Monopole Excitations Research Articles

An improved artificial spin ice structure for restoring ice degeneracy

It is known that rare-earth-based pyrochlore oxide may accommodate the well-defined two-in-two-out spin ice state with its tetrahedral unit. Low-energy excitation is argued to favor the highly concerned monopole state which attracts essential attention. However, such an excitation cannot be directly tracked and imaged using advanced characterizations, raising challenges to our understanding of the physics of monopoles. In this work, we propose an improved two-dimensional artificial spin ice structure on the Shastry–Sutherland lattice to restore the degeneracy of realistic pyrochlore systems. Such a structure avoids the deficiency of inequivalent nearest and next-nearest exchanges in the planar quadrate unit, which, however, is equivalent to the tetrahedral unit of realistic pyrochlore oxides. Therefore, this spin ice model restores state degeneracy that is lost in conventional planar artificial spin ice structures, representing an improved simulator of real spin ice systems. Our careful investigations of such improved structures reveal the rich physics of spin ice excitations, including the phase diagram, which allows different ordered phases and interesting critical phase transitions between spin ice phase I and phase II. Energy spectrum analysis suggests that restoration of state degeneracy substantially reduces monopole excitation energy, resulting in a striking monopole emergency at the critical point. Furthermore, the emergent spin dimer phase in this improved model allows high-density monopole excitations and exhibits high-correlated monopole fluid states.

On the area-averaged effective sound absorption coefficient of porous materials excited by a monopole

The area-averaged effective sound absorption coefficient (SAC) of a rigid-backed homogeneous porous material subjected to a monopole excitation is calculated as the absorbed-to-incident sound power ratio. Using Allard's model to describe the sound propagation above the porous material, an analytical model for this power-based SAC is proposed and proves to give a good approximation of the sound absorption performance under monopole excitation of sufficiently large areas of material. The impact of factors on the power-based SAC, such as monopole height, material radial dimension used to calculate the sound powers, and material properties is discussed. The power-based SAC frequency-dependent behavior is analyzed through sound intensity field assessments at the material surface and is compared to normal incident plane wave and diffuse field SACs. The sound absorption behavior of sound absorbers under monopole excitation exhibits notable distinctions and peculiar results compared to those observed under plane wave and diffuse fields, particularly at low frequencies and for sources close to the material.

Inelastic scattering reaction as a probe for monopole, dipole and quadrupole excitations

The study of collective motions within atomic nuclei, such as monopole, dipole, and quadrupole excitations, is crucial for understanding nuclear structure and properties, which provide insights into the spatial distribution and dynamic nature of protons and neutrons in nuclei. By analyzing the angular distributions of these modes excited via inelastic scattering reactions, we can determine the transition characteristics and deformation lengths of nuclei. Specific case studies, including quadrupole excitations and dipole excitations indicating exotic cluster structures in light nuclei, highlight the utility of modern experimental setups like the solenoidal spectrometer and the active target detector.

Design of volumetric sound metadiffusers using gradient-based optimization

Broadband volumetric sound diffusers are designed by using gradient-based optimization (GBO) by adjusting the position and radius of each scatterer in nonuniform configurations. The multiple scattering theory is employed to evaluate the diffusion coefficient by computing the scattered pressure field by nonuniform planar configurations of cylindrical scatterers for a monopole excitation. The analytical formulas for the gradients of the diffusion coefficient with respect to positions and radii of a cluster of cylindrical scatterers are derived which enhance the modeling when integrated with GBO and parallel computing. Sound volume metadiffusers with a high diffusion coefficient are designed by perturbatively rearranging the scatterer configurations. Single frequency and broad-octave-band optimizations are performed to maximize the diffusion coefficient while supplying analytical formulas for its gradients with respect to positions and radii. The GBO is implemented by direct optimization employing Multistart and fmincon solvers with sequential quadratic programming algorithms. The GBO approach is demonstrated providing numerical examples for nonuniform configurations of rigid cylinders embedded in the air environment. The polar plots for scattered pressure are evaluated for a wide range of ⅓ octave bands. The GBO approach can facilitate the automated metamaterial process by combining it with global optimization, generative modeling, and reinforcement learning.

Isoscalar giant monopole resonance in 40,48Ca

Starting from the Skyrme energy-density functional, the microscopic model is developed for the monopole excitations. We take into account the coupling between one-, two-, and three-phonon terms in the wave functions of excited 0+ states. As an example, the properties of the isoscalar giant monopole resonance (ISGMR) for the doubly magic nuclei 40,48Ca are discussed. The inclusion of complex configurations leads to a fragmentation of the ISGMR strength to lower energy 0+ states and also higher energy tail.

