A Hamiltonian formalism is applied for the investigation of nonlinear spin wave dynamics under the influence of antisymmetric magnetic interactions. In the framework of this formalism we account not only for symmetric magnetic interactions (exchange, dipole-dipole, magnetocrystalline anisotropy), but also for antisymmetric interactions, such as Dzyaloshinskii-Moriya exchange interaction. The account of antisymmetric exchange, in general, could lead to the appearance of an additive nonreciprocal term in the spin wave dispersion law. We present the generalization of the linear transformation for the diagonalization of quadratic part of the Hamiltonian (so-called “third Holstein-Primakoff transformation”) for the antisymmetric case, which allowed us to obtain generalized expressions for the coefficients of the nonlinear three- and four-magnon interactions. As an example, nonlinear spin wave interactions in ultrathin ferromagnetic nanowires and films subjected to interfacial Dzyaloshinskii-Moriya interaction (IDMI) are considered. It was found that three-magnon interaction coefficients in the “Damon-Eshbach” geometry are nonzero only in the case of the noncollinear interacting spin waves, and vanish in the case of the collinear spin waves. It was also found that the nonlinear spin wave frequency shift caused by the four-magnon interaction is nonreciprocal, and has the sign opposite to that of the nonreciprocal term in linear spin wave dispersion, so that the IDMI-induced nonreciprocity of the spin wave spectrum decreases with the increase of the spin wave amplitude.

Heusler compounds are promising materials for spintronics with adjustable electronic properties including 100% spin polarization at the Fermi energy. We investigate the electronic states of ${\mathrm{AlO}}_x$ capped epitaxial thin films of the ferromagnetic half-metal ${\mathrm{Co}}_2\mathrm{MnSi}$ ex situ by soft x-ray angular resolved photoemission spectroscopy (SX-ARPES). Good agreement between the experimental SX-ARPES results and photoemission calculations including surface effects was obtained. In particular, we observed in line with our calculations a large photoemission intensity at the center of the Brillouin zone, which does not originate from bulk states, but from a surface resonance. This provides strong evidence for the validity of the previously proposed model based on this resonance, which was applied to explain the huge spin polarization of ${\mathrm{Co}}_2\mathrm{MnSi}$ observed by angular-integrating UV-photoemission spectroscopy.

We present implementation of the alloy analogy model within fully relativistic density-functional theory with the coherent potential approximation for a treatment of nonzero temperatures. We calculate contributions of phonons and magnetic and chemical disorder to the temperature-dependent resistivity, anomalous Hall conductivity (AHC), and spin-resolved conductivity in ferromagnetic half-Heusler NiMnSb. Our electrical transport calculations with combined scattering effects agree well with experimental literature for Ni-rich NiMnSb with 1–2% Ni impurities on Mn sublattice. The calculated AHC is dominated by the Fermi surface term in the Kubo-Bastin formula. Moreover, the AHC as a function of longitudinal conductivity consists of two linear parts in the Ni-rich alloy, while it is nonmonotonic for Mn impurities. We obtain the spin polarization of the electrical current $P>90\%$ at room temperature and we show that $P$ may be tuned by chemical composition. The presented results demonstrate the applicability of an efficient first-principles scheme to calculate temperature dependence of linear transport coefficients in multisublattice bulk magnetic alloys.

We use the stochastic series expansion quantum Monte Carlo method to study the Heisenberg models on the square lattice with strong and weak couplings in the form of three different plaquette arrangements known as checkerboard models $\mathrm{C}2\times 2$, $\mathrm{C}2\times 4$, and $\mathrm{C}4\times 4$. The $a\times b$ here stands for the shape of the plaquette consisting of spins connected by strong couplings. Through detailed analysis of a finite-size scaling study, the critical point of the $\mathrm{C}2\times 2$ model is improved as $g_c=0.548524\left(3\right)$ compared with previous studies where $g$ is the ratio of weak and strong couplings in the models. For $\mathrm{C}2\times 4$ and $\mathrm{C}4\times 4$ we give $g_c=0.456978\left(2\right)$ and $0.314451\left(3\right)$. We also study the critical exponents $\nu$ and $\eta$ and the universal property of the Binder ratio to give further evidence that all quantum phase transitions in these three models are in the three-dimensional O(3) universality class. Furthermore, our fitting results show the importance of effective corrections in the scaling study of these models.

We study the topological property of the magnetoelastic excitation in noncollinear antiferromagnets. As a toy model, we consider the magnon-phonon coupling in a triangular antiferromagnet with a ${120}^{\circ }$ Nèel order. We find that in the presence of out-of-plane external magnetic field, the magnon-polaron bands, which arise from hybridization of magnons and phonons, can carry Chern number, even though the individual magnon and phonon bands are topologically trivial. Large Berry curvature is induced from the anticrossing regions between the magnon and phonon bands, which renormalizes the thermal Hall conductivity of phonon bands. To compute the Berry curvature and Chern number of magnon-polarons, we give a simple algorithm to diagonalize magnetoelastic Hamiltonian without diagonalizing the phonon Hamiltonian, by mapping the problem to the diagonalization of bosonic Bogoliubov-de-Gennes (BdG) Hamiltonian. This is necessary because the contribution to the Berry curvature from phonon cannot be properly captured if we compute the Berry curvature from magnetoelastic Hamiltonian whose phonon sector has been already diagonalized.

