Topologically protected, unidirectional surface magnon polaritons (SMPs) guided along the interface between air and ferrimagnetic yttrium iron garnet slab structures, biased with an external static magnetic field in the Voigt configuration, are realized experimentally by measuring the S-parameters between two small loop magnetic dipole antennas that launch and receive the SMP. Transmission is compared in the two cases where an abrupt change in the form of a step is present and absent in the interface. We find that transmission of the SMP peaks near the same level in each case, a sign of its topological protection. In addition, the dependence of isolation on slab thickness is investigated for a specified antenna configuration.

We report magnetization (χ, $M$), magnetic specific heat ($C_{\mathrm{M}}$), and neutron powder diffraction results on a quasi-two-dimensional (2D) $S=2$ square lattice antiferromagnet ${\mathrm{Ba}}_2{\mathrm{FeSi}}_2{\mathrm{O}}_7$ consisting of ${\mathrm{FeO}}_4$ tetrahedrons with highly compressive tetragonal distortion (27%). Despite of the quasi-2D lattice structure, both χ and $C_{\mathrm{M}}$ present three-dimensional magnetic long-range ordering below the Néel temperature $T_{\mathrm{N}}=5.2\mathrm{K}$. Neutron diffraction data show a collinear ${\mathit{Q}}_{\mathit{m}}=$(1,0,1/2) antiferromagnetic (AFM) structure below $T_{\mathrm{N}}$ but the ordered moment aligned in the $ab$ plane is suppressed by 26% from the ionic spin $S=2$ value ($4{\mu }_{\mathrm{B}}$). Both the AFM structure and the suppressed moments are well explained by using Monte Carlo simulations with a large single-ion in-plane anisotropy $D=1$.4 meV and a rather small Heisenberg exchange $J_{\mathrm{intra}}=0$.15 meV in the plane. The characteristic 2D spin fluctuations are recognized in the magnetic entropy release and diffuse scattering above $T_{\mathrm{N}}$. This new quasi-2D magnetic system also displays unusual nonmonotonic dependence of $T_{\mathrm{N}}$ as a function of magnetic field $H$.

Exploiting the theory of solitons in a nonlinear elastic medium we predict a phenomenon called a phononic tsunami, which is characterized by the dramatic increase of the local amplitude of phonon modes. To elucidate the possible experimental detection of this phenomenon we propose to use a wedge-shaped sample in which a sharp edge serves for the emulation of the shoaling effect and such a local enhancement can be observed. Together with eigenfrequencies of transverse and longitudinal phonon modes of a system we find the characteristic dispersion relations that can be considered as a hallmark of a phononic tsunami. We justify our predictions by means of analytical calculations and numerical simulations showing a possible realization of this nonlinear effect in such a geometry. Our results provide the framework for the implementation of kind experiments aimed at realizing and investigating a phononic tsunami phenomenon in relevant materials.

The perpendicular magnetic anisotropy properties of CoFeB/MgO films on $3d$ light-metal vanadium have been experimentally investigated. The interfacial magnetic anisotropy is $1.4\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$ for the V/CoFeB/MgO multilayers annealed at 240 °C. A sign change of the effective torque fields with the increase of V thickness has been observed, which is confirmed by current-induced magnetization switching measurements, suggesting a competition of multiple effects on current-induced torques in the weakly spin-orbit coupled V/CoFeB/MgO multilayers. These findings have implications for the fundamental understanding of current-induced torque and further device applications using $3d$ light transition-metal based heterostructures.

Well-protected magnetization, tunable quantum states, and long spin-relaxation time are desired for the use of magnetic molecules in spintronics and quantum information technologies. In this work, endohedral fullerene molecules $\mathrm{M}@{\mathrm{C}}_{28}$ with different transition-metal cores were explored through systematic first-principles calculations and spin dynamics analyses. Many of them have bias-tunable structure, stable magnetization, and sizable magnetic anisotropy energy. Furthermore, some of them may have spin-relaxation time up to several milliseconds for their quantum spin states at high temperature (∼10 K) after full consideration of spin-vibration couplings. Our results suggest that these $\mathrm{M}@{\mathrm{C}}_{28}$ provide a rich pool of single-molecule magnets for diverse applications.

