We report on element-specific measurements of ultrafast demagnetization and magnetization precession damping in permalloy thin films. Magnetization dynamics induced by an optical pump at 1.5 eV is probed simultaneously at the $M_{2,3}$ edges of Ni and Fe with high-order harmonics for moderate demagnetization rates (less than 50%). The role of the intersublattice exchange interaction in both longitudinal and transverse dynamics is analyzed with a Landau-Lifshitz-Bloch description of ferromagnetically coupled Fe and Ni sublattices. It is shown that the intersublattice exchange interaction governs the dissipation during demagnetization as well as precession damping of the magnetization vector.

Mott insulator $\mathrm{Ni}{\mathrm{Ga}}_2{\mathrm{S}}_4$ has attracted much attention because of its elusive magnetic ground state due to the geometric frustration and layered structures. To our knowledge, however, additional studies of the turning properties through external parameters are rather scarce in this system. Here, we comprehensively performed high-pressure studies for the $\mathrm{Ni}{\mathrm{Ga}}_2{\mathrm{S}}_4$ single crystal. With synchrotron x-ray diffraction and Raman-scattering results, we found the starting trigonal $P\overline{3}m1$ phase quickly transfers to the $R3m$ phase at a lower pressure of 1.6 GPa due to the intralayer sliding. With further compressing, another two structural phase transitions were observed, $\mathrm{Ni}{\mathrm{Ga}}_2{\mathrm{S}}_4$ transfers to tetragonal $P\overline{4}2m$ phase at 15.5 GPa and then enters into orthogonal $Pna{2}_1$ phase beyond 18.2 GPa. In addition, electrical transport measurements show a Mott insulator-metal transition close to 17 GPa, which can be related to the $R3m\to P\overline{4}2m$ transition where a $\sim 17.5\%$ collapse in the unit-cell volume and a sharp reduction in Raman intensity occur. The metallic $\mathrm{Ni}{\mathrm{Ga}}_2{\mathrm{S}}_4$ shows no superconducting transition down to 1.8 K. This paper enriches our understanding of $\mathrm{Ni}{\mathrm{Ga}}_2{\mathrm{S}}_4$ especially its structure information, the tuning between the Mott insulating state and metallic state simultaneously accompanied by a structural phase transition sheds light on further understanding of magnetism and Mott physics in $\mathrm{Ni}{\mathrm{Ga}}_2{\mathrm{S}}_4$.

In this paper, dynamics of a magnetic skyrmion induced by linear and circular surface acoustic waves in antiferromagnetic (AFM) systems are determined. The skyrmion is considered as a rigid disk with a repulsive constant potential that scatters the surface waves. By insertion of skyrmion profile inside the AFM Lagrangian, the equation of motion could be inferred. Conservation of linear and angular momenta results in a force and torque applied to the skyrmion by surface waves. The force and torque are calculated by using the scattering concept in the Born approximation for both linear and circular polarization. By equating the time derivative of these momenta with force and torque emerged from the Born approximation, the skyrmion dynamic is determined as a coupled nonlinear differential equation. The motion of skyrmion has no Hall effect in linear waves, confirming the physics of AFM skyrmion dynamics with no Magnus force. The skyrmion shows a ratchet motion in circular waves. This dynamic emerges because circular waves can produce a small amount of torque on spin texture. The velocity is inversely proportional to the Gilbert damping, as expected for AFM systems. We found meaningful consistency between our theoretical findings and atomistic simulation. Besides, we observe that the velocity goes up with skyrmion radius. Our findings uncover a nonlinear dynamics for skyrmions in AFM systems that can be originated by coupling to acoustic waves for future studies.

