Arranging magnetic nanoparticles (MNPs) into highly ordered structures, so-called superlattices or mesocrystals, is of great interest from a fundamental point of view, as the employment of the corresponding coupled nanoentities introduces additional degrees of freedom to manipulate the overall magnetic characteristics of such hierarchical materials. Characterizing the associated magnetic interactions on the mesoscopic scale is indispensable for obtaining a profound understanding of the relative strengths of the types of interactions involved, such as dipole-dipole interactions, which affect the collective response of a corresponding mesocrystal. In this paper, nanoparticles are deposited onto silicon substrates by spin coating, leading to two-dimensional monolayered structures showing a close-packed hexagonal arrangement. The MNPs consist of iron oxide (magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$) and are coated with a nonmagnetic polymer (polystyrene). The MNPs are synthesized such that their diameters $d_{\text{MNP}}$ are tuned in a range between 9 and 18 nm. A precise manipulation of the shell thickness $d_{\text{shell}}$ is achieved by coating the MNPs with polystyrene of different molecular weights. In this fashion, the spacing between the MNPs, $d_{\text{spacer}}=2d_{\text{shell}}$, is varied in a range between 6 and 14 nm. Within the investigated $d_{\text{spacer}}$ range, dipolar interactions govern the collective properties showing distinct distance-dependent characteristics. As $d_{\text{spacer}}$ increases, the dipolar coupling strength between the MNPs decreases, as deduced from the spectral features of ferromagnetic resonance experiments. These observations are further corroborated by numerical simulations of the dynamic properties of appropriate model systems. A comparison of the experimental and theoretical findings shows that material parameters, such as the magnetization $M_{\text{MNP}}$ and the magnetocrystalline anisotropy $C_{\text{MNP}}$ of the MNPs, are reduced compared to their bulk values.

In the framework of the spin-lattice model we investigate a role of impurity doping on the evolution of the magnetic susceptibility and the development of the magnetic moment in magnetic field in chiral magnets. We focus on a particular class of silicide—Fe- and Co-doped MnSi. We introduce two feasible models accounting for different possible impurity arrangements in a real compound. Our calculations are based on the Monte Carlo method for classical spins. We are interested in the magnetic response and the degradation of the magnetic phase transition upon doping. We compare the calculated susceptibility with available experimental data in these compounds. We illustrate features of the behavior of the magnetic susceptibility by the corresponding spin patterns showing the deterioration of the spiral structure with temperature and the corresponding formation of ring-shape patterns in Bragg intensity profiles.

Magnetic skyrmions are topologically protected chiral spin textures which present opportunities for next-generation magnetic data storage and logic information technologies. The topology of these structures originates in the geometric configuration of the magnetic spins, more generally described as the structure. While the skyrmion structure is most often depicted using a two-dimensional projection of the three-dimensional (3D) structure, recent works have emphasized the role of all three dimensions in determining the topology and their response to external stimuli. In this paper, grazing-incidence small-angle neutron scattering and polarized neutron reflectometry are used to determine the 3D structure of hybrid skyrmions. The structure of the hybrid skyrmions, which includes a combination of Néel-like and Bloch-like components along their length, is expected to significantly contribute to their notable stability, which includes ambient conditions. To interpret the neutron scattering data, micromagnetic simulations of the hybrid skyrmions were performed, and the corresponding diffraction patterns were determined using a Born approximation transformation. The converged magnetic profile reveals the magnetic structure along with the skyrmion depth profile, including the thickness of the Bloch and Néel segments and the diameter of the core.

The layered Mott insulator $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ using scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ layers in proximity to graphite. In the single-layer $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ in direct contact with graphite, distinct states in the Mott gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ upper layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott gap within the upper layers, suggesting that an unconventional Mott transition is occurring in these layers. The results show that the heterostructure comprising $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$ and graphite is a good platform for investigating the doping physics in $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin liquid candidates.

