Skyrmions are small magnetic quasiparticles, which are uniquely characterized by their topological charge and their helicity. In this Rapid Communication, we show via calculations how both properties can be determined without relying on real-space imaging. The orbital magnetization and topological Hall conductivity measure the arising magnetization due to the circulation of electrons in the bulk and the occurrence of topologically protected edge channels due to the emergent field of a skyrmion crystal. Both observables quantify the topological Hall effect and distinguish skyrmions from antiskyrmions by sign. Additionally, we predict a magnetoelectric effect in skyrmion crystals, which is the generation of a magnetization (polarization) by application of an electric (magnetic) field. This effect is quantified by spin toroidization and magnetoelectric polarizability. The dependence of the transverse magnetoelectric effect on the skyrmion helicity fits that of the classical toroidal moment of the spin texture and allows one to differentiate skyrmion helicities: It is largest for Bloch skyrmions and zero for Néel skyrmions. We predict distinct features of the four observables that can be used to detect and characterize skyrmions in experiments.

Although spin cycloids and helices are quite common, remarkably little is known about the normal modes of a spin cycloid or helix with finite length on a discrete lattice. Based on simple one-dimensional lattice models, we numerically evaluate the normal modes of a spin cycloid or helix produced by either Dzyaloshinskii-Moriya (DM) or competing exchange (CE) interactions. The normal modes depend on the type of interaction and on whether the nearest-neighbor exchange is antiferromagnetic (AF) or ferromagnetic (FM). In the AF/DM and FM/DM cases, there is only a single Goldstone mode; in the AF/CE and FM/CE cases, there are three. For FM exchange, the spin oscillations produced by non-Goldstone modes contain a mixture of tangential and transverse components. For the DM cases, we compare our numerical results with analytic results in the continuum limit. Examples are given of materials that fall into all four cases.

We consider the superconducting pairing induced by spin-wave exchange in a ferromagnet with both conduction and localized electrons, the latter being described as spins. We use the microscopic Eliashberg theory to describe the pairing of conducting electrons and the random phase approximation approach to treat the localized spins assuming an exchange coupling between the conducting electrons and spins. In the framework of a nonrelativistic Hamiltonian, we found that the spin-wave exchange results in equal spin electron pairing described by the two components of the order parameter, ${\mathrm{\Delta }}^{\uparrow }$ (both spins up) and ${\mathrm{\Delta }}^{\downarrow }$ (both spins down). Due to the conservation of total spin projection on the axis of the spontaneous ferromagnetic moment, the spin-wave exchange at low temperatures includes an emission of magnons and an absorption of thermal magnons by the conduction electrons. The absorption and emission processes depend differently on the temperature, with the absorption being progressively suppressed as the temperature drops. As a result, the superconducting pairing exists only if the electron–spin-wave exchange parameter $g$ exceeds some critical value $g_c$. At $g>g_c$, pairing vanishes if the temperature drops below the lowest point $T_{cl}$ or increases above the upper critical point $T_{ch}\approx T_m$ (the Curie temperature) where the spin waves cease to exist. This behavior inherent to the spin-carrying glue is in an obvious disagreement with the results of the conventional BCS approach, which assumes that the effective electron-electron attraction is simply proportional to the static magnetic susceptibility.

We discuss expectation values of the twist operator $U$ appearing in the Lieb-Schultz-Mattis theorem (or the polarization operator for periodic systems) in excited states of the one-dimensional correlated systems $z_L^{(q,\pm )}\equiv \langle {\mathrm{\Psi }}_{q/2}^{\pm }\left|U^q\right|{\mathrm{\Psi }}_{q/2}^{\pm }\rangle$, where $|{\mathrm{\Psi }}_p^{\pm }\rangle$ denotes the excited states given by linear combinations of momentum $2p{k}_{\mathrm{F}}$ with parity $\pm 1$. We found that $z_L^{(q,\pm )}$ gives universal values $\pm 1/2$ on the Tomonaga-Luttinger (TL) fixed point, and its signs identify the topology of the dominant phases. Therefore, this expectation value changes between $\pm 1/2$ discontinuously at a phase transition point with the U(1) or SU(2) symmetric Gaussian universality class. This means that $z_L^{(q,\pm )}$ extracts the topological information of TL liquids. We explain these results based on the free-fermion picture and the bosonization theory, and also demonstrate them in several physical systems.

