular momenta J appearing in time resolved fluorescence in conditions of quadratic and linear energy shift dependence on magnetic quantum number M and external field strength. Density matrix formalism is used to obtain in explicit form the expressions for time dependent fluorescence intensity after δ-function pulsed excitation. In case of pure quadratic Stark effect, which is typical for 1Σ state diatomics, excited state quantum beats for J≫1 exhibit a regular, or ‘‘grill’’ structure, consisting of narrow equidistant ‘‘principal’’ peaks with equal relative amplitudes on the exponential decay background. At linear polarized excitation the time intervals between the adjacent peaks are 2π/ω20, ω20 being the splitting frequency between coherently excited M-sublevels with M=2 and M′=0. If an admixture of linear contribution is present in field induced level shifts, the grill structure is superimposed by a single frequency harmonic modulation. A special geometry was found in which the quadratic beats are fully absent and the modulated grill pattern is brought into existence only by the influence of linear term. Such a case takes place when the light polarization vector in fluorescence is directed at 45° angle with respect to the exciting light polarization vector and yields the most sensitive way to separate quadratic and linear contribution. We considered the examples when the first order term appears by a combined action of electric and magnetic field, as well as due to the e–f level electric field induced mixing, with the parameters typical for the NaK molecule.

Fluid interface in two dimensions close to the critical point

Theoretical and computer simulation results for the equilibrium structure of the interface between two-dimensional fluid phases near the critical point are described. In the theoretical analysis, the equilibrium interface is assumed, in accord with current ideas, to consist of an intrinsic interface of the nonclassical van der Waals type, broadened by long-wavelength capillary wave fluctuations. It is found that in two dimensions of space, the interfacial thickness is very sensitive to the choices of the external field and of the intrinsic interface. For an intrinsic interface with a thickness proportional to the bulk correlation length ξ, the critical exponent ω describing the divergence of the interfacial thickness as the critical point is approached depends on the scale of the external field relative to ξ, and ranges from ω = 9/ 32 to ω = 17/ 32 , in contrast to the prediction ω = 1 of nonclassical van der Waals theory. For a linear external field (gravity), the interfacial thickness is found to exhibit a crossover with change of critical exponent from capillary wave behavior at low and near critical temperatures to van der Waals behavior at temperatures very close to the critical temperature. An estimate of the order of magnitude of the crossover temperature is made by combining available experimental data and theory. In the currently accessible temperature range, the capillary wave prediction prevails, in contrast to the case of three-dimensional fluids. Computer simulations are performed for continuous fluids using the method of molecular dynamics. The interfacial thickness between two-dimensional fluid phases, in which the particles interact with a Lennard-Jones potential, is determined as a function of temperature in the absence of an external field. The number of particles ranges from 242 to 1024. Due to the smallness of the interfacial areas studied, long-wavelength capillary waves are strongly suppressed, and the results for the interfacial thickness are found to be consistent with the prediction of the nonclassical van der Waals theory of the intrinsic interface.

A Study of the Separation Efficiency in the Concentric-Tube Countercurrent Separation Process under Generalized Linear Applied Fields and with Recycles at Both Ends

Abstract The mathematical model for the separation of binary mixtures has been extended to a concentric-tube continuous-contact countercurrent column under generalized linear external fields and with recycles at both ends. An analytical solution is obtained by use of the orthogonal expansion method. Numerical results for separation in a thermal diffusion column are also illustrated.

Uniform bounds of the Schwinger functions in boson field models

We study the lattice and space cutoff boson field models with periodic boundary condition in d-dimensional space–time (d-N+). We prove that if the pressures of the interaction (and also the pressures of the interaction with linear external fields) under consideration are bounded uniformly in the cutoffs, the corresponding Schwinger functions are also bounded uniformly in the cutoffs. By applying the above result we prove the uniform bounds of the space cutoff Schwinger functions for the (λφ4−σφ2−μφ)3 model and the lattice and space cutoff Schwinger functions for the exponential type interactions in d-dimensional space–time.

On non linear external field effects in quasi-superconducting films

Using an emproved Hartree-Fock approximation on the Landau Ginzburg equation we compute the extra conductivity for quasi-superconducting thin films near the phase transition in the non linear region.