We demonstrate that, even when employing above band gap excitation, photons emitted from semiconductor quantum dots can have linewidths that approach their transform-limited values. This is accomplished by using quantum dots embedded within bottom-up photonic nanowires, an approach which mitigates several potential mechanisms that can result in linewidth broadening: (i) Only a single quantum dot is present in each device, (ii) dot nucleation proceeds without the formation of a wetting layer, and (iii) the sidewalls of the photonic nanowire are comprised not of etched facets, but of epitaxially grown crystal planes. Using these structures we achieve linewidths of $2\times$ the transform limit for above band gap excitation. We also demonstrate a highly nonlinear dependence of the linewidth on both excitation power and temperature which can be described by an independent boson model that considers both deformation and piezoelectric exciton-phonon coupling. We find that for sufficiently low excitation powers and temperatures, the observed excess broadening is not dominated by phonon dephasing, a surprising result considering the high phonon occupation that occurs with above band gap excitation.

Full-shell nanowires are hybrid nanostructures consisting of a semiconducting core encapsulated in an epitaxial superconducting shell. When subject to an external magnetic flux, they exhibit the Little-Parks (LP) phenomenon of flux-modulated superconductivity, an effect connected to the physics of Abrikosov vortex lines in type-II superconductors. We show theoretically that full-shell nanowires can host subgap states that are a variant of the Caroli–de Gennes–Matricon (CdGM) states in vortices. These CdGM analogs are shell-induced Van Hove singularities in propagating core subbands. We elucidate their structure, parameter dependence, and behavior in tunneling spectroscopy through a series of models of growing complexity. Using microscopic numerical simulations, we show that CdGM analogs exhibit a characteristic skewness towards higher flux values inside nonzero LP lobes resulting from the interplay of three ingredients. First, the orbital coupling to the field shifts the energy of the CdGM analogs proportionally to the flux and to their generalized angular momentum. Second, CdGM analogs coalesce into degeneracy points at flux values for which their corresponding radial wavefunctions are threaded by an integer multiple of the flux quantum. And third, the average radii of all CdGM-analog wavefunctions inside the core are approximately equal for realistic parameters and are controlled by the electrostatic band bending at the core/shell interface. As the average radius moves away from the interface, the degeneracy points shift towards larger fluxes from the center of the LP lobes, causing the skewness. This analysis provides a transparent interpretation of the nanowire spectrum that allows to extract microscopic information by measuring the number and skewness of CdGM analogs. Moreover, it allows to derive an efficient Hamiltonian of the full-shell nanowire in terms of a modified hollow-core model at the average radius.

We have investigated the temperature dependence of the Mn local structure in $A$-site ordered $R\mathrm{Ba}{\mathrm{Mn}}_2{\mathrm{O}}_6$ ($R=\mathrm{La}$, Nd, Sm, and Y) perovskites, in parallel with their disordered counterparts, $R_{0.5}{\mathrm{Ba}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$, by means of x-ray emission (XES) and x-ray absorption spectroscopy (XAS) The end member $\mathrm{LaBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$ shows a nearly regular $\mathrm{Mn}{\mathrm{O}}_6$ octahedron independent of temperature. With decreasing the $R$ ionic radius in the ordered samples, the XAS results indicate that a local distortion develops in the $\mathrm{Mn}{\mathrm{O}}_6$ octahedron at the low-temperature charge-localized and polar phases. For $\mathrm{NdBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$, this local distortion is tiny, indicating the absence of charge segregation at the Mn site. This is followed by a bigger local distortion anticipated for $\mathrm{SmBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$ in its respective charge-localized and polar phase and finally, the biggest local distortion for the smallest $A$-site cation ordered compound, $\mathrm{YBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$, for which it even persists above the polar charge-localization transition temperatures. The high-resolution XAS spectra confirm the presence of charge segregation between two nonequivalent Mn sites in the low-temperature polar phase of Sm and Y ordered samples. Thus, our XAS study suggests a displacive mechanism for the charge-localization and polar transitions in the Nd and Sm ordered samples while a combination of displacive and order-disorder contributions is revealed for $\mathrm{YBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$. Besides, calorimetric measurements confirm the combination of the two mechanisms, order-disorder and displacive, for the ordered Sm and Y compounds. On the other hand, the $A$-site disordered $R_{0.5}{\mathrm{Ba}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ samples with $R$ cations smaller than Nd present a significant static (temperature-independent) local disorder, which explains why polar charge-localization transitions are not developed in these samples. Finally, we correlate our results about the Mn local structure and character of the transitions with the macroscopic magnetic and electric behavior of both $A$-site ordered and disordered compounds.

