Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this Rapid Communication, we report the discovery of a ferromagnetic Kondo lattice compound, ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$. As in almost all ferromagnetic Kondo lattice systems, ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. The ac magnetic susceptibility, magnetization, and specific heat measurements show that ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ has a Kondo temperature $T_{\mathrm{K}}\approx 14$ K and a Curie temperature $T_{\mathrm{C}}=2.4\mathrm{K}$. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground-state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard-axis ordering is unlikely. On the other hand, the broad second-order phase transition at $T_{\mathrm{C}}$ favors a fluctuation-induced mechanism.

We present a computationally efficient and general first-principles based method for spin-lattice simulations for solids and clusters. The method is based on a coupling of atomistic spin dynamics and molecular dynamics simulations, expressed through a spin-lattice Hamiltonian, where the bilinear magnetic term is expanded up to second order in displacement. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and we observe good agreement with previous simulations. We also illustrate the coupled spin-lattice dynamics method on a more conceptual level, by exploring dissipation-free spin and lattice motion of small magnetic clusters (a dimer, trimer, and tetramer). The method discussed here opens the door for a quantitative description and understanding of the microscopic origin of many fundamental phenomena of contemporary interest, such as ultrafast demagnetization, magnetocalorics, and spincaloritronics.

Fractionalized excitations are of considerable interest in recent condensed-matter physics. Fractionalization of the spin degrees of freedom into localized and itinerant Majorana fermions is predicted for the Kitaev spin liquid, an exactly solvable model with bond-dependent interactions on a two-dimensional honeycomb lattice. As a function of temperature, theory predicts a characteristic two-peak structure of the heat capacity as a fingerprint of these excitations. Here we report on detailed heat-capacity experiments as a function of temperature and magnetic field in high-quality single crystals of $\alpha -\mathrm{RuC}{\mathrm{l}}_3$. We undertook considerable efforts to determine the exact phonon background. We measured single-crystalline ${\mathrm{RhCl}}_3$ as a nonmagnetic reference and performed ab initio calculations of the phonon density of states for both compounds. These ab initio calculations document that the intrinsic phonon contribution to the heat capacity cannot be obtained by a simple rescaling of the nonmagnetic reference using differences in the atomic masses. Sizable renormalization is required even for nonmagnetic ${\mathrm{RhCl}}_3$ with its minute difference from the title compound. In $\alpha -\mathrm{RuC}{\mathrm{l}}_3$ in zero magnetic field, excess heat capacity exists at temperatures well above the onset of magnetic order. In external magnetic fields far beyond quantum criticality, when long-range magnetic order is fully suppressed, the excess heat capacity exhibits the characteristic two-peak structure. In zero field, the lower peak just appears at temperatures around the onset of magnetic order and seems to be connected with canonical spin degrees of freedom. At higher fields, beyond the critical field, this peak is shifted to 10 K. The high-temperature peak located around 70 K is hardly influenced by external magnetic fields, carries the predicted amount of entropy $R/2ln2$, and may resemble remnants of Kitaev physics.

A crystalline topological phase is a topological phase with spatial symmetries. In this work, we give a very general physical picture of such phases: A topological phase with spatial symmetry $G$ (with internal symmetry $G_{\mathrm{int}}\le G$) is described by a defect network, a $G$-symmetric network of defects in a topological phase with internal symmetry $G_{\mathrm{int}}$. The defect network picture works both for symmetry-protected topological (SPT) and symmetry-enriched topological (SET) phases, in systems of either bosons or fermions. We derive this picture both by physical arguments and by a mathematical derivation from the general framework of Thorngren and Else [Phys. Rev. X 8, 011040 (2018)]. In the case of crystalline SPT phases, the defect network picture reduces to a previously studied dimensional reduction picture, thus establishing the equivalence of this picture with the general framework of Thorngren and Else applied to crystalline SPTs.

Acoustic flat lensing is achieved here by tuning a phononic array to have indefinite medium behavior in a narrow frequency spectral region along the acoustic branch in the irreducible Brillouin zone (IBZ). This is confirmed by the occurrence of a flat band along an unusual path in the IBZ and by interpreting the intersection point of isofrequency contours on the corresponding isofrequency surface; coherent directive collimated beams are formed whose reflection from the array surfaces create lensing. Theoretical predictions using a mass-spring lattice approximation of the phononic crystal (PC) are corroborated by time-domain experiments, airborne acoustic waves generated by a source with a frequency centered about 10.6 kHz, placed at three different distances from one side of a finite PC slab, constructed from polymeric spheres, yielding distinctive focal spots on the other side. These experiments evaluate the pressure field using optical feedback interferometry and demonstrate precise control of the three-dimensional wave trajectory through a sonic crystal.