In real samples, bound states in the continuum (BICs) are manifested as resonances with a finite, although significantly high, quality ($Q$) factor. Control over the $Q$ factor through an asymmetry parameter (i.e., an intentional defect of the structure, allowing structural imperfections to be introduced on purpose) allows tailoring the coupling strength with the modes of the free space. In most systems, $Q$ has an inverse quadratic dependence on the asymmetry parameter. However, various applications require different scaling laws. For instance, sensors require a steeper dependence, whereas light generation needs a less steep one for robustness. Here, we consider a metasurface consisting of dielectric rods with air holes inside of them, obtaining several different scaling laws. Our analysis reveals that BIC has dominant and asymmetry-induced multipole terms. Depending on the radiation properties of the induced multipoles and their amplitude in the case of vanishing asymmetry, the exponent in the scaling law lies in the range from $-4$ to $-1.75$, including the common case of $-2$.

We show that the magnetic precession of ferromagnetic moments in a noncentrosymmetric magnetic Weyl semimetal induces a dc electric current through a mechanism analogous to an adiabatic charge pump. The dc current is a consequence of a Berry phase effect in momentum space resulting from the circular motion of Weyl nodes induced by the precession. This mechanism resembles the Faraday effect, namely, an induced magnetic field by a circular electric current. The circular motion of Weyl nodes induces a magnetic charge current in momentum space, which results in a Berry phase that describes the adiabatic pump. Experimentally, this phenomenon is similar to a spin motive force, which is an electric current induced by magnetic precision in the presence of the spatial gradient of magnetization. However, unlike a conventional spin motive force, this dc current occurs without the magnetization gradient. The result demonstrates a nontrivial interplay between the topological electronic state and magnetic dynamics.

We investigate the consequences of resonant tunneling of Cooper pairs on the quantum phase slips occurring in a Josephson junction. The amplitude for quantum tunneling under the Josephson potential barrier is modified by the Landau-Zener amplitude of adiabatic passage through an Andreev level crossing, resulting in the suppression of $2\pi$ phase slips. As a consequence, close to resonance, $4\pi$ phase slips become the dominant tunneling process. We illustrate this crossover by determining the energy spectrum of a transmon circuit, showing that a residual charge dispersion persists even at perfect transparency.

We report on nonlinear transport phenomena at high filling factor and DC current-induced electronic hydrodynamics in an ultrahigh mobility $\text{(}\mu =20\times {10}^6{\mathrm{cm}}^2/\mathrm{Vs}\text{)}$ two-dimensional electron gas in a narrow ($15-\mu \mathrm{m}$ wide) GaAs/AlGaAs Hall bar for DC current densities reaching 0.67 A/m. The various phenomena and the boundaries between the phenomena are captured together in a two-dimensional differential resistivity map as a function of magnetic field (up to 250 mT) and DC current. This map, which resembles a phase diagram, demarcate distinct regions dominated by Shubnikov-de Haas (SdH) oscillations (and phase inversion of these oscillations) around zero DC current; negative magnetoresistance and a double-peak feature (both ballistic in origin) around zero field; and Hall field-induced resistance oscillations (HIROs) radiating out from the origin. From a detailed analysis of the data near zero field, we show that increasing the DC current suppresses the electron-electron scattering length that drives a growing hydrodynamic contribution to both the differential longitudinal and transverse (Hall) resistivities. Our approach to induce hydrodynamics with DC current differs from the more usual approach of changing the temperature. We also find a significant (factor of two to four) difference between the quantum lifetime extracted from SdH oscillations, and the quantum lifetime extracted from HIROs. In addition to observing HIRO peaks up to the seventh order, we observe an unexpected HIRO-like feature close to midway between the first-order and the second-order HIRO maxima at high DC current.

We study full counting statistics of electron transport through a Majorana single-charge transistor. At low bias voltage, transport is dominated by the so-called Josephson-Majorana cycle, a sequence of normal and anomalous single-charge and Josephson tunneling. Factorial cumulants characterizing the full counting statistics elucidate the correlated nature of the charge transfers in this cycle. Moreover, we predict a topological transition in the full counting statistics from a perfect Poissonian transfer of Cooper pairs to a correlated switching between two distinct fermion parity states with increasing Josephson coupling.