The spin-orbital entangled states are of great interest as they hold exotic phases and intriguing properties. Here we use first-principles calculations to investigate the electronic and magnetic properties of ${\mathrm{RuI}}_3$ and ${\mathrm{RuCl}}_3$ in both bulk and monolayer cases. Our results show that ${\mathrm{RuI}}_3$ bulk is a paramagnetic metal, which is in agreement with recent experiments. We find that the ${\mathrm{Ru}}^{3+}$ ion of ${\mathrm{RuI}}_3$ is in the spin-orbital entangled $j_{\mathrm{eff}}=\frac{1}{2}$ state. More interestingly, a metal-insulator transition occurs from ${\mathrm{RuI}}_3$ bulk to monolayer, and this is mainly due to the band narrowing with the decreasing lattice dimensionality and to the Ru-I hybridization altered by the I $5p$ spin-orbit coupling. In contrast, ${\mathrm{RuCl}}_3$ bulk and monolayer both show Mott-insulating behavior, the ${\mathrm{Ru}}^{3+}$ ion is in the formal $S=\frac{1}{2}$ and $L=1$ state with a large in-plane orbital moment, and this result well explains the experimental large effective magnetic moment of ${\mathrm{RuCl}}_3$ and the strong in-plane magnetization. The present work demonstrates the contrasting spin-orbital states and the varying properties of ${\mathrm{RuI}}_3$ and ${\mathrm{RuCl}}_3$.

Acoustic topological insulators that host the topological boundary states, which is insensitive to defects and disorder, have become an important research topic and provide completely new ways to manipulate acoustic waves. However, most of the acoustic topological boundary states generally appear in a single or two band gaps, hindering the applications in multiband acoustic devices. Compared to previous work, in this paper, we experimentally observe the acoustic multiple topological boundary states in four band gaps, including the end (corner) states in one-dimensional (two-dimensional) phononic crystals, based on the acoustic quartic-root topological insulators. These topological boundary states originate from two consecutive square-root procedures, which is similar to the square-root topological insulators. Our paper provides a brand-new approach to achieve the multiple boundary states by simply inserting additional cavities without elaborate designing the structure, which makes the manipulation of the acoustic more flexible.

Two geometric phases of mixed quantum states, known as the interferometric phase and Uhlmann phase, are generalizations of the Berry phase of pure states. After reviewing the two geometric phases and examining their parallel-transport conditions, we specify a class of cyclic processes that are compatible with both conditions and therefore accumulate both phases through their definitions, respectively. Those processes then facilitate a fair comparison between the two phases. We present exact solutions of two-level and three-level systems to contrast the two phases. While the interferometric phase exhibits finite-temperature transitions only in the three-level system but not the two-level system, the Uhlmann phase shows finite-temperature transitions in both cases. Thus, using the two geometric phases as finite-temperature topological indicators demonstrates the rich physics of topology of mixed states.

We report on domain pattern transfer from a ferroelectric ${\mathrm{BaTiO}}_3$ substrate with a (111) orientation of the surface to an epitaxial Co film grown on a Pd buffer layer. Spatially modulated interfacial strain transfer from ferroelectric/ferroelastic domains and inverse magnetostriction in the ferromagnetic film induce stripe regions with a modulation of the in-plane uniaxial magnetic anisotropy direction. Using spin-polarized low-energy electron microscopy, we observe the formation of two distinct anisotropy configurations between stripe regions, leading to angles of ${60}^{\circ }$ or ${120}^{\circ }$ between the magnetizations of adjacent domains. Moreover, through application of a magnetic field parallel or perpendicular to these stripes, head-to-head or head-to-tail magnetization configurations are initialized. This results in four distinct magnetic domain-wall types associated with different energies and domain widths, which, in turn, affects whether domain pattern transfer can be achieved.