We study the helimagnetic ground state of the MnGe cubic alloy using small-angle neutron scattering and a high-resolution method, the so-called MIEZE spectroscopy. Upon cooling below the Néel temperature $T_{\mathrm{N}}=170\left(5\right)\mathrm{K}$, we observe the proliferation of long-wavelength gapless spin fluctuations, concomitant with a continuous evolution of the helical correlation length. These fluctuations disappear at $T_{\mathrm{com}}=32\left(5\right)\mathrm{K}$ when the helical period becomes commensurate with the lattice. We propose to describe this intermediate phase as a soliton lattice, promoting nonlinear collective modes, or phasons, over a large temperature interval. We discuss the possible relevance of our results to the previously observed magnetotransport anomalies.

Photons interact with each other in condensed matter through the same mechanism that forms Cooper pairs in superconductors—the exchange of virtual phonons [A. Saraiva et al., Phys. Rev. Lett. 119, 193603 (2017)]. It is, however, unclear which consequences of this interaction will be observable and potentially lead to further analogy with superconductivity. We investigate the energy, momentum, and production rate of correlated Stokes–anti-Stokes (SaS) photons in diamond and other transparent media, experiencing properties akin to those of electronic Cooper pairs. The rate of correlated SaS production depends on the energy shifts of the pair, which in the BCS theory determines whether there should be an attractive or repulsive interaction. With this view, we only observe correlated SaS in the case of attractive interactions. While traditional photon-phonon collisions scatter light in all directions, the correlated SaS photons follow the same path as the noninteracting laser. The observed correlated SaS photon pairs are rare, but our model indicates paths to achieve higher interaction energies.

We consider a bipartite quantum conductor and analyze fluctuations of heat quantity in a subsystem as well as self-information associated with the reduced-density matrix of the subsystem. By exploiting the multicontour Keldysh technique, we calculate the Rényi entropy, or the information-generating function, subjected to the constraint of the local heat quantity of the subsystem, from which the probability distribution of conditional self-information is derived. We present an equality that relates the optimum capacity of information transmission and the Rényi entropy of order 0, which is the number of integer partitions into distinct parts. We apply our formalism to a two-terminal quantum dot. We point out that in the steady state, the reduced-density matrix and the operator of the local heat quantity of the subsystem may be commutative.

Maximally localized Wannier functions are localized orthogonal functions that can accurately represent given Bloch eigenstates of a periodic system at a low computational cost, thanks to the small size of each orbital. Tight-binding models based on the maximally localized Wannier functions obtained from different systems are often combined to construct tight-binding models for large systems such as a semi-infinite surface. However, the corresponding maximally localized Wannier functions in the overlapping region of different systems are not identical, and this discrepancy can introduce serious artifacts to the combined tight-binding model. Here, we propose two methods to seamlessly stitch two different tight-binding models that share some basis functions in common. First, we introduce a simple and efficient method: (i) find the best matching maximally localized Wannier function pairs in the overlapping region belonging to the two tight-binding models, (ii) rotate the spin orientations of the two corresponding Wannier functions to make them parallel to each other, and (iii) make their overall phases equal. Second, we propose a more accurate and generally applicable method based on the iterative minimization of the difference between the Hamiltonian matrix elements in the overlapping region. We demonstrate our methods by applying them to the surfaces of diamond, GeTe, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and TaAs. Our methods can be readily used to construct reliable tight-binding models for surfaces, interfaces, and defects.