Beryllium is a simple hexagonal metal but exhibits anomalous low-energy acoustic plasmons on its (0001) surface, which are radically distinct from the conventional metal surface plasmons. Although the plasmons on Be(0001) have been extensively studied, the underlying mechanism remains elusive. Here, through first-principles calculations combined with model analysis, we unambiguously demonstrate that it is the topological Dirac nodal line induced surface states that result in the anomalous low-energy acoustic plasmons on Be(0001), rather than the commonly believed Shockley surface states. Our study not only clarifies the origin of the puzzling anomalous plasmons on Be(0001), but also provides a sound theoretical interpretation for the appearance of low-energy acoustic surface plasmons on the noble metal (111) surfaces. This work suggests that the topological nodal line semimetals offer a potential platform for obtaining low-energy acoustic plasmons.

Three-particle correlators are relevant for, among others, Raman, Hall, and nonlinear responses. They are also required for the next order of approximations extending dynamical mean-field theory diagrammatically. We present a general formalism on how to treat these three-particle correlators and susceptibilities, and we calculate the local three-particle response of the Anderson impurity model numerically. We find that genuine three-particle vertex corrections are sizable. In particular, it is not sufficient to just take the bare bubble terms or corrections based on the two-particle vertex. The full three-particle vertex must be considered.

Among the quasi-one-dimensional transition metal tetrachalcogenides ($M{\mathrm{Se}}_4$)${}_n\mathrm{I}$ ($M=\text{Nb}$,Ta), the $n=3$ compounds are the only ones not displaying charge density waves. Instead, they show structural transitions with puzzling transport behavior. They are semiconductors at the lowest temperatures, but their transport gaps are significantly smaller than those inferred from angle-resolved photoemission spectroscopy (ARPES) and optical conductivity. Recently, a metallic polytype of (${\mathrm{TaSe}}_4$)${}_3\mathrm{I}$ has been found with ferromagnetism and superconductivity coexisting at low temperature, in contrast to previous reports. In this work we present detailed $\mathit{ab}$ initio and tight-binding band-structure calculations for the different ($M{\mathrm{Se}}_4$)${}_3\mathrm{I}$ reported structures. We obtain good agreement with the observed transport gaps, and explain how ARPES and optics experiments effectively probe a gap between different bands due to an approximate translation symmetry, solving the controversy. Finally, we show how small extrinsic hole doping can tune the Fermi level through a Van Hove singularity in (${\mathrm{TaSe}}_4$)${}_3\mathrm{I}$ and discuss the implications for magnetism and superconductivity.

We study a system of two-dimensional Dirac electrons (as is realized on the surface of a three-dimensional topological insulator) coupled to an array of localized spins. The spins are coupled ferromagnetically to each other, forming an ordered ground state with low-energy spin-wave excitations (magnons). The Dirac electrons couple to the spins through a spin-dependent effective Zeeman field. The out-of-plane effective Zeeman field therefore serves as a Dirac mass that gaps the electronic spectrum. Once a spin is flipped, it creates a surrounding domain in which the sign of the Dirac mass is opposite to that of the rest of the sample. Therefore, an electronic bound state appears on the domain wall, as predicted by Jackiw and Rebbi. However, in a quantum magnet, a localized spin flip does not produce an eigenstate. Instead, the eigenstates correspond to delocalized spin waves (magnons). As in the case of the single flipped spin, the delocalized magnon also binds an in-gap electronic state. We name this excitation a “Jackiw-Rebbi-Magnon” (JRM) and study its signature in the dynamic spin susceptibility. When the sample is tunnel coupled to an electronic reservoir, a magnon produced in a system without any electrons hybridizes with a JRM (which binds a single electron), producing magnon-JRM polaritons. For such a system, we identify a quantum phase transition when the magnon-JRM polariton energy falls below that of the fully polarized ferromagnetic ground state.

In the present study, we have thoroughly characterized the 3D Fermi surface of a topological nodal line semimetal InBi via the Shubnikov–de Haas oscillations and density functional theory. The nitty-gritty of its full 3D Fermi surface has been discussed in detail. The Fermi surface topology and the Hall conductivity emphasized the carrier compensation as a driving force for the observed extremely high magnetoresistance. The magnetotransport revealed a unique magnetic-field-induced metal-semiconducting transition. The origin of such a phenomenon has been elaborated theoretically which has implications for layered topological nodal line semimetals with linearly dispersing bands.