In this comparative study we benchmark a recently developed nonadiabatic exchange-correlation potential within time-dependent density-functional theory (TDDFT) [Phys. Rev. Lett. 120, 157701 (2018)] by (a) validating the transient dynamics using a numerically exact density-matrix renormalization-group approach as well as by (b) comparing the $RC$ time, a typical linear-response quantity, to up to second-order perturbation theory results. As a test bed we use the dynamics of the single-impurity Anderson model. These benchmarks show that the nonadiabatic potential yields quantitatively accurate results for the transient dynamics for temperatures of the order of the hybridization strength, while the TDDFT $RC$ times quantitatively agree with those from second-order perturbation theory for temperatures which are large compared to the hybridization strength. Both results are particularly intriguing given the relatively low numerical cost of a TDDFT calculation.

Machine learning has been successfully applied to identify phases and phase transitions in condensed matter systems. However, quantitative characterization of the critical fluctuations near phase transitions is lacking. In this paper, we propose a finite-size scaling approach based on a convolutional neural network and analyze the critical behavior of a quantum Hall plateau transition. The localization length critical exponent learned by the neural network is consistent with the value obtained by conventional approaches. We show that the general-purposed method can be used to extract critical exponents in models with drastically different physics and input data, such as the two-dimensional Ising model and four-state Potts model.

Crossing from the repulsive to the attractive Casimir force is analyzed for two Au and two Si half spaces with boundary Chern-Simons layers. Boundary quantum Hall layers in external magnetic field lead to Casimir repulsion at nanoscales. We discuss features that make systems with boundary quantum Hall layers unique for precise force measurements.

Plasmonic nanoparticles resonantly couple to and confine light below the diffraction limit. This mechanism has enabled a modern renaissance in optical materials, with potential applications ranging from sensing and circuitry to renewable energies and medicines. However, these plasmonic materials are typically constrained to dilute liquids or solid two-dimensional surfaces, thereby limiting their possibilities. Here, we experimentally demonstrate a plasmonic aerosol by transitioning liquid suspensions of gold nanorods into the gas phase and simultaneously measuring their optical spectra. By measuring and modeling the evolution of the longitudinal absorbance peak of the nanorods from the liquid to the gas phase, we find that the aerosols are optically homogeneous and thermodynamically stable. We show that by tailoring the aspect ratio of the nanorods, the aerosol absorbance peak is tunable from visible to midwave infrared wavelengths. We find the sensitivity of the absorbance peak wavelength to changes in the refractive index of the gas depends linearly on the aspect ratio and can be estimated from the geometric and material properties of the nanorod. For high aspect ratio nanorods the sensitivity becomes extremely large, which may be useful in aiding geoengineering challenges. This work establishes plasmonic aerosols, potentially enabling exciting opportunities for fundamental and applied research.

The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of $\sim 1/1000e^2/h$ is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal-activation behavior with a gap of 2.8 meV (>20 K), the other being attributed to variable-range hopping (1–20 K). Magnetic-field-driven plateau-to-plateau transition gives scaling relations of ${(d{R}_{xy}/dB)}^{max}\propto T^{-\kappa }$ and $\mathrm{\Delta }{B}^{-1}\propto T^{-\kappa }$ with a consistent exponent of κ ∼ 0.2, which is half the universal value for a conventional two-dimensional electron gas. This is evidence of percolation assisted by quantum tunneling and reveals the dominance of electron-electron interaction of the topological surface states.