Using $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.975}{\mathrm{Co}}_{0.025}\right)}_2{\mathrm{As}}_2$ as an exemplar material exhibiting second-order electronic-nematic and antiferromagnetic transitions, we present measurements that reveal anomalies in the elastoresistance ($\frac{\partial {\rho }_{ij}}{\partial {\varepsilon }_{kl}}$) and elastocaloric effect ($\frac{\partial T}{\partial {\varepsilon }_{kl}}$) at both phase transitions for induced strains ${\varepsilon }_{kl}$ that do not share the symmetry of either order parameter. Both effects are understood to arise from the effect of strain on the transition temperatures; in the region close to the phase transitions this leads to (1) similarity between the strain and temperature derivatives of the resistivity and (2) similarity between the elastocaloric effect and the singular part of the specific heat. These mechanisms for elastoresistance and elastocaloric effect should be anticipated for any material in which mechanical deformation changes the transition temperature. Furthermore, these measurements provide evidence that the Fisher-Langer relation ${\rho }^{\left(c\right)}\propto U^{\left(c\right)}$ between the scattering from critical degrees of freedom and their energy density, respectively, holds near each of the transitions in the material studied under varying strain as it does for varying temperature.

In ferromagnetic metals, the interconversion of spin and charge currents via the spin Hall effect and its inverse can depend on the angle between the ferromagnet's magnetization and the spin current polarization direction. Here, such a spin-dependent (inverse) spin Hall effect is found in the ferromagnetic alloy ${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$. In a nonlocal magnon transport experiment, ${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$ is used either to excite or to detect magnonic spin currents flowing in the ferrimagnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. We find that the signal amplitude is significantly modulated by tuning the direction of the ${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$ magnetization. We design a sample structure that prevents direct magnonic coupling between the ferromagnets. Thus, we can identify unambiguously an intrinsic electronic origin of the observed effect.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is important for chiral domain walls (DWs) and for stabilizing magnetic skyrmions. We study the effects of introducing increasing thicknesses of Ir, from zero to 2 nm, into a Pt/Co/Ta multilayer between the Co and Ta layers. There is a marked increase in magnetic moment, due to the suppression of the dead layer at the interface with Ta, but the perpendicular anisotropy is hardly affected. All samples show a universal scaling of the field-driven DW velocity across the creep and depinning regimes. Asymmetric bubble expansion shows that DWs in all of the samples have the left-handed Néel form. The value of in-plane magnetic field at which the creep velocity shows a minimum drops markedly on the introduction of Ir, as does the frequency shift of the Stokes and anti-Stokes peaks in Brillouin light scattering (BLS) measurements. Despite this qualitative similarity, there are quantitative differences in the DMI strength given by the two measurements, with BLS often returning higher values. Many features in bubble expansion velocity curves do not fit simple models commonly used, namely a lack of symmetry about the velocity minimum and no difference in velocities at high in-plane fields. These features are explained by the use of a new model in which the depinning field is allowed to vary with in-plane field in a way determined from micromagnetic simulations. This theory shows that the velocity minimum underestimates the DMI field, consistent with BLS giving higher values. Our results suggest that the DMI at an Ir/Co interface has the same sign as the DMI at a Pt/Co interface.

In classical and quantum spin ice, rodlike features appear in the neutron-scattering structure factor when the pinch points characteristic of classical spin ice get washed out. We show that these features do not indicate the absence of spin correlations between planes perpendicular to the rods. Rather, they arise because neutron scattering is largely insensitive to the three-dimensional correlations which are present throughout. We present two very simple models which exhibit a pristine incarnation of such scattering rods. This provides a physical picture for their appearance, elucidates the role played by monopole excitations, and identifies conditions conducive to their observation.

The calculated electronic band structure of graphene is relatively simple, with a Fermi surface consisting only of six Dirac cones in the first Brillouin zone—one at each $\overline{K}$. In contrast, angle-resolved photoemission measurements of graphene grown on SiC(0001) often show six satellite Dirac cones surrounding each primary Dirac cone. Recent studies have reported two further Dirac cones along the $\overline{\mathrm{\Gamma }}\text{−}\overline{K}$ line, and argue that these are not photoelectron diffraction artifacts but real bands deriving from a modulation of the ionic potential in the graphene layer. Here we present measurements using linearly polarized synchrotron light which show all of these replicas as well as several additional ones. Using information obtained from dark corridor orientations and angular warping, we demonstrate that all but one of these additional features—including those previously assigned as real initial-state bands—are possible to explain by simple final-state photoelectron diffraction.

Both theoretical interest and practical significance attach to the sign and strength of Casimir forces. A famous, discouraging no-go theorem states that “the Casimir force between two bodies with reflection symmetry is always attractive.” Here, we identify a promising way to avoid the assumptions of the no-go theorem, and propose a universal way to realize repulsive Casimir forces. We show that the sign and strength of Casimir forces can be adjusted by inserting optically active or gyrotropic media between bodies, and modulated by external fields.