The effect of a three-dimensional strain state on the magnetocaloric properties of epitaxial ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_3$ (LCMO) thin films grown on two types of substrates, ${\mathrm{SrTiO}}_3$ (001) (STO) and ${\mathrm{LaAlO}}_3$ (001) (LAO), has been studied as a function of film thickness within the range of 25–300 nm. The STO substrate imposes an in-plane tensile biaxial strain, while the LAO substrate imposes an in-plane compressive biaxial strain. The in-plane biaxial strain on LCMO by the STO substrate gets relaxed more rapidly than that by the LAO substrate but both LCMO/STO and LCMO/LAO show a maximum entropy change ($\mathrm{\Delta }{S}_{max}$) of $\sim 12.1 {\mathrm{J}\mathrm{Kg}}^{-1}{\mathrm{K}}^{-1}$ $\left(76.33{\mathrm{mJ}\mathrm{cm}}^{-3}{\mathrm{K}}^{-1}\right)$ and $\sim 3.2 {\mathrm{J}\mathrm{Kg}}^{-1}{\mathrm{K}}^{-1}$ $\left(20.18{\mathrm{mJ}\mathrm{cm}}^{-3}{\mathrm{K}}^{-1}\right)$, respectively, at a critical thickness of 75 nm (at 6 T applied magnetic field). LCMO/LAO is found to exhibit a wider transition temperature region with full width at half maxima (FWHM) $\sim 40$ K of the $\frac{dM}{dT}$ vs $T$ curve compared to LCMO/STO with FWHM $\sim 33$ K of that curve. This broadening of the transition region indicates that the table-like magnetocaloric effect (MCE) is attainable by changing the strain type. The maximum relative cooling power, $\sim 361 {\mathrm{J}\mathrm{Kg}}^{-1}$ $\left(2277{\mathrm{mJ}\mathrm{cm}}^{-3}\right)$ of LCMO/STO and $\sim 339 {\mathrm{J}\mathrm{Kg}}^{-1}$ $\left(2138.5{\mathrm{mJ}\mathrm{cm}}^{-3}\right)$ of LCMO/LAO, is also observed at the thickness $\sim 75\mathrm{nm}$. The Curie temperature varies with the thickness exploring the variation of ferromagnetic interaction strength due to strain relaxation. The film thickness and substrate induced lattice strain are proved to be the significant parameters for controlling MCE. The highest MCE response at a particular thickness shows the possibility of tuning MCE in other devices by optimizing thickness.

Disorder is important in the study of quantum spin liquids, but its role on the spin dynamics remains elusive. Here, we explore the disorder effect by investigating the magnetic-field dependence of the low-energy magnetic excitations in a triangular-lattice frustrated magnet ${\mathrm{YbZnGaO}}_4$ with inelastic neutron scattering. With an intermediate field of 2.5 T applied along the $c$ axis, the broad continuum at zero field becomes more smeared both in energy and momentum. With a field up to 10 T, which fully polarizes the magnetic moments, we observe clear spin-wave excitations with a gap of $\sim 1.4\mathrm{meV}$ comparable to the bandwidth. However, the spectra are significantly broadened. The excitation spectra both at zero and high fields can be reproduced by performing classical Monte Carlo simulations which take into account the disorder effect arising from the random site mixing of nonmagnetic ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Ga}}^{3+}$ ions. These results elucidate the critical role of disorder in broadening the magnetic excitation spectra and mimicking the spin-liquid features in frustrated quantum magnets.

Correlated materials have appeared as an arena to study non-Hermitian effects as typically exemplified by the emergence of exceptional points. We show here that topological Kondo insulators are an ideal platform for studying these phenomena due to strong correlations and surface states exhibiting a nontrivial spin texture. Using numerical simulations, we demonstrate the emergence of exceptional points in the single-particle Green's function on the surface of the material while the bulk is still insulating. We reveal how quasiparticle states with long lifetimes are created on the surface by non-Hermitian effects while the Dirac cones are smeared, which explains the surface Kondo breakdown at which heavy Dirac cones disappear from the single-particle spectrum and are replaced by light states. We further show how the non-Hermiticty changes the spin texture inherent in the surface states, which might help identify exceptional points experimentally. Besides confirming the existence of non-Hermitian effects on the surface of a topological Kondo insulator, this paper demonstrates how the eigenstates and eigenvalues of the effective non-Hermitian matrix describing the single-particle Green's function help understand the properties of correlated materials.

The electron-boson spectral density (EBSD) function can be obtained from the measured optical scattering rate by solving a generalized Allen formula, which relates the two quantities with an integral equation and is an inversion problem. Thus far, numerical approaches, such as the maximum entropy method (MEM) and the least squares fitting method, have been applied for solving the generalized Allen formula. Here, we developed a method to obtain the EBSD functions from the optical scattering rate using a machine learning approach (MLA). We found that the MLA is more robust against random noise compared with the MEM. We applied the developed MLA to experimentally measured optical scattering rates and obtained reliable EBSD functions in terms of their shapes including the amplitudes. We expect that the MLA can be a useful and rapid method for solving other inversion problems, which may contain random noise.