Arboreal topological and fracton phases

We describe topologically ordered and fracton ordered states on novel geometries which do not have an underlying manifold structure. Using tree graphs such as the $k$-coordinated Bethe lattice ${\cal B}(k)$ and a hypertree called the $(k,n)$-hyper-Bethe lattice ${\cal HB}(k,n)$ consisting of $k$-coordinated hyperlinks (defined by $n$ sites), we construct multidimensional arboreal arenas such as ${\cal B}(k_1) \square {\cal B}(k_2)$ by the notion of a graph Cartesian product $\square$. We study various quantum systems such as the ${\mathbb Z}_2$ gauge theory, generalized quantum Ising models (GQIM), the fractonic X-cube model, and related X-cube gauge theory defined on these arenas. Even the simplest ${\mathbb Z}_2$ gauge theory on a 2d arboreal arena is fractonic -- the monopole excitation is immobile. The X-cube model on a 3d arboreal arena is fully fractonic, all multipoles are rendered immobile. We obtain variational ground state phase diagrams of these gauge theories. Further, we find an intriguing class of dualities in arboreal arenas as illustrated by the ${\mathbb Z}_2$ gauge theory defined on ${\cal B}(k_1) \square {\cal B}(k_2)$ being dual to a GQIM defined on ${\cal HB}(2,k_1) \square {\cal HB}(2,k_2)$. Finally, we discuss different classes of topological and fracton orders on arboreal arenas. We find three distinct classes of arboreal toric code orders on 2d arboreal arenas, those that occur on ${\cal B}(2) \square {\cal B}(2)$, ${\cal B}(k) \square {\cal B}(2), k >2$, and ${\cal B}(k_1) \square {\cal B}(k_2)$, $k_1,k_2>2$. Likewise, four classes of X-cube fracton orders are found in 3d arboreal arenas -- those on ${\cal B}(2)\square{\cal B}(2)\square {\cal B}(2)$, ${\cal B}(k) \square {\cal B}(2)\square {\cal B}(2), k>2$, ${\cal B}(k_1) \square {\cal B}(k_2) \square {\cal B}(2), k_1,k_2 >2$, and ${\cal B}(k_1) \square {\cal B}(k_2) \square {\cal B}(k_3), k_1,k_2,k_3 >2$.

Dynamical fractal and anomalous noise in a clean magnetic crystal

Fractals-objects with noninteger dimensions-occur in manifold settings and length scales in nature. In this work, we identify an emergent dynamical fractal in a disorder-free, stoichiometric, and three-dimensional magnetic crystal in thermodynamic equilibrium. The phenomenon is born from constraints on the dynamics of the magnetic monopole excitations in spin ice, which restrict them to move on the fractal. This observation explains the anomalous exponent found in magnetic noise experiments in the spin ice compound Dy2Ti2O7, and it resolves a long-standing puzzle about its rapidly diverging relaxation time. The capacity of spin ice to exhibit such notable phenomena suggests that there will be further unexpected discoveries in the cooperative dynamics of even simple topological many-body systems.

Structural magnetic glassiness in the spin ice Dy2Ti2O7

The spin ice compound Dy$_2$Ti$_2$O$_7$ is well-known to realise a three-dimensional Coulomb spin liquid with magnetically charged monopole excitations. Its fate at low temperatures, however, remains an intriguing open question. Based on a low-temperature analysis of the magnetic noise and diffuse neutron scattering under different cooling protocols, combined with extensive numerical modelling, we argue that upon cooling, the spins freeze into what may be termed a `structural magnetic glass', without an a priori need for chemical or structural disorder. Specifically, our model indicates the presence of frustration on two levels, first producing a near-degenerate constrained manifold inside which phase ordering kinetics is in turn frustrated. Our results suggest that spin ice Dy$_2$Ti$_2$O$_7$ provides one prototype of magnetic glass formation specifically, and a setting for the study of kinetically constrained systems more generally.

Defect-induced monopole injection and manipulation in artificial spin ice

Lithographically defined arrays of nanomagnets are well placed for application in areas such as probabilistic computing or reconfigurable magnonics due to their emergent collective dynamics and writable magnetic order. Among them are artificial spin ice (ASI), which are arrays of binary in-plane macrospins exhibiting geometric frustration at the vertex interfaces. Macrospin flips in the arrays create topologically protected magnetic charges, or emergent monopoles, which are bound to an antimonopole to conserve charge. In the absence of controllable pinning, it is difficult to manipulate individual monopoles in the array without also influencing other monopole excitations or the counter-monopole charge. Here, we tailor the local magnetic order of a classic ASI lattice by introducing a ferromagnetic defect with shape anisotropy into the array. This creates monopole injection sites at nucleation fields below the critical lattice switching field. Once formed, the high energy monopoles are fixed to the defect site and may controllably propagate through the lattice under stimulation. Defect programing of bound monopoles within the array allows fine control of the pathways of inverted macrospins. Such control is a necessary prerequisite for the realization of functional devices, e. g. reconfigurable waveguide in nanomagnonic applications.