We fabricate Josephson field-effect transistors in germanium quantum wells contacted by superconducting aluminum and demonstrate supercurrents carried by holes that extend over junction lengths of several micrometers. In superconducting quantum point contacts we observe discretization of supercurrent, as well as Fabry-Pérot resonances, demonstrating ballistic transport. The magnetic field dependence of the supercurrent follows a clear Fraunhofer-like pattern, and Shapiro steps appear upon microwave irradiation. Multiple Andreev reflections give rise to conductance enhancement and evidence a transparent interface, confirmed by analyzing the excess current. These demonstrations of ballistic superconducting transport are promising for hybrid quantum technology in germanium.

Hot electron transport in a quantum Hall edge channel of an AlGaAs/GaAs heterostructure is studied by investigating the energy distribution function in the channel. Ballistic hot-electron transport, its optical-phonon replicas, weak electron-electron scattering, and electron-hole excitation in the Fermi sea are clearly identified in the energy spectra. The optical-phonon scattering is analyzed to evaluate the edge potential profile. We find that the electron-electron scattering is significantly suppressed with increasing the hot-electron's energy well above the Fermi energy. This can be understood with suppressed Coulomb potential with longer distance for higher energy. The results suggest that the relaxation can be suppressed further by softening the edge potential. This is essential for studying noninteracting chiral transport over a long distance.

We derive an effective Hamiltonian system describing the low-energy dynamics of circular magnetic skyrmions and antiskyrmions. Using scaling and symmetry arguments, we model (anti)skyrmion dynamics through a finite set of coupled, canonically conjugated, collective coordinates. The resulting theoretical description is independent of both micromagnetic details as well as any specificity in the ansatz of the skyrmion profile. Based on the Hamiltonian structure, we derive a general description for breathing dynamics of (anti)skyrmions in the limit of radius much larger than the domain wall width. The effective energy landscape reveals two qualitatively different types of breathing behavior. For small energy perturbations, we reproduce the well-known small breathing mode excitations, where the magnetic moments of the skyrmion oscillate around their equilibrium solution. At higher energies, we find a breathing behavior where the skyrmion phase continuously precesses, transforming Néel to Bloch skyrmions and vice versa. For a damped system, we observe the transition from the continuously rotating and breathing skyrmion into the oscillatory one. We analyze the characteristic frequencies of both breathing types, as well as their amplitudes and distinct energy dissipation rates. For rotational (oscillatory) breathing modes, we predict on average a linear (exponential) decay in energy. We argue that this stark difference in dissipative behavior should be observable in the frequency spectrum of excited (anti)skyrmions.

Spin-orbit interactions in optics traditionally describe an influence of the polarization degree of freedom of light on its spatial properties. The most prominent example is the generation of a spin-dependent optical vortex upon focusing or scattering of a circularly polarized plane wave by a nanoparticle, converting spin to orbital angular momentum of light. Here we present a mechanism of conversion of orbital-to-spin angular momentum of light upon scattering of a linearly polarized vortex beam by a spherical silicon nanoparticle. We show that focused linearly polarized Laguerre-Gaussian beams of first order ($\ell =\pm 1$) exhibit an $\ell$-dependent spatial distribution of helicity density in the focal volume. By using a dipolar scatterer the helicity density can be manipulated locally, while influencing globally the spin and orbital angular momentum of the beam. Specifically, the scattered light can be purely circularly polarized with the handedness depending on the orbital angular momentum of the incident beam. We corroborate our findings with theoretical calculations and an experimental demonstration. Our work sheds light on the global and local properties of helicity conservation laws in electromagnetism.

Phase transitions at a finite (i.e., nonzero) temperature are typically dominated by classical correlations, in contrast to zero temperature transitions where quantum mechanics plays an essential role. Therefore, it is natural to ask if there are any signatures of a finite-temperature phase transition in measures that are sensitive only to quantum correlations. Here we study one such measure, namely, entanglement negativity, across finite-temperature phase transitions in several exactly solvable Hamiltonians and find that it is a singular function of temperature across the transition. Our results also lead to a mean-field argument that shows that negativity can distinguish spontaneous symmetry breaking with local order parameter at finite temperature from that in the ground state. Along the way, we prove certain general results which simplify the calculation of negativity for commuting projector Hamiltonians and as an aside, we also calculate the entanglement of formation exactly in an interacting model.