The nitrogen-vacancy (NV) center in diamond is a heavily studied defect due to its potential applications to quantum metrology and computation, particularly in ambient conditions. The key mechanism to using the NV in any application lies in the ability to read out the spin state of the defect which is typically done optically. The optical contrast is then the key metric for electron spin readout fidelity and one of the key limiting factors in the NV's overall performance. We present a mechanism for high contrast readout using the spin-to-charge conversion mechanism in conjunction with an electrode to improve the spin contrast by altering the NV energy levels relative to the diamond conduction band. Theoretical modeling predicts an optical spin contrast at 42% which would be the highest optical contrast for the NV at room temperature and the technique opens up a range of alternative research pathways for the NV which are discussed.

The magnetic spin Hall effect (MSHE) may produce the out-of-plane spin current with $z$-direction polarization $\left({\mathit{\sigma }}_{\mathbf{z}}\right)$, thus allowing the field-free switching of perpendicular magnetization in ferromagnetic films. Here, we fabricate the epitaxial $L{1}_1\text{−}\mathrm{CoPt}/D{0}_{19}\text{−}\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ bilayers on MgO(111) substrates and characterize their magnetic and electrical transport properties. In the $L{1}_1\text{−}\mathrm{CoPt}/D{0}_{19}\text{−}\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ bilayers, we observe the spin-orbit torque switching of perpendicular magnetization under zero external field. The magnetization switching ratio and polarity are specifically related to the Néel vector direction of the antiferromagnetic ${\mathrm{Mn}}_3\mathrm{Ga}$ as well as the crystalline orientations which the pulse current is applied along. The field-free switching in the bilayers is attributed to the ${\mathit{\sigma }}_{\mathbf{z}}$ spin current generated by the MSHE, which is driven by the reduced symmetry of ${\mathrm{Mn}}_3\mathrm{Ga}$ film with noncollinear spin structure.

In the topological superconductor with the nematic superconductivity in the $E_u$ representation, it is possible to have different types of vortices. One is associated with the vorticity in the particle-hole space and corresponds to the Abrikosov vortex. Another type corresponds to the vorticity in the spin space and is called the spin vortex. We study the interaction of the Abrikosov vortex with the spin vortices. We derive the free energy of the sample with the Abrikosov and the strain-induced spin vortices using the Ginzburg-Landau approach for the two-component superconducting order parameter. We calculate the critical strain at which the spin vortex is formed. We show that the spin vortex and the Abrikosov vortex attract each other, and as a result, they have a common core. We apply Bogoliubov–de Gennes equations to study electronic states in a combined vortex structure. We show that no zero-energy states (Majorana fermions) are localized near the common vortex core of the Abrikosov vortex and the spin vortex of any type. Possible experimental realization is discussed.

Acoustic topological insulators have revolutionized our understanding of acoustic wave propagation, providing a robust way to manipulate acoustic waves. Acoustic switches offer new possibilities for logical operations, and the topological insulators further enrich the robustness of their signal execution process. To date, most studies in this field are focused on designing a static acoustic structure. However, the experimental realization of a dynamic topological phononic logic to switch acoustic waves is still a great challenge. Here, we experimentally demonstrate a topological acoustic switch based on compressible rubber columns, where the topological propagation route can be dynamically switched by external pressure. Our simulation and experiment results show that when the acoustic system is compressed, the trivial band gap can be efficiently switched to the nontrivial band gap, resulting in dynamic modulation of acoustic communication with high transmission efficiencies. Our study provides a guide to design a soft topological acoustic switch, which exhibits enormous application potential in acoustic signal sensing and topological transistors.

Kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$,Rb,Cs) exhibit an exotic charge order (CO), involving three order parameters, with broken translation and time-reversal symmetries compatible with the presence of orbital currents. The properties of this phase are still intensely debated, and it is unclear if the origin of the CO is mainly due to electron-electron or electron-phonon interactions. Most of the experimental studies confirm the nematicity of this state, a feature that might be enhanced by electronic correlations. However, it is still unclear whether the nematic CO becomes stable at a temperature equal to ($T_{\text{nem}}=T_{\text{C}}$) or lower than ($T_{\text{nem}}<T_{\text{C}}$) that of the CO itself. Here, we systematically characterize several CO configurations, some proposed for the new member of the family ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$, by combining phenomenological Ginzburg-Landau theories, valid irrespective of the specific ordering mechanism, with mean-field analysis. We find a few configurations for the CO that are in agreement with most of the experimental findings to date and that are described by different Ginzburg-Landau potentials. We propose to use resonant ultrasound spectroscopy to experimentally characterize the order parameters of the CO, such as the number of their components and their relative amplitude, and we provide an analysis of the corresponding elastic tensors. This might help to explain which mean-field configuration found in our study is the most representative for describing the CO state of kagome metals, and it can provide information regarding the nematicity onset temperature $T_{\text{nem}}$ with respect to $T_{\text{C}}$.

Spin-orbit coupling (SOC) and spatial inversion asymmetry are together capable of hosting a variety of effects such as Rashba and SOC spin splittings accompanied with Berry phase effects. Predominantly, spin-valley coupling and valley-contrasting physics are studied in hexagonal transition-metal dichalcogenides. In this work, we explore monoclinic monochalcogenide tellurene monolayer in two stable phases ($\beta$ and ${\beta }^{\prime }$) for strain coupled spin-valleytronic properties using first-principles calculations. A high Rashba coefficient of 0.90 eVÅ is obtained in strained $\beta$-Te while it magnifies to 1.55 eV Å at 8% strain in ${\beta }^{\prime }$-Te. Time-reversal-connected valleys in tellurene lead to an induced Berry curvature (BC) in $\beta$-Te by the application of strain. The strain-induced BC of 21.27 ${\mathrm{Bohr}}^2$ is obtained, specifically at 4.5% biaxial strain in $\beta$-Te by undergoing a phase transition, whereas noncentrosymmetric ${\beta }^{\prime }$-Te has a maximum BC of 22.90 ${\mathrm{Bohr}}^2$ at $-4$% strain. Large Rashba spin splittings with spin Berry curvature lead to intrinsically large spin Hall conductivity (SHC) in tellurene. Strain tunable SHC of $\sim 44(\hslash /e)\text{S/cm}$ is obtained for $p$-type ${\beta }^{\prime }$-Te at carrier concentration $n_h\approx 1.1\times {10}^{15}{\mathrm{cm}}^{-2}$, and SHC of $\sim 3.68(\hslash /e)\text{S/cm}$ is obtained for $n$-type ${\beta }^{\prime }$-Te at $n_e\approx 2.3\times {10}^{13}{\mathrm{cm}}^{-2}$. Moreover, finite SHC of $2.48(\hslash /e)\text{S/cm}$ and two pairs of edge states crossing in the forbidden region implies inversion asymmetric $\beta {}^{\prime }$ tellurene as a potential double quantum spin Hall insulator. The comprehensive study of the mentioned intriguing properties of tellurene indicates its high technological potential in a merged spin-valleytronic multifunctional nanodevice and in low-power quantum electronics.

Topological nodal-line semimetals gain extensive attention for their peculiar linear band crossings along closed lines in the Brillouin zone leading to various interesting properties. Here, we systematically study the nodal-line plasmons of prototypical nodal-line semimetals $\mathrm{ZrSi}X$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) by high-resolution electron energy loss spectroscopy, and first-principles calculations. In energy range from near- to mid-infrared frequencies, plasmons induced by the intraband correlations ($\sim 0.8$ eV) and the interband correlations ($\sim 0.3$ eV) of surface states related to the nodal-line electrons are universally observed in all the $\mathrm{ZrSi}X$ family. The bulk plasmon ($\sim 0.6$ eV) is observed in ZrSiS but is indiscernible in both ZrSiSe and ZrSiTe owing to their stronger screening effect from the surface states and large interlayer distance. Although the plasmons in ZrSiS seem to be temperature independent, the frequencies of the intraband plasmons in ZrSiSe and ZrSiTe show large redshifts ($\sim 10\%$) with temperature increasing from 30 K to room temperature, which can be ascribed to the prominent thermal occupation effect of nodal-line electrons when the Fermi level is close to the nodal line.