Isovector density and isospin impurity in 40Ca

We study isoscalar (IS) and isovector (IV) densities in Ca40 in comparison with theoretical densities calculated by Skyrme Hatree-Fock (HF) models. The charge symmetry breaking and the charge independence breaking forces are introduced to study the effect on the IV density. The effect of isospin mixing in the ground-state density is examined by using the particle-vibration coupling model taking into account the collective IV giant monopole excitation. We show a clear correlations in the IV density and isospin impurity of Ca40 within the HF and the particle-vibration coupling model. We extract for the first time the experimental information of isospin impurity from the magnitude of IV density.

Multiprobe study of excited states in C12 : Disentangling the sources of monopole strength between the energy of the Hoyle state and Ex=13 MeV

Background: The Hoyle state is the archetypal α-cluster state which mediates the 3α reaction to produce C12 and is of great interest for both nuclear structure and astrophysics. Recent theoretical calculations predict a breathing-mode excitation of the Hoyle state at Ex≈9 MeV. Its observation is hindered by the presence of multiple broad states and potential interference effects. An analysis with Gaussian line shapes of measurements at the Research Center for Nuclear Physics (Osaka University) with the Grand Raiden spectrometer suggested that additional strength was needed at Ex≈9 MeV to reproduce the data; this analysis did not account for the well-known threshold effects observed in C12. Nevertheless, various theoretical studies have since concluded that this additional strength corresponds to the predicted breathing-mode excitation of the Hoyle state. To meaningfully identify a new source of monopole strength in this astrophysically significant region, a more appropriate phenomenological analysis which accounts for penetrability and interference effects must be used to determine whether the data can be explained with previously established states. Purpose: We aim to investigate the monopole strength in the astrophysically important excitation-energy region of C12 between Ex=7 and 13 MeV to determine whether the previously established sources of monopole strength are able to reproduce the data. Method: The C12(α,α′)C12 and C14(p,t)C12 reactions, which are expected to exhibit contrasting selectivity towards different monopole excitations, were employed at various detection angles and beam energies to populate states in C12. The inclusive excitation-energy spectra were simultaneously analyzed with multilevel, multichannel line shapes. Various scenarios with different sources of monopole strength and interference effects were considered to determine whether the ghost of the Hoyle state and the previously established broad 03+ state at Ex≈10 MeV are able to reproduce the observed monopole strength. Results: Clear evidence was found for excess monopole strength at Ex≈9 MeV, particularly in the C12(α,α′)C12 reaction at 0∘. This additional strength cannot be reproduced by the previously established monopole states between Ex=7 and 13 MeV. Coincident charged-particle decay data suggest that the strength at Ex≈9 MeV is dominantly monopole, with no evidence of a J>0 contribution. Conclusions: The data support a new source of monopole strength at Ex≈9 MeV, which cannot be described with a phenomenological parametrization of all previously established states. An additional 0+ state at Ex≈9 MeV yielded a significantly improved fit of the data and is a clear candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parametrization of the astrophysically important monopole strengths in C12 is required.17 MoreReceived 19 November 2020Revised 21 May 2021Accepted 16 June 2021DOI:https://doi.org/10.1103/PhysRevC.105.024308©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayCollective levelsDirect reactionsNuclear astrophysicsNuclear reactionsNuclear structure & decaysProperties6 ≤ A ≤ 19TechniquesCluster modelsNuclear PhysicsGravitation, Cosmology & Astrophysics

Isovector giant monopole and quadrupole resonances in a Skyrme energy density functional approach with axial symmetry