The field of topological materials science has recently been focusing on three-dimensional Dirac semimetals, which exhibit robust Dirac phases in the bulk. However, the absence of characteristic surface states in accidental Dirac semimetals (DSMs) makes it difficult to experimentally verify claims about the topological nature using commonly used surface-sensitive techniques. The chiral magnetic effect (CME), which originates from the Weyl nodes, causes an $\mathbf{E}·\mathbf{B}$-dependent chiral charge polarization, which manifests itself as negative magnetoresistance. We exploit the extended lifetime of the chirally polarized charge and study the CME through both local and nonlocal measurements in Hall bar structures fabricated from single crystalline flakes of the DSM ${\mathrm{Bi}}_{0.97}{\mathrm{Sb}}_{0.03}$. From the nonlocal measurement results we find a chiral charge relaxation time, which is over one order of magnitude larger than the Drude transport lifetime, underlining the topological nature of ${\mathrm{Bi}}_{0.97}{\mathrm{Sb}}_{0.03}$.

We study interacting two-component hard-core bosons on a square lattice for which, in the presence of staggered magnetic flux, its ground state is a bosonic integer quantum Hall (BIQH) state. By using a coupled-wire bosonization approach, we show analytically that this model exhibits a BIQH state at total charge half filling associated with a symmetry-protected topological phase under $\text{U}\left(1\right)$ charge conservation. These theoretical expectations are verified, using the infinite density matrix renormalization group method, by providing numerical evidence for (i) a quantized Hall conductance ${\sigma }_{xy}=\pm 2$, and (ii) two counterpropagating gapless edge modes. Our model is a bosonic cousin of the fermionic Haldane model and serves as an additional case of analogy between bosonic and fermionic quantum Hall states.

The values of material parameters required for quantitative electrothermal modeling of nanoscale structures typically differ strongly from those of the bulk material. In this work, we apply a simple experimental technique that allows us to estimate values for thermal conductivity of both a metal nanowire and its insulating substrate by measuring the increase in resistance due to small amounts of dc self-heating. We measure gold nanowires with widths between 24 and 55 nm using this technique, and extract relevant material parameters as a function of temperature. Electrical resistivities of our nanowires are width dependent and much higher than bulk gold values, and enhanced temperature dependence of the resistivity indicates a depressed Debye temperature due to significant phonon softening. The fit thermal conductivity versus temperature of our 21-nm ${\mathrm{SiO}}_2$ on Si substrate is highly consistent with literature values for oxide thin films. We find the thermal conductivity of the nanowires increases rapidly with temperature and width and is well below the value for bulk gold, which can be qualitatively explained by the dominance of structural scattering. The Lorenz number is relatively constant over temperature, as in the Wiedemann-Franz theory, but it is significantly lower than reported values for bulk gold and exhibits some width dependence.

The question whether ferroelectricity (FE) may coexist with a metallic or highly conducting state or rather it must be suppressed by the screening from the free charges is the focus of a rapidly increasing number of theoretical studies and is finally receiving positive experimental responses. The issue is closely related to the thermoelectric and multiferroic (also magnetic) applications of FE materials where the electrical conductivity is required or spurious. In these circumstances, the traditional methods for probing ferroelectricity are hampered or made totally ineffective by the free charges, which screen the polar response to an external electric field. This fact may explain why more than 40 years passed among the first proposals of FE metals and the present experimental and theoretical activities. The measurement of the elastic moduli, Young's modulus in the present case, versus temperature is an effective method for studying the influence of doping on a FE transition because the elastic properties are unaffected by electrical conductivity. In this manner, it is shown that the FE transitions of ${\mathrm{BaTiO}}_{3-\delta }$ are not suppressed by electron doping through O vacancies; only the onset temperatures are depressed, but the magnitudes of the softenings, and hence of the piezoelectric activity, are initially even increased.

We show that a paramagnetic CoGa compound possesses large enough spin Hall angle to allow robust spin-orbit torque switching of perpendicularly magnetized ferrimagnetic MnGa films in CoGa/MnGa/oxide heterostructures. The spin Hall efficiency estimated via spin Hall magnetoresistance and harmonic Hall measurements is $+0.05\pm 0.01$, which is surprisingly large for a system that does not contain any heavy metal element. First-principles calculations corroborate our experimental observations and suggest that the hybridized Co 3d–Ga $4p$ orbitals are responsible for the intrinsic spin Hall effect. Our results suggest that efficient spin current generation can be realized in intermetallics by alloying a transition metal with a $p$-orbital element and by Fermi-level tuning.