In this paper, we demonstrate by employing first-principles density functional theory calculations that AB or Bernal-type stacked armchair and ${\mathrm{AB}}^{\prime }$ zigzag bilayered SiC nanoribbons are converted into defect-free zigzag (zz) and armchair (ac) single-walled SiC nanotubes (SiCNTs), respectively. This formation of closed SiC nanostructures is facilitated by the establishment of interlayer bonds and the compensation of different polarity of charges at their edges. It is also understood from nudged elastic band calculations that edge dynamics is involved in the formation of SiCNTs, and growing zz-SiCNTs is found to be more feasible as compared with ac-SiCNTs. Our study of dimension-dependent electronic properties shows that both of these SiCNTs are semiconducting in nature, as the edge states of the nanoribbons have completely vanished. On the other hand, AA stacked zz bilayered SiC nanoribbons are converted into defective ac-SiCNTs, but conducting edge states are preserved owing to the presence of Si-Si and C-C bonds. Finally, we explore the critical dimension for the formation of SiCNTs. This reveals that freestanding and defect-free SiCNTs could be formed from nanoribbon with a diameter up to 1.2 nm, above which the closed edge structure possibly evolves.

We study the dynamics of hardcore spin models on the square and triangular lattice, constructed by analogy to hard spheres, where the translational degrees of freedom of the spheres are replaced by orientational degrees of freedom of spins on a lattice and the packing fraction as a control parameter is replaced by an exclusion angle. In equilibrium, models on both lattices exhibit a Kosterlitz-Thouless transition at an exclusion angle ${\mathrm{\Delta }}_{\mathrm{KT}}$. We devise compression protocols for hardcore spins and find that any protocol that changes the exclusion angle nonadiabatically, if endowed with only local dynamics, fails to compress random initial states beyond an angle ${\mathrm{\Delta }}_{\mathrm{J}}>{\mathrm{\Delta }}_{\mathrm{KT}}$. This coincides with a doubly algebraic divergence of the relaxation time of compressed states toward equilibrium. We identify a remarkably simple mechanism underpinning this divergent timescale: topological defects involved in the phase ordering kinetics of the system become incompatible with the hardcore spin constraint, leading to a vanishing defect mobility as $\mathrm{\Delta }\to {\mathrm{\Delta }}_{\mathrm{J}}$.

As a paradigm of weak ergodicity breaking in disorder-free nonintegrable models, quantum many-body scars (QMBS) can offer deep insights into the thermalization dynamics of gauge theories. Having been first discovered in a spin-$\frac{1}{2}$ quantum link formulation of the Schwinger model, it is a fundamental question as to whether QMBS persist for $S>\frac{1}{2}$ since such theories converge to the lattice Schwinger model in the large-$S$ limit, which is the appropriate version of lattice QED in one spatial dimension. In this work, we address this question by exploring QMBS in spin-$S\mathrm{U}\left(1\right)$ quantum link models (QLMs) with staggered fermions. We find that QMBS persist at $S>\frac{1}{2}$, with the resonant scarring regime, which occurs for a zero-mass quench, arising from simple high-energy gauge-invariant initial product states. We furthermore find evidence of detuned scarring regimes, which occur for finite-mass quenches starting in the physical vacua and the charge-proliferated state. Our results conclusively show that QMBS exist in a wide class of lattice gauge theories in one spatial dimension represented by spin-$S$ QLMs coupled to dynamical fermions, and our findings can be tested on near-term cold-atom quantum simulators of these models.

Multiple conduction channels interacting with a quantum impurity—a spin in the conventional “multichannel Kondo effect” or a topological mesoscopic device (“topological Kondo effect”)—has been proposed as a platform to realize anyonic quasiparticles. However, the above implementations require either perfect channel symmetry or the use of Majorana fermions. Here we propose a Majorana-free mesoscopic setup which implements the Kondo effect of the symplectic Lie group and can harbor emergent anyons (including Majorana fermions, Fibonacci anyons, and ${\mathbb{Z}}_3$ parafermions) even in the absence of perfect channel symmetry. In addition to the detailed prescription of the implementation, we present the strong coupling solution by mapping the model to the multichannel Kondo effect associated to an internal $\text{SU}\left(2\right)$ symmetry and exploit conformal field theory to predict the nontrivial scaling of a variety of observables, including conductance, as a function of temperature. This work does not only open the door for robust Kondo-based anyon platforms, but also sheds light on the physics of strongly correlated materials with competing order parameters.