[Background] Giant resonance (GR) is a typical collective mode of vibration. The deformation splitting of the isovector (IV) giant dipole resonance is well established. However, the splitting of GRs with other multipolarities is not well understood. [Purpose] I explore the IV monopole and quadrupole excitations and attempt to obtain the generic features of IV giant resonances in deformed nuclei by investigating the neutral and charge-exchange channels simultaneously. [Method] I employ a nuclear energy-density functional (EDF) method: the Skyrme-Kohn-Sham-Bogoliubov and the quasiparticle random-phase approximation are used to describe the ground state and the transition to excited states. [Results] I find the concentration of the monopole strengths in the energy region of the isobaric analog or Gamow-Teller resonance irrespective of nuclear deformation, and the appearance of a high-energy giant resonance composed of the particle-hole configurations of $2\hbar \omega_0$ excitation. Splitting of the distribution of the strength occurs in the giant monopole and quadrupole resonances due to deformation. The lower $K$ states of quadrupole resonances appear lower in energy and possess the enhanced strengths in the prolate configuration, and vice versa in the oblate configuration, while the energy ordering depending on $K$ is not clear for the $J=1$ and $J=2$ spin-quadrupole resonances. [Conclusions] The deformation splitting occurs generously in the giant monopole and quadrupole resonances. The $K$-dependence of the quadrupole transition strengths is largely understood by the anisotropy of density distribution.

Generation of Circularly Polarized Quasi-Non-Diffractive Vortex Wave via a Microwave Holographic Metasurface Integrated with a Monopole

In this paper, a novel method for generating a circularly polarized (CP) quasi-non-diffractive vortex wave carrying orbital angular momentum (OAM), based on the microwave holographic metasurface integrated with a monopole, is proposed. This method is the combination of the non-diffraction theory and the principle of waveguide-fed-based holography and is equivalent to a superposition of two scalar impedance modulation surfaces. To verify the proposed method, a holographic metasurface generating a left-handed circularly polarized (LHCP) quasi-non-diffractive vortex wave carrying −1 mode OAM at the normal direction, was simulated and analyzed. The metasurface consisted of inhomogeneous slot units on a grounded substrate and a monopole excitation. Moreover, the location distribution of slots was determined by a computed interferogram between the reference wave and the object wave with the non-diffractive feature. Compared with an ordinary vortex wave, the quasi-non-diffractive wave obtained by our proposed method possessed a smaller divergence radius and a stronger electric field strength in the 9 times wavelength range. It paved a new path for manipulating the non-diffractive vortex wave in medium distance without using an external feeding source, which holds great potential for the miniaturization devices applied in medium-distance high-capacity secure communication, high-resolution imaging and intelligent detection.

Orbital variational adiabatic hyperspherical method applied to Bose-Einstein condensates

A variational basis set motivated by mean-field theory is utilized to describe the Bose-Einstein condensate within the adiabatic hyperspherical coordinate framework. The simplest single-orbital variant of this treatment reproduces many of the ground-state properties predicted by the Gross-Pitaevskii equation. But a multiorbital improvement to the basis set yields a better representation of particle correlations and of the critical number where the condensate collapses for a negative two-body scattering length. The method also produces systematic deviations from Bogoliubov theory for the fundamental monopole excitation frequency.

A Transfer Function Measurement Setup With an Electro-Optic Sensor for MR Safety Assessment in Cascaded Media

This article introduces a transfer function (TF) measurement setup for cascaded media (CM) and demonstrates that the full immersion of an implant in a lossy medium might result in an underestimation of the RF-induced heating compared to partial immersion. A CMTF measurement setup was constructed using medium-specific local electric field sources and electro-optical sensor (EOS). TFs of bare and insulated wire samples were measured in the CM air/saline. Finite-difference time-domain (FDTD) simulations were implemented using a reciprocal approach with a monopole excitation source as well as a piecewise plane wave excitation method. As a theoretical guideline for understanding the CM behavior of the devices, an analytical model based on the buried wire models was derived. CMTFs agree with good precision between FDTD simulations, analytical modeling, and the EOS measurements. Measured TFs were calibrated and validated similar to ISO/TS 10974:Ed.2. Calculated tip SAR values under uniform incident field show an increase up to a factor of 5.7 for partially immersed wires compared to the full immersion. On average, FDTD-based TFs overestimate SAR by 25% compared to measurement-based TFs. As safety assessments for fully immersed implant leads or devices might not represent the worst-case scenario, single-medium assessments might underestimate the RF-induced heating.

Nuclear Gamma Radiation Caused by a Muon at Rest in 152Sm

Investigation of the interaction of muons with complex nuclei yields important results on the properties of nuclei and on their electromagnetic interaction with muons happening to be in the Coulomb field of the nucleus. The muon serves as a unique instrument in studies of the charge distribution and of other properties of nuclei. The following are processes which yield information relevant to the properties of nuclei. This work is the first (and unique) investigation of the dynamic E0 excitation of nucleus at µ– decay on the K-orbit of muonic atom. This excitation mechanism permits one to get per se nuclear monopole excitation. Study of nuclear excitation by the bound muon is of independent interest— the discovery of a new process and validation of ideas about it in the future can provide a new complementary method for studying nuclear monopole states. Information on the phenomenon investigated in the present work is of interest from the point of view of analysis of neutrinoless µ → ẹ conversion in Mu2e and COMET experiments.