Droplet solitons are large amplitude localized spin-wave excitations that can be created in magnetic thin films with uniaxial anisotropy by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low-temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of gigahertz). Low-frequency ($\lesssim 1\mathrm{GHz}$) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and current, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g., spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low-frequency mode modulation, characteristics observed in our experiments.

Devices based on magnetocaloric materials provide great hope for environmentally friendly and energy efficient cooling that does not rely on the use of harmful gasses. ${\mathrm{Fe}}_2\mathrm{P}$ based compounds are alloys that have shown great potential for magnetocaloric devices. The magnetic behavior in ${\mathrm{Fe}}_2\mathrm{P}$ is characterized by a strong magnetocaloric effect that coexists with a first-order magnetic transition (FOMT). Neutron diffraction and inelastic scattering, Mössbauer spectroscopy, and first-principles calculations have been used to determine the structural and magnetic state of ${\mathrm{Fe}}_2\mathrm{P}$ around the FOMT. The results reveal that ferromagnetic moments in the ordered phase are perturbed at the FOMT such that the moments cant away from the principle direction within a small temperature region. The acoustic-phonon modes reveal a temperature-dependent nonzero energy gap in the magnetically ordered phase that falls to zero at the FOMT. The interplay between the FOMT and the phonon energy gap indicates hybridization between magnetic modes strongly affected by spin-orbit coupling and phonon modes leading to magnon-phonon quasiparticles that drive the magnetocaloric effect.

An iron-based superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x(0\le x\le 0.6)$ undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x=0.6$) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the $d_{xy}$ orbital to the Fermi level, a Lifshitz transition under pressure.

We study the superfluid critical temperature in a two-band attractive Fermi system with strong pairing fluctuations associated with both interband and intraband couplings. We focus specifically on a configuration where the intraband coupling is varied from weak to strong in a shallow band coupled to a weakly interacting deeper band. The whole crossover from the Bardeen-Cooper-Schrieffer (BCS) condensation of largely overlapping Cooper pairs to the Bose-Einstein condensation (BEC) of tightly bound molecules is covered by our analysis, which is based on the extension of the Nozières-Schmitt-Rink approach to a two-band system. In comparison with the single-band case, we find a strong enhancement of the critical temperature, a significant reduction of the preformed pair region where pseudogap effects are expected, and the entanglement of two kinds of composite bosons in the strong-coupling BEC regime.

We report our point-contact spectroscopy (PCS) study on the superconducting state of the type-II Dirac semimetal ${\mathrm{PdTe}}_2$ with a superconducting transition temperature $T_{\mathrm{c}}\sim 1.65$ K. Both mechanical- and soft-PCS differential conductance curves at 0.3 K show a consistent double-peak structure and they can be perfectly fitted by a single $s$-wave gap based on the Blonder-Tinkham-Klapwijk model. The gap follows a typical Bardeen-Cooper-Schrieffer temperature behavior, yielding ${\mathrm{\Delta }}_0\sim 0.29$ meV and $2{\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_{\mathrm{c}}=4.15$ in the strong-coupling regime. A sudden suppression of the superconducting gap in a magnetic field around $H{}_{\mathrm{c}1}\sim 130$ Oe is observed for most point contacts on ${\mathrm{PdTe}}_2$, characteristic of a first-order transition for type-I superconductors in field. However, for other contacts, a smooth evolution of the PCS conductance persists up to $H{}_{\mathrm{c}2}\sim 600$ Oe, signaling a local type-II superconductivity. The observed admixture of type-I and type-II superconductivity can possibly arise from an inhomogeneous electron mean free path on the surface of ${\mathrm{PdTe}}_2$ due to its topological surface states.

We propose a mechanical graphene analog which is made of stainless steel beads placed in a periodic magnetic field by a proper design. A stable, free of mechanical borders granular structure with well-predicted wave dynamics is experimentally constructed. First, we report the dispersion relation in conjunction with the evidence of the Dirac points. Theoretical analysis shows that, compared to genuine or other artificial graphene analogs, edge modes exist in the free zigzag and armchair boundaries together with bulk modes composed of in-plane extended translations but localized rotations at the edges. We observe the existence of edge modes in free zigzag boundary, and we reveal an experimental turning effect of edge waves from the zigzag to the armchair/zigzag boundary, even in the absence of a full band gap for bulk modes. Our work shows that granular graphene can serve as an excellent experimental platform to study Dirac, topological, and nonlinear wave phenomena.