We report on magnetic-torque and resistivity measurements of the heavy-fermion compound ${\mathrm{CeCoIn}}_5$ in static magnetic fields up to 36 T and temperatures down to 50 mK. While quantum oscillations of the de Haas–van Alphen (dHvA) as well as the Shubnikov–de Haas (SdH) effect confirm the previously reported Fermi surfaces, an analysis of the field dependence reveals two anomalous features. The first is seen at about 22 T as a sharp anomaly in the resistivity for current applied along the $a$ direction. The second appears as nonmonotonic field-dependent oscillation frequencies and amplitudes in both dHvA and SdH signals. This second feature emerges at about 28 T. This field is close to that of the nematic transition reported for ${\mathrm{CeRhIn}}_5$ and the proposed Lifshitz transition in ${\mathrm{CeIrIn}}_5$. We discuss possible common grounds of these latter features that might originate from the very similar band structures of these materials.

According to energy band theory, ground states of a normal conductor and insulator can be obtained by filling electrons individually into energy levels, without any restrictions. It fails when the electron-electron correlation is taken into account. In this work, we investigate the dynamic process of building ground states of a Hubbard model. It is based on time-ordered quantum quenches for unidirectional hopping across a central and an auxiliary Hubbard model. We find that there exists a set of optimal parameters (chemical potentials and pair binding energy) for the auxiliary system, which takes the role of electron-pair reservoir. The exceptional point dynamics in non-Hermitian quantum mechanics allows the perfect transfer of electron pair from the reservoir to the central system, obtaining its ground states at different fillings. The dynamics of time-ordered pair filling not only provides a method for correlated quantum state engineering, but also reveals the feature of the ground state in an alternative way.

Nodal-line semimetals (NLSMs) support many exotic phenomena due to the unique torus-shaped Fermi surfaces in their band structure. We study the nonlinear responses of three-dimensional NLSMs in the presence of parallel dc electric and strong magnetic fields using the Boltzmann equation. At zero temperature, the second harmonic conductivity is found to exhibit $1/B$-periodic quantum oscillations which are akin to the Shubnikov–de Haas (SdH) oscillations. More interestingly, we uncover an exotic oscillation in nonlinear responses which persists even at temperatures where the typical SdH-like oscillations vanish. This oscillation originates from the periodic gap closing of a pair of Landau levels at zero Fermi energy with the variation of the magnetic field. The emergence of the oscillation is a remarkable fingerprint of ring dispersion and the corresponding frequency can be used to determine the radius of the ring. The second harmonic conductivity diverges at the gap closing points leading to the giant nonlinear response in NLSMs at finite temperature.

Using the Laughlin's argument on a torus with two pinholes, we numerically demonstrate that the discontinuities of the center of mass work well as an invariant of the pumping phenomena during the process of the flux attachment, trading the magnetic flux for a statistical one. This is consistent with the bulk-edge correspondence of the fractional quantum Hall effect of anyons. We also confirm that the general feature of the edge states remains unchanged during the process while the topological degeneracy is discretely changed. This supports the stability of the quantum Hall edge states in the adiabatic heuristic principle.

We study the antiferromagnetic XYZ spin chain with quenched bond randomness, focusing on a critical line between localized Ising magnetic phases. A previous calculation using the spectrum-bifurcation renormalization group, and assuming marginal many-body localization, proposed that critical indices vary continuously. In this work, we solve the low-energy physics using an unbiased numerically exact tensor network method named the “rigorous renormalization group.” We find a line of fixed points consistent with infinite-randomness phenomenology, with indeed continuously varying critical exponents for average spin correlations. A self-consistent Hartree–Fock-type treatment of the $z$ couplings as interactions added to the free-fermion random XY model captures much of the important physics including the varying exponents; we provide an understanding of this as a result of local correlation induced between the mean-field couplings. We solve the problem of the locally correlated XY spin chain with an arbitrary degree of correlation and provide analytical strong-disorder renormalization group proofs of continuously varying exponents based on an associated classical random walk problem. This is also an example of a line of fixed points with continuously varying exponents in the equivalent disordered free-fermion chain. We argue that this line of fixed points also controls an extended region of the critical interacting XYZ spin chain.