Enhanced magnetic moment is induced in nonmagnetic metals. Specifically, gold, silver, and copper are rendered with bulk magnetic behavior by doping these metals with ferrimagnetic strontium hexa-ferrite ($\mathrm{SrF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$) nanoparticles. The doped metals exhibit classical macroscopic permanent magnetic properties. Remarkably, detailed magnetization studies reveal that the saturation moments $\left(M_S\right)$ of these materials are enhanced by a factor of 5–8 compared to pure $\mathrm{SrF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ nanoparticles. The enhancement order is $\mathrm{Cu}>\mathrm{Ag}>\mathrm{Au}$. A mechanism is suggested for the enhancement and for its order, based on a full materials characterization of these $\mathrm{SrF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$@Metals.

We investigate experimentally and theoretically the magnetization reversal process in one-dimensional magnonic structures composed of permalloy nanostripes of the two different widths and finite length arranged in a periodic and quasiperiodic order. We showed that dipolar coupling between rectangular nanostripes is significantly reduced as compared to the analytical and numerical predictions, probably due to formation of the closure domains at the nanostripe ends. Although the main feature of the hysteresis loop is determined by different shape anisotropies of the component elements and the dipolar interactions between them, the quasiperiodic order influences the hysteresis loop by introducing additional tiny switching steps and change of the plateau width. We also showed that the dipolar interactions between nanostripes forming a ribbon can be counterintuitively decreased by reduction of the distance between the neighboring ribbons.

${\mathrm{Fe}}^{3+} S=5/2$ ions form saw-tooth-like chains along the $a$ axis of the oxoselenite ${\mathrm{Fe}}_2\mathrm{O}{\left({\mathrm{SeO}}_3\right)}_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C=105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T=150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

We study the decay rate $\mathrm{\Gamma }\left(k\right)$ of density excitations of two-component Bose-Einstein condensates at zero temperature. Those excitations, where the two components oscillate in phase, include the Goldstone mode resulting from condensation. While within Bogoliubov approximation the density sector and the spin (out-of-phase) sector are independent, they couple at the three-phonon level. For a Bose-Bose mixture we find that the Belyaev decay is slightly modified due to the coupling with the gapless spin mode. At the phase separation point the decay rate changes instead from the standard $k^5$ to a $k^{5/2}$ behavior due to the parabolic nature of the spin mode. If instead a coherent coupling between the two components is present, the spin sector is gapped and, away from the ferromagnetic-like phase transition point, the decay of the density mode is not affected. On the other hand, at the transition point, when the spin fluctuations become critical, the Goldstone mode is not well defined anymore since $\mathrm{\Gamma }\left(k\right)\propto k$. As a consequence, we show that the friction induced by a moving impurity is enhanced—a feature which could be experimentally tested. Our results apply to every nonlinear 2-component quantum hydrodynamic Hamiltonian which is time-reversal invariant and possesses an $U\left(1\right)\times {\mathbb{Z}}_2$ symmetry.

This Comment is on the paper “Effect of rotation on the elastic moduli of solid ${}^4\mathrm{He}$” by T. Tsuiki, D. Takahashi, S. Murakawa,Y. Okuda, K. Kono, and K. Shirahama [Phys. Rev. B 97, 054516 (2018)]. The authors speculate on a putative effect of the effective mass of ${}^3\mathrm{He}$ impuritons on the angular momentum of solid ${}^4\mathrm{He}$ under rotation. This is a crucial point in their work when trying to explain the variation of the share modulus, $\mu$, with temperature. They refer also to my general theory of kinetics and dynamics of quasiparticles in nonstationary moving bodies subjected to time varying deformations [cf, e.g., Phys. Rep. 354, 411 (2001)] and in particular to the effect of a strong anisotropy in the temperature dependence of the vacancy diffusion under rotation. The effect is due to Coriolis force. The authors apply by analogy the same idea to ${}^3\mathrm{He}$ impuritons. However their consideration is inconsistent. They replace the bare mass of impurity in the Coriolis force by the impuriton effective mass. The role of the Coriolis force on the impuriton diffusion becomes highly exaggerated and leads to a not existing effect.