Effective harmonic methods allow for calculating temperature-dependent phonon frequencies by incorporating the anharmonic contributions into an effective harmonic Hamiltonian. The systematic errors arising from such an approximation are explained theoretically and quantified by density-functional-theory-based numerical simulations. Two techniques with different approaches for sampling the finite-temperature phase space in order to generate the force-displacement data are compared. It is shown that the error in free energy obtained by using either can exceed that obtained from 0 K harmonic lattice dynamics analysis, which neglects the anharmonic effects.

In recent years, a lot of effort has been devoted to the quest for experimental realizations of Kitaev interactions in spin systems. Recently, many materials have been synthesized which seem to realize extended Kitaev models, where Kitaev interactions are supplemented by Heisenberg and other bond dependent terms. The crystal and electronic structure of these materials renders the determination of a minimal model a nontrivial pursuit. In this work, we will concentrate on one of these particular materials, $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, for which various minimal models have been proposed. Employing large scale Monte Carlo simulations we show how the number of prospective models can be reduced. We study in detail six models with a different range of interactions and show how only two of those reproduce the most recent experimental results for this material. We obtain two possible minimal models, one of them with nearest neighbor interactions, while the other includes interactions up to third neighbors. Furthermore, we show that strong bond anisotropies and further neighbor interactions are crucial to stabilize the tilted counterrotating spirals found in $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$. We further clarify the picture and distinguish these two models by studying the magnetization processes. We predict the magnetization behavior of these models and propose future experimental directions.

Single crystal neutron diffraction, inelastic neutron scattering, and electron spin resonance experiments are used to study the magnetic structure and spin waves in ${\mathrm{Pb}}_2\mathrm{VO}{\left({\mathrm{PO}}_4\right)}_2$, a prototypical layered $S=1/2$ ferromagnet with frustrating next-nearest neighbor antiferromagnetic interactions. The observed excitation spectrum is found to be inconsistent with a simple square lattice model previously proposed for this material. At least four distinct exchange coupling constants are required to reproduce the measured spin wave dispersion. The degree of magnetic frustration is correspondingly revised and found to be substantially smaller than in all previous estimates.

The onset of ferromagnetism in cobalt-tin alloys is investigated experimentally and theoretically. The $\mathrm{C}{\mathrm{o}}_{1+x}\mathrm{Sn}$ alloys were prepared by rapid quenching from the melt and form a modified hexagonal NiAs-type crystal structure for $0.45\le x\le 1$. The magnetic behavior is described analytically and by density-functional theory using supercells and the coherent-potential approximation. The excess of Co concentration $x$, which enters the interstitial $2d$ sites in the hypothetical NiAs-ordered parent alloy CoSn, yields a Griffiths-like phase and, above a quantum critical point $(x_c\approx 0.65)$, a quantum phase transition to ferromagnetic order. Quantum critical exponents are determined on the paramagnetic and ferromagnetic sides of the transition and related to the nature of the magnetism in itinerant systems with different types of chemical disorder.

We investigate optically excited terahertz standing spin waves in noncollinear magnetic bilayers. Using femtosecond laser-pulse excitation, a spin current is generated in the first ferromagnetic (FM) layer, and flows through a conductive spacer layer to be injected into the second (transverse) FM layer, where it exerts a spin-transfer torque on the magnetization and excites higher-order standing spin waves. We show that the noncollinear magnetic bilayer is a convenient tool that allows easy excitation of terahertz spin waves, and can be used to investigate the dispersion and thereby the spin-wave stiffness parameter in the thin-film regime. This is experimentally demonstrated using wedge-shaped Co and CoB (absorption) layers. Furthermore, the damping of these terahertz spin waves is investigated, showing a strong increase of the damping with decreasing absorption layer thickness, much stronger than expected from interface spin pumping effects. Additionally, a previously unseen sudden decrease in the damping for the thinnest films is observed. A model for the additional damping contribution incorporating both these observations is proposed.

A skyrmion can be stabilized in a nanodisk geometry in a ferromagnetic material with Dzyaloshinskii-Moriya (DM) interaction. We apply spin torque uniform in space and time and observe numerically that the skyrmion is set in steady rotational motion around a point off the nanodisk center. We give a theoretical description of the emerging auto-oscillation dynamics based on the coupling of the rotational motion to the breathing mode of the skyrmion and to the associated oscillations of the in-plane magnetization. The analysis shows that the achievement of auto-oscillations in this simple setup is due to the chiral symmetry breaking. Thus, we argue that the system is turned into a spin-torque oscillator due to the chiral DM interaction. We also show injection locking of this skyrmion oscillator demonstrating feasibility of application by using synchronization of multiple skyrmion auto-oscillators.

We address the theory of magnon-phonon interactions and compute the corresponding quasiparticle and transport lifetimes in magnetic insulators, with a focus on yttrium iron garnet at intermediate temperatures from anisotropy- and exchange-mediated magnon-phonon interactions, the latter being derived from the volume dependence of the Curie temperature. We find in general weak effects of phonon scattering on magnon transport and the Gilbert damping of the macrospin Kittel mode. The magnon transport lifetime differs from the quasiparticle lifetime at shorter wavelengths.