Effective medium theory for a photonic pseudospin- 12 system

Photonic pseudospin-1/2 systems, which exhibit Dirac cone dispersion at Brillouin zone corners in analogy to graphene, have been extensively studied in recent years. However, it is known that a linear band crossing of two bands cannot emerge at the center of Brillouin zone in a two-dimensional photonic system respecting time reversal symmetry. Using a square lattice of elliptical magneto-optical cylinders, we constructed an unpaired Dirac point at the Brillouin zone center as the intersection of the second and third bands corresponding to the monopole and dipole excitations. Effective medium theory can be applied to the two linearly crossed bands with the effective constitutive parameters numerically calculated using the boundary effective medium approach. It is shown that only the effective permittivity approaches zero while the determinant of the nonzero effective permeability vanishes at the Dirac point frequency, showing a different behavior from the double-zero index metamaterials obtained from the pseudospin-1 triply degenerate points for time reversal symmetric systems. Exotic phenomena, such as the Klein tunneling and Zitterbewegung, in the pseudospin-1/2 system can be well understood from the effective medium description. When the Dirac point is lifted, the edge state dispersion near the $\Gamma$ point can be accurately predicted by the effective constitutive parameters. We also further realized magneto-optical complex conjugate metamaterials for a wide frequency range by introducing a particular type of non-Hermittian perturbations which make the two linear bands coalescence to form exceptional points at the real frequency.

Towards a quantum Monte Carlo–based density functional including finite-range effects: Excitation modes of a K39 quantum droplet

Some discrepancies between experimental results on quantum droplets made of a mixture of $^{39}\mathrm{K}$ atoms in different hyperfine states and their analysis within extended Gross-Pitaevskii theory (which incorporates beyond mean-field corrections) have been recently solved by introducing finite-range effects into the theory. Here we study the influence of these effects on the monopole and quadrupole excitation spectrum of extremely dilute quantum droplets using a density functional built from first-principles quantum Monte Carlo calculations, which can be easily introduced in the existing Gross-Pitaevskii numerical solvers. Our results show differences of up to $20%$ with those obtained within the extended Gross-Pitaevskii theory, likely providing another way to observe finite-range effects in mixed quantum droplets by measuring their lowest excitation frequencies.

A bridge between finite and infinite nuclear matter

The bridge between finite and infinite nuclear systems is analyzed for the fundamental quantities, such as binding energy, incompressibility, and giant monopole excitation energy, using relativistic mean-field formalism. The well-known Thomas–Fermi, extended Thomas–Fermi, and Hartree approximations are used to evaluate the observables. A parametric form of the density is used to convert the infinite nuclear matter density to the mean density of a finite nucleus. The present analysis shows an approximate estimation of finite nucleus properties from information on the corresponding infinite nuclear matter quantities. In other words, it is not quite exact to get the observables of finite nuclei by converting the corresponding entities of the nuclear matter system or vice versa. If this can be achieved at all, it can be done only approximately.

Emergence of weak pyrochlore phase and signature of field induced spin ice ground state in Dy2−xLaxZr2O7; x = 0, 0.15, 0.3

The pyrochlore oxides Dy2Ti2O7 and Ho2Ti2O7 are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these, Dy2Zr2O7 is reported to crystallize in a distorted fluorite structure. We present here the magnetic and heat capacity studies of La substituted Dy2Zr2O7. Our findings suggest the absence of spin ice state in Dy2Zr2O7 but the emergence of the magnetic field induced spin freezing near T ≈ 10 K in ac susceptibility measurements which is similar to Dy2Ti2O7. The magnetic heat capacity of Dy2Zr2O7 shows a shift in the peak position from 1.2 K in zero field to higher temperatures in the magnetic field, with the corresponding decrease in the magnetic entropy. The low temperature magnetic entropy at 5 kOe field is R ln2 − (1/2)R ln(3/2) which is the same as for the spin ice state. Substitution of non-magnetic, isovalent La3+ for Dy3+ gradually induces the structural change from highly disordered fluorite to weakly ordered pyrochlore phase. The La3+ substituted compounds with less distorted pyrochlore phase show the spin freezing at lower field which strengthens further on the application of magnetic field. Our results suggest that the spin ice state can be stabilized in Dy2Zr2O7 either by slowing down of the spin dynamics or by strengthening the pyrochlore phase by suitable substitution in the system.