A ${}^{125}\mathrm{Te}$ nuclear magnetic resonance (NMR) study was carried out in the paramagnetic state of the recently discovered quasi-one-dimensional spin-1 chain compound ${\mathrm{NiTe}}_2{\mathrm{O}}_5$. We observed that the ${}^{125}\mathrm{Te}$ NMR spectrum splits into two in a magnetic field applied along the $c$ axis. Based on the strong temperature variation of the relative intensity ratio of the split lines, we infer that the line splitting arises from the two sublattice susceptibilities induced in opposite directions along the chains. In great support of this interpretation, a quantitative analysis of the spin-lattice relaxation rate $T_1^{-1}$ and the Knight shift data unravels dominant transverse spin fluctuations. We conclude that Ising-like uniaxial spin correlations persist up to surprisingly high temperatures compared to the exchange energy scales. Spin-charge coupling mechanism via a self-doping effect may be important.

The topological properties of massive and massless fermionic quasiparticles have been intensively investigated over the past decade in topological materials without magnetism. Recently, the bosonic analogs of such quasiparticles arising from spin waves have been reported in a two-dimensional (2D) honeycomb-lattice ferromagnet/antiferromagnet and a 3D antiferromagnet. Here, we use time-of-flight inelastic neutron scattering to study spin waves of the $S=1$ honeycomb-lattice antiferromagnet ${\mathrm{BaNi}}_2{\left({\mathrm{AsO}}_4\right)}_2$, which has a zigzag antiferromagnetic (AFM) ground state identical to that of the Kitaev quantum spin liquid candidate $\alpha \text{−}{\mathrm{RuCl}}_3$. We determine the magnetic exchange interactions in the zigzag AFM ordered phase, and show that spin waves in ${\mathrm{BaNi}}_2{\left({\mathrm{AsO}}_4\right)}_2$ have symmetry-protected Dirac points inside the Brillouin zone boundary. These results provide a microscopic understanding of the zigzag AFM order and associated Dirac magnons in honeycomb-lattice magnets, and are also important for establishing the magnetic interactions in Kitaev quantum spin liquid candidates.

Compressing hydrogen-rich hydrides is an effective method to search for exotic properties such as high-$T_{\text{c}}$ superconductivity. Here we show that high pressure and high temperature stabilize unique hydrogen tubes in hafnium hydrides. A combination of structural searches and first-principle calculations predict a metastable stochiometric ${\mathrm{HfH}}_9$ at 200 GPa. ${\mathrm{HfH}}_9$ is composed of H tubes intercalated within Hf-H framework, where two-thirds of the hydrogen atoms are arranged in a tubelike ${\mathrm{H}}_{12}$ structure located inside channels formed by the remainder ${\mathrm{HfH}}_3$. Each ${\mathrm{H}}_{12}$ tube gains 0.876 electrons from the ${\mathrm{HfH}}_3$ framework, indicating the ionic character of ${\mathrm{HfH}}_9$. Calculations show that ${\mathrm{HfH}}_9$ is a potential superconductor with an estimated $T_{\text{c}}$ of 110 K at 200 GPa, with the electron-phonon coupling arising mainly from the ${\mathrm{H}}_{12}$ tube and its interaction with the ${\mathrm{HfH}}_3$ framework. The current results suggest the existence of diverse hydrogen chemistries at high pressure that could be unravelled by future experimental studies.

van der Waals Josephson junctions utilize the interface between two superconducting ${\mathrm{NbSe}}_2$ crystal flakes as a tunnel barrier. We fabricate such junctions by a dry-transfer procedure and characterize them by electric transport experiments at various temperatures and under microwave irradiation at frequencies up to 224 GHz. We observe integer Shapiro steps in the current-voltage characteristics indicating a sinusoidal Josephson current-phase relation and the possibility to use this kind of junction even in the terahertz frequency range.

Demulitplexing is used to connect data or signals from one single channel stream to multiple destinations. The streams may be additionally divided through their frequency, polarizations, angular momentum, etc. Here, we show how one is able to demultiplex sound waves at different physical positions and frequency components by employing stacked valley-Hall topological insulators made of merged triangular rigid scatterers. By tuning their two relative angles, we are able to break or restore the underlying lattice symmetry within two topologically independent band gaps. Thus, we experimentally stacked various individually tuned valley-Hall insulators to enable spectrally specific radiated sound to be routed to a particular destination via scattering-immune valley-projected edge states. We foresee that these findings will advance further studies in condensed matter physics, classical wave physics, but also in technological areas of communication by capitalizing on unconventional valley-Hall physics.