Planar tunneling spectroscopy of the Kondo insulator $\mathrm{Sm}{\mathrm{B}}_6$ suggests that an interaction between the surface Dirac fermions and the bulk spin excitons results in incompletely protected topological surface states. To gain further insight into their true nature, it is necessary to study other topological Kondo insulator candidates. Calculations of electronic energy bands predict that the Kondo insulator $\mathrm{Yb}{\mathrm{B}}_{12}$ hosts topological surface states protected by crystalline mirror symmetry. In this study, we present tunneling conductance spectra obtained from the (001) surface of $\mathrm{Yb}{\mathrm{B}}_{12}$ single crystals and discuss them in comparison to $\mathrm{Sm}{\mathrm{B}}_6$. The linear conductance at low bias provides strong evidence for the existence of surface Dirac fermions. The double-hump structure in the negative-bias region is associated with hybridized band edges, in agreement with a calculated band structure. While these similarities with $\mathrm{Sm}{\mathrm{B}}_6$ are suggestive of the existence of topological surface states in $\mathrm{Yb}{\mathrm{B}}_{12}$, in agreement with other experiments, some discrepancies are also observed, which we attribute to a difference in their exact nature from those in $\mathrm{Sm}{\mathrm{B}}_6$.

In this work, we report on electron spin resonance studies of H atoms stabilized in solid ${\mathrm{H}}_2$ films at temperature $0.1$ and $0.7\mathrm{K}$ and in a magnetic field of $4.6\mathrm{T}$. We produced H atoms by two different techniques: bombarding ${\mathrm{H}}_2$ films by $0.1\mathrm{keV}$ electrons generated during an rf discharge run in the sample cell or exposing ${\mathrm{H}}_2$ films to a flux of $5.7\mathrm{keV}$ electrons released during tritium decay. We observed a faster H atom accumulation in the films made of ${\mathrm{H}}_2$ gas with a small initial ortho-${\mathrm{H}}_2$ content (0.2–3%) as compared with those made from the gas with a higher initial ortho-${\mathrm{H}}_2$ admixture. The accumulation rate difference was about 70% for the samples exposed to high energy electrons and about an order of magnitude for the samples bombarded by low-energy electrons. We propose possible explanations for the observed behavior.

We propose two machine-learning methods based on neural networks, which we respectively call the phase-classification method and the temperature-identification method, for detecting different types of phase transitions in the XXZ models without prior knowledge of their critical temperatures. The XXZ models have exchange couplings which are anisotropic in the spin space where the strength is represented by a parameter $\mathrm{\Delta }(>0)$. The models exhibit second-order phase transition when $\mathrm{\Delta }>1$, but the Berezinskii-Kosterlitz-Thouless (BKT) phase transition when $\mathrm{\Delta }<1$. In the phase-classification method, the neural network is trained using spin or vortex configurations of well-known classical spin models other than the XXZ models, e.g., the Ising models and the XY models, to classify those of the XXZ models to corresponding phases. We demonstrate that the trained neural network successfully detects the phase transitions for both $\mathrm{\Delta }>1$ and $\mathrm{\Delta }<1$, and the evaluated critical temperatures coincide well with those evaluated by conventional numerical calculations. In the temperature-identification method, on the other hand, the neural network is trained so as to identify temperatures at which the input spin or vortex configurations are generated by the Monte Carlo thermalization. The critical temperatures are evaluated by analyzing the optimized weight matrix, which coincides with the result of numerical calculation for the second-order phase transition in the Ising-like XXZ model with $\mathrm{\Delta }=1.05$ but cannot be determined uniquely for the BKT transition in the XY-like XXZ model with $\mathrm{\Delta }=0.95$.